29 research outputs found
BVI Photometry and the Luminosity Functions of the Globular Cluster M92
We present new BVI ground-based photometry and VI space-based photometry for the globular cluster M92 (NGC 6341) and examine luminosity functions in B, V, and I containing over 50,000 stars ranging from the tip of the red giant branch to several magnitudes below the main-sequence turnoff. Once corrected for completeness, the observed luminosity functions agree very well with theoretical models and do not show stellar excesses in any region of the luminosity function. Using reduced-Ï2 fitting, the new M92 luminosity function is shown to be an excellent match to the previously published luminosity function for M30. These points combine to establish that the subgiant excess found in previously published luminosity functions of Galactic globular clusters is due to deficiencies in the stellar models used at that time. Using up-to-date stellar models results in good agreement between observations and theory. Several statistical methods are presented to best determine the age of M92. These methods prove to be insensitive to the exact choice of metallicity within the published range. Using [Fe/H] = -2.17 to match recent studies we find an age of 14.2 ± 1.2 Gyr for the cluster
BVI Photometry and the Luminosity Function of the Globular Cluster M92
We present new BVI ground-based photometry and VI space-based photometry for
the globular cluster M92 (NGC 6341) and examine luminosity functions in B, V,
and I containing over 50,000 stars ranging from the tip of the red giant branch
to several magnitudes below the main sequence turn off. Once corrected for
completeness, the observed luminosity functions agree very well with
theoretical models and do not show stellar excesses in any region of the
luminosity function. Using reduced chi squared fitting, the new M92 luminosity
function is shown to be an excellent match to the previously published
luminosity function for M30. These points combine to establish that the
"subgiant excess" found in previously published luminosity functions of
Galactic globular clusters are due to deficiencies in the stellar models used
at that time. Using up to date stellar models results in good agreement between
observations and theory.
Several statistical methods are presented to best determine the age of M92.
These methods prove to be insensitive to the exact choice of metallicity within
the published range. Using [Fe/H]=-2.17 to match recent studies we find an age
of 14.2 plus or minus 1.2 Gyr for the cluster.Comment: 22 pages, 13 figures, 3 tables, accepted for publication in A
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The ACS Survey Of Globular Clusters. V. Generating A Comprehensive Star Catalog For Each Cluster
The ACS Survey of Globular Clusters has used Hubble Space Telescope's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous data set for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available point-spread function models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we ran a standard set of similar to 10(5) tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the World Wide Web.Astronom
The ACS Survey of Galactic Globular Clusters. viii. Effects of Environment on Globular Cluster Global Mass Functions
We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic Globular Clusters to construct global present-day mass functions for 17 globular clusters utilizing multi-mass King models to extrapolate from our observations to the global cluster behavior. The global present-day mass functions for these clusters are well matched by power laws from the turnoff, â0.8 M â, to 0.2-0.3 M â on the lower main sequence. The slopes of those power-law fits, α, have been correlated with an extensive set of intrinsic and extrinsic cluster properties to investigate which parameters may influence the form of the present-day mass function. We do not confirm previous suggestions of correlations between α and either metallicity or Galactic location. However, we do find a strong statistical correlation with the related parameters central surface brightness, ÎŒ V , and inferred central density, Ï0. The correlation is such that clusters with denser cores (stronger binding energy) tend to have steeper mass functions (a higher proportion of low-mass stars), suggesting that dynamical evolution due to external interactions may have played a key role in determining α. Thus, the present-day mass function may owe more to nurture than to nature. Detailed modeling of external dynamical effects is therefore a requisite for determining the initial mass function for Galactic globular clusters
The ACS Survey of Galactic Globular Clusters. VI. NGC 6366: A Heavily Stripped Galactic Globular Cluster
We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs) to construct a color-magnitude diagram for the bulge cluster, NGC 6366. The luminosity function derived from those data extends to M F606W ~ 9, or masses of ~0.3 M â. Unlike most GCs, the mass function peaks near the main-sequence turnoff with significantly fewer low-mass stars even after correction for completeness and mass segregation. Using a multimass King model, we extrapolate the global cluster behavior and find the global mass function to be poorly matched by a power law, with a particular deficit of stars with masses between 0.5 and 0.7 M â. We briefly discuss this interesting anomaly within the context of tidal stripping
The ACS survey of globular clusters. XIII. Photometric calibration in comparison with Stetson standards
In this study we compare the photometric data of 34 Milky Way globular
clusters, observed within the ACS Treasury Program (PI: Ata Sarajedini) with
the corresponding ground-based data, provided by the Photometric Standard Field
Catalogs of Stetson (2000, 2005). We focus on the transformation between the
HST/ACS F606W to V-band and F814W to I-band only. The goal is to assess the
validity of the filter transformation equations by Sirianni et al.(2005) with
respect to their dependence on metallicity, Horizontal Branch morphology, mass
