Ionic emissions in comet C/2016 R2 (Pan-STARRS)
MMNRAS , 1–12 (2019) Preprint 23 April 2020 Compiled using MNRAS L A TEX style file v3.0
Ionic emissions in comet C/2016 R2 (Pan-STARRS)
Kumar Venkataramani , (cid:63) Shashikiran Ganesh and Kiran S.Baliyan Astronomy & Astrophysics Division, Physical Research Laboratory, Ahmedabad, India. Department of Physics, Leach Science Center, Auburn University, Auburn, AL, USA.
23 April 2020
ABSTRACT
We carried out observations of a peculiar comet, C/2016 R2 (Pan-STARRS), using a lowresolution spectrograph mounted on the 1.2m telescope at Mount Abu Infrared Observatory,India. The comet was observed on two dates in January 2018, when it was at a heliocentricdistance of 2.8 AU. Study based on our observations revealed that the optical spectrum of thiscomet is quite unusual as compared to general cometary spectra. Most of the major cometaryemissions like C , C and CN were absent in comet C/2016 R2. However, the comet spectrumshowed very strong emission bands from ionic species like CO + and N + . A mean N /CO ratioof 0.09 ± ± + and N + were calculated from their emission bands. The optical spectrum suggests that the cometaryice is dominated by CO. The low depletion factor of N /CO ratio in this comet, as comparedto the solar nebula and the unusual spectrum of the comet are consequences of distinctiveprocessing at the location of its formation in the early solar nebula. Key words: molecular processes – methods: observational – techniques: spectroscopic –telescopes – comets: individual: C/2016 R2 – Oort Cloud
Cometary molecular emissions are well known and have beenstudied since a long time. A typical optical spectrum of a comet withwell developed coma shows molecular emissions (Venkataramaniet al. 2016) dominated by carbon chain radicals like C and C .NH and CN are two other species which show prominent emissionlines in the optical spectrum. In general, these molecular emissionsstart appearing sequentially when the comet comes closer than 3AU (Krishna Swamy 2010) to the Sun. The most likely emission toappear first is that of CN molecule at around 3 AU, followed by therest of the emissions. There are very few comets in which emissionsare reported beyond 3 AU and even fewer beyond 5 AU. Ionicemissions like CO + and N + are rarely seen in the coma of a comet.They are however abundantly found in the plasma tail of comets.Comet 1962 VIII, also known as comet Humason, is one whereCO + emissions were detected(Arpigny 1964) in the coma. Lot ofwork (e.g. Voelzke et al. 1997; Guineva et al. 1999, 2000; Jockerset al. 1987) has been carried out based on the CO + emissionsthat were found in the coma and tail of Comet 1P/Halley. Thereare several more comets in which significant amount of CO + hasbeen detected e.g. 109P/Swift-Tuttle (Boehnhardt & Birkle 1994),122P/1995 S1 (deVico) & C/1995 O1 (Hale-Bopp) (Cochran et al.2000), comet C/1996 B2(Hyakutake) (Wyckoff et al. 1999) to (cid:63) E-mail : [email protected] name a few. CO is the presumed dominant parent source for theproduction of CO + ions in comets, although dissociative ionizationof CO also produces CO + ions in comets (Jockers & Bonev1997a). CO has been detected in all of the comets showing CO + emissions. In fact, strong (A-X) transition CO bands in the UVregion (near 1500 Å) have been detected in all the bright cometsobserved with IUE or HST (Krishna Swamy 2010).Molecular nitrogen (N ) is also one of the most abundantspecies in the star-forming regions (Womack et al. 1992a) and inthe proto-solar nebulae(Hily-Blant et al. 2017; Wyckoff 1992). How-ever, there have been a lot of speculations about its presence incomets, even though N + has been observed in many comets withmostly low resolution spectra. Korsun et al. (2006) claim to have ob-served N + at a record heliocentric distance of 6.8 AU. Cochran et al.(2000) & Cochran (2002) have reported N + in comets 122P/de Vicoand C/2002 C1 (IKEYA-ZHANG) respectively, whereas Wyckoff& Heyd (2003) claims to have observed this emission band in cometC/1995 O1 (Hale-Bopp). A strong confirmation for the presence ofN in comets came from the in-situ measurements of comet 67P/C-G made by ROSINA mass spectrometer on board the Rosetta space-craft (Rubin et al. 2015). The N /CO ratio in comet 67P/ determinedby Rubin et al. (2015) is probably the most authentic value reported,as it was measured in-situ. Mousis et al. (2016) have compared thisN /CO ratio in comet 67P with various laboratory results and have © 2019 The Authors a r X i v : . [ a s t r o - ph . E P ] A p r Venkataramani et al. discussed the formation, agglomeration and origin of these ices oncomets.Comet C/2016 R2 (‘R2’ for all future text references in this pa-per) is an Oort cloud comet discovered by PanSTARRS on Septem-ber 7 th + and N + emissionlines in comet R2 and have estimated the N /CO ratio. However,they have not detected any emissions from the neutral species whichdominate the optical spectrum of a comet. Observations carried outby Wierzchos & Womack (2018) in the submillimeter wavelengthshave indicated that the comet is rich in CO and depleted in HCN.Biver et al. (2018) have also confirmed the lack of HCN in thiscomet. More recent reports have established the uniqueness of thiscomet. McKay et al. (2019) have discussed in detail, the peculiarcomposition of R2 using observations from various ground andspace observing facilities. Opitom et al. (2019a) have obtained highresolution optical spectra of R2. They claim the detection of variousionic species and very faint emissions from the neutrals. Detectionof major emissions from some of the neutral species is expected atsuch heliocentric distances and their non detection underlines theneed for a detailed study of this comet. In his thesis work, Venkatara-mani (2019) has observed and studied this comet in detail using thelow resolution spectrograph LISA. Here we investigate the opticalspectra of R2 by observing it with a low resolution spectrograph. Wederive and present the column densities of the cometary emissionsbased on our observations. We also discuss the implications of theN /CO ratio determined for comet R2 from our observations. The observations of comet R2 were made using a low resolutionspectrograph called LISA mounted on the 1.2m telescope at theMount Abu Infrared observatory, India. The spectrograph coversa wavelength range of 3800 Å to 7400 Å. The 35 µ m slit used inthe spectrograph results in a wavelength dispersion of 2.6 Å perpixel. The observations are described in detail by Venkataramaniet al. (2016). On the 1.2m telescope, the plate scale on the LISAspectroscopic detector plane is 0.25 arcsec per pixel. The slit hasa width of 1.75 arcsec and is 98 arcsec long. The slit was placedalong the North South Direction.The spectra of the comet were obtained on two nights, i.e. 2018January 13 and 2018 January 25. The observation log is shown intable 1. On both the epochs, the comet was observed at an elevationof 83 deg (close to zenith), at an airmass ≈
