Ronald E. Taam

Common envelope evolution: where we stand and how we can move forward

This work aims to present our current best physical understanding of common-envelope evolution (CEE). We highlight areas of consensus and disagreement, and stress ideas which should point the way forward for progress in this important but long-standing and largely unconquered problem. Unusually for CEE-related work, we mostly try to avoid relying on results from population synthesis or observations, in order to avoid potentially being misled by previous misunderstandings. As far as possible we debate all the relevant issues starting from physics alone, all the way from the evolution of the binary system immediately before CEE begins to the processes which might occur just after the ejection of the envelope. In particular, we include extensive discussion about the energy sources and sinks operating in CEE, and hence examine the foundations of the standard energy formalism. Special attention is also given to comparing the results of hydrodynamic simulations from different groups and to discussing the potential effect of initial conditions on the differences in the outcomes. We compare current numerical techniques for the problem of CEE and also whether more appropriate tools could and should be produced (including new formulations of computational hydrodynamics, and attempts to include 3D processes within 1D codes). Finally we explore new ways to link CEE with observations. We compare previous simulations of CEE to the recent outburst from V1309 Sco, and discuss to what extent post-common-envelope binaries and nebulae can provide information, e.g. from binary eccentricities, which is not currently being fully exploited.

Compact Object Modeling with the StarTrack Population Synthesis Code

We present a comprehensive description of the population synthesis code StarTrack. The original code has been significantly modified and updated. Special emphasis is placed here on processes leading to the formation and further evolution of compact objects (white dwarfs, neutron stars, and black holes). Both single and binary star populations are considered. The code now incorporates detailed calculations of all mass transfer phases, a full implementation of orbital evolution due to tides, as well as the most recent estimates of magnetic braking. This updated version of StarTrack can be used for a wide variety of problems, with relevance to observations with many current and planned observatories, e.g., studies of X-ray binaries (Chandra, XMM-Newton), gravitational radiation sources (LIGO, LISA), and gamma-ray burst progenitors (HETE-II, Swift). The code has already been used in studies of Galactic and extragalactic X-ray binary populations, black holes in young star clusters, Type Ia supernova progenitors, and double compact object populations. Here we describe in detail the input physics, we present the code calibration and tests, and we outline our current studies in the context of X-ray binary populations.

Double Core Evolution. X. Through the Envelope Ejection Phase

The evolution of binary systems consisting of an asymptotic giant branch star of mass equal to 3 M☉ or 5 M☉ and a main-sequence star of mass equal to 0.4 M☉ or 0.6 M☉ with orbital periods 200 days has been followed from the onset through the late stages of the common-envelope phase. Using a nested grid technique, the three-dimensional hydrodynamical simulations of an asymptotic giant branch star with radii ~1 AU indicate that a significant fraction of the envelope gas is unbound (~31% and 23% for binaries of 3 M☉ and 0.4 M☉, and 5 M☉ and 0.6 M☉, respectively) by the ends of the simulations and that the efficiency of the mass ejection process ~40%. During an intermediate phase, a differentially rotating structure resembling a thick disk surrounds the remnant binary briefly before energy input from the orbits of the companion and remnant core drive the mass away. While the original volume of the giant is virtually evacuated in the late stages, most of the envelope gas remains marginally bound on the grid. At the ends of our simulations, when the orbital decay timescale exceeds about 5 yr, the giant core and companion orbit one another with a period of ~1 day (2.4 days for a binary involving a more evolved giant), although this is an upper limit to the final orbital period. For a binary of 5 M☉ and 0.4 M☉, the common envelope may not be completely ejected. The results are not found to be sensitive to the degree to which the initial binary system departs from the synchronous state.

Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object. 61 refs.

An AMR Study of the Common Envelope Phase of Binary Evolution

The hydrodynamic evolution of the common-envelope (CE) phase of a low-mass binary composed of a 1.05 M ☉ red giant and a 0.6 M ☉ companion has been followed for five orbits of the system using a high-resolution method in three spatial dimensions. During the rapid inspiral phase, the interaction of the companion with the red giants extended atmosphere causes about 25% of the CE to be ejected from the system, with mass continuing to be lost at the end of the simulation at a rate ~2 M ☉ yr–1. In the process the resulting loss of angular momentum and energy reduces the orbital separation by a factor of seven. After this inspiral phase the eccentricity of the orbit rapidly decreases with time. The gravitational drag dominates hydrodynamic drag at all times in the evolution, and the commonly used Bondi-Hoyle-Lyttleton prescription for estimating the accretion rate onto the companion significantly overestimates the true rate. On scales comparable to the orbital separation, the gas flow in the orbital plane in the vicinity of the two cores is subsonic with the gas nearly corotating with the red giant core and circulating about the red giant companion. On larger scales, 90% of the outflow is contained within 30° of the orbital plane, and the spiral shocks in this material leave an imprint on the density and velocity structure. Of the energy released by the inspiral of the cores, only about 25% goes toward ejection of the envelope.

