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The formation of the extended thin disc is the most spectacular event of our Galaxy in the past ∼8 Gyr. To unveil this process, obtaining precise and accurate stellar ages for a large sample of stars is essential although challenging. In this work, we present the asteroseismic age determination of 5306 red giant branch stars using Kepler and LAMOST data, with a thorough examination of how the age determination is affected by the choice of different temperature scales and stellar models. Thanks to the high precision of the asteroseismic and spectroscopic parameters of our sample stars, we are able to achieve age determination with an average accuracy of 12 per cent. However, the age determination is sensitively dependent on the adopted temperature scale, as 50 K difference in effective temperature may cause larger than 10 per cent systematic uncertainty in the age estimates. Using the ages derived with the most plausible set of the temperature scale, we study the age distribution of the chemical thin disc stars, and present an estimate of the formation epoch of the first Galactic thin disc stars. We find that the first (oldest) thin disc stars have an age of |$9.5^{+0.5(\rm rand.)+0.5(\rm sys.)}_{-0.4(\rm rand.)-0.3(\rm sys.)}$|  Gyr, where the systematic uncertainties reflect ages estimated using different stellar evolutionary models. At this epoch, the Galactic thick disc was still forming stars, indicating there is a time window when both the thin and thick discs of our Galaxy were forming stars together. Moreover, we find that the first thin disc stars exhibit a broad distribution of Galactocentric radii, suggesting that the inner and outer thin discs began to form simultaneously. [ABSTRACT FROM AUTHOR]

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We apply a population-orbit superposition metho1d to 16 galaxies in the Fornax cluster observed with MUSE/VLT in the context of the Fornax3D project. By fitting the luminosity distribution, stellar kinematics, and age and metallicity maps simultaneously, we obtained the internal stellar orbit distribution, as well as the age and metallicity distribution of stars on different orbits for each galaxy. Based on the model, we decompose each galaxy into a dynamically cold disk (orbital circularity -z - 0:8) and a dynamically hot nondisk component (orbital circularity -z < 0:8), and obtain the surface-brightness, age, and metallicity radial profiles of each component. The galaxy infall time into the cluster is strongly correlated with galaxy cold-disk age with older cold disks in ancient infallers. We quantify the infall time tinfall of each galaxy with its cold-disk age using a correlation calibrated with TNG50 cosmological simulations. For galaxies in the Fornax cluster, we found that the luminosity fraction of cold disk in galaxies with tinfall > 8 Gyr are a factor of ~4 lower than in more recent infallers while controlling for total stellar mass. Nine of the 16 galaxies have spatially extended cold disks, and most of them show positive or zero age gradients; stars in the inner disk are ~2-5 Gyr younger than that in the outer disk, in contrast to the expectation of inside-out growth. Our results indicate that the assembly of cold disks in galaxies is strongly affected by their infall into clusters, by either removal of gas in outer regions or even tidally stripping or heating part of the pre-existing disks. Star formation in outer disks can stop quickly after the galaxy falls into the cluster, while star formation in the inner disks can last for a few Gyrs more, building the positive age gradient measured in cold disks. [ABSTRACT FROM AUTHOR]

Context. Orbital motion in binary and planetary systems is the main source of precise stellar and planetary mass measurements, and the joint analysis of data obtained using multiple observational methods can both lift degeneracies and improve precision. Aims. We set out to measure the masses of individual stars in binary systems using all the information brought by the HIPPARCOS and Gaia absolute astrometric missions. Methods. We present BINARYS, a tool that uses the HIPPARCOS and Gaia absolute astrometric data and combines them with relative astrometry and/or radial velocity measurements to determine the orbit of a binary system. This tool rigorously combines the HIPPARCOS and Gaia data (here EDR3) and can use the HIPPARCOS Transit Data as needed for binaries where HIPPARCOS detects significant flux from the secondary component. It also supports the case where Gaia has resolved the system, giving an astrometric solution for both components. Results. We determine model-independent individual masses for the first time for three systems: the two mature binaries Gl 494 (M1 = 0.584 ± 0.003 Mand M2 = 87 ± 1 MJup) and HIP 88745 (M1 = 0.96 ± 0.02 Mand M2 = 0.60+0.02 -0.01 M), and the younger AB Dor member GJ 2060 (M1 = 0.60+0.06 -0.05 M and M2 = 0.45+0.06 -0.05 M). The latter provides a rare test of evolutionary model predictions at young ages in the low-stellar-mass range and sets a lower age limit of 100 Myr for the moving group. [ABSTRACT FROM AUTHOR]

Context. The nuclear stellar disc (NSD) is a flat dense stellar structure at the heart of the Milky Way. Recent work has shown that analogous structures are common in the nuclei of external spiral galaxies, where there is evidence of an age gradient that indicates that they form inside-out. However, the characterisation of the age of the NSD stellar population along the line of sight is still missing due to its extreme source crowding and the high interstellar extinction towards the Galactic centre. Aims. We aim to characterise the age of the stellar population at different average Galactocentric NSD radii to investigate for the first time the presence of an age gradient along the line of sight. Methods. We selected two groups of stars at different NSD radii via their different extinction and proper motion distribution. We analysed their stellar population by fitting their de-reddened Ks luminosity functions with a linear combination of theoretical models. Results. We find significant differences in the stellar population at different NSD radii, indicating the presence of an age gradient along the line of sight. Our sample from the closest edge of the NSD contains a significant fraction (∼40% of its total stellar mass) of intermediate-age stars (2–7 Gyr) that is not present in the sample from stars deeper inside the NSD, in which ∼90% of the stellar mass is older than 7 Gyr. Our results suggest that the NSD age distribution is similar to the one found in external galaxies and they imply that bar-driven processes observed in external galaxies are similarly at play in the Milky Way. [ABSTRACT FROM AUTHOR]

Robust age estimates of red giant stars are now possible thanks to the precise inference of their mass based on asteroseismic constraints. However, there are cases where such age estimates can be highly precise yet very inaccurate. An example is giants that have undergone mass loss or mass transfer events that have significantly altered their mass. In this context, stars with 'apparent' ages significantly higher than the age of the Universe are candidates for stripped stars, or stars that have lost more mass than expected, most likely via interactions with a companion star or because of the poorly understood mass-loss mechanism along the red-giant branch. In this work we identify examples of such objects among red giants observed by Kepler, both at low ([Fe/H] ≲ −0.5) and solar metallicity. By modelling their structure and pulsation spectra, we find a consistent picture that confirms that they are indeed low-mass objects consisting of a He core of ≈0.5 M⊙ and an envelope of ≈0.1 − 0.2 M⊙. Moreover, we find that these stars are characterised by a rather extreme coupling (q ≳ 0.4) between the pressure-mode and gravity-mode cavities, one that is much higher than the typical value for red clump stars, thus providing a direct seismic signature of their peculiar structure. The complex pulsation spectra of these objects, if observed with sufficient frequency resolution, hold detailed information about the structural properties of likely products of mass stripping and can hence potentially shed light on their formation mechanism. On the other hand, our tests highlight the difficulties associated with reliably measuring the large frequency separation, especially in shorter datasets, which impacts the reliability of the inferred masses and ages of low-mass red clump stars with, for example, K2 or TESS data. [ABSTRACT FROM AUTHOR]

