Thermonuclear Fusion of Hydrogen in Classical Novae
Classical novae are a subclass of novae. They occur in binary systems of a white dwarf (a compact star) with a main sequence star as a companion. If they are close enough together, the white dwarf can accrete hydrogen rich material from its companion. In this way an envelope forms on the surface of the white dwarf. During the accretion process the envelope is being compressed and therefore heated until ignition temperatures for the explosive thermonuclear fusion ('combustion') of hydrogen into helium are reached. The fusion energy, which is essentially being released within minutes, yields a rise of the luminosity of the binary system of up to 15 magnitudes and the ejection of the whole accreted envelope of the white dwarf. Such a ,cataclysmic' change of properties of a binary system can be detected as a nova outburst on earth. In our galaxy the rate of classical novae is between 20 and 50 per year. There are also ,supernovae' and 'dwarf novae'. The underlying physical processes there are different from the ones causing a classical nova.
We have been studying the processes taking place in the accreted envelope on top of a white dwarf during a nova explosion by means of challenging three dimensional (3D) computer simulations.
We show some results of those calculations in the following figure.
Figure 1 shows surfaces of constant absolute value of the velocity at a certain time during the explosion, namely for 24 km/s and 110 km/s. Moreover, there is also a horizontal cut which the absolute value of the velocity is color coded on. Red means high velocities and blue low ones. The illustrated domain corresponds to a physical volume of about (1000km)3 and is, therefore, just a part of the whole computational domain.
The pictures give an impression of the gas motions in the envelope which occur during the explosion, named ,turbulent convection'. It is obvious that the high velocities correspond to large spatial (,convective') structures, whereas the low velocities correspond to small spatial structures and eddies according to the so called turbulent cascade.
Figure 2 shows snapshots of the temporal evolution of the nova explosion. We show, color coded, the distribution of the radioactive oxygen 14 (14O) Isotope which can operate as an indicator for the nuclear fusion process.
In Figure 2 we show vertical slices through the envelope and the upper shells of the white dwarf of about 1000km x 1800km. Bright colors correspond to high 14O concentrations. This isotope is produced in the hydrogen fusion (combustion) reactions almost exclusively in a very thin shell on top of the white dwarf at temperatures exceeding 100 Million degrees Kelvin. Strong turbulent motions stir it throughout the whole envelope. Therefore, 14O is an appropriate ,marker' for the gas motions, just like steam, which is used in wind channel experiments to visualize the air flows around the body of a car.
At the beginning of the sequence there is only little production of 14O on top of the white dwarf. But the production rises as a consequence of the rising intensity of the nuclear reactions and is stirred by turbulent motions which are also a consequence of enhanced nuclear energy production. In this way the 14O isotopes are gradually transported into the outer parts of the envelope until it is highly enriched in 14O as a whole at the end of the explosion.
It is, may be a little bit puzzling but the thermonuclear combustion in stars is, under certain hydrodynamical aspects, not much different from ,every day' combustion processes here on earth. There are lots of similarities to combustion in Otto engines or even forest fires. Therefore these calculations are also of a high relevance for technical combustions and could be of importance especially for pre-design of combustion engines.
The studies have been and are still being carried out by means of computer simulations on the most powerful supercomputers in the world, such as for instance the CRAY T3E with 816 Dec Alpha Processors or the Fujitsu VPP700. Our simulations are among the most time consuming ones ever done and operate todays computers at their limits.
A. Kercek, W. Hillebrandt