1992AJ 104.1543F 2 THE ASTRONOMICALJOURNAL Universities forResearchinAstronomy, Inc.,undercontractwiththeNationalScienceFoundation. Visiting Astronomer,KittPeakNational Observatory,NationalOpticalAstronomyObservatories,which are operatedbytheAssociationof Presidential YoungInvestigator. 1543 Astron.J. 104 (4),October1992 0004-6256/92/101543-14$00.90 ©1992Am.Astron. Soc.1543 © American Astronomical Society • Provided by the NASA Astrophysics Data System -1 -1 _1 -1 We presentphotometricandspectroscopicobservationsofSN1991bg,averypeculiarTypelasuper- Institute ofGeophysicsandPlanetaryPhysics,LawrenceLivermoreNationalLaboratory,P.O.Box808,L-413,Livermore, brightness SN199Ibgwas1.6magsubluminousinVand2.5B,comparedwith nova locatedintheoutskirtsofElgalaxyNGC4374(M84)Virgocluster.Atmaximum mag dratherthan0.026.Furthermore,thekneeinVlightcurveofSN199Ibgoccurs magd ratherthanthetypicalvalueof0.06magd“forSNela,andalate-timedecline0.034 The declinefrommaximumwascertainlyquitesteep;wemeasureaninitiallinearVfadingof0.10 normal SNela.Thecolorswereunusuallyred,buttheobjectwasnotsignificantlyreddenedbydust. past maximumhasarathernarrowabsorptionlineattributed toNaID;thestrengthofthisfeature elsewhere inthespectrumwhereasFeIIwasweakorabsent.Linesduetootherintermediate-mass spectroscopically peculiaratmaximumbrightness;e.g.,adeeptroughwaspresentaround4200A. Despite havingprominentSillabsorptionnear6150A,characteristicofSNela,SN199Ibgwas curvedoesnotexhibitasecondarymaximum,andtheRshowsnosignofplateau. only ~17dpastBmaximum,incontrasttotheusualvalueof30-40d.UnlikenormalSNela,/ spectra ofSNeIb/Icthanla.Weidentifyemissionlines of[CaII]AA7291,7324,OI>17773,the grew withtime.Incertainotherrespectsthespectrum3weeks pastmaximummorecloselyresembles was slightlylowerthanaverage(11000-13000kms)forSNela.Aspectrumobtained3weeks elements werealsostrongnearmaximumbrightness,buttheexpansionvelocity(—10000kms) Spectral synthesissuggeststhatmuchofthisfeaturewasproducedbyTiII.Indeed,IIpresent core, therebyleavingacompact remnant;alternatively,perhapsthecompleteexplosionof alow-mass low massfortheejecta,expandingatanearlynormalvelocity, isconsistentwiththeearlytransitionto that thisobjectwasproducedbytheslowdeflagrationof awhitedwarf.Theobservedexpansion scattering lines;apparentlythenebularphasebeganvery early inSN199Ibg.Threemonthspast Ca IIinfraredtriplet,and(tentatively)[Oi]Á5577,allsuperimposed onacomplexblendofPCygni the nebularphaseaswellwithrapiddeclineand early kneeintheVlightcurve.Thus,SN velocity, however,istoolargeunlessasub-Chandrasekharmass ofmaterialwasejected.Anunusually maximum the[Call]blendstartedtodominatespectrum. ThepropertiesofSN199Ibgsuggest ( —1.0^q)whitedwarfwas responsible. WeshowthatsomepropertiesofthepeculiarSN 1986Gin 199 Ibgmayhaveresultedfromthedeflagrationofonly outerlayerofmaterialontopaC-O THE SUBLUMINOUS,SPECTROSCOPICALLYPECULIARTYPElaSUPERNOVA1991bg Department ofAstronomy,andCenterforParticleAstrophysics,UniversityCalifornia,Berkeley,California94720 Richard R.Treffers,ThomasMatheson,LuisC.Ho,andArjunDey Palomar Observatory,105-24,CaliforniaInstituteofTechnology,Pasadena,91125 Department ofPhysicsandAstronomy,UniversityOklahoma,Norman,Oklahoma73019 Department ofAstronomy,UniversityCalifornia,Berkeley,California94720 Van VleckObservatory,WesleyanUniversity,Middletown,Connecticut06457 1 Alexei V.FilippenkoandMichaelW.Richmond Wallace L.W.SargentandToddA.Small 2 IN THEELLIPTICALGALAXYNGC4374 William HerbstandCharlesH.Ford David BranchandC.MartinGaskell (Received 4March1992;revised15June1992) Wil J.M.vanBreugel VOLUME 104,NUMBER4 California 94550 ABSTRACT OCTOBER 1992 1992AJ 104.1543F noted withincreasingfrequencyinwell-observedSNela constitute aremarkablyhomogeneoussubclass.However, lines, yettwomonthspastmaximumthespectrumwas maximum declineintheÎ7i?Fbands,Frogeletal(1987) anomalies intheopticalspectraaswellarapidpost- during thepastdecade.OneofbestexamplesisSN spectroscopic andphotometricpeculiaritieshavebeen ippenko etal.1992).Filippenko(1992;seealsoRuiz- nearly indistinguishablefromthatofaclassicalSNla(Fil- imum spectrumdidnotexhibitSiIIorCaabsorption was typical.AverystrikingcaseisSN199IT;itspremax- than didthenormalSNla1989B(Leibundgutetal. interesting object,SN1990N,hadconsiderablyweaker brightness wasnormal(Cristianietal.1992).Another nagia &Gilmozzi1991).Itisclaimed,ontheotherhand, and theearlytimeultravioletspectrumwasunusual(Pa- reported clearlydiscrepantinfrared(JHK)lightcurves, Phillips etal(1987)andCristiani(1992)observed minous relativetootherSNela,butthisisnotcertain Si IIabsorption(at6150A)oneweekpriortomaximum that theextinction-correctedBmagnitudeatmaximum 1544 FILIPPENKOETAL.:SN1991bg the lightcurvesandcolorofSN199ITdifferedonly ported byPhillipsetal(1992)showsthattheshapesof Lapuente etal1992)arguedthatSN199ITwasoverlu- trum wasclassifiedasTypelabythepresenceofstrong member oftheVirgocluster),wasdiscoveredvisually(m properties areallundeniablypeculiar.SN199Ibg,located slightly fromSNlatemplatecurves(Leibundgut1988). pared theobservedcolor(B— V—0.76mag)onUT1991 influenced bythissimilarity,Della Valleetal.(1991)com- that ofthestronglyreddenedSN 1986GinCenA.Perhaps emission bandnear4100Amade thespectrumresemble troscopically peculiarandquitesubluminous.Benettietal. Filippenko etal.(1991)statedthattheobjectlookedspec- Sill absorptionnear6150Â(Kirshneretal1991),but servatory. AsdocumentedintheIAUCirculars,spec- with a0.4mreflectorattheYatsugatakeSouthBaseOb- spectra, orluminosity). the importantparameters(e.g.,lightandcolorcurves, peculiarities weretemporary,orrestrictedtoonlysomeof Hence, onecouldarguethatinalloftheabovecases December 15.4withthatoftypical SNelaatmaximum usual strengthoftheSiIIlines, andtheabsenceofan 1986G, whichwasinthedustlaneofCenA(NGC5128). 