Recent spectroscopic and photometric surveys of Galactic Globular Clusters have shown that these stellar systems host distinct sub-populations of stars characterised by peculiar chemical patterns. In ...the following we wish to address the issue of how these specific chemical patterns affect both the structural and evolutionary properties of stars as well as their spectral energy distribution. The implications of these effects on the photometric appearance of multiple stellar populations in different photometric planes are also briefly discussed.
ABSTRACT
Updated evolutionary and pulsational model predictions are combined in order to interpret the properties of Galactic Classical Cepheids in the Gaia Data Release 2. In particular, the ...location of the instability strip boundaries and the analytical relations connecting pulsation periods to the intrinsic stellar parameters are combined with evolutionary tracks to derive reliable and accurate period–age and the first theoretical period–age–colour relations in the Gaia bands for a solar chemical abundance pattern (Z = 0.02, Y = 0.28). The adopted theoretical framework takes into account possible variations in the mass–luminosity relation for the core helium-burning stage as due to changes in the core convective overshooting and/or mass-loss efficiency, as well as the impact on the instability strip boundaries due to different assumptions for superadiabatic convection efficiency. The inferred period–age and period–age–colour relations are applied to a selected sample of both fundamental and first overtone Gaia Cepheids, and individual ages for the various adopted theoretical scenarios are derived. The retrieved age distributions confirm that a variation in the efficiency of superadiabatic convection in the pulsational model computations has a negligible effect, whereas a brighter mass–luminosity relation, as produced by mild overshooting, rotation, or mass-loss, implies significantly older age predictions. Moreover, older Cepheids are found at larger Galactocentric distances, while first overtone Cepheids are found to be systematically older than the fundamental ones. The comparison with independent age distribution analysis in literature supports the predictive capability of current theoretical framework.
In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, it is now thought that basically all globular clusters host multiple stellar ...populations, each with its own chemical abundance pattern and colour–magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the globular cluster NGC 6752 have disclosed a low Na/Fe abundance for the whole sample, suggesting that they are all first generation stars, and that all second generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second generation horizontal branch stars – all located at the hot side of the blue and extended horizontal branch of this cluster – possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the asymptotic giant branch phase, thus explaining at the same time the low value of the ratio between horizontal branch and asymptotic giant branch stars (the R2 parameter) observed in NGC 6752. We have critically discussed this mass-loss scenario, finding that the required mass-loss rates are of the order of 10-9 M⊙ yr-1, significantly higher than current theoretical and empirical constraints. By making use of synthetic horizontal branch simulations, we demonstrate that our modelling correctly predicts the R2 parameter for NGC 6752, without the need to invoke very efficient mass loss during the core He-burning stage. As a test of our stellar models we show that we can reproduce the observed value of R2 for both M 3, a cluster of approximately the same metallicity and with a redder horizontal branch morphology, and M 13, a cluster with a horizontal branch very similar to NGC 6752. However, our simulations for the NGC 6752 horizontal branch predict however the presence of a significant fraction of second generation stars (about 50%) along the cluster asymptotic giant branch. We conclude that there is no simple explanation for the lack of second generation stars in the spectroscopically surveyed sample, although the interplay between mass loss (with low rates) and radiative levitation may play a role in explaining this puzzle.
Abstract
The zero point of the reddening toward the Large Magellanic Cloud (LMC) has been the subject of some dispute. Its uncertainty propagates as a systematic error for methods that measure the ...extragalactic distance scale through knowledge of the absolute extinction of LMC stars. In an effort to resolve this issue, we used three different methods to calibrate the most widely used metric to predict LMC extinction, the intrinsic color of the red clump, (
V
−
I
)
RC
,0
, for the inner ∼3° of that galaxy. The first approach was to empirically calibrate the color zero points of the BaSTI isochrones over a wide metallicity range of ΔFe/H ≈ 1.10 using measurements of red clump stars in 47 Tuc, the solar neighborhood, and NGC 6791. From these efforts, we also measure these properties of the solar neighborhood red clump, (
V
−
I
,
G
BP
−
K
s
,
G
−
K
s
,
G
RP
−
K
s
,
J
−
K
s
,
H
−
K
s
,
M
I
,
M
Ks
)
RC
,0
= (1.02, 2.75, 2.18, 1.52, 0.64, 0.15, −0.23, −1.63). The second and third methods were to compare the observed colors of the red clump to those of Cepheids and RR Lyrae in the LMC. With these three methods, we estimated the intrinsic color of the red clump of the LMC to be (
V
−
I
)
RC
,0,LMC
= {≈
0.93
, 0.91 ± 0.02, 0.89 ± 0.02}, respectively, and similarly, using the first and third methods, we estimated (
V
−
I
)
RC
,0,SMC
= {≈
0.85
, 0.84 ± 0.02}, respectively, for the Small Magellanic Cloud. We estimate the luminosities to be
M
I
,
RC
,LMC
= −0.26 and
M
I
,
RC
,SMC
= −0.37. We show that this has important implications for recent calibrations of the tip of the red giant branch in the Magellanic Clouds used to measure
H
0
.
