120 research outputs found
The viscosity parameter alpha and the properties of accretion disc outbursts in close binaries
The physical mechanisms driving angular momentum transport in accretion discs
are still unknown. Although it is generally accepted that, in hot discs, the
turbulence triggered by the magneto-rotational instability is at the origin of
the accretion process in Keplerian discs, it has been found that the values of
the stress-to-pressure ratio (the alpha "viscosity" parameter) deduced from
observations of outbursting discs are an order of magnitude higher than those
obtained in numerical simulations. We test the conclusion about the
observation-deduced value of alpha using a new set of data and comparing the
results with model outbursts. We analyse a set of observations of dwarf-nova
and AM CVn star outbursts and from the measured decay times determine the
hot-disc viscosity parameter alpha_h. We determine if and how this method is
model dependent. From the dwarf-nova disc instability model we determine an
amplitude vs recurrence-time relation and compare it to the empirical
Kukarkin-Parenago relation between the same, but observed, quantities. We found
that all methods we tried, including the one based on the amplitude vs
recurrence-time relation, imply alpha_h ~ 0.1 - 0.2 and exclude values an order
of magnitude lower. The serious discrepancy between the observed and the
MRI-calculated values of the accretion disc viscosity parameter alpha is
therefore real since there can be no doubt about the validity of the values
deduced from observations of disc outbursts.Comment: Astronomy and Astrophysics, in press. (In Fig. 3b the upper sequence
of numbers and symbols is an artefact of the compilation on astro-ph) and
should be ignored.
Models of AM CVn star outbursts
Outbursting AM CVn stars exhibit outbursts similar to those observed in
different types of dwarf novae. Their light-curves combine the characteristic
features of SU UMa, ER UMa, Z Cam, and WZ Sge-type systems but also show a
variety of properties never observed in dwarf novae. The compactness of AM CVn
orbits and their unusual chemical composition make these systems valuable
testbeds for outburst models. We aim for a better understanding of the role of
helium in the accretion disc instability mechanism, testing the model for dwarf
novae outbursts in the case of AM CVn stars, and aim to explain the outburst
light-curves of these ultra-compact binaries. We calculated the properties of
the hydrogen-free AM CVn stars using our previously developed numerical code
adapted to the different chemical composition of these systems and supplemented
with formulae accounting for mass transfer rate variations, additional sources
of the disc heating, and the primary's magnetic field. We discovered how
helium-dominated discs react to the thermal-viscous instability and were able
to reproduce various features of the outburst cycles in the light-curves of AM
CVn stars. The AM CVn outbursts can be explained by the suitably adapted
dwarf-nova disc instability model but, as in the case of its application to
hydrogen-dominated cataclysmic variables, one has to resort to additional
mechanisms to account for the observed superoutbursts, dips, cycling states,
and standstills. We show that the enhanced mass-transfer rate, due presumably
to variable irradiation of the secondary, must not only be taken into account
but is a determining factor that shapes AM CVn star outbursts. The cause of the
variable secondary's irradiation has yet to be understood; the best candidate
is the precession of a tilted/warped disc.Comment: Astronomy and Astrophysics - in press; corrected (language) versio
The October 1985 Long Outburst of U Geminorum: Revealing the Viscous Time Scale in Long Orbital Period Dwarf Novae
We examine the AAVSO light curve of U Geminorum from 1908 to 2002, with
particular focus on the October 1985 outburst. This outburst was longer than
any other seen in U Gem by about a factor of 2, and appears to be unique among
all dwarf nova outbursts seen in systems with orbital periods longer than 3 hr
in that one can measure the decay time scale during the initial slow decay.
