SGER: Near-Infrared Spectroscopic Survey of Massive Young Stars in the Central Molecular Zone of the Galactic Center

University Of Chicago, Chicago IL

Investigators

Abstract

Dr. Oka will identify young stellar objects within 200pc of the Galactic center as probes of the dense interstellar gas of the inner Milky Way. These young stars have almost featureless near-infrared spectra, so that the absorption lines of the molecules H3+ and CO can be measured cleanly. The rather simple chemistry of H3+ then allows both the ionization rate and the temperature in the dense interstellar gas to be estimated directly. Dr. Oka's preliminary work on a few stars has shown a surprisingly large amount of H3+, indicating that warm, diffuse molecular gas fills much of the volume near the Galactic center, and implying an unexpectedly high density of cosmic rays. Dr Oka will select about 100 candidate stars using newly-available catalogs in the near-infrared at 1.2 microns, 2.2 microns and 3.6 microns. Many of these will be red giant stars, which are unsuitable because their atmospheres produce strong absorption bands in the stellar spectra. Dr. Oka will use the UK Infrared Telescope (UKIRT) in Hawaii for medium-resolution spectroscopy of his candidate stars; in red giants, the 2-micron overtone bands of CO is strong, and easily detected in short observations of 5 or 10 minutes. The young stellar objects, which lack such absorption, will then be observed at high resolution near 3.6 microns, with UKIRT and the larger Subaru and Gemini telescopes. Spectra of stars near the Galactic center will show narrow absorption lines of CO, from cold gas in the 3 spiral arms that lie between us and the center. If this work yields a sufficient sample of young stars near the center of the Milky Way, it will enable a larger project of deeper observations to probe the distribution of H3+ and the physical state of the interstellar gas there. The H3+ molecule is of interest both to astrophysicists and to physical chemists, because it is crucial in the ion-molecule chemistry of the molecular clouds from which new stars are born. The larger project would involve mentoring graduate students, in which Dr. Oka has an outstanding record.