1991a; Phillipsetal.1992),butitspostmaximumevolution 1' fromthenucleusofElgalaxyNGC4374(M84,a (Phillips etal.1992).Infact,superiorphotometryre- ~14 mag)onUT1991December9.844byR.Kushida (1991) remarkedthattheslowexpansionvelocity,un- v © American Astronomical Society • Provided by the NASA Astrophysics Data System Type lasupernovae(SNela)aregenerallybelievedto Very recently,thereappearedaSNlawhoseobserved some SNelaaremavericks,bothphotometricallyandspectroscopically.Theseobjectsmustbe magnitudes. Moreover,SN19711inNGC5055isknowntohaveresembled1986G;thus, Cen AweresimilartothoseofSN1991bg,anditmayhavebeenintrinsicallydimbyseveral carefully eliminatedfromsamplesofSNelausedindeterminationscosmologicalparameters. 1991bg wasprobablyaclosecousinofthesetwoobjects.SN199Ibgclearlydemonstratesthatatleast 1. INTRODUCTION on UT1991December15.35,buttheynotedthattheab- red color(2?—F=0.86mag)ofSN199Ibg,asmeasured close tothatófSN1957Binthesamegalaxy.Phillipsetal ber 9.860and10.804SN199Ibghadaphotovisualmag- paper, convincinglydemonstratethatSN199Ibgwasan thus, theobjectseemednearly3magfainter(in2?)than -corrected Bmagnitudeatmaximumwas11.9— normal, butsignificantlyreddened,SNlawhose intrinsically subluminous,photometricallyandspectro- SN 1957B. sence ofinterstellarNaIDlinesimplieslowextinction; maximum brightness,whichoccurredaroundUT1991 at m~14.9mag.SN199Ibgwasthereforecaughtpriorto plate obtainedonUTDecember3.777revealedtheobject nitude of14.5and14.4,respectively,aprediscovery erties ofthesemavericksareassociatedwithalow(sub- was similartothepeculiarSNela1986GinCenAand scopically discrepantobject.Insomerespects,however,it December 15inF(seeSec.3.1).Anupperlimitofm Chandrasekhar) massintheejecta. used duringtheobservationswereconsiderablylesssensi- were collectedbytwogroups,oneattheUniversityof brightness overthecourseofseveralweeks. reported inthispaperspantheintervalUT1991December tive in2?,andespeciallyU.TheWesleyanmeasurements nucleus; seeFig.1[Plate110].Cross-checkswithfainter Both groupsmadedifferentialmeasurementsbetweenSN thus, thereisreasonableoverlapamongthetwodatasets. keley spanUT1992January1throughApril20; Only theF,RandIbandsareavailable;instruments California, Berkeley,andtheotheratWesleyanUniversity. 0.9 mtelescopeatKittPeak National Observatorywith were obtainedontwonightsin 1992February,usingthe magnitudes ofthemaincomparison starforSN199Ibg the “dipperasterism”fieldinM67 (Schild1983).TheVRI transformation coefficientsderived fromobservationsof verted tothestandardJohnson-Cousins VRIsystemusing field starsrevealedthiscomparisonstartobeofconstant (1991) alsomentionedthespectroscopicpeculiaritiesand (—0.0 mag)andconcludedthatSN199Ibgmightbea 19711 inNGC5055.Wesuggestthattheobservedprop- 1991 November17.802. = 15.1magwasdeterminedfromaplateexposedonUT v 199Ibg andafieldstarlocated—75"SWofthegalaxy 16 throughUT1992February15,whilethosefromBer- F Our ownspectraandlightcurves,tobediscussedinthis According toKosaietal.(1991),onUT1991Decem- Our photometricCCDdataonSN199Ibg(Table1) The instrumentaldifferentialmagnitudeswerecon- 2. OBSERVATIONSANDREDUCTIONS 2.1 1544 1992AJ 104.1543F -1 3 3 brators andtheM67fieldwasalsoobserved.Nosignificant differences werefoundinthetransformationcoefficient slopes betweentheLandolt(1983)andSchild Landolt (1983)standardswereusedastheprimarycali- the T2KCCDandstandardKittPeakfilterset. mag. comparison starareF=14.63,Æ=13.70,and7=12.83 standards. TheadoptedmagnitudesfortheSN1991bg Vleck ObservatorywithaCCDcameraattachedtothe0.6 m Perkintelescope.AFordAerospace512Xpixelchip the telescope,buttrackingmechanismwassufficiently Vleck was2.5"FWHM.Noautoguidingpossiblewith ployed. ThescaleattheCassegrainfocuswas0.5"pixel filters (VRI)recommendedbyBessell(1990)wereem- flattened (usingtwilightskyflats)andstacked.Differential posures weretakenineachfilter,andtheimages accurate overexposurestimesof^100s.Three60sex- and thefieldofviewwas~4'x4\TypicalseeingatVan photometry ofthesupernovaerelativetoprimarycom- by 180°aboutthenucleusofgalaxy.Therotatedimages parison starwasobtainedinthefollowingmanner,with 1545 FILIPPENKOETAL.:SN1991bg were subsequentlysubtractedfromtheoriginalimagesto IRAF. Thefieldswerealigned,trimmed,andthenrotated ter subtractionofthe(verysmall)residualbackground, remove thebackgroundcontributionfromNGC4374.Af- stellar brightnessesweremeasuredthroughacircularap- (known as“PM512”)andtheJohnson-Cousinssystem erture 5"indiameter.Uncertaintieswereestimatedby brightness. Indeed,thestandarderroranalysispackagein totheoriginaldataframes,subtractinglight adding 5-10artificialstarsofthesamebrightnessas their deviationsfromtheinputmagnitudeshouldberep- same distanceasSN199IbgfromthecenterofNGC4374, ter intheresults.Sincestarswereplacedatroughly magnitudes oftheartificialstars,anddeterminingscat- of NGC4374andanyresidualbackground,measuringthe short discussionofthelightcurvesSN199IT(Balonek IRAF gavenearlythesameuncertainties.Asidefroma resentative oftheuncertaintyinmeasuringsupernova’s from theWesleyanprogramofsupernovaphotometrywith et al.1991),Table1containsthefirstdatatobepublished the Perkin0.6mreflector. Automatic ImagingTelescope(BAIT)atLeuschnerOb- servatory inLafayette,California(Richmondetal.1992), a smallerversionofthe0.8mroboticimagingreflector has aspectralresponseveryclosetothatoftheFordAero- the