Forbidden oxygen lines at various nucleocentric distances in comets

Abstract

Aims. We study the formation of the [OI] lines -that is, 5577.339 Å (the green line), 6300.304 Å and 6363.776 Å (the two red lines) -in the coma of comets and determine the parent species of the oxygen atoms using the ratio of the green-to-red-doublet emission intensity, I 5577 /(I 6300 + I 6364 ), (hereafter the G/R ratio) and the line velocity widths. Methods. We acquired high-resolution spectroscopic observations at the ESO Very Large Telescope of comets C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and 103P/Hartley 2 when they were close to Earth (<0.6 au). Using the observed spectra, which have a high spatial resolution (<60 km/pixel), we determined the intensities and widths of the three [OI] lines. We spatially extracted the spectra to achieve the best possible resolution of about 1−2 , that is, nucleocentric projected distances of 100 to 400 km depending on the geocentric distance of the comet. We decontaminated the [OI] green line from C 2 lines blends that we identified.Results. The observed G/R ratio in all four comets varies as a function of nucleocentric projected distance (between ∼0.25 to ∼0.05 within 1000 km). This is mainly due to the collisional quenching of O( 1 S) and O( 1 D) by water molecules in the inner coma. The observed green emission line width is about 2.5 km s −1 and decreases as the distance from the nucleus increases, which can be explained by the varying contribution of CO 2 to the O( 1 S) production in the innermost coma. The photodissociation of CO 2 molecules seem to produce O( 1 S) closer to the nucleus, while the water molecule forms all the O( 1 S) and O( 1 D) atoms beyond 10 3 km. Thus we conclude that the main parent species producing O( 1 S) and O( 1 D) in the inner coma is not always the same. The observations have been interpreted in the framework of the previously described coupled-chemistry-emission model, and the upper limits of the relative abundances of CO 2 were derived from the observed G/R ratios. Measuring the [OI] lines might provide a new way to determine the CO 2 relative abundance in comets.

High-resolution spectra of comet C/2013 R1 (Lovejoy)

“…The errors correspond to the standard deviation of the 12 spectra. Considering the theoretical production rates of Raghuram & Bhardwaj (2013; see Table 1 of Decock et al (2015), the 1 S/ 1 D ratio corresponding to the G/R ratio), these results confirm that H 2 O is the main parent molecule that photodissociates to produce oxygen atoms when the comet is observed at a heliocentric distance of r ∼ 1 au. This agrees with previous analyses reported for other comets (Cochran 1984(Cochran , 2008Capria et al 2010;Decock et al 2013, and references therein).…”

“…This decontamination was made by substracting a C 2 synthetic spectrum fitted on another C 2 cometary line that appears to be close to the [OI] green line (at 5577.541 Å). More details about the creation of the C 2 synthetic spectrum and its subtraction are given in Rousselot et al (2012) and Decock et al (2015). Figure 1 presents a spectrum of the 5577.339 Å cometary oxygen line region.…”

Prediction of Forbidden Ultraviolet and Visible Emissions in Comet 67p/Churyumov–gerasimenko

“…Decock et al (2015) analyzed several ESOʼs VLT observed high-resolution green-and red-doublet emission line spectra on various comets. CO 2 mixing ratios are derived in these comets by comparing the ESO VLT observations with modeled G/R ratio profiles (Decock et al 2015). The model calculated G/R ratio profile is presented on 67P in Figure 11.…”

“…This model has been applied on several comets and results have been compared with the Earth-based observations (Bhardwaj et al 1996;Bhardwaj 1999;Bhardwaj & Raghuram 2011, 2014Decock et al 2015). The model calculations for comets 103P/Hartley2 and 1P/ Halley have shown that suprathermal electron impact is an important excitation process in the formation of CO(a 3 Π), which is more important than the photodissociation of CO 2 (Bhardwaj & Raghuram 2011;.…”

“…Recently, we have applied our model for the analysis of high-resolution spectroscopic observations made from the ESO very large telescope (VLT) on four comets (viz., C/2002 T7 (LINEAR), 73P-C/SchwassmannWachmann 3, 8P/Tuttle, and 103P/Hartley 2). This study has allowed us to constrain the CO 2 volume mixing ratios in these comets (Decock et al 2015).…”