1. The sky conditionswere photometric on both the nights. The comet was observed ata relatively low solar phase angle of 15 to 18 deg. Hence, thoughthe slit was placed at comet’s photo-centre, the geometry of ob-servation was such that the comet’s tail might be in the directionalong the line of sight. As a consequence, we are integrating theflux coming from all along the tail of the comet. In order to subtractthe contribution from the sky, the telescope was moved away fromthe comet by nearly 30 arc minutes and sky spectra were taken with More details on the spectrograph are available on the web site of themanufacturer: Shelyak Instruments ( ) the same exposure times. The following flux standards, availablein the IRAF standard star database, were observed for the purposeof flux calibration : HD74721, HD81809, HD84937, BD+08d2015.Spectral extraction and data analysis were carried out using IRAFand GNU Octave software respectively. The optical spectrum of the comet R2 was obtained with the long slitof the spectrograph centred on the coma of the comet. The spectrumobtained on 25th January 2018 is shown in the figure 1. The majoremission features are marked in the spectrum and are listed in table2. The major cometary emissions such as the C Swan bands, C and CN are completely absent in the spectrum of comet R2. Eventhough the carbon chain molecules (C and C ) are expected tobe seen only at much closer heliocentric distances, as comparedto the Sun-comet distance at the time of these observations, theleast presumption would be to expect at least CN emissions at thisdistance. However, no such emissions were detected. The lack ofC , C and CN in our observed spectrum is consistent with resultsreported from observations before (Cochran & McKay 2018a,b)and after (Opitom et al. 2019a) our observation dates. The opticalspectrum on the contrary was dominated by a number of CO + emission bands (comet tail system). With low resolution spectra,it is usually difficult to resolve and differentiate CO + bands fromthe major emission features of C , C and CN. However, due tothe absence of such emission bands from the neutral species incomet R2, we were easily able to detect most of the CO + bands inthe region of 4000 Å to 5500 Å. Along with CO + , we have alsodetected a strong emission band feature with a band head centredaround 3910 Å which is attributed to N + emission. In addition tothe emissions detected by Cochran & McKay (2018a,b) with a highresolution spectrograph, we also report the detection of NH bands(0-10-0, 0-11-0, 1-7-0 being the most prominent among them) andtwo significant emission features around 5910 Å and 6200 Å,which could possibly be attributed to H O + ions. Moreover, wehave observed the comet at a heliocentric distance of 2.8 AU inJanuary 2018, whereas Cochran & McKay (2018a,b) have observedit at a much larger distance of 3.09 AU. These observations arecritical as many of the major cometary emissions are triggered atthese distances.As the CO + emissions were extremely strong, the bands wereidentified by matching the wavelengths of the observed emissionpeaks with the Π / and Π / band head emissions as mentionedin Magnani & A’Hearn (1986) (Table 3 of the paper). However, inorder to identify the weaker emissions beyond 5400 Å, cometarylines listed in the catalogues (described in section 3.3) and labspectra have been used. We have obtained the column densities ofthese species seen in the coma of the comet at two dates in January2018. + emission bands Many strong CO + emission bands were detected in the optical spec-trum of the comet (listed in table 2). All these emission bands belongto the comet-tail system of the CO + transitions. Magnani & A’Hearn(1986) have given a theoretical estimate of the relative band inten-sities of the CO + emission bands. They have compared their resultswith those on comet Humason or 1962 VIII (Arpigny 1964). Wehave calculated the band intensities of CO + emission bands in cometR2 at a heliocentric distance of 2.8 AU. MNRAS , 1–12 (2019) omet C/2016 R2 Table 1.
Observational Log Date Mid-UT Heliocentric Geocentric Solar Phase Exposure AirmassDistance r h Distance ∆ (AU) (AU) (degrees) (Seconds)13/01/2018 15:55 2.87 2.13 15 1800 1.0025/01/2018 14:30 2.82 2.23 18 1800 1.00 Figure 1.
Top: The optical spectrum of comet C/2016 R2 taken on 25th January 2018. The CO + emission lines are marked. The emission features beyond 5400Å have also been marked. These features were investigated to search for possible emissions from H O + and NH . Bottom: The spectrum of comet C/2014 Q2(Venkataramani et al. 2016) been shown in order to compare the two spectra. This shows emissions from carbon chain radicals like C , C . CN and resemblesa spectrum of a typical comet. Comet R2 does not show any of these lines and its spectrum is dominated by CO + emissions. Comet Humason was observed at around 2.6 AU and the theo-retical values are calculated for 1 AU sun-comet distance. Two of thestrongest CO + emission bands seen in the spectrum are due to the(2-0) and (3-0) A-X transition. However, the (2-0) band is contam-inated by (0-1) N + emission (Described in section 3.2). Therefore,the band intensities are normalized to (3-0) band and these areshown in table 3. The values reported in other works have also beenlisted in the table alongside our calculated values. (All the valuesare normalized to the (3-0) band).The values of the relative band intensities for comet R2 in table3 match quite well, on both epochs, with the other values reported.The emission band fluxes on both epochs were converted to columndensities using the equation 4 of Venkataramani et al. (2016). Theseare listed in table 4. The g-factors given by Magnani & A’Hearn(1986) have been used for the calculations. + band A very strong emission band of N + , with band-head centred around3910 Å was detected in our spectrum of comet R2. This emission comes from the (0-0) band transition of the first-negative system ofN + ion. Major difficulty in detecting the N + bands in this wavelengthregion is caused by the blending of this band with the strong CNemission which generally over-shadows any minor N + emissionwhich might be present. By comparing the observed spectra ofcomet R2 with that of a typical optical cometary spectra like that ofcomet C/2014 Q2 (Venkataramani et al. 2016), we can clearly seethat the CN emission band is missing in this spectrum. Due to theabsence of CN emission in comet R2, there was no contaminationpresent in the observed N + band. The cometary origin of this bandhas been well justified by Cochran & McKay (2018a,b). In ourcase, although the observations were made in the early half of thenight, we expect that the contamination due to the terrestrial N + skylines would be very low since the N + emissions from the skyare extremely weak during the night (Torr et al. 1992) and anyminimal emission would have been subtracted in the process of skysubtraction.The wavelengths of the band heads for different band tran-sitions of N + and their relative strengths have been tabulated byWallace (1962). The (0-1) and (0-2) band transitions of N + ion giverise to emissions with band heads centred around 4278 Å and Å re- MNRAS000
