1,081 research outputs found
Time-Dependent Behavior of Linear Polarization in Unresolved Photospheres, With Applications for The Hanle Effect
Aims: This paper extends previous studies in modeling time varying linear
polarization due to axisymmetric magnetic fields in rotating stars. We use the
Hanle effect to predict variations in net line polarization, and use geometric
arguments to generalize these results to linear polarization due to other
mechanisms. Methods: Building on the work of Lopez Ariste et al., we use simple
analytic models of rotating stars that are symmetric except for an axisymmetric
magnetic field to predict the polarization lightcurve due to the Hanle effect.
We highlight the effects for the variable line polarization as a function of
viewing inclination and field axis obliquity. Finally, we use geometric
arguments to generalize our results to linear polarization from the weak
transverse Zeeman effect. Results: We derive analytic expressions to
demonstrate that the variable polarization lightcurve for an oblique magnetic
rotator is symmetric. This holds for any axisymmetric field distribution and
arbitrary viewing inclination to the rotation axis. Conclusions: For the
situation under consideration, the amplitude of the polarization variation is
set by the Hanle effect, but the shape of the variation in polarization with
phase depends largely on geometrical projection effects. Our work generalizes
the applicability of results described in Lopez Ariste et al., inasmuch as the
assumptions of a spherical star and an axisymmetric field are true, and
provides a strategy for separating the effects of perspective from the Hanle
effect itself for interpreting polarimetric lightcurves.Comment: 6 pages; 4 figures. Includes an extra figure found only in this
preprint versio
HST PanCET program: A Cloudy Atmosphere for the promising JWST target WASP-101b
We present results from the first observations of the Hubble Space Telescope
(HST) Panchromatic Comparative Exoplanet Treasury (PanCET) program for
WASP-101b, a highly inflated hot Jupiter and one of the community targets
proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS)
program. From a single HST Wide Field Camera 3 (WFC3) observation, we find that
the near-infrared transmission spectrum of WASP-101b contains no significant
HO absorption features and we rule out a clear atmosphere at 13{\sigma}.
Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at
observing strong molecular transmission features. We compare WASP-101b to the
well studied and nearly identical hot Jupiter WASP-31b. These twin planets show
similar temperature-pressure profiles and atmospheric features in the
near-infrared. We suggest exoplanets in the same parameter space as WASP-101b
and WASP-31b will also exhibit cloudy transmission spectral features. For
future HST exoplanet studies, our analysis also suggests that a lower count
limit needs to be exceeded per pixel on the detector in order to avoid unwanted
instrumental systematics.Comment: 7 pages, 4 figures, 1 table, Accepted to ApJ
Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b
We have developed 1-D photochemical and thermochemical kinetics and diffusion
models for the transiting exoplanets HD 189733b and HD 209458b to study the
effects of disequilibrium chemistry on the atmospheric composition of "hot
Jupiters." Here we investigate the coupled chemistry of neutral carbon,
hydrogen, oxygen, and nitrogen species, and we compare the model results with
existing transit and eclipse observations. We find that the vertical profiles
of molecular constituents are significantly affected by transport-induced
quenching and photochemistry, particularly on cooler HD 189733b; however, the
warmer stratospheric temperatures on HD 209458b can help maintain
thermochemical equilibrium and reduce the effects of disequilibrium chemistry.
For both planets, the methane and ammonia mole fractions are found to be
enhanced over their equilibrium values at pressures of a few bar to less than a
mbar due to transport-induced quenching, but CH4 and NH3 are photochemically
removed at higher altitudes. Atomic species, unsaturated hydrocarbons
(particularly C2H2), some nitriles (particularly HCN), and radicals like OH,
CH3, and NH2 are enhanced overequilibrium predictions because of quenching and
photochemistry. In contrast, CO, H2O, N2, and CO2 more closely follow their
equilibrium profiles, except at pressures < 1 microbar, where CO, H2O, and N2
are photochemically destroyed and CO2 is produced before its eventual
high-altitude destruction. The enhanced abundances of HCN, CH4, and NH3 in
particular are expected to affect the spectral signatures and thermal profiles
HD 189733b and other, relatively cool, close-in transiting exoplanets. We
examine the sensitivity of our results to the assumed temperature structure and
eddy diffusion coefficientss and discuss further observational consequences of
these models.Comment: 40 pages, 16 figures, accepted for publication in Astrophysical
Journa
Deuterium chemistry in protoplanetary disks II The inner 30 AU
We present the results of models of the chemistry, including deuterium, in
the inner regions of protostellar disks. We find good agreement with recent gas
phase observations of several (non--deuterated) species. We also compare our
results with observations of comets and find that in the absence of other
processing e.g. in the accretion shock at the surface of the disk, or by mixing
in the disk, the calculated D/H ratios in ices are higher than measured and
reflect the D/H ratio set in the molecular cloud phase. Our models give quite
