1996AJ 111. . 32 7R l THE ASTRONOMICALJOURNAL dependently byatleastfoursetsofobservers:TimPuckett chard Berry,andReikiKushida(Puckettetal.1994).Its discovery, beinginterpretedassimilartothatofatypeII and JerryArmstrong,WayneJohnsonDougMillar,Ri- Wheeler etal.(1994)andFilippenko(1995)confirmed California atBerkeley. have beenpresentinitsatmosphere. elimination: typeIcSNeshowsnolinesofhydrogen(hence grew thatitwasatypeIcevent(Clocchiattietal.1994; spectrum causedsomeconfusioninthefirstfewdaysafter that Filippenkoetal.diddetecttheHeIXI0830lineinSN of Heintheoptical(hencearenottypelb).Note,however, 6150 Â(hencearenottypela),andalsolackabsorptionlines are nottypeII),lackthedeepSiIIabsorptiontroughnear this classification,whichislargelytheendofaprocess Kirshner etal.1994).Detailedspectroscopicstudiesby (Schmidt &Kirshner1994a),typelb(Filippenkoetal. 327 Astron.J.Ill (1),January1996 0004-6256/96/1 ll(l)/327/13/$6.00 (Turatto &Zanin1994);afteraweek,however,consensus “The Whirlpool”)wasdiscoveredon1994April2,UTin- Also associatedwiththeCenterforParticle Astrophysics,Universityof 1994), typeIc(Schmidtetal.andpeculiarla 19941, showingthatatleastasmallamountofheliummust © American Astronomical Society • Provided by the NASA Astrophysics Data System SN 19941intheface-onScgalaxyM51(NGC5194, We presentopticalphotometryforthetypeIcSN19941inM51(NGC5194)from1994March31toAugust other supemovaedifficult.Usingadistancemodulus/j=29.6±03magtoM51,wecalculateabsolute to SN19941wasconsiderableandquiteuncertain,A1.4±0.5mag,makingadetailedcomparison 6, UTstartingeightdaysbeforeÆ-bandmaximumandendingthreemonthslater.Weestimatetheextinction than thoseofotherSNe.©1996AmericanAstronomicalSociety. than “normar’typelaSNe.ThereisstrongevidencethattheejectaofSN19941weremuchlessmassive magnitudes andaquasibolometriclightcurveforthesupernova.ItappearsthatSN19941waslessluminous Schuyler D.VanDyk,WynnHo,ChienPeng,YoungPaik,RichardR.Treffers, 1. INTRODUCTION Department ofEthicsandSociology,UniversidadComplutensedeMadrid,Madrid28040,Spain Department ofAstrophysicalSciences,PrincetonUniversity,Princeton,NewJersey08544 UBVRI PHOTOMETRYOFTHETYPEIcSN19941INM51 Astronomy Department,UniversityofCalifornia,Berkeley,California94720 Michael MoellerandClausPawellek Heather TartaraandMelodySpence Oil CityHighSchool,City,Pennsylvania16301 Received 1995July31;revisedOctober10 Luebeck PublicObservatory,Luebeck,Germany Electronic mail:[email protected] 1 Javier Bustamante-Donas and AlexeiV.Filippenko Michael W.Richmond VOLUME 111,NUMBER1 ABSTRACT hmo hm hm0 J2000.0. Thesmalldifferencemaybeduetothestronggra- Leuschner andLickObservatories,reporthereourre- dient inlightfromthenucleusofM51atposition circle yielda=132954?072,extinction.Inaddition, ments ontothestandardJohnson-Cousinssystemappearsin environs inthediskofM51.Ourcalibrationmeasure- tometry weusedtoextractthelightofSN19941fromits tion 3discussesthe“templatesubtraction”methodofpho- ner asourown. supernova, asMorrisonandArgylestate. (1994) reportthatopticalmeasurementswithameridian a= l32954?06,5=+47°11'29:96.Morrison&Argyle a= 132747í731,¿=+4726'57'.'84equinox1950.0,corre- discuss thedistancetoM51,and calculateabsolutemagni- of extinction,whichwedescribe inSec.6.In7,we this eventoccurredclosetothenucleusofM51,itisnot ages takenatothersitesandreducedtheminthesameman- sults. Inaddition,wehavecollectedseveralveryearlyim- Sec. 4.In5wepresentlightcurvesofSN19941.Since surprising thatthereareseveral indications ofalargeamount ) The positionoftheSNisslightlyuncertain:Rupenetal. We initiatedaprogramofmulticolorphotometryat Section 2containsadescriptionoftheobservations.Sec- © 1996Am.Astron. Soc.327 JANUARY 1996 1996AJ 111. . 32 7R binned mode,yieldingaplate scaleof0737perpixel.The ranged from60to1080s.The frameswerecorrectedforbias we thenappliedthemtotherawimagesinusualfashion. frames fromthemedianoffiveexposurestwilightsky, from about200s(intheRand/bands)to1500 U to asR94).Thefullwidthathalfmaximum(FWHM) of mond, etal1993;Richmondal.1994,hereafterreferred telescope, bothreflectorsequippedwithCCDcameras(Rich- nitude sequenceofcomparisonstars. measured fromCCDimages,andreducedtoacommonmag- place ourconclusionsinSec.9.AnAppendixlistsdetailsof Fig. 1.V-bandimageofM51(=NGC5194)andSN19941taken1994April a 2048X2048pixelCCDcamera, whichweoperatedin2X2 the medianoffiveimageseachafternoon,andflatfield and, atlatetime,Bbands).Wecreatedbiasframesbytaking observations fromtelescopesatothersites.Alldatawere tory andoneatLickObservatory,withseveralveryearly tained mainlybythetwotelescopesatLeuschnerObserva- erence starsA,B,C,andD.Wereportonobservationsob- the equationsusedtoplaceourobservationsontostan- curves ofSN19941withthosethetypeIc1987M.We observed opticalfluxes.InSec.8,wecomparethelight we calculateaquasibolometriclightcurvebysummingthe of viewisabout53X53.SN1994119"fromthenucleusM51. and flatfieldas describedabove. seeing wastypically2"-3" FWHM, andexposuretimes dard Johnson-Cousinsphotometricsystem. 