/ (cid:73) Imaging polarimetry and spectropolarimetry of comet C 2013 R1 (LoveJoy) GalinBorisova,b,∗,StefanoBagnulob,PlamenNikolova,TanyuBoneva aInstituteofAstronomyandNationalAstronomicalObservatory,BulgarianAcademyofSciences,72,TsarigradskoChausseeBlvd.,BG-1784Sofia,Bulgaria bArmaghObservatory,CollegeHill,ArmaghBT619DG,NorthernIreland,UK Abstract WehaveobtainedimagingpolarimetryofthecometC/2013R1(Lovejoy)with2-Channel-Focal-ReducerRozheninstrumentat 2mRitchey-Chre´tien-Coude´ telescopeoftheBulgarianNationalAstronomicalObservatoryRozhenintwodustcontinuumfilters 6coveringwavelengthintervalsclearfrommolecularemissionsandcentredat4430Å in(bluefilter)andat6840Å in(redfilter).In 1 imagingmodewemeasuredthedegreeoflinearpolarisation17.01±0.09%intheblueand18.81±0.02%inthered,whichisina 0 verygoodagreementwithmeasurementsofothercometsatthesimilarphaseangle.Wehavealsoobtainedpolarisationmapsinboth 2 filters. Wefoundastrongcorrelationbetweenthespatialdistributionofthepolarisationandthedustcolour. Spectropolarimetry n ofthenucleusregionshowsanincreaseofthepolarisationwithwavelength,andadepolarisationinthespectralregionswithgas a Jemissionlines,mostnoticeableinC2emissionband,whichshowsapolarisationof6.0±1.1%. 2Keywords: comets,C/2013R1(Lovejoy),polarimetry,spectropolarimetry,dust,molecules 1 ] P1. Introduction From the theoretical side, many works have been carried E out by Kolokolova and collaborators (Kolokolova et al., 1997; . h Polarimetryissensitivetothephysicalpropertiesofthedust KolokolovaandJockers,1997;Kolokolovaetal.,2004). pparticles: size, shape, porosity, orientation and chemical com- The polarisation of the dust jet-like structures in the dust -position represented by its material complex refractive index. comaofthecometHale–Boppwasobtainedforthefirsttimeby o rPolarimetricmeasurementsgiveusthepossibilitytodetermine Hadamciketal.(1997)andwasdiscussedlateroninHadamcik stsome parameters that cannot be determined trough traditional andLevasseur-Regourd(2003). aintensitymeasurements. A recent review of all comets investigation can be found in [ Thefirstpolarimetricobservationsofcometsweremadeby thebooksbyMishchenkoetal.(2010)andKiselevetal.(2015). 1Franc¸ois Arago (1854), who discovered the polarised light in CometC/2013R1(Lovejoy)wasdiscoveredbyTerryLove- vtheGreatComet1819II. joy(Thornlands,Queensland,Australia)withimagesacquired 1 Later, observations of comets clarified some common char- 0 on 2013 September 7 and 8, using his 20-cm reflector and a acteristics of the polarised light, for example that usually the 7 CCDcamera. 2plane of polarisation is perpendicular to the scattering plane, Other polarimetric measurements of the comet C/2013 R1 0andthattherearevariationofthepolarisationindifferentparts (Lovejoy)arepresentedbyFurushoetal.(2014)(imaginepo- 1.ofthecomet(coma,tail). Contemporarypolarimetricobserva- larisation with the Subaru telescope) and by Rosenbush et al. 0tions of comets began with the work of Yngve O¨hman (1939, (2014)(linearandcircularpolarimetricmeasurementsandtheir 61941),whoobservedforthefirsttimethecontinuumpolarisa- modelling). 1 tionincometsandthepolarisationoftheemissionlines. : v Mostoftherecentpolarimetricobservationsofcometshave ibeen obtained by Kiselev and collaborators. Kiselev et al. X 2. Observations (2005) has also created a database with more than 2600 mea- r asurementsoflinearandcircularpolarisationfor64cometssince 1940s. Comet C/2013 R1 (Lovejoy) was observed during a multi- MostofthepolarimetricobservationsofsmallSolarsystem instrument campaign with the 2m Ritchey-Chre´tien-Coude´ bodiesareaimedatmeasuringthevariationofthepolarisation (RCC) telescope of the Bulgarian National Astronomical Ob- withphaseangle(whichactuallyis180◦−scatteringangle)and servatory(BNAO)Rozhenfrom20Dec2013until07Jan2014. alsoitsdependenceonwavelength. Because of the target brightness we could achieve a relatively high S/N ratio and obtain high accuracy of polarimetric