21 research outputs found
Constraints on the Origin of Manganese from the Composition of the Sagittarius Dwarf Spheroidal Galaxy and the Galactic Bulge
The trend of [Mn/Fe] in the Galactic bulge follows the solar-neighborhood
relation, but most stars in the Sagittarius dwarf spheroidal galaxy show
[Mn/Fe] deficient by approximately 0.2 dex. This leads us to conclude that the
Mn yields from both type Ia and type II SNe are metallicity-dependent. Our
observations militate against the idea, suggested by Gratton, that Mn is
over-produced by type Ia SNe, relative to type II SNe. We predict Mn/Fe ratios,
lower than the solar neighborhood relation, for the younger populations of
nearly all dwarf galaxies, and that Mn/Fe ratios may be useful for tracing the
accretion of low-mass satellites into the Milky Way.Comment: 10 pages, 3 figures, accepted for publication in ApJ
Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy: Implications for the Low-Mass Stellar Initial Mass Function at High Redshift
The stellar initial mass function at high redshift is an important defining
property of the first stellar systems to form and may also play a role in
various dark matter problems. We here determine the faint stellar luminosity
function in an apparently dark-matter-dominated external galaxy in which the
stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a
system with a particularly simple stellar population - all of the stars being
old and metal-poor - similar to that of a classical halo globular cluster. A
direct comparison of the faint luminosity functions of the UMi Sph and of
similar metallicity, old globular clusters is equivalent to a comparison of the
initial mass functions and is presented here, based on deep HST WFPC2 and STIS
imaging data. We find that these luminosity functions are indistinguishable,
down to a luminosity corresponding to 0.3 solar masses. Our results show that
the low-mass stellar IMF for stars that formed at very high redshift is
apparently invariant across environments as diverse as those of an extremely
low-surface-brightness, dark-matter-dominated dwarf galaxy and a
dark-matter-free, high-density globular cluster within the Milky Way.Comment: Accepted by New Astronomy. 64 pages, including 9 embedded postscript
tables and 20 embedded postscript figures, plus 14 separate jpeg figures.
Postscript versions of the jpeg figures and a complete version of the paper
with all figures embedded can be found at http://tarkus.pha.jhu.edu/~mlh
The Star Formation History of the Large Magellanic Cloud
Using WFPC2 aboard the Hubble Space Telescope, we have created deep
color-magnitude diagrams in the V and I passbands for approximately 100,000
stars in a field at the center of the LMC bar and another in the disk. The
main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red
clump and horizontal branch morphologies, and the differential Hess diagram of
the two fields all strongly imply that the disk and bar have significantly
different star-formation histories (SFHs). The disk's SFH has been relatively
smooth and continuous over the last 15 Gyr while the bar's SFH was dominated by
star formation episodes at intermediate ages. Comparison of the LF against
predictions based on Padova theoretical stellar evolution models and an assumed
age-metallicity relationship allows us to identify the dominant stellar
populations in the bar with episodes of star formation that occurred from 4 to
6 and 1 to 2 Gyr ago. These events accounted, respectively, for approximately
25% and 15% of its stellar mass. The disk field may share a mild enhancement in
SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with
the formation of the LMC bar.Comment: 14 pages, 5 figures, Latex, also available at
http://www.ps.uci.edu/physics/smeckerhane.html. Accepted for publication in
Ap
The Star Clusters in the Starburst Irregular Galaxy NGC 1569
We examine star clusters in the irregular, starburst galaxy NGC 1569 from HST
images. In addition to the two known super star clusters, we identify 45 other
clusters that are compact but resolved. Integrated UVI colors of the clusters
span a large range, and suggest that ages range from 3 Myrs to 1 Gyr. However,
most of the clusters were formed at the tail end of the recent starburst.
Numerous clusters in addition to the know super star clusters are similar in
luminosity to a small globular cluster. We examined the radial surface
brightness of four of the clusters. Their half-light radii and core radii are
in the range observed in present-day globular clusters. Therefore, conditions
that produced the recent starburst have also been those necessary for producing
compact, bright star clusters.
We examine resolved stars in the outer parts of the two super star clusters.
Cluster A is dominated by bright blue stars with a small population of red
supergiants. Sub-components A1 and A2 have similar colors and a two-dimensional
color map does not offer evidence that one component is dominated by red
supergiants and the other not. The contradiction of the presence of red super-
giants with Wolf-Rayet stars may instead not be a contradiction at all since
there coexistence in a coeval population is not inconsistent with the evolution
of massive stars. Cluster B is dominated by red supergiants, and this is
confirmed by the presence of the stellar CO absorption feature in an integrated
spectrum. The various age indicators are consistent with a picture in which
cluster B is of order 10--20 Myrs old, and cluster A is >4-5 Myrs old.Comment: To be published in AJ, November 200
The Metallicity Distribution Function of Red Giants in the LMC
We report new metallicity determinations for 39 red giants in a 220 square
arcminute region, 1.8 degrees southwest of the bar of the Large Magellanic
Cloud. These abundance measurements are based on spectroscopy of the Ca II
infrared triplet. The metallicity distribution function (MDF) shows a strong
peak at [Fe/H] = -0.57 +/- 0.04. Half the red giants in our field fall within
the range -0.83 < [Fe/H] < -0.41. We find a striking contrast in the shape of
the MDF below [Fe/H] < -1 between our inner disk field and the distant outer
field studied by Olszewski (1993). Our field-star MDF seems similar to that of
the intermediate-age (1-3 Gyr) star clusters. We have also obtained abundance
estimates using Stromgren photometry for ~1000 red giants in the same field.
The Stromgren measurements, which are sensitive to a combination of cyanogen
and iron lines, correlate well with the Ca II measurements, but a
metallicity-dependent offset is found (abridged).Comment: 55 pages, 16 figures, AASTeX v5.0, extended data table not included,
to appear in the October, 2000 issue of The Astronomical Journa