2, withcomparisonstarsmarked.Northisupandeasttotheleft.Thefield 328 RICHMONDETAL:SN19941 sources. Oneof us(JBD)useda20cmSchmidt-Cassegrain stellar imageswasusually3"-4".Ourexposuretimesvaried We haveimages atveryearlytimesfrom two additional Lick Observatory’s1mNickel telescopeisequippedwith At LeuschnerObservatory,weuseda50and76cm Figure 1showsSN19941,itshostgalaxyM51,andref- © American Astronomical Society •Provi 2. OBSERVATIONS d bythe NASAAstrophysics DataSystem be closesttotheCousinsRband.Specifically,wefindin- FWHM about1.5pixels.Comparingtheraw,instrumental telescope atLick Observatory,oneeachin BVRL Weuseda After theSNhad fadedfromvisibility,we acquiredimages method, alsodescribedinR95 andsummarizedbrieflyhere. mond etal(1995,hereafterreferredtoasR95),butfound referring tothesedata. rection, andawidthofabout1.5pixels.Theexposuretimes We findfc=-0.41±0.25.willdesignatethesedata“Æ.” those listedinTable1,wefindtheresponseofthissystemto obtain exposuresof160and180sonApril2UT.Theplate telescope andanSBIGST-4CCDcamerawithoutfiltersto of M51ingood seeing(FWHM«*176)with theNickel1m that thestrongspiralfeatures ofM51ledtosignificantre- der toextractinstrumentalmagnitudesfromeachimage, we galaxy’s nucleus,makingphotometryadifficulttask.In or- calculate k=—0.26±0A2.Wewillusetheterm“V”when and (D-A)=1.77.AssertingthatV=v+k(V—R)C,we instrumental differencesare(Æ—A)=1.76,(C—A)=1.72, closest totheJohnson-CousinsVband.Inthiscase, were 10,15,and10s.Althoughacorrectionwasmadefor objects showtrailsofabout4pixelsinthenorth-southdi- Public ObservatoryinGermany,equippedwithanSBIG for kusingtheinstrumentalmagnitudesandouradoptedV where Cisanarbitraryzero-pointoffset.Wecanfindavalue instrumental system,wecanwriteR=r+fc(V—)+C, differences inmagnitudebetweenstarsA,B,C,andDwith attempted tousethe“rotsub”techniquedescribedinRich- again usingtheinstrumentalmagnitudedifferencesbetween dark count,noflatfieldingwasappliedtotheimages.Once strumental differences(B-A)=1.32,(C-A)=1.73,and(D scale is377±072perpixel,andstellarimagesappeartohave see Sec.4below).Therefore, weturnedtothe“tempsub” surements ofthecomparison stars derivedbythisprocedure; siduals atthepositionof SN (however,wedidusemea- stars A,B,C,andD,wefindtheseimagestohavearesponse SN 19941onMarch31UT.Thescaleis276perpixel.All ST-6 CCDcameraandnofilter,toacquirethreeimagesof and RmagnitudesforthecomparisonstarslistedinTable1. —A)=1.49. Definingrtobethemagnitudeofastarin r v v r r A 14.14*14.0513.4413.0812.81Corwin1994 B —16.57:15.09:14.39:13.65:Corwin1994 A —14.005*±0.00413.420*0.00213.067*db0.00312.730*Lick B —16.339±0.01715.096±0.00714.331±0.00913.680±0.005Lick D —16.168±0.01615.266±0.00714.746±0.00914.2000.005Lick C —15.751±0.01715.212±0.01014.857±0.01314.497Lick star UBVRIsource 'Adopted asprimarycomparisonstarvalues. Light fromSN19941wassurroundedbylightthe MM andCPuseda49cm//4.2telescopeattheLuebeck Table 1.Johnson-Cousinsmagnitudesofcomparisonstars. 3. EXTRACTINGINSTRUMENTALMAGNITUDES 328 1996AJ 111. . 32 7R 2 2 cation ofthe“tempsub”technique.Positivefeaturesaredarkandnegative Fig. 2./-bandimageofM51andSN19941taken1994April5,afterappli- produced byothertechniqueswetried.Wethenmeasured ones white.Northisupandeasttotheleft. 329 RICHMONDETAL:SN19941 placed thecopiessymmetricallyaroundnucleusofM51, raw images.WeusedstarAtodefineapoint-spreadfunction value calculatedfortheSNinsubtractedimagewasvery the “template”fromoriginal.Thisprocedureyieldedim- convolved themwithaGaussiantoequalizetheseeingdisks, of thePSFscaledtomatchbrightnessSN; we the DAOPHOT2packageinIRAFtoaddartificialstars the close tozero. volved templateimage,andidenticalskyannuli.The lar aperturesofradius0.75timestheFWHMcon- the lightofstarA(inconvolvedtemplateimage)and ings, asFig.2shows;theresultswerecleanerthanthose a circularapertureofradius10pixels,andthensubtracted images tomatcheachimagetakenwhiletheSNwasvisible, Propagating themagnitudes through ourphotometricsolu- uncertainty intheinstrumental magnitudeofSN19941. mean ofthefourvalues.Weusedthisasanestimator the the templatesubtraction,wemeasuredbrightnessofeach SN (inthesubtractedimage),usingidenticalsyntheticcircu- ages inwhichSN19941stoodoutclearlyfromitssurround- set ofIRAFtoolstostretchandrotatethese“template” tronomy, Inc.. underçontract totheNational.Sciençe Foundation. __._ ihe Standard.system usingcolof transformation IRAF isdistributed bytheNationalOpticalAstronomy Observatories, tion yieldedthefinaluncertainties welistinTables4and5. artificial starandcalculatedthestandarddeviationfrom the Our instrumentissoinsensitive tolightinthispassbandthat at roughlythesamedistanceasSN19941.Afterperforming scaled themtomatchthesky-subtractedintensityofstarAin which areoperated bytheAssociationofUniversities forResearchinAs- (PSF) foreachimage,andaddedintotheimagefourcopies In ordertogaugetheaccuracyofsubtraction,weused The i/-banddata,however, must betreateddifferently. © American Astronomical Society •Provided bythe NASAAstrophysics Data System photometric solution. nucleus ofM51wassofeeblydetectedthatitbarelyreached A isverylow,suchaprocedurewillincreasethenoiseand visible. The“tempsub”techniquementionedaboveinvolves even verylongexposuresyieldedweaksignalsfromthe precision