4. Emission and absorption from molecular gas seen near the Cassiopeia A SNR
Fig. 3 shows 12CO J=2-1 and 13CO J=1-0 spectra averaged over the face of the nebula. The well-known separation into local gas near 0-velocity and Perseus Arm material is obvious. The local gas has a blended, red-shifted wing which is weaker in CO emission but stronger in HCO+ absorption (Liszt & Lucas, 1995) as seen in Figs. 6 and 7. Emission from the local gas is so uniform that displaying a map is not worthwhile. Averaged over the nebula, the statistics of its CO J=2-1 emission are K km s-1 with a per-point rms measurement error of 0.53 K km s-1, a maximum of 7.3 K km s-1 and a minimum of 2.0 K km s-1.
CO emission from the Perseus Arm gas occurs in three features whose relative spatial positions are shown in Figs. 4 and 5. The most highly blue-shifted gas appears in an interesting filamentary structure to the southeast, while the most red-shifted gas appears to the east and west alike. The intermediate-velocity material appears mainly to the west and the northernmost edges of the nebula are relatively free of strong emission. Statistics of the Perseus Arm J=2-1 CO emission averaged over the face of the nebula are K km s-1 with a per-point statistical rms of 1 K km s-1, a maximum of 33.7 K km s-1 and a minimum of 0 K km s-1. Using typical CO- conversion factors, the roughly-determined contributions to the mean extinction are mag. from the Perseus arm gas and mag. for the local material, values which agree with results from optical studies as derived most recently by Hurford & Fesen (1996). The maximum extinction inferred toward the nebula, in the vicinity of the bright western continuum peak where the J=2-1 CO line integral is 40 K km s-1, is AV 7-8 mag.
4.1. Using the Cassiopeia A SNR as a background source at mm-wavelengths
As mentioned in Paper I and in the Introduction here, the physical conditions in the clouds seen toward Cassiopeia are such that the expected emission brightnesses of molecules like HCO+ and HCN, small though they might be, are not negligible compared to the weak radiocontinuum emission. In Paper I we argued that the existence of HCO+ absorption profiles toward Cassiopeia is something of a coincidence because their expected depths are not a monotonic function of their opacity. Excitation via photon-trapping is important in the moderate density regime in which the lines are formed, so line brightnesses increase (and absorption depths decrease) with increasing opacity over the expected range of parameter space.
To show this effect directly, we took emission profiles of various species on and off the nebula at positions marked by crosses in Fig. 4. At the three more easterly positions (Fig. 6), the stronger portions of the continuum shell are only barely occulted; the darkest, densest gas is seen off the face of the nebula. There is no appreciable continuum at the two lower positions and all species appear in emission, including HCO+. The peak brightness of the HCO+ off the nebula is 0.15-0.25 K, comparable to the brightness of the radio shell, as expected from the discussion in Paper I. No HCO+ emission is seen from any of the local gas which clearly is of much lower hydrogen column density.
The Perseus Arm gas has progressively lower column density to the north in Fig. 6 and its 13CO/C18O intensity ratios at the three positions are 14, 21, and 45; the northernmost position obviously has fairly low extinction. But at this position there is a curious pattern to all of the HCO+ profiles. For the Perseus Arm gas, it is the -37 km s-1 feature which appears in absorption while the much stronger -47 km s-1 emission lines have no counterpart. For the local gas, it is the more weakly CO-emitting, blue-shifted wing which is stronger in HCO+ absorption, as we discussed in Paper I.
