766 research outputs found
The Upper Atmosphere of HD17156b
HD17156b is a newly-found transiting extrasolar giant planet (EGP) that
orbits its G-type host star in a highly eccentric orbit (e~0.67) with an
orbital semi-major axis of 0.16 AU. Its period, 21.2 Earth days, is the longest
among the known transiting planets. The atmosphere of the planet undergoes a
27-fold variation in stellar irradiation during each orbit, making it an
interesting subject for atmospheric modelling. We have used a three-dimensional
model of the upper atmosphere and ionosphere for extrasolar gas giants in order
to simulate the progress of HD17156b along its eccentric orbit. Here we present
the results of these simulations and discuss the stability, circulation, and
composition in its upper atmosphere. Contrary to the well-known transiting
planet HD209458b, we find that the atmosphere of HD17156b is unlikely to escape
hydrodynamically at any point along the orbit, even if the upper atmosphere is
almost entirely composed of atomic hydrogen and H+, and infrared cooling by H3+
ions is negligible. The nature of the upper atmosphere is sensitive to to the
composition of the thermosphere, and in particular to the mixing ratio of H2,
as the availability of H2 regulates radiative cooling. In light of different
simulations we make specific predictions about the thermosphere-ionosphere
system of HD17156b that can potentially be verified by observations.Comment: 31 pages, 42 eps figure
Ground-based detection of sodium in the transmission spectrum of exoplanet HD209458b
[Context] The first detection of an atmosphere around an extrasolar planet
was presented by Charbonneau and collaborators in 2002. In the optical
transmission spectrum of the transiting exoplanet HD209458b, an absorption
signal from sodium was measured at a level of 0.023+-0.006%, using the STIS
spectrograph on the Hubble Space Telescope. Despite several attempts, so far
only upper limits to the Na D absorption have been obtained using telescopes
from the ground, and the HST result has yet to be confirmed.
[Aims] The aims of this paper are to re-analyse data taken with the High
Dispersion Spectrograph on the Subaru telescope, to correct for systematic
effects dominating the data quality, and to improve on previous results
presented in the literature.
[Methods] The data reduction process was altered in several places, most
importantly allowing for small shifts in the wavelength solution. The relative
depth of all lines in the spectra, including the two sodium D lines, are found
to correlate strongly with the continuum count level in the spectra. These
variations are attributed to non-linearity effects in the CCDs. After removal
of this empirical relation the uncertainties in the line depths are only a
fraction above that expected from photon statistics.
[Results] The sodium absorption due to the planet's atmosphere is detected at
>5 sigma, at a level of 0.056+-0.007% (2x3.0 Ang band), 0.070+-0.011% (2x1.5
Ang band), and 0.135+-0.017% (2x0.75 Ang band). There is no evidence that the
planetary absorption signal is shifted with respect to the stellar absorption,
as recently claimed for HD189733b. The measurements in the two most narrow
bands indicate that some signal is being resolved.[abridged]Comment: Latex, 7 pages: accepted for publication in Astronomy & Astrophysic
Deuterium Toward WD1634-573: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
We use Far Ultraviolet Spectrocopic Explorer (FUSE) observations to study
interstellar absorption along the line of sight to the white dwarf WD1634-573
(d=37.1+/-2.6 pc). Combining our measurement of D I with a measurement of H I
from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD1634-573
of D/H=(1.6+/-0.5)e-5. In contrast, multiplying our measurements of D I/O
I=0.035+/-0.006 and D I/N I=0.27+/-0.05 with published mean Galactic ISM gas
phase O/H and N/H ratios yields D/H(O)=(1.2+/-0.2)e-5 and
D/H(N)=(2.0+/-0.4)e-5, respectively. Note that all uncertainties quoted above
are 2 sigma. The inconsistency between D/H(O) and D/H(N) suggests that either
the O I/H I and/or the N I/H I ratio toward WD1634-573 must be different from
the previously measured average ISM O/H and N/H values. The computation of
D/H(N) from D I/N I is more suspect, since the relative N and H ionization
states could conceivably vary within the LISM, while the O and H ionization
states will be more tightly coupled by charge exchange.Comment: 23 pages, 5 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by ApJ Supplemen
Hot-Jupiters and hot-Neptunes: a common origin?
We compare evolutionary models for close-in exoplanets coupling irradiation
and evaporation due respectively to the thermal and high energy flux of the
parent star with observations of recently discovered new transiting planets.
