441 research outputs found
Determining Bolometric Corrections for BATSE Burst Observations
We compare the energy and count fluxes obtained by integrating over the
finite bandwidth of BATSE with a measure proportional to the bolometric energy
flux, the phi-measure, introduced by Borgonovo & Ryde. We do this on a sample
of 74 bright, long, and smooth pulses from 55 GRBs. The correction factors show
a fairly constant behavior over the whole sample, when the
signal-to-noise-ratio is high enough. We present the averaged spectral
bolometric correction for the sample, which can be used to correct flux data.Comment: 3 pages, 3 figures, to appear in AIP proc. "Gamma-Ray Burst and
Afterglow Astronomy 2001" Woods Hole, Massachusett
Modelling Circumbinary Gas Flows in Close T Tauri Binaries
Young close binaries open central gaps in the surrounding circumbinary
accretion disc, but the stellar components may still gain mass from gas
crossing through the gap. It is not well understood how this process operates
and how the stellar components are affected by such inflows. Our main goal is
to investigate how gas accretion takes place and evolves in close T Tauri
binary systems. In particular, we model the accretion flows around two close T
Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission
lines, although their orbital characteristics are very different. In order to
derive the density and velocity maps of the circumbinary material, we employ
two-dimensional hydrodynamic simulations with a locally isothermal equation of
state. The flow patterns become quasi-stable after a few orbits in the frame
co-rotating with the system. Gas flows across the circumbinary gap through the
co-rotating Lagrangian points, and local circumstellar discs develop around
both components. Spiral density patterns develop in the circumbinary disc that
transport angular momentum efficiently. Mass is preferentially channelled
towards the primary and its circumstellar disc is more massive than the disc
around the secondary. We also compare the derived density distribution to
observed line profile variability. The line profile variability tracing the gas
flows in the central cavity shows clear similarities with the corresponding
observed line profile variability in V4046 Sgr, but only when the local
circumstellar disc emission was excluded. Closer to the stars normal
magnetospheric accretion may dominate while further out the dynamic accretion
process outlined here dominates. Periodic changes in the accretion rates onto
the stars can explain the outbursts of line emission observed in eccentric
systems such as DQ Tau.Comment: Accepted for publication in MNRA
Water production in comet 81P/Wild 2 as determined by Herschel/HIFI
The high spectral resolution and sensitivity of Herschel/HIFI allows for the detection of multiple rotational water lines and accurate determinations
of water production rates in comets. In this Letter we present HIFI observations of the fundamental 1_(10)–1_(01) (557 GHz) ortho and 1_(11)–0_(00)
(1113 GHz) para rotational transitions of water in comet 81P/Wild 2 acquired in February 2010. We mapped the extent of the water line emission
with five point scans. Line profiles are computed using excitation models which include excitation by collisions with electrons and neutrals and
solar infrared radiation. We derive a mean water production rate of 1.0 × 10^(28) molecules s^(−1) at a heliocentric distance of 1.61 AU about 20 days
before perihelion, in agreement with production rates measured from the ground using observations of the 18-cm OH lines. Furthermore, we
constrain the electron density profile and gas kinetic temperature, and estimate the coma expansion velocity by fitting the water line shapes
The Vertical Structure of Planet-induced Gaps in Proto-Planetary Discs
Giant planets embedded in circumstellar discs are expected to open gaps in
these discs. We examine the vertical structure of the gap edges. We find that
the planet excites spiral arms with significant (Mach number of a half)
vertical motion of the gas, and discuss the implications of these motions. In
particular, the spiral arms will induce strong vertical stirring of the dust,
making the edge appeared `puffed up' relative to the bulk of the disc.
Infra-red observations (sensitive to dust) would be dominated by the light from
the thick inner edge of the disc. Sub-millimetre observations (sensitive to gas
velocities) would appear to be hot in `turbulent' motions (actually the ordered
motion caused by the passage of the spiral arms), but cold in chemistry.
