Activity of the first interstellar comet 2I/Borisov around perihelion: Results from Indian observatories
Aravind Krishnakumar, Shashikiran Ganesh, Kumar Venkataramani, Devendra Sahu, Dorje Angchuk, Thirupathi Sivarani, Athira Unni
MMNRAS , 1–10 (2021) Preprint 11 January 2021 Compiled using MNRAS L A TEX style file v3.0
Activity of the first interstellar comet 2I / Borisov around perihelion:Results from Indian observatories
Aravind Krishnakumar , (cid:63) , Shashikiran Ganesh , Kumar Venkataramani ,Devendra Sahu, Dorje Angchuk, Thirupathi Sivarani, Athira Unni Physical Research Laboratory, Ahmedabad, India Institute of Technology Gandhinagar, Gandhinagar, India Auburn University, Auburn, USA Indian Institute of Astrophysics, Bangalore, India
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
Comet 2I / Borisov is the first true interstellar comet discovered. Here we present results from observational programs at twoIndian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 mtelescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at theHCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 31 / / / / H = C and C abundances. We find the production rate ratio, Q(C ) / Q(CN) = ± ) / Q(CN) = ± . ≤ r ≤ . / Borisov’s behaviour is analogous to that of the Solar system comets.
Key words: comets:general – comets: individual: 2I / Borisov – techniques:photometric – techniques:spectroscopic
Comets are made up of pristine material inherited from the proto-solar nebula and they have spent most of their time in the fartherreaches of the Solar system. In consequence, they can be consideredto be the time-capsules of the early Solar system. Hence, studyingthe various aspects of these minor bodies can help us gain insightinto the conditions that prevailed during the formation of the So-lar system. In this aspect, comparing comets from our Solar systemwith interstellar ones can shed light on the di ff erence / similarity inthe materials present in di ff erent proto-stellar systems. Even aftercenturies of comet observations and decades after the initial pre-diction by Sen & Rana (1993), no one had observed an interstellarobject, passing through the inner Solar system, until October 2017when ‘Oumuamua (1I / (cid:63) E-mail: [email protected], [email protected]
The comet possessed a very large eccentricity of e = and a veryhigh hyperbolic excess velocity of v ∼
32 Km / s (Guzik et al. 2020),further confirming the interstellar origin. The interstellar comet, ini-tially identified as C / / Borisov by IAU.Fitzsimmons et al. (2019) were the first to report the detection ofCN in the interstellar comet. Later, de León et al. (2020), Opitomet al. (2019) and Kareta et al. (2020), have all reported the clear de-tection of CN along with an upper limit to the production rate ofC (0-0) emissions. Lin et al. (2020) and Bannister et al. (2020) havereported the clear detection of both CN and C in their spectrum,with the latter work having the most detailed spectrum of 2I / Borisovreporting the detection of well-resolved C , NH and CN emissions.In this paper, we discuss the spectroscopic and imaging observationscarried out from two Indian observatories during November and De-cember 2019, to study the evolution of molecular emissions and alsoto put constraints on the physical characteristics of the rare inter-stellar comet 2I / Borisov. Section 2 describes the observations fromboth observatories. Section 3 discusses the data reduction and analy-sis methods used. Finally, we discuss the spectroscopic and imagingresults in section 4. https: // minorplanetcenter.net / mpec / K19 / K19S72.html © a r X i v : . [ a s t r o - ph . E P ] J a n Aravind K. et al.,
Observations of the interstellar comet, 2I / Borisov, were carried outusing two Indian observatories, the 2 m Himalayan Chandra Tele-scope(HCT) operated by the Indian Institute of Astrophysics atHanle, Ladakh and the 1.2 m telescope at Mount Abu InfraRed Ob-servatory (MIRO) operated by the Physical Research Laboratory atMount Abu, Rajasthan.In the following sub-sections we describe, briefly, the details ofthe observations. The observational log, including the heliocentricdistance, geocentric distance, phase angle and airmass at the time ofobservations are as given in Table 1. The NASA JPL HORIZONS service was used to generate the ephemerides for the comet for boththe observing locations. The 2 m HCT is located at Hanle, Ladakh (Longitude: 78 ◦ (cid:48) . (cid:48)(cid:48) East; Latitude : 32 ◦ (cid:48) . (cid:48)(cid:48) Altitude : 4475 m). We used theHimalaya Faint Object Spectrograph and Camera (HFOSC) on theHCT to observe the comet 2I / Borisov. HFOSC uses a 2K ×
4K CCDhaving a pixel size of 15 ×
15 microns with a CCD pixel scale of0 . (cid:48)(cid:48) per pixel. 2K ×
2K portion of the CCD is used during imag-ing and 1.5K ×
