Abstract
This is the second paper of a series devoted to presenting an updated release of the BaSTI (a Bag of Stellar Tracks and Isochrones) stellar model and isochrone library. Following the ...publication of the updated solar-scaled library, here we present the library for an
α
-enhanced heavy element distribution. These new
α
-enhanced models account for all improvements and updates in the reference solar metal distribution and physics inputs, as in the new solar-scaled library. The models cover a mass range between 0.1 and 15
M
⊙
, 18 metallicities between Fe/H = −3.20 and +0.06 with
α
/Fe = +0.4, and a He-to-metal enrichment ratio Δ
Y
/Δ
Z
= 1.31. For each metallicity, He-enhanced stellar models are also provided. The isochrones cover (typically) an age range between 20 Myr and 14.5 Gyr, including consistently the pre-main-sequence phase. The asteroseismic properties of the theoretical models have also been calculated. Models and isochrones have been compared with results from independent calculations, with the previous BaSTI release, and also with selected observations, to test the accuracy/reliability of these new calculations. All stellar evolution tracks, asteroseismic properties, and isochrones are publicly available at
http://basti-iac.oa-teramo.inaf.it
.
We performed a detailed analysis of the use of C/N measured in red giant branch stars between the completion of the first dredge up and the red giant branch bump (C/N FDU) as age indicator. C/N FDU ...cannot give accurate ages for individual stars, but may provide a general chronology for the formation of composite populations and add constraints to analyses of red giants from surface gravity-effective temperature diagrams. We provide a theoretical calibration of C/N FDU in terms of total metallicity M/H and age, for ages greater than ~1 Gyr, which we tested against variations in the initial heavy element distribution (scaled-solar vs. α-enhanced), efficiency of overshooting from MS convective cores and from the convective envelopes, variations in the initial He abundance and in the mixing length parameter. Our calibration is compared with a small sample of available measurements of C/N FDU in star clusters and halo field stars, which at least qualitatively confirm the overall trend of the predicted C/N FDU with age and M/H. The use of C/N FDU–M/H-age relations obtained from independent sets of stellar evolution calculations cause age differences (for a given C/N FDU and M/H pair) up to ~2 Gyr. More accurate spectroscopic measurements of C/N FDU in star clusters with well-established ages and metallicities are required to better test theoretical calibrations of this age indicator.
Electron conduction opacities are one of the main physics inputs for the calculation of low- and intermediate-mass stellar models. A critical question considers how to devise a bridge when ...calculating both moderate and strong degeneracy, which are necessarily performed adopting different methods. The density-temperature regime at the boundary between moderate and strong degeneracy is, in fact, crucial for modelling the helium cores of red giant branch stars and the hydrogen-helium envelopes of white dwarfs. Prompted by newly published, improved calculations of electron thermal conductivities and opacities for moderate degeneracy, we study different, physically motivated prescriptions to bridge these new computations with well-established results in the regime of strong degeneracy. We find that these varied prescriptions have a sizable impact on the predicted He-core masses at the He-flash (up to 0.01
M
⊙
for initial total masses far from the transition to non-degenerate He-cores and up to ∼0.04
M
⊙
for masses around the transition), the tip of the red giant branch (up to ∼0.1 mag), and the zero-age horizontal branch luminosities (up to 0.03 dex for masses far from the transition and up to ∼0.2 dex around the transition), and white dwarf cooling times (up to 40–45% at high luminosities, and up to ∼25% at low luminosities). Current empirical constraints on the tip of the red giant branch and the zero age horizontal branch absolute magnitudes do not yet allow for the definitive exclusion of any of these alternative options for the conductive opacities. Tests against observations of slowly-cooling faint WDs in old stellar populations will need to be performed to see whether they are capable of setting some more stringent constraints on bridging calculations of conductive opacities for moderate and strong degeneracy.
We performed a detailed study of the evolution of the luminosity of He-ignition stage and of the red giant branch bump luminosity during the red giant branch phase transition for various ...metallicities. To this purpose we calculated a grid of stellar models that sample the mass range of the transition with a fine mass step equal to 0.01 M⊙. We find that for a stellar population with a given initial chemical composition, there is a critical age (of 1.1–1.2 Gyr) around which a decrease in age of just 20–30 million years causes a drastic drop in the red giant branch tip brightness. We also find a narrow age range (a few 107 yr) around the transition, characterized by the luminosity of the red giant branch bump being brighter than the luminosity of He ignition. We discuss a possible link between this occurrence and observations of Li-rich core He-burning stars.
