Looks can be deceiving

Abstract

Context. White dwarfs (WDs) are important and abundant tools to study the structure and evolution of the Galactic environment. However, the multiplicity of WD progenitors is generally neglected. Specifically, a merger in a binary system can lead to a single WD, which could result in wrongly inferred quantities if only single stellar evolution (SSE) is considered. These mergers are linked to transients such as luminous red novae and Type Ia supernovae. Aims. We investigate the impact of binary evolution (BE) upon observable single WDs, and compare their properties to WDs formed through SSE. We assess the evolutionary channels and the age and mass distributions of the resulting single WDs. Methods. We employed SeBa to model the evolution of single star and binary populations. We synthesised the observable single WD population within 100 pc, including cooling and observational selection effects. Additionally, we constructed models with different evolution and primordial population properties to study the effects on the properties of the resulting single WDs. Results. White dwarfs from binary mergers make up about 10−30% of all observable single WDs and 30−50% of massive WDs. On average, individual WDs take 3.1−5 times longer to form through BE than SE, and so appear ∼1 Gyr younger than they are if BE is ignored. In all models, the effect of mergers on the age distribution is clearly noticeable. The median age typically increases by 85−430 Myr and 200−390 Myr for massive WDs. Although abundant, we do not find evidence that WDs from mergers significantly alter the shape of the WD mass distribution. Conclusions. Assuming SSE for inferring properties of single WDs gives rise to intrinsic errors as single WDs can also be formed following a binary merger. Strategies for mitigating the effect of mergers on the WD age distributions are discussed.

Forever young white dwarfs: When stellar ageing stops

“…Re-cent studies (Toonen et al 2017;Maoz et al 2018) suggest that binary mergers substantially contribute to the single WD population. The theoretical studies of Temmink et al (2020) estimated that 30-45% of the WDs more massive than 0.9M are formed through binary mergers, mostly via the merger of two WDs, and Cheng et al (2020) estimated observationally that the fraction of high-mass WDs (in the range of 0.8-1.3M ) that formed as a result of double WD mergers is nearly 20 %.…”

Mergers of equal-mass binaries with compact object companions from mass transfer in triple star systems

“…Hence, it could appear to have formed from a much younger progenitor star than the outer tertiary companion, which offers a potential smoking gun of a prior merger event (although we note that this is not a unique signature of this specific channel) (e.g. Temmink et al 2019).…”

SDSS J124043.01+671034.68: the partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?

“…We crudely estimate the diffusion timescales at the bottom of the deep convection zone to be ∼10 Myr (Koester 2009). 7 Assuming that SDSS J1240+6710 underwent a thermonuclear event that resulted in significant reheating (which is also postulated for a non-igniting merger event; see Wegg &Phinney 2012 andTemmink et al 2019), we can crudely estimate the time since this event by adopting a standard cooling track for a T eff = 20 500 K and 0.41 M white dwarf, resulting in ≈ 40 Myr -which is comparable to the diffusion time-scales at the bottom of the convection zone. We conclude that the photospheric abundances, reflecting the composition of the deep convective layer in the outer envelope, have probably not been altered significantly by the differential diffusion velocities of the individual elements.…”