ofphotometry.SoonafterSN19941wasdiscov- parison star;thefainterstarsB,C,andDprovidecheckson on photonandreadoutnoise,propagatethemthroughthe tainties calculatedbytheIRAFtaskqphot,whicharebased inner radius5"0andouter11"2.Weadopttheuncer- tion thatposedsuchaprobleminBVRLThesmallamountof the positionofSN19941,alleviatingbackgroundsubtrac- circular apertureofradiusr=3pixels=1"86.Lightfromthe degrade thesignalevenfurther.Wechosetoperformpure background nearstarAthandoesaperturephotometry.Since BVRI photometryinthispaper,sincewebelieveCCDpho- band oftheform reduction ofSNphotometry.Nonetheless,hisvalueswere the fieldofM51,whichweresourceseveralmagni- the lackofvariationstarA,andyieldestimates aperture photometryoneachoriginalU-bandimage,usinga signal-to-noise ratioforboththeSNandcomparisonstar suring theremaininglight.ForUband,inwhich subtracting amodelfromtheoriginalimage,andthenmea- SN, starA,andthenucleusofM51;noothersourceswere however, wemaintaintheU-band magnitudeofstarApro- We adopttheLickcalibrationforstarAasbasis all formal uncertaintiesintheAppendix.Usingsolutions, we where Vdenotesastandardmagnitude,bandvinstrumental IRAF, wecalculatedaphotometricsolutionforeachpass- the listofLandolt(1992)inBVRLUsingphotcaltask we usedthe1mNickeltelescopeatLickObservatoryto discovery, andwelisttheminTable1.On1995April24UT, very usefultousandothers,inthefirstfewmonthsafter light, andthathisvaluesshouldbeusedonlyforpreliminary tion wasdifficultforthosestarsimmersedinthegalaxy’s galaxy. Corwin’smessagewarnedthatbackgroundsubtrac- tudes listedontheThompson-Bryan(1989)chartfor ered, Corwin(1994)distributedhismeasurementsofstarsin corrected using thecolortransformation coefficientsde- Johnson-Cousins system.The LickObservatorydatawere vided byCorwin. we wereunabletoacquire U-band observationsatLick, tometry allowsmoreaccuratesubtractionofthecomplicated comparison starsA,B,C,andD,whichwelistinTable 1. were abletocalculatemagnitudesonthestandardsystem for values, andXtheairmass.Welistsolutionstheir observe thefieldofSN19941andfivemulti-starfieldsfrom scribed above. Leuschner Observatorydata were placedonto stars, wewereabletotransform allrawmagnitudesontothe “sky” thatwedetectedwasestimatedfromanannuluswith We haveadoptedstarA(seeFig.1)asourprimarycom- Having adoptedasetofmagnitudes forthecomparison V=v +v\+u2X+i;3(Z?—u)i;4X(£—u), 4. CALIBRATION coefficients 329 1996AJ 111. . 32 7R 1 330 RICHMONDETAL\SN19941 listed intheAppendix,againusingLickobservationsof for anycoloreffects.Howlargeanerrorcouldthiscause? typically lessthan0.1mag. The erroredependsonthedifferencebetweencolorsof star Atoprovideazeropoint.Thesizeofthecorrectionswas the detector;forexample,inRband, single passbandwasobservedeachnight,arenotcorrected our comparisonstarAandtheSN,oncolortermof dard system,basedonthestandard(V-R)color.Table1 where kcorrectsinstrumentalrmagnitudesintothestan- Leuschner 76cmtelescopeissubjectto^=0.02±0.01;in Let usnowconsidereachofthethreesingle-bandpassdetec- between theSNandstarAwouldhavebeenA(V—Æ)=0.37. exploded, andassumeithadatemperature7=20000K,we might havebeenblueratearliertimes.Ifweassumethatthe observations ofSN19941show(0.25,butthecolor shows thatstarAhas(V-R)=0.36.TheearliestLeuschner tions inturn.TheMarch31.43V-bandpointfromthe find (V-/?)=-0.01,inwhichcasethedifferencecolor the worstcase,^=0.03,colortermwouldintroducean SN radiatedlikeablackbodyinthefirstfewdaysafterit A: =-0.26±0.42;intheworstcase,if£„-0.68,color nal errorinextractingthemagnitudeofSN.TheMarch error of6=0.01mag,negligiblewhencomparedtotheinter- magnitude extractedfromthethreeLuebeckimagesis0.40 term couldbe6=0.25mag.Sincetheinternalerrorof from theextractionprocess.Sincewehavepushedalleffects JBD haveacolorterm£„=—0.41±0.25;intheworstcase, tainty listedinTable3.Finally,theearlyobservationsby mag, thiscontributesasmallamounttotheoveralluncer- 31.96 V-bandpointfromLeubeckcouldsuffer lated uncertaintiestobeconservativeones. to theirextremesinthesecalculations,webelievedthetabu- mag, largerthantheinternalscatterof0.08magmeasured the colorterm£„=—0.66couldintroduceanerrorof0.24 our observationsbycomparingittotheotherthreebright r i; The veryearliestdata,listedinTable2forwhichonlya © American Astronomical Society • Provided by the NASA Astrophysics Data System e=k]{V-R)s\ =kMV-R), We cancheckthatstarAdidnotvaryoverthecourseof rLAH Mar 31.969443.4615.46±0.47 Mar 31.439442.9316.41±0.25 Mar 29.399440.89>18.82 Mar 28.449439.94 Apr 02.029444.52 UT Date •Julian Date—2,440,000. Table 3.Internalaccuracyofdifferentialmagnitudes. difference BVRItelescope Table 2.EarlyobservationsofSN19941. A-B 0.110.040.020.01Leuschner50-cm A-B —0.100.080.03Leuschner76-cm A-C —0.130.070.06Leuschner76-cm A-C 0.050.030.02Leuschner50-cm A-D —0.100.050.04Leuschner76-cm A-D 0.130.050.04Leuschner50-cm JD* > 18.2Leuschner76-cm 13.97 ±0.25Bustamante20-cmunfiltered R Luebeck 49-cmunfiltered Leuschner 76-cm Leuschner 76-cm below, weuserawmagnitudesextractedfromtheframes limits totheprecisionofourphotometry.Indiscussion processed viatherotsubprocedure,sinceresidualsat stars inthefield.Thesecheckcanalsobeusedtoderive March 28-June11;thevaluesof{A—B)yieldmost positions ofstarsleftbytemplatesubtractionaredifficultto B,V,RJ, respectively.Welisttheinternalprecisionofraw interpret. NodifferencebetweenstarAandanyother than 0.08,0.01,0.005,0.006magoverthatperiodin parison starsarelocatedinmuchcleanerenvironmentsthan the precisionofphotometryfromourimages.Allcom- differential magnitudesinTable3,toprovideafeelingfor stringent limits,permittingachangeinthemeanofnomore showed asignificantlineartrendoverthe75dayperiod1994 b 73 14 c b d 3
