Publications (6)0 Total impact
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A. P. Milone,
G. Piotto,
L. R. Bedin,
A. Aparicio,
J Anderson,
A. Sarajedini,
A. F. Marino,
A. Moretti,
M. B. Davies,
B. Chaboyer,
A. Dotter,
M. Hempel, A. Marin-Franch,
S Majewski,
N. E. Q. Paust,
I. N. Reid,
A. Rosenberg,
M. Siegel
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ABSTRACT: The fraction of binary stars is an important ingredient to interpret globular
cluster dynamical evolution and their stellar population. We investigate the
properties of main-sequence binaries measured in a uniform photometric sample
of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of
the Globular Cluster Treasury project. We measured the fraction of binaries and
the distribution of mass-ratio as a function of radial location within the
cluster, from the central core to beyond the half-mass radius. We studied the
radial distribution of binary stars, and the distribution of stellar mass
ratios. We investigated monovariate relations between the fraction of binaries
and the main parameters of their host clusters. We found that in nearly all the
clusters, the total fraction of binaries is significantly smaller than the
fraction of binaries in the field, with a few exceptions only. Binary stars are
significantly more centrally concentrated than single MS stars in most of the
clusters studied in this paper. The distribution of the mass ratio is generally
flat (for mass-ratio parameter q>0.5). We found a significant anti-correlation
between the binary fraction in a cluster and its absolute luminosity (mass).
Some, less significant correlation with the collisional parameter, the central
stellar density, and the central velocity dispersion are present. There is no
statistically significant relation between the binary fraction and other
cluster parameters. We confirm the correlation between the binary fraction and
the fraction of blue stragglers in the cluster.
11/2011;
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Michael H. Siegel,
Steven R. Majewski,
David R. Law,
Ata Sarajedini,
Aaron Dotter, A. Marin-Franch,
Brian Chaboyer,
Jay Anderson,
Antonio Aparicio,
Luigi R. Bedin,
Maren Hempel,
Antonino Milone,
Nathaniel Paust,
Giampaolo Piotto,
I. Neill Reid,
Alfred Rosenberg
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ABSTRACT: We use observations from the ACS study of Galactic globular clusters to
investigate the spatial distribution of the inner regions of the disrupting
Sagittarius dwarf spheroidal galaxy (Sgr). We combine previously published
analyses of four Sgr member clusters located near or in the Sgr core (M54, Arp
2, Terzan 7 and Terzan 8) with a new analysis of diffuse Sgr material
identified in the background of five low-latitude Galactic bulge clusters (NGC
6624, 6637, 6652, 6681 and 6809) observed as part of the ACS survey. By
comparing the bulge cluster CMDs to our previous analysis of the M54/Sgr core,
we estimate distances to these background features. The combined data from four
Sgr member clusters and five Sgr background features provides nine independent
measures of the Sgr distance and, as a group, provide uniformly measured and
calibrated probes of different parts of the inner regions of Sgr spanning
twenty degrees over the face of the disrupting dwarf. This allows us, for the
first time, to constrain the three dimensional orientation of Sgr's disrupting
core and globular cluster system and compare that orientation to the
predictions of an N-body model of tidal disruption. The density and distance of
Sgr debris is consistent with models that favor a relatively high Sgr core mass
and a slightly greater distance (28-30 kpc, with a mean of 29.4 kpc). Our
analysis also suggests that M54 is in the foreground of Sgr by ~2 kpc,
projected on the center of the Sgr dSph. While this would imply a remarkable
alignment of the cluster and the Sgr nucleus along the line of sight, we can
not identify any systematic effect in our analysis that would falsely create
the measured 2 kpc separation. Finally, we find that the cluster Terzan 7 has
the most discrepant distance (25 kpc) among the four Sgr core clusters, which
may suggest a different dynamical history than the other Sgr core clusters.