Wesleyantelescope,anditsThomsonTH7895CCD currently underconstruction(Richmond&Filippenko on anoffsetstartakenfromtheHubbleSpaceTelescope space CCD.Thetelescopefinds,acquires,andautoguidés FWHM. Examplesareshown inFig.1.Biasanddark- 240 to2400s,andtheseeing wastypically2.5-3.0" Guide StarCatalog.Exposures ofSN199Ibgrangedfrom National OpticalAstronomyObservatories. IRAF (ImageReductionandAnalysis Facility)isdistributedbythe 1991). TheBAITusesaBessellfiltersetsimilartothatat © American Astronomical Society • Provided by the NASA Astrophysics Data System The WesleyandataonSN199IbgwereobtainedatVan The Berkeleydatawerecollectedbythe0.5m above. Ourresults(Table1),togetherwithFig.1,repre- tracted fromeachindividualimageexactlyasdescribed which wasthendividedbyamediantwilightskyflatfield. the WesleyanandBerkeleyphotometry.Inparticular, small, systematic,yettime-dependentdifferencebetween few illustrativeimagesshownbyRichmondetal.1992). sent thefirstpublisheddatafromBAIT(exceptfora Differential magnitudes,aswelluncertainties,wereex- current contributionsweresubtractedfromeachimage, nifies anysmalldifferencesindetectorsensitivitybetween Bessell Rfilterhasaverylongtailtothered,whichmag- very differentfromthatofa“typical”star,thecolor- tudes ontoastandardscale—derivedfromobservationsof dependent correctionsusedtoplaceinstrumentalmagni- 7500 and9000Â.Sincethespectrumofasupernovais the instrumentalpassbandsatdifferentsites,onecancor- to SNe.Moreover,sinceasupernova’sspectrumchanges lack therequiredamountofdata. rect forcoloreffects(Hamuy etal.1990);however,we enough informationonthespectrumofasupernovaand with time,anysuchdiscrepancieschangeaswell.Given “typical” stars—mayyielddiscrepantvalueswhenapplied (Filippenko etal.1991)prompted ustoobtainspectraof c b 6 “Units: magnitudes. Observers: (1)Wesleyan,(2)Berkeley. MJD =JulianDateatstartofobservationminus2,440,000. UT DateMJDVa(V)R

3.3 Reddening and Extinction The red early time colors derived for SN 199 Ibg, to- gether with the extremely low luminosity (for a SN la), might be interpreted as evidence that SN 199Ibg suffers from a large amount of dust extinction. Indeed, we have already mentioned (Sec. 1) that Della Valle et al (1991) concluded that SN 199 Ibg might be a normal but highly reddened SN la, with an extinction-corrected luminosity comparable to that of typical SNe la. Roughly 3Q-80 d past maximum, on the other hand, the observed V—R color is comparable to, but perhaps even slightly bluer than, that of classical SNe la (Sec. 3.2)—consistent with Fig. 3. Color curves of SN 1991bg. B maximum is assumed to be at low reddening. JD 2 448 603.5, 2 days earlier than V maximum. Some points are the There is, in fact, no evidence for dust in the outskirts of average of 2-3 independent observations. Points without \a error bars NGC 4374, where SN 199 Ibg is located, although the data have uncertainties smaller than the size of the plotting symbol, (a) of Gallagher (1986) show that a small ( —15"X2") dust V—R color curve, compared with those of the normal SNe la 1972E, 1980N, and 1981B. (b) R—I color curve, compared with those of lane is present in the nucleus. Another convincing argu- SNe la 1972E and 1980N. ment against the presence of substantial reddening is the absence of narrow, interstellar Na ID absorption in a moderate-resolution ( — 3 A) spectrum of SN 199Ibg ob- curves to t=0 d yields V—R—0.2 mag and R—I—0.0 tained on UT 1991 January 9 (Fig. 4). We measure an mag at B maximum. These are over 0.2 mag redder than upper limit of —0.1 Â for the equivalent width of Na I D the colors of typical SNe la; V—R ~ —0.03 mag and R —I — —0.25 mag for SN 1980N [Figs. 3(a), 3(b)]. A5893 at 5913 Â, the expected position of the line at the redshift (1040 kms“1) of NGC 4374. Also, the Galactic Following an interval of steady increase of —0.035 1 contribution to Na I D is negligible, as expected given the mag d“ in the Æ—/ curve of SN 1991bg, there seems to high Galactic latitude of NGC 4374 (see also Burstein & be an abrupt halt at t—17 d, and R —I remains essentially Heiles 1984).5 For comparison, the heavily obscured SN constant at 0.59 mag for at least the next 20 (and possibly 1986G in the dust lane of Cen A has very strong Na I D up to 60) days. This value of R—I is substantially redder absorption (equivalent width —4.3 A) in its spectrum than normal SNe la at comparable phases. A change is present at about the same epoch in the V—R curve, but it is more difficult to quantify because of an offset between 5Note, however, the absorption line at an observed wavelength of 5887 À in Fig. 4, with an equivalent width of ~0.2 À. This might be Galactic the Berkeley and Wesleyan and the larger measurement NalD A5890 with a blueshift of —150 km s1. The corresponding uncertainties. After growing progressively redder by Na I D A5896 line (at an expected wavelength of 5893 A) appears to be —0.03 mag d-1 during the first 13-17 days past B maxi- missing, but it could have been obliterated by incomplete subtraction of mum, SN 1991bg reaches V—R—0.6 mag and subse- the strong NalD emission line (from city ). Alternatively, the observed absorption line may be an artifact, though it seems visible in the quently becomes much bluer. During the approximate in- raw data. In any case, the reddening corresponding to such weak Na I D terval 30-80 d past maximum, SN 199 Ibg has nearly the absorption is low, E(B— F)æ0.05 mag, as derived from the empirical same V—R color as typical SNe la, and its rate of change relation in Barbon et al. (1990).

