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Context. EX Lupi is the prototype of EX Lup-type stars, which are classical T Tauri stars (cTTSs) with luminosity bursts and outbursts of 1–5 magnitudes that last for a few months to a few years. These events are ascribed to an episodic accretion that can occur repeatedly, but whose physical mechanism is still debated. Aims. We aim to investigate the magnetically driven accretion of EX Lup in quiescence. We include for the first time a study of the small- and large-scale magnetic field. This allows us to characterise the magnetospheric accretion process of the system completely. Methods. We used spectropolarimetric times series acquired in 2016 and 2019 with the Echelle SpectroPolarimetric Device for the Observation of Stars and in 2019 with the SpectroPolarimètre InfraRouge at the Canada-France-Hawaii telescope during a quiescence phase of EX Lup. We were thus able to perform a variability analysis of the radial velocity, the emission lines, and the surface-averaged longitudinal magnetic field in different epochs and wavelength domains. We also provide a small-scale magnetic field analysis using Zeeman intensification of photospheric lines and a large-scale magnetic topology reconstruction using Zeeman-Doppler imaging. Results. Our study reveals that typical magnetospheric accretion is ongoing on EX Lup. A main accretion funnel flow connects the inner disc to the star in a stable fashion and produces an accretion shock on the stellar surface close to the pole of the magnetic dipole component. We also measure one of the strongest fields ever observed on cTTSs. This strong field indicates that the disc is truncated by the magnetic field close to but beyond the corotation radius, where the angular velocity of the disc equals the angular velocity of the star. This configuration is suitable for a magnetically induced disc instability that yields episodic accretion onto the star. [ABSTRACT FROM AUTHOR]

This paper analyses spectropolarimetric observations of the classical T Tauri star (CTTS) GM Aurigae collected with SPIRou, the near-infrared spectropolarimeter at the Canada–France–Hawaii Telescope, as part of the SLS and SPICE Large Programs. We report for the first time results on the large-scale magnetic field at the surface of GM Aur using Zeeman Doppler imaging. Its large-scale magnetic field energy is almost entirely stored in an axisymmetric poloidal field, which places GM Aur close to other CTTSs with similar internal structures. A dipole of about 730 G dominates the large-scale field topology, while higher order harmonics account for less than 30 per cent of the total magnetic energy. Overall, we find that the main difference between our three reconstructed maps (corresponding to sequential epochs) comes from the evolving tilt of the magnetic dipole, likely generated by non-stationary dynamo processes operating in this largely convective star rotating with a period of about 6 d. Finally, we report a |$5.5\sigma$| detection of a signal in the activity-filtered radial velocity data of semi-amplitude |$110\pm 20$|  m s |$^{-1}$|  at a period of |$8.745\pm 0.009$|  d. If attributed to a close-in planet in the inner accretion disc of GM Aur, it would imply that this planet candidate has a minimum mass of |$1.10 \pm 0.30\, M_\mathrm{Jup}$| and orbits at a distance of |$0.082 \pm 0.002$|  au. [ABSTRACT FROM AUTHOR]

Context. Studies of the stellar and accretion properties of classical T Tauri stars (CTTS) require photospheric spectral templates to be compared with. The use of low-activity, slowly rotating field dwarfs or model spectra can be advantageous for the determination of stellar parameters, but it can lead to an overestimate of the mass accretion rate, since both classes of templates do not include the emission of the active chromosphere present in young stars. Observed spectra of non-accreting young stars are best suited to this purpose. Using such templates comes with the downside of a limited number of available templates and observational uncertainties on the properties of the templates. Aims. For this work, we aimed to expand the currently available grid of wide-wavelength coverage observed spectra of non-accreting stars with additional new spectra and an interpolation method that allowed us to obtain a continuous grid of low resolution spectra ranging from spectral type G8 to M9.5, while also mitigating observational uncertainties. This interpolated grid was then implemented in the self-consistent method to derive stellar and accretion properties of CTTS. With the new templates, we aimed to estimate a lower limit on the accretion luminosities that can be obtained through a study of the UV excess emission using observed templates. Methods. We analyzed the molecular photospheric features present in the VLT/X-shooter spectra of the targets to perform a spectral classification, including estimates of their extinction. We applied a non-parametric fitting method to the full grid of observed templates to obtain an interpolated grid of templates. Both the individual templates and interpolated grid are provided to the community. We implemented this grid to improve the method to self-consistently derive stellar and accretion properties of accreting stars. We used the uncertainties on our interpolated grid to estimate a lower limit on the accretion luminosity that we can measure with this method. Results. Our new method, which uses a continuous grid of templates, provides results that are consistent with using individual templates but it significantly improves the reliability of the results in the case of degeneracies associated with the peculiarities of individual observed templates. We find that the measurable accretion luminosities range from ∼2.7 dex lower than the stellar luminosity in M5.5 stars to ∼1.3 dex lower for G8 stars. For young stars with masses of ∼1 M⊙ and ages of 3–6 Myr this limit translates into an observational limit of the mass accretion rate on the order of 10−10 M⊙/yr. This limit is higher than the lower limit on the measurable mass accretion rate when using the various emission lines present in the spectra of young stars to estimate the accretion rate. An analysis of these emission lines allows us to probe lower accretion rates, pending a revised calibration of the relationships between line and accretion luminosities at low accretion rates. Conclusions. The implementation of an interpolated grid of observed templates allows us to better disentangle degenerate solutions, leading to a more reliable estimate of accretion rates in young accreting stars. [ABSTRACT FROM AUTHOR]

Utilizing a decade-long unTimely data set, supplemented by multiband data from archives, we search for young stellar objects (YSOs) with variations larger than one magnitude in W 1 band within a region of 110 square degrees in the Galactic plane, covered by VISTA Variables in the Via Lactea. A total of 641 candidate YSOs have been identified. We classified them into bursts, dips, faders, seculars, and irregulars. Within the burst category, 18 sources were identified as FUor candidates and 1 as an EXor candidate. Irregulars are the most prevalent in the sample. In both bursts and faders, the redder sources tend to show a pattern of bluer when brighter, whereas the bluer sources display the opposite trend, possibly related to the accretion structure of YSOs at different stages. Finally, we obtained the recurrence time-scale for FUor eruptions at various stages of YSO evolution. Our findings indicate that younger YSOs generally experience more frequent eruptions compared to older ones. [ABSTRACT FROM AUTHOR]

We present the overview and first results from the North-PHASE Legacy Survey, which follows six young clusters for five years, using the 2 deg |$^2$| FoV of the JAST80 telescope from the Javalambre Observatory (Spain). North-PHASE investigates stellar variability on time-scales from days to years for thousands of young stars distributed over entire clusters. This allows us to find new YSO, characterize accretion, and study inner disc evolution within the cluster context. Each region (Tr 37, Cep OB3, IC 5070, IC 348, NGC 2264, and NGC 1333) is observed in six filters (SDSS griz, u band, and J0660, which covers H |$\alpha$|⁠), detecting cluster members as well as field variable stars. Tr 37 is used to prove feasibility and optimize the variability analysis techniques. In Tr 37, variability reveals 50 new YSO, most of them proper motion outliers. North-PHASE independently confirms the youth of astrometric members, efficiently distinguishes accreting and non-accreting stars, reveals the extent of the cluster populations along Tr37/IC 1396 bright rims, and detects variability resulting from rotation, dips, and irregular bursts. The proper motion outliers unveil a more complex star formation history than inferred from Gaia alone, and variability highlights previously hidden proper motion deviations in the surrounding clouds. We also find that non-YSO variables identified by North-PHASE cover a different variability parameter space and include long-period variables, eclipsing binaries, RR Lyr, and |$\delta$| Scuti stars. These early results also emphasize the power of variability to complete the picture of star formation where it is missed by astrometry. [ABSTRACT FROM AUTHOR]

