Search

We estimate the progenitor and explosion properties of the nearby Type II SN 2023ixf using a synthetic model grid of Type II supernova light curves. By comparing the light curves of SN 2023ixf with the pre-existing grid of Type II supernovae containing about 228000 models with different combinations of the progenitor and explosion properties, we obtain the |$\chi ^2$| value for every model and evaluate the properties of the models providing small values of |$\chi ^2$|⁠. We found that the light-curve models with a progenitor zero-age main-sequence mass of |$10\, {M}_\odot$|⁠ , explosion energy of |$(2\\!-\\!3) \times 10^{51}\:\mbox{erg}$|⁠ , |$^{56}\mbox{Ni}$| mass of 0.04– |$0.06\, {M}_\odot$|⁠ , mass-loss rate of |$10^{-3}$| – |$10^{-2}\, {M}_\odot \:\mbox{yr}^{-1}$| with wind velocity of |$10\:\mbox{km}\:\mbox{s}^{-1}$|⁠ , and dense, confined circumstellar matter radius of |$(6\\!-\\!10) \times 10^{14}\:\mbox{cm}$| match well to the observed light curves of SN 2023ixf. The photospheric velocity evolution of these models is also consistent with the observed velocity evolution. We note that the progenitor mass estimate could be affected by the adopted progenitor models. Although our parameter estimation is based on a pre-existing model grid and we do not perform any additional computations, the estimated parameters are consistent with those obtained by the detailed modeling of SN 2023ixf previously reported. This result shows that comparing the pre-existing model grid is a reasonable way to obtain a rough estimate for the properties of Type II supernovae. This simple way to estimate the properties of Type II supernovae will be essential in the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) era when thousands of Type II supernovae are expected to be discovered yearly. [ABSTRACT FROM AUTHOR]

We present an investigation of the L -band emission from known massive young stellar objects in the South African Radio Astronomy Observatory MeerKAT Galactic Plane Survey to search for non-thermal radio emitters in the sample. A total of 398 massive protostars, identified from the Red MSX Source survey, are located within the survey region. Among these, 162 fields that host the protostars are isolated from nearby bright H  ii regions, allowing for the study of any ionized jets present. Seventy-one of these fields have jets with five-sigma detections or higher, corresponding to a detection rate of 44 per cent. The MeerKAT fluxes of the detections, together with the upper limits of the non-detections and any other fluxes from previous observations, were used to estimate the spectral indices of the jets, and to search for the presence of non-thermal radiation. In cases where a source manifests as single in a given observation but is resolved into multiple components in observations of higher resolutions, the sum of the fluxes of the resolved components was used in estimating the indices. Any effects from missing flux in higher resolution observations were incorporated into the index uncertainties. The spectral indices of the sample show that at least 50 per cent of the jets emit non-thermal radiation. Additionally, the spectral energy distribution of some of the sources, as well as their radio luminosities exhibit evidence of non-thermal emission, especially in extended sources. [ABSTRACT FROM AUTHOR]

The viscous decretion disc (VDD) scenario has been used to model the observables of the Be stars. Its capability to predict individual observables has been confirmed for several Be stars. Here, we simultaneously analyse the spectroscopic and |$BVI$| -band photometric data for the Be star MT91-213 with the Monte Carlo radiative-transfer code hdust to determine the stellar parameters, geometry, and physical conditions for its circumstellar disc. MT91-213 is the primary component of a binary system whose companion is the pulsar PSR J2032+4127. We find that the VDD model can simultaneously reproduce the multiple observables qualitatively, but not quantitatively. We determine the mass of the primary star to be 13.1  |${\rm M}_{\odot }$| which is smaller than reported in the literature. We present a dynamical scenario for the evolving disc density from a diffuse to a dense phase. Also, we determine that the inclination of the disc is about 40 |$^{\circ }$| which means it is 20 |$^{\circ }$| tilted from the orbit of the secondary star. Our results indicate that the mass loss rate for MT91-213 is |$\sim 10^{-7}$| to |$10^{-6} {\rm M}_{\odot }\,\mathrm{yr}^{-1}$| which is in agreement with the suggested values in the literature, required to explain the observed X-ray synchrotron luminosity, |$L_\mathrm{x}$|⁠ , for PSR J2032+4127. [ABSTRACT FROM AUTHOR]

At metallicities lower than that of the Small Magellanic Cloud, it remains essentially unexplored how fossil magnetic fields, forming large-scale magnetospheres, could affect the evolution of massive stars, thereby impacting the fundamental building blocks of the early Universe. We extend our stellar evolution model grid with representative calculations of main-sequence, single-star models with initial masses of 20 and 60 M |$_\odot$|⁠ , including appropriate changes for low-metallicity environments (⁠|$Z = 10^{-3}$| – |$10^{-6}$|⁠). We scrutinize the magnetic, rotational, and chemical properties of the models. When lowering the metallicity, the rotational velocities can become higher and the tendency towards quasi-chemically homogeneous evolution increases. While magnetic fields aim to prevent the development of this evolutionary channel, the weakening stellar winds lead to less efficient magnetic braking in our models. Since the stellar radius is almost constant during a blueward evolution caused by efficient chemical mixing, the surface magnetic field strength remains unchanged in some models. We find core masses at the terminal-age main sequence between 22 and 52 M |$_\odot$| for initially 60 M |$_\odot$| models. This large difference is due to the vastly different chemical and rotational evolution. We conclude that in order to explain chemical species and, in particular, high nitrogen abundances in the early Universe, the adopted stellar models need to be under scrutiny. The assumptions regarding wind physics, chemical mixing, and magnetic fields will strongly impact the model predictions. [ABSTRACT FROM AUTHOR]

Surveys in the Milky Way and Large Magellanic Cloud have revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, and therefore in conditions approaching those of the Early Universe, remain sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign, an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the Small Magellanic Cloud, allowing us to probe multiplicity in the lowest-metallicity conditions to date (Z = 0.2 Z⊙). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods of P ≲ 3 yr, (iii) dormant black-hole binary candidates (OB+BH), and (iv) a legacy database of physical parameters of massive stars at low metallicity. Main sequence (OB-type) and evolved (OBAF-type) massive stars are observed with the LR02 setup of the GIRAFFE instrument of the Very Large Telescope (3960–4570 Å resolving power R = 6200; typical signal-to-noise ratio(S/N) ≈70–100). This paper utilises the first nine epochs obtained over a three-month time period. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. Our detailed classification reveals that the sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5 ≲ Teff/kK ≲ 45 and 3.7 < log L/L⊙ < 6.1 and initial masses of 8 ≲ Mini ≲ 80 M⊙. The sample comprises 159 O-type stars, 331 early B-type (B0–3) dwarfs and giants (luminosity classes V–III), 303 early B-type supergiants (II–I), and 136 late-type BAF supergiants. At least 82 stars are OBe stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, the sample includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries. [ABSTRACT FROM AUTHOR]

Context. Large spectroscopic surveys of individual massive stars, such as ULLYSES and XShootU, provide observational data for hundreds of massive stars. Their analysis requires large numbers of synthetic spectra so that stellar parameters can be determined. In addition, libraries of massive stars' spectra are needed to produce population synthesis models able to reproduce the observed spectra of unresolved young stellar populations, such as those revealed by the James Webb Space Telescope (JWST) in the early Universe. Aims. Our main goal is to provide an extensive library of synthetic spectra and spectral energy distributions of OB stars at metallicities of the Magellanic Clouds. This library will offer a wealth of spectrophotometric information, making it readily applicable to a variety of astrophysical problems. Methods. We used the CMFGEN code to calculate 606 NLTE, line-blanketed, expanding atmosphere models using a comprehensive set of atomic data. An overall metallicity of 1/2 Z⊙ and 1/5 Z⊙ was adopted for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively. We produced high-resolution spectra from 30 Å to 3 µm for stars on the Main Sequence and slightly beyond. Results. We provide spectral energy distributions, normalized synthetic spectra, ionizing fluxes, and photometry in various bands: Johnson UBV, Cousins RI, Bessel JHK, selected wide JWST filters, Gaia, and LSST ugrizy filters. For each of these filters, we compute bolometric corrections for all synthetic spectra and calibrations as a function of effective temperature. Conclusions. All of our synthetic spectra are publicly available through the POLLUX database, aiming to expedite multiwavelength analyses of massive stars in low metallicity environments. [ABSTRACT FROM AUTHOR]

Context. Ultra-luminous X-ray sources (ULXs) are high-mass X-ray binaries with an X-ray luminosity above 1039 erg s−1. These ULXs can be powered by black holes that are more massive than 20 M⊙, accreting in a standard regime, or lighter compact objects accreting supercritically. There are only a few ULXs with known optical or ultraviolet (UV) counterparts, and their nature is debated. Determining whether optical/UV radiation is produced by the donor star or by the accretion disc is crucial for understanding ULX physics and testing massive binary evolution. Aims. We conduct, for the first time, a fully consistent multi-wavelength spectral analysis of a ULX and its circumstellar nebula. We aim to establish the donor star type and test the presence of strong disc winds in the prototypical ULX Holmberg II X-1 (Ho II X-1). Furthermore, we aim to obtain a realistic spectral energy distribution of the ionising source, which is needed for robust nebula analysis. We acquired new UV spectra of Ho II X-1 with the Hubble Space Telescope (HST) and complemented them with archival optical and X-ray data. We explored the spectral energy distribution of the source and analysed the spectra using the stellar atmosphere code PoWR and the photoionisation code CLOUDY. Our analysis of the X-ray, UV, and optical spectra of Ho II X-1 and its nebula consistently explains the observations. We do not find traces of disc wind signatures in the UV and the optical, rejecting previous claims of the ULX being a supercritical accretor. The optical/UV counterpart of Ho II X-1 is explained by a B-type supergiant donor star. Thus, the observations are fully compatible with Ho II X-1 being a close binary consisting of an ≳66 M⊙ black hole accreting matter from an ≃22 M⊙ B-supergiant companion. Furthermore, we propose a possible evolution scenario for the system, suggesting that Ho II X-1 is a potential gravitational wave source progenitor. [ABSTRACT FROM AUTHOR]