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1992AJ 104.1543F 1549 and themostrecentlyderivedreddeningisE(i?—F)=1.1 colors asnormalSNela,thecalculatedreddeningisE(i? mum. spectrum ofSN199Ibg.Hence,weconcludethat than thatofSN1986G.ProminentNaIDabsorption cember 14(¿=1d)isshowninFig.5.Toextendthe wavelength rangetothenear-UVregion,wehaveadded in spectralshapeisgoodtheregionofoverlap.Also the UTDecember13spectrumbelow4090Â;agreement been correctedforreddening. brightness. Noneofthespectrashowninthispaperhas illustrated, forcomparison,arespectraofthenormalSNla 1992). IfSN1991bghasintrinsicallythesamebroadband (Rich 1987;D’Odoricoetal1989;Cristiani1992), trum ofSN199Ibgismuchredder thannormal,andcom- However, thespectrumofSN 1989Mdemonstratesthat to anunambiguousTypelaclassificationforSN199Ibg. ous presenceofSiilA6355absorptionnear6150A,leads 199 Ibgwasintrinsicallyredandsubluminousatmaxi- SN 199Ibgisdistinctlyatypical. Forexample,thespec- (EW^3.2 Ä)isthereforeexpected,butnotseen,inthe ±0.1 mag(Cristianietal1992;Ruiz-Lapuente&Lucy — V)^0.8mag(DellaValleetal1991),notmuchlower been deducedfromtheobserved colorsatmaximum.This parable totheheavilyobscured SN1986G,ashasalready appears tobecausedbystrong absorptionlinesintheblue 1989M andthepeculiarSNla1986Gnearmaximum obtained onUT1992January9,comparedwiththecorresponding night skyspectrumscaleddownbyafactorof10.Theredshift tion producedbyinterstellargasinNGC4374,atanexpected marked. Anupperlimitof—0.1ÀisderivedforNaIDabsorp- NGC 4374hasnotbeenremoved.Possiblenightskyresidualsare Fig. 4.Moderate-resolution(~3À)spectrumofSN199Ibg, heliocentric wavelengthof5887A,withanequivalentwidth wavelength of5913A.Thesingleabsorptionlineatanobserved © American Astronomical Society • Provided by the NASA Astrophysics Data System The spectrumofSN199IbgobtainedonUT1991De- —0.2 À,mightbeduetoGalacticNaID. The absenceofhydrogenlines,togetherwiththeobvi- 4.1 TheSpectrumNearMaximumBrightness FILIPPENKO ETAL.:SN199Ibg 4. SPECTROSCOPICPROPERTIES Observed Wavelength(Â) 1 1 -1 by dustislow(Sec.3.3).TheSiilA6355lineinSN199Ibg is weakerthanusual(butsimilartothatofSN1990N; blend near7600Áandthebroadtrougharound4200Á. features areabnormallystrong;note,forexample,thedeep region, andbyanintrinsicallyredcontinuum;extinction This isinstarkcontrasttoanotherspectroscopicallypecu- Leibundgut etal1991a),thoughseveralotherabsorption featureless (Filippenkoetal.1992).The4200Âtroughis liar SNla,199IT,whosecontinuumwasunusually lines near5800and6150Â.NotethatSN19711inNGC near 4000Â.AnothersimilaritybetweenSNe199Ibgand defined inSN199Ibgduetotheprominentemissionline also presentinSN1986G,butitsbluesideismorewell shows thesepeculiarities(Barbonetal.1973;Phillips photosphere. TheobservedminimumintheSiilA6355 5055, whoselightcurvesresemblethoseofSN1986G,also near maximumtypicallyshowexpansionvelocitiesof km s“isfoundwiththesametechnique,andotherSNela 9900 kms”.InSN1989M,bycontrast,avalueof12300 line isat6148A,correspondingtoanexpansionvelocityof SN 1989M,suggestingamoderatelyslowvelocityforthe sion velocities. less “washedout”appearanceexpectedwithlowerexpan- Branch (1990),wehaveproduced syntheticspectrafor comparison withtheSN199Ibg spectrumobtainedat¿=1 1986G istherelativestrengthoftwoSiIIabsorption d (extendedbelow4090Áwith the¿=0spectrum,asin Fig. 5).Thesyntheticspectra areforavelocityatthe 1987). 199Ibg aresomewhatnarrowerandlessblueshiftedthanin 199 Ibg,relativetonormalSNela,isconsistentwiththe 1990). TheobservedexcessofweakabsorptionlinesinSN 11 000-13000kms(Branchetal.1988;Barbon Fig. 5.SpectrumofSN199Ibgobtainedonedayafterbluemaxi- been removed,butnocorrectionsforreddeninghavemade. Unless otherwiseindicated,allspectrainthispaperareshownthe mum, comparedwiththespectrumofnormalSNla1989M(UT The effectiveunitsontheordinatearemagnitudes(—2.5log/ rest systemoftheparentgalaxy,andtelluricabsorptionlineshave 1989 July9)nearmaximum.AlsoshownisthepeculiarSNla 1986G (r=0d;UT1986May11),fromCristianietal(1992). 199Ibg, unlikethecaseintypicalSNela.SN1986Gresembles 199 IbgmorecloselythananyotherknownSNla. + constant).Notethestrongabsorptionlinenear4100AinSN v As inSN1986G,itisclearthatthestronglines Using aproceduresimilartothat describedbyJeffery& Rest Wavelength(Â) 1549 00 LO 1550 FILIPPENKO ETAL.: SN 1991bg 1550 o tion near 8300 A). These differences may prove to be due to inhomogeneities in the SN 199 Ibg atmosphere. There is also a mismatch between the relative strengths of the two Si II absorptions near 5800 and 6150 Â. The Na I D dou- blet (which was not included in the synthetic spectrum) might contribute to the former feature, but its observed wavelength is too red for this to be likely, and the mis- match may instead be a nonLTE effect. Indeed, it is likely that the small notch at —5700 A in the extreme blue wing of the 5800 Â line is actually Na I D. The lack of absorp- tion near 4050 Á in the synthetic spectrum suggests the need for lines of another ion. The degree to which the peculiarities of the SN 199 Ibg spectrum, compared to the maximum-light spectra of or- dinary SNe, are caused by abundance anomalies rather Rest Wavelength (Â) than by excitation differences is not yet clear. An excitation temperature of 10 000 K was used for the synthetic spec- Fig. 6. Spectrum of SN 199 Ibg obtained one day after blue maxi- trum in order to get some of the higher excitation O I, mum, compared with two synthetic spectra described in Sec. 4.1 of Mg II, and Si II lines to produce significant spectral fea- the text. Prominent absorption lines are marked. The synthetic spec- tures. In LTE at 10 000 K, the strengths of the Ti II lines trum containing Ti provides a much better match to the observed in the synthetic spectrum would imply a high abundance of spectrum than does the Ti-deficient synthetic spectrum. Note the Tin absorption lines near 3600, 4150, 4650, and 5000 Á; these titanium. But the color temperature of SN 199 Ibg ( — 7000 features are normally much weaker in SNe la. The obvious mis- K) is considerably lower than that of ordinary SNe la. At match between the synthetic (with Ti) and observed spectra below low temperatures Ti II lines become quite strong in LTE ~ 3600 Á is due to incompleteness of the adopted UV line list in the synthetic spectrum. (see, e.g., Fig. 1 of Branch 1987), so temperature gradients in the SN 199 Ibg atmosphere might conspire to produce both strong lines of Ti II and the higher excitation lines of photosphere of 10 000 km s-1, a continuum color temper- the other ions. ature of 7000 K, and an excitation temperature of 10 000 K. Line optical depths very radially as exp[—p/(1200 4.2 Spectral Evolution km s-1)]. Lines of O I, Mg II, Si il, S II, and Ca II are in- cluded, as generally observed in the maximum-light spec- A montage of spectra of SN 199 Ibg is shown in Fig. 7, tra of SNe la (Branch et al 1982), with fitting parameters with each spectrum labeled by its time relative to blue to control the line strengths of each ion. We also computed maximum. The quality of the data can be judged by com- a synthetic spectrum in which lines of Ti II were included. paring the data obtained on r=0 d and i = 1 d; nearly every The resulting fit, shown in Fig. 6, is satisfactory in the feature is visible in both spectra with the same relative case of the spectrum with Ti II except at wavelengths below strength. 