08/2011;
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Ata Sarajedini,
L. Bedin,
B. Chaboyer,
A. Dotter,
M. Siegel,
J Anderson,
A. Aparicio,
I. King,
S Majewski, A. Marin-Franch,
G. Piotto,
I. N. Reid,
A. Rosenberg
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ABSTRACT: We present the first results of a large ACS Survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N > ~10 for main sequence stars with masses > ~0.2Msun in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, 6779, 5053, 6144, Palomar 2, E 3, Lynga 7, Palomar 1, and NGC 6366. Our CMDs extend reliably from the horizontal branch to as much as seven magnitudes fainter than the main sequence turnoff and represent the deepest CMDs published to-date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons along with fitting the cluster main sequences to theoretical isochrones yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E 3 which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.
01/2007;
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ABSTRACT: The faint end of the differential galaxy number counts, n(m), in the Hubble Deep Field (HDF) North has been determined for the F450W, F606W, and F814W filters by means of surface-brightness fluctuation (SBF) measurements. This technique allows us to explore n(m) beyond the limiting magnitude of the HDF, providing new, stronger constraints on the faint end of n(m). This has allowed us to test the validity of previous number count studies and to produce a new determination of the faint end of n(m) for magnitudes fainter than 28.8 in the AB system and to extend this estimate down to 31. This value represents an extension of more than two magnitudes beyond the limits of previous photometric studies. The obtained n(m) slopes are \gamma=0.27, 0.21, and 0.26 in B_{450}, V_{606}, and I_{814}, respectively. Comment: 36 pages, 7 figures and 10 tables, accepted for publication in ApJ
05/2003;
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ABSTRACT: The existence of an intergalactic globular cluster population in the Coma cluster of galaxies has been tested using surface-brightness fluctuations. The main result is that the intergalactic globular cluster surface density ($N_{\rm IGC}$) does not correlate with the distance to the center of Coma and hence with the environment. Furthermore, comparing these results with different Coma mass-distribution model predictions, it is suggested that $N_{\rm IGC}$ must in fact be zero all over Coma. On the other hand, the results for $N_{\rm IGC}$ and the faint end of the galaxy number counts (beyond $m_R=23.5$) are connected. So $N_{\rm IGC}=0$ settles the slope of this function, which turns out to be $\gamma=0.36\pm0.01$ down to $m_R=26.5$. The fact that $N_{\rm IGC}=0$ all over Coma suggests that globular clusters were formed only, or almost only, from protogalactic clouds. None, or perhaps very few, could have formed in isolated regions. It also seems inappropriate to advocate a relationship between intergalactic globular clusters and dark matter distributions, although it is true that the relationship could still exist but not be strong enough to have been detected. Finally, since our conclusion is that intergalactic globular clusters do not exist in Coma, accretion of intergalactic globular clusters might not be significant in galaxy formation and evolutionary processes in the Coma galaxies. Comment: 23 pages, 5 figures, accepted for publication in ApJ
11/2002;
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ABSTRACT: Globular cluster systems of 17 elliptical galaxies have been studied in the Coma cluster of galaxies. Surface-brightness fluctuations have been used to determine total populations of globular clusters and specific frequency (S_N) has been evaluated for each individual galaxy. Enormous differences in S_N between similar galaxies are found. In particular, S_N results vary by an order of magnitude from galaxy to galaxy. Extreme cases are the following: a) at the lower end of the range, NGC 4673 has S_N = 1.0 +/- 0.4, a surprising value for an elliptical galaxy, but typical for spiral and irregular galaxies; b) at the upper extreme, MCG +5 -31 -063 has S_N = 13.0 +/- 4.2 and IC 4051 S_N = 12.7 +/- 3.2, and are more likely to belong to supergiant cD galaxies than to "normal" elliptical galaxies. Furthermore, NGC 4874, the central supergiant cD galaxy of the Coma cluster, also exhibits a relatively high specific frequency (S_N = 9.0 +/- 2.2). The other galaxies studied have S_N in the range [2, 7], the mean value being S_N = 5.1. No single scenario seems to account for the observed specific frequencies, so the history of each galaxy must be deduced individually by suitably combining the different models (in situ, mergers, and accretions). The possibility that Coma is formed by several subgroups is also considered. If only the galaxies of the main subgroup defined by Gurzadyan & Mazure (2001) are used, a trend in S_N arises in the sense of S_N being bigger in higher density regions. This result needs further confirmation. Comment: 43 pages including 12 .ps figures, accepted for publication in ApJ
12/2001;