Top: The optical spectrum of comet C/2016 R2 taken on 25th January 2018. The CO + emission lines are marked. The emission features beyond 5400Å have also been marked. These features were investigated to search for possible emissions from H O + and NH . Bottom: The spectrum of comet C/2014 Q2(Venkataramani et al. 2016) been shown in order to compare the two spectra. This shows emissions from carbon chain radicals like C , C . CN and resemblesa spectrum of a typical comet. Comet R2 does not show any of these lines and its spectrum is dominated by CO + emissions. Comet Humason was observed at around 2.6 AU and the theo-retical values are calculated for 1 AU sun-comet distance. Two of thestrongest CO + emission bands seen in the spectrum are due to the(2-0) and (3-0) A-X transition. However, the (2-0) band is contam-inated by (0-1) N + emission (Described in section 3.2). Therefore,the band intensities are normalized to (3-0) band and these areshown in table 3. The values reported in other works have also beenlisted in the table alongside our calculated values. (All the valuesare normalized to the (3-0) band).The values of the relative band intensities for comet R2 in table3 match quite well, on both epochs, with the other values reported.The emission band fluxes on both epochs were converted to columndensities using the equation 4 of Venkataramani et al. (2016). Theseare listed in table 4. The g-factors given by Magnani & A’Hearn(1986) have been used for the calculations. + band A very strong emission band of N + , with band-head centred around3910 Å was detected in our spectrum of comet R2. This emission comes from the (0-0) band transition of the first-negative system ofN + ion. Major difficulty in detecting the N + bands in this wavelengthregion is caused by the blending of this band with the strong CNemission which generally over-shadows any minor N + emissionwhich might be present. By comparing the observed spectra ofcomet R2 with that of a typical optical cometary spectra like that ofcomet C/2014 Q2 (Venkataramani et al. 2016), we can clearly seethat the CN emission band is missing in this spectrum. Due to theabsence of CN emission in comet R2, there was no contaminationpresent in the observed N + band. The cometary origin of this bandhas been well justified by Cochran & McKay (2018a,b). In ourcase, although the observations were made in the early half of thenight, we expect that the contamination due to the terrestrial N + skylines would be very low since the N + emissions from the skyare extremely weak during the night (Torr et al. 1992) and anyminimal emission would have been subtracted in the process of skysubtraction.The wavelengths of the band heads for different band tran-sitions of N + and their relative strengths have been tabulated byWallace (1962). The (0-1) and (0-2) band transitions of N + ion giverise to emissions with band heads centred around 4278 Å and Å re- MNRAS000 , 1–12 (2019)
Venkataramani et al.
Figure 2.
The column density ratio of N + to CO + as a function of distance from the photo-centre. All the ratios have been determined with respect to the CO + (3-0) band. The N /CO ratio of the solar nebula and for a few other comets have been marked in the figure, for comparison. The line shown for comet R2 isthe mean value of the ratio determined at different distances from the photo-centre. For the other comets, the horizontal lines represent the reported value forrespective objects independent of the distance. The numbers listed in the figure (same as in Table 6) correspond to the following references: 1 Fegley & Prinn(1989), Rubin et al. (2015); 2. This Work/(Venkataramani & Ganesh 2018; Venkataramani 2019); 3. Biver et al. (2018); 4. Cochran & McKay (2018a,b); 5.Opitom et al. (2019a); 6. McKay et al. (2019); 7. Korsun et al. (2006); 8. Rubin et al. (2015); 9. Cochran et al. (2000) spectively. These emissions, if present would get strongly blendedwith the CO + (2-0) and (2-1) emission bands respectively. How-ever, their strengths are extremely weak as compared to the stronger(0-0) band. According to the relative strengths given in table 57 ofWallace (1962), the (0-1) and (0-2) bands are around 25% and 3%as intense as the (0-0) band, respectively. With this, we estimate thecontamination of CO + (2-0) band by the corresponding N2+ bandsto be about 27% and that of (2-1) band to be about 11%.The g-factor given by Lutz et al. (1993) has been used to cal-culate the column densities of the N + band. The abundance ratiosof N /CO play an important role in understanding the early solarnebula. We presume that the major source of N + and CO + is thephoto-ionization of N and CO respectively. An indirect method tocalculate the abundance ratios is to estimate the ratio of column den-sities of the corresponding observable ion species. Therefore, wehave obtained the column density ratios of N + /CO + from the spec-trum of the comet R2. Cochran & McKay (2018a,b) have reporteda value of 0.06 for the N + /CO + column density ratio. They haveused the CO + (2-0) band flux for calculating the ratio. However,due to the contamination of CO + (2-0) band by N + (0-1) band, weare forced to use the integrated flux of CO + (3-0) band to estimatethis ratio. We sum the entire flux in the CO + (3-0) band in orderto calculate the ratios. Taking advantage of the long slit, we haveobtained the N + /CO + column density ratios at different distancesfrom the photo-centre as mapped along the slit. Although the slitcovers a total of 98 arcsec on the sky, the coma profile could onlybe measured upto 12.7 arcsec on either side of the photo-centre.This translates to a distance of 18195 km from the centre. Beyond this distance, SNR was too low to be measured. The N + /CO + col-umn density ratio remains fairly constant within the distance thatwe have covered. We obtain a mean value of 0.09 ± + (0-0) band being con-taminated by the CO + (5-1) emission, while determining N + /CO + ratio. Such contamination would lead to overestimation of this ra-tio. We, therefore state that the reported value is not an exact one,but is an upper cut-off value. This could possibly explain the largervalue of this ratio, as compared to the one reported by Cochran &McKay (2018a,b). Since the relative strength of the CO + (5-1) bandis not known (it has not been explicitly determined by Magnani &A’Hearn (1986)), we assume, that its contribution is negligible. If itwere not the case, our value would have differed significantly fromthe value reported by Cochran & McKay (2018a,b) obtained withhigh resolution spectra. It is also important to note that Biver et al.(2018) have reported a mean value of 0.08 for the N + /CO + columndensity ratio. This is close to the value measured by us and is withinour error limits. In fact, Biver et al. (2018) report a value of 0.09 ± + /CO + ratio is takento be as measured by us.The conversion of ion abundance ratio to the neutral abundance ra-tio N /CO, requires the knowledge of the reaction rates. Assumingthat photo-dissociation of N and CO by solar UV flux is the major MNRAS , 1–12 (2019) omet C/2016 R2 Table 2.