On the rarity of double black hole binaries: Consequences for gravitational-wave detection

Double black hole binaries are among the most important sources of gravitational radiation for ground-based detectors such as LIGO or VIRGO. Even if formed with lower efficiency than double neutron star binaries, they could dominate the predicted detection rates, since black holes are more massive than neutron stars and therefore could be detected at greater distances. Here we discuss an evolutionary process that could very significantly limit the formation of close double black hole binaries: the vast majority of their potential progenitors undergo a common-envelope (CE) phase while the donor, one of the massive binary components, is evolving through the Hertzsprung gap. Our latest theoretical understanding of the CE process suggests that this will probably lead to a merger, inhibiting double black hole formation. Barring uncertainties in the physics of CE evolution, we use population synthesis calculations and find that the corresponding reduction in the merger rate of double black holes formed in galactic fields is so great (by ~500) that their contribution to inspiral detection rates for ground-based detectors could become relatively small (~1 in 10) compared to double neutron star binaries. A similar process also reduces the merger rates for double neutron stars, by a factor of ~5, eliminating most of the previously predicted ultracompact NS-NS systems. Our predicted detection rates for Advanced LIGO are now much lower for double black holes (~2 yr-1), but are still quite high for double neutron stars (~20 yr-1). If double black holes were found to be dominant in the detected inspiral signals, this could indicate that they mainly originate from dense star clusters (not included here) or that our theoretical understanding of the CE phase requires significant revision.

MAGNETIC BRAKING REVISITED

We present a description for the angular momentum loss rate due to magnetic braking for late-type stars, taking into account recent observational data on the relationship between stellar activity and rotation. The analysis is based on an idealized two-component coronal model subject to constraints imposed on the variation of the coronal gas density, with a rotation period inferred from the observed variation of X-ray luminosity LX with rotation rate Ω (LX ∝ Ω2) for single rotating dwarfs. An application of the model to high rotation rates leads to a gradual turnover of the X-ray luminosity that is similar to the saturation recently observed in rapidly rotating dwarfs. The resulting angular momentum loss rate, , depends on Ω in the form ∝ Ωβ, where β ~ 3 for slow rotators and ~1.3 for fast rotators. The relation at high rotation rates significantly differs from the power-law exponent for slowly rotating stars, depressing the angular momentum loss rate without necessarily requiring the saturation of the magnetic field. The application of this model to the evolution of cataclysmic variable binary systems leads to mass transfer rates that are more in accordance with those observed compared to rates based on either a Skumanich law or an empirical law based on β = 1.

Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries

The evolution of binaries consisting of evolved main-sequence stars (1 < Md/M☉ < 3.5) with white dwarf companions (0.7 < Mwd/M☉ < 1.2) is investigated through the thermal mass-transfer phase. Taking into account the stabilizing effect of a strong, optically thick wind from the accreting white dwarf surface, we have explored the formation of several evolutionary groups of systems for progenitors with initial orbital periods of 1 and 2 days. The numerical results show that CO white dwarfs can accrete sufficient mass to evolve to a Type Ia supernova, and ONeMg white dwarfs can be built up to undergo accretion-induced collapse for donors more massive than about 2 M☉. For donors less massive than ~2 M☉, the system can evolve to form an He and CO or ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1 M☉) in systems characterized by 1.6 Md/M☉ 1.9 and 0.8 Mwd/M☉ 1 such that sub-Chandrasekhar-mass models for Type Ia supernovae, involving off-center helium ignition, are possible for progenitor systems evolving via the Case A mass-transfer phase. For systems characterized by mass ratios 3, the system likely merges as a result of the occurrence of a delayed dynamical mass-transfer instability. We develop a semianalytical model to delineate these phases that can be easily incorporated in population synthesis studies of these systems.

Further Constraints on Thermal Quiescent X-Ray Emission from SAX J1808.4-3658

We observed SAX J1808.4-3658 (1808), the first accreting millisecond pulsar, in deep quiescence with XMM-Newton and (near simultaneously) Gemini-South. The X-ray spectrum of 1808 is similar to that observed in quiescence in 2001 and 2006, describable by an absorbed power law with photon index 1.74 ± 0.11 and unabsorbed X-ray luminosity LX = 7.9 ± 0.7 × 1031 ergs s–1, for NH = 1.3 × 1021 cm–2. Fitting all the quiescent XMM-Newton X-ray spectra with a power law, we constrain any thermally emitting neutron star (NS) with a hydrogen atmosphere to have a temperature less than 30 eV and L NS (0.01-10 keV) <6.2 × 1030 ergs s–1. A thermal plasma model also gives an acceptable fit to the continuum. Adding an NS component to the plasma model produces less stringent constraints on the NS; a temperature of 36+4 –8 eV and L NS (0.01-10 keV) = 1.3+0.6 –0.8 × 1031 ergs s–1. In the framework of the current theory of NS heating and cooling, the constraints on the thermal luminosity of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these NSs. We compile data from the literature on the mass transfer rates and quiescent thermal flux of the largest possible sample of transient NS low-mass X-ray binaries. We identify a thermal component in the quiescent spectrum of the accreting millisecond pulsar IGR J00291+5934, which is consistent with the standard cooling model. The contrast between the cooling rates of IGR J00291+5934 and 1808 suggests that 1808 may have a significantly larger mass. This can be interpreted as arising from differences in the binary evolution history or initial NS mass in these otherwise similar systems.

THE EXISTENCE OF INNER COOL DISKS IN THE LOW/HARD STATE OF ACCRETING BLACK HOLES

The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low/hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low/hard state of black hole transient systems for a range of luminosities that depends on the magnitude of the viscosity parameter. For α ~ 0.1-0.4, an inner disk can exist for luminosities in the range ~(0.001-0.02)LEdd. The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and SWIFT J1753.5-0127 in their low/hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low-luminosity active galactic nuclei.