Star formation in the Magellanic clouds

Because of their proximity, the Magellanic Clouds provide the opportunity to conduct a detailed study of the history and current state of star formation in dwarf irregular galaxies. There is considerable evidence that star formation in the Clouds was and is proceeding in a manner different from that found in a typical well-ordered spiral galaxy. Star formation in both Clouds appears to have undergone a number of relatively intense bursts. There exist a number of similarities and differences in the current state of star formation in the Magellanic Clouds and the Milky Way. Examination of Infrared Astronomy Satellite (IRAS) sources with ground based telescopes allows identification of highly evolved massive stars with circumstellar shells as well as several types of compact emission line objects

No Death Star -- For Now

A star passing within \sim 10^4 \au of the Sun would trigger a comet shower that would reach the inner solar system about 0.18 Myr later. We calculate a prior probability of ~0.4% that a star has passed this close to the Sun but that the comet shower has not yet reached the Earth. We search the HIPPARCOS catalog for such recent close-encounter candidates and, in agreement with Garcia-Sanchez et al. (1997), find none. The new result reported in this Letter is an estimation of the completeness of the search. Because of the relatively bright completeness limit of the catalog itself, V~8, the search is sensitive to only about half the stars that could have had such a near encounter. On the other hand, we show that the search is sensitive to nearly all of the past encounters that would lead to a major shower in the future and conclude that it is highly unlikely that one will occur during the next 0.5 Myr.Comment: 10 pages, 1 figure. In press at The Astrophysical Journal Letter

IRAS and ground-based observations of star formation regions in the Magellanic clouds

The Infrared Astronomy Satellite (IRAS) detected several hundred individual regions of star formation in the Large and Small Magellanic Clouds. Nearly two dozen of the brightest such sources were searched for from the ground at 10 microns; most of these were located and measured at wavelengths from 0.6 to 20 microns. Three principle results emerge from this study: First, the IRAS data show that star formation is considerably less active in the SMC than in the LMC, relative either to mass, luminosity, or H I content. The reduced activity in the SMC is consistent with the smaller amount of molecular material inferred from CO observations. Second, the sizes of the objects range from less than a few arcsecs (objects which look like extremely compact HII regions, with little or no extended radio, optical, or infrared emission) to some tens of arcsecs across (giant HII regions, of which the largest and brightest is 30 Doradus). Third, there are no obvious differences in the characteristics of the central portions of the LMC and SMC sources; all look like Galactic H II regions of similar luminosity

The Giant Branches of Open and Globular Clusters in the Infrared as Metallicity Indicators: A Comparison with Theory

We apply the giant branch slope-[Fe/H] relation derived by Kuchinski et al. [AJ, 109, 1131 (1995)] to a sample of open clusters. We find that the slope of the giant branch in K vs. (J-K) color-magnitude diagrams correlates with [Fe/H] for open clusters as it does for metal-rich globular clusters but that the open cluster data are systematically shifted to less negative values of giant branch slope, at constant [Fe/H]. We use isochrone models to examine the theoretical basis for this relationship and find that for a given value of [Fe/H], the slope of the relationship remains constant with decreasing population age but the relation shifts to less negative values of giant branch slope with decreasing age. Both of these theoretical predictions agree with the trends found in the data. Finally, we derive new coefficients for the giant branch slope-[Fe/H] relation for specific members of 3 populations, metal-rich globular clusters, bulge stars and open clusters.Comment: 16 pages including 3 figures (AASTEX), AJ Accepted, also available at http://www.astronomy.ohio-state.edu/~martini/pubs.htm

A comparison of the near-infrared spectral features of early-type galaxies in the Coma Cluster, the Virgo cluster and the field

Earlier researchers derived the relative distance between the Coma and Virgo clusters from color-magnitude relations of the early-type galaxies in each cluster. They found that the derived distance was color-dependent and concluded that the galaxies of similar luminosity in the two clusters differ in their red stellar populations. More recently, the color-dependence of the Coma-Virgo distance modulus has been called into question. However, because these two clusters differ so dramatically in their morphologies and kinematics, it is plausible that the star formation histories of the member galaxies also differed. If the conclusions of earlier researchers are indeed correct, then some signature of the resulting stellar population differences should appear in the near-infrared and/or infrared light of the respective galaxies. We have collected near-infrared spectra of 17 Virgo and 10 Coma early-type galaxies; this sample spans about four magnitudes in luminosity in each cluster. Seven field E/S0 galaxies have been observed for comparison. Pseudo-equivalent widths have been measured for all of the field galaxies, all but one of the Virgo members, and five of the Coma galaxies. The features examined are sensitive to the temperature, metallicity, and surface gravity of the reddest stars. A preliminary analysis of these spectral features has been performed, and, with a few notable exceptions, the measured pseudo-equivalent widths agree well with previously published values