825 research outputs found
Stellar Evolutionary Models: challenges from observations of stellar systems
We briefly review some constraints (Owing to the limited number of pages of
present review, only a sub-sample of the topics discussed during the talk are
briefly summarized. For the interested readers we are pleased to send them upon
request the complete presentation file.) for stellar models in various mass
regimes and evolutionary stages as provided by observational data from
spectroscopy to multi-wavelenghts photometry. The accuracy of present
generation of stellar models can be significantly improved only through an
extensive comparison between theory and observations.Comment: 8 pages, 4 figures, invited review at the IAU Symposium 241 "Stellar
Populations as Building Blocks of Galaxies", A. Vazdekis, et al. (eds
A critical investigation on the discrepancy between the observational and theoretical Red Giant luminosity function "Bump"
New theoretical evaluations of the RGB luminosity function 'bump' and the
ZAHB luminosity covering the range of metallicities typical of galactic
globular cluster are presented. The variation of the theoretical RGB bump and
ZAHB levels due to the metallicity, original helium content, mixing length
value, age, mass loss, bolometric corrections, opacities and equation of state
adopted in the evolutionary models is also discussed. These new prescriptions
have been taken into account for casting light on a longstanding astrophysical
problem connected with the Red Giant Branch evolutionary phase, namely the
discrepancy between the observational and the theoretical luminosity of RGB
bump. A sample of globular clusters with accurate evaluations of the bump
luminosity and spectroscopical metallicity determinations has been selected.
The Zero Age Horizontal Branch luminosity at the RR-Lyrae instability strip has
been evaluated as accurately as possible, and the observational luminosity
difference between the RGB bump and the ZAHB has been compared with the
theoretical values. It is shown that there is no significant disagreement
between observations and canonical stellar models. The possible applications of
this result are also briefly discussed.Comment: 10 pages, 5 figures, Tex file, mnrass.sty style included. To appear
in MNRA
Lithium and oxygen in globular cluster dwarfs and the early disc accretion scenario
A new scenario --early disc accretion-- has been recently proposed to explain
the discovery of multiple stellar populations in Galactic globular clusters.
According to this model, the existence of well defined (anti)-correlations
amongst light element abundances (i.e. C, N, O, Na) in the photospheres of
stars belonging to the same cluster (and the associated helium enrichment), is
caused by accretion of the ejecta of short lived interacting massive binary
systems (and single fast rotating massive stars) on fully convective pre-main
sequence low- and very low-mass stars, during the early stages of the cluster
evolution. We investigated the constraints provided by considering
simultaneously the observed spread of lithium and oxygen (and when possible
also sodium) abundances for samples of turn-off stars in NGC6752, NGC6121 (M4),
and NGC104 (47Tuc), and the helium abundance of their multiple main sequences.
These observations provide a very powerful test for the accretion scenario,
because the observed O, Li and He abundance distributions at the turn off can
be used to constrain the composition (and mass) of the accreted matter, and the
timescales of the polluting stars. In case of NGC6752 we could not find a
physically consistent solution. In case of M4, spectroscopic errors are too
large compared to the intrinsic spread, to constrain the properties of the
accreted matter. As for 47Tuc, we could find a physically consistent solution
for the abundances of He and O (and Na) in the accreted gas, and predict the
abundances of these elements in the accreted matter only if pollution happens
with timescales of ~1 Myr, hence polluters are objects with masses of the order
of several tens of solar masses (abridged).Comment: 8 pages, 8 figures, accepted for publication in A&
The effect of diffusion on the Red Giant luminosity function 'bump'
This paper investigates the effect of microscopic diffusion of helium and
heavy elements on the location of the Red Giant Branch Luminosity Function Bump
in Population II stellar models. To this aim updated evolutionary models taking
into account diffusion from the Main Sequence until the Zero Age Horizontal
Branch have been computed. The observational luminosity difference between the
RGB bump and the ZAHB, as collected for a sample of galactic globular clusters,
has been compared with the corresponding theoretical values obtained by
adopting both canonical and diffusive models. We find that the effect of
diffusion, even if slightly improving the agreement between observations and
theory, is negligible with respect to the observational uncertainties. In any
case the theoretical predictions in models with and without diffusion appear in
agreement with the observational results within the estimated errors. Thus
canonical models can be still safely adopted, at least until much more accurate
observational data will be available.Comment: TeX, 6 pages, uses mnrass.sty (included), 2 postscript figures, in
publication on MNRA
Hot Horizontal Branch Stars: Predictions for Mass Loss
We predict mass-loss rates for the late evolutionary phases of low-mass
stars, with special emphasis on the consequences for the morphology of the
Horizontal Branch (HB). We show that the computed rates, as predicted by the
most plausible mechanism of radiation pressure on spectral lines, are too low
to produce EHB/sdB stars. This invalidates the scenario recently outlined by
Yong et al. (2000) to create these objects by mass loss on the HB. We argue,
however, that mass loss plays a role in the distribution of rotational
velocities of hot HB stars, and may -- together with the enhancement of heavy
element abundances due to radiative levitation -- provide an explanation for
the so-called ``low gravivity problem. The mass loss recipe derived for hot HB
(and extreme HB, sdB, sdOB) stars may also be applied to post-HB (AGB-manque,
UV-bright) stars over a range in effective temperatures between 12500 -- 40000
K.Comment: 11 pages; Accepted by A&
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