Context. T Tauri stars (TTSs) are magnetically active stars that accrete matter from the inner border of the surrounding accretion disk; plasma becomes trapped into the large-scale magnetic structures and falls onto the star, heating the surface through the so-called accretion shocks. The X-ray spectra of the TTSs show prominent Fe Kα fluorescence emission at 6.4 keV (hereafter, Fe Kα emission) that cannot be explained in a pure accretion scenario because its excitation requires significantly more energy than the maximum available through the well-constrained free-fall velocity. Neither can it be produced by the hot coronal plasma. Aims. TTSs display all signs of magnetic activity, and magnetic reconnection events are expected to occur frequently. In these events, electrons may become accelerated to relativistic speeds, and their interaction with the environmental matter may result in Fe Kα emission. It is the aim of this work to evaluate the expected Fe Kα emission in the context of the TTS research and compare it with the actual Fe Kα measurements obtained during the flare detected while monitoring RY Tau with the XMM-Newton satellite. Methods. The propagation of high-energy electrons in dense gas generates a cascade of secondary particles that results in an electron shower of random nature, whose evolution and radiative throughput was simulated in this work using the Monte Carlo code PENELOPE. A set of conditions representing the environment of the TTSs where these showers may impinge was taken into account to generate a grid of models that can aid the interpretation of the data. Results. The simulations show that the electron beams produce a hot spot at the point of impact; strong Fe Kα emission and X-ray continuum radiation are produced by the spot. This emission is compatible with RY Tau observations. Conclusions. The Fe Kα emission observed in TTSs could be produced by beams of relativistic electrons accelerated in magnetic reconnection events during flares. [ABSTRACT FROM AUTHOR]

Lyman-α (Lyα) is the strongest emission line in the accretion-generated ultraviolet spectra from T Tauri stars and, as such, plays a critical role in regulating chemistry within the surrounding protoplanetary discs. Due to its resonant nature, the scattering of Lyα photons along the line of sight encodes information about the physical properties of the intervening H  i medium. In this work, we present the first spatially resolved spectral images of Lyα emission across a protoplanetary disc in the iconic face-on T Tauri star TW Hya, observed with HST -STIS at spatial offsets 0, ±0.2, and ±0.4 arcsec. To comprehensively interpret these Lyα spectra, we utilize a 3D Monte Carlo Lyα radiative transfer simulation considering the H  i wind and protoplanetary disc. From the simulation, we constrain the wind's properties: the H  i column density |$\sim 10^{20} {\rm \, cm^{-2}}$| and the outflow velocity |$\sim 200 {\rm \, km\, s^{-1}}$|⁠. Our findings indicate that successfully interpreting the observed spectra necessitates scattering contributions in the H  i layer within the disc. Furthermore, to explore the effect of Lyα radiative transfer on protoplanetary disc chemistry, we compute the radiation field within the scattering medium and reveal that the wind reflection causes more Lyα photons to penetrate the disc. Our results show the necessity of spatially resolved Lyα observations of a broad range of targets, which will decode the complex interactions between the winds, protoplanetary discs, and surrounding environments. [ABSTRACT FROM AUTHOR]

V1741 Sgr (= SPICY 71482/Gaia22dtk) is a Classical T Tauri star on the outskirts of the Lagoon Nebula. After at least a decade of stability, in mid-2022, the optical source brightened by ∼3 mag over 2 months, remained bright until early 2023, then dimmed erratically over the next 4 months. This event was monitored with optical and infrared spectroscopy and photometry. Spectra from the peak (October 2022) indicate an EX Lup-type (EXor) accretion outburst, with strong emission from H  i , He  i , and Ca  ii lines and CO bands. At this stage, spectroscopic absorption features indicated a temperature of T ∼ 4750 K with low-gravity lines (e.g. Ba  ii and Sr  ii). By April 2023, with the outburst beginning to dim, strong TiO absorption appeared, indicating a cooler T ∼ 3600 K temperature. However, once the source had returned to its pre-outburst flux in August 2023, the TiO absorption and the CO emission disappeared. When the star went into outburst, the source's spectral energy distribution became flatter, leading to bluer colours at wavelengths shorter than ∼1.6 |$\mu$| m and redder colours at longer wavelengths. The brightening requires a continuum emitting area larger than the stellar surface, likely from optically thick circumstellar gas with cooler surface layers producing the absorption features. Additional contributions to the outburst spectrum may include blue excess from hotspots on the stellar surface, emission lines from diffuse gas, and reprocessed emission from the dust disc. Cooling of the circumstellar gas would explain the appearance of TiO, which subsequently disappeared once this gas had faded and the stellar spectrum reemerged. [ABSTRACT FROM AUTHOR]

The Hunting Outbursting Young Stars (HOYS) project performs long-term, optical, multifilter, high cadence monitoring of 25 nearby young clusters and star-forming regions. Utilizing Gaia  DR3 data, we have identified about 17 000 potential young stellar members in 45 coherent astrometric groups in these fields. Twenty one of them are clear young groups or clusters of stars within 1 kpc and they contain 9143 Gaia selected potential members. The cluster distances, proper motions, and membership numbers are determined. We analyse long-term (≈ 7 yr) V -, R -, and I -band light curves from HOYS for 1687 of the potential cluster members. One quarter of the stars are variable in all three optical filters, and two-thirds of these have light curves that are symmetric around the mean. Light curves affected by obscuration from circumstellar materials are more common than those affected by accretion bursts, by a factor of 2–4. The variability fraction in the clusters ranges from 10 per cent to almost 100 per cent, and correlates positively with the fraction of stars with detectable inner discs, indicating that a lot of variability is driven by the disc. About one in six variables shows detectable periodicity, mostly caused by magnetic spots. Two-thirds of the periodic variables with disc excess emission are slow rotators, and amongst the stars without disc excess two-thirds are fast rotators – in agreement with rotation being slowed down by the presence of a disc. [ABSTRACT FROM AUTHOR]

During the pre-main-sequence evolution, Young Stellar Objects (YSOs) assemble most of their mass during the episodic accretion process. The rarely seen FUOr-type events are valuable laboratories to investigate the outbursting nature of YSOs. Here, we present multiwavelength detection of a high-amplitude eruptive source in the young open cluster VdBH 221 with an ongoing outburst, including optical to mid-infrared time series and near-infrared spectra. The initial outburst has an exceptional amplitude of >6.3 mag in Gaia and 4.6 mag in K s, with a peak luminosity up to 16 L⊙ and a peak mass accretion rate of 1.4 × 10−5 M⊙ yr−1. The optical to infrared spectral energy distribution of this object is consistent with a low-mass star (0.2 M⊙) with a modest extinction (A V < 2 mag). A 100-d delay between optical and infrared rising stages is detected, suggesting an outside-in origin of the instability. The spectroscopic features of this object reveal a self-luminous accretion disc, very similar to FU Orionis, with a low line-of-sight extinction. Most recently, there has been a gradual increase in brightness throughout the wavelength range, possibly suggesting an enhancement of the mass accretion rate. [ABSTRACT FROM AUTHOR]

