49 research outputs found
The Blast Energy Efficiency of GRBs
Using data mostly assembled by previous authors, we consider the linear
correlation between the apparent radiative efficiency
(defined as the ratio of isotropic equivalent radiative output to inferred
isotropic equivalent kinetic energy of the blast) and where
, for 17 of 22 GRBs (Lloyd-Ronning and Zhang, 2004). We note in a
quantitative manner that this is consistent with the hypothesis that
and are influenced by viewing angle. We suggest
a more general theoretically derived expression for this correlation that could
be tested with a richer data set. If the reduction in both
and is due to viewing angle effects, then the actual radiative
efficiency is . We also find preliminary evidence (with a small sample)
for a separate class of weak GRB afterglows.Comment: Submitted to ApJL Feb. 10, 200
Magnetic Field Effects on the Motion of Circumplanetary Dust
Hypervelocity impacts on satellites or ring particles replenish
circumplanetary dusty rings with grains of all sizes. Due to
interactions with the plasma environment and sunlight, these grains
become electrically charged. We study the motion of charged dust
grains launched at the Kepler orbital speed, under the combined
effects of gravity and the electromagnetic force.
We conduct numerical simulations of dust grain trajectories, covering
a broad range of launch distances from the planetary surface to beyond
synchronous orbit, and the full range of charge-to-mass ratios from
ions to rocks, with both positive and negative electric
potentials. Initially, we assume that dust grains have a constant
electric potential, and, treating the spinning planetary magnetic
field as an aligned and centered dipole, we map regions of radial
instability (positive grains only), where dust grains are driven to
escape or collide with the planet at high speed, and vertical
instability (both positive and negative charges) whereby grains
launched near the equatorial plane and are forced up magnetic field
lines to high latitudes, where they may collide with the planet.
We derive analytical criteria for local stability in the equatorial
plane, and solve for the boundaries between all unstable and stable
outcomes. Comparing our analytical solutions to our numerical
simulations, we develop an extensive model for the radial, vertical
and azimuthal motions of dust grains of arbitrary size and launch
location. We test these solutions at Jupiter and Saturn, both of whose
magnetic fields are reasonably well represented by aligned dipoles, as
well as at the Earth, whose magnetic field is close to an anti-aligned
dipole.
We then evaluate the robustness of our stability boundaries to more
general conditions. Firstly, we examine the effects of non-zero launch
speeds, of up to 0.5 km s, in the frame of the parent
body. Although these only weakly affect stability boundaries, we find
that the influence of a launch impulse on stability boundaries
strongly depends on its direction.
Secondly, we focus on the effects of higher-order magnetic field
components on orbital stability. We find that vertical stability
boundaries are particularly sensitive to a moderate vertical offset in
an aligned dipolar magnetic field. This configuration suffices as a
model for Saturn's full magnetic field. The vertical instability also
expands to cover a wider range of launch distances in slightly tilted
magnetic dipoles, like the magnetic field configurations for Earth and
Jupiter. By contrast, our radial stability criteria remain largely
unaffected by both dipolar tilts and vertical offsets.
Nevertheless, a tilted dipole magnetic field model introduces
non-axisymmetric forces on orbiting dust grains, which are exacerbated
by the inclusion of other higher-order magnetic field components,
including the quadrupolar and octupolar terms. Dust grains whose
orbital periods are commensurate with the spatial periodicities of a
rotating non-axisymmetric magnetic field experience destabilizing
Lorentz resonances. These have been studied by other authors for the
largest dust grains moving on perturbed Keplerian ellipses. With
Jupiter's full magnetic field as our model, we extend the concept of
Lorentz resonances to smaller dust grains and find that these can
destabilize trajectories on surprisingly short timescales, and even
cause negatively-charged dust grains to escape within weeks. We
provide detailed numerically-derived stability maps highlighting the
destabilizing effects of specific higher-order terms in Jupiter's
magnetic field, and we develop analytical solutions for the radial
locations of these resonances for all charge-to-mass ratios.
We include stability maps for the full magnetic field configurations
of Jupiter, Saturn, and Earth, to compare with our analytics. We
further provide numerically-derived stability maps for the tortured
magnetic fields of Uranus and Neptune.
Relaxing the assumption of constant electric charges on dust, we test
the effects of time-variable grain charging on dust grain motion in
two distinct environments. Firstly, we examine orbital stability in
the tenuous plasma of Jupiter's main ring and gossamer ring where
sunlight, the dominant source of grain charging, is periodically
interrupted by transit through the planetary shadow. This dramatically
expands dynamical instabilities to cover a large range of grain
sizes. Secondly, we study the motion of dust grain orbits in the dense
plasma environment of the Io torus. Here dust grain charges deviate
little from equilibrium, and our stability map conforms closely to
that of constant, negatively-charged dust grains.
