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Proceedingsofthe7thEuropeanVLBINetworkSymposium Bachiller,R.Colomer,F.,Desmurs,J.F.,deVicente,P.(eds.) October12th-15th2004,Toledo,Spain VLBI observations of young Type II supernovae MiguelA.Pe´rez-Torres1,A.Alberdi1,andJ.M.Marcaide2 1 InstitutodeAstrof´ısicadeAndaluc´ıa,CSIC,Apdo.Correos3004,E-18080Granada,Spain email:[email protected],[email protected] 5 2 DepartamentodeAstronom´ıayAstrof´ısica,UniversidaddeValencia,46100Burjassot,Spain 0 email:[email protected] 0 2 Abstract. We give an overview of circumstellar interaction in young Type II supernovae, asseen through the eyes of very- long-baseline interferometry (VLBI)observations. Theresolution attained by such observations (<∼ 1mas) isapowerful tool n toprobetheinteractionthattakesplaceafterasupernovagoesoff.Thedirectimagingofasupernovapermits,inprinciple,to a estimatethedecelerationofitsexpansion,andtoobtaininformationontheejectaandcircumstellardensityprofiles,aswellas J estimatesofthemagneticfieldintensityandrelativisticparticleenergydensityinthesupernova.Unfortunately,onlyahandful 3 ofradiosupernovaearecloseandbrightenoughastopermittheirstudywithVLBI.Wepresentresultsfromourhigh-resolution 1 observationsofthenearbyTypeIIradiosupernovaeSN1986JandSN2001gd. v 6 1. Introduction density in the supernova shell, as well as information on the 1 relevant absorption and cooling mechanisms. Since the radio 0 Radiosupernovaearemostlyassociatedwithcore-collapsesu- peaksofSNeareusuallyoftheorderofafewmilliJansky,only 1 0 pernovae(Type II and Ib/c). Here, we will discuss only Type bright,nearbyradiosupernovaearegoodVLBItargets. 5 II radio supernovae, although most results apply as well to Thedensityofthesupernovaejectacanbeapproximatedby 0 Type Ib/c radio supernovae. The progenitors of Type II su- a steep power-lawprofile(ρ ∝ r−n). Ifthe masslossparam- ej h/ pernovae are surrounded by a high-density wind, n ≈ 3 × eters(M˙,vw)stayapproximatelyconstantuptotheexplosion, p 107M˙−5r15v−11 cm−3, where M˙−5 is the mass loss rate of the the CSM density profile is then ρcsm ∝ r−2. The dense shell - presupernova wind in units of 10−5 M⊙yr−1, v1 is the pre- formed at shock breakout decelerates as it interacts with the ro supernova wind velocity in units of 10 kms−1, and r15 is CSM,anditcanbeshownthattheexpansionoftheshellradius st the supernova shock radius in units of 1015 cm. In the stan- follows a power-law with time: R ∝ tm;m = (n−3)/(n−2). a dard interaction model for radio supernovae(Chevalier 1982, More generally, ρ ∝ r−s, leading to m = (n−3)/(n− s). csm v: Nadyozhin 1985), a forward shock at velocities thar largely Therefore,bycarryingseveralVLBIobservationsofthesame i exceed ∼ 10000 kms−1 propagates into the circumstellar supernova,wemaybeabletoinferthevalueofitsdeceleration X medium(CSM)whentheblastwavereachesthedense,ionized, index,m,andindirectlythevaluesofnand sthatcharacterize ar slowly expanding wind. In addition, a reverse shock moves theejectaandCSMdensityprofiles. backintothestellarenvelopeatspeedsof∼ 1000 kms−1rela- tivetotheexpandingejecta.Asaresult,ahigh-energy-density shellisformed. 