This work aims to analyze some of the polluters proposed in the self-enrichment scenarios put forward to explain the multiple populations in globular clusters (GCs), extending previous studies. Three scenarios with different polluter stars were tested: asymptotic giant branch stars (AGBs), high-mass interacting binaries (IBs), and fast rotating massive stars (FRMSs). With abundance data available from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey and ΔY estimates from precise Hubble Space Telescope (HST) photometry, twenty-six clusters were studied, increasing the number of clusters in previous studies by more than a factor of three. We also included the study of the abundances of N, C, Mg, and Al, extending previous studies that mainly focused on the abundances of He, O, and Na. In addition, we constructed an empirical model to test whether one could explain the chemical signatures of the "enriched" population of GC stars with a fixed source and dilution process based on empirical data. In agreement with work by other authors, we found that the proposed polluters can generally predict the qualitative abundance patterns in GC stars and in some cases quantitatively predict some elements, but in most cases when we compare the model yields with the observations, we find that they cannot explain the entire set of observed abundance patterns. The empirical model succeeds in reproducing the abundances of Al for a given ΔY (and vice versa), showing that there is a direct relationship between Al and He, with one increasing proportionally to the other. However, the empirical model fails to reproduce the observed abundances of Na and N, in agreement with the results of previous works. The observed decoupling between the maximum abundances of CNO-cycle elements such as N and Na with those of Al and He provides new information and constraints for future models and could take us a step closer to understanding the origin of the peculiar abundance variations of GC stars. [ABSTRACT FROM AUTHOR]

Context. Massive stars play a crucial role in the cosmic dynamics and chemical evolution of galaxies. Despite their significance, our understanding of their evolution and properties remains limited. An accurate determination of stellar parameters, such as the mass and radius, is essential for advancing our knowledge. Detached eclipsing binaries (DEBs) are particularly valuable for these determinations due to the minimal interaction between their stellar components, allowing for precise measurements. Aims. This study aims to introduce the Young Massive Detached Binary (YMDB) catalog, designed to address the gap in the high-precision absolute parameter determination for young massive stars. By focusing on DEBs within the spectral range O9-B1, this catalog seeks to provide a reliable database for future astronomical studies and improve our understanding of massive star evolution. Methods. We conducted a photometric analysis of 87 young massive stars in detached eclipsing systems using TESS light curves (LCs) that were processed through a custom pipeline. This analysis involved determining the amplitude of magnitude variations, orbital periods, times of minima, eccentricities, and the presence of apsidal motion and heartbeat phenomena. A thorough literature review was performed to obtain MK spectral classifications. We performed our own spectral classification of 19 systems to support the sample where a new classification was lacking or inconclusive. Results. The analysis identified 20 previously unreported binary systems, with 13 newly recognized as variable stars. Among the 87 stars examined, 30 are confirmed as YMDB members, and 25 are candidates pending spectral classification. The exclusion of the remaining 32 stars is attributed to unsuitable spectral types or their nondetached binary nature. Notable findings include the identification of new LC classifications, eccentricities in 13 systems, and heartbeat phenomena in several targets. Conclusions. The YMDB catalog offers a resource of high-quality LCs and reliable stellar classifications, serving as a valuable tool for the astronomical community. [ABSTRACT FROM AUTHOR]

Context. Recently, astrometric and spectroscopic surveys of OB stars revealed a few stellar-mass black holes (BHs) with orbital periods of as low as 10 days. Contrary to wind-fed BH high-mass X-ray binaries (HMXBs), no X-ray counterpart was detected, probably because of the absence of a radiatively efficient accretion disc around the BH. Nevertheless, dissipative processes in the hot, dilute, and strongly magnetised plasma around the BH (so-called BH corona) can still lead to non-thermal X-ray emission (e.g. synchrotron). Aims. We determine the X-ray luminosity distribution from BH+OB star binaries up to orbital periods of a few thousand days. Methods. We used detailed binary evolution models computed with MESA for initial primary masses of 10–90 M⊙ and orbital periods of 1–3000 d. We computed the X-ray luminosity for a broad range of radiative efficiencies that depend on the mass accretion rate and flow geometry. Results. For typical conditions around stellar-mass BHs, we show that particle acceleration through magnetic reconnection can heat the BH corona. A substantial fraction of the gravitational potential energy from the accreted plasma is converted into non-thermal X-ray emission. Our population synthesis analysis predicts that at least 28 (up to 72) BH+OB star binaries in the Large Magellanic Cloud (LMC) produce X-ray luminosities of above 1031 erg s−1, which are observable with focused Chandra observations. We identify a population of SB1 systems in the LMC and HD96670 in the Milky Way comprising O stars with unseen companions of masses of above 2.3 M⊙, which aligns well with our predictions and may be interesting sources for follow-up observations. The predicted luminosities of the OB companions to these X-ray-emitting BHs are 104.5 − 5.5L⊙. Conclusions. These findings advocate for prolonged X-ray observations of the stellar-mass black hole candidates identified in the vicinity of OB stars. Such long exposures could reveal the underlying population of X-ray-faint BHs and provide constraints for the evolution from single to double degenerate binaries and identify the progenitors of gravitational wave mergers. [ABSTRACT FROM AUTHOR]

Context. The evolution and fate of massive stars are thought to be affected by rotationally induced internal mixing. The surface boron abundance is a sensitive tracer of this in early B-type main sequence stars. Aims. We test current stellar evolution models of massive main sequence stars which include rotational mixing through a systematic study of their predicted surface boron depletion. Methods. We construct a dense grid of rotating single star models using MESA, for which we employ a new nuclear network which follows all the stable isotopes up to silicon, including lithium, beryllium, boron, as well as the radioactive isotope aluminium-26. We also compile the measured physical parameters of the 90 Galactic early B-type stars with boron abundance information. We then compare each observed stars with our models through a Bayesian analysis, which yields the mixing efficiency parameter with which the star is reproduced the best, and the probability that it is represented by the stellar models. Results. We find that about two-thirds of the sample stars are well represented by the stellar models, with the best agreement achieved for a rotational mixing efficiency of ∼50% compared to the widely adopted value. The remaining one third of the stars, of which many are strongly boron depleted slow rotators, are largely incompatible with our models, for any rotational mixing efficiency. We investigate the observational incidence of binary companions and surface magnetic fields, and discuss their evolutionary implications. Conclusions. Our results confirm the concept of rotational mixing in radiative stellar envelopes. On the other hand, we find that a different boron depletion mechanism, and likely a different formation path, is required to explain about one-third of the sample stars. The large spread in the surface boron abundances of these stars may hold a clue to understanding their origin. [ABSTRACT FROM AUTHOR]

Context. Despite mounting evidence that extreme red supergiants (RSGs) undergo episodic mass-loss events, their role in RSG evolution remains uncertain. Critical questions remain unanswered, such as whether or not these events can strip the star, and their timescale and frequency. Aims. This study delves into [W60] B90, one of the most luminous and extreme RSGs in the Large Magellanic Cloud (LMC), with our aim being to search for evidence of episodic mass loss. Our discovery of a bar-like nebular structure at 1 pc, which is reminiscent of the bar around Betelgeuse, raised the question of whether [W60] B90 also has a bow shock, motivating the present study. Methods. We collected and analyzed proper motion data from Gaia, as well as new multi-epoch spectroscopic and imaging data, and archival time-series photometry in the optical and mid-infrared (MIR). We used MARCS models to derive the physical properties of the star from the spectra. Results. We find [W60] B90 to be a walkaway star, with a supersonic peculiar velocity in the direction of the bar. We detect shocked emission between the bar and the star, based on the [S II]/Hα > 0.4 criterion, providing strong evidence for a bow shock. The 30 yr optical light curve reveals semi-regular variability, showing three similar dimming events with ΔV ~ 1 mag, a recurrence of ~12 yr, and a rise time of 400 days. We find the MIR light curve to vary by 0.51 mag and 0.37 mag in the WISE1 and WISE2 bands, respectively, and by 0.42 mag and 0.25 mag during the last dimming event. During this event, optical spectroscopy reveals spectral variability (M3 I to M4 I), a correlation between the Teff and the brightness, increased extinction, and, after the minimum, spectral features incompatible with the models. We also find a difference of >300 K between the Teff measured from the TiO bands in the optical and the atomic lines from our J-band spectroscopy. Conclusions. [W60] B90 is a more massive analog of Betelgeuse in the LMC and therefore the first single extragalactic RSG with a suspected bow shock. Its high luminosity of log(L/L⊙) = 5.32 dex, mass-loss rate, and MIR variability compared to other RSGs in the LMC indicate that it is in an unstable evolutionary state, undergoing episodes of mass loss. Investigating other luminous and extreme RSGs in low-metallicity environments using both archival photometry and spectroscopy is crucial to understanding the mechanism driving episodic mass loss in extreme RSGs in light of the Humphreys-Davidson limit and the "RSG problem". [ABSTRACT FROM AUTHOR]

