A POSSIBLE DETECTION OF INFRARED-EMISSION FROM CARBON-MONOXIDE IN COMET AUSTIN (1989C1)

We probed the ν = 1 - 0 rovibrational band of the CO molecule in Comet Austin (1989c1), using a cryogenic infrared echelle grating spectrometer. High-resolution (λ/Δλ ≈ 104) observations were made which encompassed the P(2) and P(3) line frequencies, on UT 1990 May 16 and 18. We detected line emission (at the 5 σ level) at the Doppler-shifted frequency expected for the cometary P(3) line, but only in scans covering approximately the first 3000 sec of observing time on 16 May. If cometary in origin, we attribute this to the occurrence of an outburst of at least 3000 sec duration. We derived a CO production rate, relative to that reported for water on May 16, ranging from 0.08 for a rotational temperature of 20 K, to 0.3 for Trot = 200 K, or from 2 to 8 times the reported (quiescent) value for QCO. However, the P(2) line was not seen in any of the data. Model predictions indicate that it should have been detected at the 3.5 σ level, even for relatively high coma temperatures (T ≈ 100-200 K). The CO production rates for an optically thick coma were only marginally larger than those calculated for optically thin conditions. Our measured P(3) line fluxes increased in going from a 2′ × 3′ to a 2′ × 9′ aperture, by an amount consistent with a discrete source for CO. If the outburst was accompanied by enhanced dust production, the maximum increase consistent with our data was ∼2.5-fold, at the 2 σ confidence level. This suggests that CO/dust may have been enriched in the outbursting volume relative to other regions of the nucleus. © 1992.