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The formation pathways for gravitational-wave merger sources are predicted to include common envelope (CE) evolution. Observations of high-mass post-common envelope binaries suggest that energy transfer to the envelope during the CE phase must be highly efficient. In contrast, observations of low-mass post-CE binaries indicate that energy transfer during the CE phase must be highly inefficient. Convection, a process present in low-mass and high-mass stars naturally explains this dichotomy. Using observations of Wolf–Rayet binaries, we study the effects of convection and radiative losses on the predicted final separations of high-mass common envelopes. Despite robust convection in massive stars, the effect is minimal as the orbit decays well before convection can transport the liberated orbital energy to the surface. In low-mass systems, convective transport occurs faster then the orbit decays, allowing the system to radiatively cool, thereby lowering the efficiency. The inclusion of convection reproduces observations of low-mass and high-mass binaries and remains a necessary ingredient for determining outcomes of common envelopes. [ABSTRACT FROM AUTHOR]

A significant fraction of isolated white dwarfs host magnetic fields in excess of a MegaGauss. Observations suggest that these fields originate in interacting binary systems where the companion is destroyed thus leaving a singular, highly magnetized white dwarf. In post-main-sequence evolution, radial expansion of the parent star may cause orbiting companions to become engulfed. During the common envelope phase, as the orbital separation rapidly decreases, low-mass companions will tidally disrupt as they approach the giant's core. We hydrodynamically simulate the tidal disruption of planets and brown dwarfs, and the subsequent accretion disc formation, in the interior of an asymptotic giant branch star. Compared to previous steady-state simulations, the resultant discs form with approximately the same mass fraction as estimated but have not yet reached steady state and are morphologically more extended in height and radius. The long-term evolution of the disc and the magnetic fields generated therein require future study. [ABSTRACT FROM AUTHOR]

The formation channels and predicted populations of double white dwarfs (DWDs) are important because a subset will evolve to be gravitational-wave sources and/or progenitors of Type Ia supernovae. Given the observed population of short-period DWDs, we calculate the outcomes of common envelope (CE) evolution when convective effects are included. For each observed white dwarf (WD) in a DWD system, we identify all progenitor stars with an equivalent proto-WD core mass from a comprehensive suite of stellar evolution models. With the second observed WD as the companion, we calculate the conditions under which convection can accommodate the energy released as the orbit decays, including (if necessary) how much the envelope must spin-up during the CE phase. The predicted post-CE final separations closely track the observed DWD orbital parameter space, further strengthening the view that convection is a key ingredient in CE evolution. [ABSTRACT FROM AUTHOR]

Volume complete sky surveys provide evidence for a binary origin for the formation of isolated white dwarfs with magnetic fields in excess of a MegaGauss. Interestingly, not a single high-field magnetic white dwarf has been found in a detached system, suggesting that if the progenitors are indeed binaries, the companion must be removed or merge during formation. An origin scenario consistent with observations involves the engulfment, inspiral, and subsequent tidal disruption of a low-mass companion in the interior of a giant star during a common envelope phase. Material from the shredded companion forms a cold accretion disc embedded in the hot ambient around the proto-white dwarf. Entrainment of hot material may evaporate the disc before it can sufficiently amplify the magnetic field, which typically requires at least a few orbits of the disc. Using three-dimensional hydrodynamic simulations of accretion discs with masses between 1 and 10 times the mass of Jupiter inside the core of an Asymptotic Giant Branch star, we find that the discs survive for at least 10 orbits (and likely for 100 orbits), sufficient for strong magnetic fields to develop. [ABSTRACT FROM AUTHOR]

A widely used method for parametrizing the outcomes of common envelopes (CEs) involves defining an ejection efficiency, |$\bar{\alpha }_{\mathrm{eff}}$|⁠, that represents the fraction of orbital energy used to unbind the envelope as the orbit decays. Given |$\bar{\alpha }_{\mathrm{eff}}$|⁠, a prediction for the post-CE orbital separation is possible with knowledge of the energy required to unbind the primary's envelope from its core. Unfortunately, placing observational constraints on |$\bar{\alpha }_{\mathrm{eff}}$| is challenging as it requires knowledge of the primary's structure at the onset of the CE phase. Numerical simulations have also had difficulties reproducing post-CE orbital configurations as they leave extended, but still bound, envelopes. Using detailed stellar interior profiles, we calculate |$\bar{\alpha }_{\mathrm{eff}}$| values for a matrix of primary-companion mass pairs when the primary is at maximal extent in its evolution. We find that the ejection efficiency is most sensitive to the properties of the surface-contact convective region (SCCR). In this region, the convective transport time-scales are often short compared to orbital decay time-scales, thereby allowing the star to effectively radiate orbital energy and thus lower |$\bar{\alpha }_{\mathrm{eff}}$|⁠. The inclusion of convection in numerical simulations of CEs may aid ejection without the need for additional energy sources as the orbit must shrink substantially further before the requisite energy can be tapped to drive ejection. Additionally, convection leads to predicted post-CE orbital periods of less than a day in many cases, an observational result that has been difficult to reproduce in population studies where |$\bar{\alpha }_{\mathrm{eff}}$| is taken to be constant. Finally, we provide a simple method to calculate |$\bar{\alpha }_{\mathrm{eff}}$| if the properties of the SCCR are known. [ABSTRACT FROM AUTHOR]

