Massive Stars & Star Cluster Research Summary

Discovery of a New Milky Way Globular Cluster

Wyoming graduate student Andy Monson led the discovery of a new globuar star cluster hidden beneath the gas and dust layers of our own Milky Way galaxy ( Kobulnicky et al. 2004 ). Just when astronomers thought they might have dug up the last of our galaxy's "fossils," we've discovered a new one in the Galactic equivalent of our own backyard. Called globular clusters, these ancient bundles of stars date back to the birth of our Milky Way galaxy, 13 or so billion years ago. They are sprinkled around the center of the galaxy like seeds in a pumpkin. Astronomers use clusters as tools for studying the Milky Way's age and formation. New infrared images from NASA's Spitzer Space Telescope and the University of Wyoming Infrared Observatory reveal a never-before-seen globular cluster within the dusty confines of the Milky Way. The newfound cluster is one of about 150 known to orbit the center of the Milky Way. These tightly packed knots of stars are among the oldest objects in our galaxy, having formed about 10 to 13 billion years ago. They contain several hundred thousand stars, most of which are older and less massive than our Sun. Follow-up observations with the University of Wyoming Infrared Observatory helped set the distance of the new cluster at about 9,000 light-years from Earth -- closer than most clusters -- and set the mass at the equivalent of 300,000 Suns. The cluster's apparent size, as viewed from Earth, is comparable to a grain of rice held at arm's length. It is located in the constellation Aquila. This discovery was featured by a Spitzer Space Telescope Press Release and on Astronomy Picture of the Day

The Formation of Massive Stars and the Starburst Clusters in Galaxies

Short-lived, massive stars are the dominant source of ionizing photons, mechanical energy, and heavy element nucleosynthesis in galaxies. Most massive stars form in compact clusters just a few pc in size. We have located what we believe are the youngest stages of massive cluster formation in the form of ultra-dense HII regions in nearby galaxies ( Kobulnicky & Johnson 1999, ). These often optically-obscured super clusters are the few x10^5 year-old precursors to super starclusters and globular clusters. We've identified a population of radio-bright young star clusters in nearby galaxies like M 33 and NGC 6946 ( Johnson, Kobulnicky, Massey, & Conti (2001) ) and Haro 3 ( Johnson, Indebetouw, Watson & Kobulnicky (2004) ) They provide a glimpse of the earliest stages of massive star formation. We are currently undertaking a comprehensive radio, millimeter-wave, and infrared observational program to conduct a census of extremely young massive clusters and understand their formation mechanisms. Our goal is to chronicle the formative stages of very young massive star clusters and fill in the unknown phases of the schematic diagram shown below.

Finding Companions of the Most Massive Stars

cygob2 The evolutionary path of massive stars is sensitive to the presence of close binary companions. However, very little is known about the companions of early type stars. Population synthesis studies of double neutron star systems predict formation rates one to two orders of magnitude greater than the rate inferred from the observations. This discrepancy can be explained, in part, by the choice of the mass ratio distribution, P(q), for massive stars. Mergers of these binary systems are the most promising sources of gamma-ray bursts and gravitational wave emission. The (unknown) fraction of massive close binaries has a large effect on the predicted spectral energy distribution used in modeling starburst populations in the local and high redshift universe. We are engaged in a long-term program to measure the binary mass fraction and binary mass ratio distribution for massive stars in the Milky Way using radial velocity and astrometric measurements from the Keck, Lick, WIYN, and WIRO observatories.

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