Gaia BH3 is the first observed dormant black hole (BH) with a mass of ≈30 M⊙, and it represents the first confirmation that such massive BHs are associated with metal-poor stars. Here, we explore the isolated binary formation channel for Gaia BH3, focusing on the old and metal-poor stellar population of the Milky Way halo. We used the MIST stellar models and our open-source population synthesis code SEVN to evolve 5.6 × 108 binaries, exploring 20 sets of parameters that encompass different natal kicks, metallicities, common envelope efficiencies and binding energies, and models for the Roche-lobe overflow. We find that systems such as Gaia BH3 form preferentially from binaries initially composed of a massive star (40–60 M⊙) and a low-mass companion (<1 M⊙) in a wide (P > 103 days) and eccentric orbit (e > 0.6). Such progenitor binary stars do not undergo any Roche-lobe overflow episode during their entire evolution, so the final orbital properties of the BH-star system are essentially determined at the core collapse of the primary star. Low natal kicks (≲ 10 km/s) significantly favour the formation of Gaia BH3-like systems, but high velocity kicks up to ≈220 km/s are also allowed. We estimated the formation efficiency for Gaia BH3-like systems in old (t >10 Gyr) and metal-poor (Z < 0.01) populations to be ∼4 × 10−8 M⊙−1 (for our fiducial model), representing ~3% of the whole simulated BH-star population. We expect up to ≈4000 BH-star systems in the Galactic halo formed through isolated evolution, of which ≈100 are compatible with Gaia BH3. Gaia BH3-like systems represent a common product of isolated binary evolution at low metallicity (Z < 0.01), but given the steep density profile of the Galactic halo, we do not expect more than one at the observed distance of Gaia BH3. Our models show that even if it was born inside a stellar cluster, Gaia BH3 is compatible with a primordial binary star that escaped from its parent cluster without experiencing significant dynamical interactions. [ABSTRACT FROM AUTHOR]

Products of stellar mergers are predicted to be common in stellar populations and can potentially explain stars with peculiar properties. When the merger occurs after the initially more massive star has evolved into the Hertzsprung gap, the merger product may remain in the blue part of the Hertzsprung–Russell diagram for millions of years. Such objects could, therefore, explain the overabundance of observed blue stars, such as blue supergiants. However, it is currently not straightforward to distinguish merger products from genuine single stars or other stars with similar surface diagnostics. In this work, we made detailed asteroseismic comparisons between models of massive post-main-sequence merger products and genuine single stars to identify which asteroseismic diagnostics can be used to distinguish them. In doing so, we developed tools for the relatively young field of merger seismology. Genuine single stars in the Hertzsprung gap are fully radiative, while merger products have a convective He-burning core and convective H-burning shell while occupying similar locations in the Hertzsprung–Russell diagram. These major structural differences are reflected in lower asymptotic period spacing values for merger products and the appearance of deep dips in their period spacing patterns. Our genuine single-star models with masses above roughly 11.4 solar masses develop short-lived intermediate convective zones during their Hertzsprung gap evolution. This also leads to deep dips in their period spacing patterns. Because of the lack of a convective core, merger products and genuine single stars can be distinguished based on their asymptotic period spacing value in this mass range. We performed the comparisons with and without the effects of slow rotation included in the pulsation equations and conclude that the two types of stars are seismically distinguishable in both cases. The observability of the distinguishing asteroseismic features of merger products can now be assessed and exploited in practice. [ABSTRACT FROM AUTHOR]

Context. The outcome of the formation of massive stars is an important anchor point in the formation and evolution process of these stars. It provides insight into the physics of the assembly process, and sets the conditions for stellar evolution. For massive stars, the outcome of formation is rarely observed because the processes involved unfold deep down in highly extincted molecular clouds. Aims. We characterize a population of highly reddened stars in the very young massive star-forming region M17. The group of 18 O4.5 to B9 stars constitutes one of the best samples of almost zero-age main-sequence (ZAMS) high- and intermediate-mass stars. Their properties allow us to identify the empirical location of the ZAMS of massive stars, and the rotation and mass-loss rate of stars close to or at the onset of core-hydrogen burning. Methods. We performed quantitative spectroscopic modeling of a uniform set of over 100 spectral features in optical VLT/X-shooter spectra using the nonlocal thermal equilibrium stellar atmosphere code FASTWIND and a fitting approach based on a genetic algorithm, KIWI-GA. The spectral energy distributions of photometric observations were used to determine the line-of-sight extinction. From a comparison of their positions in the Hertzsprung-Russell diagram with MIST evolutionary tracks, we inferred the stellar masses and ages. Results. We find an age of 0.4−0.2+0.6 Myr for our sample, however we also identify a strong relation between the age and the mass of the stars. All sources are highly reddened, with AV ranging from 3.6 to 10.6 mag. The sample can be subdivided into two groups. Stars more massive than 10 M⊙ have reached the ZAMS. Their projected ZAMS spin rate distribution extends to 0.3 of the critical velocity; their mass-loss rates agree with those of other main-sequence O and early-B stars. Stars with a mass in the range 3 < M/M⊙ < 7 are still on the pre-main sequence (PMS), and some of them have circumstellar disks. Evolving their υ sin i to the ZAMS assuming angular momentum conservation yields values up to ~0.6 υcrit. For PMS stars without disks, we find tentative mass-loss rates up to 10−8.5 M⊙ yr−1. The total-to-selective extinction RV is higher for PMS stars with disks than for the remainder of the sample. Conclusions. We constrain the empirical location of the ZAMS for massive (10 < M/M⊙ < 50) stars and find it to agree with its location in MIST evolutionary tracks. The ZAMS rotation rates for intermediate-mass stars are twice as high as for massive stars, suggesting that the angular momentum gain processes differ between the two groups. The relation between the age and mass of the stars suggests a lag in the formation of more massive stars relative to lower mass stars. Taking the derived mass-loss rates at face value, stellar winds are already initiated in the PMS phase. The PMS-star winds are found to be substantially more powerful than indicated by predictions for line-driven outflows. [ABSTRACT FROM AUTHOR]

Massive binary evolution models are needed to predict massive star populations in star-forming galaxies, the supernova diversity, and the number and properties of gravitational wave sources. Such models are often computed using so-called rapid binary evolution codes, which approximate the evolution of the binary components based on detailed single star models. However, about one-third of the interacting massive binary stars undergo mass transfer during core hydrogen-burning (Case A mass transfer), whose outcome is difficult to derive from single star models. For this work, we used a large grid of detailed binary evolution models for primaries in the initial mass range 10–40 M⊙ with a Large and Small Magellanic Cloud composition, to derive analytic fits for the key quantities needed in rapid binary evolution codes, that is, the duration of core hydrogen-burning, and the resulting donor star mass. We find that systems with shorter orbital periods produce up to 50% lighter stripped donors and have a lifetime up to 30% larger than wider systems. Both quantities strongly depend on the initial binary orbital period, but the initial mass ratio and the mass-transfer efficiency of the binary have little impact on the outcome. Our results are easily parameterisable and can be used to capture the effects of Case A mass transfer more accurately in rapid binary evolution codes. [ABSTRACT FROM AUTHOR]

Context. Grids of stellar evolution models with rotation using the Geneva stellar evolution code (GENEC) have been published for a wide range of metallicities. Aims. We introduce the last remaining grid of GENEC models, with a metallicity of Z = 10−5. We study the impact of this extremely metal-poor initial composition on various aspects of stellar evolution, and compare it to the results from previous grids at other metallicities. We provide electronic tables that can be used to interpolate between stellar evolution tracks and for population synthesis. Methods. Using the same physics as in the previous papers of this series, we computed a grid of stellar evolution models with GENEC spanning masses between 1.7 and 500 M⊙, with and without rotation, at a metallicity of Z = 10−5. Results. Due to the extremely low metallicity of the models, mass-loss processes are negligible for all except the most massive stars. For most properties (such as evolutionary tracks in the Hertzsprung-Russell diagram, lifetimes, and final fates), the present models fit neatly between those previously computed at surrounding metallicities. However, specific to this metallicity is the very large production of primary nitrogen in moderately rotating stars, which is linked to the interplay between the hydrogen- and helium-burning regions. Conclusions. The stars in the present grid are interesting candidates as sources of nitrogen-enrichment in the early Universe. Indeed, they may have formed very early on from material previously enriched by the massive short-lived Population III stars, and as such constitute a very important piece in the puzzle that is the history of the Universe. [ABSTRACT FROM AUTHOR]

Context. Investigating the flow of material along filamentary structures towards the central core can help provide insights into high-mass star formation and evolution. Aims. Our main motivation is to answer the question of what the properties of accretion flows are in star-forming clusters. We used data from the ALMA Evolutionary Study of High Mass Protocluster Formation in the Galaxy (ALMAGAL) survey to study 100 ALMAGAL regions at a ∼1″ resolution, located between ∼2 and 6 kpc. Methods. Making use of the ALMAGAL ∼1.3 mm line and continuum data, we estimated flow rates onto individual cores. We focus specifically on flow rates along filamentary structures associated with these cores. Our primary analysis is centered around position velocity cuts in H2CO (30, 3–20, 2), which allow us to measure the velocity fields surrounding these cores. Combining this work with column density estimates, we were able to derive the flow rates along the extended filamentary structures associated with cores in these regions. Results. We selected a sample of 100 ALMAGAL regions, covering four evolutionary stages from quiescent to protostellar, young stellar objects (YSOs), and HII regions (25 each). Using a dendrogram and line analysis, we identify a final sample of 182 cores in 87 regions. In this paper, we present 728 flow rates for our sample (4 per core), analysed in the context of evolutionary stage, distance from the core, and core mass. On average, for the whole sample, we derived flow rates on the order of ∼10−4 M⊙ yr−1 with estimated uncertainties of ±50%. We see increasing differences in the values among evolutionary stages, most notably between the less evolved (quiescent and protostellar) and more evolved (YSO and HII region) sources and we also see an increasing trend as we move further away from the centre of these cores. We also find a clear relationship between the calculated flow rates and core masses ∼M2/3, which is in line with the result expected from the tidal-lobe accretion mechanism. The significance of these relationships is tested with Kolmogorov–Smirnov and Mann-Whitney U tests. Conclusions. Overall, we see an increasing trend in the relationships between the flow rate and the three investigated parameters, namely: evolutionary stage, distance from the core, and core mass. [ABSTRACT FROM AUTHOR]

