878 research outputs found
Impact of Solar Wind Depression on the Dayside Magnetosphere under Northward Interplanetary Magnetic Field
We present a follow up study of the sensitivity of the Earth's magnetosphere
to solar wind activity using a particles-in-cell model [Baraka and Ben Jaffel,
2007], but here during northward IMF. The formation of the magnetospheric
cavity and its elongation is obtained with the classical structure of a
magnetosphere with parallel lobes. An impulsive disturbance is then applied to
the system by changing the bulk velocity of the solar wind to simulate a
decrease in the solar wind dynamic pressure followed by its recovery. In
response to the imposed disturbance, a gap [abrupt depression] in the incoming
solar wind plasma appears moving toward the Earth. The gap's size is a ~15 RE
and is comparable to the sizes previously obtained for both Bz<0 and Bz =0.
During the initial phase of the disturbance, the dayside magnetopause (MP)
expands slower than the previous cases of IMF orientations as a result of the
depression. The size of the MP expands nonlinearly due to strengthening of its
outer boundary by the northward IMF. Also, during the initial 100 {\Delta}t,
the MP shrank down from 13.3 RE to ~9.2 RE before it started expanding; a
phenomenon that was also observed for southern IMF conditions but not during
the no IMF case. As soon as they felt the solar wind depression, cusps widened
at high altitude while dragged in an upright position. For the field's
topology, the reconnection between magnetospheric and magnetosheath fields is
clearly observed in both northward and southward cusps areas. Also, the tail
region in the northward IMF condition is more confined, in contrast to the
fishtail-shape obtained in the southward IMF case. An X-point is formed in the
tail at ~110 RE compared to ~103 RE and ~80 RE for Bz =0 and Bz <0
respectively. Our findings are consistent with existing reports from many space
observatories for which predictions are proposed to test furthermore our
simulation technique.Comment: 48 pages, 6 figures, 1 table, accepted for publication in Annales
Geophysicae (ANGEO Communicates
Re-visit of HST FUV observations of hot-Jupiter system HD 209458: No Si III detection and the need for COS transit observations
The discovery of OI atoms and CII ions in the upper atmosphere of HD 209458b,
made with the Hubble Space Telescope Imaging Spectrograph (STIS) using the
G140L grating, showed that these heavy species fill an area comparable to the
planet's Roche lobe. The derived ~10% transit absorption depths require
super-thermal processes and/or supersolar abundances. From subsequent Cosmic
Origins Spectrograph (COS) observations, CII absorption was reported with
tentative velocity signatures, and absorption by SiIII ions was also claimed in
disagreement with a negative STIS G140L detection. Here, we revisit the COS
dataset showing a severe limitation in the published results from having
contrasted the in-transit spectrum against a stellar spectrum averaged from
separate observations, at planetary phases 0.27, 0.72, and 0.49. We find
variable stellar SiIII and CII emissions that were significantly depressed not
only during transit but also at phase 0.27 compared to phases 0.72 and 0.49.
Their respective off-transit 7.5 and 3.1% flux variations are large compared to
their reported 8.2+/-1.4% and 7.8+/-1.3% transit absorptions. Significant
variations also appear in the stellar line shapes, questioning reported
velocity signatures. We furthermore present archive STIS G140M transit data
consistent with no SiIII absorption, with a negative result of 1.7+/-18.7
including ~15% variability. Silicon may still be present at lower ionization
states, in parallel with the recent detection of extended magnesium, as MgI
atoms. In this frame, the firm detection of OI and CII implying solar or
supersolar abundances contradicts the recent inference of potential x20-125
subsolar metallicity for HD 209458b.Comment: Accepted for publication in Ap
Spectral, Spatial, and Time properties of the hydrogen nebula around exoplanet HD209458b
All far ultraviolet observations of HD209458 tend to support a scenario in
which the inflated hydrogen atmosphere of its planetary companion strongly
absorbs the stellar \lya flux during transit. However, it was not clear how the
transit absorption depends on the selected wavelength range in the stellar line
profile, nor how the atomic hydrogen cloud was distributed spatially around
HD209458b. Here we report a sensitivity study of observed time and spectral
variations of the stellar flux. In particular, the sensitivity of the
absorption depth during transit to the assumed spectral range in the stellar
line profile is shown to be very weak, leading to a transit depth in the range
for all possible wavelength ranges, and thereby confirming
our initially-reported absorption rate. Taking the ratio of the line profile
during transit to the unperturbed line profile, we also show that the spectral
signature of the absorption by the exoplanetary hydrogen nebula is symmetric
and typical of a Lorentzian, optically thick medium. Our results question the
adequacy of models that require a huge absorption and/or a strong asymmetry
between the blue and red side of the absorption line during transit as no such
features could be detected in the HST FUV absorption profile. Finally, we show
that standard atmospheric models of HD209458b provide a good fit to the
observed absorption profile during transit. Other hybrid models assuming a
standard model with a thin layer of superthermal hydrogen on top remain
possible.Comment: 10 pages, 7 figures, accepted for publication in Astrophysical
Journa
Observations of Mass Loss from the Transiting Exoplanet HD 209458b
Using the new Cosmic Origins Spectrograph (COS) on the {\it Hubble Space
Telescope (HST)}, we obtained moderate-resolution, high signal/noise
ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit,
both orbital quadratures, and secondary eclipse. We compare transit spectra
with spectra obtained at non-transit phases to identify spectral features due
to the exoplanet's expanding atmosphere. We find that the mean flux decreased
by % for the C II 1334.5323\AA\ and 1335.6854\AA\ lines and by
% for the Si III 1206.500\AA\ line during transit compared to
non-transit times in the velocity interval --50 to +50 km s. Comparison
of the C II and Si III line depths and transit/non-transit line ratios shows
deeper absorption features near --10 and +15 km s and less certain
features near --40 and +30--70 km s, but future observations are needed
to verify this first detection of velocity structure in the expanding
atmosphere of an exoplanet. Our results for the C II lines and the
non-detection of Si IV 1394.76\AA\ absorption are in agreement with
\citet{Vidal-Madjar2004}, but we find absorption during transit in the Si III
line contrary to the earlier result. The % obscuration of the star
during transit is far larger than the 1.5% obscuration by the exoplanet's disk.