different abundances and molecular distributions to other inner disk models
because of the differences in physical conditions in the model disk. This
emphasizes how changes in the assumptions about the density and temperature
distribution can radically affect the results of chemical models.Comment: Accepted by Astrophysical Journa
Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548
We report here on the discovery of stellar occultations, observed with
Kepler, that recur periodically at 15.685 hour intervals, but which vary in
depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star
that is apparently being occulted is KIC 12557548, a K dwarf with T_eff = 4400
K and V = 16. Because the eclipse depths are highly variable, they cannot be
due solely to transits of a single planet with a fixed size. We discuss but
dismiss a scenario involving a binary giant planet whose mutual orbit plane
precesses, bringing one of the planets into and out of a grazing transit. We
also briefly consider an eclipsing binary, that either orbits KIC 12557548 in a
hierarchical triple configuration or is nearby on the sky, but we find such a
scenario inadequate to reproduce the observations. We come down in favor of an
explanation that involves macroscopic particles escaping the atmosphere of a
slowly disintegrating planet not much larger than Mercury. The particles could
take the form of micron-sized pyroxene or aluminum oxide dust grains. The
planetary surface is hot enough to sublimate and create a high-Z atmosphere;
this atmosphere may be loaded with dust via cloud condensation or explosive
volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind,
dragging dust grains with it. We infer a mass loss rate from the observations
of order 1 M_E/Gyr, with a dust-to-gas ratio possibly of order unity. For our
fiducial 0.1 M_E planet, the evaporation timescale may be ~0.2 Gyr. Smaller
mass planets are disfavored because they evaporate still more quickly, as are
larger mass planets because they have surface gravities too strong to sustain
outflows with the requisite mass-loss rates. The occultation profile evinces an
ingress-egress asymmetry that could reflect a comet-like dust tail trailing the
planet; we present simulations of such a tail.Comment: 14 pages, 7 figures; submitted to ApJ, January 10, 2012; accepted
March 21, 201
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Atmospheric escape has been detected from the exoplanet HD 209458b through
transit observations of the hydrogen Lyman-alpha line. Here we present
spectrally resolved Lyman-alpha transit observations of the exoplanet HD
189733b at two different epochs. These HST/STIS observations show for the first
time, that there are significant temporal variations in the physical conditions
of an evaporating planetary atmosphere. While atmospheric hydrogen is not
detected in the first epoch observations, it is observed at the second epoch,
producing a transit absorption depth of 14.4+/-3.6% between velocities of -230
to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from
radiation pressure alone and require an additional acceleration mechanism, such
as interactions with stellar wind protons. The observed absorption can be
explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9
g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K.
An X-ray flare from the active star seen with Swift/XRT 8 hours before the
second-epoch observation supports the idea that the observed changes within the
upper atmosphere of the planet can be caused by variations in the stellar wind
properties, or by variations in the stellar energy input to the planetary
escaping gas (or a mix of the two effects). These observations provide the
first indication of interaction between the exoplanet's atmosphere and stellar
variations.Comment: To be published in A&A Letters, June 28, 201
Hubble PanCET: an isothermal day-side atmosphere for the bloated gas-giant HAT-P-32Ab
We present a thermal emission spectrum of the bloated hot Jupiter HAT-P-32Ab from a single eclipse observation made in spatial scan mode with the Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST). The spectrum covers the wavelength regime from 1.123 to 1.644âÎŒm which is binned into 14 eclipse depths measured to an averaged precision of 104 parts-per million. The spectrum is unaffected by a dilution from the close M-dwarf companion HAT-P-32B, which was fully resolved. We complemented our spectrum with literature results and performed a comparative forward and retrieval analysis with the 1D radiative-convective ATMO model. Assuming solar abundance of the planet atmosphere, we find that the measured spectrum can best be explained by the spectrum of a blackbody isothermal atmosphere with T_p = 1995 ± 17âK, but can equally well be described by a spectrum with modest thermal inversion. The retrieved spectrum suggests emission from VO at the WFC3 wavelengths and no evidence of the 1.4âÎŒm water feature. The emission models with temperature profiles decreasing with height are rejected at a high confidence. An isothermal or inverted spectrum can imply a clear atmosphere with an absorber, a dusty cloud deck or a combination of both. We find that the planet can have continuum of values for the albedo and recirculation, ranging from high albedo and poor recirculation to low albedo and efficient recirculation. Optical spectroscopy of the planet's day-side or thermal emission phase curves can potentially resolve the current albedo with recirculation degeneracy
Hubble PanCET: an isothermal day-side atmosphere for the bloated gas-giant HAT-P-32Ab