Context. The inner parts of the hot discs surrounding massive young stellar objects (MYSOs) are still barely explored due to observational limitations in terms of angular resolution, scarcity of diagnostic lines, and the embedded and rare nature of these targets. Aims. We present the first K-band spectro-interferometric observations towards the MYSO G033.3891, which based on former kinematic evidence via the CO bandhead emission is known to host an accreting disc. Methods. Using the high spectral resolution mode (R∼4000) of the GRAVITY/VLTI, we spatially resolved the emission of the inner dusty disc and the crucial gaseous interface between the star and the dusty disc. Using detailed modelling on the K-band dust continuum and tracers known to be associated with the ionised and molecular gaseous interface (Brγ, CO), we report on the smallest scales of accretion and ejection. Results. The new observations in combination with our geometric and kinematic models employed to fit former high spectral resolution observations on the source (R∼30 000; CRIRES/VLTI) allowed us to constrain the size of the inner gaseous disc both spatially and kinematically via the CO overtone emission at only 2 au. Our models reveal that both Brγ and CO emissions are located well within the dust sublimation radius (5 au) as traced by the hot 2.2 µm dust continuum. Conclusions. Our paper provides the first case study where the tiniest scales of gaseous accretion around the MYSO G033.3891 are probed both kinematically and spatially via the CO bandhead emission. This analysis of G033.3891 stands as only the second instance of such an investigation within MYSOs, underscoring the gradual accumulation of knowledge regarding how massive young stars gain their mass while further solidifying the disc nature of accretion at the smallest scales of MYSOs. [ABSTRACT FROM AUTHOR]

Context. Wolf–Rayet (WR) stars are massive evolved stars that exhibit particularly fast and dense stellar winds. Although they constitute a very short phase near the end of a massive star's life, they play a crucial role in the evolution of massive stars and have a substantial impact on their surrounding environment. Aims. We present the most comprehensive and deepest X-ray study to date of the properties of the richest Wolf–Rayet population observed in a single stellar cluster, Westerlund 1 (Wd1). By examining the X-ray signatures of WR stars, we aim to shed light on the hottest plasma in their stellar winds and gain insights into whether they exist as single stars or within binary systems. Methods. This work is based on 36 Chandra observations obtained from the "Extended Westerlund 1 and 2 Open Clusters Survey" (EWOCS) project, plus 8 archival Chandra observations. The overall exposure depth Ms) and baseline of the EWOCS observations extending over more than one year enable us to perform a detailed photometric, colour, and spectral analysis, as well as to search for short- and long-term periodicity. Results. In X-rays, we detect 20 out of the 24 known Wolf–Rayet stars in Wd1 down to an observed luminosity of ~7 × 1029 erg s−1 (assuming a distance of 4.23 kpc to Wd1), with 8 WR stars being detected in X-rays for the first time. Nine stars show clear evidence of variability over the year-long baseline, with clear signs of periodicity. The X-ray colours and spectral analysis reveal that the vast majority of the WR stars are hard X-ray sources (kT≥2.0 keV). The Fe XXV emission line at ~6.7 keV, which commonly originates from the wind–wind collision zone in binary systems, is detected for the first time in the spectra of 17 WR stars in Wd1. In addition the ~6.4 keV fluorescent line is observed in the spectra of three stars, which are among the very few massive stars exhibiting this line, indicating that dense cold material coexists with the hot gas in these systems. Overall, our X-ray results alone suggest a very high binary fraction (≥80%) for the WR star population in Wd1. When combining our results with properties of the WR population from other wavelengths, we estimate a binary fraction of ≥92%, which could even reach unity. This suggests that either all the most massive stars are found in binary systems within Wd1, or that binarity is essential for the formation of such a rich population of WR stars. [ABSTRACT FROM AUTHOR]

Context. There is a growing number of peculiar events that cannot be assigned to any of the main classes. SN 1987A and a handful of similar objects, thought to be explosive outcomes of blue supergiant stars, is one of them: while their spectra closely resemble those of H-rich (IIP) SNe, their light curve (LC) evolution is very different. Aims. Here we present the detailed photometric and spectroscopic analysis of SN 2021aatd, a peculiar Type II explosion. While its early-time evolution resembles that of the slowly evolving double-peaked SN 2020faa (although at a lower luminosity scale), after ∼40 days its LC shape becomes similar to that of SN 1987A-like explosions. Methods. In addition to comparing LCs, color curves, and spectra of SN 2021aatd to those of SNe 2020faa, 1987A, and other objects, we compared the observed spectra with our own SYN++ models and with the outputs of published radiative transfer models. We also carried out a detailed modeling of the pseudo-bolometric LCs of SNe 2021aatd and 1987A with a self-developed semi-analytical code, assuming a two-component ejecta (core + shell), and involving the rotational energy of a newborn magnetar in addition to radioactive decay. Results. We find that the photometric and the spectroscopic evolution of SN 2021aatd can be well described with the explosion of a ∼15 M⊙ blue supergiant star. Nevertheless, SN 2021aatd shows higher temperatures and weaker Na I D and Ba II 6142 Å lines than SN 1987A, which is instead reminiscent of IIP-like atmospheres. With the applied two-component ejecta model (accounting for decay and magnetar energy), we can successfully describe the bolometric LC of SN 2021aatd, including the first ∼40-day phase showing an excess compared to 87A-like SNe, but being strikingly similar to that of the long-lived SN 2020faa. Nevertheless, finding a unified model that also explains the LCs of more luminous events (e.g., SN 2020faa) is still a matter of debate. [ABSTRACT FROM AUTHOR]

Context. The nature of mass-loss in massive stars is one of the most important and difficult to constrain processes in the evolution of massive stars. The largest observational uncertainties are related to the influence of metallicity and wind structure with optically thick clumps. Aims. We aim to constrain the wind parameters of sample of 18 O-type stars in the LMC, through analysis with stellar atmosphere and wind models including the effects of optically thick clumping. This will allow us to determine the most accurate spectroscopic mass-loss and wind structure properties of massive stars at sub-solar metallicity to date. This will allow us to gain insight into the impact of metallicity on massive stellar winds. Methods. Combining high signal to noise (S/N) ratio observations in the ultraviolet and optical wavelength ranges gives us access to diagnostics of multiple different ongoing physical processes in the stellar wind. We produce synthetic spectra using the stellar atmosphere modelling code FASTWIND, and reproduce the observed spectra using a genetic algorithm based fitting technique to optimise the input parameters. Results. We empirically constrain 15 physical parameters associated with the stellar and wind properties of O-type stars from the dwarf, giant and supergiant luminosity classes. These include temperature, surface gravity, surface abundances, rotation, macroturbulence and wind parameters. Conclusions. We find, on average, mass-loss rates a factor of 4–5 lower than those from theoretical predictions commonly used in stellar-evolution calculations, but in good agreement with more recent theoretical predictions. In the 'weak-wind' regime we find massloss rates orders of magnitude below any theoretical predictions. We find a positive correlation of clumping factors with effective temperature with an average fcl = 14 ± 8 for the full sample. It is clear that there is a difference in the porosity of the wind in velocity space, and interclump density, above and below a temperature of roughly 38 kK. Above 38 kK an average 46 ± 24% of the wind velocity span is covered by clumps and the interclump density is 10–30% of the mean wind. Below an effective temperature of roughly 38 kK there must be additional light leakage for supergiants. For dwarf stars at low temperatures there is a statistical preference for very low clump velocity spans, however it is unclear if this can be physically motivated as there are no clearly observable wind signatures in UV diagnostics. [ABSTRACT FROM AUTHOR]

Context. Grids of stellar evolution models with rotation using the Geneva stellar evolution code (GENEC) have been published for a wide range of metallicities. Aims. We introduce the last remaining grid of GENEC models, with a metallicity of Z = 10−5. We study the impact of this extremely metal-poor initial composition on various aspects of stellar evolution, and compare it to the results from previous grids at other metallicities. We provide electronic tables that can be used to interpolate between stellar evolution tracks and for population synthesis. Methods. Using the same physics as in the previous papers of this series, we computed a grid of stellar evolution models with GENEC spanning masses between 1.7 and 500 M⊙, with and without rotation, at a metallicity of Z = 10−5. Results. Due to the extremely low metallicity of the models, mass-loss processes are negligible for all except the most massive stars. For most properties (such as evolutionary tracks in the Hertzsprung-Russell diagram, lifetimes, and final fates), the present models fit neatly between those previously computed at surrounding metallicities. However, specific to this metallicity is the very large production of primary nitrogen in moderately rotating stars, which is linked to the interplay between the hydrogen- and helium-burning regions. Conclusions. The stars in the present grid are interesting candidates as sources of nitrogen-enrichment in the early Universe. Indeed, they may have formed very early on from material previously enriched by the massive short-lived Population III stars, and as such constitute a very important piece in the puzzle that is the history of the Universe. [ABSTRACT FROM AUTHOR]

