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601. Age and metallicity relation in MC clusters
ID:
ivo://CDS.VizieR/J/A+A/554/A16
Title:
Age and metallicity relation in MC clusters
Short Name:
J/A+A/554/A16
Date:
21 Oct 2021
Publisher:
CDS
Description:
We study small open star clusters, using Stroemgren photometry to investigate a possible dependence between age and metallicity in the Magellanic Clouds (MCs). Our goals are to trace evidence of an age metallicity relation (AMR) and correlate it with the mutual interactions of the two MCs and to correlate the AMR with the spatial distribution of the clusters. In the Large Magellanic Cloud (LMC), the majority of the selected clusters are young (up to 1Gyr), and we search for an AMR at this epoch, which has not been much studied. We report results for 15 LMC and 8 Small Magellanic Cloud (SMC) clusters, scattered all over the area of these galaxies, to cover a wide spatial distribution and metallicity range. The selected LMC clusters were observed with the 1.54m Danish Telescope in Chile, using the Danish Faint Object Spectrograph and Camera (DFOSC) with a single 2kx2k CCD. The SMC clusters were observed with the ESO 3.6m Telescope, also in Chile, using the ESO Faint Object Spectrograph and Camera (EFOSC). The obtained frames were analysed with the conventional DAOPHOT and IRAF software. We used Stroemgren filters in order to achieve reliable metallicities from photometry. Isochrone fitting was used to determine the ages and metallicities. The AMR for the LMC displays a metallicity gradient, with higher metallicities for the younger ages. The AMR for LMC-SMC star clusters shows a possible jump in metallicity and a considerable increase at about 6x10^8^yr. It is possible that this is connected to the latest LMC-SMC interaction. The AMR for the LMC also displays a metallicity gradient with distance from the centre. The metallicities in SMC are lower, as expected for a metal-poor host galaxy.
602. Age-chemical-clocks-metallicity relations
ID:
ivo://CDS.VizieR/J/A+A/639/A127
Title:
Age-chemical-clocks-metallicity relations
Short Name:
J/A+A/639/A127
Date:
21 Oct 2021
Publisher:
CDS
Description:
In the era of large spectroscopic surveys, massive databases of high-quality spectra coupled with the products of the Gaia satellite provide tools to outline a new picture of our Galaxy. In this framework, an important piece of information is provided by our ability to infer stellar ages, and consequently to sketch a Galactic timeline. We aim to provide empirical relations between stellar ages and abundance ratios for a sample of stars with very similar stellar parameters to those of the Sun, namely the so-called solar-like stars. We investigate the dependence on metallicity, and we apply our relations to independent samples, that is, the Gaia-ESO samples of open clusters and of field stars. We analyse high-resolution and high-signal-to-noise-ratio HARPS spectra of a sample of solar-like stars to obtain precise determinations of their atmospheric parameters and abundances for 25 elements and/or ions belonging to the main nucleosynthesis channels through differential spectral analysis, and of their ages through isochrone fitting. We investigate the relations between stellar ages and several abundance ratios. For the abundance ratios with a steeper dependence on age, we perform multivariate linear regressions, in which we include the dependence on metallicity, [Fe/H]. We apply our best relations to a sample of open clusters located from the inner to the outer regions of the Galactic disc. Using our relations, we are able to recover the literature ages only for clusters located at R_GC_>7kpc. The values that we obtain for the ages of the inner-disc clusters are much greater than the literature ones. In these clusters, the content of neutron capture elements, such as Y and Zr, is indeed lower than expected from chemical evolution models, and consequently their [Y/Mg] and [Y/Al] are lower than in clusters of the same age located in the solar neighbourhood. With our chemical evolution model and a set of empirical yields, we suggest that a strong dependence on the star formation history and metallicity-dependent stellar yields of s-process elements can substantially modify the slope of the [s/{alpha}]-[Fe/H]-age relation in different regions of the Galaxy. Our results point towards a non-universal relation [s/{alpha}]-[Fe/H]-age, indicating the existence of relations with different slopes and intercepts at different Galactocentric distances or for different star formation histories. Therefore, relations between ages and abundance ratios obtained from samples of stars located in a limited region of the Galaxy cannot be translated into general relations valid for the whole disc. A better understanding of the s-process at high metallicity is necessary to fully understand the origin of these variations.
