621 research outputs found
Hydrodynamic simulations of irradiated secondaries in dwarf novae
We investigate numerically the surface flow on the secondary star during
outbursts. We use a simple model for the irradiation and the geometry of the
secondary star: the irradiation temperature is treated as a free parameter and
the secondary is replaced by a spherical star with a space-dependent Coriolis
force that mimics the effect of the Roche geometry. The Euler equations are
solved in spherical coordinates with the TVD-MacCormack scheme. We show that
the Coriolis force leads to the formation of a circulation flow from high
latitude region to the close vicinity of the point. However no heat can
be efficiently transported to the region due to the rapid radiative
cooling of the hot material as it enters the equatorial belt shadowed from
irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis
force could lead to a moderate increase of the mass transfer rate by pushing
the gas in the vertical direction at the point, but only during the
initial phases of the outburst (about 15 -- 20 orbital periods). We conclude
that the Coriolis force does not prevent a flow from the heated regions of the
secondary towards the region, at least during the initial phase of an
outburst, but the resulting increase of the mass transfer rate is moderate, and
it is unlikely to be able to account for the duration of long outbursts.Comment: 11 pages, 11 figures, accepted for publication in A&
Illumination in binaries
We give a simple, but accurate method that can be used to account for
illumination in compact binary systems which have a low-mass companion, even if
spherically symmetric illumination of the secondary star (not necessarily on
the main sequence) is not assumed. This is done by introducing a multiplicative
factor Phi in the Stefan-Boltzmann surface boundary condition, which accounts
for the blocking of the intrinsic secondary flux by X-ray heating of the
photospheric layers. Numerical fits and tables for Phi are given for
unperturbed effective temperatures in the range 2500 - 5600 K and log g in the
range 1.0 - 5.0Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Supplement Serie
The disappearance and reformation of the accretion disc during a low state of FO Aquarii
FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate
polar) went into a low-state in 2016, from which it slowly and steadily
recovered without showing dwarf nova outbursts. This requires explanation since
in a low-state, the mass-transfer rate is in principle too low for the disc to
be fully ionized and the disc should be subject to the standard thermal and
viscous instability observed in dwarf novae. We investigate the conditions
under which an accretion disc in an intermediate polar could exhibit a
luminosity drop of 2 magnitudes in the optical band without showing outbursts.
We use our numerical code for the time evolution of accretion discs, including
other light sources from the system (primary, secondary, hot spot). We show
that although it is marginally possible for the accretion disc in the low-state
to stay on the hot stable branch, the required mass-transfer rate in the normal
state would then have to be extremely high, of the order of 10 gs
or even larger. This would make the system so intrinsically bright that its
distance should be much larger than allowed by all estimates. We show that
observations of FO Aqr are well accounted for by the same mechanism that we
have suggested as explaining the absence of outbursts during low states of VY
Scl stars: during the decay, the magnetospheric radius exceeds the
circularization radius, so that the disc disappears before it enters the
instability strip for dwarf nova outbursts. Our results are unaffected, and
even reinforced, if accretion proceeds both via the accretion disc and directly
via the stream during some intermediate stages; the detailed process through
which the disc disappears still needs investigations.Comment: 6 pages, 5 figures. Accepted for publication in A&
Outbursts in ultracompact X-ray binaries
Very faint X-ray binaries appear to be transient in many cases with peak
luminosities much fainter than that of usual soft X-ray transients, but their
nature still remains elusive. We investigate the possibility that this
transient behaviour is due to the same thermal/viscous instability which is
responsible for outbursts of bright soft X-ray transients, occurring in
ultracompact binaries for adequately low mass-transfer rates. More generally,
we investigate the observational consequences of this instability when it
occurs in ultracompact binaries. We use our code for modelling the
thermal-viscous instability of the accretion disc, assumed here to be hydrogen
poor. We also take into account the effects of disc X-ray irradiation, and
consider the impact of the mass-transfer rate on the outburst brightness. We
find that one can reproduce the observed properties of both the very faint and
the brighter short transients (peak luminosity, duration, recurrence times),
provided that the viscosity parameter in quiescence is slightly smaller
