2,328 research outputs found
The Initial Helium Abundance of the Galactic Globular Cluster System
We estimate the initial He content in about 30% of the Galactic globular
clusters (GGCs) from new star counts we have performed on the recently
published HST snapshot database of Colour Magnitude Diagrams (Piotto et al.
2002). More in detail, we use the so-called -parameter and estimate the He
content from a calibration based on a recently updated set of stellar models.
We performed an accurate statistical analysis in order to assess whether GGCs
show a statistically significant spread in their initial He abundances, and
whether there is a correlation with the metallicity. We do not find any
significant dependence of the He abundance on the GC metallicity; this provides
an important constraint for models of Galaxy formation and evolution. Apart
from GGCs with the bluest HB morphology, the observed spread in the individual
He abundances is statistically compatible with the individual errors. This
means that either there is no intrinsic He spread among the GGCs, or that this
is masked by the errors. In the latter case we have estimated a firm 1
upper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with
the bluest HB morphology we detect a significant spread towards higher
abundances inconsistent with the individual errors. In the hypothesis that the
intrinsic dispersion on the individual He abundances is zero, taking into
account the errors on the individual R-parameter estimates, as well as the
uncertainties on the GGC [M/H] scale and theoretical calibration, we have
determined an initial He abundance Y(GGC)=0.250\pm0.006 a value in perfect
agreement with current estimates based on CMB radiation analyses and
cosmological nucleosynthesis computations.Comment: 10 pages, 4 figures, in press on Astronomy & Astrophysic
The Luminosity and Mass Function of the Globular Cluster NGC1261
I-band CCD images of two large regions of the Galactic globular cluster NGC
1261 have been used to construct stellar luminosity functions (LF) for 14000
stars in three annuli from 1.4' from the cluster center to the tidal radius.
The LFs extend to M_I~8 and tend to steepen from the inner to the outer
annulus, in agreement with the predictions of the multimass King-Michie model
that we have calculated for this cluster. The LFs have been transformed into
mass functions. Once corrected for mass segregation the global mass function of
NGC 1261 has a slope x_0=0.8+/-0.5Comment: 9 pages, A&A macros, accepted for publication in A&
Spectroscopy of horizontal branch stars in Omega Centauri
We analyze the reddening, surface helium abundance and mass of 115 horizontal
branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the
blue edge of the instability strip to Teff~50000K. The mean cluster reddening
is E(B-V)=0.115+-0.004, in good agreement with previous estimates, but we
evidence a pattern of differential reddening in the cluster area. The stars in
the western half are more reddened than in the southwest quadrant by 0.03-0.04
magnitudes. We find that the helium abundances measured on low-resolution
spectra are systematically lower by ~0.25 dex than the measurements based on
higher resolution. No difference in helium abundance is detected between
OmegaCentauri and three comparison clusters, and the stars in the range
11500-20000K follow a trend with temperature, which probably reflects a
variable efficiency of the diffusion processes. There is mild evidence that two
families of extreme HB (EHB) stars (Teff>20000K) could exist, as observed in
the field, with ~15% of the objects being helium depleted by a factor of ten
with respect to the main population. The distribution of helium abundance above
30000K is bimodal, but we detect a fraction of He-poor objects lower than
previous investigations. The observations are consistent with these being stars
evolving off the HB. Their spatial distribution is not uniform, but this
asymmetric distribution is only marginally significative. We also find that EHB
stars with anomalously high spectroscopic mass could be present in
OmegaCentauri, as previously found in other clusters. The derived
temperature-color relation reveals that stars hotter than 11000K are fainter
than the expectations of the canonical models in the U band, while no anomaly
is detected in B and V. This behavior, not observed in NGC6752, is a new
peculiarity of OmegaCentauri HB stars.Comment: Accepted for publication in A&
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
Galactic Globular Clusters as a test for Very Low-Mass stellar models
We make use of the Next Generation model atmospheres by Allard et al. (1997)
and Hauschildt, Allard & Baron (1999) to compute theoretical models for low and
very low-mass stars for selected metallicities in the range Z= 0.0002 to 0.002.
On this basis, we present theoretical predictions covering the sequence of
H-burning stars as observed in galactic globulars from the faint end of the
Main Sequence up to, and beyond, the cluster Turn Off. The role played by the
new model atmospheres is discussed, showing that present models appear in
excellent agreement with models by Baraffe et al. (1997) as computed on quite
similar physical basis. One finds that the theoretical mass-luminosity
relations based on this updated set of models, are in good agreement with the
empirical data provided by Henry & McCarthy (1993). Comparison with HST
observation discloses that the location in the Color-Magnitude diagram of the
lower Main Sequence in galactic globular clusters appears again in good
agreement with the predicted sensitive dependence of these sequences on the
cluster metallicity.Comment: accepted for pubblication on MNRA
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