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461. AGB population of NGC 6822
ID:
ivo://CDS.VizieR/J/A+A/540/A135
Title:
AGB population of NGC 6822
Short Name:
J/A+A/540/A135
Date:
21 Oct 2021
Publisher:
CDS
Description:
NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close proximity and apparent isolation provide a unique opportunity to study galactic evolution without any obvious strong external influences. This paper aims to study the spatial distribution of the asymptotic giant branch (AGB) population and metallicity in NGC 6822. Using deep, high quality JHK photometry, taken with WFCAM on UKIRT, carbon- and oxygen-rich AGB stars have been isolated. The ratio between their number, the C/M ratio, has then been used to derive the [Fe/H] abundance across the galaxy.
462. AGB stars in Cen A dwarf galaxies
ID:
ivo://CDS.VizieR/J/A+A/530/A58
Title:
AGB stars in Cen A dwarf galaxies
Short Name:
J/A+A/530/A58
Date:
21 Oct 2021
Publisher:
CDS
Description:
We investigate the resolved stellar content of early-type dwarf galaxies in the Centaurus A group, to estimate their intermediate-age population fractions. We use near-infrared photometric data taken with the VLT/ISAAC instrument, together with previously analyzed archival HST/ACS data. The combination of the optical and infrared wavelength range permits us to firmly identify luminous asymptotic giant branch stars, which are indicative of an intermediate-age population in these galaxies. We consider one dwarf spheroidal (CenA-dE1) and two dwarf elliptical (SGC1319.1-4216 and ESO269-066) galaxies that are dominated by an old population. The most recent periods of star formation are estimated to have taken place between 2 and 5Gyr ago for SGC1319.1-4216 and ESO269-066, and approximately 9Gyr ago for CenA-dE1. For ESO269-066, we find that the intermediate-age populations are significantly more centrally concentrated than the predominantly old underlying stars. The intermediate-age population fraction is found to be small in the target galaxies, consistent with fractions of up to 15% of the total population. These values might be larger by a factor of two or three, if we considered the observational limitations and the recent discussion about the uncertainties in theoretical models. We suggest that there is a correlation between intermediate-age population fraction and proximity to the dominant group galaxy, with closer dwarfs having slightly smaller fractions, although our sample is too small to draw firm conclusions. Even when considering our results as lower limits, the intermediate-age population fractions for the studied dwarfs are clearly much smaller than those found in similar dwarfs around the Milky Way, but comparable to what is seen for the low-mass M31 companions. Our results confirm our previous work about early-type dwarfs in the Centaurus A group.
463. AGB stars in Fornax dwarf spheroidal galaxy
ID:
ivo://CDS.VizieR/J/MNRAS/394/795
Title:
AGB stars in Fornax dwarf spheroidal galaxy
Short Name:
J/MNRAS/394/795
Date:
21 Oct 2021
Publisher:
CDS
Description:
We report on a multi-epoch study of the Fornax dwarf spheroidal galaxy, made with the Infrared Survey Facility, over an area of about 42x42arcmin^2^. The colour-magnitude diagram shows a broad well-populated giant branch with a tip that slopes downwards from red to blue, as might be expected given Fornax's known range of age and metallicity. The extensive asymptotic giant branch (AGB) includes seven Mira variables and 10 periodic semiregular variables. Five of the seven Miras are known to be carbon rich. Their pulsation periods range from 215 to 470d, indicating a range of initial masses. Three of the Fornax Miras are redder than typical Large Magellanic Cloud (LMC) Miras of similar period, probably indicating particularly heavy mass-loss rates. Many, but not all, of the characteristics of the AGB are reproduced by isochrones from Marigo et al. (2008A&A...482..883M) for a 2Gyr population with a metallicity of Z=0.0025.
464. AGB stars with GALEX observations
ID:
ivo://CDS.VizieR/J/ApJ/841/33
Title:
AGB stars with GALEX observations
Short Name:
J/ApJ/841/33
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have performed a comprehensive study of the UV emission detected from asymptotic giant branch (AGB) stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the near-UV (NUV) bandpass ({lambda}_eff_~2310{AA}), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the far-UV (FUV) bandpass ({lambda}_eff_~1528{AA}). We find that NUV emission is correlated with optical to near-infrared emission, leading to higher detection fractions among the brightest, and hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves, we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission and possibly the FUV emission are anticorrelated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.
465. AGB subpopulations in NGC 6397
ID:
ivo://CDS.VizieR/J/MNRAS/475/257
Title:
AGB subpopulations in NGC 6397
Short Name:
J/MNRAS/475/257
Date:
21 Oct 2021
Publisher:
CDS
Description:
It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anticorrelations of light-element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here, we investigate the AGB of NGC 6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 red giant branch (RGB) and eight AGB stars, deriving Fe, Na, O, Mg, and Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC 6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M4 (with contrasting results), since the same tools and methods were used.
466. A GBT survey of large Galactic HII regions
ID:
ivo://CDS.VizieR/J/ApJS/234/33
Title:
A GBT survey of large Galactic HII regions
Short Name:
J/ApJS/234/33
Date:
21 Oct 2021
Publisher:
CDS
Description:
As part of our ongoing HII Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly large Galactic HII regions in radio recombination line (RRL) emission. Our targets are located at a declination of {delta}>-45{deg}, which corresponds to 266{deg}>l>-20{deg} at b=0{deg}. All sources were selected from the Wide-field Infrared Survey Explorer Catalog of Galactic HII Regions, and have infrared angular diameters >=260". The Galactic distribution of these "large" HII regions is similar to that of the previously known sample of Galactic HII regions. The large HII region RRL line width and peak line intensity distributions are skewed toward lower values, compared with that of previous HRDS surveys. We discover seven sources with extremely narrow RRLs <10km/s. If half the line width is due to turbulence, these seven sources have thermal plasma temperatures <1100K. These temperatures are lower than any measured for Galactic HII regions, and the narrow-line components may arise instead from partially ionized zones in the HII region photodissociation regions. We discover G039.515+00.511, one of the most luminous HII regions in the Galaxy. We also detect the RRL emission from three HII regions with diameters >100pc, making them some of the physically largest known HII regions in the Galaxy. This survey completes the HRDS HII region census in the Northern sky, where we have discovered 887 HII regions and more than doubled the size of the previously known census of Galactic HII regions.
467. Age and metallicity in eclipsing binaries
ID:
ivo://CDS.VizieR/J/AZh/78/1104
Title:
Age and metallicity in eclipsing binaries
Short Name:
J/AZh/78/1104
Date:
21 Oct 2021
Publisher:
CDS
Description:
Based on the set of high accuracy observational data on the components of SB2 eclipsing binaries and the grid of stellar models calculated by Geneva group for the wide range of masses and abundances, we estimate most probable ages and metallicities for the 43 systems using constraints arising from common origin of the components.
468. 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.
469. 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.
470. 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.

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