Context. With its origin coming from several sources (Big Bang, stars, cosmic rays) and given its strong depletion during its stellar lifetime, the lithium element is of great interest as its chemical evolution in the Milky Way is not well understood at present. To help constrain stellar and galactic chemical evolution models, numerous and precise lithium abundances are necessary for a large range of evolutionary stages, metallicities, and Galactic volume. Aims. In the age of stellar parametrization on industrial scales, spectroscopic surveys such as APOGEE, GALAH, RAVE, and LAMOST have used data-driven methods to rapidly and precisely infer stellar labels (atmospheric parameters and abundances). To prepare the ground for future spectroscopic surveys such as 4MOST and WEAVE, we aim to apply machine learning techniques to lithium measurements and analyses. Methods. We trained a convolution neural network (CNN), coupling Gaia-ESO Survey iDR6 stellar labels (Teff, log(g), [Fe/H], and A(Li)) and GIRAFFE HR15N spectra, to infer the atmospheric parameters and lithium abundances for ∼40 000 stars. The CNN architecture and accompanying notebooks are available online via GitHub. Results. We show that the CNN properly learns the physics of the stellar labels, from relevant spectral features through a broad range of evolutionary stages and stellar parameters. The lithium feature at 6707.8 Å is successfully singled out by our CNN, among the thousands of lines in the GIRAFFE HR15N setup. Rare objects such as lithium-rich giants are found in our sample. This level of performance is achieved thanks to a meticulously built, high-quality, and homogeneous training sample. Conclusions. The CNN approach is very well adapted for the next generations of spectroscopic surveys aimed at studying (among other elements) lithium, such as the 4MIDABLE-LR/HR (4MOST Milky Way disk and bulge low- and high-resolution) surveys. In this context, the caveats of machine-learning applications should be appropriately investigated, along with the realistic label uncertainties and upper limits for abundances. [ABSTRACT FROM AUTHOR]

Context. Determining ages of stars for reconstructing the history of the Milky Way remains one of the most difficult tasks in astrophysics. This involves knowing when it is possible to relate the stellar mass with its age and when it is not. The young α-rich (YAR) stars present such a case in which we are still not sure about their ages because they are relatively massive, implying young ages, but their abundances are α-enhanced, which implies old ages. Aims. We report the results from new observations from a long-term radial-velocity-monitoring campaign complemented with high-resolution spectroscopy, as well as new astrometry and seismology of a sample of 41 red giants from the third version of APOKASC, which includes YAR stars. The aim is to better characterize the YAR stars in terms of binarity, mass, abundance trends, and kinematic properties. Methods. The radial velocities of HERMES, APOGEE, and Gaia were combined to determine the binary fraction among YAR stars. In combination with their mass estimate, evolutionary status, chemical composition, and kinematic properties, it allowed us to better constrain the nature of these objects. Results. We found that stars with M < 1 M⊙ were all single, whereas stars with M > 1 M⊙ could be either single or binary. This is in agreement with theoretical predictions of population synthesis models. Studying their [C/N], [C/Fe], and [N/Fe], trends with mass, it became clear that many YAR stars do not follow the APOKASC stars, favoring the scenario that most of them are the product of mass transfer. Our sample further includes two likely undermassive stars, that is to say of such as low mass that they cannot have reached the red clump within the age of the Universe, unless their low mass is the signature of mass loss in previous evolutionary phases. These stars do not show signatures of currently being binaries. Both YAR and undermassive stars might show some anomalous APOGEE abundances for the elements N, Na, P, K, and Cr; although, higher-resolution optical spectroscopy might be needed to confirm these findings. Conclusions. Considering the significant fraction of stars that are formed in pairs and the variety of ways that makes mass transfer possible, the diversity in properties in terms of binarity, and chemistry of the YAR and undermassive stars studied here implies that most of these objects are likely not young. [ABSTRACT FROM AUTHOR]

Context. Stellar age determinations for field stars are crucial for studying the evolutionary history of the Galaxy. The vast majority of the research in this area has so far been focused on stars with typical disk characteristics. Aims. Nowadays, the availability of high-quality asteroseismic data for stars with typical halo characteristics makes it possible to extend such investigations. The aim of this paper is to study the precision and theoretical biases in the age determinations of halo stars adopting both asteroseismic and classic observational constraints. Methods. We adopt the well-tested SCEPtER pipeline, covering evolutionary phases up to the red giant branch (RGB). The fitting grids contain stars with mass in the range of [0.7; 1.0] M⊙ and metallicity [Fe/H] from −2.5 to −0.5, which are typical ranges seen in the halo population. We investigate several scenarios characterised by different adopted observational uncertainties. We also assess the impact of systematic discrepancies between the recovery grid models and target stars by computing several synthetic grids of stellar models with perturbed input physics. Results. We achieve more precise asteroseismic age estimates for old metal-poor stars than for more metallic stars. In our reference scenario (errors in Δν and νmax of 2.5% and 5% respectively), we recover ages for stars in the main sequence (MS) or subgiant branch (SGB) with a typical 10%−20% precision, while we recover those of RGB stars with a precision of about 60%. However, recent observations allow tighter constraints on asteroseismic parameters by about a factor of 3. With this assumption, the age precision in RGB improved to 20%, while few modifications occur in the other analysed evolutionary phases. Our investigation of the relevance of systematic discrepancies between grid models and target stars shows that a mismatch in the mixing-length parameter value between grids and targets (from 1.9 to 1.74) leads to significant bias in the age estimations for MS stars (about 10%), but this bias is smaller for SGB and RGB stars. Neglecting the microscopic diffusion effect in the recovery grid leads to a typical 40% bias in age estimates for stars on the MS. Finally, we applied the age estimation technique to stars in globular clusters, adopting typical observational uncertainties from the literature. We find a precision in age estimates of around 20% for MS stars and up to 40% for RGB stars. These uncertainties are greater than those obtained with classical methods, which are therefore still to be preferred. We also applied the SCEPtER pipeline to the age determination of the stars of the cluster M4, relying on asteroseismic data for seven RGB stars from the literature. We obtain a cluster age of 11.9 ± 1.5 Gyr and a mass at the turn-of off 0.86 ± 0.04 M⊙, which are in good agreement with literature results. [ABSTRACT FROM AUTHOR]

A long-standing problem when deriving the physical properties of stellar populations is the degeneracy between age, reddening, and metallicity. When a single metallicity is used for all the star clusters in a galaxy, this degeneracy can result in 'catastrophic' errors for old globular clusters. Typically, approximately 10–20 per cent of all clusters detected in spiral galaxies can have ages that are incorrect by a factor of 10 or more. In this paper, we present a pilot study for four galaxies (NGC 628, NGC 1433, NGC 1365, and NGC 3351) from the PHANGS-HST survey. We describe methods to correct the age-dating for old globular clusters, by first identifying candidates using their colours, and then reassigning ages and reddening based on a lower metallicity solution. We find that young 'Interlopers' can be identified from their Hα flux. CO (2-1) intensity or the presence of dust can also be used, but our tests show that they do not work as well. Improvements in the success fraction are possible at the ≈15 per cent level (reducing the fraction of catastrophic age-estimates from between 13 and 21 per cent, to between 3  and 8 per cent). A large fraction of the incorrectly age-dated globular clusters are systematically given ages around 100 Myr, polluting the younger populations as well. Incorrectly age-dated globular clusters significantly impact the observed cluster age distribution in NGC 628, which affects the physical interpretation of cluster disruption in this galaxy. For NGC 1365, we also demonstrate how to fix a second major age-dating problem, where very dusty young clusters with E (B − V) > 1.5 mag are assigned old, globular-cluster like ages. Finally, we note the discovery of a dense population of ≈300 Myr clusters around the central region of NGC 1365 and discuss how this results naturally from the dynamics in a barred galaxy. [ABSTRACT FROM AUTHOR]