4K during spectroscopy in order to get a good spatialcoverage for the comet.Spectroscopic observations were carried out on 30 th November and22 nd December, using HFOSC with grism 7 providing a wavelengthrange of 3700 - 7200 Å. A long slit, 11 (cid:48) in length and 1 . (cid:48)(cid:48) inwidth, was used for the observation of the comet and another longslit 15 . (cid:48)(cid:48) in width, was used for the observation of spectroscopicstandard star. Both the slits are placed horizontally in the E-W direc-tion. With this configuration, we have a spectral resolving power of1330 for comet observations. The comet was tracked at non-siderealrate during spectroscopic and imaging exposures, using the keystonemode available in the HCT telescope control system. In this mode atrackfile containing the comet’s altitude-azimuth coordinates at reg-ular intervals, is given as an input to the system. Exposures of 1800seconds were obtained on the photocenter of the comet in spec-troscopic mode. Separate sky frame was not obtained due to timeconstraint. Standard star, HD74721 (A0V type), from the catalog ofspectroscopic standards in IRAF was observed for flux calibration.Halogen lamp spectra, zero exposure frames and FeAr lamp spectrawere obtained for flat fielding, bias subtraction and wavelength cal-ibration respectively.Imaging observations were also carried out during the same epochsusing the Johnson-Cousins BVRI filters. Multiple frames were ob-tained for each epoch, with exposure varying over the range 120-300 s. Ru 149 photometric field was also observed in all abovementioned filters in order to perform photometric calibration of thecomet images. Twilight flats were recorded and bias frames werealso taken at regular intervals during the night to correct the pixel topixel response and remove the bias o ff set respectively. The Mount Abu InfraRed Observatory (MIRO) is located atMount Abu, Rajasthan (Longitude : 72 ◦ (cid:48) . (cid:48)(cid:48) East; Latitude: https://ssd.jpl.nasa.gov/horizons.cgi The full list of spectroscopic standards available in IRAF can be found in http://stsdas.stsci.edu/cgi-bin/gethelp.cgi?onedstds ◦ (cid:48) . (cid:48)(cid:48) North; Altitude : 1680 m). One of the backendinstruments available is a 1024 × × / pixel. A CFW-2-7 (7 position, 2 inch diameterper filter) model filter wheel, from Finger Lakes Instrumentation,holds the Johnson-Cousins ( UBVRI , as described by Bessell 2005)broadband filters. Imaging observation in the
BVRI filters werecarried out on 24 th , 25 th and 27 th of December 2019. Again, Ru 149field was chosen as a standard star field to be used for photometriccalibration. Twilight flats were obtained in all filters to normalisethe pixel to pixel response of the CCD. The non-sidereal track modebuilt into the in-house developed telescope control software wasused for comet observations. Careful reduction techniques are necessary to reduce, calibrate andextract information from the raw data. The following sub-sectionsdiscuss, in brief, the various steps used in the process of analysingthe data obtained from spectroscopy and imaging.
Standard IRAF routines were used to reduce the spectroscopic ob-servational data. The bias files taken throughout the night were me-dian combined, flat-field images were average combined and nor-malised in order to perform the basic reductions of comet and stan-dard frames. Owing to the fact that the observatory is at a very highaltitude and the comet was being observed for very long exposures,the raw files are contaminated by a large number of cosmic rays. TheLaplacian Cosmic Ray Identification (van Dokkum 2001) packagewas added to IRAF and was used to remove most of the cosmic rayspresent in the spectroscopic raw data. IRAF’ s apall module was usedto extract the 1D spectrum from the comet, calibration lamp andstandard star frames. For both epochs, an aperture of 17 . (cid:48)(cid:48) (corre-sponding to 60 pixels, centered on the comet) was used to extract thecomet spectrum. The corresponding physical distance at the photo-centre can be estimated using the distance scale column from Table1. The sky spectrum required for subtraction was extracted using asimilar aperture about 60 (cid:48)(cid:48) away from the photo-centre, free fromcometary emissions (see Appendix A for details). The spectrum foreach aperture used was extracted by tracing along the dispersionaxis. The lines in the FeAr calibration lamp spectrum were identi-fied using the identify task and the solution was used to wavelengthcalibrate the comet and standard star spectra. Using the standard and sensfun tasks, an instrument sensitivity function was derivedby comparing the observed standard star flux with the catalog val-ues available in IRAF. The resulting instrument sensitivity functionwas used to flux calibrate the comet spectrum. Extinction correc-tion was also applied along with the flux calibration. Details regard-ing the continuum subtraction using solar spectrum is explained inAppendix B. A flux calibrated spectrum of the comet 2I / Borisov isshown in Figure 1, clearly indicating the presence of emissions fromCN, C and C with the latter two highly depleted, similar to whathas been observed by Opitom et al. (2019), Lin et al. (2020), Karetaet al. (2020), de León et al. (2020) and Bannister et al. (2020). The MNRAS , 1–10 (2021) nterstellar comet 2I / Borisov Figure 1.
