Enhanced OH in C-type shock waves in molecular clouds

Cosmic-ray and X-ray ionisations in molecular gas produce a weak far-ultraviolet flux through the radiative decay of H2 molecules that have been excited by collisions with energetic electrons (the Prasad-Tarafdar mechanism). I consider the effect of this dissociating flux on the oxygen chemistry in C-type shocks. Typically a few percent of the water molecules produced within the shock front are dissociated before the gas has cooled to 50K. The resulting column density of warm OH rises from 10^15 to 10^16 cm^-2 as the ionisation rate is increased from 10^-17 (typical of dark clouds) to 10^-15 s^-1 (adjacent to supernova remnants). These column densities produce substantial emission in the far-infrared rotational transitions of OH, and are consistent with the OH/H2O ratios inferred from ISO observations of emission from molecular shocks. For high ionisation rates the column of warm OH is sufficient to explain the OH(1720 MHz) masers that occur where molecular clouds are being shocked by supernova remnants. The predicted abundance of OH throughout the shock front will enable C-type shocks to be examined with high spectral resolution through radio observations of the four hyperfine ground state transitions of OH at 18cm and heterodyne measurements of emission in the FIR (e.g. from SOFIA)Comment: 5 pp incl 3 figs, LaTeX, uses emulateapj.sty; ApJ Letters in press. Revised 2nd paragraph of discussio

Thermal OH (1667/65 MHz) Absorption and Nonthermal OH (1720 MHz) Emission Towards the W28 Supernova Remnant

The W28 supernova remnant is an excellent prototype for observing shocked gas resulting from the interaction of supernova remnants (SNRs) and adjacent molecular clouds (MCs). We present two new signatures of shocked molecular gas in this remnant. One is the detection of main-line extended OH (1667 MHz) absorption with broad linewidths. The column density of OH estimated from the optical depth profiles is consistent with a theoretical model in which OH is formed behind a C-type shock front. The second is the detection of extended, weak OH (1720 MHz) line emission with narrow linewidth distributed throughout the shocked region of W28. These give observational support to the idea that compact maser sources delineate the brightest component of a much larger region of main line OH absorption and nonthermal OH (1720 MHz) emission tracing the global structure of shocked molecular gas. Main line OH (1665/67) absorption and extended OH (1720 MHz) emission line studies can serve as powerful 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

Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC443

(abridged) - We present arcsecond resolution observations from the VLA of the satellite line of the hydroxyl molecule (OH) at 1720.53 MHz toward three Galactic supernova remnants: W28, W44 and IC443. All of our observations are consistent with a model in which the OH(1720 MHz) is collisionally excited by H2 molecules in the postshock gas heated by a non-dissociative shock. Supernova remnants with OH(1720 MHz) maser emission may be promising candidates to conduct high energy searches for the sites of cosmic ray acceleration.Comment: ApJ Let (accepted). Hardcopies available from [email protected]

VLA HI Zeeman Observations Toward the W49 Complex

We report VLA HI Zeeman observations toward the W49A star-forming region and the SNR W49B. Line of sight magnetic fields (Blos) of 60 to 300 muG at 25" resolution were detected toward W49A at velocities of ~4 and ~7 km/s. The Blos values measured toward W49A show a significant increase in field strength with higher resolution especially for the ~4 km/s HI gas. The HI gas in the velocity range -5 to 25 km/s toward W49A shows good agreement both kinematically and spatially with molecular emission intrinsically associated with W49A. Based on comparisons with molecular data toward W49A, we suggest that the 4 km/s HI gas is directly associated with the northern part of the HII region ring, while the 7 km/s HI gas seems to originate in a lower density halo surrounding W49A. We estimate that the W49A North core is significantly subvirial, and that the total kinetic +magnetic energies amount to less than 1/3 of the total W49A North gravitational energy. These magnetic field results suggest that W49A North is unstable to overall gravitational collapse in agreement with evidence that the halo is collapsing onto the W49A North ring of HII regions. The majority of the HI column density toward W49B comes from Sagittarius Arm clouds along the line of sight at ~40 km/s and ~60 km/s. No significant magnetic fields were detected toward W49B. Comparison of the spectral distribution of HI gas toward W49A and W49B suggests that evidence placing W49B 3 kpc closer to the sun (i.e. at 8 kpc) than W49A is quite weak.Comment: Accepted to ApJ (March 2001), 39 pages and 16 figures. Higher resolution images for Figs. 1,3,4,5,7 and 9 can be obtained from http://www.aoc.nrao.edu/~cbroga