A mechanism is described that promises to explain how nova outbursts take place on white dwarf of 1 solar mass or less and for accretion rates of 4 x 10 to the -10 solar mass/yr or greater

Kutter, G. S.

Sparks, W. M.

English

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SUBMITTED To: iAU Col loouium No. 108
Tokyo, Japan
Sept. 1-4, 1987
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•
NUMERICAL MODELLING OF THE CLASSICAL NOVA OUTBURST
•
G. Siegfried Kutter
NASA/Goddard Space Flight Center
and
Warren M. Sparks
Los Alamos National Laboratory
Abstract: We describe a mechanism that promises to explain how nova outbursts take
place on white dwarfs of 1 Me or less and for accretion rates of 4 x 10-1 Me yr or
greater.
Observations suggest that the average mass of white dwarfs in classical nova systems
is 0.8 Me and that the typical mass transfer rate in such systems is 10-9 - 10-8 MO
yr 1 (Patterson 1984). However, model calculations that take the accretion process
into account have been unsuccessful in reproducing the classical nova outburst for
these parameters. The reason is insufficient confinement cf the nuclear runaway zone
and, hence, insufficient release of energy (Sparks and Kutter 1987). To overcome this
problem, white dwarf masses equal to or greater than 1.25 M0 , low accretion rates, or
hybernation scenarios have been invoked (Kutter and Sparks 1960, Livio and Shara 1987,
Prialnik and Kovetz 1984, Starrfield et al. 1985).
We describe here a mechanism that promises to produce nova outbursts for the observa- 	 i
tional data listed above, though to date we have explored it only for these parameters:
white dwarf	 1 M , consisting of 12C and 160, 10-3 L ;
accreting matter 4.23 x 10-10 ME) ,'r-1 , solar r.:x with 12C and 160 each enhanced
to 10• by mass, arriving on white dwarf with Keplerian angular
momentum.
The computational method is described in Kutter and Spars (1967) and Sparks and Kutter
(1987) .
The shear created on the white dwarf's surface by the rotational velocities of the
accreting H-rich matter produces an accretion belt of mixed stellar and accreted mat-
ter. After 7,080 years, the surface of the belt rotates as raFidly as the newly ar-
riving matter and shear mixing ceases. Accretion contin-.;es to are 256,400 years, w::en
nuclear burning exceeds 10 6 erg/(g•sec) at a fractional mas-s from the star's surface
of Q = 2.57C x 10-4 and the nuclear runaway gets under wa; • . lip to this point the
•
National Research Council -- NASA,,ISFC Research Assoc:-,te, on leave from The Ever-
green State College, Olympia, WA
%
rstar's evolution resembles the models described in Sparks and Kutter (1987); and, if
the evolution calculations are continued, 7.6 years later the nuclear runaway peaks
with nuc = 2.26 x 1015 erg/(a - sec). No nova results.
According to our experiences, a nova outburst will occur on a 1 M  white dwarf and for
accretion parameters shown above only if the nuclear reactions run away at greater
depth than in the above calculations and/or if the white dwarf's envelope rids itself
of some of its angular momentum during the early phase of the runaway. The greater
depth increases the gravitational confinement of the burning region; and the loss of
angular momentum increases the mechanical confinement, due to reduction of the centri-
fugal forces.
Is there a mechanism capable of moving, during the early phase of the nuclear runaway,
the nuclear runaway zone to greater depth and of ridding the envelope of angular momen-
tum? We suggest that it is the convective instability, which in the course of the run-
away grows until it reaches the star's surface. Convection mixes matter in the enve-
lope's outer rapidly rotating layers with matter in the inner less rapidly rotating
layers and thereby transports angular momentum to the nuclear runaway zone. The resul-
ting shear between that zone and the adjacent layer interior to it creates a Kelvin-
Helmholtz instability, which in turn leads to mixing of matter. tie think that this
mixing moves both the zone of peak nuclear burning and angular momentum inward. It
also mixes H-rich matter with layers rich in carbon and oxygen.
To test whether the Kelvin-Helmholtz instability, induced by convection, is capable of
producing nova outbursts, we ran our model through a nuclear runa:.ay, artificially
moving the zone of peak nuclear burning inward to Q = 3.072 x 13 -' and removing 10% of
the envelope's angular momentum. Furthermore, we enhanced the 1` J and 46  content of
the accreting matter (see above) to simulate the mixing of H-rich and original white
dwarf matter. The nuclear reaction rates rise to 2.52 x 1017 erg/(g•sec) and the en-
tire envelope above the runaway zone is ejected with speeds of 19: to 1,600 km sec-1.
The mechanism seems to work because rotational kinetic energy of the accreting matter
is not immediately released, but only when it is needed to increase the burning re-
gion's confinement during the early phase of the nuclear runawa y . Details of the
results will be presented in an Ap. J. paper now in preparation.
BIBL:03 RAPHY:
Kutter, G. S., and Sparks, W. M. 1980, Ap. J., 22399, 988.
1967, Ap. J., 321 386.
Livio, M., and Shara, M. M. 1987, Ap. J., 319, 819.
Patterson, J. 1984, Ap. J. Suppl., 54, 443.
Prialnik, D., and Kovetz, A. 1984, Ap. J., 2M 1, 367.
Sparks, W. M., and Kutter, G. S. 1987, Ap. J., 3221, 394.
Starrfield, S., Sparks, W. M., and Truran, J. W. 1985, Ap. J., ^, 136.


Numerical modelling of the classical nova outburst

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