2. SN1986JinNGC891 ThebrightnesstemperatureinferredfromVLBIimagesof SN1986JinNGC891is oneofthe mostradioluminousSNe SNeisverylarge,indicatingthattheemissionisofnon-thermal everdiscovered.Theprecisedateofitsexplosionisnotknown, origin,namelysynchrotronradiationfromrelativisticelectrons butonthebasisoftheavailableradioandopticaldataSN1986J (NE = N0E−γ)inthehigh-energy-densityshell.Asitexpands, was estimated to have exploded around the end of 1982, or a radio supernova quickly increases its radio brightness with thebeginningof1983(e.g.,Weiler,Panagia&Sramek1990). time, due to the increasingly smaller electron column density Based uponits largeradioluminosity,Weiler et al. 1990sug- inthelineofsight.Whentheopticaldepthatcm-wavelengths gested that the progenitor star was probably a red giant with hasdroppedtoaboutunity,thesupernovareachesitsmaximum a main-sequencemassof(20– 30) M⊙ thathad lostmaterial ofemission,afterwhichtheemissionmonotonicallydecreases. rapidly(M˙ >∼2 × 10−4 M⊙yr−1)inadensestellarwind.VLBI If synchrotron self-absorption effects are negligible, then the observations made at 8.4GHz at the end of 1988 by Bartel only relevant source of absorption is free-free absorption by et al. (1991, hereafterB91) showedthatthe radiostructureof thermalelectronsintheCSM,andtheradioluminosityevolves SN1986J had the form of a shell, in agreementwith expecta- as Lν ∝ R2∆RN0Bγ2+1 ν−γ−21 e−τν,ff (τff,ν ∝ ν−2.1). Thus,a radio tions from the standard interaction model, with the minimum supernovais first seen at highfrequenciesand,as times goes, ofemissionlocatedapproximatelyatitscenter.Thoseauthors it becomes visible at increasingly lower frequencies. We see claimed the existence of several protrusions at distances of thatthefluxdensitymonitoringofradiosupernovaemayyield twicetheshellradius,andwithapparentexpansionvelocitiesas importantinformation on such fundamentalphysical parame- highas∼ 15000kms−1.Sincetheseprotrusionsweretwiceas ters as magnetic field intensity and relativistic particle energy farasthemeanradiusoftheshell,itthenfollowsthatthemain 226 MiguelA.Pe´rez-Torresetal.:VLBIobservationsofyoungTypeIIsupernovae 42 25 NGC 891 24 6 SN 1986J 23 4 22 0) 2 0 0 2 TION (J 21 RC SEC 0 A A LIN Milli C E D 20 -2 19 -4 18 -6 6 4 2 0 -2 -4 -6 MilliARC SEC 17 02 22 40 35 30 25 RIGHT ASCENSION (J2000) Fig.1. Left:5GHzVLAmapofthegalaxyNGC891anditssupernovaSN1986JmadeonFebruary21,1999.Thecontoursare (1,2,4,8,...)×106µJybeam−1.Thepeakofbrightnessofthemapcorrespondstothesupernovaandis∼8.2mJybeam−1.Right: GlobalVLBImapofSN1986JonFebruary21,1999.Thecontoursare(3,5,7,9,11,13,15,17,19)×56µJybeam−1,theoff-source rms.Thepeakofbrightnessofthemapis1.13mJybeam−1 andtherestoringbeam(bottomleftintheimage)is1.3×0.9mas2 atapositionangleof-13◦.4.Inbothpanels,northisupandeastisleft. bulk of the shell expandedat roughly7500kms−1. Such pro- sionsseeninthepreviousVLBIobservations(B91)wouldim- trusions have been successfully invoked by Chugai (1993) to plyachangeinthedensityprofileofthecircumstellarwind. explainthecoexistenceofvelocitiessmallerthan1000kms−1 implied from the observed