AstroTalk:BehindthenewsheadlinesofAugust–October2020RicharddeGrijs(何锐思)(MacquarieUniversity,Sydney,Australia)Four-hundred-year-oldstellarexplosionsrevealtheirsecretsAstronomershaveusedNASA’sChandraX-rayObservatory(CXO)torecordmaterialblastingawayfromthesiteofanexplodedstaratspeedsfasterthan20millionkilometresperhour—about25,000timesfasterthanthespeedofsoundinaironEarth.

Kepler’ssupernovaremnantisthedebrisfromadetonatedstarlocatedintheconstellationofOphiuchus,about16,000light-yearsawayfromEarthinourMilkyWaygalaxy.In1604earlyastronomers,includingJohannesKepler,sawthesupernovaexplosionthatdestroyedthestar.

WenowknowthatKepler’ssupernovaremnantistheaftermathofaso-calledTypeIasupernova,whereasmalldensestar,knownasawhitedwarf,gainsenoughmasstoexceedacriticallimit—the‘Chandrasekhar’masslimit,equivalentto1.44timesthemassofourSun—afterinteractingwithacompanionstarandundergoesathermonuclearexplosionthatshattersthewhitedwarfandlaunchesitsremainsoutwards.

Thelateststudy,publishedinAugust2020,trackedthespeedof15small‘knots’ofdebrisinKepler’ssupernovaremnant,allglowingbrightlyinX-rays.Thefastestknotwasmeasuredtohaveaspeedof37millionkilometresperhour,thehighestspeedeverdetectedofsupernovaremnantdebrisinX-rays.Theaveragespeedoftheknotsisabout16millionkilometresperhour,andtheblastwaveisexpandingatabout24millionkilometresperhour.Theseresultsindependentlyconfirmthe2017discoveryofknotstravellingatthesehighspeedsinKepler’ssupernovaremnant.

ResearchersestimatedthespeedsoftheknotsbyanalysingX-rayspectrafromtheCXO,whichgivetheintensityofX-raysatdifferentwavelengths.Theobservationswerealreadyobtainedin2016.BycomparingthewavelengthsoffeaturesintheX-rayspectrumwithlaboratoryvaluesandusingtheDopplereffect,theymeasuredthespeedofeachknotalongthelineofsightfromEarthtotheremnant.TheyalsousedCXOimagesobtainedearlier,in2000,2004,2006and2014,todetectchangesinpositionoftheknotsandmeasuretheirspeedsperpendiculartooursightline.Thesetwomeasurementscombinedtogiveanestimateofeachknot’struespeedinthree-dimensionalspace.

Theearlier,2017workappliedthesamegeneraltechniqueasthenew,2020study,butusedX-rayspectrafromadifferentCXOinstrument.Thenewstudyresultedinmoreprecisedeterminationsoftheknot’sspeedsalongthelineofsightand,therefore,thetotalspeedsinalldirections.

ThehighspeedsinKepleraresimilartothosescientistshaveseeninoptical(visual-light)observationsofsupernovaexplosionsinothergalaxiesonlydaysorweeksaftertheexplosion,wellbeforeasupernovaremnantformsdecadeslater.ThiscomparisonimpliesthatsomeknotsinKeplerhavehardlybeensloweddownbycollisionswithmaterialsurroundingtheremnantintheapproximately400yearssincetheexplosion.

BasedontheCXOspectra,eightofthe15knotsaredefinitelymovingawayfromEarth,butonlytwoareconfirmedtobemovingtowardsit.(Theotherfivedonotshowacleardirectionofmotionalongourlineofsight.)Thisasymmetryinthemotionoftheknotsimpliesthatthedebrismaynotbesymmetricalongourlineofsight,butmoreknotsneedtobestudiedtoconfirmthisresult.

ThefourknotswiththehighesttotalspeedsarealllocatedalongahorizontalbandofbrightX-rayemission.Thesefourknotsareallmovinginasimilardirectionandhavesimilaramountsofelementssuchassilicon,suggestingthatthematterinalloftheseknotsoriginatedfromthesamelayeroftheexplodedwhitedwarf.

Theexplanationforthehigh-speedmaterialisunclear.SomescientistshavesuggestedthatKepler’ssupernovaremnantisfromanunusuallypowerfulTypeIaexplosion,whichmightexplainthefast-movingmaterial.Itisalsopossiblethattheimmediateenvironmentaroundtheremnantisitselfclumpy,whichcouldallowsomeofthedebristotunnelthroughregionsoflowdensityandavoidbeingsloweddownverymuch.

The2017teamalsousedtheirdatatorefinepreviousestimatesofthelocationofthesupernovaexplosion.Thisallowedthemtosearchforacompaniontothewhitedwarfthatmayhavebeenleftbehindafterthesupernova,andlearnmoreaboutwhattriggeredtheexplosion.Theyfoundalackofbrightstarsnearthecentreoftheremnant.ThisimpliedthatastarliketheSundidnotdonatematerialtothewhitedwarfuntilitreachedcriticalmass.Amergerbetweentwowhitedwarfsisfavouredinstead.