Emission band features detected in the spectrum of comet R2. Thewavelength range specifies the total extent of the band. The NH and H O + bands mentioned in the table indicates that a search for emissions from thesespecies have been carried out in the corresponding wavelength ranges.Wavelength Molecular Band Ref(Å) Species3978-4048 CO + (3-0) 14219-4292 CO + (2-0) 14663-4739 CO + (2-1) 14803-4888 CO + (3-2) 15017-5092 CO + (1-1) 14502-4551 CO + (4-2) 14529-4581 CO + (1-0) 15450-5522 CO + (0-1) 1(blend withNH + (0-2) 1(blend withH O + ∗ NH + O + ∗ H O + + (0-2) band )1-Magnani & A’Hearn (1986), 2-Tegler & Wyckoff (1989), 3-Brown et al.(1996), 4-Lutz et al. (1993) ** Contaminated with CO + doublet contributor for N + and CO + ions respectively and that the electrondissociative recombination is the primary loss mechanism for bothions Womack et al. (1992b), the conversion factor from N + /CO + toN /CO is calculated using the reaction rates used by Womack et al.(1992b). The conversion factor turns out to be 0.98. Since the un-certainty in the measured ratio is greater than 20%, the conversionfactor would be indistinguishable from unity. Therefore we assumethat N + /CO + ≈ N /CO. The column density and the column densityratios have been listed in table 5.The N /CO ratio is of great significance as it helps tounderstand the formation processes in comets and in the earlysolar nebula. The significance and implications of this ratio havebeen discussed at length in section 4. The plot of this ratio asa function of the radial distance from photo-centre is shown infigure 2. The values of N /CO ratio for a few other comets andfor the solar nebula have also been marked, for comparison. TheN /CO ratio for comet R2 obtained from various observations(Cochran & McKay 2018a,b; Biver et al. 2018; McKay et al. 2019;Opitom et al. 2019a) and from this work, along with the value of this ratio measured for other comets have been tabulated in table 6. and H O + With the high resolution spectroscopic observations on UVES-VLT,Opitom et al. (2019a) have detected various emission lines withstrong signatures from CO+, CO + and N +. However they didnot detect any emissions from OH, OH + , NH and H O + . Ourobservations were made about one month prior to their UVES-VLT observations, when the comet was at a relatively lower phaseangle. We are limited by our spectral resolution. However, in orderto understand some of the minor emission features seen on theredder side of 5500 Å in our spectra, we have made an attempt tocompare the emission features with those from different cataloguesand laboratory spectra. Our conclusions are based on how thesefeatures matched with various cometary spectral features listed inthe catalogues. However, their presence seems to be ambiguousowing to various blends in the low resolution spectra. These areclarified with the high resolution spectra of the comet (Opitomet al. 2019a). It should be noted, though, that they have mentionedmany of the lines in these regions as ’unidentified’. The peculiarcomposition of this comet has also been studied and reported byBiver et al. (2018), using the 30m IRAM telescope. They have alsoreported complimentary observations with LISA spectrograph onthe 0.28m telescope. However, they have not reported the spectrabeyond 550 nm owing to low signal-to-noise ratio in their spectra.There have been a few comets in the past whose emissionlines have been catalogued based on their high resolution opti-cal spectrum. Brown et al. (1996) have catalogued emission linesfrom comet 109P/ (Swift-Tuttle) and comet Brorsen-Metcalf (23P/).Cochran & Cochran (2002), Zhang et al. (2001) and Cremoneseet al. (2007) have catalogued the emission lines of comets 122P/(deVico), C/1995 O1 (Hale-Bopp) and 153P/(Ikeya-Zhang) respec-tively, based on their high resolution spectra. Emission lines fromdifferent molecular and ionic species from each of these catalogueswere used to generate a synthetic spectrum, which matches the res-olution of the observed spectra of comet R2. Each emission line inthese catalogues was represented as a gaussian profile with a widthmatching the line profile width in the observed spectrum. All ofthese gaussian profiles were then summed up as a function of wave-length. The result was a spectrum with a resolution similar to that ofour observed spectrum. The synthetic spectrum generated using allof the above catalogues were matched with our observed spectrum.We note that, not all the lines are present with the same relativeintensities in all the comets with available high-resolution spectra.It was seen that the spectrum of comet 109P/ (Swift-Tuttle) formsthe best match for our observed spectrum of comet R2. Therefore,this spectrum of the comet 109P/ by Brown et al. (1996) was usedto identify the emission bands seen in our observed spectrum. Inthe low resolution spectra, the blending of various emission linesdefine the pattern of the observed emission band. Therefore, theidentification was based on wavelength coincidence and by visuallymatching the emission band patterns in the observed spectrum withthat of the synthesized one. Such an initial analysis indicated a pos-sible presence of emissions from NH and H O + in the spectrum. In The phrase ’synthetic spectra’ usually refers to spectra generated usingtheoretical calculations. However, we use this phrase for the low resolu-tion spectrum generated using the high resolution cometary lines from thecatalogueMNRAS000
Emission band features detected in the spectrum of comet R2. Thewavelength range specifies the total extent of the band. The NH and H O + bands mentioned in the table indicates that a search for emissions from thesespecies have been carried out in the corresponding wavelength ranges.Wavelength Molecular Band Ref(Å) Species3978-4048 CO + (3-0) 14219-4292 CO + (2-0) 14663-4739 CO + (2-1) 14803-4888 CO + (3-2) 15017-5092 CO + (1-1) 14502-4551 CO + (4-2) 14529-4581 CO + (1-0) 15450-5522 CO + (0-1) 1(blend withNH + (0-2) 1(blend withH O + ∗ NH + O + ∗ H O + + (0-2) band )1-Magnani & A’Hearn (1986), 2-Tegler & Wyckoff (1989), 3-Brown et al.(1996), 4-Lutz et al. (1993) ** Contaminated with CO + doublet contributor for N + and CO + ions respectively and that the electrondissociative recombination is the primary loss mechanism for bothions Womack et al. (1992b), the conversion factor from N + /CO + toN /CO is calculated using the reaction rates used by Womack et al.