- ID:
- ivo://CDS.VizieR/J/AJ/158/163
- Title:
- Li abundance values for stars in NGC 6819
- Short Name:
- J/AJ/158/163
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Spectroscopy of 333 NGC 6819 stars and Gaia astrometry are used to map Li evolution from the giant branch tip to 0.5 mag below the Li dip. Isochrone comparison with [Fe/H]=-0.04, based upon neural network spectroscopic analysis, produces an age of 2.25 (2.4) Gyr for E(B-V)=0.16 (0.14) and (m-M)=12.40 (12.29). Despite originating outside the Li dip, only 10% of single subgiants/giants have measurable Li. Above the Li dip, the limiting A(Li) for single stars is 3.2+/-0.1 but the lower range is comparable to that found within the dip. The F-dwarf Li dip profile agrees with the Hyades/Praesepe, evolved forward. The Li level among stars populating the plateau fainter than the Li dip is A(Li)=2.83+/-0.16; the dispersion is larger than expected from spectroscopic error alone. Comparison of Li and V_ROT_ distributions among turnoff stars in NGC 7789, NGC 2506, NGC 3680, and NGC 6819 indicates that rotational spindown from the main sequence is critical in defining the boundaries of the Li dip. For higher-mass dwarfs, spindown is likewise correlated with Li depletion, creating a second dip, but at higher mass and on a longer timescale. The Li distribution among evolved stars of NGC 6819 is more representative of the older M67, where subgiant and giant stars emerge from within the Li dip, than the younger NGC 7789, where a broad range in V_ROT_ among the turnoff stars likely produces a range in mass among the giants.
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- ID:
- ivo://CDS.VizieR/J/AJ/127/1513
- Title:
- Lick indices on M67 members
- Short Name:
- J/AJ/127/1513
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We construct an integrated spectrum of the intermediate-age, solar-metallicity Galactic cluster M67, from individual spectroscopic observations of bona fide cluster members. The spectrum so obtained is used as a template to test our stellar population synthesis models, in an age and metallicity regime where such models remain largely untested. As a result, we demonstrate that our models predict a spectroscopic age of 3.5+/-0.5Gyr for M67, which is the same age we obtain from fitting isochrones to the color-magnitude diagram of the cluster. Full consistency is reached when using either H{beta}, H{gamma}, or H{delta} as the age indicator. We also check if the models, when applied to the cluster integrated spectrum, predict elemental abundances in agreement with the known detailed abundance pattern of the cluster. The models also pass the latter test, by predicting the abundances of iron, magnesium, carbon, and nitrogen in agreement with detailed abundance analyses of cluster stars to within 0.1dex. Encouraged by the high degree of consistency of our models, we apply them to the study of the integrated spectrum of the central 3" of the compact elliptical galaxy M32.
- ID:
- ivo://CDS.VizieR/J/ApJS/258/43
- Title:
- Light curves of AGBs in Galactic open clusters
- Short Name:
- J/ApJS/258/43
- Date:
- 15 Feb 2022 09:28:18
- Publisher:
- CDS
- Description:
- Benefiting from the GAIA second and early third releases of photometric and astrometric data we examine the population of asymptotic giant branch (AGB) stars that appear in the fields of intermediate-age and young open star clusters. We identify 49 AGB star candidates, brighter than the tip of the red giant branch, with a good-to-high cluster membership probability. Among them we find 19 TP-AGB stars with known spectral type: 4 M stars, 3 MS/S stars and 12 C stars. By combining observations, stellar models, and radiative transfer calculations that include the effect of circumstellar dust, we characterize each star in terms of initial mass, luminosity, mass-loss rate, core mass, period and mode of pulsation. The information collected helps us shed light on the TP-AGB evolution at solar-like metallicity, placing constraints on the third dredge-up process, the initial masses of carbon stars, stellar winds, and the initial-final mass relation (IFMR). In particular, we find that two bright carbon stars, MSB 75 and BM IV 90, members of the clusters NGC 7789 and NGC 2660 (with similar ages of about 1.2-1.6 Gyr and initial masses between 2.1 and 1.9 solar masses), have unusually high core masses, about 0.67-0.7 solar masses. These results support the findings of a recent work (Marigo et al., 2020NatAs...4.1102M) that identified a kink in the IFMR, which interrupts its monotonic trend just at the same initial masses. Finally, we investigate two competing scenarios to explain the Mc data: the role of stellar winds in single-star evolution, and binary interactions through the blue-straggler channel.