The ultimate fates of binary companions to stars (including whether the companion survives and the final orbit of the binary) are of interest in light of an increasing number of recently discovered, low-mass companions to white dwarfs (WDs). In this Letter, we study the evolution of a two-body system wherein the orbit adjusts due to structural changes in the primary, dissipation of orbital energy via tides, and mass-loss during the giant phases; previous studies have not incorporated changes in the primary's spin. For companions ranging from Jupiter's mass to ∼0.3 M⊙ and primaries ranging from 1 to 3 M⊙, we determine the minimum initial semimajor axis required for the companion to avoid engulfment by the primary during post-main-sequence evolution, and highlight the implications for the ultimate survival of the known exoplanets. We present regions in secondary mass and orbital period space where an engulfed companion might be expected to survive the common envelope phase (CEP), and compare with known M dwarf+WD short-period binaries. Finally, we note that engulfed Earth-like planets cannot survive a CEP. Detection of a first-generation terrestrial planet in the WD habitable zone requires scattering from a several au orbit to a high-eccentricity orbit (with a periastron of ∼R⊙) from which it is damped into a circular orbit via tidal friction, possibly rendering it an uninhabitable, charred ember. [ABSTRACT FROM AUTHOR]

ABSTRACT The potential existence of a giant planet orbiting within a few au of a stellar remnant has profound implications for both the survival and possible regeneration of planets during post-main-sequence stellar evolution. This paper reports Hubble Space Telescope Fine Guidance Sensor and US Naval Observatory relative astrometry of GD 66, a white dwarf thought to harbour a giant planet between 2 and 3 au based on stellar pulsation arrival times. Combined with existing infrared data, the precision measurements here rule out all stellar-mass and brown dwarf companions, implying that only a planet remains plausible, if orbital motion is indeed the cause of the variations in pulsation timing. [ABSTRACT FROM AUTHOR]

ABSTRACT We present results from a suite of axisymmetric, core-collapse supernova simulations in which hydrodynamic recoil from an asymmetric explosion produces large protoneutron star (PNS) velocities. We use the adaptive mesh refinement code castro to self-consistently follow core collapse, the formation of the PNS and its subsequent acceleration. We obtain recoil velocities of up to 620 km s−1 at ∼1 s after bounce. These velocities are consistent with the observed distribution of pulsar kicks and with PNS velocities obtained in other theoretical calculations. Our PNSs are still accelerating at several hundred km s−1 at the end of our calculations, suggesting that even the highest velocity pulsars may be explained by hydrodynamic recoil in generic, core-collapse supernovae. [ABSTRACT FROM AUTHOR]

We are investigating the formation and evolution of dust around the hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars. We aim to determine the connection between the probable merger past of these stars and their current dust-production activities. We carried out high angular resolution interferometric observations of three RCB stars, namely RY Sgr, V CrA and V854 Cen, with the mid-infrared interferometer (MIDI) on the Very Large Telescope Interferometer (VLTI), using two telescope pairs. The baselines ranged from 30 to 60 m, allowing us to probe the dusty environment at very small spatial scales (50 mas or 400). The observations of the RCB star dust environments were interpreted using both geometrical models and one-dimensional radiative transfer codes. From our analysis, we find that asymmetric circumstellar material is apparent in RY Sgr, may also exist in V CrA and is possible for V854 Cen. Overall, we find that our observations are consistent with dust forming in clumps ejected randomly around the RCB star so that over time they create a spherically symmetric distribution of dust. However, we conclude that the determination of whether there is a preferred plane of dust ejection must wait until a time series of observations are obtained. [ABSTRACT FROM AUTHOR]

