The Cassiopeia A SNR, some 3 kpc distant (Braun et al., 1987) is occulted by both nearby and Perseus Arm neutral gas. As such, it has been used for absorption-line molecular spectroscopy in OH (Weinreb et al., 1963; Bieging & Crutcher, 1986), CO (Goss et al., 1984), and NH3 (Gaume et al., 1994), and even in the 3mm band where Encrenaz et al. (1980) and Linke et al. (1981) detected absorption from HCN and HCO+ using the 7m BTL antenna (HPBW 100"). Subsequent mm-wave observations with telescopes having a similarly small gain were used to put limits on the abundances of such species as CN, CS, and C2H which showed no detectable absorption (Nyman & Millar, 1989).
Sparse or small-scale maps of emission from various CO lines have been used to trace the spatial distribution of the molecular gas seen near the face of the remnant (Troland et al., 1985; Wilson et al., 1993). For the Perseus Arm features, the densities are typical of translucent clouds, with n, although the temperatures are somewhat high, K (Wilson et al., 1993). These higher temperatures, along with some suggestive aspects of the spatial distribution of the molecular gas and unusual motions of some of the fast-moving knots (FMK's) have given rise to speculation that the molecular gas is interacting with the SNR along its western border (Anderson & Rudnick, 1996; Keohane et al., 1996). Models of the remnant's morphology and evolution imply that the exterior density on the near side, typically found to be , must be larger than that behind by a substantial factor (Braun, 1987; Braun et al., 1987; Reed et al., 1995).
In a brief earlier communication (Liszt & Lucas, 1995, Paper I), we discussed the use of the Cassiopeia A SNR as a background light source for mm-wave absorption-line spectroscopy. We argued that results obtained from small, low-resolution singledish mm-wave telescopes were problematic, having been contaminated by small but inevitable amounts of foreground re-emission in the molecular gas. Given the densities and temperatures deduced for the molecular gas, we showed that the expected strength of an HCO+ absorption line would behave in a complicated fashion, and might well decline with increasing optical depth owing to excitation from photon-trapping effects in the radiative transfer. This is consistent with the fact that the weaker, nearby gas at zero-velocity is stronger in HCO+ absorption than are any of the Perseus Arm clouds.
To bear out the contentions of our earlier work, we have undertaken the various observations which are reported here. We mapped the 86 and 140 GHz radiocontinuum fluxes of Cas A (Kenney & Dent, 1985) fully for the first time to see whether any particularly bright spots across the nebula might be suitable for singledish absorption work (such is not the case with a 12m antenna, at least). We mapped 12CO J=2-1 and 13CO J=1-0 emission over a 12´-15´ region around the nebula to see whether any signs of an interaction were apparent and to show how the optical extinction might be expected to vary across the nebula. We also took profiles of HCO+ and C18O on and off the face of the nebula to trace the transition from off-source emission to on-source absorption. The strength of HCO+ emission seen away from the SNR, typically 0.1-0.2 K, is indeed the same as that of the brighter portions of the nebular continuum. Local features which have no detectable HCO+ emission and little CO emission are more easily seen in HCO+ absorption, while the more strongly-emitting Perseus Arm features are absent in absorption.
Details of the observing are described in Sect. 2. Sect. 3 discusses the continuum fluxes and Sect. 4 discusses the molecular gas. Sect. 5 is a brief summary.
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999