- ID:
- ivo://CDS.VizieR/J/A+A/632/A95
- Title:
- Light curves of NGC 6910 pulsating stars
- Short Name:
- J/A+A/632/A95
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology we derived an age of 10.6^+0.9^_-0.8_Myr for NGC 6910. We also identified the degree l for 8 of 37 modes detected in these stars and classified all modes in terms of p, g, and mixed-mode pulsations. Of the nine pulsating stars examined in the paper, eight are {beta} Cep stars, including three that are hybrid {beta} Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive {beta} Cep star, NGC 6910-38, has a mass of about 5.6 solar masses. The present theory does not predict unstable p modes in B-type stars with such a low mass. The g modes with relatively high frequencies (>3.5d^-1^), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and {chi} Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars.
- ID:
- ivo://CDS.VizieR/J/PASP/131/D4101
- Title:
- Likely Pleiades members with Gaia DR2
- Short Name:
- J/PASP/131/D4101
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This paper presents an investigation on fundamental astrophysical properties of the Pleiades cluster (M 45) using high-precision astrometric and photometric data from the Gaia Data Release 2 (Gaia-DR2). To obtain reliable cluster members, a machine-learning (ML) method is used to compute membership probabilities for 31462 sample stars within a radius of 6.5{deg} from the cluster center, both the astrometric and photometric data are taken into account. We obtain a total number of 1454 likely cluster members with membership probabilities larger than 0.6, including a well-known white dwarf (LB 1497) with a high membership probability of ~0.96. We find a well-defined relationship between the parallaxes and proper motions of the cluster members, the most likely explanation for the relationship is that the depth effect of the cluster along the line of sight must be taken into consideration. Using Monte Carlo simulations, the most likely distance, proper motion, and radial velocity of the cluster are determined to be D=136.0+/-0.1pc, (<{mu}_{alpha}_cos{delta}>, <{mu}_{delta}_>)=(+20.141+/-0.093, -45.536+/ -0.081)mas/yr, and <V_r_>=+5.8+/-0.1km/s, respectively. It is found that the likely cluster members extend outward to a limiting radius of R_lim_=310'+/-12' (12.3+/-0.5pc) from the cluster center, and the total mass of the cluster within this radius is M_tot_=721+/-93M_{sun}_. We find clear evidence for the presence of spatial mass segregation in this young cluster by analyzing the photometry and spatial positions of the likely cluster members. Interestingly, we also find that four high-mass cluster members with high membership probabilities (>0.99) are being ejected from the inner region of the cluster, they may have formed via close encounters between single and binary stars.
- ID:
- ivo://CDS.VizieR/J/MNRAS/450/4301
- Title:
- Line list for red giants in open clusters
- Short Name:
- J/MNRAS/450/4301
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have observed high-dispersion echelle spectra of red giant members in the five open clusters (OCs) NGC 1342, NGC 1662, NGC 1912, NGC 2354 and NGC 2447 and determined their radial velocities and chemical compositions. These are the first chemical abundance measurements for all but NGC 2447. We combined our clusters from this and previous papers with a sample drawn from the literature for which we remeasured the chemical abundances to establish a common abundance scale. With this homogeneous sample of OCs, we study the relative elemental abundances of stars in OCs in comparison with field stars as a function of age and metallicity. We find a range of mild enrichment of heavy (Ba-Eu) elements in young OC giants over field stars of the same metallicity. Our analysis supports that the youngest stellar generations in cluster might be underrepresented by the solar neighbourhood field stars.