Gaia Data Release 2 revealed a population of ultramassive white dwarfs on the Q branch that are moving anomalously fast for a local disc population with their young photometric ages. As the velocity dispersion of stars in the local disc increases with age, a proposed explanation of these white dwarfs is that they experience a cooling delay that causes current cooling models to infer photometric ages much younger than their true ages. To explore this explanation, we investigate the kinematics of ultramassive white dwarfs within 200 pc of the Sun using the improved Gaia Early Data Release 3 observations. We analyse the transverse motions of 0.95–1.25 M⊙ white dwarfs, subdivided by mass and age, and determine the distributions of the three-dimensional components of the transverse velocities. The results are compared to expectations based on observed kinematics of local main-sequence stars. We find a population of photometrically young (∼0.5–1.5 Gyr) ultramassive (∼1.15–1.25 M⊙) white dwarfs for which the transverse velocity component in the direction of Galactic rotation is more dispersed than for local disc stars of any age; thus, it is too dispersed to be explained by any cooling delay in white dwarfs originating from the local disc. Furthermore, the dispersion ratio of the velocity components in the Galactic plane for this population is also inconsistent with a local disc origin. We discuss some possible explanations of this kinematically anomalous population, such as a halo origin or production through dynamical effects of stellar triple systems. [ABSTRACT FROM AUTHOR]

Multiwavelength images from the Hubble Space Telescope covering the wavelength range 0.27–1.6 μ m show that the central area of the nearby dwarf galaxy NGC 4449 contains several tens of compact sources that are emitting in the hydrogen recombination line Pa β (1.2818 μ m) but are only marginally detected in H α (0.6563 μ m) and undetected at wavelengths λ ≤ 0.55 μ m. An analysis of the spectral energy distributions (SEDs) of these sources indicates that they are likely relatively young stellar clusters heavily attenuated by dust. The selection function used to identify the sources prevents meaningful statistical analyses of their age, mass, and dust extinction distributions. However, these cluster candidates have ages ∼5–6 Myr and A V > 6 mag, according to their SED fits, and are extremely compact, with typical deconvolved radii of 1 pc. The dusty clusters are located at the periphery of the dark clouds within the galaxy and appear to be partially embedded. Density and pressure considerations indicate that the H ii regions surrounding these clusters may be stalled, and that pre-supernova (pre-SN) feedback has not been able to clear the clusters of their natal cocoons. These findings are in potential tension with existing models that regulate star formation with pre-SN feedback, since pre-SN feedback acts on short timescales, ≲4 Myr, for a standard stellar initial mass function. The existence of a population of dusty stellar clusters with ages >4 Myr, if confirmed by future observations, paints a more complex picture for the role of stellar feedback in controlling star formation. [ABSTRACT FROM AUTHOR]

We present a multi-epoch spectroscopic study of LkCa 4, a heavily spotted non-accreting T Tauri star. Using SpeX at NASA's Infrared Telescope Facility (IRTF), 12 spectra were collected over five consecutive nights, spanning ≈1.5 stellar rotations. Using the IRTF SpeX Spectral Library, we constructed empirical composite models of spotted stars by combining a warmer (photosphere) standard star spectrum with a cooler (spot) standard weighted by the spot filling factor, f spot. The best-fit models spanned two photospheric component temperatures, T phot = 4100 K (K7V) and 4400 K (K5V), and one spot component temperature, T spot = 3060 K (M5V) with an A V of 0.3. We find values of f spot to vary between 0.77 and 0.94 with an average uncertainty of ∼0.04. The variability of f spot is periodic and correlates with its 3.374 day rotational period. Using a mean value for f meanspot to represent the total spot coverage, we calculated spot corrected values for T eff and L ⋆. Placing these values alongside evolutionary models developed for heavily spotted young stars, we infer mass and age ranges of 0.45–0.6 M ⊙ and 0.50–1.25 Myr, respectively. These inferred values represent a twofold increase in the mass and a twofold decrease in the age as compared to standard evolutionary models. Such a result highlights the need for constraining the contributions of cool and warm regions of young stellar atmospheres when estimating T eff and L ⋆ to infer masses and ages as well as the necessity for models to account for the effects of these regions on the early evolution of low-mass stars. [ABSTRACT FROM AUTHOR]

High-resolution sub-mm observations of some protoplanetary discs reveal non-axisymmetric features, which can often be interpreted as dust concentrations in vortices that form at the edges of gaps carved out by the embedded planets. We use recent results on the time-scale for the planet-driven vortex development in low-viscosity discs to set constraints on the mass and age of a planet producing the vortex. Knowledge of the age of the central star in a vortex-bearing protoplanetary disc system allows one to set a lower limit on the planetary mass at the level of several tens of M⊕. Also, an independent upper limit on the planetary mass would constrain the planetary age, although given the current direct imaging detection limits this constraint is not yet very stringent (it is also sensitively dependent on the disc scale height). These results can be extended to account for the history of planetary mass accretion if it is known. We apply our calculations to several protoplanetary discs harbouring vortex-like features as revealed by ALMA and set limits of (30–50) M⊕ (for disc aspect ratio of 0.1) on the minimum masses of putative planets that could be responsible for these vortices. Our vortex-based method provides an independent way of constraining the properties of embedded planets, complementary to other approaches. [ABSTRACT FROM AUTHOR]

The ultraviolet (UV) emission of stellar flares may have a pivotal role in the habitability of rocky exoplanets around low-mass stars. Previous studies have used white-light observations to calibrate empirical models which describe the optical and UV flare emission. However, the accuracy of the UV predictions of models has previously not been tested. We combined TESS optical and GALEX UV observations to test the UV predictions of empirical flare models calibrated using optical flare rates of M stars. We find that the canonical 9000-K black-body model used by flare studies underestimates the GALEX near-ultraviolet (NUV) energies of field age M stars by up to a factor of 6.5 ± 0.7 and the GALEX far-ultraviolet energies of fully convective field age M stars by 30.6 ± 10.0. We calculated energy correction factors that can be used to bring the UV predictions of flare models closer in line with observations. We calculated pseudo-continuum flare temperatures that describe both the white-light and GALEX NUV emission. We measured a temperature of 10 700 K for flares from fully convective M stars after accounting for the contribution from UV line emission. We also applied our correction factors to the results of previous studies of the role of flares in abiogenesis. Our results show that M stars do not need to be as active as previously thought in order to provide the NUV flux required for prebiotic chemistry, however, we note that flares will also provide more FUV flux than previously modelled. [ABSTRACT FROM AUTHOR]

We announce the discovery of pulsations in HD 23642, the only bright eclipsing system in the Pleiades, based on light curves from the Transiting Exoplanet Survey Satellite (TESS). We measure 46 pulsation frequencies and attribute them to δ Scuti pulsations in the secondary component. We find four ℓ = 1 doublets, three of which have frequency splittings consistent with the rotation rate of the star. The dipole mode amplitude ratios are consistent with a high stellar inclination angle and the stellar rotation period agrees with the orbital period. Together, these suggest that the spin axis of the secondary is aligned with the orbital axis. We also determine precise effective temperatures and a spectroscopic light ratio, and use the latter to determine the physical properties of the system alongside the TESS data and published radial velocities. We measure a distance to the system in agreement with the Gaia parallax, and an age of 170 ± 20 Myr based on a comparison to theoretical stellar evolutionary models. [ABSTRACT FROM AUTHOR]

