Observations of the forbidden oxygen lines in DIXI target Comet 103P/Hartley

We present analysis of observations of the forbidden oxygen lines in Comet 103P/Hartley, the target of the Deep Impact Extended Investigation (DIXI), during its Fall 2010 apparition. We obtained the observations using the ARCES echelle spectrometer mounted on the Astrophysical Research Consortium (ARC) 3.5-m telescope at Apache Point Observatory. We detected the 5577, 6300, and 6364 Å lines on all dates, and so were able to calculate the ratio of the 5577 Å line intensity to the sum of the intensities of the 6300 Å and 6364 Å lines (the oxygen line ratio). A ratio of 0.05-0.1 would suggest that water is the dominant parent, while a higher ratio suggests that another volatile, such as CO₂, is an important contributor to the O I population. We find a range of values for the oxygen line ratio varying from ∼0.05-0.13. This range includes values that are higher than those typically measured for comets observed at a similar heliocentric distance to 103P (∼1 AU). The higher measured ratio is likely a result of a higher intrinsic CO₂ abundance in the material released from 103P than typical comets, as found by direct measurements of CO₂ and H₂O made by the DIXI spacecraft. The CO₂/H₂O ratio in 103P was also observed by the DIXI spacecraft to vary significantly with rotational phase of the nucleus, which explains the large range of oxygen line ratios observed. These observations of the forbidden oxygen lines in 103P, a comet for which a directly observed CO₂/H₂O ratio is available, suggest that high quality observations of the forbidden oxygen lines in comets can serve as a proxy for CO₂/H₂O ratio measurements.