0> 2 CÖ GO How welldoourlightcurvesagreewithothersinthe Let usconsiderthelightcurves ineachbandpass(see 9440 94609480950095209540 .o* Q O SN 19941U O Julian Day-2,440,000 O LO50-cm a CfA ' I 331 r"& ooCM 332 RICHMOND ETAL: SN 19941 332
Table 7. Peak apparent magnitudes of SN 19941. u- B R I h) O LO50-cm b SN 19941 B JD 9449.5 ±0.1 9450.56 ±0.04 9451.44 ±0.03 9451.92 ±0.04 9452.44 ±0.08 x L0 76-cm Oï UT e Apr 07.0 Apr 08.06 Apr 08.94 Apr 09.42 Apr 09.94 ☆ Lick Mag d 13.82 ±0.04 13.77 ±0.02 12.91 ±0.02 12.65 ±0.02 12.50 ± 0.02 Am18 — 2.07 ±0.03 1.74 ±0.02 1.46 ±0.02 1.08 ±0.02 » ^ Cf A <□> ‘Time and value of peak based on fit to 50-cm and CfA data. bJulian Day -2,440,000, measured at peak of polynomial fit. Q) cBased on polynomial fit to points within JD 9444-9458. T3 dDecline from peak to value 15 days later. 0 % ‘S ocCG q light. Our early /?-band observations constrain the SN to m a cyoo have brightened by over 4 mag in about 5 days before being O detected. About 8 days elapsed from the first detection in R O O O band to maximum light in R. The peak of the light curve is somewhat broader than those in B and V, but the late-time decline rate similar: 0.017 mag per day. As in the V band, the last two points indicate a flattening of the light curve. The /-band light curve, shown in Fig. 9, exhibits the wid- 9450 9500 9550 est peak and weakest “knee” separating the post-maximum Julian Day - 2,440,000 fall from the late-time decline. Again using data after JD 2449500, we find a linear slope of 0.021 mag per day. The Fig. 6. B light curve of SN 19941. The data labelled “CfA” were provided final two points are substantially brighter than the extrapo- by Schmidt & Kirshner (1994b). lated curve. We next construct color curves for SN 19941, and plot maximum light. As R95 mentioned, Leuschner Observatory them in Figs. 10-13. In each figure, the color rapidly reddens ¿/-band observations can show large (up to ~0.5 mag) de- from discovery to roughly JD 2449465; after that time, the viations from the standard Johnson-Cousins system; in the (B-V) and (F-Æ) values turn back to the blue. This turn- present case, the agreement with the CfA data (and with that ing point occurs at the same time as the “knee” in the B and of Lee et al. 1994) suggests that our calibration was more V light curves. Both features probably occur as the ionization successful than it was for SN 1994D. front, or pseudophotosphere, of the supernova reaches into The 5-band light curve, shown in Fig. 6, rises and falls the core of the ejecta and the SN makes the transition from quickly, reaching an inflection point sometime around 15 being optically thick to optically thin. The (R - /) color con- days after maximum and eventually settling into linear de- tinues to redden for several more weeks, reaching what cline. Fitting a straight line to the data after JD 2449500, we
find a slope of 0.017 mag per day, albeit with substantial 1 uncertainty. There is some evidence for flattening of the light T ' ' 1 curve at late times. SN 19941 V Several early detections define clearly the very steep rise ° this paper to maximum in the V band, shown in Fig. 7. The SN must * CfA have risen at least 2.4 mag in the 2 days between the upper » Lee et ai. limit on March 29 and the first measurement, and another 0.9 * Yokooetal. mag within the next 12 h. The time from the first V-band 0) o* -o detection to V-band peak light is about 9 days. As in the B 3 band, the light curve falls steeply after maximum, then •+J ‘S reaches a linear decline at late times, at a rate of 0.021 mag COoo per day. The last two points indicate that the light curve E flattened significantly at the end of our observations. > 15 In Fig. 8, one sees another very sharp rise to maximum oes * o 8*‘ Table 6. Lick data on SN 19941. o “8^* UT Date JDb B I Apr 11.47 9453.97 — — 12.71 (01) — Apr 12.37 9454.87 14.11 (01) 13.10 (00) 12.75 (00) 12.50 (01) Apr 13.50 9456.00 14.37 (01) 13.26 (00) 12.85 (01) 12.58 (00) May 14.46 9486.96 16.78 (04) 15.77 (02) 15.25 (03) 14.54 (00) Jun 11.38 9514.88 17.48 (08) — 15.94 (04) — 9460 9480 9500 Jul 13.32 9546.82 17.95 (26) _ _ _ Julian Day - 2,440,000 Jul 14.30 9547.80 18.05 (13) 17.23 (08) 16.46 (08) 15.95 (05) Aug 07.21 9571.71 18.10 (18) 17.60 (06) 16.93 (09) 16.34 (05) Fig. 4. Comparison of the V light curves of SN 19941 obtained by this *Parantheses enclose uncertainty, in hundredths of a magnitude. paper, Schmidt & Kirshner “CfA” (1994b), Yokoo et al. (1994), and Lee bJulian Date - 2,440,000, at midpoint of sequence of images. et al. (1995).
