44 research outputs found
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&
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
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
The zoo of dwarf novae : illumination, evaporation and disc radius variation
In the framework of the dwarf nova thermal-viscous disc instability model, we
investigate the combined effects on the predicted dwarf nova lightcurves of
irradiating the accretion disc and the secondary star and of evaporating the
inner parts of the disc. We assume the standard values of viscosity. We confirm
the suggestion by Warner (1998) that the large variety of observed outbursts'
behaviour may result from the interplay of these three effects. We are able to
reproduce light curves reminiscent of those of systems such as RZ LMi or EG
Cnc. We can obtain long lasting outbursts, very similar to superoutbursts,
without assuming the presence of a tidal instability.Comment: 10 pages, 7 figures; accepted for publication in Astronomy and
Astrophysic
Disc instability models, evaporation and radius variations
We show that the outcome of disc instability models is strongly influenced by
boundary conditions such as the position of the inner and outer disc edges. We
discuss other sources of uncertainties, such as the tidal torque, and we
conclude that disc illumination, disk size variations and a proper prescription
for the tidal torque must be included in models if one wishes to extract
meaningful physical information on e.g. viscosity from the comparison of
predicted and observed lightcurves.Comment: 8 pages, 1 figure. To be published in the proceedings of the
conference "Disk Instabilities in Close Binary Systems - 25 Years of the Disk
Instability Model
Accretion disc outbursts: a new version of an old model
We have developed 1D time-dependent numerical models of accretion discs,
using an adaptive grid technique and an implicit numerical scheme, in which the
disc size is allowed to vary with time. The code fully resolves the cooling and
heating fronts propagating in the disc. We show that models in which the radius
of the outer edge of the disc is fixed produce incorrect results, from which
probably incorrect conclusions about the viscosity law have been inferred. In
particular we show that outside-in outbursts are possible when a standard
bimodal behaviour of the Shakura-Sunyaev viscosity parameter alpha is used. We
also discuss to what extent insufficient grid resolutions have limited the
predictive power of previous models. We find that the global properties
(magnitudes, etc. ...) of transient discs can be addressed by codes using a
high, but reasonable, number of fixed grid points. However, the study of the
detailed physical properties of the transition fronts generally requires
resolutions which are out of reach of fixed grid codes. It appears that most
time-dependent models of accretion discs published in the literature have been
limited by resolution effects, improper outer boundary conditions, or both.Comment: 13 pages, 12 figures; accepted for publication in MNRA
Spectra of accretion discs around white dwarfs
We present spectra of accretion discs around white-dwarfs calculated with an
improved and updated version of the Shaviv & Wehrse (1991) model. The new
version includes line opacities and convective energy transport and can be used
to calculate spectra of hot discs in bright systems (nova--like variables or
dwarf novae in outburst) as well as spectra of cold accretion discs in
quiescent dwarf novae.Comment: 10 pages. Talk presented at "Jean-Pierre Lasota, X-ray binaries,
accretion disks and compact stars" (October 2007); Abramowicz, M. Ed., New
Astron. Rev., in pres