703 research outputs found
Calcium-Rich Gap Transients: Tidal Detonations of White Dwarfs?
We hypothesize that at least some of the recently discovered class of
calcium-rich gap transients are tidal detonation events of white dwarfs (WDs)
by black holes (BHs) or possibly neutron stars. We show that the properties of
the calcium-rich gap transients agree well with the predictions of the tidal
detonation model. Under the predictions of this model, we use a follow-up X-ray
observation of one of these transients, SN 2012hn, to place weak upper limits
on the detonator mass of this system that include all intermediate-mass BHs
(IMBHs). As these transients are preferentially in the stellar haloes of
galaxies, we discuss the possibility that these transients are tidal
detonations of WDs caused by random flyby encounters with IMBHs in dwarf
galaxies or globular clusters. This possibility has been already suggested in
the literature but without connection to the calcium-rich gap transients. In
order for the random flyby cross-section to be high enough, these events would
have to be occurring inside these dense stellar associations. However, there is
a lack of evidence for IMBHs in these systems, and recent observations have
ruled out all but the very faintest dwarf galaxies and globular clusters for a
few of these transients. Another possibility is that these are tidal
detonations caused by three-body interactions, where a WD is perturbed toward
the detonator in isolated multiple star systems. We highlight a number of ways
this could occur, even in lower-mass systems with stellar-mass BHs or neutron
stars. Finally, we outline several new observational tests of this scenario,
which are feasible with current instrumentation.Comment: 10 pages, 1 figure, accepted for publication in MNRA
On the Origin of the Absorption Features in SS433
We present high-resolution optical spectroscopy of the X-ray binary system
SS433, obtained over a wide range of orbital phases. The spectra display
numerous weak absorption features, and include the clearest example seen to
date of those features, resembling a mid-A type supergiant spectrum, that have
previously been associated with the mass donor star. However, the new data
preclude the hypothesis that these features originate solely within the
photosphere of the putative mass donor, indicating that there may be more than
one region within the system producing an A supergiant-like spectrum, probably
an accretion disc wind. Indeed, whilst we cannot confirm the possibility that
the companion star is visible at certain phase combinations, it is possible
that all supergiant-like features observed thus far are produced solely in a
wind. We conclude that great care must be taken when interpreting the behaviour
of these weak features.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure
T Pyxidis: The First Cataclysmic Variable with a Collimated Jet
We present the first observational evidence for a collimated jet in a
cataclysmic variable system; the recurrent nova T Pyxidis. Optical spectra show
bipolar components of H with velocities , very similar
to those observed in the supersoft X-ray sources and in SS 433. We argue that a
key ingredient of the formation of jets in the supersoft X-ray sources and T
Pyx (in addition to an accretion disk threaded by a vertical magnetic field),
is the presence of nuclear burning on the surface of the white dwarf.Comment: 10 pages 2 figures to appear in ApJ Letter
The physical properties of AM CVn stars: new insights from Gaia DR2
AM CVn binaries are hydrogen deficient compact binaries with an orbital
period in the 5-65 min range and are predicted to be strong sources of
persistent gravitational wave radiation. Using Gaia Data Release 2, we present
the parallaxes and proper motions of 41 out of the 56 known systems. Compared
to the parallax determined using the HST Fine Guidance Sensor we find that the
archetype star, AM CVn, is significantly closer than previously thought. This
resolves the high luminosity and mass accretion rate which models had
difficulty in explaining. Using Pan-STARRS1 data we determine the absolute
magnitude of the AM CVn stars. There is some evidence that donor stars have a
higher mass and radius than expected for white dwarfs or that the donors are
not white dwarfs. Using the distances to the known AM CVn stars we find strong
evidence that a large population of AM CVn stars have still to be discovered.
As this value sets the background to the gravitational wave signal of LISA,
this is of wide interest. We determine the mass transfer rate for 15 AM CVn
stars and find that the majority have a rate significantly greater than
expected from standard models. This is further evidence that the donor star has
a greater size than expected.Comment: Accepted by A&A in main journa
Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole
We present three-dimensional (3-D) simulations of rotationally induced line
variability arising from complex circumstellar environment of classical T Tauri
stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations
of Romanova et al., who considered accretion onto a CTTS with a misaligned
dipole magnetic axis with respect to the rotational axis. The density, velocity
and temperature structures of the MHD simulations are mapped on to the
radiative transfer grid, and corresponding line source function and the
observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are
computed using the Sobolev escape probability method. We study the dependency
of line variability on inclination angles (i) and magnetic axis misalignment
angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS
observed by Folha & Emerson, we find that models with a smaller misaligngment
angle (Theta<~15 deg.) are more consistent with the observations which show
that majority of Pa-beta are rather symmetric around the line centre. For a
high inclination system with a small dipole misalignment angle (Theta ~ 15
deg.), only one accretion funnel (on the upper hemisphere) is visible to an
observer at any given rotational phase. This can cause an anti-correlation of
the line equivalent width in the blue wing (v0)
over a half of a rotational period, and a positive correlation over other half.
We find a good overall agreement of the line variability behaviour predicted by
our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version
with full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd
The absence of jets in cataclysmic variable stars
We show that the recently developed thermal model which successfully
describes how jets are launched by young stellar objects, when applied to
system containing disk-accreting white dwarfs naturally explain the otherwise
surprising absence of jets in cataclysmic variable stars. Our main argument
uses the crucial element of the thermal model, namely that the accreted
material is strongly shocked due to large gradients of physical quantities in
the boundary layer, and then cools on a time scale longer than its ejection
time from the disk. In our scenario the magnetic fields are weak, and serve
only to recollimate the outflow at large distances from the source, or to
initiate the shock, but not as a jet-driving agent. Using two criteria in that
model, for the shock formation and for the ejection of mass, we find the mass
accretion rate above which jets could be blown from accretion disks around
young stellar objects and white dwarfs. We find that these accretion mass rates
are ~10^{-7} Mo/yr, and ~10^{-6} Mo/yr for young stellar objects and white
dwarfs respectively. Considering the uncertainties of the model, these limits
could overestimate the critical value by a factor of ~10.Comment: Submitted to Astronomy and Astrophysic
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