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A large population of intermediate-separation binaries, consisting of a main-sequence (MS) star and a white dwarf (WD), recently emerged from Gaia’s third data release (DR3), posing challenges to current models of binary evolution. Here we examine the s -process element abundances in these systems using data from GALAH DR3. Following refined sample analysis with parameter estimates based on GALAH spectra, we find a distinct domain where enhanced s -process elemental abundances depend on both the WD mass and metallicity, consistent with parameter spaces identified in previous asymptotic giant branch (AGB) nucleosynthesis studies having higher s -process yields. Notably, these enhanced abundances show no correlation with the systems’ orbital parameters, supporting a history of accretion in intermediate-separation MS+WD systems. Consequently, our results form direct observational evidence of a connection between AGB masses and s -process yields. We conclude by showing that the GALAH DR3 survey includes numerous Ba dwarf stars, within and beyond the mass range covered in our current sample, which can further elucidate s -process element distributions in MS+WD binaries.

The third data release of Gaia introduced a large catalog of astrometric binaries, out of which about 3200 are likely main-sequence stars with a white dwarf (WD) companion. These binaries are typically found with orbital separations of ∼1 au, a separation range that was largely unexplored due to observational challenges. Such systems are likely to have undergone a phase of stable mass transfer while the WD progenitor was on the asymptotic giant branch. Here we study the WD mass distribution of a volume-complete sample of binaries with K/M dwarf primaries and orbital separations of ∼1 au. We find that the number of massive WDs relative to the total number of WDs in these systems is smaller by an order of magnitude compared to their occurrence among single WDs in the field. One possible reason can be an implicit selection of the WD mass range if these are indeed post-stable-mass-transfer systems. Another reason can be the lack of merger products in our sample compared to the field, due to the relatively tight orbital separations of these systems. In addition, we find that about 14% of these systems have distant tertiary companions within 1 pc.

We present a detailed model atmosphere analysis of massive white dwarfs with M > 0.9 M _⊙ and T _eff ≥ 11,000 K in the Montreal White Dwarf Database 100 pc sample and the Pan-STARRS footprint. We obtained follow-up optical spectroscopy of 109 objects with no previous spectral classification in the literature. Our spectroscopic follow-up is now complete for all 204 objects in the sample. We find 118 normal DA white dwarfs, including 45 massive DAs near the ZZ Ceti instability strip. There are no normal massive DBs: the six DBs in the sample are strongly magnetic and/or rapidly rotating. There are 20 massive DQ white dwarfs in our sample, and all are found in the crystallization sequence. In addition, 66 targets are magnetic (32% of the sample). We use magnetic white dwarf atmosphere models to constrain the field strength and geometry using offset dipole models. We also use magnetism, kinematics, and rotation measurements to constrain the fraction of merger remnant candidates among this population. The merger fraction of this sample increases from 25% for 0.9–1 M _⊙ white dwarfs to 49% for 1.2–1.3 M _⊙ . However, this fraction is as high as ${78}_{-7}^{+4}$ % for 1.1–1.2 M _⊙ white dwarfs. Previous works have demonstrated that 5%–9% of high-mass white dwarfs stop cooling for ∼8 Gyr due to the ^22 Ne distillation process, which leads to an overdensity of Q-branch stars in the solar neighborhood. We demonstrate that the overabundance of the merger remnant candidates in our sample is likely due to the same process.