This rate is ~26+-6 d/mag. Using estimates of the rate of accretion during
outburst taken from Froning et al., one can show that ~1e24 g of gas was
accreted onto the white dwarf during the outburst. When coupled with the
viscous time inferred from the (short orbital period) SU UMa stars, the U Gem
viscous time scale lends support to the standard model in which the decays in
dwarf novae can either be viscous or thermal, with the ratio between them being
roughly h/r where h is the vertical pressure scale height in the disk.Comment: 11 pages, 3 figure
Modeling Eclipses in the Classical Nova V Persei: The Role of the Accretion Disk Rim
Multicolor (BVRI) light curves of the eclipsing classical nova V Per are
presented, and a total of twelve new eclipse timings are measured for the
system. When combined with previous eclipse timings from the literature, these
timings yield a revised ephemeris for the times of mid-eclipse given by HJD =
2,447,442.8260(1) + 0.107123474(3) E. The eclipse profiles are analyzed with a
parameter-fitting model that assumes four sources of luminosity: a white dwarf
primary star, a main-sequence secondary star, a flared accretion disk with a
rim, and a bright spot at the intersection of the mass-transfer stream and the
disk periphery. A matrix of model solutions are computed, covering an extensive
range of plausible parameter values. The solution matrix is then explored to
determine the optimum values for the fitting parameters and their associated
errors. For models that treat the accretion disk as a flat structure without a
rim, optimum fits require that the disk have a flat temperature profile.
Although models with a truncated inner disk (R_in >> R_wd) result in a steeper
temperature profile, steady-state models with a temperature profile
characterized by T(r) \propto r^{-3/4} are found only for models with a
significant disk rim. A comparison of the observed brightness and color at
mid-eclipse with the photometric properties of the best-fitting model suggests
that V Per lies at a distance of ~ 1 kpc.Comment: Accepted for publication in The Astrophysical Journal. Thirty-nine
pages, including 9 figures. V2 - updated to include additional references and
related discussion to previous work overlooked in the original version, and
to correct a typo in the ephemeris given in the abstract. V3 - Minor typos
corrected. The paper is scheduled for the 20 June 2006 issue of the ApJ. V4 -
An error in equation (9) has been corrected. The results presented in the
paper were not affected, as all computations were made using the correct
formulation of this equatio
The dependence of the viscosity-parameter on the disk scale height profile
It is shown that the height scale for accretion disks is a constant whenever
hydrostatic equilibrium and sub-sonic turbulence regime hold in the disk. In
order to have a variable height scale, processes that do contribute with an
extra term to the continuity equation are needed. This makes the viscosity
parameter much greater in the outer region and much smaller in the inner
region. Under these circumstances, turbulence is a presumable source of
viscosity in the disk.Comment: 8 pages, submitted to Apj
The 2001 Superoutburst of WZ Sagittae: A Clue to the Dynamics of Accretion Disks
We examine the light curve of the July-August 2001 superoutburst of WZ
Sagittae. During the decline from maximum light the locally defined decay time
increases from ~4 d/mag to ~12 d/mag over the first ~15 d of the ~25 d
superoutburst, as the system faded from m_V~8.5 to m_V~10. The superoutburst is
caused by the sudden accretion of ~10^{24} g of gas onto the white dwarf, and
the deviation from exponentiality in the decay light curve is expected
qualitatively during a ``viscous decay'' in which the dominant mode of
depletion of the gas stored in the accretion disk is accretion onto the central
object. In other words, as the mass of the accretion disk decreases, the
viscous time scale increases. We show that the data are also quantitatively
consistent with the theoretical viscous decay time, both calculated via a
simple scaling and also from time dependent calculations, when one adopts
standard model parameters for WZ Sge.Comment: 13 pages, 2 figure
Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries
Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer
deep survey photometer and in 2001 January with the Chandra X-ray Observatory
Low Energy Transmission Grating Spectrograph, we investigate the
extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg
in outburst. We find quasi-periodic oscillations (QPOs) at nu_0 ~ 0.012 Hz and
nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and
possibly nu_2 ~ nu_0 + nu_1 ~ 0.12 Hz in the soft X-ray flux. These data,
combined with the optical data of Woudt & Warner for VW Hyi, extend the
Psaltis, Belloni, & van der Klis nu_high-nu_low correlation for neutron star
and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude
in frequency, with nu_low ~ 0.08 nu_high. This correlation identifies the
high-frequency quasi-coherent oscillations (so-called ``dwarf nova
oscillations'') of cataclysmic variables (CVs) with the kilohertz QPOs of
LMXBs, and the low-frequency QPOs of CVs with the horizontal branch
oscillations (or the broad noise component identified as such) of LMXBs.