Context. The first stars formed over five orders of magnitude in mass by accretion in primordial dark matter halos. Aims. We study the evolution of massive, very massive and supermassive primordial (Pop III) stars over nine orders of magnitude in accretion rate. Methods. We use the stellar evolution code GENEC to evolve accreting Pop III stars from 10−6–103M⊙ yr−1 and study how these rates determine final masses. The stars are evolved until either the end central Si burning or they encounter the general relativistic instability (GRI). We also examine how metallicity affects the evolution of the star at one accretion rate. Results. At rates below ∼2.5 × 10−5M⊙ yr−1 the final mass of the star falls below that required for pair-instability supernovae. The minimum rate required to produce black holes with masses above 250 M⊙ is ∼5 × 10−5M⊙ yr−1, well within the range of infall rates found in numerical simulations of halos that cool via H2, ≲10−3M⊙ yr−1. At rates of 5 × 10−5M⊙ yr−1 to 4 × 10−2M⊙ yr−1, like those expected for halos cooling by both H2 and Lyα, the star collapses after Si burning. At higher accretion rates the GRI triggers the collapse of the star during central H burning. Stars that grow at above these rates are cool red hypergiants with effective temperatures log(Teff) = 3.8 and luminosities that can reach 1010.5 L⊙. At accretion rates of 100–1000 M⊙ yr−1 the gas encounters the general relativistic instability prior to the onset of central hydrogen burning and collapses to a black hole with a mass of ∼106M⊙ without ever having become a star. Conclusions. Our models corroborate previous studies of Pop III stellar evolution with and without hydrodynamics over separate, smaller ranges in accretion rate. They also reveal for the first time the critical transition rate in accretion above which catastrophic baryon collapse, like that which can occur during galaxy collisions in the high-redshift Universe, produces supermassive black holes via dark collapse. [ABSTRACT FROM AUTHOR]

Context. The first stars formed over five orders of magnitude in mass by accretion in primordial dark matter halos. Aims. We study the evolution of massive, very massive and supermassive primordial (Pop III) stars over nine orders of magnitude in accretion rate. Methods. We use the stellar evolution code GENEC to evolve accreting Pop III stars from 10−6–103M⊙ yr−1 and study how these rates determine final masses. The stars are evolved until either the end central Si burning or they encounter the general relativistic instability (GRI). We also examine how metallicity affects the evolution of the star at one accretion rate. Results. At rates below ∼2.5 × 10−5M⊙ yr−1 the final mass of the star falls below that required for pair-instability supernovae. The minimum rate required to produce black holes with masses above 250 M⊙ is ∼5 × 10−5M⊙ yr−1, well within the range of infall rates found in numerical simulations of halos that cool via H2, ≲10−3M⊙ yr−1. At rates of 5 × 10−5M⊙ yr−1 to 4 × 10−2M⊙ yr−1, like those expected for halos cooling by both H2 and Lyα, the star collapses after Si burning. At higher accretion rates the GRI triggers the collapse of the star during central H burning. Stars that grow at above these rates are cool red hypergiants with effective temperatures log(Teff) = 3.8 and luminosities that can reach 1010.5 L⊙. At accretion rates of 100–1000 M⊙ yr−1 the gas encounters the general relativistic instability prior to the onset of central hydrogen burning and collapses to a black hole with a mass of ∼106M⊙ without ever having become a star. Conclusions. Our models corroborate previous studies of Pop III stellar evolution with and without hydrodynamics over separate, smaller ranges in accretion rate. They also reveal for the first time the critical transition rate in accretion above which catastrophic baryon collapse, like that which can occur during galaxy collisions in the high-redshift Universe, produces supermassive black holes via dark collapse. [ABSTRACT FROM AUTHOR]

Strong metallicity-dependent winds dominate the evolution of core He-burning, classical Wolf–Rayet (cWR) stars, which eject both H and He-fusion products such as |$^{14}$| N, |$^{12}$| C, |$^{16}$| O, |$^{19}$| F, |$^{22}$| Ne, and |$^{23}$| Na during their evolution. The chemical enrichment from cWRs can be significant. cWR stars are also key sources for neutron production relevant for the weak s-process. We calculate stellar models of cWRs at solar metallicity for a range of initial Helium star masses (12–50  |$\rm M_{\odot }$|⁠), adopting recent hydrodynamical wind rates. Stellar wind yields are provided for the entire post-main sequence evolution until core O-exhaustion. While literature has previously considered cWRs as a viable source of the radioisotope |$^{26}$| Al, we confirm that negligible |$^{26}$| Al is ejected by cWRs since it has decayed to |$^{26}$| Mg or proton-captured to |$^{27}$| Al. However, in Paper I, we showed that very massive stars eject substantial quantities of |$^{26}$| Al, among other elements including N, Ne, and Na, already from the zero-age-main-sequence. Here, we examine the production of |$^{19}$| F and find that even with lower mass-loss rates than previous studies, our cWR models still eject substantial amounts of |$^{19}$| F. We provide central neutron densities (N |$_{n}$|⁠) of a 30  |$\rm M_{\odot }$| cWR compared with a 32  |$\rm M_{\odot }$| post-VMS WR and confirm that during core He-burning, cWRs produce a significant number of neutrons for the weak s-process via the |$^{22}$| Ne(⁠|$\alpha$|⁠ ,n) |$^{25}$| Mg reaction. Finally, we compare our cWR models with observed [Ne/He], [C/He], and [O/He] ratios of Galactic WC and WO stars. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) stellar evolution models are widely used across various astrophysical fields, however they are still dominated by important uncertainties that deeply affect their predictive power. Among those, the merging of independent convective regions is a poorly understood phenomenon predicted by some 1D models but whose occurrence and impact in real stars remain very uncertain. Being an intrinsically multi-D phenomenon, it is challenging to predict the exact behaviour of shell mergers with 1D models. In this work, we conduct a detailed investigation of a multiple shell merging event in a 20 M |$_\odot$| star using 3D hydrodynamic simulations. Making use of the active tracers for composition and the nuclear network included in the 3D model, we study the merging not only from a dynamical standpoint but also considering its nucleosynthesis and energy generation. Our simulations confirm the occurrence of the merging also in 3D, but reveal significant differences from the 1D case. Specifically, we identify entrainment and the erosion of stable regions as the main mechanisms that drive the merging, we predict much faster convective velocities compared to the mixing-length theory velocities, and observe multiple burning phases within the same merged shell, with important effects for the chemical composition of the star, which presents a strongly asymmetric (dipolar) distribution. We expect that these differences will have important effects on the final structure of massive stars and thus their final collapse dynamics and possible supernova explosion, subsequently affecting the resulting nucleosynthesis and remnant. [ABSTRACT FROM AUTHOR]

Understanding the explosion mechanism and hydrodynamic evolution of core-collapse supernovae (SNe) is a long-standing quest in astronomy. The asymmetries caused by the explosion are encoded into the line profiles which appear in the nebular phase of the SN evolution – with particularly clean imprints in He star explosions. Here, we carry out nine different supernova simulations of He-core progenitors, exploding them in 3D with parametrically varied neutrino luminosities using the prometheus-hotb code, hydrodynamically evolving the models to the homologous phase. We then compute nebular phase spectra with the 3D Non-Local Thermodynamic Equilibrium spectral synthesis code extrass (EXplosive TRAnsient Spectral Simulator). We study how line widths and shifts depend on progenitor mass, explosion energy, and viewing angle. We compare the predicted line profile properties against a large set of Type Ib observations, and discuss the degree to which current neutrino-driven explosions can match observationally inferred asymmetries. With self-consistent 3D modelling – circumventing the difficulties of representing |$^{56}$| Ni mixing and clumping accurately in 1D models – we find that neither low-mass He cores exploding with high energies nor high-mass cores exploding with low energies contribute to the Type Ib SN population. Models which have line profile widths in agreement with this population give sufficiently large centroid shifts for calcium emission lines. Calcium is more strongly affected by explosion asymmetries connected to the neutron star kicks than oxygen and magnesium. Lastly, we turn to the near-infrared spectra from our models to investigate the potential of using this regime to look for the presence of He in the nebular phase. [ABSTRACT FROM AUTHOR]

Neutron stars (NSs) are observed with high space velocities and elliptical orbits in binaries. The magnitude of these effects points to natal kicks that originate from asymmetries during the supernova (SN) explosions. Using a growing set of long-time 3D SN simulations with the Prometheus-Vertex code, we explore the interplay of NS kicks that are induced by asymmetric neutrino emission and by asymmetric mass ejection. Anisotropic neutrino emission can arise from a large-amplitude dipolar convection asymmetry inside the proto-NS (PNS) termed LESA (Lepton-number Emission Self-sustained Asymmetry) and from aspherical accretion downflows around the PNS, which can lead to anisotropic neutrino emission (absorption/scattering) with a neutrino-induced NS kick roughly opposite to (aligned with) the kick by asymmetric mass ejection. In massive progenitors, hydrodynamic kicks can reach up to more than 1300 km s−1, whereas our calculated neutrino kicks reach (55–140) km s−1 (estimated upper bounds of (170–265) km s−1) and only ∼(10–50) km s−1, if LESA is the main cause of asymmetric neutrino emission. Therefore, hydrodynamic NS kicks dominate in explosions of high-mass progenitors, whereas LESA-induced neutrino kicks dominate for NSs born in low-energy SNe of the lowest-mass progenitors, when these explode nearly spherically. Our models suggest that the Crab pulsar with its velocity of ∼160 km s−1, if born in the low-energy explosion of a low-mass, single-star progenitor, should have received a hydrodynamic kick in a considerably asymmetric explosion. Black holes, if formed by the collapse of short-lived PNSs and solely kicked by anisotropic neutrino emission, obtain velocities of only some km s−1. [ABSTRACT FROM AUTHOR]