Episodic accretion is one of the competing models to explain the observed luminosity spread in young stellar clusters. These short-lived high accretion events could also have a strong impact on planet formation. Observations of high-amplitude variability in young stellar objects (YSOs) due to large changes in the accretion rate provide direct observational evidence for episodic accretion. However, there are still uncertainties in the frequency of these events and if episodic accretion is universal among YSOs. To determine the frequency of outbursts in Class I YSOs, we built a large and robust sample of objects at this evolutionary stage, and searched for high-amplitude near-infrared (Δ K S > 2 mag) variability in the VIRAC2 database of the Vista Variables in the Via Lactea survey. By complementing with near-IR (2MASS and DENIS) and mid-IR (WISE /Neo- WISE) data, we find that from ∼7000 Class I YSOs, 97 objects can be classified as eruptive variable YSOs. The duration of the outbursts vary from a few months to longer than 9 yr, and cover a similar range of amplitudes. Values of Δ K S > 5 mag, however, are only observed in outbursts with duration longer than 9 yr. When considering different effects of completeness and contamination, we estimate that the incidence of episodic accretion in Class I YSOs is between 2 and 3 per cent. Finally, we determine a recurrence time-scale of long-term outbursts (a.k.a FUors) of |$\tau =1.75^{+1.12}_{-0.87}$| kyr. The latter value agrees with previous estimates and is in line with the expectations of higher frequency of FUor outbursts during younger stages of evolution. [ABSTRACT FROM AUTHOR]

We have performed a comprehensive search of a VISTA Variables in the Via Lactea (VVV) data base of 9.5 yr light curves for variable sources with Δ Ks ≥ 4 mag, aiming to provide a large sample of high amplitude eruptive young stellar objects (YSOs) and detect unusual or new types of infrared variable source. We find 222 variable or transient sources in the Galactic bulge and disc, most of which are new discoveries. The sample mainly comprises novae, YSOs, microlensing events, Long Period Variable stars (LPVs), and a few rare or unclassified sources. Additionally, we report the discovery of a significant population of aperiodic late-type giant stars suffering deep extinction events, strongly clustered in the Nuclear Disc of the Milky Way. We suggest that these are metal-rich stars in which radiatively driven mass loss has been enhanced by super-solar metallicity. Among the YSOs, 32/40 appear to be undergoing episodic accretion. Long-lasting YSO eruptions have a typical rise time of ∼2 yr, somewhat slower than the 6–12 month time-scale seen in the few historical events observed on the rise. The outburst durations are usually at least 5 yr, somewhat longer than many lower amplitude VVV events detected previously. The light curves are diverse in nature, suggesting that multiple types of disc instability may occur. Eight long-duration extinction events are seen wherein the YSO dims for a year or more, attributable to inner disc structure. One binary YSO in NGC 6530 displays periodic extinction events (P =59 d) similar to KH 15D. [ABSTRACT FROM AUTHOR]

During the pre-main-sequence (pre-MS) evolution stage of a star, significant amounts of stellar mass are accreted during episodic accretion events, such as multidecade FUor-type outbursts. Here, we present a near-infrared spectroscopic follow-up study of 33 high-amplitude (most with Δ Ks > 4 mag) variable sources discovered by the Vista Variables in the Via Lactea (VVV) survey. Based on the spectral features, 25 sources are classified as eruptive young stellar objects (YSOs), including 15 newly identified FUors, six with long-lasting, but EXor-like bursts of magnetospheric accretion and four displaying outflow-dominated spectra. By examining the photometric behaviours of eruptive YSOs, we found most FUor-type outbursts have higher amplitudes (Δ Ks and Δ W 2), faster eruptive time-scales and bluer infrared colours than the other outburst types. In addition, we identified seven post-MS variables apparently associated with deep dipping events and an eruptive star with deep aluminium monoxide absorption bands resembling those seen in the V838 Mon stellar merger. [ABSTRACT FROM AUTHOR]

We present a survey of molecular line emission towards the molecular cloud surrounding Herbig Be star V645 Cyg. The survey was performed with the 20-m Onsala space telescope at 3 and 4 mm. We detected emission lines of 33 molecules and their isotopologues from diatomic molecules to four COMs up to seven atoms. Using detected lines, we estimated molecular column densities and abundances relative to molecular hydrogen in local thermodynamic equilibrium (LTE) approximation for all molecules except for methanol, for which we obtained physical parameters using a non-LTE model. Moreover, in the basement of the non-LTE model of methanol line emission, we consider that there is a weak maser effect in the additional spectral component of 51–40 E methanol line at 84.521 GHz. We compared the molecular abundances with values found in several astrochemical templates: molecular clouds, hot cores, and photodissociation regions, and found that signatures of these different types can be found towards V645 Cyg. We also obtained maps of the cloud in several molecular emission lines. The peaks of CO and CH3OH emission are shifted from the direction of the star, but the CS, HCO+, HNC, HCN, and N2H+ emission peaks are observed directly towards the star. Exploring the gas kinematics around V645 Cyg, we found that velocity structure in the ambient molecular cloud on the scale ≈1.6–2.0 pc is not the same as within ≈0.5 pc found previously by other authors. [ABSTRACT FROM AUTHOR]

We examine the long-term spectroscopic and photometric variability of EX Lupi and TW Hya, studying the presence of stable accretion and the role it plays in the observed variability. Analysing the velocity modulations of the emission lines with star - melt , we obtain information on the structure of the accretion columns and the disc–star connection. The emission-line radial velocities reveal that TW Hya, like EX Lupi, has a remarkably stable or slow-varying accretion column footprint, locked to the star for several years. The line-emitting regions are non-polar for both EX Lupi and TW Hya, and species with different energies differ in position. In contrast, the continuum emission as observed in the photometry is very variable and can be modelled by hotspot(s) that change over time in phase, shape, temperature, size, and location with respect to the emission-line region. The continuum emission region may not be limited to the stellar surface, especially during episodes of high accretion. The broad-line emission observed in EX Lupi during episodes of increased accretion reveals a further structure, which can be fitted by non-axisymmetric disc in Keplerian rotation inwards of the corotation radius. Since the radial velocity modulation due to accretion footprints is so stable, we used it to search for further velocity modulations. While no residual modulation (other than caused by stellar rotation) is found in these objects, a similar analysis could help to identify young planets/companions. Therefore, determining whether stable accretion footprints are common among young stars is a key to detect young planets. [ABSTRACT FROM AUTHOR]

NGC 2071 is a star-forming region that overlaps with three γ-ray sources detected by the Fermi Space Telescope. We propose that strong flare activity in T Tauri stars could produce γ-ray emission in a way that makes them a counterpart to some unidentified sources detected by the Large Area Telescope aboard the Fermi satellite. We have performed a spectral and temporal analysis for two Fermi data sets: the first 2 yr and the entire 14 yr of observations. We have found that the γ-ray source is detectable at 3.2σ above the background at energies above 100 GeV during the first 2 yr of observation. The analysis of the expected frequency of the highest energy flares occurring in T Tauri stars is consistent with our estimate. In addition, we have determined the minimum energy of the flare that would produce γ-ray emission, which is ∼5 × 1037 erg. This agreement becomes a hard observational constraint supporting previous hypotheses about rare flares as the origin of unidentified γ-ray sources in star-forming regions. [ABSTRACT FROM AUTHOR]

