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Register a new userEstimating the outcomes of common envelope evolution in triple stellar systems
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We present a new model describing the evolution of triple stars which undergo common envelope evolution, using a combination of analytic and numerical techniques. The early stages of evolution are driven by dynamical friction with the envelope, which causes the outer triple orbit to shrink faster than the inner binary. In most cases, this leads to a chaotic dynamical interaction between the three stars, culminating in the ejection of one of the stars from the triple. This ejection and resulting recoil on the remnant binary are sufficient to eject all three stars from the envelope, which expands and dissipates after the stars have escaped. These results have implications for the properties of post-common envelope triples: they may only exist in cases where the envelope was ejected before the onset of dynamical instability, the likelihood of which depends on the initial binary separation and the envelope structure. In cases where the triple becomes dynamically unstable, the triple does not survive and the envelope dissipates without forming a planetary nebula.
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Common envelope (CE) is an important phase in the evolution of interacting evolved binary systems. The interaction of the binary components during the CE evolution (CEE) stage gives rise to orbital inspiral and the formation of a short-period binary or a merger, on the expense of extending and/or ejecting the envelope. CEE is not well understood, as hydrodynamical simulations show that only a fraction of the CE-mass is ejected during the dynamical inspiral, in contrast with observations of post-CE binaries. Different CE models suggest different timescales are involved in the CE-ejection, and hence a measurement of the CE-ejection timescale could provide direct constraints on the CEE-process. Here we propose a novel method for constraining the mass-loss timescale of the CE, using post-CE binaries which are part of wide-orbit triple systems. The orbit/existence of a third companion constrains the CE mass-loss timescale, since rapid CE mass-loss may disrupt the triple system, while slower CE mass-loss may change the orbit of the third companion without disrupting it. As first test-cases we examine two observed post-CE binaries in wide triples, Wolf-1130 and GD-319. We follow their evolution due to mass-loss using analytic and numerical tools, and consider different mass-loss functions. We calculate a wide grid of binary parameters and mass-loss timescales in order to determine the most probable mass-loss timescale leading to the observed properties of the systems. We find that mass-loss timescales of the order of $10^{3}-10^{5}{rm yr}$ are the most likely to explain these systems. Such long timescales are in tension with most of the CE mass-loss models, which predict shorter, dynamical timescales, but are potentially consistent with the longer timescales expected from the dust-driven winds model for CE ejection.
I study a triple star common envelope evolution (CEE) of a tight binary system that is spiraling-in inside a giant envelope and launches jets that spin-up the envelope with an angular momentum component perpendicular to the orbital angular momentum of the triple star system. This occurs when the orbital plane of the tight binary system and that of the triple star system are inclined to each other, so the jets are not along the triple star orbital angular momentum. The merger of the tight binary stars also tilts the envelope spin direction. If the giant is a red supergiant (RSG) star that later collapses to form a black hole (BH) the BH final spin is misaligned with the orbital angular momentum. Therefore, CEE of neutron star (NS) or BH tight binaries with each other or with one main sequence star (MSS) inside the envelope of an RSG, where the jets power a common envelope jets supernova (CEJSN) event, might end with a NS/BH - NS/BH close binary system with spin-orbit misalignment. Such binaries can later merge to be gravitational waves sources. I list five triple star scenarios that might lead to spin-orbit misalignments of NS/BH - NS/BH binary systems, two of which predict that the two spins be parallel to each other. In the case of a tight binary system of two MSSs inside an asymptotic giant branch star the outcome is an additional non-spherical component to the mass loss with the formation of a messy planetary nebula.
Binary neutron stars have been observed as millisecond pulsars, gravitational-wave sources, and as the progenitors of short gamma-ray bursts and kilonovae. Massive stellar binaries that evolve into merging double neutron stars are believed to experience a common-envelope episode. During this episode, the envelope of a giant star engulfs the whole binary. The energy transferred from the orbit to the envelope by drag forces or from other energy sources can eject the envelope from the binary system, leading to a stripped short-period binary. In this paper, we use one-dimensional single stellar evolution to explore the final stages of the common-envelope phase in progenitors of neutron star binaries. We consider an instantaneously stripped donor star as a proxy for the common-envelope phase and study the stars subsequent radial evolution. We determine a range of stripping boundaries which allow the star to avoid significant rapid re-expansion and which thus represent plausible boundaries for the termination of the common-envelope episode. We find that these boundaries lie above the maximum compression point, a commonly used location of the core/envelope boundary. We conclude that stars may retain fractions of a solar mass of hydrogen-rich material even after the common-envelope episode. We show that, under the standard energy formalism, all of our models require additional energy sources in order to successfully eject the common envelope.
Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the {alpha}-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)- main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions.
The discovery via gravitational waves of binary black hole systems with total masses greater than $60M_odot$ has raised interesting questions for stellar evolution theory. Among the most promising formation channels for these systems is one involving a common envelope binary containing a low metallicity, core helium burning star with mass $sim 80-90M_odot$ and a black hole with mass $sim 30-40M_odot$. For this channel to be viable, the common envelope binary must eject more than half the giant stars mass and reduce its orbital separation by as much as a factor of 80. We discuss issues faced in numerically simulating the common envelope evolution of such systems and present a 3D AMR simulation of the dynamical inspiral of a low-metallicity red supergiant with a massive black hole companion.
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