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Helium star - carbon-oxygen white dwarf (CO WD) binaries are potentialsingle-degenerate progenitor systems of thermonuclear supernovae. Revisiting aset of binary evolution calculations using the stellar evolution code$\texttt{MESA}$, we refine our previous predictions about which systems canlead to a thermonuclear supernova and then characterize the properties of thehelium star donor at the time of explosion. We convert these model propertiesto NUV/optical magnitudes assuming a blackbody spectrum and support thisapproach using a matched stellar atmosphere model. These models will bevaluable to compare with pre-explosion imaging for future supernovae, though weemphasize the observational difficulty of detecting extremely blue companions.The pre-explosion source detected in association with SN 2012Z has beeninterpreted as a helium star binary containing an initially ultra-massive WD ina multi-day orbit. However, extending our binary models to initial CO WD massesof up to $1.2\,M_{\odot}$, we find that these systems undergo off-center carbonignitions and thus are not expected to produce thermonuclear supernovae. Thistension suggests that, if SN 2012Z is associated with a helium star - WDbinary, then the pre-explosion optical light from the system must besignificantly modified by the binary environment and/or the WD does not have acarbon-rich interior composition.

Pre-Explosion Properties of Helium Star Donors to Thermonuclear Supernovae