3600 A, where the line list is incomplete. Prominent ab- The visual (4000-6500 Ä) spectrum at r=19 d and sorption features are identified beneath the synthetic spec- t=21 d is considerably redder than at maximum bright- trum. The broad absorption trough near 4200-4400 A, one ness, supporting our conclusions based on the V—R color of the most conspicuous peculiarities of the SN 199 Ibg curve (Sec. 3.2), but it subsequently becomes bluer than at spectrum, can be matched reasonably well by a blend of maximum brightness. At t=9\ d, in fact, the visual spec- Ti II lines, with a contribution from Mg II A4481; compare trum is very blue, while the near-IR spectrum is still quite with the synthetic spectrum not containing Ti II. In the red. synthetic spectrum Ti II is also primarily responsible for The peculiar, relatively narrow absorption line near the features in the range 4500-5000 A. Although we can- 5700 Á in the t= 19 d spectrum is probably Na I D. Its not guarantee uniqueness for this interpretation, many observed wavelength supports our earlier conclusion that other possibilities were explored and rejected. It does ap- the Si II line near 5800 A is largely uncontaminated by pear that the SN 199 Ibg spectrum, like the maximum-light Na I at t=\ d; the Na I line contributes only the small spectra of ordinary SNe la, is dominated by lines of notch in the blue wing of the feature. Note that the intermediate-mass elements, provided that we extend the strengths of the Na I and Ca II H + K absorption lines in- definition of “intermediate mass” (generally O through crease dramatically as the object ages. What appears to be Ca) from Ca (Z=20) to Ti (Z=22). unusually narrow Na I D emission is conspicuous in the Inspection of Fig. 6 shows that the synthetic spectrum ¿=48 d spectrum; on the other hand, the obvious displace- with Ti II differs from the SN 199 Ibg spectrum in a num- ment of the corresponding narrow blueshifted component ber of details. The synthetic spectrum is generally too makes the overall profile differ markedly from a classical P smooth; it has a deficit of weak features, and several closely Cygni profile. By ¿=91 d the Na ID “emission line” is spaced features are too blended (e.g., the Oi and Mgil more prominent and perhaps substantially broader than it absorption blend near 7500 A, and the Ca II triplet absorp- was previously. The associated absorption component has

© American Astronomical Society • Provided by the NASA Astrophysics Data System 00 LO 1551 FILIPPENKO ETAL.: SN 1991bg 1551 o

CM (T\(S\

Rest Wavelength (Â)

Fig. 8. Comparison of the spectrum of SN 199 Ibg at t= 19 d with spectra of other SNe I roughly 3 weeks past maximum brightness. The spectrum of SN 1990N (i=21 d; UT 1990 July 31) was obtained by A. V. F. and J. C. Shields with the 3 m reflector at Lick Observatory, and is part of the extensive data set to be pub- lished by Leibundgut et al (1992). That of SN Ic 1990aa (UT 1990 September 27) is from Filippenko ( 1992); the exact epoch is uncertain, but it is estimated to be 2-3 weeks past maximum. The narrow emission lines are produced by the superposed H II region. SN 1986G is an average of the i=21 d (UT 1986 June 1) and i=23 d (UT 1986 June 3) spectra from Cristiani et al (1992). Rest Wavelength (Â) mal SNe la; see Figs. 8 and 9. Identifying the emission lines Fig. 7. Montage of spectra of SN 1991bg, showing the spectral on the basis of wavelength coincidence alone is risky, how- evolution over the course of three months. Days are indicated ever, for several reasons. First, the underlying spectrum relative to blue maximum, which is assumed to have occurred on obviously is not merely a smooth continuum, but a com- UT 1991 December 13 (JD 2 448 603.5; two days earlier than the plex blend of P Cygni scattering lines that varies rapidly observed date of V maximum). with wavelength, so the wavelengths of the emission peaks may be affected by the slopes in the underlying spectrum. considerable structure, and may be contaminated by other Moreover, the widths of the emission lines are consistent lines. with the widths and blueshifts associated with the P Cygni As SN 1991bg aged, it appears to have entered its neb- lines, which means that the emission lines are formed in ular phase very early, consistent with a low mass in ejecta the same region as the P Cygni lines. Thus the emission- expanding at a nearly normal velocity. For example, at line photons are subject to occultation by the photosphere, t= 19 d we already begin to see a peak in the spectrum near which will produce an emission-line blueshift, and also to 5600 À which is tentatively identified with [O i] A5577; it is scattering by the P Cygni lines before escaping from the quite strong by ¿=48 days. The emergence of this emission atmosphere, which can redshift the emission-line photons. feature may be partly responsible for the narrow width of Although the presence of Ha is a possibility, the ramifica- the NalD absorption line in SN 199 Ibg. The [O l] tions for the physical models are so serious that it is best AA6300, 6364 doublet is weak or absent at the same epoch, not to speculate until more convincing evidence is found. implying that 109 cm-3 and 8000 K if the 5600 Á Figure 8 shows a comparison between SN 199 Ibg, SN feature is [O l] A5577. [A five-level atom and atomic data la 1990N, SN Ic 1990aa, and SN 1986G, in each case from Osterbrock (1989) were used to calculate a grid of about three weeks past maximum brightness. Despite being line-intensity ratios.] Other emission lines also seem to be similar in some wavelength regions (e.g., prominent emis- visible in the t= 19 d and later spectra. At red wavelengths, sion near 6500 A), there are noticeable differences between net emission may be present in the Ca II triplet, O IA7773, SN 199 Ibg and SN la 1990N which actually make SN and [Call] AA7291, 7324 at ¿=19 d and ¿=27 d. In the 199 Ibg vaguely resemble SN Ic 1990aa. The most obvious blue region, there may be net emission in the Ca II H-f-K ones are as follows. ( 1 ) A strong absorption line near 7500 feature and in Mg i] A4571. In a future paper we will Â, primarily due to O I kill A (with some contribution present synthetic nebular spectra of SN 199Ibg. from Mg H in the red wing), is present in SNe 199Ibg and Aside from the emission line previously ascribed to 1990aa but not in SN 1990N. (2) SNe l991bg and 1990aa Na I D, perhaps the most intriguing emission features in both have deep troughs around 4200 Â, whereas this fea- the ¿= 19-48 d spectra are the strong one near 6500 Â and ture is partially filled by an emission line in SN 1990N. (3) the weaker one near 4850 A, which could be Ha and H0, The continua of SNe 199 Ibg and 1990aa are very red and respectively. These are also present in the spectra of nor- similar in shape, whereas SN 1990N is quite blue. In ad-

