Magnetic wind-driven accretion in dwarf novae. (arXiv:1812.02076v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Scepi_N/0/1/0/all/0/1">Nicolas Scepi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dubus_G/0/1/0/all/0/1">Guillaume Dubus</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lesur_G/0/1/0/all/0/1">Geoffroy Lesur</a>
Dwarf novae (DNe) and X-ray binaries exhibit outbursts thought to be due to a
thermal-viscous instability in the accretion disk. The disk instability model
(DIM) assumes that accretion is driven by turbulent transport, customarily
attributed to the magneto-rotational instability (MRI). Recent results point
out that MRI turbulence alone fails to reproduce the light curves of DNe. We
aim to study the impact of wind-driven accretion on the light curves of DNe.
Local and global simulations show that magneto-hydrodynamic winds are present
when a magnetic field threads the disk, even for relatively high ratios of
thermal pressure to magnetic pressure ($beta approx 10^{5}$). These winds are
very efficient in removing angular momentum but do not heat the disk; they do
not behave as MRI-driven turbulence. We add wind-driven transport in the
angular momentum equation of the DIM, assuming a fixed magnetic configuration:
dipolar or constant with radius. We use prescriptions for the wind torque and
the turbulent torque derived from shearing box simulations. The wind torque
enhances the accretion of matter, resulting in light curves that look like DNe
outbursts when assuming a dipolar field with a moment
$muapprox10^{30},mathrm{G,cm^{3}}$. In the region where the wind
dominates, the disk is cold, optically thin and the accretion speed is sonic.
This acts as if the inner disk was truncated, leading to higher quiescent X-ray
luminosities from the white dwarf boundary layer than expected with the
standard DIM. The disk is stabilized if the wind-dominated region is large
enough, potentially leading to `dark’ disks emitting little radiation.
Wind-driven accretion can play a key role in shaping the light curves of DNe
and X-ray binaries. Future studies will need to include the time evolution of
the magnetic field threading the disk to fully assess its impact on the
dynamics of the accretion flow.
Dwarf novae (DNe) and X-ray binaries exhibit outbursts thought to be due to a
thermal-viscous instability in the accretion disk. The disk instability model
(DIM) assumes that accretion is driven by turbulent transport, customarily
attributed to the magneto-rotational instability (MRI). Recent results point
out that MRI turbulence alone fails to reproduce the light curves of DNe. We
aim to study the impact of wind-driven accretion on the light curves of DNe.
Local and global simulations show that magneto-hydrodynamic winds are present
when a magnetic field threads the disk, even for relatively high ratios of
thermal pressure to magnetic pressure ($beta approx 10^{5}$). These winds are
very efficient in removing angular momentum but do not heat the disk; they do
not behave as MRI-driven turbulence. We add wind-driven transport in the
angular momentum equation of the DIM, assuming a fixed magnetic configuration:
dipolar or constant with radius. We use prescriptions for the wind torque and
the turbulent torque derived from shearing box simulations. The wind torque
enhances the accretion of matter, resulting in light curves that look like DNe
outbursts when assuming a dipolar field with a moment
$muapprox10^{30},mathrm{G,cm^{3}}$. In the region where the wind
dominates, the disk is cold, optically thin and the accretion speed is sonic.
This acts as if the inner disk was truncated, leading to higher quiescent X-ray
luminosities from the white dwarf boundary layer than expected with the
standard DIM. The disk is stabilized if the wind-dominated region is large
enough, potentially leading to `dark’ disks emitting little radiation.
Wind-driven accretion can play a key role in shaping the light curves of DNe
and X-ray binaries. Future studies will need to include the time evolution of
the magnetic field threading the disk to fully assess its impact on the
dynamics of the accretion flow.
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