I10: From evolving binaries to the merging of compact objects

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  • More than 70% of massive stars experience a binary interaction at least once in their life (Sana et al. 2012). In the course of their evolution, one of the stars first becomes a compact object (white dwarf, neutron star or black hole), and, if close enough, attracts matter from its companion. The stars thus exchange both matter and angular momentum, through an energetic process called accretion: they become accreting, compact binaries (Chaty 2013). Such a pair of massive stars eventually evolves towards the merging of two compact objects.

     

    This phenomenon, leading to the emission of gravitational waves, has been beautifully revealed on the 14th of September 2015 by the LIGO collaboration, arising from the merging of two heavy stellar mass black holes of ~30 solar masses (Abbott et al 2016ab). The two firm gravitational wave detections already announced likely constitute the tip of the iceberg: indeed, close binaries exist everywhere in our Universe, and should be detected when they merge and emit gravitational waves!

     

    Most evolutionary models of binary stellar systems are based on the coupled evolution of two single, isolated stars. However, these evolutionary models are incomplete: while we do not fully understand the mechanisms governing stellar evolution, we know even less about the physical processes occurring in close binary systems, where both stars exchange matter and angular momentum. First, the common envelope phase, occurring very early in the evolution of a compact binary, is still both theoretically and observationally highly unknown. Second, the natal kick received at the supernova event is not constrained, especially for the black holes. Finally, the metallicity plays an important role in the strength of the stellar winds, which can cause the star to lose much of its mass. Therefore, as the full evolution of binaries towards merging is not fully understood, the current population synthesis models of binary systems in galaxies have a high degree of uncertainty, implying that the search to identify the merger progenitors is flawed.

     

    In this interface project between AIM and APC, we propose to tackle this problem by computing the evolution of the current population of compact binaries known in our Galaxy, using new data obtained from the Gaia satellite, revolutionizing the field of astrometry by providing a totally new 6D view (position and velocity) of our Galaxy. Then, by comparing the latest evolutionary stages of compact objects with the predictions of current population synthesis models, we will be able to constrain the three biggest uncertainties of these models: the common envelope phase, the natal kicks, and the metallicity. We then plan to extrapolate our results to low-metallicity galactic environments, computing updated population synthesis models, to improve the predicted rates of compact object mergers, and thus of gravitational wave detections. In short, “yesterday’s binaries are today’s gravitational waves”!

     

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    POSITION NAME SURNAME LABORATORY NAME GRADE, EMPLOYER
    WP leader CHATY Sylvain AIM PR, Université Paris Diderot, USPC
    WP co-leader PORTER Edward APC DR, CNRS
    WP member CHASSANDE-MOTTIN Eric APC CR, CNRS
    WP member COLEIRO Alexis APC Post-doc
    WP member FOGLIZZO Thierry AIM IR, CEA
    WP member FORTIN Francis AIM PhD, Ecole Doctorale ED 560
    WP member MARSHALL Douglas AIM MCF, Université Paris Diderot, USPC
    WP member MIRABEL Felix AIM IR, CEA

     

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    The aim of the project is to reveal various aspects of the evolution of binary systems hosting black holes and neutron stars, in order to characterize the merger progenitors which are the potential emitters of gravitational waves. The project will include multi-wavelength observations to constrain the metallicity of binary systems, modeling of the evolution of binaries, use of population synthesis models, and prediction of merging rates in various environments.

    We unanimously agreed to propose the position to Federico Garcia, who arrived to begin his 2-year contract on the 4th of September 2017.

    Since the beginning of his contract, Federico Garcia has already looked at the Gaia catalogue that corresponds to the 1st release, and he found that nearly 30 accreting binaries are present within this catalogue, with accurate position and proper motion. He began to work on producing a catalogue of accreting binaries.

    Summary of WP of our project:

    1. Build a 6D-catalogue of known compact binaries, taking into account the new observational parameters obtained with the Gaia satellite: position and velocity, along with distance, proper motion, spectral type;
    2. compute the evolution of known accreting binaries in our Galaxy, for which we have enough parameters, through the classical isolated binary evolution channel (using the public code MESA or private StarTrack);
    3. compare the accurate evolution of these binaries with the current output of population synthesis models, to better constrain the 3 most uncertain parameters: common enveloppe phase, natal kick, metallicity;
    4. Extrapolate from this template, with better constrained parameters, to compute the population of binary systems present in low-metallicity environments, with the binary star evolution (BSE) model;

     

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