Hubble Space Telescope Imaging of the Outburst Site of M31 RV. II. No Blue Remnant in Quiescence

TL;DR

This study re-examines HST data of M31 RV's 1988 outburst site, debunking previous claims of a hot remnant and concluding no detectable blue remnant exists in the observed field.

Contribution

The paper provides a detailed re-analysis of archival HST images, correcting prior misinterpretations and clarifying the nature of the remnant of M31 RV after its outburst.

Findings

No evidence of a hot, blue remnant in the HST images.

Previous UV and hot star detections were due to cosmic rays and misinterpretation.

M31 RV's remnant is either undetectable, a red giant companion, or an unresolved red giant.

Abstract

M31 RV is a red transient that erupted in 1988 in the Andromeda bulge, reaching a luminosity intermediate between novae and supernovae. It was cool throughout its outburst, unlike a normal classical nova. In 2006, Bond & Siegel examined archival HST optical images of the M31 RV site, obtained in 1999. We found only old red giants at the site, and no stars of unusual color. However, Shara et al. recently claimed to have detected (a) a bright UV source within the error box in HST UV images taken in 1995, (b) a hot (Teff>40,000 K) optical source in the same 1999 images that we examined, and (c) cooling of this source from 1999 to 2008. Shara et al. argue that this source's behavior is consistent with a classical-nova outburst occurring on a low-mass white dwarf. I have re-examined all of the HST frames, including new ones obtained in 2009-2010. I find that: (a) the bright 1995 UV source reported by Shara et al. was actually due to cosmic rays striking the same pixel in two successive exposures; (b) the claim that an optically bright star in the error box is very hot is actually due to misinterpretation of red-giant colors in the STmagnitude system; (c) there is no evidence for variability of any source within the error box from 1999 to 2010; and (d) there are no stars of unusually blue or red color in the error box. Our 2006 conclusions remain valid: either M31 RV had faded below HST detectability by 1999, or its remnant is an unresolved companion of a red giant in the field, or the remnant is one of the red giants.