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1552 FILIPPENKO ET AL. : SN 1991bg 1552

Rest Wavelength (Â) Rest Wavelength (Â)

Fig. 9. Comparison of the spectrum of SN 1991bg at i=48 d with spectra of other SNe I roughly 7 weeks past maximum brightness. Fig. 10. Comparison of the spectrum of SN 199Ibg at t=91 d The spectrum of SN 1987M (UT 1987 November 22) is taken with spectra of other SNe I roughly 13 weeks past maximum from Filippenko et al ( 1990); it corresponds to i~62 d past max- brightness. The spectrum of SN 1987M (UT 1987 December 26) imum. That of SN 1986G is a composite made from the i=54 d is taken from Filippenko et al (1990); it corresponds to t~96 d (UT 1986 July 4) spectrum of Cristiani et al (1992) and the past maximum. That of SN 1986G (r=90 d; UT 1986 August 9) i=56 d (UT 1986 July 6) spectrum of Phillips et al (1987). The is from Cristiani et al ( 1992); we have interpolated over telluric spectra of SN 1989B (UT 1989 March 29) and SN 1990N (UT bands near 6860 and 7600 Â. The spectrum of SN 1989B (¿=80 d; 1990 August 29), both corresponding to i=50 d, were obtained by UT 1989 April 28), obtained by A. V. F. and J. C. Shields with the A. V. F. and J. C. Shields with the 3 m reflector at Lick Obser- 3 m reflector at Lick Observatory, is part of an extensive dataset to vatory; they are part of extensive datasets to be published by Wells be published by Wells et al (1992). et al (1992; SN 1989B) and by Leibundgut et al (1992; SN 1990N). mal SN la 1989B (Barbon et al 1990; Wells et al 1992). Despite some of the early-time spectroscopic peculiarities dition, the spectrum continues to rise at the near-IR end of of SN 1986G, by this stage its spectrum is quite similar to SNe 199 Ibg and 1990aa, but not in SN 1990N. those of SNe 1990N and 1989B. SN 1991bg broadly re- There are also more important similarities between the sembles these SNe la, but substantial differences are spectra of SNe 199 Ibg and 1986G. For example, the blue present. The Na I D and Ca II H+K absorption lines in shoulder of the emission line near 5500 Á differs signifi- SN 199Ibg are very conspicuous, and the trough in the cantly from the corresponding feature in SN 1990N, yet it range 4100-4500 A is considerably deeper than in typical closely resembles that of SN 1986G. Moreover, the shape SNe la. The narrow emission line at ~5900 Á (Na I D?) of the 6500 Â emission is almost identical in SNe 199 Ibg is also quite unusual. Unfortunately, the data for SN and 1986G. On the other hand, the absorption line near 199 Ibg end at 7000 A, beyond which we might expect to 5700 Á (assumed to be Na I D) is deeper and narrower in see additional discrepancies between SN 199 Ibg and other SN 1991bg than in SN 1986G. Unfortunately, the spectra SNe la (cf. Fig. 8). Comparison with the spectrum of SN of SN 1986G published by Phillips et al. ( 1987) and Cris- 1987M shows that the resemblance between SN 199 Ibg tiani et al ( 1992) do not show the near-IR region at and SNe Ic is not as strong as at weeks (Fig. 8), weeks, so we cannot check for O I A7774 absorption. The though both objects exhibit deep Na I D and Ca il H + K presence of this line is SN 1986G would substantially im- absorption. prove the overall spectral similarity with SN 199 Ibg and Figure 10 shows spectra of SNe I obtained —13 weeks help distinguish SN 1986G from normal SNe la. SN past maximum brightness. There are several differences be- 1986G appears considerably redder than SN 199Ibg, but tween SN 1991bg and the normal SN la 1989B, including this is partly due to reddening by the dust lane of Cen A. the generally bluer colors of SN 199 Ibg in the visual region Spectra obtained even longer past maximum brightness (4000-6500 Á). As in Fig. 9, at this phase the spectrum of (t~l weeks) are illustrated in Fig. 9. Except for having a SN 1986G differs little from those of typical SNe la, espe- somewhat peculiar premaximum spectrum, SN 1990N was cially after correcting SN 1986G for reddening. Some de- a typical SN la (Leibundgut et al. 1991a); its spectrum at tails in the spectrum of SN 1986G more closely resemble this phase is nearly indistinguishable from that of the nor- those of SN 199 Ibg than of normal SNe la (e.g., the shape

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1992AJ 104.1543F 4500 Á). Á), butothersdonot(e.g.,thesectionbetween4000and also presenttosomeextentinSN1986G,isnotasstrong and isolatedinnormalSNela.InSN199Ibgthislineis of thebroademissionfeaturebetween5100Áand5600 Preliminary inspectionofuncalibrateddataobtainedat almost undoubtedlyproducedby[Call]AA7291,7324. trum. ThisindicatesthatthedepletionofgaseousCaonto reveals thatthe[Call]linegreatlydominatesspec- Lick Observatory—5monthspastmaximumbrightness grains islow(Kingdonetal.1992).Itthereforereason- /IA6300, 6364doubletseemstobeabsent.Thus,aswasthe form intheejectaofSN199Ibgduringfirsthalfyear able toconcludethatsignificantamountsofdustdidnot is substantiallyweakerinSN199Ibg,andthe[Ol] after theexplosion. 1553 FILIPPENKOETAL.:SN199Ibg case atearlierphases(Figs.8and9),SN199Ibgexhibits SN le1987M(Fig.10),notethattheCaIInear-IRtriplet spectrum seemstobesomewhatofacrossbetweenthose only afewofthemainfeaturesSNeIc;infact, normal SNelaandIc.Nevertheless,thespectroscopic similarities (especiallyinFig.8)areintriguing,andthey the behaviorofSNeIb/Icin/,H,andKbands(Elias light curveofSN1991bg(Sec.5.2);thisisreminiscent mentioned theabsenceofasecondarymaximuminI should betakenseriously.Furthermore,wehavealready et al.1985).ItcouldturnoutthatthepeculiarSNla itor, willprovideimportantcluesforourunderstandingof SNe Ib/Ic,whoseprogenitorsareusually(butnotalways) spectra ofSN199Ibgtopursuethisinterestingpossibility. thought tobemassivestars(e.g.,Wheeler&Harkness acteristics (especiallyatearlytimes)andinthelightcurve shapes ofSN1991bgand1986G(and,byextension, have arguedthatSN199Ibgwas1.6magsubluminousin SN 19711;seePhillipsetal.1987).Ontheotherhand,we corrected luminosityasnormalSNela.Thus,itinitially 199 Ibg,whichalmostcertainlyhadawhitedwarfprogen- appears asthoughSN199Ibgand1986Gdifferedfrom 1990). Weareattemptingtoobtainadditionallate-time each otherinatleastoneveryimportantrespect. color excessofE(i?