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

奥卡博士将确定银河系中心200 pc以内的年轻恒星天体，作为银河系内部致密星际气体的探测器。 这些年轻的恒星几乎没有特征的近红外光谱，因此可以干净地测量分子H3+和CO的吸收线。 H3+的化学性质相当简单，因此可以直接估计稠密星际气体的电离率和温度。 奥卡博士对几颗恒星的初步研究显示，H3+的含量惊人地高，这表明温暖、弥漫的分子气体充满了银河系中心附近的大部分体积，这意味着宇宙射线的密度出乎意料地高。 Oka博士将使用1.2微米，2.2微米和3.6微米的近红外线新目录选择大约100颗候选恒星。 其中许多将是红巨星，这是不合适的，因为它们的大气层在恒星光谱中产生强烈的吸收带。 奥卡博士将使用位于夏威夷的英国红外望远镜（UKIRT）对他的候选恒星进行中等分辨率的光谱分析;在红巨星中，CO的2微米泛音带很强，很容易在5或10分钟的短时间观测中被发现。 年轻的恒星物体，缺乏这种吸收，然后将被观察到在3.6微米附近的高分辨率，与UKIRT和更大的斯巴鲁和双子座望远镜。 银河系中心附近恒星的光谱将显示CO的窄吸收线，来自位于我们和中心之间的3个螺旋臂中的冷气体。 如果这项工作在银河系中心附近获得足够的年轻恒星样本，它将使更大规模的更深入观察项目能够探测H3+的分布和那里星际气体的物理状态。 H3+分子对天体物理学家和物理化学家都很感兴趣，因为它在新恒星诞生的分子云的离子-分子化学中至关重要。 更大的项目将涉及指导研究生，奥卡博士在这方面有着出色的记录。

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.