narrow optical lines (Leibundgut et al. 1991), and the large velocitiesindicatedfrom the VLBI Atadistanceof9.6Mpc(Tully1998),1mascorresponds measurements. toalinearsizeof∼1.4×1017cm ≈0.05pc.Basedon8.4GHz VLBIobservations,B91foundanangularsizeof∼3.7masfor WeusedarchivalVLAandglobalVLBIobservationsofsu- the shell of SN1986J,5.74yr after its explosion(assumingit pernova1986Jat5GHz,takenabout16yraftertheexplosion, tookplaceon1983.0).Thecorrespondingmeanlinearvelocity to obtain the images shown in Fig. 1. The right panel corre- wouldthenbe∼14700kms−1forthefirst5.74yr.However,this spondstothe5GHzVLBIimageofSN1986J.Itshowsahighly velocityappliesonlytotheprotrusionsfoundbyB91,nottothe distorted shell of radio emission, indicativeof a strong defor- shell. Indeed, the velocities reported in B91 were calculated mationoftheshockfront.Theapparentanisotropicbrightness for the protrusions, and assuming that these originated in the distribution is very suggestive of the forward shock colliding centreatthetimeoftheexplosion.Theseauthorsalsopointed withaclumpy,orfilamentarywind.Notethatthereareseveral out thatthe protrusionsextendedfrom the centre to twice the “protrusions” outside the shell, though just above three times radiusofthe shell,i.e., theprotrusionswereoutsidetheshell. thenoiselevelandatdifferentpositionanglesfromthoseprevi- Therefore,avalueof∼1.85masfortheangularsizeoftheshell ouslyreportedbyB91.Therefore,theseprotrusionscouldnot of SN1986Jat epoch1988.74is indicated,and a mean linear be real, but must be just artifacts of the image reconstruction velocityoftheradioshellof∼7400kms−1ismoreappropriate procedure.If this is the case, the disappearanceof the protru- to characterizethefirst 5.74yrof the expansionof SN1986J, MiguelA.Pe´rez-Torresetal.:VLBIobservationsofyoungTypeIIsupernovae 227 1 2 3 0.5 1.0 1.5 2.0 4 SN 2001gd 2 36 39 SN 2001gd MilliARC SEC 0 -2 38 -4 0) 26 June 2002 00 -6 N (J2 6 4 2 MilliARC0 SEC -2 -4 O 37 TI A 200 400 600 800 1000 N LI C DE 4 SN 2001gd 36 2 NGC 5033 35 MilliARC SEC 0 -2 8 April 2003 -4 34 8 April 2003 13 13 40 35 30 25 20 15 -6 RIGHT ASCENSION (J2000) 6 4 2 0 -2 -4 MilliARC SEC Fig.2. Left: 8.4 GHz VLA map of the galaxy NGC5033 and its supernova SN2001gd, from observations on 8 April 2003. The lowest contour is at 48µJy beam−1, corresponding to three times the off-source rms. The peak of brightness of the map correspondsto thenucleusofNGC5033andis2.44mJy beam−1.Thesupernovais thebrightpoint-likesourcenorthwardsof thenucleusofNGC5033.Right:(Top)VLBImapofSN2001gdon26June2002.Contoursaredrawnat10,20,30,40,50,60, 70,80,and90%ofthepeakofbrightnessof3.64mJybeam−1.(Bottom)VLBI mapofSN2001gdon8April2003.Contours aredrawnat10,20,30,40,50,60,70,80,and90%ofthepeakofbrightnessof947µJybeam−1.Inallpanels,northisupand eastisleft. ashasbeenpreviouslynoticedbyChevalier(1998)andHouck we are sampling the progenitor wind about 11000 yr prior to etal.