ABSTRACT The presence of a close, low-mass companion is thought to play a substantial and perhaps necessary role in shaping post-asymptotic giant branch and planetary nebula outflows. During post-main-sequence evolution, radial expansion of the primary star, accompanied by intense winds, can significantly alter the binary orbit via tidal dissipation and mass-loss. To investigate this, we couple stellar evolution models (from the zero-age main sequence through the end of the post-MS) to a tidal evolution code. The binary's fate is determined by the initial masses of the primary and the companion, the initial orbit (taken to be circular), and the Reimers mass-loss parameter. For a range of these parameters, we determine whether the orbit expands due to mass-loss or decays due to tidal torques. Where a common envelope (CE) phase ensues, we estimate the final orbital separation based on the energy required to unbind the envelope. These calculations predict period gaps for planetary and brown dwarf companions to white dwarfs. The upper end of the gap is the shortest period at which a CE phase is avoided. The lower end is the longest period at which companions survive their CE phase. For binary systems with 1 M⊙ progenitors, we predict no Jupiter-mass companions with periods ≲270 d. Once engulfed, Jupiter-mass companions do not survive a CE phase. For binary systems consisting of a 1 M⊙ progenitor with a companion 10 times the mass of Jupiter, we predict a period gap between ∼0.1 and ∼380 d. These results are consistent with both the detection of a ∼50 MJ brown dwarf in a ∼0.003 au (∼0.08 d) orbit around the white dwarf WD 0137−349 and the tentative detection of a ∼2 MJ planet in a ≳2.7 au (≳4 yr) orbit around the white dwarf GD66. [ABSTRACT FROM AUTHOR]

HD 179821 is an evolved star of unknown progenitor mass range (either post-asymptotic giant branch or post-red supergiant) exhibiting a double-peaked spectral energy distribution (SED) with a sharp rise from ∼8 to . Such features have been associated with ejected dust shells or inwardly truncated circumstellar discs. In order to compare SEDs from both systems, we employ a spherically symmetric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 μm, we find that both models produce equivalent fits to the data. However, longward of 40 μm, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and observations. For HD 179821, our best fit consists of a central source characterized by and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colours so that they resemble a photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modelling. [ABSTRACT FROM AUTHOR]

The origin, evolution and role of magnetic fields in the production and shaping of proto-planetary nebulae (PPNe) and planetary nebulae (PNe) are a subject of active research. Most PNe and PPNe are axisymmetric with many exhibiting highly collimated outflows; however, it is important to understand whether such structures can be generated by isolated stars or require the presence of a binary companion. Towards this end, we study a dynamical, large-scale α−Ω interface dynamo operating in a 3.0 M⊙ Asymptotic Giant Branch (AGB) star in both an isolated setting and a setting in which a low-mass companion is embedded inside the envelope. The back reaction of the fields on the shear is included and differential rotation and rotation deplete via turbulent dissipation and Poynting flux. For the isolated star, the shear must be resupplied in order to sufficiently sustain the dynamo. Furthermore, we investigate the energy requirements that convection must satisfy to accomplish this by analogy to the Sun. For the common envelope case, a robust dynamo results, unbinding the envelope under a range of conditions. Two qualitatively different types of explosion may arise: (i) magnetically induced, possibly resulting in collimated bipolar outflows and (ii) thermally induced from turbulent dissipation, possibly resulting in quasi-spherical outflows. A range of models is presented for a variety of companion masses. [ABSTRACT FROM AUTHOR]

A significant fraction of planetary nebulae (PNe) and protoplanetary nebulae (PPNe) exhibit aspherical, axisymmetric structures, many of which are highly collimated. The origin of these structures is not entirely understood, however, recent evidence suggests that many observed PNe harbour binary systems, which may play a role in their shaping. In an effort to understand how binaries may produce such asymmetries, we study the effect of low-mass companions (planets, brown dwarfs and low-mass main-sequence stars) embedded into the envelope of a star during three epochs of its evolution [red giant branch, asymptotic giant branch (AGB), interpulse AGB]. We find that common envelope evolution can lead to three qualitatively different consequences: (i) direct ejection of envelope material resulting in a predominately equatorial outflow, (ii) spin-up of the envelope resulting in the possibility of powering an explosive dynamo-driven jet and (iii) tidal shredding of the companion into a disc which facilitates a disc-driven jet. We study how these features depend on the secondary's mass and discuss observational consequences. [ABSTRACT FROM AUTHOR]

We perform numerical simulations of a stellar galactic disc with initial conditions chosen to represent an unrelaxed population which might have been left following a merger. Stars are unevenly distributed in radial action angle, though the disc is axisymmetric. The velocity distribution in the simulated solar neighbourhood exhibits waves travelling in the direction of positive v, where u, v are the radial and tangential velocity components. As the system relaxes and structure wraps in phase space, the features seen in the u– v plane move closer together. We show that these results can be obtained also by a semi-analytical method. We propose that this model could provide an explanation for the high-velocity streams seen in the solar neighbourhood at approximate v in km s−1, of −60 (HR 1614), −80, −100 (Arcturus) and −160. In addition, we predict four new features at and 60 km s−1. By matching the number and positions of the observed streams, we estimate that the Milky Way disc was strongly perturbed ∼1.9 Gyr ago. This event could have been associated with Galactic bar formation. [ABSTRACT FROM AUTHOR]