29 research outputs found
Mass transfer in eccentric binaries: the new Oil-on-Water SPH technique
To measure the onset of mass transfer in eccentric binaries we have developed
a two-phase SPH technique. Mass transfer is important in the evolution of close
binaries, and a key issue is to determine the separation at which mass transfer
begins. The circular case is well understood and can be treated through the use
of the Roche formalism. To treat the eccentric case we use a newly-developed
two phase system. The body of the donor star is made up from high-mass "water"
particles, whilst the atmosphere is modelled with low-mass "oil" particles.
Both sets of particles take part fully in SPH interactions. To test the
technique we model circular mass-transfer binaries containing a 0.6 Msun donor
star and a 1 Msun white dwarf; such binaries are thought to form cataclysmic
variable (CV) systems. We find that we can reproduce a reasonable CV
mass-transfer rate, and that our extended atmosphere gives a separation that is
too large by aproximately 16%, although its pressure scale height is
considerably exaggerated. We use the technique to measure the semi-major axis
required for the onset of mass transfer in binaries with a mass ratio of q=0.6
and a range of eccentricities. Comparing to the value obtained by considering
the instantaneous Roche lobe at pericentre we find that the radius of the star
required for mass transfer to begin decreases systematically with increasing
eccentricity.Comment: 9 pages, 8 figures, accepted by MNRA
Distortion of secondaries in semi-detached binaries and the cataclysmic variable period minimum
Based on SPH simulations, we quantify the geometrical distortion effect due
to tidal and rotational forces on polytropic secondaries in semi-detached
binaries. The main effect is an expansion of the polytropic star, with an
effect on the radius of 5%-12%, depending on the polytropic index and
the mass ratio. We apply such distortion effects to the secular evolution of
secondaries in cataclysmic variable systems. We focus on systems below the 2-3h
period gap and that approach the minimum period. We find a significant increase
of the predicted minimum period ( 4% if changes in the secondary's
thermal relaxation are approximately taken into account). Though an
improvement, the effect is not big enough to solve the mismatch between
predicted and observed minimum period at 80 min.Comment: 9 pages, Latex file, uses aa.cls, accepted for publication in A&
Structure and evolution of rotationally and tidally distorted stars
This paper aims to study the configuration of two components caused by
rotational and tidal distortions in the model of a binary system. The
potentials of the two distorted components can be approximated to 2nd-degree
harmonics. Furthermore, both the accretion luminosity () and the
irradiative luminosity are included in stellar structure equations. The
equilibrium structure of rotationally and tidally distorted star is exactly a
triaxial ellipsoids. A formula describing the isobars is presented, and the
rotational velocity and the gravitational acceleration at the primary surface
simulated. The results show the distortion at the outer layers of the primary
increases with temporal variation and system evolution. Besides, it was
observed that the luminosity accretion is unstable, and the curve of the
energy-generation rate fluctuates after the main sequence in rotation
sequences. The luminosity in rotation sequences is slightly weaker than that in
non-rotation sequences. As a result, the volume expands slowly. Polar ejection
is intensified by the tidal effect. The ejection of an equatorial ring may be
favoured by both the opacity effect and the -effect in
the binary system.Comment: 10 pages, 17 figures,Accepted by Astronomy and Astrophysic
High-resolution spectroscopy of the intermediate polar EX Hydrae. I. Kinematic study and Roche tomography
EX Hya is one of the few double-lined eclipsing cataclysmic variables that
allow an accurate measurement of the binary masses. We analyze orbital
phase-resolved UVES/ VLT high resolution spectroscopic observations of EX Hya
with the aims of deriving the binary masses and obtaining a tomographic image
of the illuminated secondary star. We present a novel method for determining
the binary parameters by directly fitting an emission model of the illuminated
secondary star to the phase-resolved line profiles of NaI lambda 8183/ 8195 in
absorption and emission and CaII lambda 8498 in emission. The fit to the NaI
and CaII line profiles, combined with the published K1, yields a white-dwarf
mass M1 = 0.790 +/- 0.026 Msun, a secondary mass M2 = 0.108 +/- 0.008 Msun, and
a velocity amplitude of the secondary star K2 = 432.4 +/- 4.8 km s-1. The
secondary is of spectral type dM5.5 +/- 0.5 and has an absolute K-band
magnitude of MK = 8.8. Its Roche radius places it on or very close to the main
sequence of low-mass stars. It differs from a main sequence star by its
illuminated hemisphere that faces the white dwarf. The secondary star
contributes only 5% to the observed spin-phase averaged flux at 7500 A, 7.5% at
8200 A, and 37% in the K-band. We present images of the secondary star in the
light of the NaI doublet and the CaII emission line derived with a simplified
version of Roche tomography. We have discovered narrow spectral lines from the
secondary star in EX Hya that delineate its orbital motion and allow us to
derive accurate masses of both components. The primary mass significantly
exceeds recently published values. The secondary is a low-mass main sequence
star that displays a rich emission line spectrum on its illuminated side, but
lacks chromospheric emission on its dark side.Comment: 15 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic
Dwarf novae in the Hamburg quasar survey : rarer than expected
Aims. We report the discovery of five new dwarf novae that were spectroscopically identified in the Hamburg Quasar Survey (HQS),and discuss the properties of the sample of new dwarf novae from the HQS.