Optical spectrum of 2I / Borisov observed with the HFOSC instrument on HCT on 2019-12-22.96 UT. Inset shows the RGB colour composite view ofthe comet on the same night using images taken with HFOSC.
Table 1.
Observational Log
Telescope Heliocentric Geocentric Distance scale PhaseDate Time Facility Distance (r H ) Distance ( ∆ ) at photo-centre angle Airmass [UT] [UT] [AU] [AU] [Km / arcsecond] [ ◦ ]30 / / / / / / / / / / inset image represents the RGB view of the comet using the sameinstrument.The area under the curve within the wavelength range coveringthe full emission bands of di ff erent molecules, as mentioned inLangland-Shula & Smith (2011), was used to obtain the total fluxof the observed emissions. The uncertainties in the flux were ob-tained from the noise in the parts of the spectrum adjacent to theemission bands. The total number of molecules (N) present in theaperture used was calculated using N = π ∆ g × F , (1)where g is the fluorescence e ffi ciency, ∆ is the geocentric distanceand F is the total flux inside the aperture used for extraction. Thetotal number of molecules present in the aperture was converted intothe number present in the entire coma with the help of the Haserfactor (further described in Appendix C). The Haser factor for both the epochs were obtained from Schleicher’s website for an apertureradius of 8.88 (cid:48)(cid:48) (corresponding to 30 pixel, half the total apertureused). Since the Haser factor is derived for a circular aperture, theobtained value was adjusted for our aperture area of 1 . (cid:48)(cid:48) × . (cid:48)(cid:48) .The total number of molecules was then divided by the lifetime ofthe daughter molecule ( τ ) to obtain the production rate. The valuesfor g and τ at 1 AU were taken from A’Hearn et al. (1995). Schle-icher (2010) have tabulated the g-factor of CN for di ff erent heliocen-tric distances and velocities, which we have used in our calculations.The scale lengths were scaled by r h and the fluorescence e ffi ciencyof other molecules by r − h in order to obtain the appropriate values tobe used at the corresponding heliocentric distance ( r h ). A self scripted Python routine was used to perform all basic data re-duction techniques (bias subtraction, flat fielding and median com- https://asteroid.lowell.edu/comet/ MNRAS , 1–10 (2021)
Aravind K. et al.,
CNCN C ( ) Δ ν = 0 C C ( ) Δ ν = 0 C Figure 2.
Optical spectra of 2I / Borisov as observed on the pre and post peri-helion epochs. bining) on both comet and standard star images. Aperture photom-etry was performed on the comet and standard stars (Ru 149, Ru149B, Ru 149D, Ru 149E) using the PHOTUTILS package in As-tropy. The instrumental magnitudes of the standard stars were thencorrected for both extinction and color. Extinction coe ffi cient val-ues (magnitude / airmass) of various filters, as given in Stalin et al.(2008), were used to apply the extinction correction to the instru-mental magnitudes of the standard stars as well as the comet imagesobtained from HCT. The coe ffi cients for Mount Abu were takenfrom an in house project carried out for computing the extinctionvalues for the site. The zero point o ff set in magnitude was then com-puted with the help of Landolt’s standard star magnitudes as givenin Landolt (1992). The comet instrumental magnitude were then cor-rected for zero point to obtain its apparent magnitude in various fil-ters. These are tabulated in Table 3. & DISCUSSION4.1 Spectroscopy
From successful spectroscopic observations of the interstellar visi-tor, carried out on two epochs, pre and post perihelion, we detect thepresence of CN radical, C ( ∆ ν =
0) Swan band and C emission(latter two being highly depleted) as shown in Figure 2. The produc-tion rates of CN, C and C computed for both epochs, as mentionedin Section 3.1, are listed in Table 2. The comet has been monitoredin spectroscopy by various groups with heliocentric distance rangingfrom 2.7 to 2.02 AU. The current work reports the production ratesfor the comet pre and post perihelion and hence contributes a valu-able data point towards studying the characteristics of the emissionsin the comet post perihelion.The production rate of CN, C and C , reported in this work, pre-perihelion, is comparable with the values reported in other observa-tions, as shown in Figure 3, with slight increase in the rate which canbe accounted by the fact that the comet was approaching perihelion.Among the clear detection of C as reported by Lin et al. (2020),Bannister et al. (2020) and the current work, an increasing trend inthe production rate can be observed, while it is di ffi cult to comparethe same with the upper limits reported in the other observations.Adding an important data point to the spectroscopic observation ofthe rare visit of the interstellar comet, the production rate of CN andC shows drastic change with only a slight variation in that of C ,post-perihelion. The production rate ratio, Q(C ) / Q(CN), was com-puted for both the epochs and the comet was seen to be depleted in carbon chain molecules, according to the classification