(1992b). The conversion factor turns out to be 0.98. Since the un-certainty in the measured ratio is greater than 20%, the conversionfactor would be indistinguishable from unity. Therefore we assumethat N + /CO + ≈ N /CO. The column density and the column densityratios have been listed in table 5.The N /CO ratio is of great significance as it helps tounderstand the formation processes in comets and in the earlysolar nebula. The significance and implications of this ratio havebeen discussed at length in section 4. The plot of this ratio asa function of the radial distance from photo-centre is shown infigure 2. The values of N /CO ratio for a few other comets andfor the solar nebula have also been marked, for comparison. TheN /CO ratio for comet R2 obtained from various observations(Cochran & McKay 2018a,b; Biver et al. 2018; McKay et al. 2019;Opitom et al. 2019a) and from this work, along with the value of this ratio measured for other comets have been tabulated in table 6. and H O + With the high resolution spectroscopic observations on UVES-VLT,Opitom et al. (2019a) have detected various emission lines withstrong signatures from CO+, CO + and N +. However they didnot detect any emissions from OH, OH + , NH and H O + . Ourobservations were made about one month prior to their UVES-VLT observations, when the comet was at a relatively lower phaseangle. We are limited by our spectral resolution. However, in orderto understand some of the minor emission features seen on theredder side of 5500 Å in our spectra, we have made an attempt tocompare the emission features with those from different cataloguesand laboratory spectra. Our conclusions are based on how thesefeatures matched with various cometary spectral features listed inthe catalogues. However, their presence seems to be ambiguousowing to various blends in the low resolution spectra. These areclarified with the high resolution spectra of the comet (Opitomet al. 2019a). It should be noted, though, that they have mentionedmany of the lines in these regions as ’unidentified’. The peculiarcomposition of this comet has also been studied and reported byBiver et al. (2018), using the 30m IRAM telescope. They have alsoreported complimentary observations with LISA spectrograph onthe 0.28m telescope. However, they have not reported the spectrabeyond 550 nm owing to low signal-to-noise ratio in their spectra.There have been a few comets in the past whose emissionlines have been catalogued based on their high resolution opti-cal spectrum. Brown et al. (1996) have catalogued emission linesfrom comet 109P/ (Swift-Tuttle) and comet Brorsen-Metcalf (23P/).Cochran & Cochran (2002), Zhang et al. (2001) and Cremoneseet al. (2007) have catalogued the emission lines of comets 122P/(deVico), C/1995 O1 (Hale-Bopp) and 153P/(Ikeya-Zhang) respec-tively, based on their high resolution spectra. Emission lines fromdifferent molecular and ionic species from each of these catalogueswere used to generate a synthetic spectrum, which matches the res-olution of the observed spectra of comet R2. Each emission line inthese catalogues was represented as a gaussian profile with a widthmatching the line profile width in the observed spectrum. All ofthese gaussian profiles were then summed up as a function of wave-length. The result was a spectrum with a resolution similar to that ofour observed spectrum. The synthetic spectrum generated using allof the above catalogues were matched with our observed spectrum.We note that, not all the lines are present with the same relativeintensities in all the comets with available high-resolution spectra.It was seen that the spectrum of comet 109P/ (Swift-Tuttle) formsthe best match for our observed spectrum of comet R2. Therefore,this spectrum of the comet 109P/ by Brown et al. (1996) was usedto identify the emission bands seen in our observed spectrum. Inthe low resolution spectra, the blending of various emission linesdefine the pattern of the observed emission band. Therefore, theidentification was based on wavelength coincidence and by visuallymatching the emission band patterns in the observed spectrum withthat of the synthesized one. Such an initial analysis indicated a pos-sible presence of emissions from NH and H O + in the spectrum. In The phrase ’synthetic spectra’ usually refers to spectra generated usingtheoretical calculations. However, we use this phrase for the low resolu-tion spectrum generated using the high resolution cometary lines from thecatalogueMNRAS000 , 1–12 (2019)
Venkataramani et al.
Table 3.
Relative intensities of CO + emission bands (Comet-tail system). All the band intensities are normalized to the (3-0) bandBand Transition This work (Comet C/2016 R2) Arpigny(observations) A’Hearn (Theory) Krishnaswamy (Theory) Arpigny (1964); Magnani & A’Hearn (1986); Krishna Swamy (1979)
Table 4. CO + emission bands in comet C/2016 R2Band Transition Wavelength range g-factor Column Density(Å) ( × − ergs mol − sec − ) ( × mol cm − )13/01/2018 25/01/2018(3-0) 3978-4048 2.20 2.20 1.88(2-0) 4219-4292 1.79 < < < < Table 5. N + emission band [B-X(0-0) transition] in comet C/2016 R2 - Wavelength range : 3810-3935 ÅDate Column Density Column Density Ratio( × mol cm − ) N ( N + ) N ( CO + ) order to further investigate our results and search for the presence ofthese species, we have used the CO + laboratory spectra (Kim 1994,and personal communication with Dr.Ivanova, Astronomical Insti-tute of the Slovak Academy of Sciences) to compare the emissionfeatures seen in the observed spectrum. There are certain featuresof H O + and NH which blend completely with some of the CO + emissions. However, there are also certain emission features whichstand out and which do not blend with the CO + emission bands.This is explained in more detail in the sub-sections below. Search for NH emission features The figure 3 shows the observed spectrum plotted along with theNH synthetic spectrum (Brown et al. 1996) and the CO + lab spec- trum (Kim 1994). Two strong emission band features in the observedspectrum significantly matched the NH synthetic spectrum. One ofthem is the NH (0-10-0) band near 5700 Å, and second one is the(0-11-0) band near 5430 Å. The (0-10-0) band coincides with a CO + doublet at 5700 Å. The (0-11-0) band is relatively less contaminatedand better resolved. There is a minor blend towards the end of itsprofile from the nearby CO + (0-1) doublet emission feature centredaround 5460 Å and 5500 Å. A few other minor features were alsodetected, which could be attributed to NH emissions e.g (1-7-0)band near 5380 Å. However, strong emissions from (0-9-0) bandcentred around 6000 Å are missing in the observed spectrum. Thecolumn densities and production rates of 0-11-0 emission bandswere calculated with 3 σ (photon-noise of 10%) upper limits. Theseare listed in table 7. The g-factors from Tegler & Wyckoff (1989) MNRAS , 1–12 (2019) omet C/2016 R2 Table 6.
The N /CO ratio for the Solar nebula, various comets and comet R2 obtained from various observations. The numbers listed in the final column (sameas in Fig. 2) correspond to the following references: 1 Fegley & Prinn (1989), Rubin et al. (2015); 2. This Work/(Venkataramani & Ganesh 2018; Venkataramani2019); 3. Biver et al. (2018); 4. Cochran & McKay (2018a,b); 5. Opitom et al. (2019a); 6. McKay et al. (2019); 7. Korsun et al. (2006); 8. Rubin et al. (2015);9. Cochran et al. (2000) Object r ∆ Instrument Observatory N /CO Reference (AU) (AU)Solar Nebula – – – – 0.145 1C/2016 R2 2.8 2.1 LISA (R=1000) 1.2m Mount Abu 0.09 ± ± ± ± ± † ROSINA Rosetta 5.70 ± × − × − × − † Rosetta observed the comet 67P/C-G at a distance of 10km from the nucleus in Oct 2014. Observations carried out from May to Oct 2014.
Figure 3.
