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A Comprehensive OH Maser Survey of Star-forming Regions

Interstellar OH masers are bright signposts for recently formed massive stars, and the maser emission can be used to study the kinematic and physical conditions of dense molecular material surrounding these stars. Using the VLA we have interferometrically mapped 91 interstellar OH maser sources in one or both of the ground-state, main-line, ²Π_3/2 J=3/2 transitions near 18 cm wavelength. The maps comprising this large, uniformly processed survey have a spectral resolution of 0.14 km s^-1 and an angular resolution of ≈1.5 arcsec. We measured the absolute positions of the masers to an accuracy of ≈0.4 arcsec, except for sources with declinations below about -30°, and relative positions of isolated OH maser spots within each source to an accuracy of ≈0.01 arcsec. This survey forms a nearly complete sample of interstellar OH masers stronger than 1 Jy in both right- and left-circular polarization in at least one of the ground-state OH transitions. Future work includes a statistical analysis of the maser properties and the interpretation of the results in the framework of massive star formation research. Also possible will be the determination of magnetic field strengths and directions in the maser regions which, among other things, will allow important conclusions on the large-scale magnetic field of the Milky Way.

A New Type of OH Maser

We have detected weak OH maser emission near the intermediate-to-high-mass protostellar Turner-Welch source in the W3 OH region. Unlike typical interstellar OH masers that are associated with ultra-compact HII regions, these OH masers appear to be associated with a bipolar outflow traced by strong H₂O masers. These OH masers may indicate a new class of interstellar masers that are associated with very young stars which have yet to ionize their surroundings. The OH molecules may form in sufficient density for maser action near the edges of material giving rise to H₂O maser emission, where interstellar UV radiation is sufficient to dissociate the H₂O molecules. Alternatively, the OH masers may mark the spot where the unique synchrotron-emitting jet in the TW source now impinges on external material, possibly leading to OH formation.

SiO Masers and Galactic Structure

In a project led by scientists from Leiden Univerity and using the IRAM 30 m telescope, late-type stars in the inner Galaxy selected from the ISOGAL and MSX catalogs were searched for maser emission in the 86 GHz v=1, J=2-1 transition of SiO in order to significantly increase the number of stars with known radial velocities. Our 255 detections double the number of line-of-sight velocities known towards the inner Galaxy. Combining our measurements with those from other surveys, in particular OH maser surveys, will allow a kinematic analysis of the stellar orbits in the inner Galaxy. Analyzing the colours and luminosities of OH and SiO maser emitters also yields new clues on the conditions under which different masers may occur.