(1998). its explosion. Since the mass loss rate of SN1986J seems to The angularsize of the shellof SN 1986Jon21 February have been >∼ 2 × 10−4 M⊙yr−1 (Weiler, Panagia & Sramek 1999is∼4.7mas,equivalentto≈ 6.8 × 1017 cm.Combining 1990),the swept-up mass must have been Msw ≈ 2.2M⊙ for this angular size measurement with that obtained by B91 for a standard density profile of the progenitorwind (ρcs ∝ r−2). epoch 1988.74 (θ ≈ 1.85mas), we obtain a mean angular Since the expansionof the supernovahasnotdeceleratedsig- expansion velocity of the shell between 29 September 1988 nificantly between the two VLBI observations,it followsthat (1988.74)and21February1999(1999.14)of≈0.14masyr−1, the swept-upmassbythe shockfrontmustbe muchless than whichcorrespondstoalinearvelocityof∼ 6300kms−1.Ifwe the mass of the ejected hydrogen-richenvelope,Menv, as oth- assumethatSN1986Jfreelyexpandedforthefirst5.74yrofits erwiseweshouldhaveobservedamuchstrongerdeceleration. life(r ∝ tm,m = 1)andthenstartedtodecelerate,theexpan- Infact,momentumconservationimpliesthatMenv >∼ 12 M⊙, sionbetweenthetwoepochsofVLBIobservationsischaracter- significantlylargerthanMsw. Ifthehydrogen-richmassenve- izedbym=0.90±0.06.Thismilddecelerationcontrastswith lopewasashighas12 M⊙,thisisahintthattheprogenitorof thecaseofseveralothersupernovae,forwhichastrongdecel- SN1986Jwasprobablyasingle,massiveRedSuperGiant(as eration has been measured (SN1979C: Marcaide et al. 2002; previously suggested by Weiler et al. 1990), which lost mass SN1987A: Staveley-Smith et al. 1993, Gaensler et al. 1997; rapidly,but managedto keep intact most of its hydrogen-rich SN1993J:Marcaideetal.1997,Barteletal.2000). envelope by the time of explosion. This result contrasts with Forastandardpresupernovawindvelocity,v =10 kms−1, thecasesofSN1993JandSN1979C,whosehydrogen-richen- w the linear size of SN1986J at epoch t=16.14 yr implies that velopeshadmassesof0.2–0.4 M⊙(Woosleyetal.1994,Houck 228 MiguelA.Pe´rez-Torresetal.:VLBIobservationsofyoungTypeIIsupernovae & Fransson 1996)and <∼ 0.9 M⊙ (Marcaideet al. 2002),and in the particle density, the post-shock magnetic field is also whoseprogenitorstarswerepartofbinarysystems. fourfold increased. Hence, compression alone of the circum- Since the radio emission is of synchrotronorigin, we can stellar wind magnetic field cannot accountfor the large mag- estimate a minimum total energy (in magnetic fields, elec- neticfieldsexistinginSN1986J,andotherfieldamplification trons, and heavy particles) and a minimummagnetic field for mechanismsare neededto be invoked,e.g.,turbulentamplifi- SN1986J.Ifweassumeequipartition,thentheminimumtotal cation(Chevalier1982;Chevalier&Blondin1995).Thesame energyis(Pacholczyk1970)E(tot) =c (1+k)4/7φ3/7R9/7L4/7, conclusion was reached for the case of SN1993J (Fransson min 13 where L is the radio luminosity of the source, R is a charac- & Bjo¨rnsson1998,Pe´rez-Torresetal. 2001), where magnetic teristicsize,c isaslowly-dependentfunctionofthespectral fieldamplificationfactors∼100 werefoundtobenecessary. 13 index,α,φisthefractionofthesupernova’svolumeoccupied by the magnetic field and by the relativistic particles (filling 3. SN2001gdinNGC5033 factor), and k is the ratio of the (total) heavy particle energy to the electron energy. This ratio depends on the mechanism NGC 2001gd in NGC 5033 was discovered by Nakano et thatgeneratestherelativisticelectrons,rangingfromk ≈ 1up al. (2001) on 24.820 November 2001, although its explosion to k = m /m ≈ 2000, where m and m are the proton and date is uncertain. The supernova had a visual magnitude of p e p e electronmass,respectively. 