412 research outputs found
What We Learn from Quantitative Ultraviolet Spectroscopy of Naked White D warfs in Cataclysmic Variables
Using the Hopkins Ultraviolet Telescope and Hubble Space Telescope, observers
have now obtained UV spectra with sufficient signal to noise and resolution to
allow quantitative spectroscopic analyses of the WDs in several DNe. In the
``cleanest'' DNe, such as U Gem, the observations are allowing the basic
physical parameters of the WD -- temperature, radius, gravity, rotation rate,
and surface abundances -- to be established. A second component also exists in
these systems, which may either be the disk or may be related to the WD itself.
Here I summarize the current state of the observations and our understanding of
the data, highlighting some of the uncertainties in the analyses as well the
prospects for fundamentally advancing our understanding of DNe and WDs with
future observations.Comment: 6 pages, 8 figures, to be published in the proc. for Cataclysmic
Variables: A 60th Birthday Symposium in Honour of Brian Warne
Disc wind models for FU Ori objects
We present disc wind models aimed at reproducing the main features of the
strong Na I resonance line P-Cygni profiles in the rapidly-accreting pre-main
sequence FU Ori objects. We conducted Monte Carlo radiative transfer
simulations for a standard magnetocentrifugally driven wind (MHD) model and our
own "Genwind" models, which allows for a more flexible wind parameterisation.
We find that the fiducial MHD wind and similar Genwind models, which have flows
emerging outward from the inner disc edge, and thus have polar cavities with no
absorbing gas, cannot reproduce the deep, wide Na I absorption lines in FU Ori
objects viewed at low inclination. We find that it is necessary to include an
"inner wind" to fill this polar cavity to reproduce observations. In addition,
our models assuming pure scattering source functions in the Sobolev
approximation at intermediate viewing angles () do not yield sufficiently deep line profiles. Assuming complete
absorption yields better agreement with observations, but simple estimates
strongly suggest that pure scattering should be a much better approximation.
The discrepancy may indicate that the Sobolev approximation is not applicable,
possibly due to turbulence or non-monotonic velocity fields; there is some
observational evidence for the latter. Our results provide guidance for future
attempts to constrain FU Ori wind properties using full MHD wind simulations,
by pointing to the importance of the boundary conditions necessary to give rise
to an inner wind, and by suggesting that the winds must be turbulent to produce
sufficiently deep line profiles.Comment: 12 pages, 17 figures, accepted for publication in MNRA
An Improved Model for the Spectra of Disks of Nova-like Variables
The spectra arising from the disks of nova-like variables show many of the
features seen in stellar atmospheres. They are typically modelled either from
an appropriated weighted set of stellar atmospheres or a disk atmosphere with
energy is dissipated near the disk plane, with the effective temperature
distribution expected from a steady state accretion disk. However these models
generally over-predict the depth of the Balmer jump and the slope of the
spectrum in the ultraviolet. The problem is likely due to energy dissipation in
the disk atmosphere, which produces a flatter vertical temperature profile than
is observed in stars. Here, we provide validation for this hypothesis in the
form of spectra generated using the stellar atmosphere code TLUSTY using a
parametric prescription for energy dissipation as a function of depth and
closely match the spectrum of the nova-like IX Vel over the wavelength range
1150-6000 \AA.Comment: 12 pages, 7 figure
The Geometry and Ionization Structure of the Wind in the Eclipsing Nova-like Variables RW Tri and UX UMa
The UV spectra of nova-like variables are dominated by emission from the
accretion disk, modified by scattering in a wind emanating from the disk. Here
we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-likes
which have been observed with the Hubble Space Telescope in the
far-ultraviolet, in an attempt to constrain the geometry and the ionization
structure of their winds. Using our Monte Carlo radiative transfer code we
computed spectra for simply-parameterized axisymmetric biconical outflow models
and were able to find plausible models for both systems. These reproduce the
primary UV resonance lines - N V, Si IV, and C IV - in the observed spectra in
and out of eclipse. The distribution of these ions in the wind models is
similar in both cases as is the extent of the primary scattering regions in
which these lines are formed. The inferred mass loss rates are 6% to 8% of the
mass accretion rates for the systems. We discuss the implication of our point
models for our understanding of accretion disk winds in cataclysmic variables.Comment: 13 pages, 15 figures and 4 tables. Published in Ap
The Impact of Accretion Disk Winds on the Optical Spectra of Cataclysmic Variables
Many high-state non-magnetic cataclysmic variables (CVs) exhibit blue-shifted
absorption or P-Cygni profiles associated with ultraviolet (UV) resonance
lines. These features imply the existence of powerful accretion disk winds in
CVs. Here, we use our Monte Carlo ionization and radiative transfer code to
investigate whether disk wind models that produce realistic UV line profiles
are also likely to generate observationally significant recombination line and
continuum emission in the optical waveband. We also test whether outflows may
be responsible for the single-peaked emission line profiles often seen in
high-state CVs and for the weakness of the Balmer absorption edge (relative to
simple models of optically thick accretion disks). We find that a standard disk
wind model that is successful in reproducing the UV spectra of CVs also leaves
a noticeable imprint on the optical spectrum, particularly for systems viewed
at high inclination. The strongest optical wind-formed recombination lines are
H and He II . We demonstrate that a higher-density outflow
model produces all the expected H and He lines and produces a recombination
continuum that can fill in the Balmer jump at high inclinations. This model
displays reasonable verisimilitude with the optical spectrum of RW Trianguli.
No single-peaked emission is seen, although we observe a narrowing of the
double-peaked emission lines from the base of the wind. Finally, we show that
even denser models can produce a single-peaked H line. On the basis of
our results, we suggest that winds can modify, and perhaps even dominate, the
line and continuum emission from CVs.Comment: 15 pages, 13 figures. Accepted to MNRA
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