Spectral and photometric variability of the Classical T Tauri stars RY Tau and SU Aur from 2013 to 2022 is analysed. We find that in SU Aur the H α line's flux at radial velocity RV  = −50 ± 7  km s−1 varies with a period P = 255 ± 5 d. A similar effect previously discovered in RY Tau is confirmed with these new data: P = 21.6 d at RV  = −95 ± 5  km s. In both stars, the radial velocity of these variations, the period, and the mass of the star turn out to be related by Kepler's law, suggesting structural features on the disc plane orbiting at radii of 0.2 au in RY Tau and 0.9 au in SU Aur, respectively. Both stars have a large inclination of the accretion disc to the line of sight – so that the line of sight passes through the region of the disc wind. We propose there is an azimuthal asymmetry in the disc wind, presumably in the form of 'density streams,' caused by substructures of the accretion disc surface. These streams cannot dissipate until they go beyond the Alfven surface in the disc's magnetic field. These findings open up the possibility to learn about the structure of the inner accretion disc of CTTS on scales less than 1 au and to reveal the orbital distances related to the planet's formation. [ABSTRACT FROM AUTHOR]

Gaia21bty, a pre-main-sequence star that previously had shown aperiodic dips in its light curve, underwent a considerable Δ G ≈ 2.9 mag brightening that occurred over a few months between 2020 October and 2021 February. The Gaia light curve shows that the star remained near maximum brightness for about 4–6 months, and then started slowly fading over the next 2 yr, with at least three superimposed ∼1 mag sudden rebrightening events. Whereas the amplitude and duration of the maximum is typical for EX Lupi-type stars, optical and near-infrared spectra obtained at the maximum are dominated by features which are typical for FU Ori-type stars (FUors). Modelling of the accretion disc at the maximum indicates that the disc bolometric luminosity is 43 L⊙ and the mass accretion rate is 2.5 × 10−5 M⊙ yr−1, which are typical values for FUors even considering the large uncertainty in the distance (⁠|$1.7_{-0.4}^{+0.8}$| kpc). Further monitoring is necessary to understand the cause of the quick brightness decline, the rebrightening, and the other post-outburst light changes, as our multicolour photometric data suggest that they could be caused by a long and discontinuous obscuration event. We speculate that the outburst might have induced large-scale inhomogeneous dust condensations in the line of sight leading to such phenomena, whilst the FUor outburst continues behind the opaque screen. [ABSTRACT FROM AUTHOR]

We present results from long-term spectroscopic monitoring of 21 T-Tauri stars located in the Taurus–Auriga star-forming region (SFR). We combine medium and high-dispersion Echelle spectroscopy obtained at the Stará Lesná, Skalnaté Pleso (both in Slovakia), and Tautenburg (Germany) observatories with low-resolution flux-calibrated spectra from Asiago (Italy) observatory all taken between 2015 and 2018. We extend the coverage by additional medium-resolution spectra from Stará Lesná obtained in 2022. In the previous paper, we measured photometric periods of these targets in a range of 0.7–3.1 d, which could be due to the rotation of a spotted surface or binarity. Here, we use the broadening-function technique to determine the radial and projected rotational velocities to reveal any close binary companion. Our analysis concludes that no such companion is present with an orbital period equal to the photometric period. We focus our analysis primarily on determining atmospheric parameters such as surface gravity log  g , effective temperature T eff, and metallicity [Fe/H]. Additionally, we measure the equivalent width of H α, Li i , and interstellar Na i lines. We also investigate the effect of possible reddening on individual targets and construct the HR diagram of our sample. Using pre-main-sequence evolutionary models, we determine the age of our targets. This analysis hints at ages younger than 50 Myr with mean age 5 ± 3 Myr, masses between 0.75 and 2.10 M⊙, and minimum radii in the range 0.60–3.17 R⊙. Altogether, the results are consistent with expected young stars with larger radii than those of main-sequence stars. [ABSTRACT FROM AUTHOR]

FU Orionis objects (FUors) are eruptive young stars, which exhibit outbursts that last from decades to a century. Due to the duration of their outbursts, and to the fact that only about two dozens of such sources are known, information on the end of their outbursts is limited. Here we analyse follow-up photometry and spectroscopy of Gaia21elv, a young stellar object, which had a several decades long outburst. It was reported as a Gaia science alert due to its recent fading by more than a magnitude. To study the fading of the source and look for signatures characteristic of FUors, we have obtained follow-up near-infrared (NIR) spectra using Gemini South/IGRINS, and both optical and NIR spectra using VLT/X-SHOOTER. The spectra at both epochs show typical FUor signatures, such as a triangular shaped H -band continuum, absorption-line dominated spectrum, and P Cygni profiles. In addition to the typical FUor signatures, [O  i ], [Fe  ii ], and [S  ii ] were detected, suggesting the presence of a jet or disc wind. Fitting the spectral energy distributions with an accretion disc model suggests a decrease of the accretion rate between the brightest and faintest states. The rapid fading of the source in 2021 was most likely dominated by an increase of circumstellar extinction. The spectroscopy presented here confirms that Gaia21elv is a classical FUor, the third such object discovered among the Gaia science alerts. [ABSTRACT FROM AUTHOR]

Context. Classical T Tauri stars (cTTs) accrete from their circumstellar disk. The material falls onto the stellar surface, producing an accretion shock, which generates veiling in a star's spectra. In addition, the shock causes a localized accretion spot at the level of the chromosphere. Aims. Our goal is to investigate the accretion, particularly the mass accretion rates (Ṁacc), for the cTTs DK Tau, over two periods of 17 and 29 days, using two different procedures for comparison purposes. Methods. The first method relies on the derivation of the accretion luminosity via accretion-powered emission lines. The second compares the variability of the optical veiling with accretion shock models to determine mass accretion rates. We used observations taken in 2010 and 2012 with the ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) spectropolarimeter at the CFHT (Canada-France-Hawaii Telescope). Results. We find peak values of the veiling (at ∼550 nm) ranging from 0.2 to 1.3, with a steeper trend across the wavelength range for higher peak values. When using the accretion-powered emission lines, we find mass accretion rate values ranging from log (Ṁacc[M⊙ yr−1]) = −8.20 to log (Ṁacc[M⊙ yr−1]) = −7.40. This agrees with the values found in the literature, as well as the values calculated using the accretion shock models and the veiling. In addition, we identify a power-law correlation between the values of the accretion luminosity and the optical veiling. For the 2010 observations, using the values of the filling factors (which represent the area of the star covered by an accretion spot) derived from the shock models, we infer that the accretion spot was located between +45° and +75° in latitude. Conclusion. We show that both methods of determining the mass accretion rate yield similar results. We also present a helpful means of confirming the accretion luminosity values by measuring the veiling at a single wavelength in the optical. [ABSTRACT FROM AUTHOR]