Assuming that the same mechanisms produce the QPOs in white dwarf, neutron
star, and black hole binaries, we find that the data exclude the relativistic
precession model and the magnetospheric and sonic-point beat-frequency models
(as well as any model requiring the presence or absence of a stellar surface or
magnetic field); more promising are models that interpret QPOs as
manifestations of disk accretion onto any low-magnetic field compact object.Comment: 15 pages including 4 encapsulated postscript figures; LaTeX format,
uses aastex.cls; accepted on 2002 July 23 for publication in The
Astrophysical Journa
On the turbulent -disks and the intermittent activity in AGN
We consider effects of the MHD turbulence on the viscosity during the
evolution of the thermal-viscous ionization instability in the standard
-accretion disks. We consider the possibility that the accretion onto a
supermassive black hole proceeds through an outer standard accretion disk and
inner, radiatively inefficient and advection dominated flow. In this scenario
we follow the time evolution of the accretion disk in which the viscosity
parameter is constant throughout the whole instability cycle, as
implied by the strength of MHD turbulence. We conclude that the hydrogen
ionization instability is a promising mechanism to explain the intermittent
activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte
Stability and Evolution of Supernova Fallback Disks
We show that thin accretion disks made of Carbon or Oxygen are subject to the
same thermal ionization instability as Hydrogen and Helium disks. We argue that
the instability applies to disks of any metal content. The relevance of the
instability to supernova fallback disks probably means that their power-law
evolution breaks down when they first become neutral. We construct simple
analytical models for the viscous evolution of fallback disks to show that it
is possible for these disks to become neutral when they are still young (ages
of a few 10^3 to 10^4 years), compact in size (a few 10^9 cm to 10^11 cm) and
generally accreting at sub-Eddington rates (Mdot ~ a few 10^14 - 10^18 g/s).
Based on recent results on the nature of viscosity in the disks of close
binaries, we argue that this time may also correspond to the end of the disk
activity period. Indeed, in the absence of a significant source of viscosity in
the neutral phase, the entire disk will likely turn to dust and become passive.
We discuss various applications of the evolutionary model, including anomalous
X-ray pulsars and young radio pulsars. Our analysis indicates that metal-rich
fallback disks around newly-born neutron stars and black holes become neutral
generally inside the tidal truncation radius (Roche limit) for planets, at
\~10^11 cm. Consequently, the efficiency of the planetary formation process in
this context will mostly depend on the ability of the resulting disk of rocks
to spread via collisions beyond the Roche limit. It appears easier for the
merger product of a doubly degenerate binary, whether it is a massive white
dwarf or a neutron star, to harbor planets because it can spread beyond the
Roche limit before becoming neutral.[Abridged]Comment: 34 pages, 2 figures, accepted for publication in Ap
Superhumps in Cataclysmic Binaries. XXV. q_crit, epsilon(q), and Mass-Radius
We report on successes and failures in searching for positive superhumps in
cataclysmic variables, and show the superhumping fraction as a function of
orbital period. Basically, all short-period systems do, all long-period systems
don't, and a 50% success rate is found at P_orb=3.1+-0.2 hr. We can use this to
measure the critical mass ratio for the creation of superhumps. With a
mass-radius relation appropriate for cataclysmic variables, and an assumed mean
white-dwarf mass of 0.75 M_sol, we find a mass ratio q_crit=0.35+-0.02.
We also report superhump studies of several stars of independently known mass
ratio: OU Virginis, XZ Eridani, UU Aquarii, and KV UMa (= XTE J1118+480). The
latter two are of special interest, because they represent the most extreme
mass ratios for which accurate superhump measurements have been made. We use
these to improve the epsilon(q) calibration, by which we can infer the elusive
q from the easy-to-measure epsilon (the fractional period excess of P_superhump
over P_orb). This relation allows mass and radius estimates for the secondary
star in any CV showing superhumps. The consequent mass-radius law shows an
apparent discontinuity in radius near 0.2 M_sol, as predicted by the disrupted
magnetic braking model for the 2.1-2.7 hour period gap. This is effectively the
"empirical main sequence" for CV secondaries.Comment: PDF, 45 pages, 9 tables, 12 figures; accepted, in press, to appear
November 2005, PASP; more info at http://cba.phys.columbia.edu
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