This study presents observations of two possible substellar survivors of post-main sequence engulfment, currently orbiting white dwarf stars. Infrared and optical spectroscopy of GD 1400 reveals a 9.98 h orbital period, where the benchmark brown dwarf has M 2 = 68 ± 8  M Jup, T eff ≈ 2100 K, and a cooling age under 1 Gyr. A substellar mass in the lower range of allowed values is favoured by the gravitational redshift of the primary. Synthetic brown dwarf spectra are able to reproduce the observed CO bands, but lines below the bandhead are notably overpredicted. The known infrared excess towards PG 0010+281 is consistent with a substellar companion, yet no radial velocity or photometric variability is found despite extensive searches. Three independent stellar mass determinations all suggest enhanced mass-loss associated with binary evolution, where the youngest total age for an isolated star is 7.5 ± 2.5 Gyr. A possible solution to this conundrum is the cannibalization of one or more giant planets, which enhanced mass-loss post-main sequence, but were ultimately destroyed. PG 0010 + 281 is likely orbited by a debris disc that is comfortably exterior to the Roche limit, adding to the growing number of non-canonical discs orbiting white dwarfs. At present, only L-type (brown) dwarfs are known to survive direct engulfment during the post-main sequence, whereas T- and Y-type substellar companions persist at wide separations. These demographics indicate that roughly 50  M Jup is required to robustly avoid post-main sequence annihilation, suggesting all closely orbiting giant planets are consumed, which may contribute to mass-loss and magnetic-field generation in white dwarfs and their immediate progenitors. [ABSTRACT FROM AUTHOR]

The spectral analysis and kinematic studies of faint blue stars of O and B types above the mid-plane of the Milky Way can lead to a better understanding of their populations and origins. In this paper, we present an analysis of the chemical composition and kinematics of 17 B-type stars. Our sample includes 13 B-type stars with | Z | > 0.5 kpc and | b | ≥ 20°, three B-type stars with | Z | ≤ 0.5 kpc and | b | < 20°, and a hot post-asymptotic giant branch (post-AGB) star without infrared excess, in the Milky Way. Our analysis rests on medium-resolution spectra and non-local thermodynamic equilibrium tlusty model atmospheres. The ages and masses of the stars are obtained using solar-metallicity PARSEC isochrones, and we calculate their velocities and Galactic trajectories using galpy and data from the Gaia EDR3. The larger projected rotational velocities and results of abundance analysis of 16 B-type stars indicate that they are likely to be B-type stars of normal Population I. The three B-type stars with | Z | ≤ 0.5 kpc and | b | < 20° can be formed in the Galactic disc. The origins of the 13 B-type stars with | Z | > 0.5 kpc and | b | ≥ 20° are as follows. (i) Analysis of the C and Mg abundances, kinematics and orbits show that 10 of them originate in the Galactic disc at different Galactocentric distances. They may be scattered from the disc via the binary ejection mechanism or the dynamical ejection mechanism. (ii) The three stars presenting abnormal C or Mg abundances and counter-rotating V ϕ might be post-AGB stars accreted from a disrupted satellite of the Milky Way. [ABSTRACT FROM AUTHOR]

Equations governing the evolution of a star involve multiple coupling constants. Thus, the time it spends as a main-sequence star can be expected to depend on whether or not such constants vary over the time scale of stellar evolution. When the star belongs to a globular cluster, the star's age cannot exceed that of the globular cluster, and the latter cannot exceed the age of the Universe. This fact can be used to constrain or verify the variation of the coupling constants, i.e., the speed of light c , the gravitational constant G , the Planck constant h , and the Boltzmann constant k. We have estimated the age of the main-sequence star analytically from the time it takes to synthesize all its hydrogen into helium under fixed and varying coupling constants scenarios. When we permitted the interrelated variation of the four constants ( G ~ c 3 ~ h 3 ~ k 3 / 2 ) and differentiated between the cosmological energy and local energy conservation laws, we could show that the variation of the constants established in our earlier studies, i.e., G ˙ / G = 3 c ˙ / c = 3 h ˙ / h = 1.5 k ˙ / k = 3.90 ± 0.04 × 10 − 10   yr − 1 at the current cosmic time is consistent with the present work. Nevertheless, the challenge remains to come up with an experiment, astrometric or terrestrial, that can unequivocally prove or falsify the predicted variation. [ABSTRACT FROM AUTHOR]

The star formation and metal enrichment histories of galaxies – at any epoch – constitute one of the key properties of galaxies, and their measurement is a core aim of observational extragalactic astronomy. The lack of deep rest-frame optical coverage at high redshift has made robust constraints elusive, but this is now changing thanks to JWST. In preparation for the constraints provided by JWST , we explore the star formation and metal enrichment histories of galaxies at z  = 5–13 using the First Light And Reionization Epoch Simulations (flares) suite. Built on the EAGLE model, the unique strategy of flares allows us to simulate galaxies with a wide range of stellar masses (and luminosities) and environments. While we predict significant redshift evolution of average ages and specific star formation rates, our core result is mostly a flat relationship of age and specific star formation rate with stellar mass. We also find that galaxies in this epoch predominantly have strongly rising star formation histories, albeit with the normalization dropping with redshift and stellar mass. In terms of chemical enrichment, we predict a strong stellar mass–metallicity relation present at z  = 10 and beyond alongside significant α-enhancement. Finally, we find no large-scale environmental dependence of the relationship between age, specific star formation rate, or metallicity with stellar mass. [ABSTRACT FROM AUTHOR]

The detected exoplanet population displays a dearth of planets with sizes of about two Earth radii, the so-called radius gap. This is interpreted as an evolutionary effect driven by a variety of possible atmospheric mass-loss processes of exoplanets. For mass loss driven by an exoplanet's irradiation by stellar X-ray and extreme-UV photons, the time evolution of the stellar magnetic activity is important. It is known from observations of open stellar clusters that stars of the same age and mass do not all follow the same time evolution of activity-induced X-ray and extreme-UV luminosities. Here, we explore how a realistic spread of different stellar activity tracks influences the mass loss and radius evolution of a simulated population of small exoplanets and the observable properties of the radius gap. Our results show qualitatively that different saturation time-scales, i.e. the young age at which stellar high-energy emission starts to decline, and different activity decay tracks over moderate stellar ages can cause changes in the population density of planets in the gap, as well as in the observable width of the gap. We also find that while the first 100 million years of mass loss are highly important to shape the radius gap, significant evolution of the gap properties is expected to take place for at least the first 500–600 million years, i.e. the age of the Hyades cluster. Observations of exoplanet populations with defined ages will be able to shed more light on the radius gap evolution. [ABSTRACT FROM AUTHOR]

The ages of stars have historically relied on isochrone fitting of standardized grids of models. While these stellar models have provided key constraints on observational samples of massive stars, they inherit many systematic uncertainties, mainly in the internal mixing mechanisms applied throughout the grid, fundamentally undermining the isochrone method. In this work, we utilize the mass–lumiosity (M–L) plane of Higgins & Vink as a method of determining stellar age, with mixing-corrected models applying a calibrated core overshooting αov and rotation rate to fit the observational data. We provide multiple test-beds to showcase our new method, while also providing comparisons to the commonly used isochrone method, highlighting the dominant systematic errors. We reproduce the evolution of individual O stars, and analyse the wider sample of O and B supergiants from the VLT-FLAMES Tarantula Survey, providing dedicated models with estimates for αov, Ω/Ωcrit, and ultimately stellar ages. The M–L plane highlights a large discrepancy in the spectroscopic masses of the O supergiant sample. Furthermore the M–L plane also demonstrates that the evolutionary masses of the B supergiant sample are inappropriate. Finally, we utilize detached eclipsing binaries, VFTS 642 and VFTS 500, and present their ages resulting from their precise dynamical masses, offering an opportunity to constrain their interior mixing. For the near-TAMS system, VFTS 500, we find that both components require a large amount of core overshooting (αov ≃ 0.5), implying an extended main-sequence width. We hence infer that the vast majority of B supergiants are still burning hydrogen in their cores. [ABSTRACT FROM AUTHOR]