This study presents the largest mid-infrared spectral catalogue of Herbig Ae/Be stars to date, containing the Spitzer Infrared Spectrograph spectra of 126 stars. Based on the catalogue analysis, two prominent infrared vibrational modes of C60 bands at 17.4 and 18.9 μm are detected in the spectra of nine sources, while 7.0 μm feature is identified in the spectra of HD 319896. The spectral index analysis and the comparison of the known sources with C60 features indicated that there exist two different types of emission classes among the sample of stars. The infrared spectra of six Herbig Ae/Be stars in this study resemble that of reflection nebulae, and their association with previously known reflection nebulae is confirmed. In the case of three Herbig Ae/Be stars, we report the tentative evidence of C60 emission features originating from the circumstellar disc or nearby diffused emission region. The detection fraction of C60 in the total HAeBe star sample is ∼7 per cent, whereas the detection fraction is 30 per cent for HAeBe stars associated with nebulosity. In the catalog, C60 is exclusively present in the circumstellar regions of B type Herbig Ae/Be stars, with no evidence of its presence detected in stars with later spectral types. The present study has increased the number of young stellar objects and reflection nebulae detected with C60 multifold, which can help in understanding the excitation and formation pathway of the species. [ABSTRACT FROM AUTHOR]

Mid-infrared (mid-IR) variability in young stellar objects (YSOs) is driven by several physical mechanisms, which produce a variety of amplitudes and light-curve shapes. One of these mechanisms, variable disc accretion, is predicted by models of episodic accretion to drive secular variability, including in the mid-IR. Because the largest accretion bursts are rare, adding new objects to the YSO eruptive variable class aids our understanding of the episodic accretion phenomenon and its possible impact on stellar and planetary formation. A previous analysis of 6.5 yr of NeoWISE light curves (3–5 μm) of ∼7000 nearby YSOs found an increase in the fraction of variability and variability amplitude for objects at younger stages of evolution. To help interpret these light curves, we have obtained low- and high-resolution near-IR spectra of 78 objects from this sample of YSOs. In this work, we present the analysis of nine nearby YSOs (d <1 kpc) that show the characteristics of known classes of eruptive variable YSOs. We find one FU Orionis (FUor)-like source, one EX Lupi type object, and six YSOs with mixed characteristics or V1647 Ori like objects. The varied characteristics observed in our sample are consistent with recent discoveries of eruptive YSOs. We discuss how a wide range in YSO outburst parameters (central mass, maximum accretion rate during outburst, evolutionary stage, and/or instability leading to the outburst) may play a significant role in the observed spectrophotometric properties of YSO outbursts. [ABSTRACT FROM AUTHOR]

We reanalyse archival X-ray data of 16 Ae/Be Herbig stars obtained by the XMM–Newton and Chandra satellites. Stellar X-ray spectra in the energy range 0.2–8 keV were fitted with the use of APEC and MEKAL hot plasma emission models, and with models with an additional power-law component. We find that for Herbig stars, the dependence of the unabsorbed X-ray luminosity on stellar mass and radius, L X ∝  R α M β with α ≈ 3 and β ≈ 2, is similar to that for T Tauri stars. The independently determined accretion rates, rotation periods, and the surface magnetic fields follow a tight correlation predicted by the standard magnetospheric accretion theory. We suggest that X-ray emission from Herbig stars is powered by magnetic reconnection events in the tenuous corona at the disc–magnetosphere boundary. [ABSTRACT FROM AUTHOR]

A naive Bayes classifier for identifying Class II YSOs has been constructed and applied to a region of the Northern Galactic Plane containing 8 million sources with good quality Gaia EDR3 parallaxes. The classifier uses the five features: Gaia G -band variability, WISE mid-infrared excess, UKIDSS and 2MASS near-infrared excess, IGAPS Hα excess, and overluminosity with respect to the main sequence. A list of candidate Class II YSOs is obtained by choosing a posterior threshold appropriate to the task at hand, balancing the competing demands of completeness and purity. At a threshold posterior greater than 0.5, our classifier identifies 6504 candidate Class II YSOs. At this threshold, we find a false positive rate around 0.02 per cent and a true positive rate of approximately 87 per cent for identifying Class II YSOs. The ROC curve rises rapidly to almost one with an area under the curve around 0.998 or better, indicating the classifier is efficient at identifying candidate Class II YSOs. Our map of these candidates shows what are potentially three previously undiscovered clusters or associations. When comparing our results to published catalogues from other young star classifiers, we find between one quarter and three quarters of high probability candidates are unique to each classifier, telling us no single classifier is finding all young stars. [ABSTRACT FROM AUTHOR]

We present the results of the UBVI C variability survey in the young open cluster NGC 6611 based on observations obtained during 34 nights spanning one year. In total, we found 95 variable stars. Most of these stars are classified as periodic and irregular pre-main sequence (PMS) stars. The analysis of the JHK S 2MASS photometry and four-colour IRAC photometry revealed 165 Class II young stellar sources, 20 of which are irregular variables and one is an eclipsing binary. These classifications, complemented by JHK UKIDSS photometry and riH α VPHAS photometry, were used to identify 24 candidates for classical T Tauri stars and 30 weak-lined T Tauri stars. In addition to the PMS variables, we discovered eight δ Scuti candidates. None of these were previously known. Furthermore, we detected 17 eclipsing binaries where two were previously known. Based on the proper motions provided by the Gaia EDR3 catalogue, we calculated the cluster membership probabilities for 91 variable stars. For 61 variables, a probability higher than 80 per cent was determined, which makes them cluster members. Only 25 variables with a probability less than 20 per cent were regarded to be non-members. [ABSTRACT FROM AUTHOR]

We present measurements of spot properties on 31 young stellar objects, based on multiband data from the HOYS (Hunting Outbursting Young Stars) project. On average the analysis for each object is based on 270 data points during 80 d in at least three bands. All the young low-mass stars in our sample show periodic photometric variations. We determine spot temperatures and coverage by comparing the measured photometric amplitudes in optical bands with simulated amplitudes based on atmosphere models, including a complete error propagation. 21 objects in our sample feature cool spots, with spot temperatures 500–2500 K below the stellar effective temperature (T eff), and a coverage of 0.05–0.4. Six more have hot spots, with temperatures up to 3000 K above T eff and coverage below 0.15. The remaining four stars have ambiguous solutions or are AA Tau-type contaminants. All of the stars with large spots (i.e. high coverage >0.1) are relatively cool with T eff < 4500 K, which could be a result of having deeper convection zones. Apart from that, spot properties show no significant trends with rotation period, infrared excess, or stellar properties. Most notably, we find hot spots in stars that do not show K − W 2 infrared excess, indicating the possibility of accretion across an inner disc cavity or the presence of plage. [ABSTRACT FROM AUTHOR]

We present stellar variability towards the young open cluster NGC 6823. Time series V - and I -band CCD photometry led to identification and characterization of 88 variable stars, of which only 14 have been previously recognized. We ascertain the membership of each variable with optical UBVI and infrared photometry, and with Gaia EDR3 parallax and proper motion data. Seventy two variable stars are found to be cluster members, of which 25 are main sequence stars and 48 are pre-main-sequence stars. The probable cluster members collectively suggest an isochrone age of the cluster to be about 2 Myrs based on the GAIA photometry. With the colour and magnitude, as well as the shape of the light curve, we have classified the main sequence variables into β Cep, δ Scuti, slowly pulsating B type, and new class variables. Among the pre-main-sequence variables, eight are classical T Tauri variables, and four are Herbig Ae/Be objects, whereas the remaining belong to the weak-lined T Tauri population. The variable nature of 32 stars is validated with TESS light curves. Our work provides refined classification of variability of pre-main-sequence and main-sequence cluster members of the active star-forming complex, Sharpless 86. Despite no strong evidence of the disc-locking mechanism in the present sample of TTSs, one TTS with larger Δ(I − K) is found to be a slow rotator. [ABSTRACT FROM AUTHOR]

