Probing the Milky Way Disk with Massive Stars

Trustees Of Boston University, Boston

Investigators

Abstract

Galaxies live and die by their star formation. Galaxies, like our Milky Way, are rich in interstellar gas and dust clouds. Gravity causes some interstellar clouds to collapse and form stars. The largest, brightest, hottest, and most massive stars that form are called "OB stars." OB stars are seen at their birthplaces because in their short lifetimes they do not have time to move very far from the collapsing interstellar clouds from which they formed. The light from these OB stars is energetic enough to create "HII Regions." HII regions are hot bubbles of fully ionized gas that surround their parent OB stars. OB star formation and the HII regions that go with it are mostly confined to the spiral arms of galaxies. HII regions are tracers of Galactic spiral structure. Collaborations with Haystack Observatory and Green Bank Observatory (GBO) will bring the excitement of radio astronomy to Boston University undergraduates and provide mentors at the Physics Inspiring the Next Generation (PING) Summer Camp for 9th grade girls and boys held at Green Bank. PING is a collaboration between GBO and the National Society of Black Physicists. The OB star chemical abundances indicate the present state of the interstellar gas and reveal the elemental enrichment caused by the nuclear processing of many stellar generations. They thus provide unique and important tracers of billions of years of Galactic chemical evolution (GCE). The Southern HII Region Discovery Survey (SHRDS) will use the Australian Telescope Compact Array to find hundreds of heretofore unknown HII regions. The SHRDS extends the successful Green Bank Telescope and Arecibo HRDS projects to the Southern sky; nearly half of the Milky Way disk cannot be observed from the Northern Hemisphere. The SHRDS will improve the Galactic HII region census in the largest unexplored part of the Milky Way disk where the location of the spiral arms and the galactic bar is especially uncertain. It will give the first complete map of where massive stars are forming in the entire Milky Way disk. This map will allow a detailed analysis of the overall structure of OB-star formation sites in the Milky Way and how the physical properties of HII regions change across the disk. New Galactic HII regions are discovered by detecting radio recombination line (RRL) radiation from candidate HII region targets. These targets are identified by comparing infrared and radio continuum images from existing surveys of the sky. Comparing far-IR and radio continuum survey data leads to a list of some 2,000 candidate HII regions in the Galaxy. RRL spectroscopic confirmation is needed to prove that a candidate is an H II region and to give its velocity. The recombination lines give kinematic distances and electron temperatures for the nebulae. Together, these allow derivation of the properties of the ionizing stars and the heavy element abundances, the metallicities, of the HII region gas. A Galactic scale map of HII region metallicities can inform both GCE and the merger history of the Milky Way. This project will achieve an areal density of sources, distributed throughout the Galactic disk, that can show local zones of metal enrichment (or depletion). This would provide evidence for mergers, thus adding a new tool for the study of Galactic archaeology.