© American Astronomical Society • Provided by the NASA Astrophysics Data System r"& ooCM 333 RICHMOND ETAL: SN 19941 333 h) Oï
Fig. 7. V light curve of SN 19941. The data labelled “CfA” were provided Fig. 9. I light curve of SN 19941. The data labelled “CfA” were provided by by Schmidt & Kirshner (1994b). Schmidt & Kirshner (1994b).
might be a plateau at (/?-/) ^0.7 mag at the end of our blackbody of temperature T=7950 K; but such a cool atmo- observations. sphere should also produce more lines than were seen in the Note that the earliest measured colors of SN 19941 are earliest spectrum. Hotter blackbodies yield bluer colors: considerably redder than is typical for most SNe. The first 7= 15 000 K corresponds to (#-10=0.00 mag, and (B — V) datum, for example, is 0.38 mag. According to Fil- r=20 000 K corresponds to (#-V) = -0.09 mag. If one ippenko et al (1995), the spectrum at this time was a rela- postulates that SN 19941 resembled a perfect blackbody in tively smooth and featureless continuum, suggesting that the the first days after discovery, then its observed colors suggest SN’s atmosphere was hot—hot enough, perhaps, to resemble a moderate amount of reddening. a blackbody. Using the Bessell (1979) flux/magnitude rela- The colors at maximum remain redder than those of some tionships, we find that (B — V) =0.38 mag corresponds to a types of SNe. As Fig. 11 shows, at the time of maximum
I 1 I 1 I i I SN 19941 (U-B) 1.5 O LO 50-cm
O ^ 0.5 O O
0 <9 I ... I ... I ... I ... I 9440 9460 9480 9500 9520 9540 Julian Day - 2,440,000
Fig. 8. R light curve of SN 19941. The data labelled “CfA” were provided Fig. 10. U-B color curve of SN 19941. No reddening corrections have by Schmidt & Kirshner (1994b). been applied.
© American Astronomical Society • Provided by the NASA Astrophysics Data System ¡T"pc 00CM 334 RICHMOND ET AL. \ SN 19941 334
h—^—1—^—r h) < SN 19941 (B-V) SN 19941 (R-I) UD O LO 50-cm O) x LO 76-cm *x9 * ° <á° o ■ SN 1987M 00 oo°° ° 73 +-> CuOß ■ CE?x ß © cö 6> O o 0 E > O á5 I x T m O o 0 Q>* O O O O LO 50-cm *< X54 x x LO 76-cm
9460 9480 9500 9520 9460 9480 9500 9520 Julian Day - 2,440,000 Julian Day - 2,440,000
Fig. 11. B-V color curve of SN 19941. No reddening corrections have been Fig. 13. R-I color curve of SN 19941. No reddening corrections have been applied. We also include points for the type Ic SN 1987M, which have been applied. shifted in time slightly (see Sec. 8). Each of the BVRI light curves exhibits a linear decline of about 0.020 mag per day (and becoming less steep at the end light in 5, JD 24499450, (B- 0.8 mag. Even after cor- of our observations), corresponding to an exponential de- recting for extinction of E(Æ- V)=0.45 mag (see Sec. 6), the crease in luminosity. The radioactive decay of 56Co yields a intrinsic color of SN 19941 was (Æ-V)°~0.35. The “nor- smaller slope, 0.0098 mag per day. Therefore, if the late-time mal” type la SNe, such as SNe 1980N (Hamuy et al. 1991) light curve of SN 19941 is powered by 56Co, a substantial and 1994D (Richmond et al. 1995), show (5 - V) ^ 0.0. The fraction of the high-energy photons must escape directly newly recognized class of “subluminous” SNe la, such as from the ejecta, rather than interacting with matter and being SNe 1991bg (Filippenko et al 1992; Leibundgut et al 1993) degraded to optical photons. This suggests that the mass of and 1992K (Hamuy et al 1994), are even more red than SN the ejecta is small, and/or the radioactive material has been 19941, with SN 1991bg reaching {B - V)°~ 0.7 at maximum. mixed throughout the ejecta. The peculiar “type lib” SN 1993J, which faded at a similar rate of 0.02 mag per day (R94), is thought to have had an envelope mass of only 1.6- 4.6 M0 (see Table 4 of Wheeler & Filippenko 1995). SN 19941 (V-R) O LO 50-cm Another clue to the mass of the ejecta in SN 19941 is the width of the peak of the light curve. As Fig. 14 shows, the peak of its light curve is substantially narrower than those of two other SNe with relatively low-mass envelopes: SN 1993J (see above) and SN 1994D (which, as a type la, pre- cu sumably had M j ^1.4M ). Since the light curve at this ß 0.6 e ecta 0 O x® time is driven by the diffusion of energy produced by the Ö0ß O op O 56 56 CO x O 0 Qod radioactive decay of Ni to Co, its width is dependent on a 0 o the amount of material through which the energy must flow. ^ 0.4 A massive envelope increases the diffusion time and leads to © > o a broad peak near maximum light. A skimpy envelope, on the other hand, allows photons to escape more quickly from the inner, 56Ni-rich layers in which they are produced, yield- ing a narrow peak. Other factors can affect the light curve shape: if radioactive material is mixed into the outer layers in the explosion, its energy will escape more quickly from the envelope, producing a short-lived maximum. 9460 9480 9500 9520 Julian Day - 2,440,000 6. EXTINCTION
Fig. 12. V—R color curve of SN 19941. No reddening corrections have been color Curves of SN 19941 suggest that the object was applied. observed through significant amounts of interstellar material.
