May 6, 1999 CfA Colloquium

Title: X Marks the Spot: Masers and Dynamics in Regions of High-Mass Star Formation

Speaker: Lincoln Greenhill

Abstract: Thirty years ago, the first detection of astronomical water maser emission was accompanied by the exclamation, "It is raining in Orion!" Indeed regions of high-mass star formation commonly exhibit strong maser emission from water and other species, such as hydroxyl, methanol, and in several cases, silicon monoxide. The masers generally arise in distinct clumps of dense gas that move with the mass flows driven by protostars and young stellar objects, which are otherwise hidden from view at optical and infrared wavelengths by high extinction columns. Because of excitation requirements, water and silicon monoxide masers are most closely associated with the hot central objects. The water masers populate warm dust-laden material while silicon monoxide masers exist in hotter, relatively dust-free surroundings. In contrast to regions of low-mass star formation, the birthplaces of high-mass stars are poorly understood, in part because of higher extinction, greater distance, and greater complexity. However, we may study in detail the structure and dynamics of particular regions via the angular distribution of masers, and their line-of-sight velocities and proper motions. Resolutions in angle and velocity of < 1 milliarcsecond (< 0.5 AU at 500 pc) and < 1 km/s are routinely achieved with radio interferometry. Recent maps of several regions, including Orion-KL, show good evidence of well collimated bipolar winds and disks on spatial scales of 1 - 1000 AU.

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