mea- surements. C/2013 R1 (Lovejoy) was a new comet which ap- (cid:73)Basedondatacollectedwith2mRCCtelescopeatRozhenNationalAs- proachtotheinnerSolarSystemforthefirsttimeandgivingus tronomicalObservatory. ∗Correspondingauthor anopportunitytoinvestigatethepristinematerialfromtheera Emailaddress:[email protected](GalinBorisov) oftheSolarSystemformation. PreprintsubmittedtoPlanetaryandSpaceScience January13,2016 2.1. Instrumentation Date ra,AU ∆b,AU αc,deg Obs. mode Polarimetric observations were performed with the 2- 20Dec2013 0.8132 0.8562 72.2 HRSd Channel-Focal-Reducer Rozhen (FoReRo2) (Jockers et al., 21Dec2013 0.8123 0.8765 71.1 HRS 2000) attached at the Cassegrain focus of the 2m RCC tele- 22Dec2013 0.8118 0.8965 70.1 HRS scope. In polarimetric mode, FoReRo2 is equipped with a 23Dec2013 0.8118 0.9161 69.1 HRS Wollastonprism,placedbeforeadichroicbeamsplitter,which 24Dec2013 0.8122 0.9355 68.1 HRS splitsthesignalsintotwodifferentchannels,allowingustore- 29Dec2013 0.8210 1.0305 63.0 ImPole constructpolarimetricmapsofextendedobjectsintwospectral 30Dec2013 0.8240 1.0495 62.0 NBFf regionssimultaneously,usingnarrowbandfilters. Byreplacing 31Dec2013 0.8275 1.0675 61.0 H2O+ thefilterswithtwogrisms,wecanperformspectropolarimetric 03Jan2014 0.8405 1.1205 58.1 SPolg measurements. An example of raw spectropolarimetric image 08Jan2014 0.8631 1.1865 54.6 NBF&H2O+ canbeseeninFig.1. Table1:ObservingLog aHeliocentricdistance bGeocentricdistance cPhaseangle dHighresolutionspectroscopy eImagingpolarimetry fGasanddustcomaimaginginNarrowbandfilters gSpectropolarimetry Figure1:SpectropolarimetricimageofcometC/2013R1(Lovejoy). At the time of our observations, the polarimetric optics of FoReRo2includedaWollastonprismbutnotaretarderwave- Imagingpolarimetrywasobtainedintwodustcontinuumfil- plate,preventingusfromadoptingabeam-swappingtechnique ters covering wavelength intervals clear from molecular emis- tominimiseinstrumentaleffects(see,e.g.Bagnuloetal.,2009). sion and centred at 4430Å and 6840Å, having a passband of Previousexperienceshowedthatpolarimetricobservationswith 35Å, and 71Å and hereafter called IF443 and IF684, respec- FoReRo2wereaffectedbynon-negligibleandnon-constantin- tively(seeFig.2). strumental polarisation. In an attempt to mitigate this prob- lem,wedecidedtoobtainobservationsattwoinstrumentposi- tionangles,onewiththeprincipalplanoftheWollastonprism alignedtothescatteringplan(i.e.,theplandefinedbythesun, thecometandtheobserver)andoneperpendiculartoit. Byde- notingwith f(cid:107) and f⊥ thefluxesintheparallelandintheper- pendicularbeams,andwithk andk thetransmissionfunctions (cid:107) ⊥ intheparallelandintheperpendicularbeamoftheWollaston prismrespectively,theobservedquantityis (cid:32) (cid:33) (cid:32) (cid:33)  QIˆ = 21 ff(cid:107)(cid:107)−+ ff⊥⊥ − ff(cid:107)(cid:107)−+ ff⊥⊥  PA=φ+90◦ PA=φ = 1(cid:34)k(cid:107)(I+Q)−k⊥(I−Q) − k(cid:107)(I−Q)−k⊥(I+Q)(cid:35) 2 k (I+Q)+k (I−Q) k (I−Q)+k (I+Q) (cid:107) ⊥ (cid:107) ⊥ Figure2: Continuumfilterstransmissioncurves(IF443&IF684)overplotted = (k(cid:107)+k⊥)2IQ−(k(cid:107)−k⊥)2IQ onacometspectrum (k +k )2I2−(k −k )2Q2 (cid:107) ⊥ (cid:107) ⊥ whereφistheangleistheanglebetweenthedirectionObject- NorthPoleandthedirectionObject-Sun. Ifk (cid:39)k weobtain 2.2. CometC/2013R1(Lovejoy) (cid:107) ⊥ ImagingandspectropolarimetricdatawereobtainedonDe- Q Qˆ cember29andJanuary3respectively,withFoReRo2. Thege- (cid:39) . I I ometrical conditions during the observations are shown in ta- ble1. whereQ/IisthereducedStokesparameterQmeasuredassum- ing as a reference direction that one perpendicular to the scat- 2.3. DataReduction teringplane. WedidnotmeasureU/I,assumingthatforsym- Allimageswerepre-processedthroughastandardbiassub- metricreasonsitisprobablyzero. Ofcourse,theimageswere tractionandflatfieldcorrection. combinedtogetheraftera90degreesrotation. 