2,224 research outputs found
The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
Photometry with HST's ACS reveals that the subgiant branch (SGB) of the
globular cluster NGC 1851 splits into two well-defined branches. If the split
is due only to an age effect, the two SGBs would imply two star formation
episodes separated by 1 Gyr. We discuss other anomalies in NGC 1851
which could be interpreted in terms of a double stellar population. Finally, we
compare the case of NGC 1851 with the other two globulars known to host
multiple stellar populations, and show that all three clusters differ in
several important respects.Comment: 22 pages, 7 figures, accepted for pubblication on Ap
Multiple stellar populations in the Galactic globular cluster NGC 6752
We have carried out high-precision photometry on a large number of archival
HST images of the Galactic globular cluster NGC 6752, to search for signs of
multiple stellar populations. We find a broadened main sequence, and
demonstrate that this broadening cannot be attributed either to binaries or to
photometric errors. There is also some indication of a main-sequence split. No
significant spread could be found along the subgiant branch, however.
Ground-based photometry reveals that in the U vs. (U-B) color-magnitude
diagram the red-giant branch exhibits a clear color spread, which we have been
able to correlate with variations in Na and O abundances. In particular the
Na-rich, O-poor stars identified by Carretta et al. (2007) define a sequence on
the red side of the red-giant branch, while Na-poor, O-rich stars populate a
bluer, more dispersed portion of the red-giant branch.Comment: 31 pages, 12 figures; Accepted for Publication in the Astrophysical
Journa
Global and non-global parameters of horizontal branch morphology of globular clusters
The horizontal branch (HB) morphology of globular clusters (GCs) is mainly
determined by metallicity. However, the fact that GCs with almost the same
metallicity exhibit different HB morphologies demonstrates that at least one
more parameter is needed to explain the HB morphology. It has been suggested
that one of these should be a global parameter that varies from GC to GC, and
the other a non-global parameter that varies within the GC. In this study we
provide empirical evidence corroborating this idea. We used the photometric
catalogs obtained with the Advanced Camera for Surveys (ACS) of the Hubble
Space Telescope (HST) and analyse the CMDs of 74 GCs. The HB morphology of our
sample of GCs has been investigated on the basis of the two new parameters L1
and L2 that measure the distance between the RGB and the coolest part of the
HB, and the color extension of the HB, respectively. We find that L1 correlates
with both metallicity and age, whereas L2 most strongly correlates with the
mass of the hosting GC. The range of helium abundance among the stars in a GC,
characterised by Delta Y and associated with the presence of multiple stellar
populations, has been estimated in a few GCs to date. In these GCs we find a
close relationship among Delta Y, GC mass, and L2. We conclude that age and
metallicity are the main global parameters while the range of helium abundance
within a GC is the main non-global parameter defining the HB morphology of
Galactic GCs.Comment: 34 pages, 13 figures, accepted for publication in Ap
The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters
We present new measurements of the centers for 65 Milky Way globular
clusters. Centers were determined by fitting ellipses to the density
distribution within the inner 2\arcmin of the cluster center, and averaging
the centers of these ellipses. The symmetry of clusters was also analyzed by
comparing cumulative radial distributions on opposite sides of the cluster
across a grid of trial centers. All of the determinations were done with
stellar positions derived from a combination of two single-orbit ACS images of
the core of the cluster in and . We find that the
ellipse-fitting method provides remarkable accuracy over a wide range of core
sizes and density distributions, while the symmetry method is difficult to use
on clusters with very large cores, or low density. The symmetry method requires
a larger field, or a very sharply peaked density distribution.Comment: 17 pages, 8 figures, Accepted for publication in AJ, supplementary
material will be available upon publicatio
Mapping the differential reddening in globular clusters
We build differential-reddening maps for 66 Galactic globular clusters (GCs)
with archival HST WFC/ACS F606W and F814W photometry. Because of the different
GC sizes (characterised by the half-light radius ) and distances to the
Sun, the WFC/ACS field of view (200\arcsec\times200\arcsec) coverage
() lies in the range 1\la R_{obs}/R_h\la15 for about 85% of the
sample, with about 10% covering only the inner (R_{obs}\la R_h) parts. We
divide the WFC/ACS field of view across each cluster in a regular cell grid,
and extract the stellar-density Hess diagram from each cell, shifting it in
colour and magnitude along the reddening vector until matching the mean
diagram. Thus, the maps correspond to the internal dispersion of the reddening
around the mean. Depending on the number of available stars (i.e. probable
members with adequate photometric errors), the angular resolution of the maps
range from \approx7\arcsec\times7\arcsec to
\approx20\arcsec\times20\arcsec. We detect spatially-variable extinction in
the 66 globular clusters studied, with mean values ranging from
\mEBV\approx0.018 (NGC\,6981) up to \mEBV\approx0.16 (Palomar\,2).