Methods. Follow-up time-resolved spectroscopy and photometry have been obtained to characterise the new systems.
Results. The orbital periods determined from analyses of the radial velocity variations and/or orbital photometric variability are Porb 105.1min or Porb 109.9min for HS 0417+7445, Porb = 114.3 ± 2.7min for HS 1016+3412, Porb = 92.66 ± 0.17 min for HS 1340+1524, Porb = 272.317 ± 0.001 min for HS 1857+7127, and Porb = 258.02 ± 0.56 min for HS 2214+2845. HS 1857+7127 is found to be partially eclipsing. In HS 2214+2845 the secondary star of spectral type M3 ± 1 is clearly detected, and we estimate the distance to the system to be d = 390 ± 40 pc. We recorded one superoutburst of HS 0417+7445, identifying the system as a SUUMatype
dwarf nova. HS 1016+3412 and HS 1340+1524 have rare outbursts, and their subtype is yet undetermined. HS 1857+7127 frequently varies in brightness and may be a ZCam-type dwarf nova. HS 2214+2845 is a UGem-type dwarf nova with a most likely cycle length of 71 d.
Conclusions. To date, 14 new dwarf novae have been identified in the HQS. The ratio of short-period (3 h)systems of this sample is 1.3, much smaller compared to the ratio of 2.7 found for all known dwarf novae. The HQS dwarf novae display typically infrequent or low-amplitude outburst activity, underlining the strength of spectroscopic selection in identifying new
CVs independently of their variability. The spectroscopic properties of short-period CVs in the HQS, newly identified and previously known, suggest that most, or possibly all of them are still evolving towards the minimum period. Their total number agrees with the predictions of population models within an order of magnitude. However, the bulk of all CVs is predicted to have evolved past the minimum period, and those systems remain unidentified. This suggests that those post-bounce systems have markedly weaker Hβ emission lines compared to the average known short-period CVs, and undergo no or extremely rare outbursts
Long-term evolution of compact binaries with irradiation feedback
We resume the discussion about irradiation-driven mass transfer cycles in
semi- detached compact binary systems. The analytical model that describes the
onset of these cycles, which occur on a thermal timescale of the donor star, is
reexamined. We take into account a contribution of the thermal relaxation which
is not related to the irradiation of the donor star and which was neglected in
previous studies. Cataclysmic variables (CVs) containing extended giant donors
are more stable than previously thought. CVs close to the upper edge of the
period gap can undergo cycles for low angular momentum loss rates, as they have
been suggested by recent magnetic braking prescriptions, while they are stable
for high braking rates. A model for the irradiation geometry that takes into
account surface elements near the terminator of the donor star indicates that
possibly also low-mass X-ray binaries (LMXBs) can undergo mass transfer cycles.
Regarding the braking rate, which is necessary to drive cycles, basically the
same restrictions apply for short period LMXBs as they do for short period CVs.
We confirm that LMXBs containing giants can undergo cyles. In terms of an
irradiation efficiency parameter \alpha CVs are susceptible to the irradiation
instability for \alpha >= 0.1 while LMXBs are susceptible for \alpha <= 0.1.
The predictions of the analytical model are checked by the first long-term
evolutionary computations of systems undergoing mass transfer cycles with full
1D stellar models. For unevolved main sequence (MS) and giant donors the
analytic model provides reasonable values for the boundaries of the stable and
unstable regions while CVs containing highly evolved MS donors are more stable
at high braking rates than expected.Taking into account irradiation the minimum
period of CVs is increased by up to 1-2 minutes, depending on \alpha.Comment: 19 pages, 24 figures, accepted by A&
A precise HST parallax of the cataclysmic variable EX Hydrae, its system parameters, and accretion rate
Using the HST Fine Guidance Sensor, we have measured a high precision
astrometric parallax of the cataclysmic variable EX Hydrae, pi=15.50+-0.29mas.
From the wavelength-integrated accretion-induced energy flux, we derive a
quiescent accretion luminosity for EX Hya of Lacc = (2.6+-0.6)x10e32 erg. The
quiescent accretion rate then is Mdot=(6.2\+-1.5)x10e-11
(M1/0.5Msun)^(-1.61})Msun/yr. The time-averaged accretion rate, which includes
a small correction for the rare outbursts, is 6% higher. We discuss the system
parameters of EX Hya and deduce M1=0.4-0.7Msun, M2=0.07-0.10Msun, and
i=76.0deg-77.6deg, using recent radial velocity measurements of both components
and restrictions imposed by other observational and theoretical constraints. We
conclude that the secondary is undermassive, overluminous, and expanded over a
ZAMS star of the same mass. Near the upper limit to M1, the accretion rate of
the white dwarf coincides with that due to near-equilibrium angular momentum
loss by gravitational radiation and angular momentum transfer from the orbit
into the spin-up of the white dwarf. Near the lower mass limit, the
correspondingly higher accretion rate requires that either an additional
angular momentum loss process is acting besides gravitational radiation or that
accretion occurs on a near-adiabatic time scale. The latter possibility would
imply that EX Hya is in a transient phase of high mass transfer and the
associated spin-up of the white dwarf.Comment: 7 pages A&A-Latex, 1 Figure, accepted for publication in A&