Finally, we focus on the poorly understood spokes in Saturn's B ring,
highlighting the observational constraints on spokes, and present our
hypothesis for spoke formation
TESS Observations of Kepler systems with Transit Timing Variations
We identify targets in the Kepler field that may be characterized by transit
timing variations (TTVs) and are detectable by the Transiting Exoplanet Survey
Satellite (TESS). Despite the reduced signal-to-noise ratio of TESS transits
compared to Kepler, we recover 48 transits from 13 systems in Sectors 14, 15,
26, 40 and 41. We find strong evidence of a nontransiting perturber orbiting
Kepler-396 (KOI-2672) and explore two possible cases of a third planet in that
system that could explain the measured transit times. We update the ephemerides
and mass constraints where possible at KOI-70 (Kepler-20), KOI-82 (Kepler-102),
KOI-94 (Kepler-89), KOI-137 (Kepler-18), KOI-244 (Kepler-25), KOI-245
(Kepler-37), KOI-282 (Kepler-130), KOI-377 (Kepler-9), KOI-620 (Kepler-51),
KOI-806 (Kepler-30), KOI-1353 (Kepler-289) and KOI-1783 (Kepler-1662).Comment: 26 pages, 9 figure
A Ground-Based Albedo Upper Limit for HD 189733b from Polarimetry
We present 50 nights of polarimetric observations of HD 189733 in band
using the POLISH2 aperture-integrated polarimeter at the Lick Observatory Shane
3-m telescope. This instrument, commissioned in 2011, is designed to search for
Rayleigh scattering from short-period exoplanets due to the polarized nature of
scattered light. Since these planets are spatially unresolvable from their host
stars, the relative contribution of the planet-to-total system polarization is
expected to vary with an amplitude of order 10 parts per million (ppm) over the
course of the orbit. Non-zero and also variable at the 10 ppm level, the
inherent polarization of the Lick 3-m telescope limits the accuracy of our
measurements and currently inhibits conclusive detection of scattered light
from this exoplanet. However, the amplitude of observed variability
conservatively sets a upper limit to the planet-induced polarization
of the system of 58 ppm in band, which is consistent with a previous upper
limit from the POLISH instrument at the Palomar Observatory 5-m telescope
(Wiktorowicz 2009). A physically-motivated Rayleigh scattering model, which
includes the depolarizing effects of multiple scattering, is used to
conservatively set a upper limit to the geometric albedo of HD
189733b of . This value is consistent with the value derived from occultation observations with HST STIS (Evans et al.
2013), but it is inconsistent with the large albedo
reported by (Berdyugina et al. 2011).Comment: 10 pages, 9 figures, submitted to Ap
Time variation of Kepler transits induced by stellar spots - a way to distinguish between prograde and retrograde motion. II. Application to KOIs
Mazeh, Holczer, and Shporer (2015) have presented an approach that can, in
principle, use the derived transit timing variation (TTV) of some transiting
planets observed by the mission to distinguish between prograde and
retrograde motion of their orbits with respect to their parent stars' rotation.
The approach utilizes TTVs induced by spot-crossing events that occur when the
planet moves across a spot on the stellar surface, looking for a correlation
between the derived TTVs and the stellar brightness derivatives at the
corresponding transits. This can work even in data that cannot temporally
resolve the spot-crossing events themselves. Here we apply this approach to the
KOIs, identifying nine systems where the photometric spot modulation
is large enough and the transit timing accurate enough to allow detection of a
TTV-brightness-derivatives correlation. Of those systems five show highly
significant prograde motion (Kepler-17b, Kepler-71b, KOI-883.01, KOI-895.01,
and KOI-1074.01), while no system displays retrograde motion, consistent with
the suggestion that planets orbiting cool stars have prograde motion. All five
systems have impact parameter , and all systems
within that impact parameter range show significant correlation, except
HAT-P-11b where the lack of a correlation follows its large stellar obliquity.
Our search suffers from an observational bias against detection of high impact
parameter cases, and the detected sample is extremely small. Nevertheless, our
findings may suggest that stellar spots, or at least the larger ones, tend to
be located at a low stellar latitude, but not along the stellar equator,
similar to the Sun.Comment: V2: accepted to Ap