14.5, and is located ∼3’ north-northwest of the nucleus of Based onsnapshotVLAobservationsofSN1986Jcarried NGC 5033.Nakanoetal. (2001) reportedthe followingposi- outon13June1999,we determinedanspectralindexof α = tionforSN2001gd:α=13h13m23s.89,δ=+36◦38’17”.7.They −0.69±0.06(S ∝ να)between1.6and8.5GHz.Thisvalue alsoreportedthattherewasnostarvisibleattheaboveposition ν is very close to α = −0.67 obtained by B91, and is a typical on earlier framestaken from 1996to April 2001.A spectrum valueforsupernovaethatareintheopticallythinradioregime. obtained by P. Berlind ten days later (Matheson et al. 2001), Withα= −0.69andS = 7.2mJyfromourobservations, showedSN2001gdtobeaTypeIIbsupernovawellpastmax- 4.9GHz we obtain a radio luminosity L ≈ 1.38× 1037(D/9.6Mpc)2 imum light. Matheson et al. (2001) pointed outthat the spec- ν ergs−1 forthefrequencyrange107–1010 Hz.Thevalueofthe trumwasalmostidenticaltooneofSN1993Jobtainedonday functionc isapproximately2.39×104.Asthecharacteristic 93afterexplosionMathesonetal.(2000).SinceSN1993Jwas 13 sizeforSN1986J,wetakehalfthelargestdiameteroftheshell, astrongradioemitter,itwasnaturaltoexpectSN2001gdtobe θ=2.35mas,whichcorrespondstoalinearsizeofR= D·θ ≈ alsostronginradio.Stockdaleetal.(2002)detectedSN2001gd 3.4 ×1017cm.Withthesevalues,theminimumtotalenergyis on 8 February 2002 at cm-wavelengths with the Very Large then Array(VLA),thusconfirmingthesuggestionthatSN 2001gd shouldbe a strongradioemitter.Theircontinuousmonitoring E(tot) ≈1.13×1048(1+k)4/7φ3/7 θ9/7 D17/7erg ofSN2001gdsinceitsfirstradiodetectionhasconfirmedthat min 0.66 2.35 9.6 SN 2001gdisverysimilarin its radiopropertiestoSN 1993J whereφ=0.66φ ,θ=2.35θ mas,andD=9.6D Mpc.The 0.66 2.35 9.6 (Stockdaleetal.2003),assuggestedbytheiropticalsimilarity. valueofthemagneticfieldthatyieldsE(min) isthenequalto tot WeobservedSN2001gdon26June2002and8April2003 B ≈10.6(1+k)2/7φ−2/7θ−6/7D−2/7mG atafrequencyof8.4GHz,usinganearth-wideglobalVLBIar- min 0.66 2.35 9.6 ray (Pe´rez-Torreset al. 2004). Unfortunately,we were unable Since1≤ k ≤2000,E(tot) canhavevaluesbetween∼ 2×1048 toresolvethefineradiostructureofSN2001gd(Fig.2),despite min the milliarcsecond resolution of our VLBI array. Therefore, and∼9×1049erg,whilethecorrespondingvaluesofthemag- we had to resort to non-imagingdata analysis to make fits to neticfieldcanliebetween∼13and∼93mG(forφ=0.66and thevisibilitydataandestimatethetruesupernovasizeateach D=9.6Mpc).ThesevaluesofthemagneticfieldforSN1986J epoch.Weassumedsphericalsymmetryforourmodelfits,and areinagreementwith,e.g.,thoseobtainedforSN1993Jatsim- usedthefollowingmodels:(i)anopticallythinsphere;(ii)an ilar radii. For example, Pe´rez-Torres, Alberdi, and Marcaide 2001showedthatB≈ 0.58(r/3× 1016cm)−1G,whichresults uniformlybright,circulardisk;and(iii)anopticallythinshell ofwidth25%theinnerradius.Formally,allthreemodelsgave