criteriondefined by A’Hearn et al. (1995). Also, the comet can be classifiedas depleted in carbon chain molecules according to the criterion de-fined by Cochran et al. (2012), where the production rates of bothC and C with respect to CN are considered. Figure 4 compiles thevalues of Q(C ) / Q(CN) as reported from all the other observationswith our own observations. It is interesting to observe that there is anincrease in the production rate ratio with heliocentric distance (un-til perihelion), which is not common among Solar system cometsfor a minimal change in heliocentric distance (A’Hearn et al. 1995;Cochran et al. 2012). Along the orbit of the comet, the behaviourhas changed from highly depleted to a moderately depleted comet,as it approached perihelion. Even though the production rate ratio,reported in our work, pre-perihelion, Q(C ) / Q(CN) = ) / Q(CN) = ) / Q(CN)) with increas-ing heliocentric distance in a sample of Solar system comets. How-ever, A’Hearn et al. (1995), Cochran et al. (1992) and Cochran et al.(2012) did not observe any variation in the production rate ratios fora minimal change in the heliocentric distance. In the current work,even though the production rate ratios are comparable within the er-rors, there is an indication of a possible asymmetry post-perihelion(see Figure 4). However, this cannot be confirmed with the limitedpost-perihelion data currently available.As shown in Figure 3, we also notice an asymmetry in the produc-tion rates of CN and C post-perihelion. Generally, such asymme-tries in production rates are observed among the short period cometsof our Solar system (eg. Opitom et al. 2017; A’Hearn et al. 1984a),close to perihelion. This asymmetry is expected either due to theillumination of di ff erent areas of the nucleus having di ff erent sur-face processing during their orbit or due to the presence of a lessvolatile surface, depleted in most of the molecules. Once these lay-ers get disintegrated by the solar radiation, the less depleted surfaceof the comet gets exposed resulting in an increased flux in emis-sions. However, Bodewits et al. (2020) and Cordiner et al. (2020)report that 2I / Borisov has an extremely high abundance of carbonmonoxide, implying that the surface of the comet has not undergonea su ffi ciently intense heat processing to cause the depletion of thetop volatile surface. In addition, such high CO abundance is usuallyuncommon among the short period comets (Dello Russo et al. 2016).All these reported facts and the observed asymmetry in productionrates around perihelion, makes us raise a question on the chemicalhomogeneity of the material present in the nucleus of 2I / Borisov orthe di ff erence in the volatile nature of the molecules present in thecomet nucleus. Bannister et al. (2020) suggests a possibility of het-erogeneous composition in the comet based on the observed increasein C activity close to perihelion. Based on the observed very highabundance of CO in 2I / Borisov, Bodewits et al. (2020) points out thepossibility of the comet having formed beyond the CO ice line of itsparent stellar system. Since the results from the current work arealso in agreement with the suggestions regarding the heterogeneity,it is possible that the comet was formed in a stellar system beyondthe CO ice line undergoing a very inhomogeneous mixing of variousvolatile compounds present in the proto-stellar nebula.A’Hearn & Cowan (1980) states that, since the parent moleculesof C are primarily contained in the grains of H O ice, the pro-duction of C is directly related to the activity in the icy grainsof H O, while production of CN and C are not. Combi & Fink(1997) discuss the possibility of C being produced from a primary MNRAS , 1–10 (2021) nterstellar comet 2I / Borisov parent molecule frozen in the icy mix of the nucleus and alsodirectly from CHON grains at temperatures ∼ K . In the currentscenario where the perihelion distance of the comet is 2.0066 AU,the temperature from solar radiation would not be high enough forCHON grains to be a primary source for C . Also, the influence ofCHON grains would result in a flattening of the spatial profile ofC as per the CHON grain halo (CGH) model proposed by Combi& Fink (1997). Such a spatial flattening has not been reported yetin the case of 2I / Borisov. On the other hand, Xing et al. (2020)reports that the water production in 2I / Borisov had increaseddrastically from November to December, close to perihelion andthen decreased rapidly by December 21 st . With the contributionfrom CHON grains being ruled out, only the activity in H O ice canexplain the increase in production rate of C close to perihelion andhence the initial increase in Q(C ) / Q(CN). The drop in the ratio postperihelion is due to an increased activity of CN while C activityhad not changed substantially. Results from our work also supportthe possibility reported by Bannister et al. (2020), regarding theheterogeneity in the comet nucleus. This would have resulted in afresh layer, rich in carbon chain parent molecules trapped in the icygrains, being