We present the first large-scale study that demonstrates how ages can be determined for large samples of stars through Galactic chemical evolution. Previous studies found that the elemental abundances of a star correlate directly with its age and metallicity. Using this knowledge, we derive ages for 214 577 stars in GALAH DR3 using only overall metallicities and chemical abundances. Stellar ages are estimated via the machine learning algorithm XGBoost for stars belonging to the Milky Way disc with metallicities in the range −1 < [Fe/H] < 0.5, using main-sequence turn-off stars as our training set. We find that stellar ages for the bulk of GALAH DR3 are precise to 1–2 Gyr using this method. With these ages, we replicate many recent results on the age-kinematic trends of the nearby disc, including the solar neighbourhood's age–velocity dispersion relationship and the larger global velocity dispersion relations of the disc found using Gaia and GALAH. These results show that chemical abundance variations at a given birth radius are small, and that strong chemical tagging of stars directly to birth clusters may prove difficult with our current elemental abundance precision. Our results highlight the need to measure abundances for as many nucleosynthetic production sites as possible in order to estimate reliable ages from chemistry. Our methods open a new door into studies of the kinematic structure and evolution of the disc, as ages may potentially be estimated to a precision of 1–2 Gyr for a large fraction of stars in existing spectroscopic surveys. [ABSTRACT FROM AUTHOR]

Nuclear reaction rates are a fundamental yet uncertain ingredient in stellar evolution models. The astrophysical S -factor pertaining to the initial reaction in the proton–proton (pp) chain is uncertain at the 1 per cent level, which contributes a systematic but generally unpropagated error of similar order in the theoretical ages of stars. In this work, we study the prospect of improving the measurement of this and other reaction rates in the pp chain and CNO cycle using helioseismology and solar neutrinos. We show that when other aspects of the solar model are improved, then it shall be possible using current solar data to improve the precision of this measurement by nearly an order of magnitude and hence the corresponding uncertainty on the ages of low-mass stars by a similar amount. [ABSTRACT FROM AUTHOR]

Old stellar populations in our Galaxy provide fossil records of the metal enrichment in the first few billion years of the cosmic history. Growing elemental abundance data of individual stars combined with stellar ages and kinematics allow us to make constraints on characteristic properties of the metal-enrichment sources in the early Universe, such as the first stars. In order to interpret observed chemical abundances in the oldest stellar populations in terms of metal-enrichment sources, stellar and supernova yield models are crucial. In this article, we review how we can interpret observed chemical abundances in old stars in terms of the nature of metal enrichment sources. We discuss the limitations and the prospects of empirically constraining supernova yield models based on a large sample of extremely metal-poor stars. At the same time, we emphasize the importance of hunting old stars beyond the Solar neighborhood, which can be achieved with the next-generation multi-object spectrographs at large telescopes. [ABSTRACT FROM AUTHOR]

Westerlund 1 (Wd 1) is one of the most relevant star clusters in the Milky Way to study massive star formation, although it is still poorly known. Here, we used photometric and spectroscopic data to model the eclipsing binary W36, showing that its spectral type is O6.5 III  +  O9.5 IV, hotter and more luminous than thought before. Its distance d W36 = 4.03 ± 0.25 kpc agrees, within the errors, with three recent Gaia-EDR3-based distances reported in Paper I, Beasor & Davies, and by Negueruela's group. However, they follow different approaches to fix the zero-points for red sources such as those in Wd 1, and to select the best approach, we used an accurate modelling of W36. The weighted mean distance of our parallax (Paper I) and binary distances results in d wd1 = 4.05 ± 0.20 kpc, with an unprecedented accuracy of 5 per cent. We adopted isochrones based on the Geneva code with supersolar abundances to infer the age of W36B as 6.4 ± 0.7 Myr. This object seems to be part of the prolific star formation burst represented by OB giants and supergiants that occurred at 7.1 ± 0.5 Myr ago, which coincides with the recently published PMS isochrone with age 7.2 Myr. Other BA-type luminous evolved stars and yellow hypergiants spread in the age range of 8–11 Myr. The four red supergiants discussed in paper I represent the oldest population of the cluster with an age of 10.7 ± 1 Myr. The multiple episodes of star formation in Wd 1 are reminiscent of that reported for the R136/30 Dor LMC cluster. [ABSTRACT FROM AUTHOR]

We present a catalogue of 3354 candidate young stars within 500 pc that appear to have been ejected from their parent associations with relative speeds of >5 km s−1. These candidates have been homogeneously selected through performing a 2D spherical traceback of previously identified pre-main-sequence candidates to various star-forming regions, ensuring that the traceback age as well as the estimated age of a star is consistent with the age of the population, and excluding contaminants from the nearby moving groups that follow the dominant velocity currents in the field. Among the identified candidates we identify a number of pairs that appear to have interacted in the process of the ejection; these pairs have similar traceback time, and their trajectory appears to be diametrically opposite from each other, or they have formed a wide binary in the process. As the selection of these candidates is performed solely in 2D, spectral follow-up is necessary for their eventual confirmation. Unfortunately, recently released Gaia DR3 radial velocities appear to be unsuitable for characterizing the kinematics of low-mass stars with ages <100 Myr, as the accretion, activity, and a variety of other spectral features that make them distinct from the more evolved stars do not appear to have been accurately accounted for in the data, resulting in significant artificially inflated scatter in their RV distribution. [ABSTRACT FROM AUTHOR]

K2 was a community-driven NASA mission where all targets were proposed through guest observer programmes. Here we provide an overview of one of the largest of these endeavours, the K2 Galactic Archaeology Programme (K2GAP), with about 25 per cent of the observed targets being allocated to this programme. K2GAP provides asteroseismic parameters for about 23 000 giant stars across the Galaxy, which together with spectroscopic stellar parameters can give age and masses of stars. We discuss in detail the target selection procedure and provide a python program that implements the selection function (github.com/sanjibs/k2gap). Broadly speaking, the targets were selected on 2MASS colour J − Ks > 0.5, with finely tuned adjustments for each campaign. We discuss the detection completeness of the asteroseismic parameters νmax and Δν. About 14 per cent of giants were found to miss νmax detections and it was difficult to detect Δν for RC stars. Making use of the selection function, we compare the observed distribution of asteroseismic masses to theoretical predictions. The median asteroseismic mass is higher by about 4 per cent compared to predictions. We provide a selection-function-matched mock catalogue of stars based on a synthetic model of the Galaxy for the community to use in subsequent analyses of the K2GAP data set (physics.usyd.edu.au/k2gap/download/). [ABSTRACT FROM AUTHOR]

Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots and show strong chromospheric emission lines because of their strong surface magnetic field. We discuss the dynamo activities of ZAMS stars with respect to their periodic light variation caused by a starspot and with respect to the strength of the chromospheric emission lines. The light curves of 33 ZAMS stars in IC 2391 and IC 2602 were obtained from TESS (Transiting Exoplanet Survey Satellite) photometric data. The light curves can be grouped into the following four categories: single frequency, possible shape changer, beater, and complex variability. The amplitudes of the light curves are 0.001–0.145 mag, similar to those of ZAMS stars in Pleiades. The starspot coverages are |$0.1\%$| – |$21\%$|⁠. We found that the light variations and Ca  ii emission line strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as large as those of the most active superflare stars and two orders larger than those of the Sun, and are located on the extensions of the superflare stars. These results suggest that superflare stars link the properties of the Sun to those of the ZAMS stars of ages between 30 and 120 Myr. ZAMS stars with a single frequency or possible shape change in the light curve tend to have both large light variation, indicating large spot coverage, and saturated Ca  ii emission line strength. ZAMS stars with beat or complex variability have small spot coverage and a faint Ca  ii emission line. We also detected 21 flares in the TESS light curves of 12 ZAMS stars in IC 2391 and IC 2602, where most of these stars have saturated chromospheric Ca  ii emission lines. The energies of the flares are estimated to be ∼1033–1035 erg, which is comparable with the energy of a superflare. [ABSTRACT FROM AUTHOR]