603. Age dissection of the Milky Way discs
ID:
ivo://CDS.VizieR/J/A+A/645/A85
Title:
Age dissection of the Milky Way discs
Short Name:
J/A+A/645/A85
Date:
21 Oct 2021
Publisher:
CDS
Description:
Ensemble studies of red-giant stars with exquisite asteroseismic (Kepler), spectroscopic (APOGEE), and astrometric (Gaia) constraints offer a novel opportunity to recast and address long-standing questions concerning the evolution of stars and of the Galaxy. Here, we infer masses and ages for nearly 5400 giants with available Kepler light curves and APOGEE spectra using the code PARAM, and discuss some of the systematics that may affect the accuracy of the inferred stellar properties. We then present patterns in mass, evolutionary state, age, chemical abundance, and orbital parameters that we deem robust against the systematic uncertainties explored. First, we look at age-chemical-abundances ([Fe/H] and [alpha/Fe]) relations. We find a dearth of young, metal-rich ([Fe/H]>0.2) stars, and the existence of a significant population of old (8-9Gyr), low-[alpha/Fe], super-solar metallicity stars, reminiscent of the age and metallicity of the well-studied open cluster NGC 6791. The age-chemo-kinematic properties of these stars indicate that efficient radial migration happens in the thin disc. We find that ages and masses of the nearly 400 alpha-element-rich red-giant-branch (RGB) stars in our sample are compatible with those of an old (~11Gyr), nearly coeval, chemical-thick disc population. Using a statistical model, we show that the width of the observed age distribution is dominated by the random uncertainties on age, and that the spread of the inferred intrinsic age distribution is such that 95% of the population was born within ~1.5Gyr. Moreover, we find a difference in the vertical velocity dispersion between low- and high-[alpha/Fe] populations. This discontinuity, together with the chemical one in the [alpha/Fe] versus [Fe/H] diagram, and with the inferred age distributions, not only confirms the different chemo-dynamical histories of the chemical-thick and thin discs, but it is also suggestive of a halt in the star formation (quenching) after the formation of the chemical-thick disc. We then exploit the almost coeval alpha-rich population to gain insight into processes that may have altered the mass of a star along its evolution, which are key to improving the mapping of the current, observed, stellar mass to the initial mass and thus to the age. Comparing the mass distribution of stars on the lower RGB (R<11R_{sun}_) with those in the red clump (RC), we find evidence for a mean integrated RGB mass loss DM=0.10+/-0.02M_{sun}_. Finally, we find that the occurrence of massive (M>~1.1M_{sun}_) alpha-rich stars is of the order of 5% on the RGB, and significantly higher in the RC, supporting the scenario in which most of these stars had undergone an interaction with a companion.
604. Age estimates for massive SFR stellar populations
ID:
ivo://CDS.VizieR/J/ApJ/787/108
Title:
Age estimates for massive SFR stellar populations
Short Name:
J/ApJ/787/108
Date:
21 Oct 2021
Publisher:
CDS
Description:
A major impediment to understanding star formation in massive star-forming regions (MSFRs) is the absence of a reliable stellar chronometer to unravel their complex star formation histories. We present a new estimation of stellar ages using a new method that employs near-infrared (NIR) and X-ray photometry, Age_JX_. Stellar masses are derived from X-ray luminosities using the L_X_-M relation from the Taurus cloud. J-band luminosities are compared to mass-dependent pre-main-sequence (PMS) evolutionary models to estimate ages. Age_JX_ is sensitive to a wide range of evolutionary stages, from disk-bearing stars embedded in a cloud to widely dispersed older PMS stars. The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) project characterizes 20 OB-dominated MSFRs using X-ray, mid-infrared, and NIR catalogs. The Age_JX_ method has been applied to 5525 out of 31784 MYStIX Probable Complex Members. We provide a homogeneous set of median ages for over 100 subclusters in 15 MSFRs; median subcluster ages range between 0.5 Myr and 5 Myr. The important science result is the discovery of age gradients across MYStIX regions. The wide MSFR age distribution appears as spatially segregated structures with different ages. The Age_JX_ ages are youngest in obscured locations in molecular clouds, intermediate in revealed stellar clusters, and oldest in distributed populations. The NIR color index J-H, a surrogate measure of extinction, can serve as an approximate age predictor for young embedded clusters.
605. Age estimates for NGC 2024 and ONC stars
ID:
ivo://CDS.VizieR/J/ApJ/787/109
Title:
Age estimates for NGC 2024 and ONC stars
Short Name:
J/ApJ/787/109
Date:
21 Oct 2021
Publisher:
CDS
Description:
We analyze age distributions of two nearby rich stellar clusters, the NGC 2024 (Flame Nebula) and Orion Nebula cluster (ONC) in the Orion molecular cloud complex. Our analysis is based on samples from the MYStIX survey and a new estimator of pre-main sequence (PMS) stellar ages, Age_JX_, derived from X-ray and near-infrared photometric data. To overcome the problem of uncertain individual ages and large spreads of age distributions for entire clusters, we compute median ages and their confidence intervals of stellar samples within annular subregions of the clusters. We find core-halo age gradients in both the NGC 2024 cluster and ONC: PMS stars in cluster cores appear younger and thus were formed later than PMS stars in cluster peripheries. These findings are further supported by the spatial gradients in the disk fraction and K-band excess frequency. Our age analysis is based on Age_JX_ estimates for PMS stars and is independent of any consideration of OB stars. The result has important implications for the formation of young stellar clusters. One basic implication is that clusters form slowly and the apparent age spreads in young stellar clusters, which are often controversial, are (at least in part) real. The result further implies that simple models where clusters form inside-out are incorrect and more complex models are needed. We provide several star formation scenarios that alone or in combination may lead to the observed core-halo age gradients.