The massive binary HDE 228766 is composed of an O type primary and an evolved secondary. However, previous qualitative analyses of the composite spectrum have led to a wide discussion about whether the secondary is an Of or a Wolf–Rayet star. We use new observations and our novel QER20 package to disentangle for the first time the spectra of the two stellar components and obtain artefact |$-$| free reconstructed spectra, yielding the more accurate and reliable spectral classifications of O7.5 V((f))z for the primary and O6 Iaf for the secondary. The emission features of the P |$-$| Cygni profiles of the H  |$\beta$| and He i 5876 Å lines, present in the reconstructed spectrum of the secondary, show that this star is at an initial phase of its transition to the WN evolutionary stage. A previously unobserved variable emission, composed of at least four independent features, is seen since 2014 superposed to the H  |$\gamma$| absorption line. Our analysis reveals that these emission features originate from a physically extended region. This could be explained by an episode of enhanced mass-loss in the scenario of a non |$-$| conservative evolution of the binary. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive multiwavelength investigation of the intermediate-age open cluster NGC 1912. We identified 401 member stars for NGC 1912 using K-nearest neighbour and Gaussian Mixture Model machine learning techniques. The fundamental parameters determined for the cluster are metallicity (z) = 0.0141 |$\pm$| 0.0006, log(age) = 8.519 |$\pm$| 0.040, binary fraction (⁠|$b_{\text{frac}}$|⁠) = 0.445 |$\pm$| 0.035, visual extinction (⁠|$A_V$|⁠) = 0.864 |$\pm$| 0.033 mag, total-to-selective extinction ratio (⁠|$R_V$|⁠) = 3.007 |$\pm$| 0.096, distance = 1096 |$\pm$| 15 pc. The central density (⁠|$\rho _o$|⁠), core radius (⁠|$r_c$|⁠), tidal radius (⁠|$r_t$|⁠), and background density (⁠|$\rho _{\mathrm{bg}}$|⁠) for the cluster are obtained as 0.643 |$\pm$| 0.067 stars arcmin |$^{-2}$| , 7.743 |$\pm$| 0.784 arcmin, 84.633 |$\pm$| 7.188 arcmin, and 0.001 |$\pm$| 0.000 stars arcmin |$^{-2}$| , respectively. Dynamical mass segregation study revealed a concentration of some apparently massive stars towards the cluster centre. Polarimetric analysis of 90 stars in |$BVR_cI_c$| wavelength bands revealed that the observed polarization in NGC 1912 is not solely attributed to interstellar dust. The Galactic magnetic field is dominant in this region. Dust grain sizes within the intracluster region are found to be relatively smaller than that of the general ISM. The extinction map revealed the existence of a potential 'interstellar bubble' located close to the cluster region. Towards the periphery of the probable bubble region, which displays an ovoid-like geometry, 16 probable young stellar objects have been detected, suggesting the ongoing triggering of secondary star formation events in this region. [ABSTRACT FROM AUTHOR]

The galactic tidal interaction is a possible mechanism to trigger active star formation in galaxies. Recent analyses using H  i data in the Large Magellanic Cloud (LMC) proposed that the tidally driven H  i flow, the L-component, is colliding with the LMC disk, the D-component, and is triggering high-mass star formation toward the active star-forming regions R136 and N44. In order to explore the role of the collision over the entire LMC disk, we investigated the I-component, the collision-compressed gas between the L- and D-components, over the LMC disk, and found that |$74\%$| of the O/WR stars are located toward the I-component, suggesting their formation in the colliding gas. We compared four star-forming regions (R136, N44, N11, and the N77–N79–N83 complex). We found a positive correlation between the number of high-mass stars and the compressed gas pressure generated by collisions, suggesting that pressure may be a key parameter in star formation. [ABSTRACT FROM AUTHOR]

The properties of blue supergiants are key for constraining the end of the main sequence (MS) of massive stars. Whether the observed drop in the relative number of fast-rotating stars below ≈21 kK is due to enhanced mass-loss rates at the location of the bistability jump, or the result of the end of the MS is still debated. Here, we combine newly derived estimates of photospheric and wind parameters with Gaia distances and wind terminal velocities from the literature to obtain upper limits on the mass-loss rates for a sample of 116 Galactic luminous blue supergiants. The parameter space covered by the sample ranges between 35–15 kK in Teff and 4.8–5.8 dex in log(L/L⊙). Our results show no increase in the mass-loss rates over the bistability jump. Therefore, we argue that the drop in rotational velocities cannot be explained by enhanced mass loss. Since a large jump in the mass-loss rates is commonly included in evolutionary models, we suggest an urgent revision of the default prescriptions currently in use. [ABSTRACT FROM AUTHOR]

Aims. We aim to measure the impact of our mass-loss recipe in the evolution of massive stars at the metallicity of the Small Magellanic Cloud (SMC). Methods. We used the Geneva-evolution code (GENEC) to run evolutionary tracks for stellar masses ranging from 20 to 85 M⊙ at SMC metallicity (ZSMC = 0.002). We upgraded the recipe for stellar winds by replacing Vink's formula with our self-consistent m-CAK prescription, which reduces the value of the mass-loss rate, Ṁ, by a factor of between two and six depending on the mass range. Results. The impact of our new [weaker] winds is wide, and it can be divided between direct and indirect impact. For the most massive models (60 and 85 M⊙) with Ṁ ≳ 2 × 10−7M⊙ yr−1, the impact is direct because lower mass loss make stars remove less envelope, and therefore they remain more massive and less chemically enriched at their surface at the end of their main sequence (MS) phase. For the less massive models (20 and 25 M⊙) with Ṁ ≲ 2 × 10−8M⊙ yr−1, the impact is indirect because lower mass loss means the stars keep high rotational velocities for a longer period of time, thus extending the H-core burning lifetime and subsequently reaching the end of the MS with higher surface enrichment. In either case, given that the conditions at the end of the H-core burning change, the stars will lose more mass during their He-core burning stages anyway. For the case of Mzams = 20–40 M⊙, our models predict stars will evolve through the Hertzsprung gap, from O-type supergiants to blue supergiants (BSGs), and finally red supergiants (RSGs), with larger mass fractions of helium compared to old evolution models. New models also sets the minimal initial mass required for a single star to become a Wolf-Rayet (WR) at metallicity Z = 0.002 at Mzams = 85 M⊙. Conclusions. These results reinforce the importance of upgrading mass-loss prescriptions in evolution models, in particular for the earlier stages of stellar lifetime, even for Z ≪ Z⊙. New values for Ṁ need to be complemented with upgrades in additional features such as convective-core overshooting and distribution of rotational velocities, besides more detailed spectroscopical observations from projects such as XShootU, in order to provide a robust framework for the study of massive stars at low-metallicity environments. [ABSTRACT FROM AUTHOR]

The kinetic temperature structure of the massive filament DR21 within the Cygnus X molecular cloud complex has been mapped using the IRAM 30 m telescope. This mapping employed the para-H2CO triplet (JKaKc = 303−202, 322−221, and 321–220) on a scale of ~0.1 pc. By modeling the averaged line ratios of para-H2CO 322–221/303–202 and 321–220/303 –202 with RADEX under non local thermodynamic equilibrium (LTE) assumptions, the kinetic temperature of the dense gas was derived, which ranges from 24 to 114 K, with an average temperature of 48.3 ± 0.5 K at a density of n(H2)= 105 cm−3. In comparison to temperature measurements using NH3 (1, 1)/(2,2) and far-infrared (FIR) wavelengths, the para-H2CO(3–2) lines reveal significantly higher temperatures. The dense clumps in various regions appear to correlate with the notable kinetic temperature (Tkin ≳ 50 K) of the dense gas traced by H2CO. Conversely, the outskirts of the DR21 filament display lower temperature distributions (Tkin < 50 K). Among the four dense cores (N44, N46, N48, and N54), temperature gradients are observed on a scale of ~0.1–0.3 pc. This suggests that the warm dense gas traced by H2CO is influenced by internal star formation activity. With the exception of the dense core N54, the temperature profiles of these cores were fitted with power-law indices ranging from −0.3 to −0.5, with a mean value of approximately −0.4. This indicates that the warm dense gas probed by H2CO is heated by radiation emitted from internally embedded protostar(s) and/or clusters. While there is no direct evidence supporting the idea that the dense gas is heated by shocks resulting from a past explosive event in the DR21 region on a scale of ~0.1 pc, our measurements of H2CO toward the DR21W1 region provide compelling evidence that the dense gas in this specific area is indeed heated by shocks originating from the western DR21 flow. Higher temperatures as traced by H2CO appear to be associated with turbulence on a scale of ~0.1 pc. The physical parameters of the dense gas as determined from H2CO lines in the DR21 filament exhibit aremarkable similarity to the results obtained in OMC-1 and N113, albeit on a scale of approximately 0.1–0.4 pc. This may imply that the physical mechanisms governing the dynamics and thermodynamics of dense gas traced by H2CO in diverse star formation regions may be dominated by common underlying principles despite variations in specific environmental conditions. [ABSTRACT FROM AUTHOR]

Context. The first stars might have been fast rotators. This would have important consequences for their radiative, mechanical, and chemical feedback. Aims. We discuss the impact of fast initial rotation on the evolution of massive Population III models and on their nitrogen and oxygen stellar yields. Methods. We explore the evolution of Population III stars with initial masses in the range of 9 M⊙ ≤ Mini ≤ 120 M⊙, starting with an initial rotation on the zero-age main sequence equal to 70% of the critical one. Results. We find that with the physics of rotation considered here, our rapidly rotating Population III stellar models do not follow a homogeneous evolution. They lose very little mass in the case in which mechanical winds are switched on when the surface rotation becomes equal to or larger than the critical velocity. The impact on the ionising flux appears to be modest when compared to moderately rotating models. Fast rotation favours, in models with initial masses above ∼20 M⊙, the appearance of a very extended intermediate convective zone around the H-burning shell during the core He-burning phase. This shell has important consequences for the sizes of the He- and CO-cores, and thus impacts the final fate of stars. Moreover, it has a strong impact on nucleosynthesis, boosting the production of primary 14N. Conclusions. Fast initial rotation significantly impacts the chemical feedback of Population III stars. Observations of extremely metal-poor stars and/or starbursting regions are essential to provide constraints on the properties of the first stars. [ABSTRACT FROM AUTHOR]

