Ultra-soft sources as supernova type Ia progenitors

TL;DR

This paper investigates ultra-soft sources as potential Type Ia supernova progenitors, conducting a survey in the SMC to detect their signatures, but finds no conclusive evidence, highlighting uncertainties in their optical spectra.

Contribution

It introduces the concept of ultra-soft sources and presents a survey searching for their optical signatures, providing constraints on their prevalence and characteristics.

Findings

No strong He II lines detected, suggesting rarity or different spectra of USS.

Detected some unusual objects, but none confirmed as USS.

Discussion of LMC N66 as a possible USS example.

Abstract

Missing from the usual considerations of nuclear burning white dwarfs as Type Ia supernova progenitors are systems with very higher mass transfer rates, where more material than is needed for steady burning accretes on the white dwarf. This will move out the photosphere of the white dwarf, causing it to emit at longer wavelengths. Thus, we propose the name ultra-soft source (USS) for these objects. We present a VLT/FLAMES survey looking for USS in the SMC, selected to be bright in the far UV and with blue far UV-V colors. While we find some unusual objects, and recover known planetary nebula and WR stars, we detect no objects with strong He II lines, which should be a signature of USS. This null result either puts an upper limit on the number of USS in the SMC, or shows that we do not understand what the optical spectra of such objects will look like. We also discuss the unusual LMC [WN] planetary nebula LMC N66 as a possible example of an USS. It has a luminosity consistent with that expected, and its spectra show incompletely CNO-processed material - strong helium lines, some hydrogen, enhanced nitrogen and depleted carbon. It also shows periodic outbursts. USS may resemble N66 in quiescence. However, it lacks a FUV excess, contrary to our predictions.