BP Psc is an active late-type (sp:G9) star with unclear evolutionary status lying at high-galactic latitude b  = −57°. It is also the source of the well collimated bipolar jet. We present results of the proper motion and radial velocity study of BP Psc outflow based on the archival H α imaging with the GMOS camera at 8.1-m Gemini-North telescope as well as recent imaging and long-slit spectroscopy with the SCORPIO multimode focal reducer at 6-m BTA telescope of SAO RAS. The 3D kinematics of the jet revealed the full spatial velocity up to ∼140 km s−1 and allows us to estimate the distance to BP Psc system as D  = 135 ± 40 pc. This distance leads to an estimation of the central source luminosity L * ≈ 1.2  L ⊙, indicating that it is the ≈1.3M⊙ T Tauri star with an age t ≲ 7 Myrs. We measured the electron density of order Ne ∼ 102 cm−3 and mean ionization fraction f ≈ 0.04 within the jet knots and estimated upper limit of the mass-loss rate in NE lobe as |$\dot{M}_{\rm out}\approx 1.2\cdot 10^{-8}{\rm M}_{\odot }\,{\rm yr}^{-1}$|⁠. The physical characteristics of the outflow are typical for the low-excitation YSO jets and consistent with the magnetocentrifugal mechanism of its launching and collimation. Prominent wiggling pattern revealed in H α images allowed us to suppose the existence of a secondary substellar companion in a non-coplanar orbit and estimate its most plausible mass as Mp ≈ 30 M Jup. We conclude that BP Psc is one of the closest to the Sun young jet-driving systems and its origin is possibly related to the episode of star formation triggered by expanding supershells in second Galactic quadrant. [ABSTRACT FROM AUTHOR]

The analysis results of spectral energy distribution (SED) curves in the range 0.36–100 μm for 45 young WTTS-type stars and 9 comparison CTTS-type stars are presented. Two types of SED curves are observed according to the classification of the spectra of young stars. Stars with type III spectra show excess emission only in the far IR range, at λ ≥ 12 µm. Stars with type II spectra, in addition to emission in the far IR range, also show excess emission in the near IR range, at 2 μm < λ < 12 μm. A new subtype of IIId stars from the group of stars with type III spectra has been identified. These stars have a weak IR excess in the spectrum in the range λ ≥ 60 μm, signs of debris disks, and often show chromospheric activity. It is shown that 15 out of 45 WTTSs have UV excesses, and the same number of stars have near-IR excesses. Two of these stars are type II in their spectra and show signs of disk accretion. The physical parameters and parameters of excess radiation are determined, and the masses and ages of program stars are estimated. [ABSTRACT FROM AUTHOR]

We studied the open star cluster Berkeley 76 using Gaia DR3 and 2MASS data sets. We obtained 314 most probable cluster members with a membership probability greater than 80%. We have found the cluster center as α = 106.67 ± 0.07 deg and δ = − 11. 75 ± 0.07 deg. The mean PMs of cluster members in right ascension and declination are − 0. 60 ± 0.22 and 1.35 ± 0.19 mas yr−1, respectively. The radial density profile estimates the cluster's radius to be 4.5 arcmin. The cluster's heliocentric distance is estimated as 4.27 ± 0.91 kpc, and its age is determined as 1. 60 ± 0. 35 Gyr. Using the most probable cluster members, we determined the mass function slope for Berkeley 76 as 1.57 ± 0.19. This value is close to Salpeter's slope within the uncertainty. Berkeley 76 is a dynamically relaxed cluster with a relaxation time of 23.4 Myr. • We have estimated the membership probability of stars and used those stars for further analysis. • The main astrophysical parameters, like age, distance, reddening, etc., are estimated precisely. • Performed the mass function study and we found the cluster is dynamically relaxed. [ABSTRACT FROM AUTHOR]

We identify 20 F-type Herbig stars and provide a list of 22 pre-main-sequence candidates from LAMOST DR8. The effective temperature, distance, extinction, stellar luminosity, mass, age, and radius are derived for each Herbig star based on optical spectra, photometry, Gaia EDR3 parallaxes, and pre-main-sequence evolutionary tracks. According to spectral energy distribution, 19 F-type Herbig stars belong to Class II YSOs, and one belongs to the flat-spectrum class. Four have Spitzer IRS spectra, of which three show extremely weak polycyclic aromatic hydrocarbon emissions, and three with both amorphous and crystalline silicate emissions share similar parameters and are at the same evolutionary stage. We detect a solar-nearby outbursting EXor Herbig star J034344.48+314309.3, a possible precursor of a Herbig Ae star. Intense emission lines of H i, He i, O i, Na i, and Ca ii originating from the rapid accretion during the outbursts are detected in its optical spectra, and silicate emission features are detected in its infrared spectrum. We also conduct a statistical analysis of the disk properties of all known Herbig stars using the defined infrared spectral indices ( α J âˆ' K S and α K S âˆ' W 3 ). The proportion of Herbig stars with moderate infrared excesses decreases as effective temperature increases. The majority of the precursors (F, G, or K type) have moderate infrared excesses. Hotter Herbig stars tend to have a larger proportion with large infrared excesses. The trends may be due to the fact that hotter stars have larger areas of re-emitting dust, although there is some scatter due to the particularities of each disk. [ABSTRACT FROM AUTHOR]

Current spectroscopic surveys are producing large catalogs of chemical abundances for stars of all types. The yttrium-to-magnesium ratio, [Y/Mg], has emerged as a candidate age indicator for solar twins in the local stellar neighborhood. However, it is unclear whether it is a viable age diagnostic for more diverse stellar types, so we investigate [Y/Mg] as an age indicator for the FGK-type planet host stars observed by Kepler. We find that the [Y/Mg] “Clock” is most precise for solar twins, with a [Y/Mg]/age slope of m = âˆ'0.0370 ±0.0071 dex Gyrâˆ'1 and σ Age = 2.6 Gyr. We attribute the lower precision compared to literature results to nonsolar twins contaminating our solar twin sample and recommend a 1.5 Gyr systematic uncertainty for stellar ages derived with any [Y/Mg]â€"Age relation. We also analyzed the [Y/Mg] Clock as a function of T eff, log g , and metallicity individually and find no strong trends, but we compute statistically significant [Y/Mg]â€"Age relations for subsamples defined by ranges in T eff, log g , and metallicity. Finally, we compare [Y/Mg] and rotation ages and find statistically similar trends as for isochrone ages, although we find that rotation ages perform better for GK dwarfs while isochrones perform better for FG subgiants. We conclude that the [Y/Mg] Clock is most precise for solar twins and analogs but is also a useful age diagnostic for FGK stars. [ABSTRACT FROM AUTHOR]

We present a suite of galaxy formation simulations that directly model star cluster formation and disruption. Starting from a model previously developed by our group, here we introduce several improvements to the prescriptions for cluster formation and feedback, then test these updates using a large suite of cosmological simulations of Milky Way mass galaxies. We perform a differential analysis with the goal of understanding how each of the updates affects star cluster populations. Two key parameters are the momentum boost of supernova feedback f boost and star formation efficiency per free-fall time ϵ ff. We find that f boost has a strong influence on the galactic star formation rate, with higher values leading to less star formation. The efficiency ϵ ff does not have a significant impact on the global star formation rate, but dramatically changes cluster properties, with increasing ϵ ff leading to a higher maximum cluster mass, shorter age spread of stars within clusters, and higher integrated star formation efficiencies. We also explore the redshift evolution of the observable cluster mass function, finding that most massive clusters have formed at high redshift z > 4. Extrapolation of cluster disruption to z = 0 produces good agreement with both the Galactic globular cluster mass function and age–metallicity relation. Our results emphasize the importance of using small-scale properties of galaxies to calibrate subgrid models of star cluster formation and feedback. [ABSTRACT FROM AUTHOR]

