7,722 research outputs found
Theory for the Secondary Eclipse Fluxes, Spectra, Atmospheres, and Light Curves of Transiting Extrasolar Giant Planets
We have created a general methodology for calculating the
wavelength-dependent light curves of close-in extrasolar giant planets (EGPs)
as they traverse their orbits. Focussing on the transiting EGPs HD189733b,
TrES-1, and HD209458b, we calculate planet/star flux ratios during secondary
eclipse and compare them with the Spitzer data points obtained so far in the
mid-infrared. We introduce a simple parametrization for the redistribution of
heat to the planet's nightside, derive constraints on this parameter (P_n), and
provide a general set of predictions for planet/star contrast ratios as a
function of wavelength, model, and phase. Moreover, we calculate average
dayside and nightside atmospheric temperature/pressure profiles for each
transiting planet/P_n pair with which existing and anticipated Spitzer data can
be used to probe the atmospheric thermal structure of severely irradiated EGPs.
We find that the baseline models do a good job of fitting the current secondary
eclipse dataset, but that the Spitzer error bars are not yet small enough to
discriminate cleanly between all the various possibilities.Comment: 14 figures, 7 text pages (in two-column emulateapj format); Accepted
to the Ap.J. June 26, 2006; one cosmetic change made to astro-ph version
Studies of high latitude current systems using MAGSAT vector data
The magnetic disturbance fields caused by global external current systems are considered with particular emphasis on improving the understanding of the physical processes which control high latitude current systems. Following processing the MAGSAT data were routinely plotted in the Universal Time (UT) format as well as in a polar plot format. The H'D'U' coordinate system, was adopted as the standard for representing the MAGSAT residual magnetic field vectors. A data file was generated and the TPOLAR computer code was developed to determine from the orbital elements, the time, latitude, and MLT of the extremum latitude of each transpolar segment of orbit. The precision of the vector data set from MAGSAT prompted an extended exploratory phase for data analysis procedures, modeling techniques and phenomenology
Studies of high latitude current systems using Magsat vector data
Disturbance fields caused by global external current systems are analyzed in order to gain an improved understanding of the phydical processes which control high latitude current systems and to increase the confidence level in the identification of internal field levels. The basic approach is to: (1) categorize the vector data by those physical parameters important for investigation of external current systems; (2) map the disturbances for appropriate conditions; (3) model the currents which might cause the mapped disturbances; and (4) correlate results with data from other sources. It is concluded that the Magsat data set appears to have remarkably high precision and quality and should permit major advances to be made in modeling current distribution at high latitudes in the ionosphere and magnetosphere
GRB Energetics in the Swift Era
We examine the rest frame energetics of 76 gamma-ray bursts (GRBs) with known
redshift that were detected by the Swift spacecraft and monitored by the
satellite's X-ray Telescope (XRT). Using the bolometric fluence values
estimated in Butler et al. 2007b and the last XRT observation for each event,
we set a lower limit the their collimation corrected energy Eg and find that a
68% of our sample are at high enough redshift and/or low enough fluence to
accommodate a jet break occurring beyond the last XRT observation and still be
consistent with the pre-Swift Eg distribution for long GRBs. We find that
relatively few of the X-ray light curves for the remaining events show evidence
for late-time decay slopes that are consistent with that expected from post jet
break emission. The breaks in the X-ray light curves that do exist tend to be
shallower and occur earlier than the breaks previously observed in optical
light curves, yielding a Eg distribution that is far lower than the pre-Swift
distribution. If these early X-ray breaks are not due to jet effects, then a
small but significant fraction of our sample have lower limits to their
collimation corrected energy that place them well above the pre-Swift Eg
distribution. Either scenario would necessitate a much wider post-Swift Eg
distribution for long cosmological GRBs compared to the narrow standard energy
deduced from pre-Swift observations. We note that almost all of the pre-Swift
Eg estimates come from jet breaks detected in the optical whereas our sample is
limited entirely to X-ray wavelengths, furthering the suggestion that the
assumed achromaticity of jet breaks may not extend to high energies.Comment: 30 pages, 10 figures, Accepted to Ap
Analysis of the X-ray Emission of Nine Swift Afterglows
The X-ray light-curves of 9 Swift XRT afterglows (050126, 050128, 050219A,
050315, 050318, 050319, 050401, 050408, 050505) display a complex behaviour: a
steep t^{-3.0 \pm 0.3} decay until ~400 s, followed by a significantly slower
t^{-0.65+/-0.20} fall-off, which at 0.2--2 d after the burst evolves into a
t^{-1.7+/-0.5} decay. We consider three possible models for the geometry of
relativistic blast-waves (spherical outflows, non-spreading jets, and spreading
jets), two possible dynamical regimes for the forward shock (adiabatic and
fully radiative), and we take into account a possible angular structure of the
outflow and delayed energy injection in the blast-wave, to identify the models
which reconcile the X-ray light-curve decay with the slope of the X-ray
continuum for each of the above three afterglow phases. By piecing together the
various models for each phase in a way that makes physical sense, we identify
possible models for the entire X-ray afterglow. The major conclusion of this
work is that a long-lived episode of energy injection in the blast-wave, during
which the shock energy increases at t^{1.0+/-0.5}, is required for five
afterglows and could be at work in the other four as well. Optical observations
in conjunction with the X-ray can distinguish among these various models. Our
simple tests allow the determination of the location of the cooling frequency
relative to the X-ray domain and, thus, of the index of the electron power-law
distribution with energy in the blast-wave. The resulting indices are clearly
inconsistent with an universal value.Comment: 10 pages, minor changes, to be published in the MNRA
Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands
We find that a theoretical fit to all the HD 209458b data at secondary
eclipse requires that the dayside atmosphere of HD 209458b have a thermal
inversion and a stratosphere. This inversion is caused by the capture of
optical stellar flux by an absorber of uncertain origin that resides at
altitude. One consequence of stratospheric heating and temperature inversion is
the flipping of water absorption features into emission features from the near-
to the mid-infrared and we see evidence of such a water emission feature in the
recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere
optical absorber may help explain both the weaker-than-expected Na D feature
seen in transit and the fact that the transit radius at 24 m is smaller
than the corresponding radius in the optical. Moreover, it may be a factor in
why HD 209458b's optical transit radius is as large as it is. We speculate on
the nature of this absorber and the planets whose atmospheres may, or may not,
be affected by its presence.Comment: Accepted to the Astrophysical Journal Letters on August 28, 2007, six
pages in emulateapj forma
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