exposed and hence resulting in the steep increase ofC production rate along with the water production rate. We alsoinfer a possibility that, as the southern hemisphere of the nucleuswas illuminated, after perihelion, a fresh unexposed area of thecomet started sublimating as proposed by Kim et al. (2020). Thisresulted in the drastic increase in production rates of CN and C with only a minimal change in the C production rate owing to thereduced water production rate. Even though A’Hearn et al. (1986)and Fray et al. (2005) discusses the prospect of CHON grains beinga possible parent source of CN, it can be ruled out in this casesince the contribution would be very less due to the periheliondistance of the comet as discussed earlier. We also observe anabrupt discontinuity in the production rate of CN as compared tothat for C (see Figure 3). This strongly suggests that both of themcome primarily from di ff erent sources as discussed by A’Hearn& Cowan (1980). This behaviour can also be considered as aconfirmation, in this work, that the parent molecules of both CN andC resides mainly in the comet nucleus whereas that of C is mostlypresent in the icy grains of the coma. The di ff erence in activity ofC and C also confirms that the parent molecules of these emis-sions should be entirely di ff erent as mentioned in Yamamoto (1981). The comet was observed in imaging mode on 5 epochs (Table. 1) inthe Bessells’s
BVRI filters from both HCT and MIRO. The imageswere reduced and apparent magnitudes were computed as explainedin section 3.2. ρ From the magnitudes computed, as given in Table 3, the opticalcolours of the comet were found to be; (B-V) = ± (V-R) = ± (R-I) = ± (B-R) = ± / ’Oumuamua (Jewitt et al.2017). The colours of the interstellar comet are also surprisinglysimilar to the mean colours of long period comets in the Solar sys-tem (Jewitt 2015). The (B-V) , (V-R) and (R-I) colours, after transfor-mation to the SDSS photometric system as described in Jordi et al. (2006), also compares within uncertainties in measurement to thecolours reported by Bolin et al. (2020) and Hui et al. (2020). Theavailable magnitudes were also used to compute A f ρ , a proxy tothe amount of dust produced (A’Hearn et al. 1984b). The obtainedvalues of A f ρ (see Table 3), for 30 th November and 22 nd Decem-ber, in V band is found to be similar to the values reported by Xinget al. (2020), for the same wavelength band, during nearby epochs(1 st December and 21 st December respectively). The equation foraverage slope of the curve of reflectivity, as mentioned in A’Hearnet al. (1984b), when used with the observed magnitudes provides aslope S (cid:48) = (9 . ± . / Å for the red-end (6400-7900 Å) and S (cid:48) = (13 . ± . / Å for the blue-end (4200 - 5500 Å). The ob-served average slope at the red end is consistent with the values re-ported by Lin et al. (2020) [ S (cid:48) = . / Å], de León et al. (2020)[ S (cid:48) = (10 ± / Å], Kareta et al. (2020) [ S (cid:48) = / Å] andHui et al. (2020) [ S (cid:48) = (10 . ± . / Å]. These values of spec-tral slope of 2I / Borisov suggests that the dust composition presentin the cometary coma could be similar to those observed in the D-type asteroids (Licandro et al. 2018), a suggestion first proposedby de León et al. (2019) from their spectroscopic observations of2I / Borisov. The observed slope at the blue end cannot be comparedwith the values reported through spectroscopy, since the magnitudesmeasured using the broad band filters, B & V , would be largely af-fected by the emissions from CN and C respectively. The dust-gasratio, as shown in Table 2 is also similar to the dust-gas ratio ofcarbon chain depleted Solar system comets (A’Hearn et al. 1995).These are clear indications of the similarity in dust composition of2I / Borisov with Solar system comets implying a high possibility ofthe comet formation process similar to Solar system happening inother stellar systems. ff ective scattering cross section The apparent magnitude is a function of the heliocentric distance,geocentric distance and the phase angle at the time of observation.Hence, the absolute magnitude (H), which corresponds to the mag-nitude of the comet at a heliocentric and geocentric distance of 1 AUand a phase angle of 0 ◦ , given by H = m − log ( r H ∆ ) + . log [ φ ( α )] , (2)was computed. Here m is the apparent magnitude in the respectivefilter, r H is the heliocentric distance, ∆ is the geocentric distanceand φ ( α ) is the phase function corresponding to the phase angleat the time of observation, as defined in Schleicher & Bair (2011).The R band absolute magnitude can be used to compute the e ff ectivescattering cross section in order to investigate the nature of activityin the comet. The e ff ective scattering cross section (C e ) is computedusing the following equation; C e = π r p . m (cid:12) , R − H R ] , (3)where r is the mean Earth-Sun distance in Km, p is the geometricalbedo of the cometary dust and m (cid:12) , R is the solar apparent magni-tude in the R band. Using the values of r = . × Km and m (cid:12) , R = − .