The observed spectrum of comet C/2016 R2 (blue) plotted along with the synthetic spectra generated using the catalogued NH lines (black) incomet 109P/Swift-Tuttle (Brown et al. 1996). The red spectrum is the CO + laboratory spectra (Kim 1994). The NH (0-10-0) band is strongly blended with aCO + feature. The NH (0-11-0) emission feature is relatively better resolved. However there is a minor blend with the (0-1) CO + doublet towards the edge ofthe profile.MNRAS000
The observed spectrum of comet C/2016 R2 (blue) plotted along with the synthetic spectra generated using the catalogued NH lines (black) incomet 109P/Swift-Tuttle (Brown et al. 1996). The red spectrum is the CO + laboratory spectra (Kim 1994). The NH (0-10-0) band is strongly blended with aCO + feature. The NH (0-11-0) emission feature is relatively better resolved. However there is a minor blend with the (0-1) CO + doublet towards the edge ofthe profile.MNRAS000 , 1–12 (2019) Venkataramani et al. and daughter scale-lengths from Fink et al. (1991) were used in theprocess. We assume that NH is the only parent of NH which isproduced through the photolytic process (Wyckoff et al. 1989) NH + h ν −→ NH + H. Since this process has a branching ratio of 95%,it would be safe to assume that the production rate of NH willbe equivalent to that obtained from the NH band fluxes. A syn-thetic spectrum was generated (as described in section 3.3) usingthe strengths of the lines in NH bands given in Brown et al. (1996)in order to examine the blending of different emission bands in theobserved low resolution spectrum. The 0-11-0 band was contami-nated by the presence of 1-7-0 band. The intensity ratio of 1-7-0 to0-11-0 emissions within the selected wavelength region was calcu-lated based on the strengths given by Brown et al. (1996). This ratiowas then used to remove the flux contribution of 1-7-0 emission inthe 0-11-0 band of the observed comet spectrum. The 0-11-0 bandis also contaminated at the tail end of its profile by Π / emissionof the (0-1) band of CO + which is centred at 5461. The relativeintensity of this band is unknown and has also not been explicitlycalculated by Magnani & A’Hearn (1986). It is therefore difficult toquantify the amount of contamination in the (0-11-0) band of NH .Since we are overestimating the (0-11-0) band flux, the calculatedcolumn densities and production rates for this band represent anupper limit.Prior to comet R2, the only comet in which N + , CO + and NH have been detected simultaneously is comet Halley. Womack et al.(1992b) have determined the average N /NH ratio in comet Halleyand in various star-forming regions. They report a value of 0.1 forthis ratio in comet Halley which differs significantly from the valuesfor the star forming regions. All of these three species have beendetected in comet R2, although the detection of NH seems to beonly marginal. Search for H O + emission features Figure 4 shows the observed spectrum plotted along with H O + (Lew 1976) and CO + (Kim 1994) lab spectra. The lab spectra arescaled to relative strengths in the vertical axis in order to lookfor matching features. The (0-8-0) H O + emission feature centredaround 6200 Å and the (0-9-0) feature centred at around 5910 Å inthe lab spectrum matches well with the observed spectrum. The(0-8-0) is severely contaminated by the CO + (0-2) doublet andtherefore, their individual contributions cannot be determined for(0-8-0) H O + and (0-2) CO + band at 6200 Å. However, the (0-9-0)H O + feature does not have any contamination from the CO + emis-sions.The observed spectrum was also compared to the H O + emissionsfrom other comet observations. It was seen, that both the (0-8-0)and (0-9-0) emission features matched quite well with the corre-sponding H O + emission features listed in the catalogues for othercomets. This can be seen in figures 5 and 6, where the spectrumof comet R2 is compared with the H O + synthetic spectra createdby degrading the high resolution spectra of comets 109P/ (Swift-Tuttle) and C/1996 B2 (Hyakutake) observed by (Brown et al. 1996)and Wyckoff et al. (1999) respectively. The NH synthetic spectrumhas also been plotted, in the figure, to check for possible blend be-tween the emissions from the two species in this wavelength region.Though there is a very weak blend of the (0-9-0) feature at 5910Å with one NH band at 5931 Å (the blend occurs at the tail of theline profile), the two features can be significantly resolved in theobserved spectrum.Based on the above analysis, the emission feature at 5910 Å appearsto be a relatively better indicator for the presence of H O + ions. As seen from figure 4, the emission feature centred around 6200 Å isexpected to have a strong blend with the Π / (0-2) emission of CO + at 6189 Å. Owing to the low resolution of the observed spectrum,it is not possible to estimate the extent of contamination using fluxratios with the stronger CO + bands. However, this blend and thematch with the synthetic spectra (figure 5) can be understood in thefollowing way.The H O + synthetic spectrum has been generated using thecatalogued (Brown et al. 1996) high resolution H O + emissionlines of comet 109P/Swift-Tuttle. The same comet has been imagedby Jockers & Bonev (1997b) in CO + (2-0) band and H O + (0-8-0)band filters centred around 4260 Å and 6200 Å respectively. Usingthe g-factors from Magnani & A’Hearn (1986), they claim that theH O + filter is contaminated by CO + (0-2) band up to an extent of ≈ + /H O + , inthe tail-ward direction at distances of 10 km, to be ≈ + to bediffused close to the nucleus, looking at figure 10 of Jockers & Bonev(1997b), the CO + /H O + ratio varies from 50 to 60% at the nuclearcentre, which could be a significant value. As discussed above, theH O + synthetic spectra created based on the spectral catalogue inBrown et al. (1996) matches quite well with the emission featurearound 6200 Å in the observed spectrum of R2. Brown et al. (1996)obtained the high resolution spectra by centring the comet image onthe slit, which implies, that they are looking at the photo-centre ofthe comet. Though they have catalogued the H O + lines, there is nomention or indication of CO + emissions in their spectra. They haveidentified the H O + emission lines based on the laboratory spectragiven by Lew (1976). Therefore, it is possible that the cataloguedhigh resolution emission lines of H O + might be contaminated withCO + emissions and subsequently, the synthetic spectra derived fromthis catalogue might also have blended emissions from both CO + and H O + , thereby matching well with the observed spectra.It is difficult to conclude the presence of H O + based on the 6200Å features because of the blend with the CO + doublet. The emissionat 5910 Å stands out as a separate feature free of any contaminationfrom CO + . The integrated flux ratio of this emission feature withrespect to the CO + (3-0) doublet is 0.18 ± O/CO ratio to be 0.32%, which is significantlylower than the flux ratio that we have measured. However, since theg-factors for the H O + (0-9-0) is unknown, the conversion of theflux ratio into ratio of production rates of H O + /CO + is not feasible.Although it does not conclusively prove the detection of emissionsfrom H O + ions, above evidences suggest that their presence incomet R2 is highly probable. The unusual spectrum of comet R2 raises a lot of questions on thecomet’s origin and its formation in the solar system. The implica-tions of such a spectrum and the likely origin of the comet has beendiscussed by Venkataramani (2019) at length. The detection of N + or the parent N in comets is vital, as they provide key informationon the formation of ices in the early solar nebula and their agglom-eration in comets. The abundance of N in a comet will stronglydepend on the location (and hence temperature), where the cometwas formed. This would define the process of comet agglomerating MNRAS , 1–12 (2019) omet C/2016 R2 Figure 4.
