Faint calcium-rich transient from the double-detonation of a $0.6\,M_\odot$ carbon-oxygen white dwarf star

TL;DR

This paper models a faint, calcium-rich transient resulting from a double detonation in a white dwarf merger, providing insights into its spectral and luminosity characteristics and proposing a progenitor scenario for Ca-rich transients.

Contribution

It presents the first 3D hydrodynamic simulation of a white dwarf merger leading to a faint, Ca-rich transient, linking such events to specific merger scenarios.

Findings

Produces a low-luminosity SN Ia-like lightcurve with <0.01 M_sun of Ni-56.

Spectra show strong calcium lines in early weeks.

Resembles observed Ca-rich transients in lightcurve and spectral features.

Abstract

We have computed a three-dimensional hydrodynamic simulation of the merger between a massive ($0.4\,M_\odot$) helium white dwarf (He WD) and a low-mass ($0.6\,M_\odot$) carbon-oxygen white dwarf (CO WD). Despite the low mass of the primary, the merger triggers a thermonuclear explosion as a result of a double detonation, producing a faint transient and leaving no remnant behind. This type of event could also take place during common-envelope mergers whenever the companion is a CO WD and the core of the giant star has a sufficiently large He mass. The spectra show strong Ca lines throughout the first few weeks after the explosion. The explosion only yields $<0.01\,M_\odot$ of $^{56}$Ni, resulting in a low-luminosity SN Ia-like lightcurve that resembles the Ca-rich transients within this broad class of objects, with a peak magnitude of $M_\mathrm{bol} \approx -15.7\,$mag and a rather slow decline rate of $\Delta m_{15}^\mathrm{bol}\approx 1.5\,$mag. Both, its lightcurve-shape and spectral appearance, resemble the appearance of Ca-rich transients, suggesting such mergers as a possible progenitor scenario for this class of events.