2 3. Results 3.1. Aperturepolarimetry The aperture photometry with a circular aperture with a ra- dius of 13×103km on the comet was performed to measure the intensity of the two orthogonal polarised beams f(cid:107) and f⊥. The background was measured in the farthest possible place in the sunwards direction. The results for both contin- uum filter IF443 and IF684 are P = 17.01 ± 0.09% and 443 P = 18.81±0.02%respectively. Theircomparisonwiththe 684 Kiselev database (Kiselev et al., 2005) is shown in Fig. 3 and isinagoodagreementwiththedataforthecomet1P/Halleyat similarphaseangles(65◦). Figure3:ComparisonofpolarimetricmeasurementofcometC/2013R1(Love- joy)andKiselevdatabase(Kiselevetal.,2005)(errorbarsofourmeasurements aresmallerthanthesymbols) 3.2. Imagingpolarimetry In order to investigate the spatial distribution of the linear polarisation of the light reflected from cometary dust we per- Figure4:Polarisationmapwithamarkedcometphotocentre(+)anddirection totheSun((cid:12)←)(toppanel)andradialprofilesindifferentdirections(bottom formedimagingpolarimetryofthecomet’scoma. panel)inIF684 Weconstructpolarisationmapsbycalculatingthedegreeof linear polarisation, using the beam swapping technique, for each pixel in a pre-selected region around comet photo cen- tre. Theresultantpolarimetricmapsandradialprofilesforthe dustcontinuumfilterIF684arepresentedinthetopandbottom panelofFig.4,respectively. unsteadyoutflowfromthenucleus. Thisprocedureremovethe Duetothelowsignal-to-noiseratio,thepolarimetricmapin averagecoma,modelledbyfittingtheradialprofilesofthedust IF443dosenotdisplayanyremarkablestructures. Bycontrast, atdifferentazimuthswithapowerlow. Afterwardsthismodel thepolarisationmapinIF684showsclearlyahighlypolarised wassubtractedandonlythejet-likestructureremainsintheim- structureapartfromthenucleus. age.InthetoppanelofFig.5,suchastructureisclearlyvisible, Thepolarisationdegreeinasquareregion11×11103kmen- butitisshiftedfromthepolarisationone,presentedwithover- veloped this structure is slightly higher than that of the total plottedcontours. coma,i.e. Pjet =18.83±0.045% 684 Inordertoinvestigatethenatureofthisstructure,wefirsttry Next, we compared it with the distribution of the dust nor- to connect this highly polarised structure with a jet-like struc- malised reflectivity gradient, or so called reddening. That is ture in the dust coma. Therefore we used an enhanced proce- why we construct the so called colour map of the dust by cal- duretorevealanyinhomogeneityaroundthenucleuscausedby culatingthequantity 3 Figure6: SpectropolarisationofcometC/2013R1(Lovejoy)andafittedcon- tinuumpolarisation. We carried out the the polynomial fit to the dust continuum polarisation. Theresultforcomet1P/Halleyatdifferentphase angles are also presented (Kiselev et al., 2005) and our obser- vations are very well fitted to the phase angle and wavelength trendofthecomet1/PHalley(seeFig.7). Figure5: EnhancedcomastructureinIF684(toppanel)andcolourmap(bot- tompanel)comparedwithpolarisation(overplayedcontours) Figure7:ComparisonofthecometC/2013R1(LoveJoy)withthetypicalcomet 1P/Halleymeasurementsatdifferentphaseangles. 1 ∂S S(cid:48)(λ ,λ )= , 1 2 S¯ ∂λ The polarisation in molecular spectral lines is lower than that in the continuum. According to the theory of diatomic whereS¯ =(Sλ2−Sλ1)/2andisusuallyexpressedin%/1000Å, molecules it should be a constant of 1/7 (Feofilov, 1961). To for each pixel in both dust continuum images in IF443 and estimatethepolarisationoftheemissionlineswefirstsubtract IF684. The resultant colour map is presented in the bottom the continuum contribution from both spectra in the ordinary panel of figure 5. The connection between high polarisation andextraordinarybeams. Then,usingtheremainingfluxfrom andlowreddeningisclearlyvisible. Theinterpretationisthat resonance fluorescence of the molecules, we calculate the de- lowreddeningmeanssmallparticles,orverydarkones,bothof greeoflinearpolarisationfollowingtheproceduredescribedin whichgivehighpolarisation. section2.3foreachwavelengthbin. TheresultforthemainC 2 molecularbandispresentedinFig.8. 