The observed spectrum of comet C/2016 R2 (blue) plotted along with the synthetic spectra generated using the laboratory spectra (Lew 1976) ofH O + lines (green). The CO + (red) laboratory spectra (Kim 1994) is shown for comparison. The (0-8-0) band at 6200 Åblends strongly with the CO + feature.However, the (0-9-0) feature at 5910 Åis seen without any blend. Figure 5.
The observed spectrum of comet C/2016 R2 plotted along with the synthetic spectrum generated using the catalogued H O + lines in comet109P/Swift-Tuttle (Brown et al. 1996). The NH synthetic spectrum has also been plotted in the figure to check for any kind of blend between the emissionsfrom both the species. Table 7. NH emission in comet C/2016 R2:.The 3 σ (photon noise of 10%) upper limits of Column Density and Production rate have been listed.Band Transition Wavelength range g-factor Column Density Production rate(Å) ( × − photons mol − sec − ) ( × mol cm − ) ( × mol sec − )13/01/2018 25/01/2018 13/01/2018 25/01/2018(0-11-0) 5410-5495 4.17 < < < <000
The observed spectrum of comet C/2016 R2 plotted along with the synthetic spectrum generated using the catalogued H O + lines in comet109P/Swift-Tuttle (Brown et al. 1996). The NH synthetic spectrum has also been plotted in the figure to check for any kind of blend between the emissionsfrom both the species. Table 7. NH emission in comet C/2016 R2:.The 3 σ (photon noise of 10%) upper limits of Column Density and Production rate have been listed.Band Transition Wavelength range g-factor Column Density Production rate(Å) ( × − photons mol − sec − ) ( × mol cm − ) ( × mol sec − )13/01/2018 25/01/2018 13/01/2018 25/01/2018(0-11-0) 5410-5495 4.17 < < < <000 , 1–12 (2019) Venkataramani et al.
Figure 6.
The observed spectrum of comet C/2016 R2 plotted along with the synthetic spectra generated using the catalogued H O + lines in comet 109P/Swift-Tuttle (Brown et al. 1996) and H O + lines in comet C/1996 B2 (Wyckoff et al. 1999) the condensed ices from the nebula, trapping the gases and the chem-istry thenceforth. Since N and CO condense and evolve at similartemperatures (Mousis et al. 2010), measuring the N /CO ratio be-comes vital. The N /CO ratio calculated for comet R2 (see table 6for all references) is the highest value to be reported as compared toany of the other comets in which this ratio has been measured. Thecomparison of this ratio with some of the other comets and its valuein the solar nebula (as calculated by Rubin et al. 2015, in the gasphase proto-solar Nebula) is shown in figure 2. Cochran et al. (2000)determined the N + /CO + ratios in comets 122P/de Vico and cometHale-Bopp and derived the upper limits as 3 × − and 9.9 × − respectively. They have also referenced many other works in whichN + detection has been reported and the N + /CO + ratio has beenmeasured. Korsun et al. (2006) have detected N + in comet C/2002VQ94 at a heliocentric distance of 6.8 AU and have reported a sig-nificantly larger value of N + /CO + = 0.04. However, they claim N + as only a tentative detection. In general, the N /CO ratios measuredin comets seem to be significantly depleted as compared to the ratiosof the proto-solar nebula (PSN) itself. Iro et al. (2003) have pointedout, that the deficiency of molecular nitrogen as compared to CO isbecause of the fact that CO forms clathrate hydrate more easily thanN . This also depends on the amount of water ice available for theclathration to take place. One of the recent and probably the mostauthentic in-situ measurements of N /CO ratio in comet 67P/CGwas made by ROSINA mass spectrometer on board the Rosettaspacecraft(Rubin et al. 2015). They report an average value for theratio N /CO = 5.70 × − . Assuming a proto-solar gas phase valueof N /CO = 0.145, they have estimated the depletion factor of thisratio to be ≈ in amorphous water ice or the clathrate cagesformed by crystallised water ice. However, they also quote, that thisratio might increase below a temperature of 24 K due to increasedefficiency of N trapping. The measured value of N /CO ratio incomet R2 is larger than the mean value of comet 67P/ by about anorder of magnitude. Assuming an N /CO ratio of 0.145 for the solarnebula, the depletion factor based on our calculated value of this ratio turns out to be about ≈ ± /CO = 0.06) is consistent with the laboratoryexperiment results Owen & Bar-Nun (1995), where a similar ratiois expected in the gases trapped in amorphous water ice, assumingthat the icy planetesimals formed in the solar nebula at around 50Kand N /CO ≈ − µ m min − reaching an ice thickness of 0.1 µ m as compared to afaster rate of 0.3 µ m min − reaching an ice thickness of 10 µ m in theearly experiments (Lofer 2018, priv. comm). They quote, that theabundance of trapped gases would highly depend on ice depositionrates and have found that the rapid deposition may be inappropriatefor the conditions in the interstellar medium or the PSN. With theslower deposition rate and assuming a nebular N /CO value of 0.22Bar-Nun et al. (2007) have estimated N /CO ratio in comets to be6.6 × − using the experimentally found depletion factor of 3 × − . This is much smaller than the value obtained for comet R2 andis not consistent with the model of N being trapped by amorphouswater ice. Rubin et al. (2015) and Mousis et al. (2016) have bothcited Bar-Nun et al. (2007) in order to compare and interpret N /COratio of comet 67P. We refer to the work of Bar-Nun et al. (2007),Rubin et al. (2015) and Mousis et al. (2016) in order to explainthe large N /CO ratio found in comet R2. There are two possiblescenarios:(i) N trapped in amorphous water ice: If N in comet R2 wastrapped by amorphous water ice, the trapping efficiency has to bemuch greater than what has been observed in other comets and this This depletion factor corresponds to a value of 33.3 in terms of the samefactor calculated by Rubin et al. (2015), which is an inverse as compared tothe one calculated by Bar-Nun et al. (2007). MNRAS , 1–12 (2019) omet C/2016 R2 would be possible if the temperature is much lower than 24K (Rubinet al. 2015). At 24K, the depletion factor is found to be about ≈ ≈ Condensation of N and CO as pure ices: Figure 1 ofMousis et al. (2016) gives a detailed comparison of N /CO ratiomeasured in comet 67P/ to that of the one sampled from the (i) gasestrapped in amorphous ice, (ii) case where N and CO are crystallisedas pure ices in the proto-solar nebula and (iii) as gases trapped in theclathrate cages. The ratio measured for comet 67P is consistent withthe laboratory experiment results of N being trapped in amorphousice at around 24-30 K range. If we consider trapping in clathratecages, the 67P/ data is consistent with this result in the temperaturerange of 44-50 K. The higher N /CO ratio