and integrated (V-I) colour of the various globular clusters. Such a dependence
is expected due to the fact that the transformation equations are based on the
observations of only one globular cluster, i.e., NGC 2419. Surprisingly, the
correlation between offset and metallicity is found to be weak, with a low
level significance. The correlation between offset and Horizontal Branch
structure, as well as total cluster mass is still weaker. Based on the
available data we do not find the photometric offset to be linked to multiple
stellar populations, e.g., as found in NGC 0288, NGC 1851, and NGC 5139. The
results of this study show that there are small systematic offsets between the
transformed ACS- and observed ground based photometry, and that these are only
weakly correlated, if at all, with various cluster parameters and their
underlying stellar populations. As a result, investigators wishing to transform
globular cluster photometry from the Sirianni et al.(2005) ground-based V, I
system onto the Stetson (2000) system simply need to add 0.040 (+/-0.012) to
the V-band magnitudes and 0.047 (+/-0.011) to the I-band magnitudes. This in
turn means that the transformed ACS (V-I) colours match the ground-based values
from Stetson (2000) to within ~0.01 mag.Comment: 28 pages, 14 figures, accepted for publication in ApJ
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The ACS Survey Of Galactic Globular Clusters. VI. NGC 6366: A Heavily Stripped Galactic Globular Cluster
We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs) to construct a color-magnitude diagram for the bulge cluster, NGC 6366. The luminosity function derived from those data extends to M(F606W) similar to 9, or masses of similar to 0.3 M(circle dot). Unlike most GCs, the mass function peaks near the main-sequence turnoff with significantly fewer low-mass stars even after correction for completeness and mass segregation. Using a multimass King model, we extrapolate the global cluster behavior and find the global mass function to be poorly matched by a power law, with a particular deficit of stars with masses between 0.5 and 0.7 M(circle dot). We briefly discuss this interesting anomaly within the context of tidal stripping.NASA GO-10775, 5-26555Space Telescope Science InstituteInstituto de Astrofisica de Canarias P3-94Ministry of Education and Science of the Kingdom of Spain AYA-2008-67913Astronom
The ACS Survey of Galactic Globular Clusters XI: The Three-Dimensional Orientation of the Sagittarius Dwarf Spheroidal Galaxy and its Globular Clusters
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.Comment: 41 pages, 16 figures, accepted to Ap
Deep ACS Imaging in the Globular Cluster NGC 6397: The Cluster Color Magnitude Diagram and Luminosity Function
We present the CMD from deep HST imaging in the globular cluster NGC 6397.
The ACS was used for 126 orbits to image a single field in two colors (F814W,
F606W) 5 arcmin SE of the cluster center. The field observed overlaps that of
archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM)
clean the data. Applying the PM corrections produces a remarkably clean CMD
which reveals a number of features never seen before in a globular cluster CMD.
In our field, the main sequence stars appeared to terminate close to the
location in the CMD of the hydrogen-burning limit predicted by two independent
sets of stellar evolution models. The faintest observed main sequence stars are
about a magnitude fainter than the least luminous metal-poor field halo stars
known, suggesting that the lowest luminosity halo stars still await discovery.
At the bright end the data extend beyond the main sequence turnoff to well up
the giant branch. A populous white dwarf cooling sequence is also seen in the
cluster CMD. The most dramatic features of the cooling sequence are its turn to
the blue at faint magnitudes as well as an apparent truncation near F814W = 28.
The cluster luminosity and mass functions were derived, stretching from the
turn off down to the hydrogen-burning limit. It was well modeled with either a
very flat power-law or a lognormal function. In order to interpret these fits
more fully we compared them with similar functions in the cluster core and with
a full N-body model of NGC 6397 finding satisfactory agreement between the
model predictions and the data. This exercise demonstrates the important role
and the effect that dynamics has played in altering the cluster IMF.Comment: 43 pages including 4 tables and 12 diagrams. Figures 2 and 3 have
been bitmapped. Accepted for publication in the Astronomical Journa
The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models
The ACS Survey of Galactic Globular Clusters, an HST Treasury Project, will
deliver high quality, homogeneous photometry of 65 globular clusters. This
paper introduces a new collection of stellar evolution tracks and isochrones
suitable for analyzing the ACS Survey data. Stellar evolution models were
computed at [Fe/H]= -2.5, -2.0, -1.5, -1.0, -0.5, and 0; [alpha/Fe]= -0.2, 0,
0.2, 0.4, 0.6, and 0.8; and three initial He abundances for masses from 0.1 to
1.8 Msun and ages from 2 to 15 Gyr. Each isochrone spans a wide range in
luminosity from Mv~14 up to the tip of the red giant branch. These are
complemented by a set of He-burning tracks that extend from the zero age
horizontal branch to the onset of thermal pulsations on the asymptotic giant
branch. In addition, a set of computer programs are provided that make it
possible to interpolate the isochrones in [Fe/H], generate luminosity functions
from the isochrones, and create synthetic horizontal branch models. The tracks
and isochrones have been converted to the observational plane with two
different color-Teff transformations, one synthetic and one semi-empirical, in
ground-based B, V, and I, and F606W and F814W for both ACS-WFC and WFPC2
systems. All models and programs presented in this paper are available from
http://stellar.dartmouth.edu/~models/Comment: 46 pages, 12 figures, AJ in press; figures 11 and 12 are reduced in
siz