COLLISIONAL PUMPING OF OH MASERS IN THE 2II1/2 LADDER

We have studied the collisional pumping of OH masers in star-forming regions and report the first detailed calculations of such pumping since collision rates with the correct parities have become available. We find that collisions of OH with H2 pump not only the J = 1/2 Λ-doublet but also the J = 3/2 and J = 5/2 Λ-doublets of 2Π1/2. Using parameters inferred by VLBI for W3(OH), we find that collisional pumping can easily explain the observed brightness temperatures and luminosities of the J = 1/2 Λ-doublet masers. Furthermore, we predict what the brightness temperature and luminosity should be for the J = 3/2 and J = 5/2 Λ-doublets in the same region.