Calcium-rich explosions are very faint ( M _B ∼ −15.5), type I supernovae (SNe) showing strong Ca lines, mostly observed in old stellar environments. Several models for such SNe have been explored and debated, but none were able to reproduce consistently the observed properties of Ca-rich SNe, nor their rates and host-galaxy distributions. Here we show that the disruptions of low-mass carbon–oxygen (CO) white dwarfs (WDs) by hybrid helium–CO (HeCO) WDs during their merger could explain the origin and properties of such SNe. We make use of detailed multidimensional hydrodynamical-thermonuclear ( FLASH ) simulations to characterize such explosions. We find that the accretion of CO material onto an HeCO-WD heats its He shell and eventually leads to its “weak” detonation and ejection and the production of a sub-energetic ∼10 ^49 erg Ca-rich SN, while leaving the CO core of the HeCO-WD intact as a hot remnant WD, possibly giving rise to X-ray emission as it cools down. We model the detailed light curves and spectra of such explosions and find excellent agreement with the observations of Ia/c Ca-rich, and potentially Ib Ca-rich, SNe. We thereby provide a viable, consistent model for the origins of Ca-rich SNe. These findings can shed new light on the role of Ca-rich SNe in the chemical evolution of galaxies and the intracluster medium, and their contribution to the observed 511 Kev signal in the galaxy originating from positrons produced from ^44 Ti decay. Finally, the origins of such SNe points to the key role of HeCO-WDs as SN progenitors and their potential role as progenitors of other thermonuclear SNe including normal Ia.

Double white dwarf (double-WD) binaries may merge within a Hubble time and produce high-mass WDs. Compared to other high-mass WDs, the double-WD merger products have higher velocity dispersion because they are older. With the power of Gaia data, we show strong evidence for double-WD merger products among high-mass WDs by analyzing the transverse-velocity distribution of more than 1000 high-mass WDs (0.8–1.3 M⊙). We estimate that the fraction of double-WD merger products in our sample is about 20%. We also obtain a precise double-WD merger rate and its mass dependence. Our merger rate estimates are close to binary population synthesis results and support the idea that double-WD mergers may contribute to a significant fraction of type Ia supernovae. [ABSTRACT FROM AUTHOR]

We consider the formation of binary black hole (BH) mergers through the evolution of field massive triple stars. In this scenario, favorable conditions for the inspiral of a BH binary are initiated by its gravitational interaction with a distant companion, rather than by a common-envelope phase invoked in standard binary evolution models. We use a code that follows self-consistently the evolution of massive triple stars, combining the secular triple dynamics (Lidov–Kozai cycles) with stellar evolution. After a BH triple is formed, its dynamical evolution is computed using either the orbit-averaged equations of motion, or a high-precision direct integrator for triples with weaker hierarchies for which the secular perturbation theory breaks down. Most BH mergers in our models are produced in the latter non-secular dynamical regime. We derive the properties of the merging binaries and compute a BH merger rate in the range (0.3–1.3) Gpc−3 yr−1, or up to ≈2.5 Gpc−3 yr−1 if the BH orbital planes have initially random orientation. Finally, we show that BH mergers from the triple channel have significantly higher eccentricities than those formed through the evolution of massive binaries or in dense star clusters. Measured eccentricities could therefore be used to uniquely identify binary mergers formed through the evolution of triple stars. While our results suggest up to ≈10 detections per year with Advanced-LIGO, the high eccentricities could render the merging binaries harder to detect with planned space based interferometers such as LISA. [ABSTRACT FROM AUTHOR]

Binary mass transfer (MT) is at the forefront of some of the most exciting puzzles of modern astrophysics, including SNe Ia, gamma-ray bursts, and the formation of most observed exotic stellar populations. Typically, the evolution is assumed to proceed in isolation, even in dense stellar environments such as star clusters. In this paper, we test the validity of this assumption via the analysis of a large grid of binary evolution models simulated with the SeBa code. For every binary, we calculate analytically the mean time until another single or binary star comes within the mean separation of the mass-transferring binary, and compare this timescale to the mean time for stable MT to occur. We then derive the probability for each respective binary to experience a direct dynamical interruption. The resulting probability distribution can be integrated to give an estimate for the fraction of binaries undergoing MT that are expected to be disrupted as a function of the host cluster properties. We find that for lower-mass clusters ( ), on the order of a few to a few tens of percent of binaries undergoing MT are expected to be interrupted by an interloping single, or more often binary, star, over the course of the cluster lifetime, whereas in more massive globular clusters we expect to be interrupted. Furthermore, using numerical scattering experiments performed with the FEWBODY code, we show that the probability of interruption increases if perturbative fly-bys are considered as well, by a factor ∼2. [ABSTRACT FROM AUTHOR]