Because the -47 km s-1 gas is known to occult the continuum (Goss et al., 1984) its absence in absorption cannot be explained geometrically. Another explanation would be to suppose that N(HCO+) declines as N(CO) and N() increase but this is the opposite of what is seen (Lucas & Liszt, 1996; Liszt & Lucas, 1998). Instead, we argue as follows. The -47 km s-1 feature is absent in absorption because its emission and absorption interfere destructively as discussed in Paper I; to a much lesser extent, its absorption profile may have been distorted by the upward-going image of the -37 km s-1 absorption feature after folding the frequency-switched spectrum. The -37 km s-1 feature is absent in emission because its number density is too low to produce a detectable emission signal at its low optical depth. And the blue-shifted wing of the local gas is stronger in HCO+ absorption and weaker in CO emission because either the CO excitation is weak (at low pressure) or the N(CO)/N(HCO+) ratio is low (at low extinction) conditions which occur in our prior studies of HCO+ (Lucas & Liszt, 1996) and CO (Liszt & Lucas, 1998).
Profiles from the two more westerly positions marked in Fig. 4 are shown in Fig. 7. There, CO emission from the Perseus Arm is slightly stronger toward the nebula ( = 0.21 K for the HCO+ spectrum at top) and HCO+ is present in emission where the continuum is absent. Again, HCO+ absorption does not occur at the velocities of the stronger emission components, instead appearing in the red wing of the local gas and perhaps in the wing of one of the Perseus Arm features. The local gas has a substantially weaker 13CO line, but is, if anything, stronger in HCO+ absorption.
The -47 km s-1 feature appears the most strongly in HCO+ emission on both sides of the nebula and is obviously the denser gas, even to the West where its weak 13CO emission and high 12CO/13CO intensity ratio indicate that N(CO)/N(C+) and AV are relatively small. In Fig. 5, the width of the HCO+ emission at -47 km s-1 is intermediate between those of 12CO and 13CO, and noticeably greater than that of the C18O. The -37 km s-1 gas is stronger in 13CO to the West and presumably opaque to optical radiation (owing to its small 12CO/13CO intensity ratio) but seems no denser, given the weakness of its HCO+ emission.
4.2. Is the Cassiopeia A SNR interacting with ambient molecular gas?
The higher than normal kinetic temperature derived for some molecular gas (Wilson et al., 1993) and a variety of behaviour near the brightest western portion of the continuum shell (Anderson & Rudnick, 1996; Keohane et al., 1996) have led to the conjecture that the Cassiopeia A SNR may be interacting with the ambient molecular gas. Alternatively, the molecular gas might show the (presumably smaller) prior influence of the stellar system which was the SNR progenitor.
The strongest and most direct indicators of an on-going interaction are absent. Koralesky et al. (1998) did not detect shock-excited 1720 MHz OH maser emission and our data show no sign of the 30-50 km s-1-wide CO lines which are often characteristic of such interaction (Frail & Mitchell, 1998). Of course the Cas A remnant is so young that these characteristic markers may simply not have had time to develop.
Clearly, any warm gas occurs only near -47 km s-1 to the East and South of the nebula; there is no suggestion of elevated kinetic temperatures in the western regions sampled in Fig. 7, where an interaction was suggested. The warm gas is not particularly dense, as shown by the weakness of HCO+ in emission or and its linewidths are relatively small. None of the gas appears to have been highly compressed, as is apparent from earlier derivations of the density by Wilson et al. (1993), or from the weakness of HCO+ emission
13CO J=1-0 linewidths are larger to the west in the -38 km s-1 gas (FWHM = 4.3-4.5 km s-1 for 13CO in Fig. 7 1.4-1.9 km s-1 in Fig. 6) but much of the width of the 12CO must be due to simple saturation broadening since the apparent optical depths are so large. To the east the 12CO J=2-1 line is about twice as broad as 13CO J=1-0 and to the west the same ratio is about 1.5-1.7.
The molecular gas frames the nebular continuum suggestively in Fig. 4 and there are suggestions of interesting kinematic structure in Fig. 5. For instance, in the frames at " the oval shape might be interpreted as a kinematic shell centered somewhat to the south of the center of the nebula. Yet this is at least partly coincidental: emission at the velocity extremes, which in a shell would be localized toward its center, also occurs well off the nebula in ambient material in the -47 km s-1 and -37 km s-1 features.
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999