3.3. Spectropolarimetry The average value of linear polarisation over the whole Polarisation spectrum of the comet C/2013 R1 (Lovejoy) is molecular band of C is 6.0 ± 1.1%. This value is less than 2 showninFig.6. half that of the theoretical one, which is based on theory of 4 redwasfoundandcomparedwithjet-likestructuresinthe dust coma, and connected with low values in the map of thedustcolour,whichshowsthatthisregionispopulated ratherwithsmallorverydarkgrains. • Furushoetal.(2014)presentedimagingpolarisationofthe cometLovejoywiththeSubarutelescope,andtheirresults (includingthedetectionofcomafeatureswithhighpolar- isationvalues)areconsistentwithourresults. • We have obtained a spectropolarimetry of the comet and foundthechangeofthedegreeoflinearpolarisationofthe dust with wavelength. This is in a good agreement with theresultsobtainedforcomet1P/Halley. • In the C molecule emission band we have measured 2 6.0±1.1% degree of linear polarisation. The results de- viate from the theoretical value for diatomic molecules 14.3% (Feofilov, 1961), but are in good agreement with a value measured for other comets P =7.7% (Krishna max Swamy, 2010). This can be used to explain the depolari- sationeffectinthemolecularcoma. FcoigmuerteC8/:20P1o3laRri1sa(tLioonvesJpoeyc)trum of the C2 emission line in the coma of the Acknowledgements Theauthorsgratefullyacknowledgetheobservinggrantsup- portfromtheInstituteofAstronomyandRozhenNationalAs- anisotropic rotating oscillators. Also these values are for pure tronomicalObservatory,BulgarianAcademyofSciences. gas without the presence of other species. The theory says GB and SB also gratefully acknowledge financial support that polarisation of the fluorescence of diatomic molecules is fromtheCOSTActionMP1104”Polarisationasatooltostudy reduced when foreign gases are introduced, and collisions oc- theSolarSystemandbeyond”. cur,resultingadepolarisation. Thereforeweconcludethatthis GB also gratefully acknowledge financial support from the lower value of linear polarisation in the C molecular band is 2 Federation of Finnish Learned Societies and the organisers of duetocollisionswithotherspeciesinthecomet’scoma. theACM2014. Theexcitationbyunidirectionalnaturalradiationleadingto PN acknowledge financial support from ESF and Bulgar- linearpolarisationofanemissionlinehasamaximumpolarisa- ian Ministry of Education and Science under the contract tion value for a phase angle of 90◦. The expected variation of BG051PO001-3.3.06-0047. gaspolarisationwithphaseangleαisgivenbytheexpression: P sin2α References P(α)= max 1+P cos2α max Arago,F.,1854.Astronomiepopulaire.GideetBaudry,Paris. FromourmeasurementswecalculateP = 8.5+2.5%. Kr- Bagnulo,S.,Landolfi,M.,Landstreet,J.D.,LandiDegl’Innocenti,E.,Fossati, max −1.6 L.,Sterzik,M.,Sep.2009.StellarSpectropolarimetrywithRetarderWave- ishnaSwamy(2010)saysthatthetheoreticalvalueofPmax for plateandBeamSplitterDevices.PASP121,993–1015. the C and CN molecules is 7.7%, which is within our confi- Feofilov,P.,1961.ThePhysicalBasisofPolarizedEmission.ConsultantsBu- 2 denceinterval6.9–11.2. reau,NewYork. Furusho,R.,Terai,T.,Shinoda,T.,Watanabe,J.,Jul.2014.C/2012S1(ISON), C/2013R1(Lovejoy),andupdatesoftheimagingpolarimetricsurvey.In: 4. Conclusions Muinonen,K.,Penttila¨,A.,Granvik,M.,Virkki,A.,Fedorets,G.,Wilkman, O.,Kohout,T.(Eds.),Asteroids,Comets,Meteors2014.p.173. • We have measured the degree of linear polarisation in Hadamcik,E.,Levasseur-Regourd,A.C.,May2003.Dustevolutionofcomet C/1995O1(Hale-Bopp)byimagingpolarimetricobservations.A&A403, thecontinuumfiltersIF443andIF684ofthedustejected 757–768. fromcometC/2013R1(Lovejoy),andourresults(P443 = Hadamcik,E.,Levassuer-Regourd,A.C.,Renard,J.B.,Jul.1997.CcdPolari- 17.01 ± 0.09% and P = 18.81 ± 0.02%) are in a metricImagingOfCometHale-Bopp(C/1995O1).EarthMoonandPlanets 684 good agreement with measurements of the typical comet 78,365–371. 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