Shocked Molecular Gas in the Supernova Remnants W28 and W44: Near‐Infrared and Millimeter‐Wave Observations

Abstract

High resolution millimeter-wave and near-infrared observations of the supernova remnants W28 and W44 reveal extensive shocked molecular gas where supernova blast waves are propagating into giant molecular clouds. New CO observations were carried out with the IRAM 30-m and ARO 12-m telescopes, and the near-infrared observations were with Palomar 200-inch telescope. The near-infrared observations reveal shocked H2 emission from both supernova remnants, showing intricate networks of filaments on arcsec scales, following the bright ridges of the radio shells. The CO and CS linewidths, indicative of the shock speed, are 20-30 km/s. Both the near-infrared and millimeter-wave emission are attributed to shocks into gas with density >1e3 cm-3. Individual shock structures are resolved in the H2 emission, with inferred edge-on shock thickness ~1e17 cm, consistent with non-dissociative shocks into gas densities of 1e3-1e4 cm-3. Bright 1720 MHz OH masers are located within the shocked H2 gas complexes and highlight only localized areas where the conditions for masing are optimal. The Halpha and X-ray emission, have morphologies very different from the radio. We find a detailed correlation of the radio and H2 emission for some long filaments, indicating cosmic ray acceleration or re-acceleration due to the shocks into moderately dense gas. The different morphologies of these two remnants at different wavelengths is explained by a highly nonuniform structure for giant molecular clouds.