Absorption during transit at velocities between --50 and +50 km s in the
C II and Si III lines requires high-velocity ion absorbers, but models that
assume that the absorbers are high-temperature thermal ions are inconsistent
with the COS spectra. Assuming hydrodynamic model values for the gas
temperature and outflow velocity at the limb of the outflow as seen in the C II
lines, we find mass-loss rates in the range (8--40) g s.Comment: 25 pages, 4 figures, Astrophysical Journal in pres
Searching for Far-Ultraviolet Auroral/Dayglow Emission from HD209458b
We present recent observations from the HST-Cosmic Origins Spectrograph aimed
at characterizing the auroral emission from the extrasolar planet HD209458b. We
obtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectra
at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar
baseline measurement at secondary eclipse. This analysis includes a catalog of
stellar emission lines and a star-subtracted spectrum of the planet. We present
an emission model for planetary H2 emission, and compare this model to the
planetary spectrum. No unambiguously identifiable atomic or molecular features
are detected, and upper limits are presented for auroral/dayglow line
strengths. An orbital velocity cross-correlation analysis finds a statistically
significant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of the
planet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9)
P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required to
excite this transition is unclear. We compare limits on relative line strengths
seen in the exoplanet spectrum with models of ultraviolet fluorescence to
constrain the atmospheric column density of neutral hydrogen between the star
and the planetary surface. These results support models of short period
extrasolar giant planets with weak magnetic fields and extended atomic
atmospheres.Comment: Accepted to ApJ. 12 pages, 5 figures, 4 table
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
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
Saturn's atmospheric response to the large influx of ring material inferred from Cassini INMS measurements
During the Grand Finale stage of the Cassini mission, organic-rich ring
material was discovered to be flowing into Saturn's equatorial upper atmosphere
at a surprisingly large rate. Through a series of photochemical models, we have
examined the consequences of this ring material on the chemistry of Saturn's
neutral and ionized atmosphere. We find that if a substantial fraction of this
material enters the atmosphere as vapor or becomes vaporized as the solid ring
particles ablate upon atmospheric entry, then the ring-derived vapor would
strongly affect the composition of Saturn's ionosphere and neutral
stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing
data from the final few years of the mission, however, reveal none of these
predicted chemical consequences. We therefore conclude that either (1) the
inferred ring influx represents an anomalous, transient situation that was
triggered by some recent dynamical event in the ring system that occurred a few
months to a few tens of years before the 2017 end of the Cassini mission, or
(2) a large fraction of the incoming material must have been entering the
atmosphere as small dust particles less than ~100 nm in radius, rather than as
vapor or as large particles that are likely to ablate. Future observations or
upper limits for stratospheric neutral species such as HCN, HCN, and CO
at infrared wavelengths could shed light on the origin, timing, magnitude, and
nature of a possible vapor-rich ring-inflow event.Comment: accepted in Icaru
Hubble PanCET: An isothermal day-side atmosphere for the bloated gas-giant HAT-P-32Ab
This is the author accepted manuscript. The final version is available from OUP via the DOI in this recordWe 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 1.644 microns 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 +/- 17K, 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 1.4 micron 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.NN, DKS and TME acknowledge funding from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 336792. JG acknowledges support from a Leverhulme Trust Research Project Grant. G.W.H. and M.H.W. acknowledge long-term support from Tennessee State University and the State of Tennessee through its Centers of Excellence program and from the Space Telescope Science Institue under HST-GO-14767. 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). DE and VB acknowledge the financial support of the SNSF. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (project FOUR ACES; grant agreement No 724427)
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