for comet R2 does notseem to match both the above mentioned results. However, it doesmatch the results in the case where N and CO are crystallised aspure ices. This is possible when there is scarcity of water ice toeither form clathrate hydrates or amorphous ice to trap these gases.The implications of both the above scenarios would be quitedifferent. We refer to the introductory section of Mousis et al. (2016)and references therein to understand the formation of two reservoirsof ices in the proto-solar nebula (PSN). In the proto-solar nebula,the icy planetesimals within 30 AU of Sun, which agglomeratedices from the interstellar medium were vaporised due to their solarvicinity. In the process of cooling of the proto-solar nebula, watercondensed at around 150 K to form crystalline ice. As long ascrystalline water was available, the planetesimals formed in thisregion agglomerated ices in the form of clathrates. In the case wherewater was not sufficient or clathration was not possible, pure CO andN ices would have possibly formed. If we assume that the secondscenario is true in case of comet R2, there is definitive probabilitythat this Oort cloud comet formed and originated in the inner regionsof the solar system, within 30 AU of Sun. This is not surprising, asthere have been many theories and models, which predict that theOort cloud comets initially originated very close to the sun and werethen propelled into the Oort cloud by the gravitational influence ofgiant planets e.g. (Brasser & Morbidelli 2013; Kaib & Quinn 2008).In case of the first scenario, the comet must have formed at verylarge heliocentric distance, either at the edge of the PSN, or in theouter interstellar medium, which is then captured by the Oort cloud.However, in case of both the formation scenarios mentioned above,the unusual spectrum of the comet is still not justified. Not only doescomet R2 come under the category of carbon poor comets (absenceof C and C emissions, but not accounting for the Carbon in CO),but also the absence of the cyanogen emission and presence of NH once again poses intriguing questions on the formation and origin ofthe comet. If we ignore the two major ionic emissions in this comet,it becomes quite similar to the unusual comet 1988r (Yanaka) whichwas studied in detail by Fink (1992). This comet showed a lot ofNH emissions and also possibly CO emissions (not confirmed, butreported in the article), but no signatures of major emissions fromC or CN. Fink (1992) explains the possible origin of this cometas coming from the outer molecular clouds with a compositionconsiderably different from that of the known ones. Studies on therecently seen interstellar comet 2I/Borisov (Opitom et al. 2019b;Kareta et al. 2020) show that the prediction by Fink (1992) could,indeed, be true. In fact interstellar comets may be more commonthan they appear to be (Gibbs 2019; Hands & Dehnen 2020). Asimilar conclusion could be drawn for comet R2 due to its unusualcomposition questioning the location of its formation. The studyby Mousis et al. (2019) also explains the condensation scenarios ofcomet R2 based on the recent observations. Our spectroscopic observations of the comet C/2016 R2 from theMount Abu Infra-red Observatory have revealed its unusual behav-ior. The comet spectra did not exhibit any of the major cometaryemissions from the neutral species (except NH ), which are ex-pected in an Oort cloud comet at the observed heliocentric distance.The following conclusions are drawn from the present spectroscopicobservations of the comet :(i) The optical spectrum is strongly dominated by emissions fromCO + and N + .(ii) The comet spectrum also exhibited emission features on theredder side of 5400 Å . A search for possible emissions from H O + and NH were carried out by comparing the observed spectrum withthe CO + lab spectrum and spectral features from other comets. The(0-9-0) H O + and (0-11-0) NH emissions matched the observedspectra without any contamination from CO + emissions.(iii) The N /CO ratio estimated for this comet (0 . ± .
02) ishighest ever reported for any comet for which such ratio has beenmeasured. The ratio is found to be uniform, within the measurementerrors, over a distance of ∼ /CO ratio forcomet R2 could be an indication of chemically different conditionsthat persisted at the location of formation of the comet in the proto-solar nebula.(v) Two possible formation scenarios have been proposed inwhich either N is trapped in amorphous water ice at low tem-peratures below 24K or it has condensed as pure ice. ACKNOWLEDGMENTS
This work is supported by the Dept. of Space, Govt. of India. We thankthe referee, Dr Michael DiSanti, for critically reviewing the manuscript andproviding comments which have improved this paper. We would like tospecially thank Dr. S. Raghuram (PRL) and K. Aravind (PRL) for usefuldiscussions. We acknowledge the local staff at the Mount Abu Infra-RedObservatory for their help and a special thanks to Mr. Prashant Chauhan &Mr. Jinesh Jain, for their assistance in the observations. We would like tothank Dr. Diana Lofer and Prof. Dennis Bodewits for their invaluable inputsand suggestions. We would like to specially thank Dr. Oleksandra Ivanovafor providing the lab spectra for our analysis. We also thank our colleaguesin the Astronomy & Astrophysics division at PRL for their comments andsuggestions.This research has made use of ephemerides from NASA HORIZONS systemand NASA’s Astrophysics Data System Bibliographic Services
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The production rate of NH have been derived using the following formula (Krishna Swamy 2010) Q = π ∆ g τ × Flux (A1)where ∆ is the earth-comet distance, g is the fluorescence efficiency of the molecule and τ is the lifetime of the molecule.The observed column density is calculated as given below. N = π g F Ω (A2)where g is the fluorescence efficiency in ergs per molecule per second, and Ω is the solid angle subtended by the aperture in steradians. Ω is calculated as theproduct of the slit width and size of the aperture used to extract the spectrum.The calculated column densities of the CO + , NH and N + in comet C/2016 R2 have been tabulated in tables 4, 7 and 5 respectively.The column density ratios are calculated as N ( N + ) N ( CO + ) = F ( N + ) g ( CO + ) g ( N + ) F ( CO + ) (A3)The calculated column density ratios in comet C/2016 R2 have been tabulated in table 5. ERROR ESTIMATION
The errors in the calculations have been estimated using the error formula (photon noise) √ n , where n is the number counts in the raw spectrum (Assuming aPoisson distribution for a large number of photons). This results in ≈
10% errors in the calculated fluxes, column densities and production rates.MNRAS000