inavalueof∼ 50mGforaradiusof3.4 × 1017cm.Sinceitis equally good fits to the (u,v) data, so in principle we could veryunlikelythatthemagneticfieldenergydensityinthewind not rule out any of them. Although we suggest the uniformly is larger than its kinetic energy density, i.e., B2/8π ≤ ρv2/2, w bright, optically thick disk might be ruled out as a physical wecanthenobtainanupperlimitforthemagneticfieldinthe model in 2003.27, since the supernova was optically thin at circumstellarwindofSN1986J: 8.4GHz,wehaveleftitforcomparisonpurposes.Thesystem- Bcs <∼(M˙ vw)1/2r−1 ≈ 0.78(M˙−4v10)1/2r1−71mG atic increase in angulardiameterin all three modelsindicates thatthesupernovaisexpanding. whereM˙=10−4M˙−4M⊙yr−1,vw=10v10 kms−1,r=1017r17cm, At a distance of 13 Mpc, 100µas correspond to a linear and we have assumed a standard wind density profile. For distance of 2×1016cm. The averageexpansionvelocityofthe SN1986J, M˙ = 2 × 10−4 M⊙yr−1 and r = 3.4 × 1017cm, supernovacanthenbewrittenas22600D13θt−1kms−1,where andweobtainB <∼ 0.32mG.Theseargumentssuggestthat D=13D Mpcistheapparentdistancetothesupernova,θthe csm 13 themagneticfieldintheshellofSN1986Jisintherange13–93 angularradiusin µas, and t the time since explosion,in days. mG,orabout40to300timesthemagneticfieldinthecircum- Hence,theaverageexpansionspeedofSN2001gdon26June stellar wind. Since a strong shock yields a fourfold increase 2002 (t = 296d), ranges from v = 12700±2600kms−1 exp MiguelA.Pe´rez-Torresetal.:VLBIobservationsofyoungTypeIIsupernovae 229 Table1.PropertiesofsomeTypeIIradiosupernovae SN1979C1 SN1986J2 SN2001gd3 SN1993J4 Distance(Mpc) 16.1 9.6 21.6 3.63 Timesinceexplosion5(yr) ∼20.1 ∼16 <∼ 2 ∼8.6 (L/L ) ∼1.6 ∼13 ∼2 1 SN1993J 6cm,peak Radiobrightnessstructure Shell(likely) Distortedshell ? ∼Smoothshell M˙/10−5M⊙yr−1 ∼(12−16) ∼20 ? ∼5 Decelerationparameter(m) ∼0.62 ∼0.90 ? ∼0.82 t (years) 6±2 Notyet ∼? ∼0.5 break Asymmetricexpansion? No Yes ? No(<∼ 5%) Circumstellarmedium ? Clumpy ? Approx.smooth Mswept/M⊙ ∼1.6 ∼2.2 ∼? ∼0.4 Menv/M⊙ ∼0.9 ∼12 ∼? ∼0.6 Explosionscenario Binary Single Binary? Binary Magneticfieldamplification Turbulent Turbulent Turbulent Turbulent 1FromMarcaideetal.(2002).2FromPe´rez-Torresetal.(2002).3FromPe´rez-Torresetal.(2004).4FromMarcaideetal.(1995),Marcaide etal.(1995),Marcaideetal.(1997),Pe´rez-Torresetal.(2001,2002).5Timesinceexplosionatwhichwearediscussingthepropertyofthe supernova. (opticallythinshell),upto14900±3000kms−1 (opticallythin tition field. Thus, it seems that to explain the radio emission sphere). For our second epoch (t = 583d), we obtain veloci- fromSN 2001gd,we alsoneedtoinvokeamplificationmech- ties between 7200±700kms−1 (shell) and 8600±1800kms−1 anisms other than compression of the circumstellar magnetic (sphere). For any given model, the expansion speeds seem to field,e.g.,turbulentamplification. decrease,butthisdecrementisonlymarginal,sotheexpansion iscompatiblewithanon-deceleratedejecta.Thus,futureVLBI 4. Summary observationsarenecessarytoconfirm,orreject,thevalidityof thedeceleration. WehavepresentedresultsfromVLBIobservationsoftwoType We can estimate a minimum total