Two- and Three-Dimensional Simulations of the Thermonuclear Runaway in an Accreted Atmosphere of a C+O White Dwarf
ABSTRACT . We present the results of two- and three-dimensional calculations of turbulent nuclear burning of hydrogen-rich material accreted onto a white dwarf of 1.0 M fi . The main aim of the present paper is to investigate the question as to whether and how the general properties of the burning are affected by dimensionality and numerical resolution effects. In particular, we want to see whether or not convective overshooting into the surface layers of the C+O white dwarf can lead to self-enrichment of the initially solar composition of the hydrogen-rich envelope with carbon and oxygen from the underlying white dwarf core. 1 Introduction Today, we know that novae are white dwarfs in close binary systems with a main sequence star as their companion. The white dwarf collects hydrogenrich material from the companion which first settles into an accretion disk and is then accreted onto the white dwarf's surface. Due to compressional heating this envelope can reach temperatures sufficiently high...
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Article: On the C/O Enrichment of Nova Ejecta[Show abstract] [Hide abstract]
ABSTRACT: Using the results of recent work in shear instabilities in stratified fluids, we show that the resonant interaction between large-scale flows in the accreted H/He envelope of white dwarf stars and interfacial gravity waves can mix the star's envelope with the white dwarf's surface material, leading to the enhancement of the envelope's C/O abundance to levels required by extant models for nova outbursts.The Astrophysical Journal 12/2008; 562(2):L177. · 6.73 Impact Factor
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ABSTRACT: Observational and theoretical studies of the outbursts of classical novae have provided critical insights into a broad range of astrophysical phenomena. Thermonuclear runaways (TNRs) in accreted hydrogen-rich envelopes on the white dwarf (WD) components of close binary systems constitute not only the outburst mechanism for a classical nova explosion, but also for recurrent novae and a fraction of the symbiotic novae explosions. Studies of the general characteristics of these explosions, both in our own galaxy and in neighboring galaxies of varying metallicity, can teach us about binary stellar evolution, while studies of the evolution of nova binary systems can constrain models for the (as yet unidentified) progenitors of Type Ia supernovae. Further, the empirical relation between the peak luminosity of a nova and the rate of decline, which presents a challenge to theoretical models, allows novae to be utilized as standard candles for distance determinations out to the Virgo Cluster. Extensive studies of novae with IUE and the resulting abundance determinations have revealed the existence of oxygen–neon white dwarfs in some systems. The high levels of enrichment of novae ejecta in elements ranging from carbon to sulfur confirm that there is significant dredge-up of matter from the core of the underlying white dwarf and enable novae to contribute to the chemical enrichment of the interstellar medium. Observations of the epoch of dust formation in the expanding shells of novae allow important constraints to be placed on the dust formation process and confirm that graphite, SiC, and SiO2 grains are formed by the outburst. It is possible that grains from novae were injected into the pre-solar nebula and can be identified with some of the pre-solar grains or “stardust” found in meteorites. Finally, γ-ray observations during the first several years of their outburst, using the next generation of satellite observatories, could confirm the presence of decays from 7Be and 22Na.New Astronomy Reviews 04/2000; · 1.82 Impact Factor
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ABSTRACT: This paper summarizes our present understanding of the nucleosynthesis and energy generation in the carbon-nitrogen-oxygen (CNO) cycles and the influence and consequences of possible break-out reactions from the CNO range. It gives an overview of the present status of the experimental and theoretical reaction rates involved, as well as the stellar conditions at which the CNO cycle occurs. Besides the role of the CNO cycles and break-out reactions in stellar hydrogen burning, the paper discusses possible CNO depletion processes by neutron, alpha and heavy-ion induced reactions in post-main-sequence star environments.Journal of Physics G Nuclear and Particle Physics 12/1998; 25(6):R133. · 5.33 Impact Factor
theThermon uclearRunaway in anAccreted
Abstract?We presentthe resultsoftwo? andthree?dimensionalcalculationsof
turbulentn uclearburningofhydrogen?richmaterial accretedontoawhitedwarf
? Themain aimof thepresentpap eristoinvestigatethequestionasto
alit yandnumericalresolution e?ects?Inparticular?wewant toseewhetherornot
convectiveovershooting in tothe surfacelayers of theC?Owhite dwarfcanlead
withcarb onandoxygenfromthe underlyingwhitedwarfcore?