Differential-reddening correction decreases the observed foreground reddening
and the apparent distance modulus but, since they are related to the same value
of \EBV, the distance to the Sun is conserved. Fits to the mean-ridge lines of
the highly-extincted and photometrically scattered globular cluster Palomar\,2
show that age and metallicity also remain unchanged after the
differential-reddening correction, but measurement uncertainties decrease
because of the reduced scatter. The lack of systematic variations of \mEBV\
with both the foreground reddening and the sampled cluster area indicates that
the main source of differential reddening is interstellar.Comment: Accepted by MNRA
The helium spread in the Globular cluster 47 Tuc
Spectroscopy has shown the presence of the CN band dicothomy and the Na-O
anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc,
otherwise considered a "normal" prototype of high metallicity clusters from the
photometric analysis. Very recently, the re-analysis of a large number of
archival HST data of the cluster core has been able to put into evidence the
presence of structures in the Sub Giant Branch: it has a brighter component
with a spread in magnitude by 0.06 mag and a second one, made of about
10% of stars, a little fainter (by 0.05 mag). These data also show that
the Main Sequence of the cluster has an intrinsic spread in color which, if
interpreted as due to a small spread in helium abundance, suggests
Y0.027. In this work we examine in detail whether the Horizontal
Branch morphology and the Sub Giant structure provide further independent
indications that a real --although very small-helium spread is present in the
cluster. We re--analyze the HST archival data for the Horizontal Branch of 47
Tuc, obtaining a sample of 500 stars with very small photometric errors,
and build population synthesis based on new models to show that its particular
morphology can be better explained by taking into account a spread in helium
abundance of 2% in mass. The same variation in helium is able to explain the
spread in luminosity of the Sub Giant Branch, while a small part of the second
generation is characterized by a small C+N+O increase and provides an
explanation for the fainter Sub Giant Branch. We conclude that three
photometric features concur to form the paradigm that a small but real helium
spread is present in a cluster that has no spectacular evidence for multiple
populations like those shown by other massive clusters.Comment: Accepted for publication in the MNRAS on 2010 June 8. Received 2010
May 19; in original form 2010 February 9. 7 pages and 3 figures. No table
Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results
We present the first study of high-precision internal proper motions (PMs) in
a large sample of globular clusters, based on Hubble Space Telescope (HST) data
obtained over the past decade with the ACS/WFC, ACS/HRC, and WFC3/UVIS
instruments. We determine PMs for over 1.3 million stars in the central regions
of 22 clusters, with a median number of ~60,000 stars per cluster. These PMs
have the potential to significantly advance our understanding of the internal
kinematics of globular clusters by extending past line-of-sight (LOS) velocity
measurements to two- or three-dimensional velocities, lower stellar masses, and
larger sample sizes. We describe the reduction pipeline that we developed to
derive homogeneous PMs from the very heterogeneous archival data. We
demonstrate the quality of the measurements through extensive Monte-Carlo
simulations. We also discuss the PM errors introduced by various systematic
effects, and the techniques that we have developed to correct or remove them to
the extent possible. We provide in electronic form the catalog for NGC 7078 (M
15), which consists of 77,837 stars in the central 2.4 arcmin. We validate the
catalog by comparison with existing PM measurements and LOS velocities, and use
it to study the dependence of the velocity dispersion on radius, stellar
magnitude (or mass) along the main sequence, and direction in the plane of the
sky (radial/tangential). Subsequent papers in this series will explore a range
of applications in globular-cluster science, and will also present the PM
catalogs for the other sample clusters.Comment: 34 pages, 22 figures (3 in low res), 30 tables, accepted for
publication in ApJ on October 20, 201
MCAO near-IR photometry of the Globular Cluster NGC 6388: MAD observations in crowded fields
Deep photometry of crowded fields, such as Galactic Globular Clusters, is