One crucial piece of information to study the origin of multiple stellar populations in globular clusters is the range of initial helium abundances ...Y amongst the sub-populations hosted by each ...cluster. These estimates are commonly obtained by measuring the width in colour of the unevolved main sequence in an optical colour-magnitude diagram (CMD). The measured colour spread is then compared with predictions from theoretical stellar isochrones with varying initial He abundances to determine ...Y. The availability of UV/optical magnitudes, thanks to the Hubble Space Telescope UV Legacy Survey of Galactic GCs project, will allow the homogeneous determination of ...Y for a large Galactic globular cluster sample. From a theoretical point of view, accurate UV CMDs can efficiently disentangle the various sub-populations, and main sequence colour differences in the ACS F606W - (F606W - F814W) diagram allow an estimate of ...Y. We demonstrate that from a theoretical perspective, the (F606W - F814W) colour is an extremely reliable He-abundance indicator. The derivative dY/d(F606W - F814W), computed at a fixed luminosity along the unevolved main sequence, is largely insensitive to the physical assumptions made in stellar model computations, being more sensitive to the choice of the bolometric correction scale, and is only slightly dependent on the adopted set of stellar models. From a theoretical point of view, the (F606W - F814W) colour width of the cluster main sequence is therefore a robust diagnostic of the ...Y range. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT
This is the fourth paper of our new release of the BaSTI (a Bag of Stellar Tracks and Isochrones) stellar model and isochrone library. Following the updated solar-scaled, α-enhanced, and ...white dwarf model libraries, we present here α-depleted (α/Fe = −0.2) evolutionary tracks and isochrones, suitable to study the α-depleted stars discovered in Local Group dwarf galaxies and in the Milky Way. These calculations include all improvements and updates of the solar-scaled and α-enhanced models, and span a mass range between 0.1 and 15 M⊙, and 21 metallicities between Fe/H = −3.20 and +0.45 with a helium-to-metal enrichment ratio ΔY/ΔZ = 1.31, homogeneous with the solar-scaled and α-enhanced models. The isochrones – available in several photometric filters – cover an age range between ∼20 Myr and 14.5 Gyr, including the pre-main-sequence phase. We have compared our isochrones with independent calculations of α-depleted stellar models, available for the same α-element depletion adopted in the present investigation. We have also discussed the effect of an α-depleted heavy element distribution on the bolometric corrections in different wavelength regimes. Our α-depleted evolutionary tracks and isochrones are publicly available at our BaSTI website.
ABSTRACT
Based on updated pulsation models for classical Cepheids, computed for various assumptions about the metallicity and helium abundance, roughly representative of pulsators in the Small ...Magellanic Cloud (Z = 0.004 and Y = 0.25), Large Magellanic Cloud (Z = 0.008 and Y = 0.25), and M31 (Z = 0.03 and Y = 0.28), and self-consistent updated evolutionary predictions, we derived period-age and multi-band period-age-colour relations that also take into account variations in the mass–luminosity (ML) relation. These results, combined with those previously derived for Galactic Cepheids, were used to investigate the metallicity effect when using these variables as age indicators. In particular, we found that a variation in the metal abundance affects both the slope and the zero-point of the above-mentioned relations. The new relations were applied to a sample of Gaia Early Data Release 3 classical Cepheids. The retrieved distribution of the individual ages confirms that a brighter ML relation produces older ages and that first overtone pulsators are found to be concentrated towards older ages with respect to the fundamental ones at a fixed ML relation. Moreover, the inclusion of a metallicity term in the period-age and period-age-colour relations slightly modifies the predicted ages. In particular, the age distribution of the selected sample of Galactic Cepheids is found to be shifted towards slightly older values, when the F-mode canonical relations are considered, with respect to the case at a fixed solar chemical composition. A marginally opposite dependence can be found in the non-canonical F-mode and canonical FO-mode cases.
ABSTRACT
The existence of star-to-star light-element abundance variations (multiple populations, MPs) in massive Galactic and extragalactic star clusters older than about 2 Gyr is by now well ...established. Photometry of red giant branch (RGB) stars has been and still is instrumental in enabling the detection and characterization of cluster MPs, through the appropriate choices of filters, colours, and colour combinations that are mainly sensitive to N and – to a lesser degree – C stellar surface abundances. An important issue not yet properly addressed is that the translation of the observed widths of the cluster RGBs to abundance spreads must account for the effect of the first dredge-up on the surface chemical patterns, hence on the spectral energy distributions of stars belonging to the various MPs. We have filled this gap by studying theoretically the impact of the dredge-up on the predicted widths of RGBs in clusters hosting MPs. We find that for a given initial range of N abundances, the first dredge-up reduces the predicted RGB widths in N-sensitive filters compared to the case when its effect on the stellar spectral energy distributions is not accounted for. This reduction is a strong function of age and has also a dependence on metallicity. The net effect is an underestimate of the initial N-abundance ranges from RGB photometry if the first dredge-up is not accounted for in the modelling, and also the potential determination of spurious trends of N-abundance spreads with age.
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![Post first dredge-up [C/N] ...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-6361/small "Post first dredge-up [C/N] ratio as age indicator. Theoretical calibration")