Planet formation takes place in protoplanetary discs around young T-Tauri stars. PDS 70 is one of the first confirmed examples of a system where the planets are currently forming in gaps in the disc, and can be directly imaged. One of the main early influences on planet formation is the lifetime of the protoplanetary disc, which is limited by the intense stellar X-ray and UV radiation. Stellar coronal activity and accretion of material onto the star are both potential sources of XUV radiation. Previous Swift observations detected UV emission, which were consistent with a low rate of accretion. We present follow up observations with the XMM-Newton observatory, which observed PDS 70 simultaneously in X-ray and UV in order to determine intensity of XUV radiation in the system, and identify if the source is coronal, accretion, or both. We detect a strong source in both X-ray and UV, with an average X-ray 0.2–12 keV luminosity of 1.37 × 1030 erg s−1, and a possible flare which increased the luminosity to 2.8 × 1030 erg s−1. The UV flux density is in excess of what would be expected from chromospheric emission, and supports the interpretation that PDS 70 has continuing weak accretion less than ∼10−10 M⊙ yr−1. The implications of the detected X-ray and UV radiation are that the disc is likely to be in the final stages of dispersal, and will be completely evaporated in the next million years, bringing an end to the primary planet formation process. [ABSTRACT FROM AUTHOR]

The dynamics of magnetic flux tubes (MFTs) in the accretion disk of typical Herbig Ae/Be star (HAeBeS) with fossil large-scale magnetic field is modeled taking into account the buoyant and drag forces, radiative heat exchange with the surrounding gas, and the magnetic field of the disk. The structure of the disk is simulated using our magnetohydrodynamic model, taking into account the heating of the surface layers of the disk with the stellar radiation. The simulations show that MFTs periodically rise from the innermost region of the disk with speeds up to 10–12 km s−1{{\rm{s}}}^{-1}. MFTs experience decaying magnetic oscillations under the action of the external magnetic field near the disk’s surface. The oscillation period increases with distance from the star and initial plasma beta of the MFT, ranging from several hours at r=0.012aur=0.012\hspace{0.33em}{\rm{au}} up to several months at r=1aur=1\hspace{0.33em}{\rm{au}}. The oscillations are characterized by pulsations of the MFT’s characteristics including its temperature. We argue that the oscillations can produce observed IR-variability of HAeBeSs, which would be more intense than in the case of T Tauri stars, since the disks of HAeBeSs are hotter, denser, and have stronger magnetic field.

Eclipses of UX Ori stars by compact gas–dust clouds and large-scale circumstellar disk perturbations are modeled. A flared disk and a disk with a puffing-up in the dust sublimation zone are considered. The disk puffing-up explains several observed features of eclipses. The linear polarization degree can remain unchanged during the eclipse. There might be no star reddening in the blue and ultraviolet spectral regions. Strong changes (up to 90°90\text{°}) in the positional angle of the linear polarization may happen when passing from one spectral band to another. An eclipse by a large-scale disk perturbation can be noticeably deeper than an eclipse by a compact gas–dust cloud. The polarization degree in such an eclipse can also be significantly higher. In addition, an eclipse by a large-scale perturbation of a star with a puffed-up disk explains the significant scatter of eclipse parameters at the same fading level. A significant change in the positional angle of the linear polarization may occur during a large-scale disk perturbation eclipse minimum and after it. We attempted parametric identification of long-lasted deep minima of UX Ori and WW Vul. The considered simple model can only partially explain the observed phenomena and need further development.

We perform 3D hydrodynamic simulations of disc–disc stellar flybys with on-the-fly Monte Carlo radiative transfer. We show that pre-existing circumstellar discs around both stars result in fast rising (∼years) outbursts lasting two to five times longer than for a star–disc flyby. The perturber always goes into outburst (⁠|$\dot{M}\gt 10^{-5}~{\rm M_{\odot }~ yr^{-1}}$|⁠). Whereas we find that the primary goes into a decades long outburst only when the flyby is retrograde to the circumprimary disc rotation. High accretion rates during the outburst are triggered by angular momentum cancellation in misaligned material generated by the encounter. A large fraction of accreted material is alien. [ABSTRACT FROM AUTHOR]

We investigate the formation and evolution of 'primordial' dusty rings occurring in the inner regions of protoplanetary discs, with the help of long-term, coupled dust-gas, magnetohydrodynamic simulations. The simulations are global and start from the collapse phase of the parent cloud core, while the dead zone is calculated via an adaptive α formulation by taking into account the local ionization balance. The evolution of the dusty component includes its growth and back reaction on to the gas. Previously, using simulations with only a gas component, we showed that dynamical rings form at the inner edge of the dead zone. We find that when dust evolution, as well as magnetic field evolution in the flux-freezing limit are included, the dusty rings formed are more numerous and span a larger radial extent in the inner disc, while the dead zone is more robust and persists for a much longer time. We show that these dynamical rings concentrate enough dust mass to become streaming unstable, which should result in a rapid planetesimal formation even in the embedded phases of the system. The episodic outbursts caused by the magnetorotational instability have a significant impact on the evolution of the rings. The outbursts drain the inner disc of grown dust, however, the period between bursts is sufficiently long for the planetesimal growth via streaming instability. The dust mass contained within the rings is large enough to ultimately produce planetary systems with the core accretion scenario. The low-mass systems rarely undergo outbursts, and, thus, the conditions around such stars can be especially conducive for planet formation. [ABSTRACT FROM AUTHOR]

We present the results of our new observations of the young binary ZZ Tau with a circumbinary disc. The system was found to consist of two coeval (age < 2 Myr) classical T Tauri stars with the total mass 0.86 ± 0.09 M⊙, orbital period 46.8 ± 0.8 yr, semimajor axis 88.2 ± 2.1 mas, eccentricity 0.58 ± 0.02, and the orbital inclination 123 |${_{.}^{\circ}}$| 8 ± 1 |${_{.}^{\circ}}$| 0. The accretion rate of ZZ Tau A and ZZ Tau B is approximately 7 × 10−10 and 2 × 10−10 M⊙ yr−1, respectively. No correlation was found between the long-term photometric variability of ZZ Tau and orbital position of its components. The periodic light variations with P  = 4.171 ± 0.002 d were observed in the BVRI bands presumably connected with an accretion (hot) spot on the surface of the primary (ZZ Tau A). At the same time, no periodicity was observed in the U band nor in the emission line profile variations probably, due to the significant contribution of ZZ Tau B's emission, which dominates shortward of λ ≈ 0.4  |$\mu$| m. We argue that the extinction in the direction to the primary is noticeably larger than that to the secondary. It appeared that the rotation axis of the primary is inclined to the line of sight by ≈31° ± 4°. We also concluded that ZZ Tau is the source of a CO molecular outflow; however, ZZ Tau IRS rather than ZZ Tau is the source of the Herbig–Haro object HH393. [ABSTRACT FROM AUTHOR]