© American Astronomical Society • Provided by the NASA Astrophysics Data System 1996AJ 111. . 32 7R 335 RICHMONDETAL.:SN19941 We examineheretheseveralmethodswhichhavebeenused Fig. 14.ComparisonoftheV-bandlightcurvesSNe19941,1994D,and to estimatetheamountofextinction;impatientreaderscan must beinM51.High-resolutionobservationsoftheNalD line ofsighttoM51,basedontheHiemissioninsideour most ofthematerialliesinM51.Theymeasureequivalent absorption featuresbyHo&Filippenko(1995)confirmthat own MilkyWayGalaxy,implyingthatmostofthematerial skip directlytoTable8forasummary. one weretoapplytherelationshipderivedinR94relating m bysummingmanycomponentsalongthelineofsight.If widths W(D1)=1952+11mÀandW\(Z>2)=2461±11 1993J. rated NaIlinesinthespectrumofSN19941arethreetimes clouds inanothergalaxy;addition,thewidthsofsatu- regions intheMilkyWay,itmightnotapplytohigh-density late E(Z?-V)=0.92mag.However,sincethatrelationship width ofthesodiumlinestocolorexcess,onewouldcalcu- larger thananyusedtoderivetheaboveformulae. was extractedfromobservationsofstarsbehindlow-density column densityineach.Theythenconvertto of eachcomponentintheNaIlinestocalculatesodium of neutralhydrogen,anduserelationshipsbetweenlogN(H) and colorexcesstoderiveAy=3.líi5magSN19941. X fits tomodelSNlightcurves0.45±0.16 Iwamoto etal.1994 fits tomodelSNspectra0.29-0.45 Baron etal.1995 log N(NaI)toN(H)E(B-V)1.0ÍJ;?Ferlet etal.1985Ho&Filippenko1995 Wx (NaD)toE(B-V)0.92Sembachetal.1993Richmond1994 © American Astronomical Society • Provided by the NASA Astrophysics Data System Burstein &Heiles(1984)derivezeroextinctionalongthe Ho &Filippenko,awareoftheseproblems,fittheshape Table 8.Estimatesofcolorexcess(mag)toSN19941. method E(B-V)calibrationsource Spitzer 1978 prefer 1.0=^4^2.0mag. They pointoutseveralpossiblesystematicerrorsintheir values neartheupperlimitbeingfavoredatlatetimes. From theearliestspectra,theyfind0.9^A^1.4mag,with analysis, andarguethatthisvalueisanoverestimate;they cess toextinctionlistedinR94,thiscorresponds£(5-V) mainder ofthispaper.Usingtheconversionsfromcolorex- Table 9.Thelargeuncertaintyintheextinctiontothisobject several epochstomodelsofanatmosphereaC+Ostar. to absoluteones,andcomparetheintrinsicluminosityof will limitourabilitytoconverttheapparentpeakmagnitudes star. TheyquoteavalueofA=1.4±025mag. SN 19941tothatofothersupemovae. the subjectofmanyinquiries.Wewereabletofindonlytwo they determinedadistancemodulusof/x=29.9magtoM51. Group galaxiestocalibratearelationshipwithsomedepen- the sizesofthreelargestHnregionsinasetLocal and H)intheliterature.Sandage&Tammann(1974b)used distance determinations(otherthanthoseusingtheredshift 19941 totheirownmodels,basedontheexplosionofaC+O brightness fluctuations(SBF).Hefindsadistancemodulusof kindly providedapreliminarydistancetoNGC5195,the Georgiev etal.(1990)comparedthedistributionofapparent dence onthetypeandluminosityclassofparentgalaxy; and M51toderive/u=29.2mag.Inaddition,Tonry(1995) =0.45 ±0.16mag.Welisttheextinctionineachpassband sizes andluminositiesofyoungstellarassociationsinM33 The resultsdependheavilyonmodelsoftheexplosionand use itspropertiestocalculatethedistancehostgalaxy. yu=29.59±0.15 toNGC5195. smaller galaxyinteractingwithM51,basedonsurface- their assumptions.Iwamotoetal.(1994)findthattheob- y external uncertaintiesincalibration andreddening;theyas- for formalfitsofthedatatotheir model,and,second,the where tistherisetimeofbolometriclightcurve,in to theeventandfind¿¿=29.6+5log(i/9)±0.5±0.7mag, it peaksintheBband.Aslightly earlierobservationplacesa that ourearliestdetectionofSN 19941occurs7.5daysbefore days, andthequoteduncertaintiesare,first,thoseappropriate ¿1=29.2±0.3 magandA=1.4±025mag.Baronetal. sume E(J5-V)=0.45mag.Looking atTables2and7,wesee sphere method(Kirshner&Kwan1974;Schmidtetal.1992) (1995) apply,withwell-statedcaveats,theexpandingphoto- served lightcurveshapesbestmatchtheoreticaloneswhen v strong upperlimit ontheSN’sbrightness9.5days beforethe 0 r r y We willadoptavalueofA^1.4±0.5magforthere- Baron etal.(1995)fittheobservedspectraofSN19941at Iwamoto etal.(1994)comparetheobservedcolorsofSN The distancetoM51,surprisinglyenough,hasnotbeen Since SN19941appeared,severalauthorshavetriedto Table 9.Adoptedextinction(mag)toSN19941. E(B-V) AuAbAAi vr 0.45 2.211.851.401.040.68 7. ABSOLUTEMAGNITUDE 335 1996AJ 111. . 