369 research outputs found
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
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 observational properties of He-burning stars: some clues on the tilt of the HB in metal rich clusters
We investigate the predicted Color-Magnitude distribution of metal-rich
Horizontal Branch (HB) stars, discussing selected theoretical models computed
under various assumptions about the star metallicity and the efficiency of
super-adiabatic convection. We find that canonical Zero Age Horizontal Branches
with metallicity larger or of the order of Z=0.002 should be all affected by a
tilt, by an amount which increases when the metallicity is increased and/or the
mixing length is decreased, reaching a tilt of 0.2 mag in the
case of solar metallicity when a mixing length value =1.6 is assumed
( is the magnitude difference between the top of the blue HB and the
fainter magnitude reached by the red HB). Uncertainties in the luminosity of
the red HB due to uncertainty in the mixing length value are discussed. We
finally discuss the much larger tilt observed in the clusters NGC 6441 and NGC
6388, reporting additional evidence against suggested non-canonical
evolutionary scenarios. Numerical experiments show that differential reddening
could produce such sloped HBs. Further, HST-PC imaging of NGC 6441 gives clear
indications about the occurrence of differential reddening across the cluster.
However, the same imaging shows that the observed slope of the red HB {\em is
not} an artifact of differential reddening. We finally show that sloping red
HBs in metal rich clusters are a common occurrence not necessarily correlated
with the appearance of extended blue HB.Comment: 14 pages, 9 figures, Accepted by Ap
Detection of Radial Surface Brightness Fluctuation and Color Gradients in elliptical galaxies with ACS
We study surface brightness fluctuations (SBF) in a sample of 8 elliptical
galaxies using Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) data
drawn from the Hubble Space Telescope (HST) archive. SBF magnitudes in the
F814W bandpass, and galaxy colors from F814W, F435W, and F606W images -- when
available -- are presented. Galaxy surface brightness profiles are determined
as well. We present the first SBF--broadband color calibration for the ACS/WFC
F814W bandpass, and (relative) distance moduli estimates for 7 of our galaxies.
We detect and study in detail the SBF variations within individual galaxies
as a probe of possible changes in the underlying stellar populations.
Inspecting both the SBF and color gradients in comparison to model predictions,
we argue that SBF, and SBF-gradients, can in principle be used for unraveling
the different evolutionary paths taken by galaxies, though a more comprehensive
study of this issue would be required. We confirm that the radial variation of
galaxy stellar population properties should be mainly connected to the presence
of radial chemical abundance gradients, with the outer galaxy regions being
more metal poor than the inner ones.Comment: 47 pages, 13 figures, ApJ, accepte
Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study
We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in
the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The
sample includes 14 galaxies covering a wide range of physical properties:
morphology, total absolute magnitude, integrated color. We take advantage of
the latter characteristic of the sample to check existing empirical
calibrations of absolute SBF magnitudes both in the I- and V-passbands.
Additionally, by comparing our SBF and color data with the Teramo-SPoT simple
stellar population models, and other recent sets of population synthesis
models, we discuss the feasibility of stellar population studies based on
fluctuation magnitudes analysis. The main result of this study is that
multiband optical SBF data and integrated colors can be used to significantly
constrain the chemical composition of the dominant stellar system in the
galaxy, but not the age in the case of systems older than 3 Gyr.
SBF color gradients are also detected and analyzed. These SBF gradient data,
together with other available data, point to the existence of mass dependent
metallicity gradients in galaxies, with the more massive objects showing a
non--negligible SBF versus color gradient. The comparison with models suggests
that such gradients imply more metal rich stellar populations in the galaxies'
inner regions with respect to the outer ones.Comment: ApJ Accepte
HST observations of the LMC field around SN87A: distance determinations with Red Clump and Tip of the Red Giant Branch stars
We have used HST-WFPC2 multiband observations of a field around SN 1987A in
the Large Magellanic Cloud to measure its distance from the Sun. The
observations allowed us to carefully determine the interstellar extinction
along the line of sight to a large number of stars and to measure the LMC
distance by using two stellar distance indicators: the Red Clump and the Tip of
the Red Giant Branch. From an application of the Red Clump method we obtain a
distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus
systematic error), in good agreement with the distance derived by using the Tip
of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag
(statistical plus systematic error). Both values agree well with the distance
to the SN 1987A as determined from a study of its inner ring fluorescent echo
((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli
lower than 18.43 to a 99.7% significance level. Differences with respect to
previous results obtained using the same distance indicators are discussed.Comment: 7 pages including 3 figures; Astrophysical Journal, accepted for
publicatio
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