Radiative transfer modelling offers a powerful tool for understanding the enigmatic hydrogen emission lines from T Tauri stars. This work compares optical and near-IR spectroscopy of 29 T Tauri stars with our grid of synthetic line profiles. The archival spectra, obtained with VLT/X-Shooter, provide simultaneous coverage of many optical and infrared hydrogen lines. The observations exhibit similar morphologies of line profiles seen in other studies. We used the radiative transfer code torus to create synthetic H α, Pa β, Pa γ, and Br γ emission lines for a fiducial T Tauri model that included axisymmetric magnetospheric accretion and a polar stellar wind. The distribution of Reipurth types and line widths for the synthetic H α lines is similar to the observed results. However, the modelled infrared lines are narrower than the observations by |${\approx}80\,{~\rm km\,s}^{-1}$|⁠ , and our models predict a significantly higher proportion (≈90 per cent) of inverse P-Cygni profiles. Furthermore, our radiative transfer models suggest that the frequency of P-Cygni profiles depends on the ratio of the mass-loss to mass accretion rates and blue-shifted sub-continuum absorption was predicted for mass-loss rates as low as 10−12 M⊙ yr−1. We explore the effect of rotation, turbulence, and the contributions from red-shifted absorption in an attempt to explain the discrepancy in widths. Our findings show that, singularly, none of these effects is sufficient to explain the observed disparity. However, a combination of rotation, turbulence, and non-axisymmetric accretion may improve the fit of the models to the observed data. [ABSTRACT FROM AUTHOR]

Studies of T Tauri discs inform planet formation theory; observations of variability due to occultation by circumstellar dust are a useful probe of unresolved, planet-forming inner discs, especially around faint M dwarf stars. We report observations of 2M0632, an M dwarf member of the Carina young moving group that was observed by Transiting Exoplanet Survey Satellite over two 1-yr intervals. The combined light curve contains >300 dimming events, each lasting a few hours, and as deep as 40 per cent (0.55 magnitudes). These stochastic events are correlated with a distinct, stable 1.86-d periodic signal that could be stellar rotation. Concurrent ground-based, multiband photometry show reddening consistent with interstellar medium-like dust. The star's excess emission in the infrared and emission lines in optical and infrared spectra reveal a T Tauri-like accretion disc around the star. We confirm membership of 2M0632 in the Carina group by a Bayesian analysis of its Galactic space motion and position. We combine stellar evolution models with Gaia photometry and constraints on T eff, luminosity, and the absence of detectable lithium in the photosphere to constrain the age of the group and 2M0632 to 40–60 Myr, consistent with earlier estimates. 2M0632 joins a handful of long-lived discs which challenge the canon that disc lifetimes are ≲10 Myr. All known examples surround M dwarfs, suggesting that lower X-ray/ultraviolet irradiation and slower photoevaporation by these stars can dramatically affect disc evolution. The multiplanet systems spawned by long-lived discs probably experienced significant orbital damping and migration into close-in, resonant orbits, and perhaps represented by the TRAPPIST-1 system. [ABSTRACT FROM AUTHOR]

The VISTA Variables in the Via Lactea (VVV) survey obtained near-infrared photometry towards the Galactic bulge and the southern disc plane for a decade (2010–2019). We designed a modified Lomb–Scargle method to search for large-amplitude (⁠|$\Delta K_{s, 2-98{{\ \rm per\ cent}}}$|  > 1.5 mag) mid to long-term periodic variables (P > 10 d) in the 2nd version of VVV Infrared Astrometric Catalogue (VIRAC2-β). In total, 1520 periodic sources were discovered, including 59 candidate periodic outbursting young stellar objects (YSOs), based on the unique morphology of the phase-folded light curves, proximity to Galactic H  ii regions and mid-infrared colours. Five sources are spectroscopically confirmed as accreting YSOs. Both fast-rise/slow-decay and slow-rise/fast-decay periodic outbursts were found, but fast-rise/slow-decay outbursts predominate at the highest amplitudes. The multiwavelength colour variations are consistent with a variable mass accretion process, as opposed to variable extinction. The cycles are likely to be caused by dynamical perturbations from stellar or planetary companions within the circumstellar disc. An additional search for periodic variability amongst YSO candidates in published Spitzer -based catalogues yielded a further 71 candidate periodic accretors, mostly with lower amplitudes. These resemble cases of pulsed accretion but with unusually long periods and greater regularity. The majority of other long-period variables are pulsating dusty Miras with smooth and symmetric light curves. We find that some Miras have redder W 3 − W 4 colours than previously thought, most likely due to their surface chemical compositions. [ABSTRACT FROM AUTHOR]

We simulate the polarized radiative transfer in the vicinity of the UX Ori type stars during their minima. Our model of an eclipse by an extended disc perturbation generalizes the compact gas-dust cloud eclipse model. We apply the radiative transfer method based on enumeration using the directions grid to model the influence of the perturbation extensions along azimuth and radius on the eclipse depth and parameters of the linear polarization. We investigate eclipses both for the flared disc and for the disc with a puffing-up in the dust sublimation zone. The puffing-up is obtained by adding a dusty disc wind to the model. Comparison with a compact cloud eclipse model reveals that the eclipse by a large-scale azimuthally extended perturbation may be significantly deeper and show a greater linear polarization degree. We also demonstrate that the perturbation extension together with the disc puffing-up can strongly affect the degree of polarization and colour index of the star during the eclipse. The position angle of the linear polarization may also change markedly during and after an eclipse by a large-scale perturbation for the model with a puffed-up inner rim. Also, in this model, the maximum degree of the linear polarization can be achieved not at the brightness minimum but closer to the end of the eclipse. We discuss the modelling results in the context of the photopolarimetric observations of UX Ori stars. [ABSTRACT FROM AUTHOR]

We present results from photometric monitoring of V900 Mon, one of the newly discovered and still under-studied object from the FU Orionis type. The FUor phenomenon is very rarely observed, but it is essential for stellar evolution. Since we only know about twenty stars of this type, the study of each new object is very important for our knowledge. Our data were obtained in optical spectral region with the BVRI Johnson-Cousins set of filters during the period from September 2011 to April 2021. In order to follow the photometric history of the object, we measured its stellar magnitudes on available plates from the Mikulski Archive for Space Telescopes. The collected archival data suggest that the rise in brightness of V900 Mon began after January 1989 and the outburst goes on so far. In November 2009, when the outburst was registered, the star had already reached the level of brightness close to the current one. Our observations indicate that during the period 2011-2017 the stellar magnitude increased gradually in each pass band. The observed amplitude of the outburst is about 4 magnitudes (R). During the last three years, the increase in brightness has stopped and there has even been a slight decline. The comparison of light curves of the known FUor objects shows that they are very diverse and are rarely repeated. However, the photometric data we have so far show that V900 Mon's light curve is somewhat similar to those of V1515 Cyg and V733 Cep.