—F)=1.1magforSN1986G.Thisis known spectroscopicpeculiarities. (2)Theempiricalrela- those ofnormalSNelashould notbetrusted,giventhe or theoverallspectralshape,ofSN1986Gareequalto methods whichassumethattheintrinsicbroadbandcolors, actually quiteuncertainforthefollowingreasons.(1)The tion betweentheequivalentwidth oftheNaIDlineand the colorexcess[E(i?—V)~EW/4], foundforasampleof SNe byBarbonetal.(1990),has considerablescatter.(3) (1992) claimthatSN1986Ghadthesameextinction- For Galacticstars,scatterinthe relationshipbetweenthe V and2.5magsubluminousin2?,yetCristianietal. The prominentemissionlineat~7250AinSN199Ibg, © American Astronomical Society • Provided by the NASA Astrophysics Data System Although the[Call]lineinSN199Ibgresemblesthat We havenotedthesimilaritiesinspectroscopicchar- Cristiani etal(1992)basetheirconclusiononaderived 5. SNe1986GAND19711:CLOSECOUSINSOFSN1991bg? 1 1 -1 56 56 56 which Phillipsetal.(1987)estimatethatSN1986Gwas example, beaslow0.65±0.05mag—theamountby might besignificantlysmallerthan1.1mag.Itmay,for NaiD equivalentwidthandE(i?—F)islarge(Hobbs reddened relativetoSN19711intheoutskirtsofNGC Note thatthisisconsistentwiththevalueofE(i?—F) the spectrumofSN1986G. 5055 (whoseGalacticreddeningisnegligible,at6^74°). the ratiooftotaltoselectiveextinction,R=A/E(B—V), 1974, 1978).Thus,thetruecolorexcessofSN1986G with canonicalvaluesofRyzz3andRzz4.Cristianietal. their assumeddistanceofCenA(4.4Mpc)andthemea- absorbing grainsinCenAmaybesmallerthanthe mag), yieldsAf^=—19.47±0.50mag.Branch&Tam- sured maximumbrightnessofSN1986G{B=12.45±0.05 Milky WayGalaxy,implyingR=3.4;this,togetherwith mann (1992),however,summarizeintriguingevidence that RmaybemuchsmallerforSNelathanthecanonical (1987) fromananalysisofthediffuseinterstellarbandsin =0.63 ±0.11magfoundbydiSeregoAlighieri&Ponz values, adoptingR=1.5astheirbestestimate.Withthis assumption andE(2?—F)=l.lmag,wederiveM%~ 1986G byHoughetal.(1987)showthatthesizeof (1992) mentionthatpolarizationmeasurementsofSN average <=-19.0magderivedforSNelabyMiller& gut &Tammann(1990;M°b=—19.8mag).[Notethatthe Branch (1990),and2.4magfainterthanthatofLeibund- galaxy distancesadoptedbyMiller&Branch(1990)cor- B ing from3Mpc(Hesseretal.1984)to6.3(Sandage A isquiteuncertain,withrelativelyrecentestimatesrang- km s“Mpc“.] respond toHq—15kmsMpc“\whilethoseofLeibund- & Tammann1987).ThevaluesofM°quotedaboveas- gut &Tammann(1990)areclosertoi/o=50 Mpc, assuggestedbyPhillipsetal.(1987). sume i/=4.4Mpc;lowerluminositiesarefoundifd=3.3 5 interesting thatRuiz-Lapuente&Lucy(1992),inarecent mag relativetonormalSNela,aswasSN199Ibg.Itis Cen Amayhavebeenintrinsicallysubluminousby1-3 0.38 ±0.03accordingtodeflagrationmodelW7of same conclusionbasedontheirderivedNimassofonly analysis ofthenebularspectrumSN1986G,cameto —17.4 magforSN1986G.Thisis1.6fainterthanthe 5 Nomoto etal(1984)andThielemannal.(1986),0.58 nondetection ofgamma-rayemissionfromCoinSN some respects.Giventhespectroscopic andphotometric Ni mass,togetherwithaderiveddistanceof3.3Mpc resemblance ofSN19711and SN1986G(Phillipsetal. member ofthispeculiarnew subcategory ofSNela.In- B deed, SN19711seemstohave beensubluminous;itsB 199 Ibg;thetwoobjectsappear tohavebeensimilarin 1986G (Matz&Share1990). (Ruiz-Lapuente &Lucy1992),isalsoconsistentwiththe 1987), wearguethatSN19711 shouldalsobeconsidereda 56 A potentiallymoreimportantsourceofuncertaintyis These argumentsleadustobelievethatSN1986Gin Finally, itisworthmentioningthatthedistanceofCen Q ofNiisproducedbyanormalSNla.Suchlow Thus, SN1986Gwasprobablyaclosecousinof 1553 1992AJ 104.1543F 56 6 6 together withthedistancemodulusofNGC5055(29.0- atmaximum(12.1;Leibundgutetal1991b), maverick SNla,butapparentlyitwasnotunique! but afewpreliminaryremarkscanalreadybemade.We bles thatseeninSNeIb/Ic.Detailedmodelingisnecessary times, (8)theearlytransitiontoanebularspectrum,and pansion velocityoftheejecta,(6)strongTiIIlinesin colors nearmaximum,(3)therapidandmonotoniede- the mostimportantcharacteristicstobeexplainedinclude give risetotheobservedpropertiesofSN199Ibg?Many this assumption,whatkindofexplosionmechanismwould SN 1991bgprobablyhadawhitedwarfprogenitor.Under and byLeibundgut&Tammann(1990).SN1991bgwasa average M°atmaximumgivenbyMiller&Branch(1990) gible, thesevaluesareseveralmagnitudesfainterthanthe 29.9 mag;Sandage&Tammann1975;deVaucouleurs for definitiveresultsthataccountallofthesefeatures, Fe II,(7)thepeculiar,narrowNaIDlineatintermediate the earlytimespectrum,alongwithpossibledeficitof changes inthecolorevolution,(5)somewhatlowex- knee intheVlightcurve,whichcoincidedwithabrupt cline ofthelightcurves,including/band,(4)early will firstfocusourdiscussionontheexplosionof pecially Till)intheearlytimespectrum,togetherwith Chandrasekhar-mass (~1.4^q)whitedwarfs. 1554 FILIPPENKOETAL.:SN1991bg luminosity, theamountofradioactiveNisynthesizedby for SN199Ibgatmaximumisindicativeofitsbolometric tions of(C-O)whitedwarfsmayproduceSNelband apparent deficitofFellandtherelativelylowexpansion Woosley (1990)hasalreadymentionedthatslowdeflagra- in somerespects,thatofSNeIb/Ic,asobserved.Indeed, would alsogiverisetoalate-timespectrumthatresembles, oxygen (C-O)whitedwarfproducedSN199Ibg.This velocity, suggestthata“lazydeflagration”ofcarbon- would becoveredbyathickouterlayerofCandO.Thus cially small.Furthermore,theburningfrontwoulddie energy, beingthedifferencebetweennuclearenergy osynthesis wouldbecorrespondinglylow,andthekinetic gration model.Thenuclearenergyreleasedduringnucle- the explosionmustbesmallinanystraightforwarddefla- Ic. However,iftheverylowopticalluminosityobserved 1979; Aaronson&Mould1983),yieldsM=—16.8to for tworeasonsthemaximum velocity oftheintermediate- and thebindingenergyofwhitedwarf,wouldbeespe- quickly, sothesynthesizedintermediate-masselements sion; see,e.g.,Leibundgut&Pinto(1992). (1) theintrinsicallydimmaximum,(2)unusuallyred rived fromthermonuclearfusion,isconsumed throughadiabaticexpan- almost entirelybyradioactivedecay. The originalexplosionenergy,de- Note thatthepeakoflightcurve in SNelaisbelievedtobepowered (9) thestrong[Caii]emissionatlatetimes,whichresem- —17.7 mag.Ifweassumethatinternalextinctionisnegli- B 5 © American Astronomical Society • Provided by the NASA Astrophysics Data System Given itslocationintheoutskirtsofanellipticalgalaxy, The strengthoflinesintermediate-masselements(es- 6. THEORETICALRUMINATIONS 56 56 56 -61 56 mass