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

Raghuram, Susarla; Bhardwaj, Anil; Galand, M.

doi:10.3847/0004-637x/818/2/102

Cited by 5 publications

(14 citation statements)

References 78 publications

(159 reference statements)

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“…In our modelling calculations on different comets at different heliocentric distances, we have shown that H 2 O is the major source of O( 1 D) (Bhardwaj & Raghuram 2012;Raghuram & Bhardwaj 2013Decock et al 2015;Raghuram et al 2016). Even though the photodissociation rate of H 2 O producing O( 1 D) is comparable (a factor of 1.5 higher) to that of CO 2 (see, Huebner et al 1992), CO 2 became the prominent source of green and red-doublet emission lines due to the very low water abundance in this comet.…”

Section: Effect Of the Cross Sectionsmentioning

confidence: 99%

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Raghuram

Hutsemékers

Opitom

et al. 2020

A&A

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Context. The N 2 and CO-rich and water-depleted comet C/2016 R2 (Pan-STARRS) (hereafter 'C/2016 R2') is a unique comet for detailed spectroscopic analysis. Aims. We aim to explore the associated photochemistry of parent species, which produces different metastable states and forbidden emissions, in this cometary coma of peculiar composition. Methods. We re-analyzed the high-resolution spectra of comet C/2016 R2, which were obtained in February 2018, using the UVES spectrograph of the European Southern Observatory (ESO) Very Large Telescope (VLT). Various forbidden atomic emission lines of [CI], [NI], and [OI] were observed in the optical spectrum of this comet when it was at 2.8 au from the Sun. The observed forbidden emission intensity ratios are studied in the framework of a couple-chemistry emission model. Results. The model calculations show that CO 2 is the major source of both atomic oxygen green and red-doublet emissions in the coma of C/2016 R2 (while for most comets it is generally H 2 O), whereas, CO and N 2 govern the atomic carbon and nitrogen emissions, respectively. Our modelled oxygen green to red-doublet and carbon to nitrogen emission ratios are higher by a factor of 3, when compared to the observations. These discrepancies can be due to uncertainties associated with photon cross sections or unknown production/loss sources. Our modelled oxygen green to red-doublet emission ratio is close to the observations, when we consider an O 2 abundance with a production rate of 30% relative to the CO production rate. We constrained the mean photodissociation yield of CO producing C( 1 S) as about 1%, a quantity which has not been measured in the laboratory. The collisional quenching is not a significant loss process for N( 2 D) though its radiative lifetime is significant (∼10 hrs). Hence, the observed [NI] doublet-emission ratio ([NI] 5198/5200) of 1.22, which is smaller than the terrestrial measurement by a factor 1.4, is mainly due to the characteristic radiative decay of N( 2 D).

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Section: Effect Of the Cross Sectionsmentioning

confidence: 99%

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Raghuram

Hutsemékers

Opitom

et al. 2020

A&A

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“…According to our calculations, the DR of CO + may be the major source of metastable O( 1 S) and O( 1 D) oxygen atoms responsible for the green (5577 Å) and red doublet (6300, 6364 Å) emission lines observed in the cometary coma (Bhardwaj & Raghuram 2012;Raghuram & Bhardwaj 2014). Moreover, the dissociative recombination process can be considered as a source of excited C( 1 D) and C( 3 P) atoms, whose emission has been detected in the Hale-Bopp comet (Feldman 1978;Raghuram et al 2016).…”

Section: Resultsmentioning

confidence: 99%

“…The main channels for producing various atomic/neutral species such as O( 3 P), O( 1 D), O( 1 S), C( 3 P), and C( 1 D) in the inner cometary coma (Raghuram et al 2016) are the dissociative excitation of the neutral molecular species by photons and supra-thermal electrons such as photoelectrons, as well as the DR of the molecular ions. For example, the oxygen atoms (O( 1 D) and O( 1 S)) are produced by the photodissociation of CO, CO 2 and H 2 O molecules coming from the sublimation of the cometary ices (Bhardwaj & Raghuram 2012;Decock et al 2013;Raghuram & Bhardwaj 2014;Decock et al 2015).…”