97 from Willmer (2018), the above equation reduces to C e = (1 . × / p ) × − (0 . H R ) . For this work, the albedo (p) of thecomet was chosen as 0.1, typical for comet dust (Zubko et al. 2017), Composite Dust Phase Function for Comets https://asteroid.lowell.edu/comet/dustphaseHM_table.txt
MNRAS , 1–10 (2021)
Aravind K. et al.,
Table 2.
Gas production rates of comet 2I / Borisov at di ff erent heliocentric distances Date Exposure r H ∆ Production Rate (molec / sec) Production rate ratio Dust to gas ratio [UT] [s] [AU] [AU]
CN C ( ∆ ν = C Q(C ) / Q(CN) log[(
A f ρ ) R / Q(CN)] × × × ± ± ± ± ± ± ± ± ± ± Table 3.
Apparent magnitude (m), absolute magnitude (H), e ff ective scattering cross section (C e ) and A f ρ computed for various observational epochs Date m a H bR C e [Km ] A f ρ [cm] [UT] B V R I B V R I ± ± ± ± ± ±
12 107 ± ± ± ± ± ± ± ± ± ± ± ± ± ± − − ± ± ± ± − − ± ± ± ± ± − ± ±
12 66 ± ± ± − − − ± − ± ± − − ± − a An aperture size of 10,000 Km has been used on all epochs for all filters to compute the magnitude b Absolute magnitude computed using Eq.2 from the corresponding apparent magnitude in R filter
Figure 3.
Comparison of pre-perihelion production rates (measurements / upper limits) of CN (upper panel), C (middle panel) and C (lower panel) reported inFitzsimmons et al. (2019), Opitom et al. (2019), Kareta et al. (2020), de León et al. (2020), Lin et al. (2020) and Bannister et al. (2020) with the pre and postperihelion production rates of same molecules as computed in this work. as used in Jewitt & Luu (2019), Hui et al. (2020) and Bolin et al.(2020). Figure 5 depicts the decreasing trend in the scattering crosssection as a function of days in the year 2019. The grey dashed line at342 nd day represents the perihelion of the comet (8 / / d ( C e ) / dt = − . ± .
22 Km d − . A mean nuclear radius (r) can be computed for C e = Km as r ≤ . MNRAS000
22 Km d − . A mean nuclear radius (r) can be computed for C e = Km as r ≤ . MNRAS000 , 1–10 (2021) nterstellar comet 2I / Borisov Figure 4.
Cumulative comparison of the pre and post perihelion productionrate ratio of C and CN of 2I / Borisov observed in the current work with thevalues reported by Fitzsimmons et al. (2019), Opitom et al. (2019), Karetaet al. (2020), de León et al. (2020), Lin et al. (2020) and Bannister et al.(2020) while the comet was in an in-bound orbit. The shaded region rep-resents the area of carbon chain depleted comets in our Solar system, forwhich Q(C ) / Q(CN) < th December2019 with q ∼ reported by Jewitt & Luu (2019), is also included in Figure 5, wherean increasing trend is observed. As per the variation of cross sec-tion with heliocentric distance reported by Bolin et al. (2020), thecross section is seen to be increasing till the day when 2I crossed thewater-ice line at 2.5 AU and decreasing later on. Clubbing the shortrange trends reported in Jewitt & Luu (2019) (before 2I crossed thewater-ice line) and in this work (after 2I crossed the water-ice line)along with the larger range trend reported in Bolin et al. (2020), it isclear that there was a steady increase in the scattering cross sectiontill the water-ice line beyond which it decreased systematically. Thisobservation is not in agreement with the variation in scattering crosssection reported by Hui et al. (2020), where the cross section is seento be continuously reducing.Further, making use of the rate of change in e ff ective cross section,the rate of dust production can be calculated as, dMdt = ρ ¯ a d ( C e ) dt , (4)where ρ is the particle density, ¯ a is the mean particle size. In thiswork we have accepted the values of ρ = g / cm (Jewitt & Luu2019) and ¯ a = µ m (Jewitt & Luu 2019; Hui et al. 2020).Substituting these values, we get the average net mass loss rate dM / dt = − . ± . kg s − . This value depicts the rate of change indust mass over the observed period. The value is negative since thedust produced from the comet is not able to compensate for the dustlost from the photometric aperture ( ρ ∼ Km), implying that theabsolute amount of dust production is reducing over the time period,resulting in the comet getting fainter. ρ , dust production rate and sublimation flux As mentioned in Cremonese et al. (2020), the following equation,
A f ρ = AQ d ρ v d s , (5)where A is the geometric albedo, v d is the dust ejection velocity, ρ is the particle density and s is the average particle size, can be usedto convert the observed A f ρ (in meter) (see Section 4.2.1) into thedust production rate (Q d ). Using the same values for the parameters,as used in the previous section, we get a relation, Q d = . v d ( A f ρ ) . Figure 5.
