Revised Stellar Parameters for V471 Tau, A Post-common Envelope Binary in the Hyades

V471 Tau is a post-common-envelope binary consisting of an eclipsing DA white dwarf and a K-type main-sequence star in the Hyades star cluster. We analyzed publicly available photometry and spectroscopy of V471 Tau to revise the stellar and orbital parameters of the system. We used archival K2 photometry, archival Hubble Space Telescope spectroscopy, and published radial-velocity measurements of the K-type star. Employing Gaussian processes to fit for rotational modulation of the system flux by the main-sequence star, we recovered the transits of the white dwarf in front of the main-sequence star for the first time. The transits are shallower than would be expected from purely geometric occultations owing to gravitational microlensing during transit, which places an additional constraint on the white-dwarf mass. Our revised mass and radius for the main-sequence star is consistent with single-star evolutionary models given the age and metallicity of the Hyades. However, as noted previously in the literature, the white dwarf is too massive and too hot to be the result of single-star evolution given the age of the Hyades, and may be the product of a merger scenario. We independently estimate the conditions of the system at the time of common envelope that would result in the measured orbital parameters today.

Simulating the Outcome of a Binary Neutron Star Merger in a Common Envelope Jets Supernova

We simulate the influence of the energy that the merger process of two neutron stars (NSs) releases inside a red supergiant (RSG) star on the RSG envelope inner to the merger location. In the triple-star common envelope evolution (CEE) that we consider, a tight binary system of two NSs spiraling in inside an RSG envelope and because of mass accretion and dynamical friction, the two NSs merge. We deposit merger-explosion energies of 3 × 1050 and 1051 erg at distances of 25 and 50 R ⊙ from the center of the RSG, and with the three-dimensional hydrodynamical code FLASH we follow the evolution of the RSG envelope in inner regions. For the parameters we explore, we find that more than 90% of the RSG envelope mass inward of the merger site stays bound to the RSG. NSs that experience CEE are likely to accrete RSG envelope mass through an accretion disk that launches jets. These jets power a luminous transient event, a common envelope jets supernova (CEJSN). The merger process adds to the CEJSN energy. Our finding implies that the interaction of the merger product, a massive NS or a BH, with the envelope can continue to release more energy, both by further inspiraling and by mass accretion by the merger product. Massive RSG envelopes can force the merger product to spiral into the core of the RSG, leading to an even more energetic CEJSN.

Binary Black Hole Formation with Detailed Modeling: Stable Mass Transfer Leads to Lower Merger Rates

Rapid binary population synthesis codes are often used to investigate the evolution of compact-object binaries. They typically rely on analytical fits of single-star evolutionary tracks and parameterized models for interactive phases of evolution (e.g., mass transfer on a thermal timescale, determination of dynamical instability, and common envelope) that are crucial to predict the fate of binaries. These processes can be more carefully implemented in stellar structure and evolution codes such as MESA. To assess the impact of such improvements, we compare binary black hole mergers as predicted in models with the rapid binary population synthesis code COSMIC to models ran with MESA simulations through mass transfer and common-envelope treatment. We find that results significantly differ in terms of formation paths, the orbital periods and mass ratios of merging binary black holes, and consequently merger rates. While common-envelope evolution is the dominant formation channel in COSMIC, stable mass transfer dominates in our MESA models. Depending upon the black hole donor mass, and mass-transfer and common-envelope physics, at subsolar metallicity, COSMIC overproduces the number of binary black hole mergers by factors of 2–35 with a significant fraction of them having merger times orders of magnitude shorter than the binary black holes formed when using detailed MESA models. Therefore we find that some binary black hole merger rate predictions from rapid population syntheses of isolated binaries may be overestimated by factors of ∼ 5–500. We conclude that the interpretation of gravitational-wave observations requires the use of detailed treatment of these interactive binary phases.

Prospects for Detecting Exoplanets around Double White Dwarfs with LISA and Taiji

Recently, Tamanini & Danielski discussed the possibility of detecting circumbinary exoplanets (CBPs) orbiting double white dwarfs (DWDs) with the Laser Interferometer Space Antenna (LISA). Extending their methods and criteria, we discuss the prospects for detecting exoplanets around DWDs not only by LISA, but also by Taiji, a Chinese space-borne gravitational-wave (GW) mission that has slightly better sensitivity at low frequencies. We first explore how different binary masses and mass ratios affect the abilities of LISA and Taiji to detect CBPs. Second, for certain known detached DWDs with high signal-to-noise ratios, we quantify the possibility of CBP detections around them. Third, based on the DWD population obtained from the Mock LISA Data Challenge, we present basic assessments of the CBP detections in our Galaxy during a 4 yr mission time for LISA and Taiji. We discuss the constraints on the detectable zone of each system, as well as the distributions of the inner/outer edge of the detectable zone. With the DWD population, we further inject two different planet distributions with an occurrence rate of 50% and constrain the total detection rates. We briefly discuss the prospects for detecting habitable CBPs around DWDs with a simplified model. These results can provide helpful inputs for upcoming exoplanetary projects and help analyze planetary systems after the common envelope phase.

Simulating the Negative Jet Feedback Mechanism in Common Envelope Jet Supernovae

We use the stellar evolution code MESA to study the negative jet feedback mechanism in common envelope jet supernovae (CEJSNe), in which a neutron star (NS) launches jets in the envelope of a red supergiant (RSG). We find that the feedback reduces the mass accretion rate to be χ j ≃ 0.04–0.3 times the mass accretion rate without the operation of jets. We mimic the effect of the jets on the RSG envelope by depositing the energy that the jets carry into the envelope zones outside the NS orbit. The energy deposition inflates the envelope, therefore reducing the density in the NS vicinity, which in turn reduces the mass accretion rate in a negative feedback cycle. In calculating the above values for the negative jet feedback coefficient (the further reduction in the accretion rate) χ j, we adopt the canonical ratio of jet power to actual accretion power of 0.1, and the results of numerical simulations that show the actual mass accretion rate to be a fraction of 0.1–0.5 of the Bondi–Hoyle–Lyttleton mass accretion rate.

Unexpected Short-period Variability in Dwarf Carbon Stars from the Zwicky Transient Facility

Dwarf carbon (dC) stars, main-sequence stars showing carbon molecular bands, are enriched by mass transfer from a previous asymptotic-giant-branch (AGB) companion, which has since evolved to a white dwarf. While previous studies have found radial-velocity variations for large samples of dCs, there are still relatively few dC orbital periods in the literature and no dC eclipsing binaries have yet been found. Here, we analyze photometric light curves from DR5 of the Zwicky Transient Facility for a sample of 944 dC stars. From these light curves, we identify 34 periodically variable dC stars. Remarkably, of the periodic dCs, 82% have periods less than two days. We also provide spectroscopic follow-up for four of these periodic systems, measuring radial velocity variations in three of them. Short-period dCs are almost certainly post-common-envelope binary systems, because the periodicity is most likely related to the orbital period, with tidally locked rotation and photometric modulation on the dC either from spots or from ellipsoidal variations. We discuss evolutionary scenarios that these binaries may have taken to accrete sufficient C-rich material while avoiding truncation of the thermally pulsing AGB phase needed to provide such material in the first place. We compare these dCs to common-envelope models to show that dC stars probably cannot accrete enough C-rich material during the common-envelope phase, suggesting another mechanism like wind-Roche lobe overflow is necessary. The periodic dCs in this paper represent a prime sample for spectroscopic follow-up and for comparison to future models of wind-Roche lobe overflow mass transfer.