860 research outputs found
Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies
We present the results of a search for clusters in dwarf irregular galaxies
in the Virgo and Fornax Cluster using HST WFPC2 snapshot data. The galaxy
sample includes 28 galaxies, 11 of which are confirmed members of the Virgo and
Fornax clusters. In the 11 confirmed members, we detect 237 cluster candidates
and determine their V magnitudes, V-I colors and core radii. After statistical
subtraction of background galaxies and foreground stars, most of the cluster
candidates have V-I colors of -0.2 and 1.4, V magnitudes lying between 20 and
25th magnitude and core radii between 0 and 6 pc. Using H-alpha observations,
we find that 26% of the blue cluster candidates are most likely HII regions.
The rest of the cluster candidates are most likely massive (>10^4 Msol) young
and old clusters. A comparison between the red cluster candidates in our sample
and the Milky Way globular clusters shows that they have similar luminosity
distributions, but that the red cluster candidates typically have larger core
radii. Assuming that the red cluster candidates are in fact globular clusters,
we derive specific frequencies (S_N) ranging from ~0-9 for the galaxies.
Although the values are uncertain, seven of the galaxies appear to have
specific frequencies greater than 2. These values are more typical of
ellipticals and nucleated dwarf ellipticals than they are of spirals or Local
Group dwarf irregulars.Comment: 46 pages, 14 figures, 3 tables, accepted by AJ. Higher quality PS
version of entire paper available at
http://www.astro.washington.edu/seth/dirr_gcs.htm
Identification of a Class of Low-Mass Asymptotic Giant Branch Stars Struggling to Become Carbon Stars in the Magellanic Clouds
We have identified a new class of Asymptotic Giant Branch (AGB) stars in the
Small and Large Magellanic Clouds (SMC/LMC) using optical to infrared
photometry, light curves, and optical spectroscopy. The strong dust production
and long-period pulsations of these stars indicate that they are at the very
end of their AGB evolution. Period-mass-radius relations for the
fundamental-mode pulsators give median current stellar masses of 1.14 M_sun in
the LMC and 0.94 M_sun in the SMC (with dispersions of 0.21 and 0.18 M_sun,
respectively), and models suggest initial masses of <1.5 M_sun and <1.25 M_sun,
respectively. This new class of stars includes both O-rich and C-rich
chemistries, placing the limit where dredge-up allows carbon star production
below these masses. A high fraction of the brightest among them should show S
star characteristics indicative of atmospheric C/O ~ 1, and many will form
O-rich dust prior to their C-rich phase. These stars can be separated from
their less-evolved counterparts by their characteristically red J-[8] colors.Comment: 16 pages, 18 figures, accepted for publication in Ap
Distances to Populous Clusters in the LMC via the K-Band Luminosity of the Red Clump
We present results from a study of the distances and distribution of a sample
of intermediate-age clusters in the Large Magellanic Cloud. Using deep
near-infrared photometry obtained with ISPI on the CTIO 4m, we have measured
the apparent K-band magnitude of the core helium burning red clump stars in 17
LMC clusters. We combine cluster ages and metallicities with the work of
Grocholski & Sarajedini to predict each cluster's absolute K-band red clump
magnitude, and thereby calculate absolute cluster distances. An analysis of
these data shows that the cluster distribution is in good agreement with the
thick, inclined disk geometry of the LMC, as defined by its field stars. We
also find that the old globular clusters follow the same distribution,
suggesting that the LMC's disk formed at about the same time as the globular
clusters, ~ 13 Gyr ago. Finally, we have used our cluster distances in
conjunction with the disk geometry to calculate the distance to the LMC center,
for which we find (m-M)o = 18.40 +/- 0.04_{ran} +/- 0.08_{sys}, or Do = 47.9
+/- 0.9 +/- 1.8 kpc.Comment: 31 pages including 5 figures and 7 tables. Accepted for publication
in the August 2007 issue of A
Stellar Crowding and the Science Case for Extremely Large Telescopes
We present a study of the effect of crowding on stellar photometry. We
develop an analytical model through which we are able to predict the error in
magnitude and color for a given star for any combination of telescope
resolution, stellar luminosity function, background surface brightness, and
distance. We test our predictions with Monte Carlo simulations of the LMC
globular cluster NGC 1835, for resolutions corresponding to a seeing-limited
telescope, the , and an AO-corrected 30-m (near diffraction limited)
telescope. Our analytically predicted magnitude errors agree with the
simulation results to within 20%. The analytical model also predicts that
errors in color are strongly affected by the correlation of crowding--induced
photometric errors between bands as is seen in the simulations. Using
additional Monte Carlo simulations and our analytical crowding model, we
investigate the photometric accuracy which 30-m and 100-m Extremely Large
Telescopes (ELTs) will be able to achieve at distances extending to the Virgo
cluster. We argue that for stellar populations work, ELTs quickly become
crowding-limited, suggesting that low--Strehl AO systems may be sufficient for
this type of science.Comment: 25 pages, 19 figures in 35 separate files, Astronomical Journal,
accepte
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