3,702 research outputs found
Hot Jupiters and Cool Stars
Close-in planets are in jeopardy as their host stars evolve off the main
sequence to the subgiant and red giant phases. In this paper, we explore the
influences of the stellar mass (in the range 1.5--2\Mso ), mass-loss
prescription, planet mass (from Neptune up to 10 Jupiter masses), and
eccentricity, on the orbital evolution of planets as their parent stars evolve
to become subgiants and Red Giants. We find that planet engulfment during the
Red Giant Branch is not very sensitive to the stellar mass or mass-loss rates
adopted in the calculations, but quite sensitive to the planetary mass. The
range of initial separations for planet engulfment increases with decreasing
mass-loss rates or stellar mass and increasing planetary masses. Regarding the
planet's orbital eccentricity, we find that as the star evolves into the red
giant phase, stellar tides start to dominate over planetary tides. As a
consequence, a transient population of moderately eccentric close-in Jovian
planets is created, that otherwise would have been expected to be absent from
main sequence stars. We find that very eccentric and distant planets do not
experience much eccentricity decay, and that planet engulfment is primarily
determined by the pericenter distance and the maximum stellar radius.Comment: 38 pages, 15 figures, accepted for publication in Ap
RS Ophiuchi: Thermonuclear Explosion or Disc Instability?
Sokoloski et al (2008) have recently reported evidence that the recurrent
nova RS Ophiuchi produced a pair of highly collimated radio jets within days of
its 2006 outburst. This suggests that an accretion disc must be present during
the outburst. However in the standard picture of recurrent novae as
thermonuclear events, any such disc must be expelled from the white dwarf
vicinity, as the nuclear energy yield greatly exceeds its binding energy. We
suggest instead that the outbursts of RS Oph are thermal--viscous instabilities
in a disc irradiated by the central accreting white dwarf. The distinctive
feature of RS Oph is the very large size of its accretion disc. Given this, it
fits naturally into a consistent picture of systems with unstable accretion
discs. This picture explains the presence and speed of the jets, the brightness
and duration of the outburst, and its rise time and linear decay, as well as
the faintness of the quiescence. By contrast, the hitherto standard picture of
recurrent thermonuclear explosions has a number of severe difficulties. These
include the presence of jets, the faintness of quiescence, and the fact the the
accretion disc must be unstable unless it is far smaller than any reasonable
estimate.Comment: MNRAS, in pres
Is the Bardeen-Petterson effect responsible for the warping and precession in NGC 4258?
Strong evidence for the presence of a warped Keplerian accretion disc in
NGC4258 (M 106) has been inferred from the kinematics of water masers detected
at sub-parsec scales. Assuming a power-law accretion disc and using constraints
on the disc parameters derived from observational data, we have analyzed the
relativistic Bardeen-Petterson effect driven by a Kerr black hole as the
potential physical mechanism responsible for the disc warping. We found that
the Bardeen-Petterson radius is comparable to or smaller than the inner radius
of the maser disc (independent of the allowed value for the black hole spin
parameter). Numerical simulations for a wide range of physical conditions have
shown that the evolution of a misaligned disc due to the Bardeen-Petterson
torques usually produces an inner flat disc and a warped transition region with
a smooth gradient in the tilt and twist angles. Since this structure is similar
to that seen in NGC 4258, we propose that the Bardeen-Petterson effect may be
responsible for the disc warping in this galaxy. We estimated the time-scale
necessary for the disc inside of the Bardeen-Petterson radius to align with the
black hole's equator, as a function of the black hole spin. Our results show
that the Bardeen-Petterson effect can align the disc within a few billion years
in the case of NGC 4258. Finally, we show that if the observed curvature of the
outer anomalous arms in the galactic disc of NGC 4258 is associated with the
precession of its radio jet/counterjet, then the Bardeen-Petterson effect can
provide the required precession period.Comment: 10 pages, 5 figures, 1 table, accepted for publication in The Monthly
Notices of the Royal Astronomical Societ
Evidence for a merger of binary white dwarfs: the case of GD 362
GD 362 is a massive white dwarf with a spectrum suggesting a H-rich
atmosphere which also shows very high abundances of Ca, Mg, Fe and other
metals. However, for pure H-atmospheres the diffusion timescales are so short
that very extreme assumptions have to be made to account for the observed
abundances of metals. The most favored hypothesis is that the metals are
accreted from either a dusty disk or from an asteroid belt. Here we propose
that the envelope of GD 362 is dominated by He, which at these effective
temperatures is almost completely invisible in the spectrum. This assumption
strongly alleviates the problem, since the diffusion timescales are much larger
for He-dominated atmospheres. We also propose that the He-dominated atmosphere
of GD 362 is likely to be the result of the merger of a binary white dwarf.Comment: 4 pages, 3 figures. Accepted for publication in Astrophysical Journal
Letter
The Formation and Role of Vortices in Protoplanetary Disks
We carry out a two-dimensional, compressible, simulation of a disk, including
dust particles, to study the formation and role of vortices in protoplanetary
disks. We find that anticyclonic vortices can form out of an initial random
perturbation of the vorticity field. Vortices have a typical decay time of the
order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk
aspect ratio of H/r = 0.15). If vorticity is continuously generated at a
constant rate in the flow (e.g. by convection), then a large vortex can form
and be sustained (due to the merger of vortices).
We find that dust concentrates in the cores of vortices within a few orbital
periods, when the drag parameter is of the order of the orbital frequency.
Also, the radial drift of the dust induces a significant increase in the
surface density of dust particles in the inner region of the disk. Thus,
vortices may represent the preferred location for planetesimal formation in
protoplanetary disks.
We show that it is very difficult for vortex mergers to sustain a relatively
coherent outward flux of angular momentum.Comment: Sumitted to the Astrophysical Journal, October 20, 199
A Characterization of the Brightness Oscillations During Thermonuclear Bursts From 4U 1636-536
The discovery of nearly coherent brightness oscillations during thermonuclear
X-ray bursts from six neutron-star low-mass X-ray binaries has opened up a new
way to study the propagation of thermonuclear burning, and may ultimately lead
to greater understanding of thermonuclear propagation in other astrophysical
contexts, such as in Type Ia supernovae. Here we report detailed analyses of
the ~580 Hz brightness oscillations during bursts from 4U 1636-536. We
investigate the bursts as a whole and, in more detail, the initial portions of
the bursts. We analyze the ~580 Hz oscillations in the initial 0.75 seconds of
the five bursts that were used in a previous search for a brightness
oscillation at the expected ~290 Hz spin frequency, and find that if the same
frequency model describes all five bursts there is insufficient data to require
more than a constant frequency or, possibly, a frequency plus a frequency
derivative. Therefore, although it is appropriate to use an arbitrarily
complicated model of the ~580 Hz oscillations to generate a candidate waveform
for the ~290 Hz oscillations, models with more than two parameters are not
required by the data. For the bursts as a whole we show that the
characteristics of the brightness oscillations vary greatly from burst to
burst. We find, however, that in at least one of the bursts, and possibly in
three of the four that have strong brightness oscillations throughout the
burst, the oscillation frequency reaches a maximum several seconds into the
burst and then decreases. This behavior has not been reported previously for
burst brightness oscillations, and it poses a challenge to the standard burning
layer expansion explanation for the frequency changes.Comment: 18 pages including three figures, uses aaspp4.sty, submitted to The
Astrophysical Journal on April
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