energyand a minimum II supernovae, SN 1986J and SN 2001gd. Those results are magneticfieldforSN2001gd,aswehavedonepreviouslyfor summarizedinTable1,wherewehavealsoincludedforcom- SN 1986J. From our VLA observations on 8 April 2003, we parisonresults obtainedfromradioobservationson two other determined an spectral index of α = −1.05±0.08 (S ∝ να) ν TypeIIsupernovae,SN1979CandSN1993J. between1.4and43GHz,andS =1.02±0.02.Withthose 8.4GHz VLBI observations of Type II supernovae suggest that if values,weobtainL = (6.0±0.1)×1036D2 ergs−1between ν 13 particlesandfieldsarenotfarfromequipartition,thenitseems 1.4 and 43GHz. As R, we take the linear size corresponding clear that amplification mechanisms other than compression tohalftheangularsizeofSN2001gdmeasuredinthe8April fromthe existing circumstellarmagneticfield mustbe acting, 2003epoch,θ=(190±40)µas,whichtranslatesintotoalinear in order to explain the observed synchrotron radio emission. sizeofR = D · θ = (3.8±0.8)×1016cm.Withthesevalues, Turbulentamplificationseemstobethemostpromisingmech- weget anism. E ≈ 1.6×1046(1+k)4/7φ3/7θ9/7D17/7erg min 0.5 190 13 We would like to end this contribution by stressing that Bmin ≈ 43(1+k)2/7φ−0.25/7θ1−960/7D−132/7mG very-long-baseline interferometry (VLBI) observations of ra- Since 1 ≤ k ≤ 2000, Emin is in the range (2.4 × 1046– dio supernovaeare a powerfultool to probe the circumstellar 1.3 × 1048)erg, and the equipartition magnetic field in the interactionthattakesplaceafterasupernovaexplodes.Were- range(52–376)mG. Thus,for a typicalenergyof 1051ergfor call the VLBI community that we have had to wait 11 years the explosion of SN 2001gd, the fraction of energy neces- to have a really ”SN1993J”-like event, and even so at a dis- sary to powerthe radio emissionof SN 2001gdis quitemod- tancefourtimesgreater.Sincethosesupernovaebelongtothe est. The upper range of the equipartition magnetic field is in raregroupofradiosupernovaethatpermittheirfollow-upwith marginal agreement with that obtained for SN 1993J at the VLBI,wecannotmissthefewopportunitieswehaveofobserv- same radius(B ≈1.8G, 1998; B ≈0.5G, 2001).We notethat ingandmonitoringthem.Unexpectedresults,likethediscov- our inferred equipartition magnetic field cannot be explained eryofapulsarnebulaortheaccretionofmaterialontoablack by compression of the pre-existing field in the circumstellar hole,mightbewaitingforustounveilthem,sometimesmany wind. Following the same reasoning made for SN 1986J, we yearsafterthesupernovaexplosion(e.g.Bietenholz,Bartel& obtain that Bcsm<∼ (M˙ vw)1/2r−1 ≈ 8(M˙−4v10)1/2r1−61mG For Rupen2004). r = 3.8 × 1016cm and a mass loss rate similar to that of SN1993J(M˙ = 5×10−5 M⊙yr−1),weobtain Bcsm<∼ 1.5mG, Acknowledgements. MAPT acknowledges support of the Spanish whichisafactorabout35to250timessmallerthantheequipar- National programme Ramoń y Cajal. The European VLBI Network 230 MiguelA.Pe´rez-Torresetal.:VLBIobservationsofyoungTypeIIsupernovae isajointfacilityofEuropean,Chinese,SouthAfricanandotherradio astronomyinstitutesfundedbytheirnationalresearchcouncils. 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