- ID:
- ivo://CDS.VizieR/J/A+A/372/862
- Title:
- Lithium abundances in IC 2602 and IC 2391
- Short Name:
- J/A+A/372/862
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Lithium abundances and [Fe/H] for a sample of late-type stars in the young open clusters IC 2602 and IC 2391. Abundances are based on high resolution spectra obtained at ESO (3.6m + CASPEC) and CTIO (4m + echelle spectrograph)
- ID:
- ivo://CDS.VizieR/J/A+A/613/A63
- Title:
- Lithium content for 148 Pleiades stars
- Short Name:
- J/A+A/613/A63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The evolution of lithium abundance over a star's lifetime is indicative of transport processes operating in the stellar interior. Aims. We revisit the relationship between lithium content and rotation rate previously reported for cool dwarfs in the Pleiades cluster. Methods. We derive new LiI 670.8nm equivalent width measurements from high-resolution spectra obtained for low-mass Pleiades members. We combine these new measurements with previously published ones, and use the Kepler/K2 rotational periods recently derived for Pleiades cool dwarfs to investigate the lithium-rotation connection in this 125Myr-old cluster. Results. The new data confirm the correlation between lithium equivalent width and stellar spin rate for a sample of 51 early K-type members of the cluster, where fast rotating stars are systematically lithium-rich compared to slowly rotating ones. The correlation is valid for all stars over the (J-Ks) color range 0.50-0.70mag, corresponding to a mass range from about 0.75 to 0.90M_{sun}_, and may extend down to lower masses. Conclusions. We argue that the dispersion in lithium equivalent widths observed for cool dwarfs in the Pleiades cluster reflects an intrinsic scatter in lithium abundances, and suggest that the physical origin of the lithium dispersion pattern is to be found in the pre-main sequence rotational history of solar-type stars.
- ID:
- ivo://CDS.VizieR/J/A+A/541/A150
- Title:
- Lithium in M67 and Hyades
- Short Name:
- J/A+A/541/A150
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Lithium abundances in open clusters are a very effective probe of mixing processes, and their study can help to understand the large depletion of lithium in the Sun. Due to its age and metallicity, the open cluster M67 is especially interesting on this regard. Many studies on lithium abundances in M67 have already been performed, but a homogeneous global analysis of lithium in stars from subsolar up to the most massive members, was never accomplished for a large sample based on high-quality spectra. We tested our non-standard models, which were calibrated using the Sun with observational data. We collected literature data to follow, for the first time in a homogeneous way, NLTE lithium abundances of all observed single stars in M67 more massive than about 0.9 solar masses.
- ID:
- ivo://CDS.VizieR/J/AJ/151/136
- Title:
- LITTLE THINGS dwarf irregular galaxies FUV regions
- Short Name:
- J/AJ/151/136
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We examine FUV images of the LITTLE THINGS sample of nearby dwarf irregular (dIrr) and Blue Compact Dwarf galaxies to identify distinct young regions in their far outer disks. We use these data, obtained with the Galaxy Evolution Explorer satellite, to determine the furthest radius at which in situ star formation can currently be identified. The FUV knots are found at distances from the center of the galaxies of 1-8 disk scale lengths and have ages of =<20 Myr and masses of 20 M_{sun}_ to 1x10^5^ M_{sun}_. The presence of young clusters and OB associations in the outer disks of dwarf galaxies shows that dIrrs do have star formation taking place there in spite of the extreme nature of the environment. Most regions are found where the H I surface density is ~1 M_{sun}_/pc^2^, though both the H I and dispersed old stars go out much further. This limiting density suggests a cutoff in the ability to form distinct OB associations and perhaps even stars. We compare the star formation rates in the FUV regions to the average rates expected at their radii and beyond from the observed gas, using the conventional correlation for gas-rich regions. The localized rates are typically 10% of the expected average rates for the outer disks. Either star formation in dIrrs at surface densities <1 M_{sun}_/pc^2^ occurs without forming distinct associations, or the Kennicutt-Schmidt relation over-predicts the rate beyond this point. In the latter case, the stellar disks in the far-outer parts of dIrrs result from scattering of stars from the inner disk.