32 7R 3 present inthestellarassociationsstudiedbyGeorgievetal 5-band peak.Weassumethatthetimeofbolometricmaxi- can beveryprecise,duetothelargeuncertaintyinred- ing theirvalueofi=9days,weareleftwith/¿=29.6. mum occursatthetimeofpeaklightinBband(asR94 mag). TheworkbySandage&Tammann(1974b)shouldbe derestimation ofthedistancetoM51(implyingjul>29.2 dening tothesupernova.Ifsimilarlylargeextinctionwere rected eachmagnitudeforextinction,convertedthemagni- based on4CepheidsandMiras(Alves&Cook1995).We values (Sandage&Tammann1974a)forthedistancesto of Hnregionsmayhavesystematicerrors.Comparingtheir less affectedbyextinction,buttheircalibrationofthesizes show islikelyforthesecondarypeakofSN1993J).Accept- 336 RICHMONDETAL:SN19941 creating a“quasibolometric”lightcurve(Fig.15).Wecor- convert themeasuredfluxesfromSN19941intoluminosity, mal” typelaSN(R95,andreferencestherein). bands except/.Therangeoflikelyvaluesfortheluminosity which wewillattachaverylargeuncertaintyof0.3mag,in ered here,inanycase,andsogiveitthegreatestweight feel theSBFmethodissuperiortoothermethodsconsid- for M101isslightlyhigherthanavalueof¿¿=29.1mag find evidenceforoverestimationofthedistancescale(sug- results forE(5-V)=0.08arecorrect as printedinR94.WethankBrian modulus of¿¿=29.6magtoM51,wethentranslatedthe tude intofluxusingthecalibrationofBessell(1979),and ties aredominatedbytheunknownextinctionforallpass- 7, wecancalculatethepeakabsolutemagnitudesofSN two methods. considerable differencebetweenthoseproducedbytheother view ofthepreliminarynatureSBFdistanceand calculating adistancemodulustoM51.Weadopt¿¿=29.6, a recentreviewofnumbermethods(deVaucouleurs gesting ¿¿<29.9):theyplaceM31at¿¿=24.84mag,whereas some oftheircalibratinggalaxiestomorerecentones,we (1990), andnottakenintoaccount,itwouldleadtoanun- one shouldreplacethelabel“E(B-V)=0.32” with“E(£-V)=0.40.”The our routinethatcorrectsforextinction.This erroraffectedtheanalysisofSN In thecourseofperformingcalculations forSN19941,wefoundanerrorin the areaofoverlapbetweenfilters).Assumingadistance of thistypeIcsupernovaissolargethatdetailedcompari- Schmidt forhisattention todetail,whichledthediscovery ofthiserror. summed allthefluxes(makingaveryminorcorrectionfor SN 19941appearstohavebeenlessluminousthana“nor- sons toothereventsseempointless.Wecansay,atleast,that section, andtheapparentmagnitudesatpeaklistedinTable 1993J inR94thefollowingmanner: in Tables14and15,Fig.11, 19941, whichweplaceinTable10.Notethattheuncertain- 1993) finds¿¿=24.3mag,andtheirestimateof¿¿=29.3mag r 3 © American Astronomical Society • Provided by the NASA Astrophysics Data System It isclearthatnomeasurementbasedonSN19941itself Using themachineryofR94,wecanusethisdistanceto Given thisdistance,theextinctionadoptedinprevious -17.99 ±0.84—17.68±0.73-18.09±0.58±0.48-17.78±0.38 “Assuming fi=29.6±0.3magandE(B-V)0.45mag. ü BVRI Table 10.AbsolutepeakmagnitudesforSN19941. bolometric lightcurveatveryearlytimesdependssignifi- bolometric luminositydoesnotincludeanycontribution Note thedifferencebetweenUBVRIandBVRIsumsforhighvaluesofcolor Fig. 15.Quasibolometriclightcurves,formedbysummingopticalfluxes. tween theUBVRIandBVRIsumsforhighestvalueof to theopticalfluxes. from theUVtoIR,norisitderivedanyblackbodyfit excess. points measureddirectlyfromimagesareintheGunn- curves togetherwiththeirdetailedanalysisofthisobject’s than ahandfulofphotometricdataintheliteratureisSN measured atleast2?,V,R,and/-bandmagnitudes. color excess.Evenaftermaximumbrightness,on1994April cantly ontheextinction.NotedifferenceinFig.15be- cess totheSN:0.30,0.45,and0.60mag.Theshapeof Filippenko etalsuggestthatthe lightcurvesareconsistent Thuan gandrpassbands;itisthereforedifficulttomakea lometric luminosityofSN19941foreachnightinwhichwe indication thatanymeasurement wasmadebeforethepeak. confidently assignadatefor maximum light;thereisno detailed comparisonwithourJohnson-CousinsUBVRIdata. a syntheticpassbandtofluxcalibratedspectra,andthose spectra. Mostoftheirphotometryisconstructedbyapplying summed opticalflux.Tables11and12containthequasibo- summed opticalfluxintoaluminosity.Notethatthisquasi- to findthebest match tothelightcurvesforSN 19941,we with maximumbrightnessoccurring atthetimeofdiscovery, 1987M. Filippenkoetal(1990)publishedasetoflight 19 UT,theUbandstillcontributesabout10%of 1987 September21.Whenshifting thepointsforSN1987M Table 12. BVRI quasibolometric luminosity of SN 19941 (ergs/s). b C 12 I ' I h) JD E(B-V)=0.30 E(B-V)=0.45 E(B-V)=0.60 UD 9444.95 6.450e+41 1.015e+42 1.619e+42 Type Ic SNe: red (J) 9446.92 1.091e+42 1.700e+42 2.682e+42 (J) 9447.89 1.214e+42 1.879e+42 2.944e+42 9449.86 1.571e+42 2.427e+42 3.795e+42 13 x 941 R 9452.95 1.605e+42 2.453e+42 3.798e+42 9453.94 1.399e+42 2.124e+42 3.265e+42 • 87M r (shifted) 9453.95 1.444e+42 2.194e+42 3.374e+42 9454.82 1.324e+42 2.003e+42 3.065e+42 9454.85 1.311e+42 1.978e+42 3.020e+42 QJ 14 9454.87 1.367e+42 2.069e+42 3.172e+42 T) 9455.88 1.084C+42 1.624e+42 2.462e+42 3 9455.88 1.256e+42 1.889e+42 2.874«+42 9456.84 1.084e+42 1.624e+42 2.462e+42 CIOa 9457.82 9.650c+41 1.440e+42 2.172e+42 Table 11. UBVRI quasibolometric luminosity of SN 19941 (ergs/s). 