Investigations of the formation of young stellar objects (YSOs) and planets require the detailed analysis of individual sources as well as statistical analysis of a larger number of objects. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) project provides such a unique opportunity by establishing a UV spectroscopic library of young high- and low-mass stars in the local universe. Here, we analyse optical photometry of the three ULLYSES targets (TX Ori, V505 Ori, V510 Ori) and other YSOs in the σ Ori cluster taken at the time of the HST observations to provide a reference for those spectra. We identify three populations of YSOs along the line of sight to σ Ori, separated in parallax and proper motion space. The ULLYSES targets show typical YSO behaviour with pronounced variability and mass accretion rates of the order of 10−8 M⊙ yr−1. Optical colours do not agree with standard interstellar reddening and suggest a significant contribution of scattered light. They are also amongst the most variable and strongest accretors in the cluster. V505 Ori shows variability with a 7 d period, indicating an inner disc warp at the co-rotation radius. Uncovering the exact nature of the ULLYSES targets will require improved detailed modelling of the HST spectra in the context of the available photometry, including scattered light contributions as well as non-standard reddening. [ABSTRACT FROM AUTHOR]

We examine whether stellar flyby simulations can initiate FU Orionis outbursts using 3D hydrodynamics simulations coupled to live Monte Carlo radiative transfer. We find that a flyby where the secondary penetrates the circumprimary disc triggers a 1–2 yr rise in the mass accretion rate to 10−4 M⊙ yr−1 that remains high (≳10−5 M⊙ yr−1) for more than a hundred years, similar to the outburst observed in FU Ori. Importantly, we find that the less massive star becomes the dominant accretor, as observed. [ABSTRACT FROM AUTHOR]

We introduce the star-melt   python package that we developed to facilitate the analysis of time-resolved emission-line spectroscopy of young stellar objects. star-melt automatically extracts, identifies, and fits emission lines. We summarize our analysis methods that utilizes the time domain of high-resolution stellar spectra to investigate variability in the line profiles and corresponding emitting regions. This allows us to probe the innermost disc and accretion structures of YSOs. Local temperatures and densities can be determined using Boltzmann statistics, the Saha equation, and the Sobolev large velocity gradient approximation. star-melt allows for new results to be obtained from archival data, as well as facilitating timely analysis of new data as it is obtained. We present the results of applying star-melt to three YSOs, using spectra from UVES, XSHOOTER, FEROS, HARPS, and ESPaDOnS. We demonstrate what can be achieved for data with disparate time sampling, for stars with different inclinations and variability types. For EX Lupi, we confirm the presence of a localized and stable stellar-surface hotspot associated with the footprint of the accretion column. For GQ Lupi A, we find that the maximum infall rate from an accretion column is correlated with lines produced in the lowest temperatures. For CVSO109. we investigate the rapid temporal variability of a redshifted emission wing, indicative of rotating and infalling material in the inner disc. Our results show that star-melt is a useful tool for such analysis, as well as other applications for emission lines. [ABSTRACT FROM AUTHOR]

We study the formation and the kinematic evolution of the early-type Herbig Be star IL Cep and its environment. The young star is a member of the Cep OB3 association, at a distance of 798 ± 9 pc, and has a 'cavity' associated with it. We found that the B0V star HD 216658, which is astrometrically associated with IL Cep, is at the centre of the cavity. From the evaluation of various pressure components created by HD 216658, it is established that the star is capable of creating the cavity. We identified 79 co-moving stars of IL Cep at 2-pc radius from the analysis of Gaia EDR3 astrometry. The transverse velocity analysis of the co-moving stars shows that they belong to two different populations associated with IL Cep and HD 216658, respectively. Further analysis confirms that all the stars in the IL Cep population are mostly coeval (∼0.1 Myr). Infrared photometry revealed that there are 26 Class II objects among the co-moving stars. The stars without circumstellar disc (Class III) are 65 per cent of all the co-moving stars. There are nine intense H  α emission candidates identified among the co-moving stars using IPHAS H  α narrow-band photometry. The dendrogram analysis on the Hydrogen column density map identified 11 molecular clump structures on the expanding cavity around IL Cep, making it an active star-forming region. The formation of the IL Cep stellar group due to the 'rocket effect' by HD 216658 is discussed. [ABSTRACT FROM AUTHOR]

We present results from long-term optical photometric and spectroscopic observations of five pre-main sequence stars, located in the vicinity of the bright nebula NGC 7129. We obtained UBVRI photometric observations in the field centered on the star V391 Cep, north-west of the bright nebula NGC 7129. Our multicolor CCD observations spanned the period from February 1998 to November 2016. At the time of our photometric monitoring, a total of thirteen medium-resolution optical spectra of the stars were obtained. The results from our photometric study show that all stars exhibit strong variability in all optical passbands. Long-term light curves of the five stars indicate the typical classical T Tauri star variations in brightness with large amplitudes. We did not find any reliable periodicity in the brightness variations of all five stars. The results from spectral observations showed that all studied stars can be classified as classical T Tauri stars with rich emission line spectra and strong variability in profiles and intensity of emission lines.

V555 Ori is a T Tauri star, whose 1.5 mag brightening was published as a Gaia science alert in 2017. We carried out optical and near-infrared (NIR) photometric, and optical spectroscopic observations to understand the light variations. The light curves show that V555 Ori was faint before 2017, entered a high state for about a year, and returned to the faint state by mid-2018. In addition to the long-term flux evolution, quasi-periodic brightness oscillations were also evident, with a period of about 5 d. At optical wavelengths both the long-term and short-term variations exhibited colourless changes, while in the NIR they were consistent with changing extinction. We explain the brightness variations as the consequence of changing extinction. The object has a low accretion rate whose variation in itself would not be enough to reproduce the optical flux changes. This behaviour makes V555 Ori similar to the pre-main sequence star AA Tau, where the light changes are interpreted as periodic eclipses of the star by a rotating inner disc warp. The brightness maximum of V555 Ori was a moderately obscured (AV  = 2.3 mag) state, while the extinction in the low state was AV  = 6.4 mag. We found that while the Gaia alert hinted at an accretion burst, V555 Ori is a standard dipper, similar to the prototype AA Tau. However, unlike in AA Tau, the periodic behaviour was also detectable in the faint phase, implying that the inner disc warp remained stable in both the high and low states of the system. [ABSTRACT FROM AUTHOR]

We find that dust clouds that eclipse young stars obscure the stellar disc inhomogeneously. In the particular case of CQ Tau, we find isolated optically thick structures with sizes ≲0.6 R * and derive the typical AV gradient in the plane of the sky, finding it to be as high as a few magnitudes per stellar radius. The large extinction gradients and complex structure of the obscuring clouds lead not only to a noticeable Rossiter–McLaughlin effect but also to complex and variable shaping of stellar absorption lines. [ABSTRACT FROM AUTHOR]

We report the first characterization of the individual discs in the intermediate-separation binary systems KK Oph and HD 144668 at millimetre wavelengths. In both systems, the circumprimary and the circumsecondary discs are detected in the millimetre continuum emission, but not in 13CO nor C18O lines. Even though the disc structure is only marginally resolved, we find indications of large-scale asymmetries in the outer regions of the primary discs, most likely due to perturbation by the companion. The derived dust masses are firmly above debris disc level for all stars. The primaries have about three times more dust in their discs than the secondaries. In the case of HD 144668, the opacity spectral index of the primary and secondary differ by a large margin of 0.69, which may be a consequence of the secondary disc being more compact. Upper limits on the gas masses imply less than 0.1  M jup in any of these discs, meaning that giant planets can no longer form in them. Considering that there have been no massive gas discs identified to date in intermediate-separation binaries (i.e. binaries at a few hundred au separation), this opens space for speculation whether their binarity causes the removal of gas, with tidal interaction truncating the discs and hence shortening the accretion time-scale. More systematic studies in this respect are sorely needed. [ABSTRACT FROM AUTHOR]