Variation of scattering cross section, computed using equation 3as a function of time, expressed as Day of Year 2019, with Day 1 beingJanuary 1 2019. The top-axis is labelled with the distance to perihelion. Theblack dashed line is the best linear least-squares fit having a gradient -1.77 ± / day. The vertical grey dashed line represents the perihelion of thecomet. The red points are the scattering cross section as reported by Jewitt &Luu (2019) while the comet was approaching perihelion. Accepting the value of dust ejection velocity as v d ∼ − (forepochs close to perihelion as reported by Hui et al. 2020), using Af ρ values for V band, we obtain a dust production rate, Q d ∼
30 kg s − .This absolute value of dust production rate is in agreement to thevalues reported in Cremonese et al. (2020), Jewitt et al. (2020b) andKim et al. (2020). Considering this dust to be produced by water-icesublimation, the patch of area supplying this dust can be computedas A = Q d / f s , where f s is the specific rate of mass sublimation fluxat equilibrium. According to Jewitt et al. (2015), the specific rate ofmass sublimation flux at equilibrium, f s ( Kg m − s − ), for a body ata heliocentric distance R is obtained from the equation, F (cid:12) (1 − A ) R cos θ = (cid:15)σ T + L ( T ) f s , (6)where F (cid:12) is the solar constant, A is the albedo, (cid:15) is the emissivityand L ( T ) is the latent heat of sublimation of ice at temperature T.For the comet 2I / Borisov at 2.7 AU, Jewitt & Luu (2019) obtains f s = × − kg m − s − . Considering the change in temperaturefrom 2.7 to 2.013 AU to be only ∆ T ∼ K (Henning & Wei-dlich 1988), the value for latent heat of sublimation of water icedoes not change significantly. Hence from Equation. 6, for the cur-rent work, we obtain the specific rate of mass sublimation flux tobe f s = . × − kg m − s − . Inserting this value in the abovedefined relation for area, provides A = . , which is equal tothe surface area of a sphere of radius r = .
18 Km. The computednuclear radius is in good agreement with the lower limits reportedin observations using the Neil Gehrels-S wift
Observatory’s Ultra-violet / Optical Telescope (Xing et al. 2020) and the Hubble SpaceTelescope (Jewitt et al. 2020a).Assuming that the empirical relation mentioned in Jorda et al. (2008)holds for this interstellar comet, we can compute the water produc-tion rate from the visual V band magnitude (reduced to a geocentricdistance of 1 AU). Using the observed reduced magnitudes, the ex-pected water production rates (Q(H O)) for 30 th November and 22 nd December 2019 are (9 . ± . × molec / s and (7 ± . × molec / s respectively. Even though the expected water productionrate for 30 th November is consistent, within uncertainties, with theobserved rate reported by Xing et al. (2020) for 1 st December, theirreported rate on 21 st December is much lower than expected fromour observations for 22 nd December. Emphasising on the fact thatthe empirical relation cannot be used to get precise measurements of
MNRAS , 1–10 (2021)
Aravind K. et al., water production rates, but only for order of magnitude estimates, wewould like to point out that, such an unexpected drastic decrease inwater production rate may be due to the heterogeneous compositionof the nucleus, as discussed earlier, along with the low abundanceof H O, uncommon in Solar system comets, as reported in Bodewitset al. (2020).
