While it is generally agreed that type Ia supernovae are thermonuclear explosions of carbon-oxygen white dwarfs, there is still an ongoing debate whether the explosions originate from two white dwarfs merging (the double degenerate or DD scenario) or a white dwarf accreting material off of a normal star companion (the single degenerate or SD scenario). In addition, observed number of suitable progenitor systems is significantly smaller than what is required to match the observed supernova rate.
White dwarfs with high mass transfer rates from their companion objects have been suggested as one type of type Ia supernova progenitor. I performed a survey looking for rapidly accreting white dwarfs (RAWD) in the central core of the SMC. I detect no candidate RAWD. The upper limits from this non-detection are 10 – 14 RAWD in the SMC, assuming they resemble the LMC source LHA 120-N 66 or fainter versions of Wolf-Rayet stars.
I conducted a second survey of the SMC to identify new symbiotic stars – white dwarfs accreting off of a giant star companion, usually via winds. They are another possible type Ia supernova progenitor. All of the stars identified as candidates in the survey are false positives. We conclude that there are about 12 – 18 bright symbiotic stars in the SMC. Even under optimistic assumptions, this is too few objects to make up for the 25 – 130 missing SD progenitors in the SMC.
I also briefly discuss LHA 115-S 18, a peculiar B[e] supergiant in the SMC, which was observed in the symbiotic star survey. S18 is both photometrically and spectroscopically variable. The system likely contains an approximately 12 solar mass B supergiant, along with a hot compact object. The identity of the hot object remains unclear since either a neutron star or a white dwarf could be formed through normal stellar evolutionary processes and are consistent with the observed properties of the hot object.