Thermonuclear eruptions on the surface of white dwarf stars

Novae are the most common thermonuclear explosions in the Universe. We expect of order 35 novae per year in our own Milky Way Galaxy (Shafter 1997), scalling with galaxy mass (Shafter et al. 2014 - for example, we expected one type Ia supernova per year per galaxy). The nova eruption occurs in a binary system following extensive accretion of hydrogen rich material, from a less evolved star, on to the surface of a white dwarf star. The eruption ejects of order 10-7 to 10-3 solar masses of matter at velocities ranging from a few hundreds to thousands of kilometers per second (Bode &Evans 2008).


The white dwarf in this eruption is not destroyed, accreting more matter from the secondary star, going into a cycle of eruptions. These eruptions can recur on timescales of a year to millions of years, governed by the proprieties of the white dwarf and the accretion rate (Starrfield et al. 1972; Truran & Livio 1986; Yaron et al. 2005). Given that the eruption recur, two outcomes may be expected depending on the details of the ejected mass and accretion rate. If more mass is ejected than accreted then the white dwarf will eventually erode, while if less mass is ejected then a helium layer may build up from hydrogen burning (Wolf et al. 2013) and this helium layer can detonate and send a quasi-spherical shock wave in to the white dwarf core converging at an off-center location, igniting carbon (Shen & Bildsten 2014).

Determining the morphology of the nova eruption

The observed morphologies have been suggested to arise from several mechanisms. A common envelope phase during the eruption, the presence of a magnetized white dwarf, and an asymmetric thermonuclear runaway (e.g. Bode &Evans 2008). The common envelope phase is the most widely accepted formation mechanism, where the ejecta engulfs the secondary star within a matter of minutes following the outburst. The secondary then transfers energy and angular momentum to the eject. Recent smoothed particle hydrodynamic calculations show that the mass loss from the secondary, during quiescence, is highly concentrated in the orbital plane and that this produces naturally the bipolar structures of the ejecta, with possibly an equatorial waist (Mohamed & Podsiadlowski 2012; Mohamed et al. 2013).


Following the enova eruption we observe a myriad of emission lines at diffrent ionisation states and shapes. The observed emission line shapes are due to Doppler velocity broadening and have been linked to the ejection geometry (e.g. Shore et al. 1993; Gill & O'Brien 1999) and also provide elemental abundances (Gehrz et al. 1998). Perhaps the best way to retrieve the true morphology of the ejecta is from the combination of 2D imaging, providing the morphology of the ejecta on the plane of the sky, combined with the emission line (e.g. Gill & O'Brien 2000; Harman & O'Brien 2003; Ribeiro et al. 2009). The combination of 2D imaging and line profile fitting furhter allow us to the detmine distances via the nebular expansion parallax method.

Measuring the ejected masses

Under development

Does the mass of the white dwarf grow?

Under development