414 research outputs found
CCD PHOTOMETRY OF RR LYRAE STARS IN M5 AS A TEST FOR THE PULSATIONAL SCENARIO
In this paper we present new CCD investigations of RR Lyrae pulsators in the
Oo.I globular cluster M5. B V curves of light for 15 RR Lyrae are presented.
With the addition of further 11 curves of light by Storm, Carney and Beck
(1991) one is dealing with a sample of 26 well studied cluster pulsators whose
properties have been implemented with similar data for RR Lyrae in clusters M3,
M15, M68 to allow a comparison with the theoretical scenario recently presented
by Bono and Stellingwerf (1994). On this basis, we discuss the distribution of
stars in the period amplitude diagram, disclosing a substantial reduction of
Sandage's period shift. We suggest that theoretical constraints concerning
periods and amplitudes could allow information on masses and luminosity of the
pulsators directly from Bailey's diagram only. Static temperatures have been
derived for all stars in the sample, discussing the dependence on the
temperature of the observed pulsational properties.Comment: postscript file of 17 pages of text plus 7 figures and 2 tables. For
any problem please write to [email protected]
Stellar models for very low mass main sequence stars: the role of model atmospheres
We present Very Low Mass stellar models as computed including non-grey model
atmospheres for selected assumptions about the star metallicities. The role of
atmospheres is discussed and the models are compared with models based on the
Eddington approximation and with similar models appeared in the recent
literature. Theoretical predictions concerning both the HR diagram location and
the mass-luminosity relation are presented and discussed in terms of
expectations in selected photometric bands. Comparison with available
observational data concerning both galactic globular clusters and dwarfs in the
solar neighborhood reveals a satisfactory agreement together with the existence
of some residual mismatches.Comment: 10 pages including 13 figures, gzip postscript file, To be published
in MNRA
Predicted HST FOC and broad band colours for young and intermediate Simple Stellar Populations
This paper presents theoretical HST and broad band colours from population
synthesis models based on an homogeneous set of stellar evolutionary tracks as
computed under canonical (no overshooting) assumptions, covering the range of
cluster ages from t=8 Myr to t=5 Gyr for three different metallicities (Z=0.02,
0.006, and 0.001). Statistical fluctuations in the cluster population have been
investigated, assessing the predicted fluctuations of the various colours as a
function of the cluster integrated absolute magnitude. We show that the red
leak in HST UV filters deeply affects the predicted fluxes and colours.
However, we find that for F152M-F307M < 0.5 and for F170M-F278M < 0.5 (which
means ages lower than 1 Gyr) the HST UV colours can still be used to infer
reliable indications on the age of distant clusters. Moreover, one finds that
the age calibration of these colours is scarcely affected by the amount of
original helium or by the assumed IMF. On this basis, we present a calibration
of the HST UV two-colours (F152M-F307M vs F170M-F278M) in terms of cluster ages
for the three above quoted metallicities. We suggest the combined use of HST UV
colours and IR colours (V-K in particular) to disentangle the metallicity-age
effect in integrated colours of young stellar populations (t< 1 Gyr).Comment: Latex 18 pages, 16 encapsulated figures, 6 tables, A&ASS accepte
Synthetic Stellar Clusters for Pop III
We present preliminary results of an incoming theoretical work concerning the
integrated properties of the Population III clusters of stars. On the basis of
synthetic Color-Magnitude Diagrams, we provide a grid of optical and near-IR
colors of Simple Stellar Populations with very low metallicity (Z=10
and Z=10) and age which spans from 10 Myr to 15 Gyr. A comparison with
higher metallicities up to 0.006 is also shown, disclosing sizable differences
in the CMD morphology, integrated colors and Spectral Energy Distribution
(SED).Comment: 2 pages, incl. 2 figures, "The First Stars", Proceedings of the
second MPA/ESO workshop, Eds.: Weiss, Abel, Hill, Springer, Heidelberg, 200
On the impact of Helium abundance on the Cepheid Period-Luminosity and Wesenheit relations and the Distance Ladder
This work analyses the effect of the Helium content on synthetic
Period-Luminosity Relations (PLRs) and Period-Wesenheit Relations (PWRs) of
Cepheids and the systematic uncertainties on the derived distances that a
hidden population of He-enhanced Cepheids may generate. We use new stellar and
pulsation models to build a homogeneous and consistent framework to derive the
Cepheid features. The Cepheid populations expected in synthetic color-magnitude
diagrams of young stellar systems (from 20 Myr to 250 Myr) are computed in
several photometric bands for Y = 0.25 and Y = 0.35, at a fixed metallicity (Z
= 0.008). The PLRs appear to be very similar in the two cases, with negligible
effects (few %) on distances, while PWRs differ somewhat, with systematic
uncertainties in deriving distances as high as about 7% at log P < 1.5.
Statistical effects due to the number of variables used to determine the
relations contribute to a distance systematic error of the order of few
percent, with values decreasing from optical to near-infrared bands. The
empirical PWRs derived from multi-wavelength datasets for the Large Magellanic
Cloud (LMC) is in a very good agreement with our theoretical PWRs obtained with
a standard He content, supporting the evidence that LMC Cepheids do not show
any He effect
Surface Brightness Fluctuations: a theoretical point of view
We present new theoretical evaluations of optical and near-IR Surface
Brightness Fluctuations (SBF) magnitudes for single-burst stellar populations
in the age range t=5-15 Gyr and metallicity from Z_{\sun}/200 to 2Z_{\sun}.
Our theoretical predictions can be successfully used to derive reliable
distance evaluations. They also appear to be a new and valuable tool to trace
the properties of unresolved stellar populations.Comment: 2 pages, incl. 1 figure, uses newpasp.sty, to be published in ``New
Horizons in Globular Cluster Astronomy'', ASP Conference Series, 2002; Eds.:
G. Piotto, G. Meylan, G. Djorgowski and M. Riello, in pres
Multipopulation aftereffects on the color-magnitude diagram and Cepheid variables of young stellar systems
Context: The evidence of a multipopulation scenario in Galactic globular
clusters raises several questions about the formation and evolution of the two
(or more) generations of stars. These populations show differences in their age
and chemical composition. These differences are found in old- and intermediate-
age stellar clusters in the Local Group. The observations of young stellar
systems are expected to present footprints of multiple stellar populations.
Aims: This theoretical work intends to be a specific step in exploring the
space of the observational indicators of multipopulations, without covering all
the combinations of parameters that may contribute to the formation of multiple
generations of stars in a cluster or in galaxy. The goal is to shed light on
the possible observational features expected by core He-burning stars that
belong to two stellar populations with different original He content and ages.
Methods: The tool adopted was the stellar population synthesis. We used new
stellar and pulsation models to construct a homogeneous and consistent
framework. Synthetic color-magnitude diagrams (CMDs) of young- and
intermediate-age stellar systems (from 20 Myr up to 1 Gyr) were computed in
several photometric bands to derive possible indicators of double populations
both in the observed CMDs and in the pulsation properties of the Cepheids.
Results: We predict that the morphology of the red/blue clump in VIK bands
can be used to photometrically indicate the two stellar populations in a rich
assembly of stars if there is a significant difference in their original He
content. Moreover, the period distribution of the Cepheids appears to be widely
affected by the coeval multiple generations of stars within stellar systems. We
show that the Wesenheit relations may be affected by the helium content of the
Cepheids.Comment: in press on A&
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