elementswouldbeverylow—whichisnottheob- becomes unboundatall)wouldhaveaverylowejection gravitational collapseratherthanthermonuclearexplo- had concludedthatsuchwhitedwarfseventuallyundergo served case.Itmaybepossibletosavethishypothesiswith that theymightexplodeinsomecases(Isernetal1991). sions (Miyaji&Nomoto1987),butrecentresultssuggest the explosionofanO-Ne-Mgwhitedwarf.Previousstudies velocity oftheburningfront. a combinationofradialmixing(neededtoexplainSN velocity. Also,sinceSN199Ibgwasquitesubluminousat ,andtheChandrasekharmassofmaterial(ifit would becreatedthroughfusionofnucleiinanO-Ne-Mg However, onlyarelativelysmallamountofnuclearenergy synthesized intermediate-masselementsto was small;thus,theejectionvelocityshouldbeeven maximum, weknowthattheamountofsynthesizedNi almost normalvelocityshouldbecometransparenttoop- rapid declineintheobservedopticallightcurves.More- low kineticenergy.GammaraysproducedbyNiand smaller. Ni. Thelattermightbeaccomplishedbymodifyingthe possible mechanismswhichmight,inprinciple,achieve nebular spectrum,asobserved.Webrieflyexplorethree the kneeinlightcurveearlyandbegintodevelopa tical radiationmorequicklythanusual—itwouldreach over, asub-Chandrasekharmassofejectaexpandingat intermediate-mass elementscouldbeobtainedevenwitha low theChandrasekharlimit,moderatevelocitiesof invoked forSN199IbgbecauseitproducesamixtureofNi front thatbeginsattheinterfacebetweenaC-Ocoreand ejection ofasub-Chandrasekharmass. white dwarfundergoingrapid(£3xlOJtqyr)ac- evolution mightcloselymatchthedata. the otherhand,predictedspectrumanditstemporal spectrum. Iftheouterlayerundergoesadeflagration,on and Heintheouterlayers,starkcontrasttoobserved intact aftertheexplosion.Adetonationprobablycannotbe Such amechanismwouldleavethecoreofwhitedwarf explain theearlytime(photospheric)spectraofSNelb. an outerenvelopeofHe,asproposedbyBranch&Nomoto Co wouldescapemoreeasily,therebyexplainingthe cretion ofaC-Omixturefromdisruptedcompanion white dwarf(e.g.,Iben&Tutukov1984).Nomoto 1990N, too;seeJefferyetal.1992)andahighratioof an inwardpropagationofacarbon-burningfrontwhich tion ofC-Onearthewhitedwarfsurface.Theypredicted hydrogen-line identificationsbriefly mentionedinSec.4.2 layers wereejected,perhapsthe resultwouldresembleSN neutron star.However,ifsome oftheoutercarbon-burned created anO-Ne-Mgwhitedwarf, destinedtocollapsea are correct. ( 1986;seealsoNomoto1982,andWoosleyetal1986)to (1985) foundthatrapidaccretioncausesprematureigni- 199 Ibg.Thismodelisnot tenable, ofcourse,ifthe Another possibilityisthatSN199Ibgmayhavebeen Alternatively, iftheejectedmassweresignificantlybe- One methodistohaveanoutward-movingburning A differentpossibilityisthattheprogenitorwasaC-O 1554 1992AJ 104.1543F being moreapplicabletodeviantssuchasSN199Ibg. This hypothesishasrecentlybeensuggestedforSNla more detailisthattheprogenitoritselfh&damassofonly Thielemann etal.(1986),andthedelayeddetonation the currentlymostfavoredmodelsofnormalSNela,such but withobviouspeculiaritiesthatareassociatedthe model DD3ofWoosley(1991);seealsothemodels as deflagrationmodelW7ofNomotoetal.(1984)and theme, ratherthanfundamentallynewexplosionmecha- the widelyacceptedframeworkofthermonuclearburning pact remnantsandthechemicalevolutionofgalaxies.SN mass oftheejectaanddetailsburningmechanism. nisms. WespeculatethatSN199Ibgisbasicallyala, of whitedwarfs;inthissensetheyarevariantsontheusual Khokhlov (1991a,b).Nevertheless,theyareallbasedon has potentiallyimportantimplicationsforthebirthofcom- Depending onwhichphysicalmodeliscorrect,SN199Ibg teristics ofseveralotherSNela(SN1986G,SN19711) especially sincewehaveshownthattheobservedcharac- be carefullymonitoredbothphotometricallyandspectro- were insomewayssimilartothoseofSN199Ibg. 1990N byShigeyamaetal.(1992),butitmightendup basis forSNelaasstandardcandles,andwhenseekingthe observed duringthepastdecade(e.g.,SNe1986G,199IT, majority ofSNelareallyarehomogeneous;peculiarexam- not befoundinflux-limitedsearchesfordistantSNe,un- nately, SNeassubluminousSN199Ibgwillgenerally as “standardcandles”fordistancemeasurements.Fortu- SNe ladoesnotsignificantlydecreasetheutilityof — 1^q,substantiallybelowtheChandrasekharlimit. This shouldbekeptinmindwhenevaluatingthephysical scopically todeterminewhethertheyappearnormal. ples seemtohavebeencommonamongthenearestSNela produce. However,itremainstobedemonstratedthata ones canberejectedbecauseofthediscrepantresultsthey of SNelainthesamplearenormal,clearlyabnormal like overluminousones.Furthermore,aslongamajority white dwarfscanendtheirexistenceinmorethanoneway. 1555 FILIPPENKOETAL.:SN199Ibg 199 Ibgisamaverickthatdeservestobeunderstood— Branch, D.,&Bettis,C.1978,AJ,83, 224 Bertola, F.1964,AJ,69,236 Benetti, S.,Cappellaro,E.,&Turatto, M.1991,IAUCircularNo.5405 Barbon, R.,Ciatti,F.,&Rosino,L.1973,MSAIt,42,65 Branch, D.1987,ApJ,320,L121 Bessell, M.S.1990,PASP,102,1181 Barbon, R.,Benetti,S.,Cappellaro,E.,Rosino,L.,&Turatto,M.1990, Balonek, T.J.,Jameel,A.,Herbst,W.,Ford,C,Chromey,F.R.,& Aaronson, M.,&Mould,J.1983,ApJ,265,1 Branch, D.,etal.1982,ApJ,252,L61 199Ibg). AllSNelausedforcosmologicalstudiesshould A&A, 237,79 Ratcliff, S.1991,BAAS,23,1405 All ofthescenariosdiscussedabovearedistinctfrom Yet anotherideathatmustbeinvestigatedinmuch We hopethattheexistenceofrare,butverypeculiar In anycase,objectssuchasSN199Ibgdemonstratethat © American Astronomical Society • Provided by theNASA Astrophysics Data System REFERENCES bundgut formachine-readabletemplatelightcurvesofSNe dwarfs. still havemuchtolearnabouttheexplosivedeathsofwhite identity oftheprogenitorsystemsSNelbandIc.We estimates madebyYoshioandReikiKushida,BrunoLei- of SN1986G,RichardB.C.Henryforhelpfuldiscussions, Observatories isappreciated.TheBerkeleygroupgrate- ful suggestions.AssistanceofthestaffatLickandPalomar and theanonymousrefereefromHarvardforhisveryuse- la, StefanoBenettiandMarkM.Phillipsfordigitalspectra botic imagingtelescopes,andJackBorde’sadviceonopti- ful toPhotometriesLtd.fordonatingpartoftheCCD has providedmuchencouragement,assistance,andinspi- the 3mShanereflectoratLickObservatory.RussGenet that ledtotheconstructionofdoublespectrographon camera usedtoobtainphotometricmeasurementsofSN cal systemshasbeeninvaluable.TheCCDandassociated ration duringtheBerkeleygroup’seffortstodevelopro- data, andtoWilliamMarinaKastforagenerousgift some ofthecomputersneededtoacquireandreduce Inc. 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Theexposuretimewas400sineachcase, obtained roboticallyonUT1992January 1withthe0.5mBerkeleyAutomaticIm- a( 1950)=122231.54,5(1950) 13'08'52.4"(Kosaietal.1991),about58"Sand Fig. 1.Broadband(VandR)CCD imagesofSN1991bgintheElgalaxyNGC4374, 1 "Eofthegalacticnucleus. S N1991bginNGC4374 V N