Forbidden oxygen lines in Comets C/2006 W3 Christensen and C/2007 Q3 Siding Spring at large heliocentric distance: Implications for the sublimation of volatile ices

We present observations and analysis of the forbidden oxygen lines in Comets C/2006 W3 (Christensen) and C/2007 Q3 (Siding Spring) at heliocentric distances of 3.13 and 2.96 AU, respectively. We obtained the observations using the ARCES echelle spectrometer, which is mounted on the Astrophysical Research Consortium 3.5-m telescope at Apache Point Observatory. We detected the 5577, 6300, and 6364 Å atomic oxygen lines in Christensen and we calculated the ratio of the green (5577 Å) line flux to that of the red doublet (6300 and 6364 Å). We obtained a ratio of 0.24 ± 0.08, a number higher than previous measurements for other comets at smaller heliocentric distance. This shows that CO and/or CO₂ made detectable contributions to the O I population in the coma of Christensen. We only detected the 6300 Å line in Siding Spring, resulting in an upper limit for the oxygen line ratio of 0.20. Based on the derived flux for the 6300 Å line, we constrain the CO₂ and H₂O production rate in Siding Spring. A comparison of our results at ∼3 AU to previous findings for comets at this approximate heliocentric distance suggests that a heliocentric distance of 3 AU is a transition region between H₂O and CO₂ dominated activity in comets.