The Early Interaction of the Planetary Nebula NGC 40 with the Interstellar Medium

We report the detection of an Halpha emission-line structure in the upstream side of the planetary nebula NGC 40, which is predicted by numerical simulations, and which is attributed to Rayleigh-Taylor instability. Such a Rayleigh-Taylor instability is exp

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aTHEEARLYINTERACTIONOFTHEPLANETARYNEBULANGC40WITHTHEINTERSTELLARMEDIUMJodieMartin,KiriakiXilouris,andNoamSoker1UniversityofVirginia,DepartmentofAstronomy,P.O.Box3818,Charlottesville,VA22903-0818;jrm7w@virginia.edu;kx8u@virginia.edu;soker@physics.technion.ac.ilABSTRACTWereportthedetectionofanHαemission-linestructureintheupstreamsideoftheplanetarynebulaNGC40,whichispredictedbynumericalsimulations,andwhichisattributedtoRayleigh-Taylorinstability.SuchaRayleigh-Taylorinstabilityisexpectedtooccuratearlystagesoftheinteractionprocessbetweentheinterstellarmedium(ISM)andafastmovingplanetarynebula,asisthecaseforNGC40.WeresolvedtheRayleigh-Taylorinstability‘tongues’,aswellasthe atnessofthenebulaaroundthe‘tongues’,whichresultsfromthedecelerationbytheISM.Subjectheadings:planetarynebulae:general ISM:structure planetarynebulae:individual(NGC40) instabilities1.INTRODUCTIONPlanetarynebulae(PNe)areexpandingionizednebulaeexpelledfromasymptoticgiant

branch(AGB)stars.TheremnantoftheAGBstar,i.e.,itscore,ionizesthenebulaasitevolvestobecomeawhitedwarf.Asthegasexpands,itsdensitydropsanditsexpansionstartstobein uencedbytheISM.IfthereisarelativemotionbetweenthePNandtheISM,theISMsignatureonthePNstructurewillemerge rstonthenebularsidefacingtheISM,i.e.,theupstreamdirection,wheretheISMrampressureonthenebulaisthelargest.Atlatetimes,whenthenebulaisrelativelylargeandoflowdensity,thistypeofinteractionformsabow-shapednebula,e.g.,Sh2-216(Tweedy,Martos,&Noriega-Crespo1995),KeWe5(Kerberetal.1998),KFR1(Rauchetal.2000),andSB51(PNG357.4-07.2;Beaulieu,Dopita,&Freeman1999),sometimeswithalongextensionofthebow-shapedregiononthe

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