Inthesesystems,whenatleastoneofthestars(withthehighestmass)reachestheendofitslifeandbecomesawhitedwarf,theothercanbegintotransfermatteruptotheChandrasekharlimit.Thisprocessleadstothecentralignitionofcarboninthewhitedwarf,producinganexplosionthatcanmultiplyitsoriginalbrightness100,000times.Kepler’ssupernovaisknowntohavearisenfromtheexplosionofawhitedwarfinabinarysystem.Therefore,adifferentteamofresearcherssearchedforthepossiblesurvivingcompanionofthewhitedwarf,whichallegedlytransferredmassuptotheleveloftheexplosion.Theimpactofthisexplosionwouldhaveincreasedthebrightnessandspeedofthemissingcompanion;itcouldevenhavemodifieditschemicalcomposition.Theteam,therefore,searchedforstarswithsomeanomalythatwouldallowthemtoidentifyoneofthemasthecompanionofthewhitedwarfthatexplodedin1604.

PilarRuizLapuente,researcheratUniversityofBarcelona,Spain,says,

“WewerelookingforapeculiarstarasapossiblecompanionoftheprogenitoroftheKeplersupernova,andwecharacterisedallstarsaroundthecentreoftheremnant,butwehavenotfoundanywiththeexpectedcharacteristics.Soeverythingpointstotheexplosionbeingcausedbythemergingmechanismofthewhitedwarfwithanotherorwiththecoreofthealreadyevolvedcompanion.”

Tocarryoutthisinvestigation,theresearchersstudiedimagestakenwiththeHubbleSpaceTelescope.

“Thegoalwastodeterminethepropermotions(theapparentmotionsintheskyasseenfromEarth)ofagroupof32starsaroundthecenterofthesupernovaremnantthatstillexiststoday,”saysLuigiBedin,researcheratPadovaObservatoryinItaly.

Theyalsouseddataobtainedwiththe8.2m(diameter)VeryLargeTelescope(VLT),attheEuropeanSouthernObservatoryinChiletocharacterisestars,anddeterminetheirdistanceandtheirradialvelocitywithrespecttotheSun.

“ThestarsoftheKeplersupernovafieldareveryweakstars,onlyaccessiblefromtheSouthernHemispherewithalargediametertelescopesuchasVLTtelescopes,”saysJohnPritchard,aresearcherattheEuropeanSouthernObservatory.

“Thereisanalternativemechanismtoproducetheexplosion.Itconsistsofthemergingoftwowhitedwarfs,orthewhitedwarfwiththecarbonandoxygencoreofthecompanionstar,inalatestageofitsevolution,inbothcasesgivingrisetoasupernova,”explainsJonayGonzálezHernándezoftheAstrophysicalInstituteoftheCanaryIslands(Spain).“IntheKeplerfield,wedonotseeanystarthatshowsanomalies.However,wefoundevidencethattheexplosionwascausedbythemergingoftwowhitedwarfsorawhitedwarfwiththecoreofthecompanionstar,possiblyexceedingtheChandrasekharlimit.”

The Kepler supernova is one of five ‘historical’ thermonuclear supernovas.Among theother four, themost famous isTychoBrahe’s supernova.When thestarthatcreatedthissupernovaremnantexplodedin1572,itwassobrightthatitwasvisibleduringtheday.Andalthoughhewasn’tthefirstoronlypersontoobservethisstellarspectacle,theDanishastronomerTychoBrahewroteabookabout his extensive observations of the event, gaining the honour of it beingnamedafterhim.

Inmoderntimes,astronomershaveobservedthedebrisfieldfromthisexplosion—whatisnowknownasTycho’ssupernovaremnant—usingdatafromtheCXO,theKarlG.JanskyVeryLargeArray(VLA),aradiotelescopebasedintheUSA,andmanyothertelescopes.Today,theyknowthattheTychoremnant

wascreatedbytheexplosionofawhitedwarfstar,thereforealsomakingitpartoftheTypeIaclassofsupernovasusedtotracktheexpansionoftheuniverse.

Sincemuchofthematerialbeingflungoutfromtheshatteredstarhasbeenheatedbyshockwaves—similartosonicboomsfromsupersonicplanes—passingthroughit,theremnantglowsstronglyinX-raylight.AstronomershavenowusedCXOobservationsfrom2000through2015tocreatethelongestmovieoftheTychoremnant’sX-rayevolutionovertime,usingfivedifferentimages.Thisshowstheexpansionfromtheexplosionisstillcontinuingabout450yearslater,asseenfromEarth’svantagepointroughly10,000light-yearsaway.

BycombiningtheX-raydatawithsome30yearsofobservationsinradiowaveswiththeVLA,astronomershavealsoproducedamovie,usingthreedifferentimages.AstronomershaveusedtheseX-rayandradiodatatolearnnewthingsaboutthissupernovaanditsremnant.

Theresearchersmeasuredthespeedoftheblastwaveatmanydifferentlocationsaroundtheremnant.Thelargesizeoftheremnantenablesthismotiontobemeasuredwithrelativelyhighprecision.Althoughtheremnantisapproximatelycircular,therearecleardifferencesinthespeedoftheblastwaveindifferentregions.Thisdifferencewasalsoseeninearlierobservations.