(typically a factor of between two and four) than in bright soft X-ray
transients and normal dwarf nova outbursts, the viscosity in outburst being
unchanged. This possibly reflects the impact of chemical composition on
non-ideal MHD effects affecting magnetically driven turbulence in poorly
ionized discs.Comment: 9 pages, 7 figures, Astronomy and Astrophysics, in pres
Dwarf nova outbursts in intermediate polars
The disc instability model (DIM) has been very successful in explaining the
dwarf nova outbursts observed in cataclysmic variables. When, as in
intermediate polars (IP), the accreting white dwarf is magnetized, the disc is
truncated at the magnetospheric radius, but for mass-transfer rates
corresponding to the thermal-viscous instability such systems should still
exhibit dwarf-nova outbursts. Yet, the majority of intermediate polars in which
the magnetic field is not large enough to completely disrupt the accretion
disc, seem to be stable, and the rare observed outbursts, in particular in
systems with long orbital periods, are much shorter than normal dwarf-nova
outbursts. We investigate the predictions of the disc instability model for
intermediate polars in order to determine which of the observed properties of
these systems can be explained by the DIM. We use our numerical code for the
time evolution of accretion discs, modified to include the effects of the
magnetic field, with constant or variable mass transfer from the secondary
star. We show that intermediate polars have mass transfer low enough and
magnetic fields large enough to keep the accretion disc stable on the cold
equilibrium branch. We show that the infrequent and short outbursts observed in
long period systems, such as e.g., TV Col, cannot be attributed to the
thermal-viscous instability of the accretion disc, but instead have to be
triggered by an enhanced mass-transfer from the secondary, or, more likely, by
some instability coupling the white dwarf magnetic field with that generated by
the magnetorotational instability operating in the accretion disc. Longer
outbursts (a few days) could result from the disc instability.Comment: 7 pages, 5 figures; submitted to Astronomy & Astrophysic
Structure and properties of transition fronts in accretion discs
We use high-resolution time-dependent numerical simulations of accretion
discs around white dwarfs to study the structure and properties of transition
fronts in the context of the thermal-viscous disc instability model. The
thermal structure of cooling and heating fronts is dominated by radiative
cooling and viscous heating, respectively, except in a very narrow precursor
region in heating fronts where advection and radial transport of energy
dominate. Cooling fronts are much broader than heating fronts, but the widths
of both types of fronts scale with the local vertical scale height of the disc.
We confirm that during a fair fraction of the propagation time of a cooling
front, the structure of the inner disc is close to self-similar. The speed of
heating fronts is ~ a few km/s, while the speed of cooling fronts is ~ a
fraction of a km/s. We show that direct measurements of the speed of transition
fronts probably cannot discriminate between various prescriptions proposed for
the viscosity parameter alpha. A natural prediction of the disc instability
model is that fronts decelerate as they propagate in the disc, independent of
the prescription for alpha. Observation of this effect would confirm that dwarf
nova outbursts are driven by the thermal-viscous instability. Most of our
results also apply to low mass X-ray binaries in which the accreting object is
a neutron star or a black hole.Comment: LateX, 12 pages, 10 figures, uses mn.sty; Submitted to MNRA
Anomalous Z Cam stars: a response to mass-transfer outbursts
Recent observations of two unusual Z Cam systems, V513 Cas and IW And have
shown light curves that seem to contradict the disc-instability model for dwarf
novae: outbursts are appearing during standstills of the system when according
to the model, the disc is supposed to be in a hot quasi-equilibrium state. We
investigate what additional physical processes need to be included in the model
to reconcile it with observations of such anomalous Z Cam systems. We used our
code for modeling thermal-viscous outbursts of the accretion discs and
determined what types of mass-transfer variations reproduce the observed light
curves. Outbursts of mass transfer (with a duration of a few days, with a short
rise time and an exponential decay) from the stellar companion will account for
the observed properties of V513 Cas and IW And, provided they are followed by a
short but significant mass-transfer dip. The total mass involved in outbursts
is of the order of 10g. We studied the possible origins of these mass
transfer outbursts and showed that they most probably result from a giant flare
near the secondary star surface, possibly due to the absence of star spots in
the region.Comment: 5 pages, 2 figures. Misprints corrected, in particular "inside-out"
should have read "outside-in
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