Observational Signatures of Carbon-Oxygen White Dwarf Merger Remnants

TL;DR

This paper explores how to observationally identify remnants of double Carbon-Oxygen white dwarf mergers, focusing on their unique nebular signatures and potential detectability in nearby galaxies.

Contribution

It introduces a novel method using spectral features of surrounding nebulae to detect white dwarf merger remnants, supported by CLOUDY simulations.

Findings

Merger remnants resemble extreme AGB stars for ~10^4 years.

They produce distinctive spectral lines, including weak hydrogen and strong carbon/oxygen lines.

Potential candidates can be observed in nearby galaxies like M31 and M87.

Abstract

Many double white dwarf (WD) mergers likely do not lead to a prompt thermonuclear explosion. We investigate the prospects for observationally detecting the surviving remnants of such mergers, focusing on the case of mergers of double Carbon-Oxygen WDs. For $\sim 10^4$ yr, the merger remnant is observationally similar to an extreme AGB star evolving to become a massive WD. Identifying merger remnants is thus easiest in galaxies with high stellar masses (high WD merger rate) and low star formation rates (low birth rate of $\sim 6-10 \,{\rm M_{\odot}}$ stars). Photometrically identifying merger remnants is challenging even in these cases because the merger remnants appear similar to He stars and post-outburst classical novae. We propose that the most promising technique for discovering WD merger remnants is through their unusual surrounding photoionized nebulae. We use CLOUDY photoionization calculations to investigate their unique spectral features. Merger remnants should produce weak hydrogen lines and strong carbon and oxygen recombination and fine-structure lines in the UV, optical and IR. With narrow-band imaging or integral field spectrographs, we predict that multiple candidates are detectable in the bulge of M31, the outskirts of M87 and other nearby massive galaxies, and the Milky Way. Our models roughly reproduce the WISE nebula surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict detectable [Ne\,VI] and [Mg\,VII] lines with JWST but that the mid-IR WISE emission is dominated by dust not fine-structure lines.