We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of the young M5 primary star UCAC4 374âˆ'046899 and the very red L6 dwarf WISEA J081322.19âˆ'152203.2. These two objects have a projected separation of 61 ′ ′ (1891 au) and are physically associated given their common proper motions and estimated distances. The primary star, COCONUTS-3A, has a mass of 0.123 ± 0.006 M ⊙, and we estimate its age as 100 Myr to 1 Gyr based on its stellar activity (via H α and X-ray emission), kinematics, and spectrophotometric properties. We derive its bulk metallicity as 0.21 ± 0.07 dex using empirical calibrations established by older and higher-gravity M dwarfs and find that this [Fe/H] could be slightly underestimated according to PHOENIX models given COCONUTS-3A’s younger age. The companion, COCONUTS-3B, has a near-infrared spectral type of L6 ± 1 int-g, and we infer physical properties of T eff = 1362 âˆ' 73 + 48 K, log (g) = 4.96 âˆ' 0.34 + 0.15 dex, R = 1.03 âˆ' 0.06 + 0.12 R Jup, and M = 39 âˆ' 18 + 11 M Jup using its bolometric luminosity, its host star’s age, and hot-start evolution models. We construct cloudy atmospheric model spectra at the evolution-based physical parameters and compare them to COCONUTS-3B’s spectrophotometry. We find that this companion possesses ample condensate clouds in its photosphere (f sed = 1) with the dataâ€"model discrepancies likely due to the models using an older version of the opacity database. Compared to field-age L6 dwarfs, COCONUTS-3B has fainter absolute magnitudes and a 120 K cooler T eff. Also, the J âˆ' K color of this companion is among the reddest for ultracool benchmarks with ages older than a few hundred megayears. COCONUTS-3 likely formed in the same fashion as stellar binaries given the companion-to-host mass ratio of 0.3 and represents a valuable benchmark to quantify the systematics of substellar model atmospheres. [ABSTRACT FROM AUTHOR]

The origin of young star clusters represents a major challenge for modern stellar astrophysics. While stellar rotation partially explains the colour spread observed along main-sequence turn-offs, i.e. where stars leave the main-sequence after the exhaustion of hydrogen in their core, and the multiple main sequences in the colour-magnitude diagrams of stellar systems younger than approximately 2 Gyr, it appears that an age difference may still be required to fulfill the observational constraints. Here we introduce an alternative approach that exploits the main-sequence turn-on, i.e. the point alongside the colour-magnitude diagram where pre-main-sequence stars join the main-sequence, to disentangle between the effects of stellar rotation and age to assess the presence, or lack thereof, of prolonged star formation in the approximately 40-Myr-old cluster NGC1818. Our results provide evidence for a fast star formation, confined within 8 Myr, thus excluding age differences as responsible for the extended main-sequence turn-offs, and leading the way to alternative observational perspectives in the exploration of stellar populations in young clusters. The nature of young star clusters in the Magellanic Clouds is debated. Here, the authors show an alternative approach that exploits data to exclude the presence of age differences greater than a few million years among cluster stars in a very young cluster. [ABSTRACT FROM AUTHOR]

The colorâ€"magnitude diagrams (CMDs) of intermediate-age star clusters (≲2 Gyr) are much more complex than those predicted by coeval, nonrotating stellar evolution models. Their observed extended main-sequence turnoffs (eMSTOs) could result from variations in stellar age, stellar rotation, or both. The physical interpretation of eMSTOs is largely based on the complex mapping between stellar modelsâ€"themselves functions of mass, rotation, orientation, and binarityâ€"and the CMD. In this paper, we compute continuous probability densities in three-dimensional color, magnitude, and v e sin i space for individual stars in a cluster’s eMSTO, based on a rotating stellar evolution model. These densities enable the rigorous inference of cluster properties from a stellar model, or, alternatively, constraints on the stellar model from the cluster’s CMD. We use the MIST stellar evolution models to jointly infer the age dispersion, the rotational distribution, and the binary fraction of the Large Magellanic Cloud cluster NGC 1846. We derive an age dispersion of ∼70â€"80 Myr, approximately half the earlier estimates due to nonrotating models. This finding agrees with the conjecture that rotational variation is largely responsible for eMSTOs. However, MIST models do not provide a satisfactory fit to all stars in the cluster and achieve their best agreement at an unrealistically high binary fraction. The lack of agreement near the main-sequence turnoff suggests specific physical changes to the stellar evolution models, including a lower mass for the Kraft break and potentially enhanced main-sequence lifespans for rapidly rotating stars. [ABSTRACT FROM AUTHOR]

Dust emission at 8 μ m has been extensively calibrated as an indicator of current star formation rate for galaxies and ∼kpc-size regions within galaxies. Yet, the exact link between the 8 μ m emission and the young stellar populations in galaxies is still under question, as dust grains can be stochastically heated also by older field stars. In order to investigate this link, we have combined midinfrared images from the Spitzer Space Telescope with a published star cluster candidates catalog for the Local Group galaxy M33. M33 is sufficiently close that the Spitzer's 8 μ m images resolve individual regions of star formation. Star clusters represent almost-single-age stellar populations, which are significantly easier to model than more complex mixtures of stars. We find a decrease in the 8 μ m luminosity per unit stellar mass as a function of age of the star clusters, with a large scatter that is consistent with varying fractions of stellar light absorbed by dust. The decrease and scatter both confirm findings based on more distant galaxies and are well described by simple models for the dust emission of a young stellar population. We conclude that the dust emission at 8 μ m depends sensitively on the age of the stellar population, out to at least the oldest age analyzed here: ∼400 Myr. This dependence complicates the use of the 8 μ m emission as a star formation rate indicator, at least for small galactic regions and individual star-forming regions. By leveraging the Spitzer legacy, this investigation paves the way for future explorations with the James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

The Serpens Molecular Cloud is one of the most active star-forming regions within 500 pc, with over 1000 young stellar objects (YSOs) at different evolutionary stages. The ages of the member stars inform us about the star formation history of the cloud. In this paper, we develop a spectral energy distribution (SED) fitting method for nearby evolved (diskless) young stars from members of the Pleiades to estimate their ages, with a temperature scale adopted from APOGEE spectra. When compared with literature temperatures of selected YSOs in Orion, the SED fits to cool (<5000 K) stars have temperatures that differ by an average of ≲50 K and have a scatter of ∼210 K for both disk-hosting and diskless stars. We then apply this method to YSOs in the Serpens Molecular Cloud to estimate ages of optical members previously identified from Gaia DR2 astrometry data. The optical members in Serpens are concentrated in different subgroups with ages from ∼4 to ∼22 Myr; the youngest clusters, W40 and Serpens South, are dusty regions that lack enough optical members to be included in this analysis. These ages establish that the Serpens Molecular Cloud has been forming stars for much longer than has been inferred from infrared surveys. [ABSTRACT FROM AUTHOR]

In this work, we aim at constraining the age of the young open cluster Melotte 20, known as α  Per, using seismic indices. The method consists of the following steps: (1) Extract the frequency content of a sample of stars in the field of an open cluster. (2) Search for possible regularities in the frequency spectra of δ Sct stars candidates, using different techniques, such as the Fourier transform, the autocorrelation function, the histogram of frequency differences and the échelle diagram. (3) Constrain the age of the selected stars by both the physical parameters and seismic indices by comparing them with a grid of asteroseismic models representative of δ Sct stars. (4) Find possible common ages between these stars to determine the age of the cluster. We performed the pulsation analysis with MultiModes , a rapid, accurate and powerful open-source code, which is presented in this paper. The result is that the age of α  Per could be between 96 and 100 Myr. This is an improvement over different techniques in the past. We therefore show that space astroseismology is capable of taking important steps in the dating of young open clusters. [ABSTRACT FROM AUTHOR]