The majority of massive stars are formed in multiple systems, and at some point during their life, they interact with their companions via mass transfer. This interaction typically leads to the primary shedding its outer envelope, resulting in the formation of a "stripped star". Classically, stripped stars are expected to quickly contract to become hot and UV-bright helium stars. Surprisingly, recent optical spectroscopic surveys have unveiled many stripped stars that are much larger and cooler, appearing "puffed up" and overlapping with the Main Sequence (MS) in the Hertzsprung–Russell diagram. Here, we study the evolutionary nature of puffed-up stripped (PS) stars and the duration of this enigmatic phase using the stellar-evolution code MESA. We computed grids of binary models at four metallicities: Solar (Z⊙ = 0.017), Large Magellanic Cloud (LMC, Z = 0.0068), Small Magellanic Cloud (SMC, Z = 0.0034), and Z = 0.1 Z⊙. Contrary to previous assumptions, we find that stripped stars regain thermal equilibrium shortly after the end of mass transfer and maintain it during most of the PS phase. Their further contraction towards hot and compact He stars is determined by the rate at which the residual H-rich envelope is depleted, with the main agents being H-shell burning (dominant for M ≲ 50 M⊙) and mass-loss in winds. The duration of the PS star phase is approximately 10% of the core-He burning lifetime (1% total lifetime) and up to 100 times more than the thermal timescale. It decreases with increasing mass and luminosity and increases with metallicity. We explored several factors influencing PS star lifetimes: orbital period, mass ratio, winds, and semiconvection. We further carried out a simple binary population synthesis estimation, finding that ∼0.5–0.7% of all the stars with log (L/L⊙) > 3.7 may, in fact, be PS stars. Our results indicate that tens to hundreds of PS stars in post-interaction binaries may be hiding in the MS population, disguised as 'normal' stars: ∼100 (∼280) in the SMC (LMC) and ∼1500 in the Milky Way. Their true nature may be revealed by spectroscopically measured low surface gravities, high N enrichment, and likely slow rotation rates. [ABSTRACT FROM AUTHOR]

Context. Massive star winds are known to be responsible for X-ray emission arising from wind plasma heated by the strong shocks up to temperatures of 106–107 K in the case of colliding wind binaries. The investigation of X-ray emission from massive stars thus constitutes a valuable tool for identifying binaries, which is otherwise a difficult task using classical techniques. Aims. We investigated thermal and nonthermal X-ray emission from the massive O-type star HD 93250 to unveil its binary orbital parameters independently. Methods. To meet our goal, we analyzed X-ray data obtained with European Photon Imaging Camera on board XMM-Newton spanning over ~19 yr. Additionally, we analyzed NuSTAR observations of HD 93250 taken at various epochs. Results. We determine the variability timescale of the X-ray emission to be 193.8±1.3d, in full agreement with the 194.3±0.4d period derived from the astrometric orbit. The X-ray spectrum of HD 93250 is well explained by a three-temperature thermal plasma emission model with temperatures of 0.26, 1.0, and 3.3 keV. The resulting X-ray flux varies in compliance with the typical colliding wind emission from eccentric massive binaries where it enhances near periastron passage and decreases gradually close to apastron in proportion to the inverse of the binary separation. The periastron-to-apastron X-ray emission ratio points to an eccentricity range of 0.20–0.25, once again in agreement with the previously determined astrometric orbit. Finally, we do not detect any hard X-ray emission attributable to nonthermal emission above 10 keV. Conclusions. Given the derived plasma temperature, the strong phase-locked variability, and the significant over-luminosity in X-rays, we establish that the X-ray emission from HD 93250 is dominated by the colliding-wind region. Our results lend support to the idea that X-ray time analysis of massive stars constitutes a relevant tool for investigating their multiplicity and for extracting relevant information on their basic orbital parameters – such as period and eccentricity – independently of any orbital solution derived from classical techniques. [ABSTRACT FROM AUTHOR]

Context. Blue supergiants (BSGs) are key objects for understanding the evolution of massive stars, which play a crucial role in the evolution of galaxies. However, discrepancies between theoretical predictions and empirical observations have opened up important questions yet to be answered. Studying statistically significant and unbiased samples of these objects can help to improve the situation. Aims. We perform a homogeneous and comprehensive quantitative spectroscopic analysis of a large sample of Galactic luminous blue stars (a majority of which are BSGs) from the IACOB spectroscopic database, providing crucial parameters to refine and improve theoretical evolutionary models. Methods. We derived the projected rotational velocity (υ sin i) and macroturbulent broadening (υmac) using IACOB-BROAD, which combines Fourier transform and line-profile fitting techniques. We compared high-quality optical spectra with state-of-the-art simulations of massive star atmospheres computed with the FASTWIND code. This comparison allowed us to derive effective temperatures (Teff), surface gravities (log 푔), microturbulences (ξ), surface abundances of silicon and helium, and to assess the relevance of stellar winds through a wind-strength parameter (log Q). Results. We provide estimates and associated uncertainties of the above-mentioned quantities for the largest sample of Galactic luminous O9 to B5 stars spectroscopically analyzed to date, comprising 527 targets. We find a clear drop in the relative number of stars at Teff ≈ 21 kK, coinciding with a scarcity of fast rotating stars below that temperature. We speculate that this feature (roughly corresponding to B2 spectral type) might be roughly delineating the location of the empirical terminal-age main sequence in the mass range between 15 and 85 M⊙. By investigating the main characteristics of the υ sin i distribution of O stars and BSGs as a function of Teff, we propose that an efficient mechanism transporting angular momentum from the stellar core to the surface might be operating along the main sequence in the high-mass domain. We find correlations between ξ,υmac and the spectroscopic luminosity 퓛 (defined as Teff4 / g). We also find that no more than 20% of the stars in our sample have atmospheres clearly enriched in helium, and suggest that the origin of this specific subsample might be in binary evolution. We do not find clear empirical evidence of an increase in the wind strength over the wind bi-stability region toward lower Teff. [ABSTRACT FROM AUTHOR]

Context. The star formation process leads to an increased chemical complexity in the interstellar medium. Sites associated with high-mass star and cluster formation exhibit a so-called hot core phase, characterized by high temperatures and column densities of complex organic molecules. Aims. We aim to systematically search for and identify a sample of hot cores toward the 15 Galactic protoclusters of the ALMA-IMF Large Program and investigate their statistical properties. Methods. We built a comprehensive census of hot core candidates toward the ALMA-IMF protoclusters based on the detection of two CH3OCHO emission lines at 216.1 GHz. We used the source extraction algorithm GExt2D to identify peaks of methyl formate (CH3OCHO) emission, a complex species commonly observed toward sites of star formation. We performed a cross-matching with the catalog of thermal dust continuum sources from the ALMA-IMF 1.3 mm continuum data to infer their physical properties. Results. We built a catalog of 76 hot core candidates with masses ranging from ~0.2 M⊙ to ~80 M⊙, of which 56 are new detections. A large majority of these objects, identified from methyl formate emission, are compact and rather circular, with deconvolved full width at half maximum (FWHM) sizes of ~2300 au on average. The central sources of two target fields show more extended, but still rather circular, methyl formate emission with deconvolved FWHM sizes of ~6700 au and 13 400 au. About 30% of our sample of methyl formate sources have core masses above 8 M⊙ and range in size from ~1000 au to 13 400 au, which is in line with measurements of archetypical hot cores. The origin of the CH3OCHO emission toward the lower-mass cores may be explained as a mixture of contributions from shocks or may correspond to objects in a more evolved state (i.e., beyond the hot core stage). We find that the fraction of hot core candidates increases with the core mass, suggesting that the brightest dust cores are all in the hot core phase. Conclusions. Our results suggest that most of these compact methyl formate sources are readily explained by simple symmetric models, while collective effects from radiative heating and shocks from compact protoclusters are needed to explain the observed extended CH3OCHO emission. The large fraction of hot core candidates toward the most massive cores suggests that they rapidly enter the hot core phase and that feedback effects from the forming protostar(s) impact their environment on short timescales. [ABSTRACT FROM AUTHOR]

Massive stars in the red supergiant (RSG) phase are known to undergo strong mass-loss through winds and observations indicate that a substantial part of this mass-loss could be driven by localized and episodic outflows. Various mechanisms have been considered to explain this type of mass-loss in RSGs, but these models often focus on single-star evolution. However, massive stars commonly evolve in binary systems, potentially interacting with their companions. Motivated by observations of the highly asymmetric circumstellar ejecta around the RSG VY CMa, we investigate a scenario where a companion on an eccentric orbit grazes the surface of an RSG at periastron. The companion ejects part of the outer RSG envelope, which radiatively cools, reaching the proper conditions for dust condensation and eventually giving rise to dust-driven winds. Using simple treatments for radiative cooling and dust-driven winds, we perform three-dimensional smoothed particle hydrodynamic simulations of this scenario with a |$20\, {\rm M}_\odot$| RSG and a |$2\, {\rm M}_\odot$| companion. We follow the evolution of the binary throughout a total of 14 orbits and observe that the orbit tightens after each interaction, in turn enhancing the mass-loss of subsequent interactions. We show that one such grazing interaction yields outflows of |$3\times 10^{-4}\, {\rm M}_\odot$|⁠ , which later results in wide asymmetric dusty ejecta, carrying a total mass of |$0.185\, {\rm M}_\odot$| by the end of simulations. We discuss the implications for the evolution of the binary, potential observational signatures, as well as future improvements of the model required to provide sensible predictions for the evolution of massive binaries. [ABSTRACT FROM AUTHOR]

