2,263 research outputs found
Hot Stellar Populations in Globular Clusters: a Photometrist's View
We briefly review the recent results on hot horizontal-branch stars in
globular clusters. Since the first Ivanfest, in 1992, there have been a number
of new observational lines of evidence which have allowed significant progress
in our understanding of blue-tail stars, though new, even more intriguing
questions arise. Despite this progress, we still do not know the answer to the
main question: why are there blue-HB-tail stars? The new photometric data bases
collected in the last few years, and the forthcoming multi-fiber observational
campaigns on 10m-class telescopes, might be the key to solve this puzzle. We
will show an example of how these data bases can disclose important properties
of blue HB stars.Comment: 12 pages, 5 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200
Comparison between Observed and Theoretical Red Giant Branch Luminosity Functions of Galactic Globular Clusters
V-band luminosity functions have been obtained for the upper main-sequence,
sub-giant branch and red giant branch of 18 galactic globular clusters from HST
data. A comparison with four sets of theoretical models has been performed. In
contrast with what was found in several previous works, a Good general
agreement has been found between the observed and theoretical LF at any
metallicity [M/H]<-0.7. Possible discrepancies at higher metallicity, in the
upper part of the RGB, need to be confirmed with further observational data and
by extending all the models to the most metal rich regime. The SGB shape has
been used to set an upper limit to the cluster age, and consequently a lower
limit on the cluster distance. A discussion on the still open problem of the
mismatch between the observed and theoretical RGB bump location is also
presented.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
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
Photometric catalog of nearby globular clusters (I)
We present the first part of the first large and homogeneous CCD
color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular
clusters (GGC) imaged in the V and I bands using only two telescopes (one for
each hemisphere). The observed clusters represent 75% of the known Galactic
globulars with (m-M)v<=16.15 mag, cover most of the globular cluster
metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances
from ~1.2 to ~18.5 kpc.
In this paper, the CMDs for the 39 GGCs observed in the southern hemisphere
are presented. The remaining 13 northern hemisphere clusters of the catalog are
presented in a companion paper. For four clusters (NGC 4833, NGC 5986, NGC
6543, and NGC 6638) we present for the first time a CMD from CCD data. The
typical CMD spans from the 22nd V magnitude to the tip of the red giant branch.
Based on a large number of standard stars, the absolute photometric calibration
is reliable to the ~0.02 mag level in both filters.
This catalog, because of its homogeneity, is expected to represent a useful
data base for the measurement of the main absolute and relative parameters
characterizing the CMD of GGCs.Comment: 34 pages, 108 figures, Astronomy & Astrophysics Supplement Series, in
press. Full resolution figures can be obtained from the authors upon reques
A PSF-based approach to Kepler/K2 data. I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158
Kepler and K2 data analysis reported in the literature is mostly based on
aperture photometry. Because of Kepler's large, undersampled pixels and the
presence of nearby sources, aperture photometry is not always the ideal way to
obtain high-precision photometry and, because of this, the data set has not
been fully exploited so far. We present a new method that builds on our
experience with undersampled HST images. The method involves a point-spread
function (PSF) neighbour-subtraction and was specifically developed to exploit
the huge potential offered by the K2 "super-stamps" covering the core of dense
star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed
during the K2 Campaign 0. We present our PSF modeling and demonstrate that, by
using a high-angular-resolution input star list from the Asiago Schmidt
telescope as the basis for PSF neighbour subtraction, we are able to reach
magnitudes as faint as Kp~24 with a photometric precision of 10% over 6.5
hours, even in the densest regions. At the bright end, our photometric
precision reaches ~30 parts-per-million. Our method leads to a considerable
level of improvement at the faint magnitudes (Kp>15.5) with respect to the
classical aperture photometry. This improvement is more significant in crowded
regions. We also extracted raw light curves of ~60,000 stars and detrended them
for systematic effects induced by spacecraft motion and other artifacts that
harms K2 photometric precision. We present a list of 2133 variables.Comment: 27 pages (included appendix), 2 tables, 25 figures (5 in low
resolution). Accepted for publication in MNRAS on November 05, 2015. Online
materials will be available on the Journal website soo
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