[Paper Review] Looks can be deceiving: underestimating the age of single white dwarfs due to binary mergers

Karel Temmink, Silvia Toonen+3 more|UvA-DARE (University of Amsterdam)|Oct 11, 2019

Gamma-ray bursts and supernovaePhysics and Astronomy108 references18 citations

TL;DR

This study demonstrates that binary mergers produce single white dwarfs (WDs) that appear significantly younger than they truly are when age is estimated via single stellar evolution (SSE) models. Using population synthesis with the SeBa code, the authors find that merger-formed WDs take 3–5 times longer to form, leading to age underestimations of up to ~1 Gyr, with 10–30% of observable WDs and up to 50% of massive WDs originating from mergers.

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 exttt{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 ${\sim} 1$ 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.

Motivation & Objective

• To investigate how binary mergers affect the inferred properties of single white dwarfs (WDs), particularly their ages and masses.
• To quantify the fraction of single WDs formed via binary mergers versus isolated single stellar evolution (SSE).
• To assess the impact of merger-formed WDs on the observed age and mass distributions of WDs within 100 pc.
• To evaluate whether the observed excess of WDs near 0.8 M☉ can be explained by binary mergers.
• To propose mitigation strategies for age inference errors in WD-based studies of galactic evolution and star formation history.

Proposed method

• Employed the SeBa binary population synthesis code to model the evolution of both single stars and binary systems.
• Simulated the formation of single WDs through both isolated single stellar evolution (SSE) and binary merger channels.
• Applied cooling tracks and observational selection effects (magnitude limit, distance cut) to synthesize a realistic observable WD population within 100 pc.
• Varied primordial binary properties and star formation histories to test robustness of results across different models.
• Compared age and mass distributions of WDs formed via SSE versus binary mergers, focusing on formation times and cooling times.
• Analyzed the initial-final mass relation (IFMR) and merger channel contributions (e.g., WD+MS, MS+MS, WD+WD) to identify dominant formation pathways.

Experimental results

Research questions

• RQ1 What fraction of observable single white dwarfs within 100 pc are formed through binary mergers rather than isolated single stellar evolution?
• RQ2 How much younger do white dwarfs appear when their true formation history involves a binary merger, compared to the age inferred via standard SSE models?
• RQ3 Does the observed excess of white dwarfs near 0.8 M☉ arise from binary merger remnants, or is it due to other mechanisms such as the initial-final mass relation?
• RQ4 How do the formation times of merger-formed white dwarfs compare to those formed via single stellar evolution, and what is the resulting age bias?
• RQ5 What strategies can mitigate the age inference error introduced by ignoring binary merger origins in white dwarf population studies?

Key findings

• Approximately 10–30% of all observable single white dwarfs within 100 pc are formed through binary mergers, increasing to 30–50% for massive WDs (M > 0.9 M☉).
• On average, white dwarfs formed via binary mergers take 3.1–5 times longer to form than their SSE counterparts, leading to a systematic age underestimation of ~1 Gyr if SSE is assumed.
• The median age of the WD population is underestimated by 85–430 Myr in models assuming constant star formation, with the effect being most pronounced for massive WDs.
• The mass distribution of white dwarfs shows no distinct bump at ~0.8 M☉ due to binary mergers; the distribution declines smoothly with increasing mass, suggesting the excess, if real, has a different origin.
• For massive WDs (M > 0.9 M☉), WD+WD mergers are the dominant channel, contributing up to 45% of single WDs, and the age bias can be a factor of ten or more in extreme cases.
• The effect of binary mergers on age distribution is robust across different observational selection criteria (magnitude limit, distance cut), though the magnitude of the age bias increases slightly with larger survey volumes.