Core-collapse supernovae (CCSNe) are considered the primary magnetar formation channel, with 15 magnetars associated with supernova remnants (SNRs). A large fraction of these should occur in massive stellar binaries that are disrupted by the explosion, meaning that |$\sim 45~{{\ \rm per\ cent}}$| of magnetars should be nearby high-velocity stars. Here, we conduct a multiwavelength search for unbound stars, magnetar binaries, and SNR shells using public optical (uvgrizy bands), infrared (J, H, K , and Ks bands), and radio (888 MHz, 1.4 GHz, and 3 GHz) catalogues. We use Monte Carlo analyses of candidates to estimate the probability of association with a given magnetar based on their proximity, distance, proper motion, and magnitude. In addition to recovering a proposed magnetar binary, a proposed unbound binary, and 13 of 15 magnetar SNRs, we identify two new candidate unbound systems: an OB star from the Gaia catalogue we associate with SGR J1822.3−1606, and an X-ray pulsar we associate with 3XMM J185246.6 + 003317. Using a Markov Chain Monte Carlo simulation that assumes all magnetars descend from CCSNe, we constrain the fraction of magnetars with unbound companions to |$5\lesssim f_u \lesssim 24~{{\ \rm per\ cent}}$|⁠ , which disagrees with neutron star population synthesis results. Alternate formation channels are unlikely to wholly account for the lack of unbound binaries as this would require |$31\lesssim f_{nc} \lesssim 66~{{\ \rm per\ cent}}$| of magnetars to descend from such channels. Our results support a high fraction (⁠|$48\lesssim f_m \lesssim 86~{{\ \rm per\ cent}}$|⁠) of pre-CCSN mergers, which can amplify fossil magnetic fields to preferentially form magnetars. [ABSTRACT FROM AUTHOR]

Nuclear reactions are key to our understanding of stellar evolution, particularly the |$^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$| rate, which is known to significantly influence the lower and upper ends of the black hole (BH) mass distribution due to pair-instability supernovae (PISNe). However, these reaction rates have not been sufficiently determined. We use the mesa stellar evolution code to explore the impact of uncertainty in the |$^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$| rate on PISN explosions, focusing on nucleosynthesis and explosion energy by considering the high resolution of the initial mass. Our findings show that the mass of synthesized radioactive nickel (56Ni) and the explosion energy increase with |$^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$| rate for the same initial mass, except in the high-mass edge region. With a high (about twice the starlib standard value) rate, the maximum amount of nickel produced falls below 70 M⊙, while with a low rate (about half of the standard value) it increases up to 83.9 M⊙. These results highlight that carbon 'preheating' plays a crucial role in PISNe by determining core concentration when a star initiates expansion. Our results also suggest that the onset of the expansion, which means the end of compression, competes with collapse caused by helium photodisintegration, and the maximum mass that can lead to an explosion depends on the |$^{12}{\rm C}(\alpha ,\gamma)^{16}{\rm O}\,$| reaction rate. [ABSTRACT FROM AUTHOR]

Recent photometric observations of massive stars have identified a low-frequency power excess which appears as stochastic low-frequency variability in light-curve observations. We present the oscillation properties of high-resolution hydrodynamic simulations of a |$25\,\,{\rm{M}_\odot }$| star performed with the PPMstar code. The model star has a convective core mass of |$\approx 12\,\,{\rm{M}_\odot }$| and approximately half of the envelope simulated. From this simulation, we extract light curves from several directions, average them over each hemisphere, and process them as if they were real photometric observations. We show how core convection excites waves with a similar frequency as the convective time-scale in addition to significant power across a forest of low and high angular degree l modes. We find that the coherence of these modes is relatively low as a result of their stochastic excitation by core convection, with lifetimes of the order of 10s of days. Thanks to the still significant power at higher l and this relatively low coherence, we find that integrating over a hemisphere produces a power spectrum that still contains measurable power up to the Brunt–Väisälä frequency. These power spectra extracted from the stable envelope are qualitatively similar to observations, with the same order of magnitude yet lower characteristic frequency. This work further shows the potential of long-duration, high-resolution hydrodynamic simulations for connecting asteroseismic observations to the structure and dynamics of core convection and the convective boundary. [ABSTRACT FROM AUTHOR]

The large number of gravitational wave (GW) detections have revealed the properties of the merging black hole binary population, but how such systems are formed is still heavily debated. Understanding the imprint of stellar physics on the observable GW population will shed light on how we can use the GW data, along with other observations, to constrain the poorly understood evolution of massive binaries. We perform a parameter study on the classical isolated binary formation channel with the population synthesis code seba to investigate how sensitive the properties of the coalescing binary black hole population are on the uncertainties related to first phase of mass transfer and stellar winds. We vary five assumptions: (1) and (2) the mass transfer efficiency and the angular momentum loss during the first mass transfer phase, (3) the mass transfer stability criteria for giant donors with radiative envelopes, (4) the effective temperature at which an evolved star develops a deep convective envelope, and (5) the mass-loss rates of stellar winds. We find that current uncertainties related to first phase of mass transfer have a huge impact on the relative importance of different dominant channels, while the observable demographics of GW sources are not significantly affected. Our varied parameters have a complex, interrelated effect on the population properties of GW sources. Therefore, inference of massive binary physics from GW data alone remains extremely challenging, given the large uncertainties in our current models. [ABSTRACT FROM AUTHOR]

Results of the long-term monitoring observations by the Hitachi 32 m radio telescope of the 6.7 GHz Class II methanol masers associated with four high-mass star-forming regions are presented. We detected periodic flux variability in G06.795−0.257, G10.472+0.027, G12.209−0.102, and G13.657−0.599 with the periods of 968, 1624, 1272, and 1266 d, respectively, although the detected period is tentative due to the short monitoring term relative to the estimated period. The facts that the flux variation patterns show the symmetric sine curves and that the luminosities of the central protostar and periods of maser flux variation are consistent with the expected period–luminosity (PL) relation suggest that the mechanisms of maser flux variability of G10.472+0.027 and G12.209−0.102 can be explained by protostellar pulsation instability. From the PL relation, the central stars of these two sources are expected to be very high-mass protostars with a mass of |$\sim 40\, M_{\odot }$| and to have a mass accretion rate of |$\sim 2 \times 10^{-2}\, M_{\odot }\:$| yr−1. On the other hand, G06.795−0.257 and G13.657−0.599 have intermittent variation patterns and have luminosities that are an order of magnitude smaller than those expected from the PL relation, suggesting that the variation mechanisms of these sources originated from a binary system. Since almost all the maser features vary with the same period regardless of the geometry, periodic accretion models may be appropriate mechanisms for flux variability in G06.795−0.257 and G13.657−0.599. [ABSTRACT FROM AUTHOR]

Herbig Ae/Be (HAeBe) stars are intermediate-mass pre-main-sequence stars, characterized by infrared (IR) excess and emission lines. They are observed to emit X-rays, whose origin is a matter of discussion and not settled yet. X-ray emission is not expected in HAeBe stars, as they lack the subsurface convective zone. In this study, we retrieved observations from the Chandra archive for 62 HAeBe stars, among which 44 sources (detection fraction ∼71  per cent) were detected in X-rays, with 7 being new detections. We use this sample as a test bed to conduct a comparative analysis of the X-ray properties of HAeBe stars and their low-mass counterparts, T Tauri stars (TTSs). Further, we compare the X-ray properties of HAeBe stars and TTSs with optical and IR properties to constrain the X-ray emission mechanism in HAeBe stars. We found no correlation between X-ray emission and disc properties of HAeBe stars, confirming that X-rays are not related to accretion shocks. About 56  per cent of HAeBe stars without any known subarcsec companions have lower plasma temperatures (kT ≤ 2 keV). We observe flaring/variability in HAeBe stars with confirmed low-mass companions. These stars show plasma temperatures > 2 keV, similar to TTSs. Guided by this information, we discuss the role of a T Tauri companion for X-ray emission seen in our sample of HAeBe stars. From the results obtained in this paper, we suggest that X-ray emission from HAeBe stars may not be related to accretion shocks or hidden TTS, but rather can be due to magnetically driven coronal emission. [ABSTRACT FROM AUTHOR]

The formation mechanism of massive stars remains one of the main open problems in astrophysics, in particular the relationship between the mass of the most massive stars and that of the cores in which they form. Numerical simulations of the formation and evolution of molecular clouds, within which dense cores and stars form self-consistently, show that the core mass increases in time, and also that the most massive stars tend to appear later than lower mass stars. We present an idealized model that incorporates accretion onto the cores as well as onto the stars, in which the core mass growth is regulated by a 'gravitational choking' mechanism that does not involve any form of support. This process is of purely gravitational origin, and causes some of the mass accreted onto cores to stagnate there, rather than being transferred to the stars. In addition, we estimate the mass of the most massive allowed star before its photoionizing radiation is capable of overcoming the accretion flow. This model constitutes a proof of concept for the simultaneous growth of the gas reservoir and the stellar mass, the delay in the formation of massive stars observed in numerical simulations, the need for massive, dense cores in order to form massive stars, and the observed correlation between the mass of the most massive star and the mass of the cluster it resides in. Also, our model implies that by the time massive stars begin to form in a core, a number of low-mass stars are expected to have already formed. [ABSTRACT FROM AUTHOR]