To day?weknow that novaearewhitedwarfsinclose binarysystemswith a
rich materialfrom thecompanion which ?rstsettles into an accretion diskand
isthen accreted on to thewhite dwarf ?ssurface? Duetocompressional heating
this env elopecan reach temperaturessu?cien tly high to burnhydrogen into
helium? ?rstbythe proton?protonc hainandlaterby theCNO?cycles?where
C?N andOnucleiactascatalysts forthe burningofh ydrogen?Numeri?
calsimulations have shownthatthese reactionscan fuseallH intoHeand
elope? providedthe matteris electrondegenerateand theCNO?
abundances aresu?ciently high?severaltimes theirsolarv alues?Starr?eld
etal??????Starr?eld etal???????Iftheseconditionsare met?the outburst
resemblesthe observ edproperties ofclassicalnov aev erywell?
Itis animp ortantfeature thatthe presence of C? N?andOnucleienhances
theenergyproduction duringtheviolentstagesof thenovaoutburst dramat?
ically?Sphericallysymmetrichydro?dynamicalsim ulationsofthe accretion
processandthe follo wing outburst?Starr?eld etal??????Fujimoto????? have
shownthatitis crucialto havelargeoverabundanceofCNOnucleiinthe
H?richenvelopealreadyat theonset of the violent burning phaseinorderto
getathermonuclearrunaway?TNR? atall?According tothesecalculations
amassfraction of CNOnucleiclose to ??? isrequired forastrongoutburst
anda fastnov a?Thecrucialquestion isthen? CanweexplainthehugeCNO
abundancesinthe accreted env elopeof thewhitedw arf? HereNis nota
problemsinceitis directlyproduced fromCandOduringH?burning? The
problem areC andO?
a fractionofthewhite dwarf?ssurfacewascov eredbythecomputationalgrid?
calculated domainw as????x????km?radialxlateral? and ????x????x
sim ulation?resp ectiv ely?
The ere doneOMETHEUS?an
h ydrocode? The
orderGo method exactnamelya
and than one
byop eratorrealistic ofstate
equations tomodeln uclearreactionsbyaNewton?Raphsontechnique?
bloc ks eachcorresp ondingtoa processorelement ?PE??Themost c hallenging
in termsparallelizationhsolver sinceh ydro
grid? ingeneral?ThePPM?sceme is anappro ximation which needs only?ad?
ditionalso?calledghost zonestouniquelyde?ne theboundaryv alueproblem
singlec k? Beforetegrationsh ydro
each blockfor a timestep aredone theghostzonesare?lledwithinformation
from theneighbouringblocks?Thismeansthattherehastobe communi?
cationb etweenneighb ouringPEs precedingeachh ydrodynamicaltimestep?
Thisis donebyexplicitmessagepassingusingtheMPP? and? ina more
recentversion ofPROMETHEUS?the MPI?library?
Thehydrodynamicaltimestep isgivenbytheCFL?Courant?Friedric h?
Levy??condition ?Colella?Woo dw ard??????Thetimestep iscomputed in
ev eryzone andtheminimaltimestep accoun tsfor thefollowing globalh y?
Allcalculationsw ereperformedontheCRAYT?EoftheRechenzen trum
Garching?Thethree?dimensionalsim ulations consumed about?????? PE
hoursonthe full???PE system?Theparallele?ciencyisov er????