Resolved sub-millimetre maps of circumstellar discs might even be able to
detect the spiral arms directly.Comment: Revision adds new data, and corrects physical intepretatio
Modelling Circumbinary Gas Flows in Close T Tauri Binaries
Young close binaries open central gaps in the surrounding circumbinary
accretion disc, but the stellar components may still gain mass from gas
crossing through the gap. It is not well understood how this process operates
and how the stellar components are affected by such inflows. Our main goal is
to investigate how gas accretion takes place and evolves in close T Tauri
binary systems. In particular, we model the accretion flows around two close T
Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission
lines, although their orbital characteristics are very different. In order to
derive the density and velocity maps of the circumbinary material, we employ
two-dimensional hydrodynamic simulations with a locally isothermal equation of
state. The flow patterns become quasi-stable after a few orbits in the frame
co-rotating with the system. Gas flows across the circumbinary gap through the
co-rotating Lagrangian points, and local circumstellar discs develop around
both components. Spiral density patterns develop in the circumbinary disc that
transport angular momentum efficiently. Mass is preferentially channelled
towards the primary and its circumstellar disc is more massive than the disc
around the secondary. We also compare the derived density distribution to
observed line profile variability. The line profile variability tracing the gas
flows in the central cavity shows clear similarities with the corresponding
observed line profile variability in V4046 Sgr, but only when the local
circumstellar disc emission was excluded. Closer to the stars normal
magnetospheric accretion may dominate while further out the dynamic accretion
process outlined here dominates. Periodic changes in the accretion rates onto
the stars can explain the outbursts of line emission observed in eccentric
systems such as DQ Tau.Comment: Accepted for publication in MNRA
High Precision Photometry for K2 Campaign 1
The two reaction wheel K2 mission promises and has delivered new discoveries
in the stellar and exoplanet fields. However, due to the loss of accurate
pointing, it also brings new challenges for the data reduction processes. In
this paper, we describe a new reduction pipeline for extracting high precision
photometry from the K2 dataset, and present public light curves for the K2
Campaign 1 target pixel dataset. Key to our reduction is the derivation of
global astrometric solutions from the target stamps, from which accurate
centroids are passed on for high precision photometry extraction. We extract
target light curves for sources from a combined UCAC4 and EPIC catalogue --
this includes not only primary targets of the K2 campaign 1, but also any other
stars that happen to fall on the pixel stamps. We provide the raw light curves,
and the products of various detrending processes aimed at removing different
types of systematics. Our astrometric solutions achieve a median residual of ~
0.13". For bright stars, our best 6.5 hour precision for raw light curves is
~20 parts per million (ppm). For our detrended light curves, the best 6.5 hour
precisions achieved is ~15 ppm. We show that our detrended light curves have
fewer systematic effects (or trends, or red-noise) than light curves produced
by other groups from the same observations. Example light curves of transiting
planets and a Cepheid variable candidate, are also presented. We make all light
curves public, including the raw and de-trended photometry, at
http://k2.hatsurveys.org.Comment: submitted to MNRA
The subcritical baroclinic instability in local accretion disc models
(abridged) Aims: We present new results exhibiting a subcritical baroclinic
instability (SBI) in local shearing box models. We describe the 2D and 3D
behaviour of this instability using numerical simulations and we present a
simple analytical model describing the underlying physical process.
Results: A subcritical baroclinic instability is observed in flows stable for
the Solberg-Hoiland criterion using local simulations. This instability is
found to be a nonlinear (or subcritical) instability, which cannot be described
by ordinary linear approaches. It requires a radial entropy gradient weakly
unstable for the Schwartzchild criterion and a strong thermal diffusivity (or
equivalently a short cooling time). In compressible simulations, the
instability produces density waves which transport angular momentum outward
with typically alpha<3e-3, the exact value depending on the background
temperature profile. Finally, the instability survives in 3D, vortex cores
becoming turbulent due to parametric instabilities.
Conclusions: The subcritical baroclinic instability is a robust phenomenon,
which can be captured using local simulations. The instability survives in 3D
thanks to a balance between the 2D SBI and 3D parametric instabilities.
Finally, this instability can lead to a weak outward transport of angular
momentum, due to the generation of density waves by the vortices.Comment: 12 pages, 17 figures, Accepted in A&
Edge modes in self-gravitating disc-planet interactions
We study the stability of gaps opened by a giant planet in a self-gravitating
protoplanetary disc. We find a linear instability associated with both the
self-gravity of the disc and local vortensity maxima which coincide with gap
edges. For our models, these edge modes develop and extend to twice the orbital
radius of a Saturn mass planet in discs with disc-to-star mass ratio >0.06,
corresponding to a Toomre Q < 1.5 at the outer disc boundary. Unlike the local
vortex-forming instabilities associated with gap edges in weakly or
non-self-gravitating low viscosity discs, the edge modes are global and exist
only in sufficiently massive discs, but for the typical viscosity values
adopted for protoplanetary discs. Analytic modelling and linear calculations
show edge modes may be interpreted as a localised disturbance associated with a
gap edge inducing activity in the extended disc, through the launching of
density waves excited at Lindblad resonances. Nonlinear hydrodynamic
simulations are performed to investigate the evolution of edge modes in
disc-planet systems. The form and growth rates of unstable modes are consistent
with linear theory. Their dependence on viscosity and gravitational softening
is also explored. We also performed a first study of the effect of edge modes
on planetary migration. We found that if edge modes develop, then the average
disc-on-planet torque becomes more positive with increasing disc mass. In
simulations where the planet was allowed to migrate, although a fast type III
migration could be seen that was similar to that seen in non-self-gravitating
discs, we found that it was possible for the planet to interact gravitationally
with the spiral arms associated with an edge mode and that this could result in
the planet being scattered outwards. Thus orbital migration is likely to be
complex and non monotonic in massive discs of the type we consider.Comment: 26 pages, 21 figures. Accepted by MNRAS. Abstract displayed is
shortene
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