Emission line diagnostics of the progenitors of type Ia supernova

Field: Physics and Astronomy (Physical Sciences)

In this dissertation, we consider the origin of thermonuclear supernovae, known by their observational classification as type Ia (hereafter SNe Ia). In particular, we develop an entirely new means to test the ``single-degenerate'' hypothesis, in which the progenitors of these tremendous explosions are suggested to be hot and luminous accreting white dwarfs. We then strongly constrain the role of any such ``hot-mode'' SN Ia progenitor channel using both a population-based argument and an individual case study, before concluding with some more general considerations of nebulae ionized by accreting white dwarfs. Type Ia supernovae have now been the subject of intensive study for decades, particularly in light of their role as standard(-izable) candles in measuring cosmological distances. However, there remains no consensus model for the evolutionary channel(s) by which they originate. In the so-called ``double-degenerate'' scenario, a binary pair of white dwarfs shed angular momentum through gravitational-wave radiation, until they inspiral and merge, triggering an explosion. Alternatively, in the classic picture of the single-degenerate scenario, a white dwarf accretes hydrogen-rich material from some main sequence or red giant companion, and grows through nuclear burning of this material at its surface until reaching sufficient mass to trigger an explosion. This suggests that single-degenerate progenitors should be extremely luminous sources in the EUV and soft X-ray bands during the accretion phase (lasting $\sim 10^{5}$--$10^{6}$ years). For this reason, such objects are generally associated with observed ``supersoft X-ray sources'' (SSSs). Previous efforts to detect or constrain the role of any such channel have focused on detecting these objects directly in the soft X-ray band (photon energies in the range 0.3 -- 0.7 keV), either on an individual basis or as the combined emission of a diffuse population. Such an approach has yielded important constraints, but only if white dwarfs accrete principally at very high temperatures (T $\sim$ $5\times 10^{5}$K). However, observed SSSs are understood to lie in a broad range of temperatures, with a possible range of at least $2 \times 10^{5}$--$10^{6}$K, and some theoretical models suggest even lower temperatures are possible. This necessitates the development of an alternative, complimentary test which can constrain the luminosity of accreting white dwarfs across a wider range of photospheric temperatures. In this work, we demonstrate that if the single-degenerate model is correct, then accreting, nuclear-burning white dwarfs should provide the dominant source of ionizing radiation in passively-evolving galaxies, roughly 40\% of which are known to host extended low-ionization emission-line regions (so-called ``retired'' galaxies, i.e. emission-line galaxies without either a central AGN or significant ongoing star formation). Therefore, one can search for the presence of any high-temperature single-degenerate progenitor population in these galaxies by looking for emission lines characteristic of ionization by very high-temperature ($10^{5}$ K -- $10^{6}$ K) sources. In particular, we find that recombination lines of He II, and forbidden lines of [N I] and [O I], provide the most sensitive diagnostics in retired galaxies to assess the role of accreting white dwarfs as SN Ia progenitors in any ``{\bf hot}-mode'' (T $\gtrsim 1.2\times 10^{5}$K) accretion regime. Following this, we limit the contribution of any high-temperature single-degenerate channel to the SN Ia rate at relatively early delay-times (1 Gyr $\leq$ t $\leq$ 4 Gyr) to $