We present a thermal emission spectrum of the bloated hot Jupiter HAT-P-32Ab from a single eclipse observation made in spatial scan mode with the Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST). The spectrum covers the wavelength regime from 1.123 to 1.644âÎŒm which is binned into 14 eclipse depths measured to an averaged precision of 104 parts-per million. The spectrum is unaffected by a dilution from the close M-dwarf companion HAT-P-32B, which was fully resolved. We complemented our spectrum with literature results and performed a comparative forward and retrieval analysis with the 1D radiative-convective ATMO model. Assuming solar abundance of the planet atmosphere, we find that the measured spectrum can best be explained by the spectrum of a blackbody isothermal atmosphere with Tp = 1995 ± 17âK, but can equally well be described by a spectrum with modest thermal inversion. The retrieved spectrum suggests emission from VO at the WFC3 wavelengths and no evidence of the 1.4âÎŒm water feature. The emission models with temperature profiles decreasing with height are rejected at a high confidence. An isothermal or inverted spectrum can imply a clear atmosphere with an absorber, a dusty cloud deck or a combination of both. We find that the planet can have continuum of values for the albedo and recirculation, ranging from high albedo and poor recirculation to low albedo and efficient recirculation. Optical spectroscopy of the planet\u27s day-side or thermal emission phase curves can potentially resolve the current albedo with recirculation degeneracy
Upper atmospheres and ionospheres of planets and satellites
The upper atmospheres of the planets and their satellites are more directly
exposed to sunlight and solar wind particles than the surface or the deeper
atmospheric layers. At the altitudes where the associated energy is deposited,
the atmospheres may become ionized and are referred to as ionospheres. The
details of the photon and particle interactions with the upper atmosphere
depend strongly on whether the object has anintrinsic magnetic field that may
channel the precipitating particles into the atmosphere or drive the
atmospheric gas out to space. Important implications of these interactions
include atmospheric loss over diverse timescales, photochemistry and the
formation of aerosols, which affect the evolution, composition and remote
sensing of the planets (satellites). The upper atmosphere connects the planet
(satellite) bulk composition to the near-planet (-satellite) environment.
Understanding the relevant physics and chemistry provides insight to the past
and future conditions of these objects, which is critical for understanding
their evolution. This chapter introduces the basic concepts of upper
atmospheres and ionospheres in our solar system, and discusses aspects of their
neutral and ion composition, wind dynamics and energy budget. This knowledge is
key to putting in context the observations of upper atmospheres and haze on
exoplanets, and to devise a theory that explains exoplanet demographics.Comment: Invited Revie
An optical transmission spectrum for the ultra-hot Jupiter WASP-121b measured with the Hubble Space Telescope
This is the author accepted manuscript. The final version is available from American Astronomical Society / IOP Publishing via the DOI in this record.We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). Across the 0.47-1 micron wavelength range, the data imply an atmospheric opacity comparable to - and in some spectroscopic channels exceeding - that previously measured at near-infrared wavelengths (1.15-1.65 micron). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.8-sigma and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500K and metal enrichment of 10-30x solar matches these data well. Using a free-chemistry retrieval analysis, we estimate a VO abundance of -6.6(-0.3,+0.2) dex. We find no evidence for TiO and place a 3-sigma upper limit of -7.9 dex on its abundance, suggesting TiO may have condensed from the gas phase at the day-night limb. The opacity rises steeply at the shortest wavelengths, increasing by approximately five pressure scale heights from 0.47 to 0.3 micron in wavelength. If this feature is caused by Rayleigh scattering due to uniformly-distributed aerosols, it would imply an unphysically high temperature of 6810+/-1530K. One alternative explanation for the short-wavelength rise is absorption due to SH (mercapto radical), which has been predicted as an important product of non-equilibrium chemistry in hot Jupiter atmospheres. Irrespective of the identity of the NUV absorber, it likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere.Support for program GO-14767 was provided by
NASA through a grant from the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS 5-26555. T.M.E., D.K.S., and N.N. acknowledge funding from the European
Research Council under the European Unions Seventh Framework Programme
(FP7/2007-2013)/ERC grant agreement no. 336792. G.W.H. and M.H.W. acknowledge
support from Tennessee State University and the State of Tennessee through its
Centers of Excellence program. J.S.F. acknowledges funding by the Spanish MINECO
grant AYA2016-79425-C3-2-P. J.K.B. is supported by a Royal Astronomical Society
Research Fellowship. This work has been carried out in the frame of the National
Centre for Competence in Research PlanetS supported by the Swiss National Science
Foundation (SNSF). V.B. and D.E. have received funding from the European
Research Council (ERC) under the European Unions Horizon 2020 research and innovation
programme (project Four Aces; grant agreement no. 724427)
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