In this work we present the optical spectroscopic and imaging ob-servations of the interstellar comet 2I / Borisov, before and after per-ihelion, using the 2-m HCT, Hanle and MIRO, 1.2 m Mt.Abu tele-scopes. Spectroscopic study shows clear emissions from CN, C andC pre and post perihelion and detects a drop in production rateratio, Q(C ) / Q(CN) post perihelion. Imaging study reveals a sys-tematically reducing
A f ρ and e ff ective cross section, using whicha possible size range of the nucleus has been computed. Using theseobservational results, we arrive at the following conclusions:(i) The computation of production rates of molecules CN, C and C shows an increase, comparable to the observations by othergroups, as the comet approached perihelion with an asymmetry inthe emission observed post perihelion.(ii) The low value of the production rate ratio, Q(C ) / Q(CN), im-plies that the comet is depleted in carbon-chain molecules. The ratiohad increased as the comet moved closer to perihelion, making ita moderately depleted one, with a later decrease in the ratio afterperihelion passage.(iii) We infer a chemical heterogeneity in the comet surface dueto which there was a drastic surge in the C emissions as the cometapproached perihelion. We also infer that an initially unexposed sur-face of the comet would have been exposed to solar radiation postperihelion resulting in the substantial increase in the production ratesof CN and C .(iv) The A f ρ values computed from the imaging observations,and hence the dust-to-gas ratio, are consistent with the numbers ob-served for Solar system comets, depleted in carbon-chain molecules.This may be an indication that the parent stellar system of 2I / Borisovwould have undergone a formation process somewhat similar to thatof the Solar system.(v) The optical colours of the comet, (B-V) = ± (V-R) = ± (R-I) = ± (B-R) = ± / Borisov, an interstellar comet withvery low abundance in H O.(vii) The possible size of the nucleus was deduced to be 0 . ≤ r ≤ . / Borisov was formed in a proto-stellar system under-going a very inhomogeneous mixing of various volatile compoundsbeyond the CO ice line.
ACKNOWLEDGEMENTS
We thank the referee for the valuable comments and suggestionswhich have improved the manuscript. We acknowledge the localsta ff at the Mount Abu InfraRed Observatory for their help. We thankthe sta ff of IAO, Hanle and CREST, Hoskote, that made these obser-vations possible. The facilities at IAO and CREST are operated bythe Indian Institute of Astrophysics, Bangalore. Work at PRL is sup-ported by the Dept of Space, Govt. of India. DATA AVAILABILITY
The data underlying this article will be shared on reasonable requestto the corresponding author.
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APPENDIX A: SKY SUBTRACTION
During the observations for this work, a separate sky frame was notobtained due to time constraints. On analysing the profile along theslit, in the spatial axis, it appears that 2I / Borisov has an extent ofabout 20 (cid:48)(cid:48) . Taking this into account, we used an equal sized aperturefor the comet and the sky about 60 (cid:48)(cid:48) apart. Figure A1 depicts thepositions of both these apertures over-plotted on the profile along theslit, in the spatial axis. The normalised wavelength calibrated spectraof both comet and sky, with a constant o ff set in the sky spectrum isshown in the top panel of Figure A2. The bottom panel depicts thecorresponding sky corrected spectrum, with the detected emissionsmarked. APPENDIX B: CONTINUUM CORRECTION
In order to extract the total flux of the various molecules from thecometary emission spectrum it is necessary to remove the contribu- C C CN Figure A2.
Top panel : Comet spectrum (blue solid line) and sky spectrum(red dashed line, with a constant o ff set) illustrating the presence of strong at-mospheric telluric lines . Bottom panel : Comet spectrum after sky correctionwith the detected emissions marked. C C CN Figure B1.
Top panel : Comet spectrum (blue solid line) overplotted witha standard solar spectrum (red dashed line) scaled to the continuum level.
Bottom panel : Comet spectrum after continuum correction with the detectedemissions marked. tion from the continuum. For this, either a solar analog star is ob-served along with the comet observations or a standard solar spec-trum is used. In the present case, we have used a standard solar spec-trum, scaled, re-sampled to match the resolution of the instrumentand corrected for slope to take into account the redder nature of thecomet dust (see Figure B1).
APPENDIX C: HASER FACTOR
During long slit spectroscopic observations of comets only a partof the total coma is being observed. Hence, the observed flux ofeach molecular species is to be extrapolated to obtain the total fluxof the same in the entire coma of the comet. Haser model (Haser1957), provides a factor called the Haser factor which is the ratioof total number of molecules present in the aperture used to thetotal number of molecules present in the whole coma. The recip-rocal of this factor, the Haser correction, can be used to extrapo-late the observed flux so as to estimate the molecular abundance inthe entire coma. Since the Haser model assumes a spherically sym-metric coma with uniform outflow of gas, we compute the Haserfactor for a circular aperture by making use of the radius of theaperture (in arcseconds). This factor can be computed using the webcalculator implemented by Prof. Schleicher on his website( https://asteroid.lowell.edu/comet/ ). In the current work, we haveused a rectangular slit (1 . (cid:48)(cid:48) × . (cid:48)(cid:48) , 60 pixel along the slit with MNRAS , 1–10 (2021) Aravind K. et al., comet at the centre). Hence, we have chosen an aperture radius of8 . (cid:48)(cid:48) (corresponding to 30 pixel) in order to compute the Haser fac-tor. Since the computed Haser factor is for a circular aperture, thefactor was normalised to unit area and then multiplied by the area ofthe rectangular aperture used, so as to obtain the factor required forthis work. This paper has been typeset from a TEX / L A TEX file prepared by the author.MNRAS000