Firstsimulations weredone intwospatialdimensions?assuming thatall
ph ysicalquan titiesare independentofthe thirddimension?Inthissymmetry
conv ective eddiesarerepresentedbyin?nitelylong rolls?Some ofourresults
thev elocities obtained inthis ?Dsimulation?Asmall temperatureincrease
soundwav esemerging from the ignitionregionignited thewholeb ottom
layerof theenvelope ?Fig??a?? As timeprogressesquasi?stationaryaxially
about ???s?Fig??c??Thesevorticesarevery stable andcarrya considerable
Theappearance ofaxi?symmetric quasi?stablestructuresin?D ?owsis
parallel geometry? andhas alsob een observed in the Earth?soceans?McWilli?
dredge?upofCandO? mainlybecauseonaverage lessturbulentkineticen?
seconds afterignitionaconsiderableamount of
velope ?????themixing processhasbeentoo slow toyieldastrong nova
to ethatveb eenresolv inour
didnot?nd majordi?erences? asfar astheglobal properties
are concerned?Mixing w asslightlylesse?cien tthan inthe modeldescrib ed in
physicsand similarresolution asb efore?butfound ev enless mixing?therefore
less energy generation anda completelydi?eren t ?owstructure ?Fig???than
in?D asmight haveb eensusp ected?Thestablevorticesdo notapp earin
the?Dcalculationssincethe?owis notrestrictedtotwo spacialdimensions
Werather observeconvectiveand turbulentpatternsasone mightexpectfor
Itisob vious?accordingtothedi?erencesbetween the?Dand ?Dcalcu?
lations? that?Dsimulationsof the TNRontopofa whitedwarf areindeed
necessarytomo delthe turbulent combustionprocess?
Wehave demonstratedthatdirectn umerical simulations ofnovaoutbursts?
as faras thephysics isconcerned? require severe enric hmentofthe H?rich
env elope priortothe TNR?If forsomereasonthis does nothapp en the
whitedw arfwill enterintoa phaseof quieth ydrostaticburningandwill? if
the env elopeful?llscertain requiremen ts?resem bleasuper?soft X?raysource
more thananova?Wealso have sho wnthatdirectn umericalsimulations
of reactionhydro dynamicshavebecome feasible?evenin?D? andshouldbe
donewhen fuelandashesarew ell mixedandburning times arem uch longer
thanthoseof turbulent mixing?
Colella P?andWoodw ardP?R?ThePiecewiseParabolic Method ?PPM?for
Gas?DynamicalSimulations? J?Comp?Physics??????? ??????
FujimotoM?Y??ATheory ofHydrogenShell Flasheson AccretingWhiteDwarfs?
I?Their ProgressandtheExpansionof the Env elope?Astrophysic al Journal? ????
Glasner S?A? andLivneE?? ConvectiveHydrogen BurningDuringa Nova Out?
burst?Astr ophysicalJournal? ???????L???????
GlasnerS?A?? LivneE?and Truran J?W??Reactive Flow inNovaOutbursts?
Astrophysic alJournal? ??????? ???????
McWilliamsJ?C?? TheEmergence of IsolatedCoherentVortices inTurbulent
Flo w? J? FluidMe ch????????? ???????
StarrfieldS?? TruranJ?W?? SparksW?M? andKutterG?G?? CNOAbun?
dancesandHydrodynamic Models oftheNovaOutburst? Astrophysic alJournal?
StarrfieldS??Sp arksW?M?and TruranJ?W?CNOAbundancesand Hy?
drodynamic Modelsof theNova Outburst?II?????M
Models with Enhanced
CarbonandOxygen? Astr ophysicalJournalSupplement??????????????
StarrfieldS?? TruranJ?W? andSparksW?M??CNOAbundancesandHydro?
dynamic Models ofthe NovaOutburst?V?????M
Fig???V elocity?eld at di?erent stagesoftheevolution forthe ?Dmo delexplained
inthe text? Giv enincoloristhe absolutev alue ofthevelocity ateachpoin t?T?
denotesthetemperature of the hottestindividualzone inunits of??