There are currently two “standard” models for the progenitors of SNe Ia. In the single degenerate scenario, a white dwarf accretes matter from a co-orbiting companion star until enough mass has accumulated to trigger an explosion. In the double degenerate scenario, a binary pair of white dwarfs sheds angular momentum due to gravitational radiation and merges, giving rise to a SN Ia. Observationally speaking, the clearest difference between the two is that in the single degenerate scenario, the accreting white dwarf must process a considerable amount of mass through steady nuclear burning, making it a highly luminous source of X-ray and extreme ultraviolet emission for up to a million years prior to the explosion.

Therefore, the most obvious way to distinguish between the two formation channels is to look for some evidence of the existence of such hot, luminous sources, allowing one to test the viability of the single degenerate scenario. Past work has focused on looking for X-ray emission, e.g. in the integrated X-ray luminosity of nearby galaxies [1]. However, some type Ia supernova progenitor models predict that much of the emission from accreting white dwarfs may be radiated in the extreme ultraviolet, where it is totally absorbed by interstellar matter. In order to move forward, an ideal test for the presence of a significant single degenerate progenitor population would need to circumvent this issue.

Rather than looking for emission from any putative single degenerate progenitors directly, we can search instead for evidence of their effect on the interstellar medium. For example, one could attempt to find signatures of the gas ionized by such sources. In early-type galaxies without ongoing star formation, we expect only post-asymptotic giant branch stars (pAGBs) to be a significant source of ionizing radiation, at least outside of the inner galactic nuclei. These stars likely power the nebular emission-line regions now found in many ellipticals [2]. However, in a recent paper, Tyrone Woods and Marat Gilfanov at the MPA have demonstrated that if the single degenerate hypothesis is correct, accreting white dwarfs should provide the dominant contribution to the ionizing background in such galaxies, in particular for relatively young stellar populations [3]. This is especially true for the ionizing continuum beyond the second ionization edge of Helium at 54.4 eV.

For a given photo-ionized nebula, the total luminosity emitted in any recombination line is roughly proportional to the incident flux of ionizing photons. This suggests that one can confirm, or strongly constrain, the presence of a significant contribution of single degenerate progenitors to the SN Ia rate by searching for recombination lines of ionized helium in the spectra of early-type galaxies. Performing numerical calculations using the photo-ionization code MAPPINGS III [4], the expected luminosity of the He II 4686 Angstrom line (the strongest He II line seen in the optical) can be computed given reasonable assumptions regarding the composition and distribution of the ionized gas. For a 1 billion year old starburst galaxy, the inclusion of the accreting white dwarf population implied by a plausible single degenerate channel increases the predicted He II 4686 Angstrom line luminosity by almost 2 orders of magnitude (see Fig. 2)!

At present, no line at 4686 Angstroms has been detected in the extended emission-line regions of early-type galaxies. In part, this is because of the intrinsic weakness of this line (though the far-ultraviolet He II line at 1640 Angstroms is roughly 6 times stronger, and may be of use here). However, if there exists a large population of accreting, nuclear-burning white dwarfs in early-type galaxies which is consistent with the single degenerate channel, then such a line should be detectable by ongoing integral field spectroscopic surveys, such as CALIFA, or through stacking analysis of available SDSS galaxy spectra [3].

For young, post-starburst galaxies, an upper limit on the He II 4686 Angstrom line luminosity of roughly 10^28 erg/s/solar mass would rule out any high temperature population consistent with the single degenerate scenario. Therefore, scientists working at the MPA hope that, in the very near future, the SN Ia community will be able to confidently detect, or place strong upper limits on, the presence of He II recombination lines in early-type galaxies.

Tyrone Woods and Marat Gilfanov

References

Gilfanov M., Bogdan. A., 2010, Nature, 463, 924

Sarzi M., Shields J. C., Schawinski K. e. a., 2010, Monthly Notices of the Royal Astronomical Society, 402, 2187

Woods, T. E., Gilfanov, M., 2013, Monthly Notices of the Royal Astronomical Society, 1254

Groves, B. A., Dopita, M. A., Sutherland, R. S. 2004, Astrophysical Journal Supplements, 153, 9