severely limited by the actual resolution of ground-based telescopes. On the
other hand, the Hubble Space Telescope does not provide the near-infrared (NIR)
filters needed to allow large color baselines. In this work we aim at
demonstrating how ground based observations can reach the required resolution
when using Multi-Conjugated Adaptive Optic (MCAO) devices in the NIR, such as
the experimental infrared camera (MAD) available on the VLT. This is
particularly important since these corrections are planned to be available on
all ground--based telescopes in the near future. We do this by combining the
infrared photometry obtained by MAD/VLT with ACS/HST optical photometry of our
scientific target, the bulge globular cluster NGC 6388, in which we imaged two
fields. In particular, we constructed color-magnitude diagrams with an
extremely wide color baseline in order to investigate the presence of multiple
stellar populations in this cluster. From the analysis of the external field,
observed with better seeing conditions, we derived the deepest optical-NIR CMD
of NGC 6388 to date. The high-precision photometry reveals that two distinct
sub-giant branches are clearly present in this cluster. We also use the CMD
from the central region to estimate the distance ((m-M)=15.33) and the
reddening (E(B-V)=0.38) for this cluster. We estimate the age to be ~11.5+/-
1.5 Gyr. The large relative-age error reflects the bimodal distribution of the
SGB stars. This study clearly demonstrates how MCAO correction in the NIR bands
implemented on ground based telescopes can complement the high-resolution
optical data from HST.Comment: 9 pages, 10 figures. Accepted for publication on A. &
A WFC3/HST view of the three stellar populations in the Globular Cluster NGC6752
Multi-band Hubble Space Telescope photometry reveals that the main sequence,
sub-giant, and the red giant branch of the globular cluster NGC6752 splits into
three main components in close analogy with the three distinct segments along
its horizontal branch stars. These triple sequences are consistent with three
stellar groups: a stellar population with a chemical composition similar to
field halo stars (population a), a population (c) with enhanced sodium and
nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y
~0.03), and a population (b) with an intermediate (between population a and c)
chemical composition and slightly helium enhanced (Delta Y ~0.01). These
components contain ~25% (population a), ~45% (population b), and ~30%
(population c) of the stars. No radial gradient for the relative numbers of the
three populations has been identified out to about 2.5 half mass radii.Comment: 42 pages, 24 figures, accepted for publication in Ap
The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. III. A quintuple stellar population in NGC2808
In this study we present first results from multi-wavelength Hubble Space
Telescope (HST) observations of the Galactic globular cluster GC NGC2808 as an
extension of the Hubble Space Telescope UV Legacy Survey of Galactic GCs
(GO-13297 and previous proprietary and HST archive data). Our analysis allowed
us to disclose a multiple-stellar-population phenomenon in NGC2808 even more
complex than previously thought. We have separated at least five different
populations along the main sequence and the red giant branch (RGB), that we
name A, B, C, D and E (though an even finer subdivision may be suggested by the
data). We identified the RGB bump in four out of the five RGBs. To explore the
origin of this complex CMD, we have combined our multi-wavelength HST
photometry with synthetic spectra, generated by assuming different chemical
compositions. The comparison of observed colors with synthetic spectra suggests
that the five stellar populations have different contents of light elements and
helium. Specifically, if we assume that NGC2808 is homogeneous in [Fe/H] (as
suggested by spectroscopy for Populations B, C, D, E, but lacking for
Population A) and that population A has a primordial helium abundance, we find
that populations B, C, D, E are enhanced in helium by Delta Y~0.03, 0.03, 0.08,
0.13, respectively. We obtain similar results by comparing the magnitude of the
RGB bumps with models. Planned spectroscopic observations will test whether
also Population A has the same metallicity, or whether its photometric
differences with Population B can be ascribed to small [Fe/H] and [O/H]
differences rather than to helium.Comment: 49 pages, 19 figure, accepted for publication in Ap
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