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تسجيل مستخدم جديدTransients following white dwarfs mergers
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تأليف
Maxim Lyutikov
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Mergers of white dwarfs (WDs) may lead to a variety of transient astrophysical events, SNIa being one possible outcome. Lyutikov & Toonen (2017, 2019) argued that mergers of WDs result, under various parameter regimes, in unusual central engine-powered supernova and a type of short Gamma Ray Bursts that show extended emission tails. Observations by Gvaramadze et al. (2019) of the central star and the nebula J005311 match to the details the model of Lyutikov & Toonen (2017, 2019) for the immediate product of a merger of a heavy ONeMg WD with CO WD (age, luminosity, stellar size, hydrogen deficiency and chemical composition).
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We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class AT2018cow result from an electron-capture collapse to a NS following a merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolution
ary channels lead to the disruption of the less massive WD during the merger and the formation of a shell burning non-degenerate star incorporating the ONeMg core. During the shell burning stage a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after $sim 10^2-10^4$ years. During the formation of a neutron star as little as $sim 10^{-2} M_odot $ of the material is ejected at the bounce-off with mildly relativistic velocities and total energy $sim$ few $ 10^{50}$ ergs. This ejecta becomes optically thin on a time scale of days - this is the FBOT. During the collapse, the neutron star is spun up and magnetic field is amplified. The ensuing fast magnetically-dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high energy emission in a manner similar to Pulsar Wind Nebulae. If the secondary WD was of the DA type, the wind will likely have $sim 10^{-4} M_odot$ of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high energy emission of similar luminosity to the optical, and late infra-red features.
The violent merger of two carbon-oxygen white dwarfs has been proposed as a viable progenitor for some Type Ia supernovae. However, it has been argued that the strong ejecta asymmetries produced by this model might be inconsistent with the low degree
of polarisation typically observed in Type Ia supernova explosions. Here, we test this claim by carrying out a spectropolarimetric analysis for the model proposed by Pakmor et al. (2012) for an explosion triggered during the merger of a 1.1 M$_{odot}$ and 0.9 M$_{odot}$ carbon-oxygen white dwarf binary system. Owing to the asymmetries of the ejecta, the polarisation signal varies significantly with viewing angle. We find that polarisation levels for observers in the equatorial plane are modest ($lesssim$ 1 per cent) and show clear evidence for a dominant axis, as a consequence of the ejecta symmetry about the orbital plane. In contrast, orientations out of the plane are associated with higher degrees of polarisation and departures from a dominant axis. While the particular model studied here gives a good match to highly-polarised events such as SN 2004dt, it has difficulties in reproducing the low polarisation levels commonly observed in normal Type Ia supernovae. Specifically, we find that significant asymmetries in the element distribution result in a wealth of strong polarisation features that are not observed in the majority of currently available spectropolarimetric data of Type Ia supernovae. Future studies will map out the parameter space of the merger scenario to investigate if alternative models can provide better agreement with observations.
The purpose of this thesis is to obtain more realistic equations of state to describe the matter forming magnetized white dwarfs, and use them to solve its structure equations. The equations of state are determined by considering the weak magnetic
field approximation $B<B_c$ ($B_c=4.41times10^{13}text{ G}$) for the electron gas of the star. The magnetic field introduces anisotropic pressures, even for the moderate values present in white dwarfs. Also, we consider the energy and pressure correction due to the Coulomb interaction of the electron gas with the ions located in a crystal lattice. Moreover, spherically symmetric Tolman-Oppenheimer-Volkoff structure equations are solved independently for the perpendicular and parallel pressures, confirming the necessity of using axisymmetric structure equations, more adequate to describe the anisotropic system. Therefore, we study the solutions in cylindrical coordinates. In this case, the mass per longitude unit is obtained instead of the total mass of the white dwarf.
We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We f
ind that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.
Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure, and the disintegration of the whole white dwarf. It is usuall
y thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and our suggestion that these supernovae instead result from mergers of carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then turn to possible observational tests, in particular those that test the absence or presence of electron captures during the burning.
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