44 research outputs found
The red giant branch phase transition: Implications for the RGB luminosity function bump and detections of Li-rich red clump stars
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 . 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 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.Comment: 5 pages, 4 figures, Astronomy & Astrophysics in pres
The BaSTI Stellar Evolution Database: models for extremely metal-poor and super-metal-rich stellar populations
We present an extension of the BaSTI stellar evolution database to extremely
metal poor () and super-metal-rich (Z=0.05) metallicities,
with both scaled-solar and -enhanced ([/Fe]=0.4) heavy element
distributions. These new tracks (from the pre-main sequence to the
early-asymptotic giant branch phase), horizontal branch models and isochrones,
will enable the use of the BaSTI database to study, i.e., the most metal poor
populations found in Local Group faint dwarf galaxies, and the metal rich
component of the Galactic bulge. An overview of several fundamental predictions
of stellar evolution over the full metallicity range of BaSTI is presented,
together with comparisons with literature calculations at and
Z=0.05.Comment: 11 pages, 15 Figures, A&A in pres
The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods
The impact that unrecognised differences in the chemical patterns of Galactic
globular clusters have on their relative age determinations is studied. The two
most widely used relative age-dating methods, horizontal and vertical, together
with the more recent relative MS-fitting method, were carefully analyzed on a
purely theoretical basis. The BaSTI library was adopted to perform the present
analysis. We find that relative ages derived using the horizontal and vertical
methods are largely dependent on the initial He content and heavy element
distribution. Unrecognized cluster-to-cluster chemical abundance differences
can lead to an error in the derived relative ages as large as ~0.5 (or ~6 Gyr
if an age of 12.8 Gyr is adopted for normalization), and even larger for some
extreme cases. It is shown that the relative MS-fitting method is by far the
age-dating technique for which undetected cluster-to-cluster differences in the
He abundance have less impact. Present results are used in order to pose
constraints on the maximum possible spread in the He and CNONa elements
abundances on the basis of the estimates - taken from the literature - of the
Galactic globular clusters relative age dispersion obtained with the various
relative age-dating techniques. Finally, it is shown that the age-metallicity
relation found for young Galactic globular clusters by the GC Treasury program
is a real age sequence and cannot be produced by variations in the He and/or
heavy element distribution.Comment: 26 pages, 8 figures, accepted for publication in ApJ
Photometric properties of stellar populations in Galactic globular clusters: the role of the Mg-Al anticorrelation
We have computed low-mass stellar models and synthetic spectra for an initial
chemical composition that includes the full C-N, O-Na, and Mg-Al abundance
anticorrelations observed in second generation stars belonging to a number of
massive Galactic globular clusters. This investigation extends a previous study
that has addressed the effect of only the C-N and O-Na anticorrelations, seen
in all globulars observed to date. We find that the impact of Mg-Al abundance
variations at fixed [Fe/H] and Helium abundance is negligible on stellar models
and isochrones (from the main sequence to the tip of the red giant branch) and
bolometric corrections, when compared to the effect of C-N and O-Na variations.
We identify a spectral feature at 490-520 nm, for low-mass main sequence stars,
caused by MgH molecular bands. This feature has a vanishingly small effect on
bolometric corrections for Johnson and Stroemgren filters that cover that
spectral range. However, specific narrow-band filters able to target this
wavelength region can be powerful tools to investigate the Mg-poor unevolved
stars and highlight possible splittings of the MS due to variations of Mg
abundances.Comment: 8 pages, 9 figures, Astronomy & Astrophysics in pres
A large stellar evolution database for population synthesis studies. IV. Integrated properties and spectra
This paper is the 4th in a series describing the latest additions to the
BaSTI stellar evolution database, which consists of a large set of homogeneous
models and tools for population synthesis studies. Here we present a new set of
low and high resolution synthetic spectra based on the BaSTI stellar models,
covering a large range of simple stellar populations (SSPs) for both scaled
solar and alpha-enhanced metal mixtures. This enables a completely consistent
study of the photometric and spectroscopic properties of both resolved and
unresolved stellar populations, and allows us to make detailed tests on their
integrated properties. Our low resolution spectra are suitable for deriving
broadband magnitudes and colors in any photometric system. These spectra cover
the full wavelength range (9-160000nm) and include all evolutionary stages up
to the end of AGB evolution. Our high resolution spectra are suitable for
studying the behaviour of line indices and we have tested them against a large
sample of Galactic globular clusters. We find that the range of ages, iron
abundances [Fe/H], and degree of alpha-enhancement predicted by the models
matches observed values very well. We have also tested the global consistency
of the BaSTI models by making detailed comparisons between ages and
metallicities derived from isochrone fitting to observed CMDs, and from line
index strengths, for the Galactic globular cluster 47Tuc and the open cluster
M67. For 47Tuc we find reasonable agreement between the 2 methods, within the
estimated errors. From the comparison with M67 we find non-negligible effects
on derived line indices caused by statistical fluctuations, which are a result
of the specific method used to populate an isochrone and assign appropriate
spectra to individual stars. (abridged)Comment: 21 pages including 13 figures. Accepted for publication in ApJ. Low
and high resolution integrated spectra, magnitudes, and mass-to-light ratios
will appear on the BaSTI website by 1st November 2008 - see
http://193.204.1.62/index.htm
Old and new issues in stellar evolution modelling
The knowledge of the evolutionary and structural properties of stars has achieved an high level of accuracy and maturity, thanks to an improved understanding of the physics at work in real stars. This notwithstanding, the current generation of stellar models is still affected by several - not always negligible - shortcomings related to our poor knowledge of some thermodynamical processes, nuclear reaction rates, as well as the efficiency of mixing/diffusive processes. These drawbacks have to be properly taken into account, when comparing theory with observations to derive evolutionary properties of both resolved and unresolved stellar populations. This paper reviews (some of) the major sources of uncertainty for the main evolutionary stages. <P /
The updated BaSTI stellar evolution models and isochrones.IV. alpha-depleted calculations
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, alpha-enhanced, and white dwarf model libraries, we
present here alpha-depleted ([alpha/Fe] = -0.2) evolutionary tracks and
isochrones, suitable to study the alpha-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 alpha-enhanced models, and
span a mass range between 0.1 and 15 Msun, 21 metallicities between [Fe/H] =
-3.20 and +0.45 with a helium-to-metal enrichment ratio dY/dZ = 1.31,
homogeneous with the solar-scaled and alpha-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 alpha-depleted stellar models,
available for the same alpha-element depletion adopted in present
investigation. We have also discussed the effect of an alpha-depleted heavy
element distribution on the bolometric corrections in different wavelength
regimes. Our alpha-depleted evolutionary tracks and isochrones are publicly
available at the BaSTI website.Comment: 7 pages, 4 figures, 3 tables. Accepted for publication in MNRA