Thisrangeinspeedoftheblastwave’soutwardmotioniscausedbydifferencesinthedensityofgassurroundingthesupernovaremnant.Thiscausesanoffsetinpositionoftheexplosionsitefromthegeometriccentre,determinedbylocatingthecentreofthecircularremnant.Theastronomersfoundthatthesizeoftheoffsetisabout10%oftheremnant’scurrentradius.Theteamalsofoundthatthemaximumspeedoftheblastwaveisabout19millionkilometresperhour.

Offsetssuchasthisbetweentheexplosioncentreandthegeometriccentrecouldexistinothersupernovaremnants.UnderstandingthelocationoftheexplosioncentreforTypeIasupernovasisimportantbecauseitnarrowsthesearchregionforasurvivingcompanionstar.Anysurvivingcompanionstarwouldhelpidentifythetriggermechanismforthesupernova,showingthatthewhitedwarfpulledmaterialfromthecompanionstaruntilitreachedtheChandrasekharmasslimitandexploded.Thelackofacompanionstarwouldfavourtheothermaintriggermechanism,wheretwowhitedwarfsmergecausingthecriticalmasstobeexceeded,leavingnostarbehind.

Thesignificantoffsetfromthecentreoftheexplosiontotheremnant’sgeometriccentreisarelativelyrecentphenomenon.Forthefirstfewhundredyearsoftheremnant,theexplosion’sshockwassopowerfulthatthedensityofgasitwasrunningintodidnotaffectitsmotion.Thedensitydiscrepancyhasincreasedastheshockmovedoutwards,causingtheoffsetinpositionbetweentheexplosioncentreandthegeometriccentretogrowwithtime.So,iffutureX-rayastronomers,say1,000yearsfromnow,dothesameobservation,theyshouldfindamuchlargeroffset.

Figure1:CXOimageoftheTychosupernovaremnant.InthelowerleftregionofTychoisabluearcofX-rayemission.Severallinesofevidencesupporttheconclusionthatthisarcisduetoashockwavecreatedwhenawhitedwarfexplodedandblewmaterialoffthesurfaceofanearbycompanionstar.(Credit:NASA/CXC/ChineseAcademyofSciences/F.Luetal.)

Figure2:Artist’simpressionshowinganexplanationfortheoriginoftheblueX-rayarcinTycho’ssupernovaremnant.ItisbelievedthatmaterialwasstrippedoffthecompanionstarbytheexplosionofthewhitedwarfintheTypeIasupernovaexplosion,formingtheshockwaveseeninthearc.Thearchasblockeddebrisfromtheexplosion,creatinga‘shadow’behindthearc.Theforceoftheexplosionimpartedakicktothecompanionstar,andthiscombinedwiththeorbitalvelocityofthecompanionbeforetheexplosiontogivethe‘observed’motionofthecompanion.Previously,studieswithopticaltelescopeshaverevealedastarwithintheremnantthatismovingmuchmorequicklythanitsneighbours,showingthatitcouldbethecompaniontothesupernova.Thesizeofthecompanion’sorbitisnotshowntoscalehere:theseparationbetweenitandthewhitedwarfbeforetheexplosionisestimatedtohaveonlybeenaboutamillionthofalight-year,whilethefullscaleoftheillustrationisover10light-years.(Credit:NASA/CXC/M.Weiss)

Figure3:ThisimageshowsirondebrisinTycho’ssupernovaremnant.Thesiteofthesupernovaexplosionisshown,asinferredfromthemotionofthepossiblecompaniontotheexplodedwhitedwarf.Thepositionofmaterialstrippedoffthecompanionstarbytheexplosion,andforminganX-rayarc,isshownbythewhitedottedline.ThisstructureismosteasilyseeninanimageshowingX-raysfromthearc’sshockwave.Finally,thearchasblockeddebrisfromtheexplosioncreatinga‘shadow’inthedebrisbetweenthereddottedlines,extendingfromthearctotheedgeoftheremnant.(Credit:Credit:NASA/CXC/ChineseAcademyofSciences/F.Luetal.)

Figure4:Kepler’ssupernovaremnant.(Credit:NASA/CXC/UnivofTexasatArlington/M.Millardetal.)

Figure5:Kepler’ssupernovaremnant.Thered,greenandbluecoloursshowlow,intermediateandhigh-energyX-raysobservedwiththeCXO;thestarfieldisfromtheDigitizedSkySurvey.(Credit:X-rays:NASA/CXC/NCSU/M.Burkeyetal.;infrared:NASA/JPL-Caltech)

Figure6:JohannesKepler’soriginaldrawingfromDeStellaNova(1606)depictingthelocationofthesupernova,markedwithanN.

Figure7:TypeIasupernovae.(Credit:NASA/CXC/M.Weiss)

Figure8:Artist’sconceptionofawhitedwarfslowlyaccretingmatterfromacompanionstar.(Credit:DavidA.Hardy&STFC)