9500 9550 b e Julian Day - 2,440,000 JD E(B-V)=0.30 E(B-V)=0.45 E(B-V)=0.60 9447.89 1.465e+42 2.373e+42 3.917e+42 9454.82 1.455e+42 2.260e+42 3.571e+42 9455.88 1.182e+42 1.815e+42 2.839e+42 Fig. 16. Comparison of the type Ic SNe 19941 and 1987M in blue pass- 9461.82 6.285e+41 9.468e+41 1.450e+42 bands. The data for 1987M have been shifted by (+5 days, -2.3 mag) in B, (+5 days, -1.0 mag) in g. ‘Assuming distance to M51 is 8.32 Mpc (fi = 29.6 mag). bJulian Date - 2,440,000, at midpoint of sequence of images. ‘Preferred value. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1996AJ 111. . 32 7R 56 NGC 5194donotmatchthosein2715. gas-to-dust ratioand/orthepropertiesofinterstellardustin that ofSN1987M.Thispuzzleseemstoimplyeither 338 RICHMONDETAL:SN19941 bulge ofM51,wescaledandsubtractedimagesthegalaxy le SN19941inM51,includingtheearliestknowndetections and earlierupperlimits,reducedthemtoacommon jandro Clocchiatti,TaichiKato,andBjornGranslo.David hypothesis thatthemassofejectainSN19941wasvery document thebehaviorofSN19941inopticalfrom9 alone fromthosecontainingthesupernova.Ourlightcurves passbands fadedatarateabouttwiceasfastthedecay days beforemaximumlightto120later. system. Inordertominimizethecontaminationfrom those ofotherSNe,eventypela.Second,lightintheVRI moto etal.1994;Nomotoal1995)favorejectamassesof of Co.TheoreticalmodelsanexplodingC+Ostar(Iwa- reliable. ItisprobablethatSN19941wasredderandfainter only 0.5-0.9M. small. First,thelightcurveshavepeaksmuchlessbroadthan little morewithconfidence.Althoughtheshapesoflight renders thecolorsandabsolutemagnitudesofSN19941un- 1987M wasintrinsicallyredderthanSN19941,orthatthe proved thispaper,eventhoughhemadeusdoalotofextra kept usallinformed.WeespeciallythankBrianSkiff,Ale- colors ofthesetwoSNemayhavebeenquitedifferent. low typeleSN1987M,thereisevidencethattheintrinsic curves ofSN19941arereasonablyclosetothoseitsfel- at peaklightthan“typical”typelaSNe,butonecansay and uncertainextinctionduetomaterialinitshostgalaxy background information.JostJahnandTimSpuckprovided ences atPrincetonUniversity.WearegratefultoSunMicro- Leuschner Observatoryfacilities.Wethankthereferee,Brian Hands-On UniverseprojectgaveHTandMSaccesstothe vital communicationsandgoodadvicetotheauthors.The Corwin andGerarddeVaucouleursprovidedveryuseful the acquisitionandreductionofdataatLeuschnerObserva- Photometries, Ltd.forequipmentdonationsthatwerevitalto work. MWRthankstheDepartmentofAstrophysicalSci- Schmidt, forprovidingmanysuggestionswhichhaveim- tory. FinancialsupportforthisresearchwasprovidedtoAVF Schlegel acquiredalate-timeimageatLickforus.Harold systems, Inc.(AcademicEquipmentGrantProgram)andto — y)=0.45±0.16,cannotbemadeveryprecise.Thelarge and hisgroupatUCBerkeleythroughNSFGrantNos.AST- Lick Observatory toobservethefieldofSN19941 andfive 8957063, AST-9115174,andAST-9417213. 0 © American Astronomical Society • Provided by the NASA Astrophysics Data System We havecollectedalargesetofobservationsthetype We contributetwopiecesofevidencethatsupportthe Many peoplehelpedtospreadthenewsofSN19941and Our bestestimateforthereddeningtosupernova,E(B A. PHOTOMETRICSOLUTIONSFOR LEUSCHNERANDLICK On 1995April24UT,weusedthe 1mNickeltelescopeat 9. CONCLUSIONS (V-R)=0M{v-r) +C, (B-V) =l.l6(b°-v)+C, I=i +0.05(v—i)C[. (V—R) =0.Sl(v—r)+C, multistar fieldsfromthelistofLandolt(1992)inBVRI.Us- formal uncertaintiesbelow: ing thephotcaltaskinIRAF,wecalculatedaphotometric VR BV VR values, andXtheairmass.Welistsolutionstheir where Vdenotesastandardmagnitude,bandvinstrumental solution foreachpassbandoftheform January 7,October8,and29,todeterminethefirst- night tonight.Sincewealwaysmeasuredthebrightnessof instrumental andstandardmagnitudes,whichvaryfrom to thestandardJohnson-Cousinssystem.ThetermsC, order colortermswhichtransforminstrumentalmagnitudes Leuschner 50cmtelescope,wefind mental B-bandmeasurements,asdescribedintheAppendix involved inthephotometricsolutions.Weincludesecond- SN 1994DrelativetothatofstarA,thesetermswerenot mental valuesbyb°intheequationsbelow.For to R94;wedenotetheextinction-correctedB-bandinstru- order color-dependentextinctioncorrectionsonlyforinstru- C, etc.,arethedifferencesbetweenzeropointsof “dipper” asterismon1993April1and61994 vember 8,November11,and13,tocalculatethe color termsfortheLeuschner76cmtelescope,finding v BV B =i;+(£—)5.230.3IX—0.06(bv R =u-(r)+4.86+0.13X+0.04( 7=i; —(ur)(r—i)+4.91+0.1lX+0.07(r—/) For theLeuschner50cmtelescope,weobservedM67 V=v +vlv2X+v3(b-v)v4X(b-v) V=v +4.92+0.20X+0M(b-v)-0.005X(b-v), We usedimagesofM67takenon1994October31,No- (U—B)= l.20(u—b°)+C, (B—V)= 1.15(¿>°—u)+Cy, 7=/ +0.02(i;—/) +Q* V= v+0.02(i;—/)Cy, V=v +0M(v-i)C, UB B v — 0.05X(r—/). -O.OOSX(b-v), + 0.0008X(u-r), 338 1996AJ 111. . 32 7R Bessell, M.S.1979,RASP,91,589 Baron, E.,Hauschildt,RH.,Branch,D.,Kirshner,R.R,&Filippenko,A.V. Alves, D.R.,&Cook,K.H.1995,AJ,110,192 Burstein, D.,&Heiles,C.1984,ApJS,54,33 339 RICHMONDETAL:SN19941 Morrison, L.V.,«feArgyle,R.W.1994,IAUCirc.No.5989 Corwin, H.1994(privatecommunication) Clocchiatti, A.,Brotherton,M.,Harkness,R.R,&Wheeler,J.C.1994,IAU Lee, M.G.,Kim,E.,S.C.,S.-L.,Park,W.-K.,