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The Evolution of Compact Binary Star Systems

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The Evolution of Compact Binary Star Systems The Evolution of Compact Binary Star Systems Konstantin A. Postnov Moscow M.V.Lomonosov State University Sternberg Astronomical Institute 13 Universitetskij Pr. 119992 Moscow Russia email: [email protected] http://www.sai.msu.ru Lev R. Yungelson Institute of Astronomy of Russian Academy of Sciences 48 Pyatnitskaya Str. 119017 Moscow Russia email: [email protected] http://www.inasan.ru Abstract We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Mergings of compact binary stars are expected to be the most important sources for the forthcoming gravitational-wave (GW) as- tronomy. In the first part of the review, we discuss observational manifestations of close binary stars with NS and/or black components and their merger rate, crucial points in the formation and evolution of compact stars in binary systems, including the treatment of the natal kicks which NSs and BHs acquire during the core collapse of massive stars and the common envelope phase of binary evolution, which are most relevant to the merging rates of NS-NS, NS-BH and BH-BH binaries. The second part of the review is devoted mainly to formation and evolution of binary WDs and their observational manifestations, including their role as progenitors of cosmologically important thermonuclear SN Ia. We also consider AM CVn-stars which are thought to be the best verification binary GW sources for future low-frequency GW space interferometers. Update (11 October 2013) arXiv:1403.4754v2 [astro-ph.HE] 21 Mar 2014 Major revision, updated and expanded. References are updated by 2007 { early-2014 publications, some outdated items removed. The reference list contains now 881 items instead of 475. The new version of the review has 39 figures. In Section 1, stellar evolution is outlined in more detail, special attention is paid to electron- capture supernovae. More attention than before is paid to the uncertainties in the estimates of the ranges of precursors of white dwarfs, neutron stars and black holes and their masses. In Section 2, a subsection (2.3) on black holes in binaries is added and an independent estimate of double black hole/neutron star coalescence rate is given. In Section 3, a subsection on mass transfer between binary components is added, discussion of common envelopes is expanded. 1 In Section 7, the discussion of the scenario for formation of short-period binaries is appended by a more detailed description of \intermediate" stages, like post-common envelope binaries, the discussion of properties of binary WD is expanded, the comparison of results of population synthesis calculations for close WD with observations is updated, thanks to significant increase of the number of observed objects. A much more extended comparison of expectations from scenarios for single- degenerate and double-degenerate SNe Ia (including violent mergers) is presented. In Section 8 all published information on double-degenerates with measured periods and esti- mates of masses of components and some subgiant+[white dwarf] systems is summarised (up to February 2014). In Section 9 more attention than previously is paid to initial stages of mass exchange in the \white dwarf" channel of formation of AM CVn stars and to comparison of \white dwarf" and \helium star" channels. Final stages of evolution of AM CVn stars and SN .Ia are discussed. In Section 10 the GW-signals of detached and interacting double-degenerates are compared in more detail, as well as their detection probabilities with planned \short-arm" (eLISA) interferom- eter. The former section on overlap of EM- and GW-signals from AM CVn stars is deleted because of (temporary?) cancellation of LISA-mission. 2 1 Introduction Close binary stars consisting of two compact stellar remnants { white dwarfs (WDs), neutron stars (NSs) or black holes (BHs) are considered as primary targets of the forthcoming field of gravitational wave (GW) astronomy (see, for a review, [751, 248, 666, 665]), since their orbital evolution is entirely controlled by emission of gravitational waves and leads to ultimate coalescence (merger) and possible explosive disruption of the components. Emission of gravitational waves accompanies the latest stages of evolution of stars and manifests instabilities in relativistic objects [9, 12]. Close compact binaries can thus serve as testbeds for theories of gravity [217]. The double NS(BH) mergers which release ∼ 1052 erg as GWs [111, 112] should be the brightest GW events in the 10 { 1000 Hz frequency band of the existing or future ground-based GW detectors like LIGO [22], VIRGO [5], GEO600 [629], KAGRA(LCGT) [721] (see also [636] for review of the current state of existing and 2nd- and 3rd-generation ground-based detectors). Mergers of double NS(BH) can be accompanied by the release of a huge amount of electromagnetic energy in a burst and manifest themselves as short