Convective-core overshoot mixing is a significant uncertainty in stellar evolution. Because numerical simulations and turbulent convection models predict exponentially decreasing radial rms turbulent velocity, a popular treatment for overshoot mixing is to apply a diffusion process with an exponentially decreasing diffusion coefficient. It is important to investigate the parameters of the diffusion coefficient because they determine the efficiency of the mixing in the overshoot region. In this paper, we have investigated the effects of the core overshoot mixing on the properties of the core in solar models. We have constrained the parameters of the overshoot model by using helioseismic inferences and the observation of the solar 8B neutrino flux. For solar-mass stars, the core overshoot mixing helps to prolong the lifetime of the convective core developed at the zero-age main sequence. If the strength of the mixing is sufficiently high, then the convective core in a solar model could survive until the present solar age, leading to large deviations of the sound speed and density profiles compared with the helioseismic inferences. The 8B neutrino flux also favours a radiative solar core. These observations provide a constraint on the parameters of the exponential diffusion model of the convective overshoot mixing. A limited asteroseismic investigation of 13 Kepler low-mass stars with 1.0 < M /M⊙ < 1.5 shows a mass-dependent range of the overshoot parameter. The overshoot mixing processes for different elements are analysed in detail. It is found that the exponential diffusion overshoot model leads to different effective overshoot mixing lengths for elements with different nuclear equilibrium time-scales. [ABSTRACT FROM AUTHOR]

This paper investigates the extended main-sequence turnoffs (eMSTOs) and stellar populations of three clusters with different ages using Gaia Data Release 2 (DR2) data in detail. The colour–magnitude diagrams (CMDs) of three Galactic clusters show broad main sequences, eMSTOs, blue stragglers, and red giant clumps. We explore the effects of age spread, binaries, and rotating stars on the morphology of the entire CMD, paying particular attention to the effects of binary stars and rotating stars on eMSTOs. Some synthetic CMDs are built on the basis of the advanced stellar population synthesis (ASPS) model to study the observed CMDs and search for the best-fitting stellar population models. The age of the youngest stars, age spread, metallicity, colour excess, distance modulus, binary fraction, and rotating star fraction of clusters are determined. The results show that both binaries and rotating stars are responsible for the eMSTO of intermediate-age cluster LP 585, but binary stars affect more than rotating stars on the eMSTO morphologies of the youngest (M11) and oldest (NGC 6819) clusters in our samples. It suggests that M11 (∼0.3 Gyr) is possibly a composite stellar population of rotating and binary stars, while clusters LP 585 (∼1.1 Gyr) and NGC 6819 (∼2.0 Gyr) are simple stellar populations of rotating and binary stars. [ABSTRACT FROM AUTHOR]

We present a comparison of the nitrogen-to-oxygen ratio (N/O) in 37 high-redshift galaxies at z ∼ 2 taken from the KMOS Lensed Emission Lines and VElocity Review (KLEVER) Survey with a comparison sample of local galaxies, taken from the Sloan Digital Sky Survey (SDSS). The KLEVER sample shows only a mild enrichment in N/O of +0.1 dex when compared to local galaxies at a given gas-phase metallicity (O/H), but shows a depletion in N/O of −0.35 dex when compared at a fixed stellar mass (M *). We find a strong anticorrelation in local galaxies between N/O and SFR in the M *–N/O plane, similar to the anticorrelation between O/H and SFR found in the mass–metallicity relation (MZR). We use this anticorrelation to construct a fundamental nitrogen relation (FNR), analogous to the fundamental metallicity relation (FMR). We find that KLEVER galaxies are consistent with both the FMR and the FNR. This suggests that the depletion of N/O in high- z galaxies when considered at a fixed M * is driven by the redshift evolution of the mass–metallicity relation in combination with a near redshift-invariant N/O–O/H relation. Furthermore, the existence of an fundamental nitrogen relation suggests that the mechanisms governing the fundamental metallicity relation must be probed by not only O/H, but also N/O, suggesting pure-pristine gas inflows are not the primary driver of the FMR, and other properties such as variations in galaxy age and star formation efficiency must be important. [ABSTRACT FROM AUTHOR]

The study of young planetary systems is of great importance for understanding their formation and evolution. Therefore, today, a great effort is placed on the observation and modeling of such systems. Of course, planets do not evolve in isolation, but their behavior depends on the host star and the environment in which they are embedded. In particular, we know that at young ages, solar‐type stars are much more active than sun‐age stars with all the phenomena that this involves, including strong emission at high energies, from ultraviolet (UV) to X‐rays, stellar winds, frequent flares, and coronal mass ejection emission. Such an energetic environment has an important influence on planetary atmospheres. It is now widely accepted that high‐energy radiation can drive planetary evaporation, especially of hot low‐density planets, and may also have a strong impact on the thermal structure and chemical compositions of the atmospheres. X‐rays and extreme ultraviolet (EUV) photons have different roles, since the former produce cascades of energetic secondary electrons, and penetrate deep in the atmospheres, reaching in some cases the planetary surface. Their effects are strongly dependent on metallicity since they are absorbed mainly from high z elements. On the contrary, the effects of EUV are concentrated on the upper layers of the atmospheres and are fairly independent of the presence of metals since they are efficiently absorbed by hydrogen and helium. In this paper, we will review some of the recent outcomes of star‐planet interactions studies during the young stellar ages. [ABSTRACT FROM AUTHOR]

A relationship between chromospheric activity and age is calibrated for FGK dwarf stars using GALEX F U V magnitudes and Gaia (G B P − G) colors. Such a calibration between GALEX F U V magnitudes and stellar age has utility in population studies of dwarfs for further understanding of the chemical evolution of the Milky Way. As an illustration of one such application we have investigated a population of Sun-like, solar neighborhood stars for their metallicities and velocity dispersions; a cross-matched sample of FGK type dwarf stars from Casagrande et al. (2011) with the Gaia and GALEX catalogs. Using calibrated relationships between F U V magnitudes and age, we determined a chromospheric activity indicator, Q , and stellar age, τ , for each dwarf. We constructed age-velocity (AVR) and age-metallicity (AMR) relations with empirically-determined F U V ages. Power law fits to AVR plots are consistent with heating mechanism models within the literature. We further demonstrate that perigalactic distance and eccentricity versus F U V -age plots are consistent with an "inside out" formation history model. [ABSTRACT FROM AUTHOR]

Grid-based modelling is widely used for estimating stellar parameters. However, stellar model grid is sparse because of the computational cost. This paper demonstrates an application of a machine-learning algorithm using the Gaussian Process (GP) Regression that turns a sparse model grid on to a continuous function. We train GP models to map five fundamental inputs (mass, equivalent evolutionary phase, initial metallicity, initial helium fraction, and the mixing-length parameter) to observable outputs (effective temperature, surface gravity, radius, surface metallicity, and stellar age). We test the GP predictions for the five outputs using off-grid stellar models and find no obvious systematic offsets, indicating good accuracy in predictions. As a further validation, we apply these GP models to characterize 1000 fake stars. Inferred masses and ages determined with GP models well recover true values within one standard deviation. An important consequence of using GP-based interpolation is that stellar ages are more precise than those estimated with the original sparse grid because of the full sampling of fundamental inputs. [ABSTRACT FROM AUTHOR]