Section: Dissociationmentioning

confidence: 99%

“…The high density of electrons and molecular ions in the cometary coma facilitates the reactive processes among them, such as the dissociative recombination (DR). This process plays an important role in producing numerous carbon and oxygen atoms in metastable states (Raghuram et al 2016;Decock et al 2013;Bhardwaj & Raghuram 2012). Moreover, Feldman (1978) showed that the DR of CO + by low-energy electron impact is the dominant source of carbon atoms rather than the photodissociation of CO in comets Kohoutek (1973 XII) and West (1976 VI).…”

Section: Introductionmentioning

confidence: 99%

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Reactive collision of electrons with CO⁺ in cometary coma

Moulane

Mezei

Laporta

et al. 2018

A&A

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Context. In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional coma, inelastic collisions between thermal electrons and molecular ions result in recombination and vibrational excitation, the rates of these processes being particularly elevated due to the high charged particle densities in the inner region. Aims. This work addresses the dissociative recombination, vibrational excitation, and vibrational de-excitation of electrons with CO + molecular cations. The aim of this study is to understand the importance of these reactive collisions in producing carbon and oxygen atoms in cometary activity. Methods. The cross-section calculations were based on Multichannel Quantum Defect Theory. The molecular data sets, used here to take into account the nuclear dynamics, were based on ab initio R-matrix approach. Results. The cross sections for the dissociative recombination, vibrational excitation, and vibrational de-excitation processes, for the six lowest vibrational levels of CO + -relevant for the electronic temperatures observed in comets -are computed, as well as their corresponding Maxwell rate coefficients. Moreover, final state distributions for different dissociation pathways are presented. Conclusions. Among all reactive collisions taking place between low-energy electrons and CO + , the dissociative recombination is the most important process at electronic temperatures characterizing the comets. We have shown that this process can be a major source of O( 3 P), O( 1 D), O( 1 S), C( 3 P) and C( 1 D) produced in the cometary coma at small cometocentric distances.

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“…heliocentric distances on the emission intensities RPC-IES observations at large (∼3 AU) heliocentric distance have shown that suprathermal electron flux was dynamically varying in the cometary coma (Clark et al, 2015;Broiles et al, 2016;Madanian et al, 2016Madanian et al, , 2017. The initial plasma studies of Galand et al (2016), when the comet was at 3 AU from the Sun, have shown that electron impact ionization rate is higher than photoionization rate by a factor 2 to 10. Using RPC-IES measured suprathermal electron flux and H 2 O electron impact ionization cross section, Galand et al (2016) have also shown that electron impact H 2 O ionization frequency significantly varies (between 0.1-8 × 10 −7 s −1 ) within a few hours of observation.…”

Section: The Effect Of Electron Impact Excitation At Large (∼3 Au)mentioning

confidence: 99%

A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

Raghuram

Bhardwaj

2020

Icarus

Self Cite

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Alice ultraviolet spectrometer onboard Rosetta space mission observed several spectroscopic emissions emanated from volatile species of comet 67P/Churyumov-Gerasimenko (hear after 67P/C-G) during its entire escorting phase. The measured emission intensities, when the comet was at around 3 AU pre-perihelion, have been used to derive electron densities in the cometary coma assuming that H I and O I lines are solely produced by electron impact dissociative excitation of cometary parent species (Feldman et al., 2015). We have developed a photochemical model for line emissions by accounting for major production pathways. The developed model has been used to calculate the emission intensities of these lines as a function of nucleocentric projected distance and also along the nadir view by varying the input parameters, viz., neutral abundances and cross sections. We have quantified the percentage contributions of photon and electron impact dissociative excitation processes to the total intensity of the emission lines, which has an important relevance for the analysis of Alice observed spectra. It is found that in comet 67P/C-G, which is having neutral gas production rate of about 10 27 s −1 when it was at 1.56 AU from the Sun, photodissociative excitation processes are more significant compared to electron impact reactions in determining the atomic emission intensities. Based on our model calculations, we suggest that the observed atomic hydrogen, oxygen, and carbon emission intensities can be used to derive H 2 O, O 2 , and CO, abundances, respectively, rather than electron density in the coma of 67P/C-G, when comet has a gas production rate of ≥ 10 27 s −1 .

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

Cited by 5 publications

References 78 publications

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Reactive collision of electrons with CO⁺ in cometary coma

A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

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

Cited by 5 publications

References 78 publications

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Reactive collision of electrons with CO+ in cometary coma

A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

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Reactive collision of electrons with CO⁺ in cometary coma