Thermal OH (1667/1665 MHz) Absorption and Nonthermal OH (1720 MHz) Emission toward the W28 Supernova Remnant

The W28 supernova remnant is an excellent prototype for observing shocked gasresulting from the interaction of supernova remnants (SNRs) and adjacentmolecular clouds (MCs). We present two new signatures of shocked molecular gasin this remnant. One is the detection of main-line extended OH (1667 MHz)absorption with broad linewidths. The column density of OH estimated from theoptical depth profiles is consistent with a theoretical model in which OH is formed behind a C-type shock front. The second is the detection ofextended, weak OH (1720 MHz) line emission with narrow linewidth distributedthroughout the shocked region of W28. These give observational support to theidea that compact maser sources delineate the brightest component of a muchlarger region of main line OH absorption and nonthermal OH (1720 MHz) emissiontracing the global structure of shocked molecular gas. Main line OH (1665/67)absorption and extended OH (1720 MHz) emission line studies can serve aspowerful tools to detect SNR-MC interaction even when bright OH (1720 MHz)masers are absent.Comment: 14 pages, 3 figures, one table, to appear in ApJ (Jan 10, 2003