606. Age estimates of SMC clusters
ID:
ivo://CDS.VizieR/J/MNRAS/478/784
Title:
Age estimates of SMC clusters
Short Name:
J/MNRAS/478/784
Date:
21 Oct 2021
Publisher:
CDS
Description:
The Small Magellanic Cloud (SMC) has recently been found to harbour an increase of more than 200 per cent in its known cluster population. Here, we provide solid evidence that this unprecedented number of clusters could be greatly overestimated. On the one hand, the fully automatic procedure used to identify such an enormous cluster candidate sample did not recover ~50 per cent, on average, of the known relatively bright clusters located in the SMC main body. On the other hand, the number of new cluster candidates per time unit as a function of time is noticeably different from the intrinsic SMC cluster frequency (CF), which should not be the case if these new detections were genuine physical systems. We found additionally that the SMC CF varies spatially, in such a way that it resembles an outside-in process coupled with the effects of a relatively recent interaction with the Large Magellanic Cloud. By assuming that clusters and field stars share the same formation history, we showed for the first time that the cluster dissolution rate also depends on position in the galaxy. The cluster dissolution becomes higher as the concentration of galaxy mass increases or if external tidal forces are present.
607. Age estimation for solar-type dwarfs
ID:
ivo://CDS.VizieR/J/ApJ/687/1264
Title:
Age estimation for solar-type dwarfs
Short Name:
J/ApJ/687/1264
Date:
21 Oct 2021
Publisher:
CDS
Description:
While the strong anticorrelation between chromospheric activity and age has led to the common use of the CaII H and K emission index (R'_HK_=L_HK_/L_bol_) as an empirical age estimator for solar-type dwarfs, existing activity-age relations produce implausible ages at both high and low activity levels. We have compiled R'_HK_, data from the literature for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity age index, we derive an improved activity-age calibration for F7-K2 dwarfs (0.5<B-V<0.9mag). We also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_HK_ values through the Rossby number to rotation periods and then makes use of improved gyrochronology relations.
608. Age-metallicity relation for nearby stars
ID:
ivo://CDS.VizieR/J/A+A/394/927
Title:
Age-metallicity relation for nearby stars
Short Name:
J/A+A/394/927
Date:
21 Oct 2021
Publisher:
CDS
Description:
Ages, Fe and Ca abundances, orbits, and populations for 1658 solar neighbourhood stars are presented. Stars are selected from Hipparcos (Cat. <II/239> Catalogue.
609. Age-metallicity relation in solar neighbourhood
ID:
ivo://CDS.VizieR/J/A+A/377/911
Title:
Age-metallicity relation in solar neighbourhood
Short Name:
J/A+A/377/911
Date:
21 Oct 2021
Publisher:
CDS
Description:
We derive stellar ages, from evolutionary tracks, and metallicities, from Stroemgren photometry, for a sample of 5828 dwarf and sub-dwarf stars from the Hipparcos (Cat. <I/239>) Catalogue. This stellar disk sample is used to investigate the age-metallicity diagram in the solar neighbourhood. Such diagrams are often used to derive a so called age-metallicity relation. Because of the size of our sample, we are able to quantify the impact on such diagrams, and derived relations, due to different selection effects. Some of these effects are of a more subtle sort, giving rise to erroneous conclusions. In particular we show that [1] the age-metallicity diagram is well populated at all ages and especially that old, metal-rich stars do exist, [2] the scatter in metallicity at any given age is larger than the observational errors, [3] the exclusion of cooler dwarf stars from an age-metallicity sample preferentially excludes old, metal-rich stars, depleting the upper right-hand corner of the age-metallicity diagram, [4] the distance dependence found in the Edvardsson et al. (1993, Cat. <J/A+A/275/101>) sample by Garnett & Kobulnicky (2000ApJ...532.1192G) is an expected artifact due to the construction of the original sample. We conclude that, although some of it can be attributed to stellar migration in the galactic disk, a large part of the observed scatter is intrinsic to the formation processes of stars.
610. Age-metallicity relation of {omega} Cen
ID:
ivo://CDS.VizieR/J/ApJ/647/1075
Title:
Age-metallicity relation of {omega} Cen
Short Name:
J/ApJ/647/1075
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a metallicity distribution based on photometry and spectra for 442 Omega Centauri cluster members that lie at the main-sequence turnoff region of the color-magnitude diagram. This distribution is similar to that found for the red giant branch. The distribution shows a sharp rise to a mean of [Fe/H]=-1.7 with a long tail to higher metallicities. Ages have then been determined for the stars using theoretical isochrones enabling the construction of an age-metallicity diagram. Interpretation of this diagram is complicated by the correlation of the errors in the metallicities and ages. Nevertheless, after extensive Monte Carlo simulations, we conclude that our data show that the formation of the cluster took place over an extended period of time: the most metal-rich stars in our sample ([Fe/H]~-0.6) are younger by 2-4Gyr than the most metal-poor population.

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