gamma-ray bursts (GRBs). A lot of observational support to NS-NS/NS-BH mergers as sources of short GRBs have been obtained (see, e.g., studies of short GRB locations in the host galaxies [200, 744] and references therein). As well, relativistic jets, associated with GRB of any nature may be sources of GW in the ground-based detectors range [50]. Double WDs, especially interacting binary WDs observed as AM CVn-stars and ultracompact X-ray binaries (UCXB), are potential GW sources within the frequency band (10{4 { 1) Hz of the space GW interferometers like currently cancelled LISA [183] 1, NGO (eLISA) [9, 10], DEGIGO [851] and other proposed or planned low-frequency GW detectors [120, 40]. At the moment, eLISA is selected for the third Large-class mission in ESAs Cosmic Vision science program (L3). Its first step should be the launch of ESA's LISA Pathfinder (LPF) mission in 2015; the launch of eLISA itself is currently planned for 2034 (https://www.elisascience.org/news/top-news/gravitationaluniverseselectedasl3). The double WD mergers are among the primary candidate mechanisms for type Ia supernovae (SNe Ia) explosions. The NIR magnitudes of the latter are considered as the best \standard candles" [25] and, in this guise, are crucial in modern cosmological studies [632, 573]. Further improvements of precision of standardisation of SNe Ia fluxes is possible, e.g., by account of their environmental dependence [634, 635, 814]. On the other hand, usually, as an event beneath the \standard candle", an explosion of a non-rotating WD with the Chandrasekhar mass (' 1:38 M ) is considered. Rotation of progenitors may increase the critical mass and makes SNe Ia less reliable for cosmological use [151, 152]. SNe Ia are suggested to be responsible for production of about 50% of heavy elements in the Universe [754]. Mergers of binary WD are, probably, one of the main-mechanisms of formation of massive (> 0:8 M ) WD and WD with strong magnetic fields (see, e.g., [318, 388] for the recent studies and review of earlier work). A comparison of SN Ia rates (for the different models of their progenitors) with observations may, in principle, shed light on both the star formation history and on the nature of the progenitors (see, e.g., [656, 867, 449, 868, 865, 246, 453, 245, 454]). Compact binaries are the end products of the evolution of binary stars, and the main purpose of the present review is to describe the astrophysical knowledge on their formation and evolution. 1 http://lisa.gsfc.nasa.gov/cosmic_vision_changes.html, http://sci.esa.int/science-e/www/object/ index.cfm?fobjectid=48661. 3 We shall discuss the present situation with the main parameters determining their evolution and the rates of coalescence of double NSs/BHs and WDs. The problem is to evaluate as accurately as possible (i) the physical parameters of the coalescing binaries (masses of the components and, if possible, their spins, magnetic fields, etc.) and (ii) the occurrence rate of mergers in the Galaxy and in the local Universe. Masses of NSs in binaries are known with a rather good accuracy of 10% or better from, e.g., pulsar studies [753, 363]; see also [406, 545] for recent summaries of NS mass measurements. The case is not so good with the rate of coalescence of relativistic binary stars. Unfortunately, there is no way to derive it from the first principles { neither the formation rate of the progenitor binaries for compact double stars nor stellar evolution are known well enough. However, the situ- ation is not completely hopeless, especially in the case of double NS systems. Natural appearance of rotating NSs with magnetic fields as radio pulsars allows searching for binary pulsars with sec- ondary compact companion using powerful methods of modern radio astronomy (for example, in dedicated pulsar surveys such as the Parkes multi-beam pulsar survey [451, 188]). Based on the observational statistics of the Galactic binary pulsars with NS companions, one can evaluate the Galactic rate of binary NS formation and merging [580, 499]. On the other hand, a direct simulation of binary star evolution in the Galaxy (the population synthesis method) can also predict the formation and merger rates of close compact binaries as a function of (numerous) parameters of binary star formation and evolution. Both kinds of estimates are plagued by badly constrained parameters or selection effects, but it is, nevertheless, encouraging that most likely Galactic rates of events obtained in two ways currently differ by a factor of ≈ 3 only: 80/Myr from observations and 30/Myr from population synthesis; see [452] for a recent review of observational and theoretical estimates and Section 6. It is important and encouraging that both estimates (observational, as inferred from recent measurements of binary pulsars [76, 332], and theoretical from the population synthesis; see Section 6) now give very close estimates for the double NS star merger rate in the Galaxy of about one event per 10 000 years.
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