Atomic oxygen in the comae of comets

Cochran, A. L.

doi:10.1016/j.icarus.2008.06.007

Cited by 32 publications

(70 citation statements)

References 19 publications

(19 reference statements)

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“…A comparison between the line shapes computed with and without expansion shows that the radial bulk motion of the coma produces the main contribution to the Doppler broadening of a thermal emission, as already pointed out by Cochran (2008). However, in the case of O( 1 D) and O( 1 S), the suprathermal distribution of the emitted photons produces a Doppler broadening larger than that caused by the expansion.…”

Section: Comparison With Observationssupporting

confidence: 63%

“…We apply the model to the case of the comet C/1996 B2 (Hyakutake), at 1 AU under minimum solar activity conditions. We compare the spectral width of both emissions and compare our results with the observations of Decock et al (2013) and Cochran (2008).…”

Section: Introductionmentioning

confidence: 93%

“…It also follows that the Doppler profile of the green and red lines must be broader than the classical Gaussian profile at ambient temperature. This broadening will thus combine with that associated with the expansion motion of the coma, which is usually considered the main spectral broadening process (Cochran 2008).…”

Section: The Monte Carlo Modelmentioning

confidence: 99%

“…Festou & Feldman (1981) analyzed the production of metastable oxygen by photodissociation of CO 2 and H 2 O and proposed that the different branching ratios of the O( 1 D) and O( 1 S) atoms from these parent molecules offer a mean to estimate the CO 2 abundance of the coma, relative to H 2 O. Cochran (2008) observed the O( 1 S) and O( 1 D) emissions of the comae of six different comets and estimated their FWHM. In particular, they estimated that comet C/1996 B2 (Hyakutake), located at a heliocentric distance of 1.37 AU at the time of observation, presented velocity FWHM's of ∼1 km s −1 for the red line and 2.4 km s −1 for the green line, with an intensity ratio between the green line and the red doublet ranging between 1.3 and 1.6.…”

Section: Introductionmentioning

confidence: 99%

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Monte Carlo Simulation of Metastable Oxygen Photochemistry in Cometary Atmospheres

Bisikalo

Shematovich

Gérard

et al. 2014

ApJ

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Cometary atmospheres are produced by the outgassing of material, mainly H 2 O, CO, and CO 2 from the nucleus of the comet under the energy input from the Sun. Subsequent photochemical processes lead to the production of other species generally absent from the nucleus, such as OH. Although all comets are different, they all have a highly rarefied atmosphere, which is an ideal environment for nonthermal photochemical processes to take place and influence the detailed state of the atmosphere. We develop a Monte Carlo model of the coma photochemistry. We compute the energy distribution functions (EDF) of the metastable O( 1 D) and O( 1 S) species and obtain the red (630 nm) and green (557.7 nm) spectral line shapes of the full coma, consistent with the computed EDFs and the expansion velocity. We show that both species have a severely non-Maxwellian EDF, that results in broad spectral lines and the suprathermal broadening dominates due to the expansion motion. We apply our model to the atmosphere of comet C/1996 B2 (Hyakutake) and 103P/Hartley 2. The computed width of the green line, expressed in terms of speed, is lower than that of the red line. This result is comparable to previous theoretical analyses, but in disagreement with observations. We explain that the spectral line shape does not only depend on the exothermicity of the photochemical production mechanisms, but also on thermalization, due to elastic collisions, reducing the width of the emission line coming from the O( 1 D) level, which has a longer lifetime.

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Section: Comparison With Observationssupporting

confidence: 63%

Section: Introductionmentioning

confidence: 93%

Section: The Monte Carlo Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Monte Carlo Simulation of Metastable Oxygen Photochemistry in Cometary Atmospheres

Bisikalo

Shematovich

Gérard

et al. 2014

ApJ

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“…In the optical region of cometary spectra, three forbidden O transitions are usually detectable, while they are not always separable from the corresponding telluric lines: the green line at 5577.339 Å ( 1 S-1 D) and the red doublet at 6300.304 Å and 6363.776 Å ( 1 D-3 P) (Feldman et al 2004;Cochran & Cochran 2001;Cochran 2008). In the spectra JMEC0039, taken on October 25, and JODF0017, taken on December 2, some of these emission lines are visible.…”

Section: Analysis Of the Atomic Oxygen Emission Linesmentioning

confidence: 99%

High resolution observation of 17P/ Holmes during the outburst event in 2007

Capria

Cremonese

Sanctis

2010

A&A

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Aims. On October 24, 2007 the comet 17P/ Holmes underwent an outburst that was extraordinary in both duration and size. We attempted to determine the cause of the outburst by acquiring and analyzing high resolution spectra of the comet. Methods. On two nights, October 25 and December 2, we acquired four high resolution spectra in the visible range at the Telescopio Nazionale Galileo in La Palma. The oxygen lines intensity ratio was estimated for the observation performed on October 25, and computed for the observation performed on December 2. Results. Many emission lines were detected in the spectra, most of them attributable to C 2 , NH 2 , CN and CH. The atomic O green line and one of the red doublet lines were detected in a spectrum acquired on October 25. All the three atomic O lines were detected in a spectrum acquired on December 2. The value obtained for the first night, 0.1 ± 0.11, implies that the H 2 O molecule was the main parent of the emission lines, while the value obtained for the second night, 0.3 ± 0.1, implies that the main parent was CO or CO 2 instead than H 2 O. Conclusions. Our results imply that in the days around the outburst the water production was exceptionally high for that heliocentric distance. By December, however, the behavior of 17P/ Holmes had returned to that of a comet orbiting at 2.6 AU from the Sun, in which state the activity is probably sustained mostly by gases more volatile than water, such as CO and CO 2 .

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Photochemistry of forbidden oxygen lines in the inner coma of 67P/Churyumov‐Gerasimenko

et al. 2016

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Observations of the green and red‐doublet emission lines have previously been realized for several comets. We present here a chemistry‐emission coupled model to study the production and loss mechanisms of the O(1S) and O(1D) states, which are responsible for the emission lines of interest for comet 67P/Churyumov‐Gerasimenko. The recent discovery of O2 in significant abundance relative to water 3.80 ± 0.85% within the coma of 67P has been taken into consideration for the first time in such models. We evaluate the effect of the presence of O2 on the green to red‐doublet emission intensity ratio, which is traditionally used to assess the CO2 abundance within cometary atmospheres. Model simulations, solving the continuity equation with transport, show that not taking O2 into account leads to an underestimation of the CO2 abundance within 67P, with a relative error of about 25%. This strongly suggests that the green to red‐doublet emission intensity ratio alone is not a proper tool for determining the CO2 abundance, as previously suggested. Indeed, there is no compelling reason why O2 would not be a common cometary volatile, making revision of earlier assessments regarding the CO2 abundance in cometary atmospheres necessary. The large uncertainties of the CO2 photodissociation cross section imply that more studies are required in order to better constrain the O(1S) and O(1D) production through this mechanism. Space weather phenomena, such as powerful solar flares, could be used as tools for doing so, providing additional information on a good estimation of the O2 abundance within cometary atmospheres.

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Atomic oxygen in the comae of comets

Cited by 32 publications

References 19 publications

Monte Carlo Simulation of Metastable Oxygen Photochemistry in Cometary Atmospheres

Monte Carlo Simulation of Metastable Oxygen Photochemistry in Cometary Atmospheres

High resolution observation of 17P/ Holmes during the outburst event in 2007

Photochemistry of forbidden oxygen lines in the inner coma of 67P/Churyumov‐Gerasimenko

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