FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs. (arXiv:1812.07569v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lyutikov_M/0/1/0/all/0/1">Maxim Lyutikov</a> (Purdue University), <a href="http://arxiv.org/find/astro-ph/1/au:+Toonen_S/0/1/0/all/0/1">Silvia Toonen</a> (Astronomical Institute Anton Pannekoek)
We suggest that fast-rising blue optical transients (FBOTs), and the
brightest event of the class AT2018cow, result from electron-capture collapse
following a merger of a massive ONeMg white dwarf (WD) with another WD. Two
distinct evolutionary channels lead to the disruption of the less massive WD
during the merger and formation of a shell burning non-degenerate star. 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 (NS) 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 time scales of days – this is the FBOT.
During the collapse the NS is spun up and magnetic field is amplified. The
ensuing fast magnetically-dominated relativistic wind from the newly formed NS
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 Pulsar Wind Nebulae-like manner. If the secondary WD was of the DA type – the
most frequent – the wind will have hydrogen, of the order of $10^{-4} M_odot$:
this explains appearance of hydrogen late in the afterglow spectrum. The model
explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies,
fast and light anisotropic ejecta producing bright optical peak, afterglow with
high energy emission of similar luminosity to optical, hard X-ray and infra-red
features, presence of dense wind environment, late powerful radio emission.
We suggest that fast-rising blue optical transients (FBOTs), and the
brightest event of the class AT2018cow, result from electron-capture collapse
following a merger of a massive ONeMg white dwarf (WD) with another WD. Two
distinct evolutionary channels lead to the disruption of the less massive WD
during the merger and formation of a shell burning non-degenerate star. 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 (NS) 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 time scales of days – this is the FBOT.
During the collapse the NS is spun up and magnetic field is amplified. The
ensuing fast magnetically-dominated relativistic wind from the newly formed NS
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 Pulsar Wind Nebulae-like manner. If the secondary WD was of the DA type – the
most frequent – the wind will have hydrogen, of the order of $10^{-4} M_odot$:
this explains appearance of hydrogen late in the afterglow spectrum. The model
explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies,
fast and light anisotropic ejecta producing bright optical peak, afterglow with
high energy emission of similar luminosity to optical, hard X-ray and infra-red
features, presence of dense wind environment, late powerful radio emission.
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