Stellar evolution tracks and isochrones are key inputs for a wide range of astrophysical studies; in particular, they are essential to the interpretation of photometric and spectroscopic observations ...of resolved and unresolved stellar populations. We have made available to the astrophysical community a large, homogenous database of up-to-date stellar tracks and isochrones and a set of programs useful in population synthesis studies. In this paper we first summarize the main properties of our stellar model database (BaSTI ) already introduced by Pietrinferni and coworkers. We then discuss an important update of the database, i.e., the extension of all stellar models and isochrones until the end of the thermal pulses along the asymptotic giant branch. This extension of the library is particularly relevant for stellar population analyses in the near-infrared or longer wavelengths, where the contribution to the integrated photometric properties by cool and bright asymptotic giant branch stars is significant. A few comparisons with empirical data are also presented and briefly discussed. We then present three Web tools that allow an interactive access to the database and make it possible to compute user-specified evolutionary tracks, isochrones, stellar luminosity functions, and synthetic color-magnitude diagrams and integrated magnitudes for arbitrary star formation histories. All these Web tools are available at the BaSTI database official Web site.
ABSTRACT
Homogeneous multiwavelength observations of classical Cepheids from the forthcoming Rubin-LSST have the potential to significantly contribute to our understanding of the evolutionary and ...pulsation properties of these pulsating stars. Updated pulsation models for classical Cepheid stars have been computed under various assumptions about chemical compositions, including relatively low metallicity (Z = 0.004 with Y = 0.25 and Z = 0.008 with Y = 0.25), solar metallicity (Z = 0.02 with Y = 0.28), and supersolar metallicity environments (Z = 0.03 with Y = 0.28). From the predicted periods, intensity-weighted mean magnitudes, and colours, we have derived the first theoretical pulsation relations in the Rubin-LSST filters (ugrizy), including period–luminosity–colour, period–Wesenheit, and period–age–colour relations. We find that the coefficients of these relations are almost insensitive to the efficiency of superadiabatic convection but are significantly affected by the assumption of the mass–luminosity relation and the adopted chemical composition. Metal-dependent versions of these relations are also derived, representing valuable tools for individual distance determinations and correction for metallicity effects on the cosmic distance scale.
Abstract
A number of studies based on the data collected by the Hubble Space Telescope (HST) GO-13297 program “HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations ...and Formation” have investigated the photometric properties of a large sample of Galactic globular clusters and revolutionized our understanding of their stellar populations. In this paper, we expand upon previous studies by focusing our attention on the stellar clusters’ internal kinematics. We computed proper motions for stars in 56 globular clusters and one open cluster by combining the GO-13297 images with archival HST data. The astrophotometric catalogs released with this paper represent the most complete and homogeneous collection of proper motions of stars in the cores of stellar clusters to date, and expand the information provided by the current (and future) Gaia data releases to much fainter stars and into the crowded central regions. We also census the general kinematic properties of stellar clusters by computing the velocity dispersion and anisotropy radial profiles of their bright members. We study the dependence on concentration and relaxation time, and derive dynamical distances. Finally, we present an in-depth kinematic analysis of the globular cluster NGC 5904.
Slowly Cooling White Dwarfs in NGC 6752 Chen, Jianxing; Ferraro, Francesco R.; Cadelano, Mario ...
The Astrophysical journal,
08/2022, Letnik:
934, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
Recently, a new class of white dwarfs (“slowly cooling WDs”) has been identified in the globular cluster M13. The cooling time of these stars is increased by stable thermonuclear hydrogen ...burning in their residual envelope. These WDs are thought to be originated by horizontal branch (HB) stars populating the HB blue tail that skipped the asymptotic giant branch phase. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in NGC 6752, another Galactic globular cluster with a metallicity, age, and HB morphology similar to M13. The normalized WD luminosity function derived in NGC 6752 turns out to be impressively similar to that observed in M13, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. As in the case of M13, the comparison with theoretical predictions is consistent with ∼70% of the investigated WDs evolving at slower rates than standard, purely cooling WDs. Thanks to its relatively short distance from Earth, NGC 6752 photometry reaches a luminosity 1 order of a magnitude fainter than the case of M13, allowing us to sample a regime where the cooling time delay, with respect to standard WD models, reaches ∼300 Myr. The results presented in this paper provide new evidence for the existence of slowly cooling WDs and further support to the scenario proposing a direct causal connection between this phenomenon and the HB morphology of the host stellar cluster.
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