The models provide an overall good agreement with observations, although at the
very limit of the quoted error bars of OGLE-TR-10, depending on its age. Using
the same general theory, we show that the three recently detected hot-Neptune
planets (GJ436, Cancri, Ara) may originate from more massive gas
giants which have undergone significant evaporation. We thus suggest that
hot-Neptunes and hot-Jupiters may share the same origin and evolution history.
Our scenario provides testable predictions in terms of the mass-radius
relationships of these hot-Neptunes.Comment: 5 pages, 2 figures, accepted in A&A Lette
High-energy environment of super-Earth 55 Cnc e I: Far-UV chromospheric variability as a possible tracer of planet-induced coronal rain
The irradiation of close-in planets by their star influences their evolution
and might be responsible for a population of ultra-short period planets eroded
to their bare core. In orbit around a bright, nearby G-type star, the
super-Earth 55 Cnc e offers the possibility to address these issues through UV
transit observations. We used the Hubble Space Telescope to observe the transit
in the FUV over 3 epochs in Apr. 2016, Jan. 2017, and Feb. 2017. These
observations reveal significant short- and long-term variability in 55 Cnc
chromospheric emission lines. In the last 2 epochs, we detected a larger flux
in the C III, Si III, and Si IV lines after the planet passed the approaching
quadrature, followed by a flux decrease in the Si IV doublet. In the second
epoch these variations are contemporaneous with flux decreases in the Si II and
C II doublet. All epochs show flux decreases in the N V doublet as well, albeit
at different orbital phases. These flux decreases are consistent with
absorption from optically thin clouds of gas, are mostly localized at low and
redshifted radial velocities in the star rest frame, and occur preferentially
before and during the transit. These 3 points make it unlikely that the
variations are purely stellar, yet we show that the occulting material is also
unlikely to originate from the planet. We tentatively propose that the motion
of 55 Cnc e at the fringes of the stellar corona leads to the formation of a
cool coronal rain. The inhomogeneity and temporal evolution of the stellar
corona would be responsible for the differences between the visits. Additional
variations are detected in the C II doublet in the first epoch and in the O I
triplet in all epochs with a different behavior that points toward intrinsic
stellar variability. Further observations at FUV wavelengths are required to
disentangle between star-planet interactions and the activity of the starComment: 22 pages, 20 figures, accepted for publication in A&
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
Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. I. Observations toward delta Orionis by the Interstellar Medium Absorption Profile Spectrograph
Studies of the abundances of deuterium in different astrophysical sites are
of fundamental importance to answering the question about how much deuterium
was produced during big bang nucleosynthesis and what fraction of it was
destroyed later. With this in mind, we used the Interstellar Medium Absorption
Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a
wavelength resolution of 4 km/s (FWHM) the L-delta and L-epsilon absorption
features produced by interstellar atomic deuterium in the spectrum of delta Ori
A. A chi-square analysis indicated that 0.96 < N(D I)< 1.45e15 cm^{-2} at a 90%
level of confidence, and the gas is at a temperature of about 6000K. To obtain
an accurate value of N(H I) needed for a determination of the atomic ratio of D
to H, we measured the L-alpha absorption features in 57 spectra of delta Ori in
the IUE archive. From our measurement of N(H I)= 1.56e20 cm^{-2}, we found that
N(D I)/N(H I)= 7.4(+1.9,-1.3)e-6 (90% confidence). Our result for D/H contrasts
with the more general finding along other lines of sight that D/H is
approximately 1.5e-5. The underabundance of D toward delta Ori A is not
accompanied by an overabundance of N or O relative to H, as one might expect if
the gas were subjected to more stellar processing than usual.Comment: 37 pages, 6 figures. Submitted to the Astrophysical Journa
The long egress of GJ~436b's giant exosphere
The M dwarf GJ 436 hosts a transiting warm Neptune known to experience
atmospheric escape. Previous observations revealed the presence of a giant
hydrogen exosphere transiting the star for more than 5 h, and absorbing up to
56% of the flux in the blue wing of the stellar Lyman-{\alpha} line of neutral
hydrogen (H i Ly{\alpha}). The unexpected size of this comet-like exosphere
prevented observing the full transit of its tail. In this Letter, we present
new Ly{\alpha} observations of GJ 436 obtained with the Space Telescope Imaging
Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The
stability of the Ly{\alpha} line over six years allowed us to combine these new
observations with archival data sets, substantially expanding the coverage of
the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep
occulting the star for 10-25 h after the transit of the planet, remarkably
confirming a previous prediction based on 3D numerical simulations with the
EVaporating Exoplanet code (EVE). This result strengthens the interpretation
that the exosphere of GJ 436b is shaped by both radiative braking and charge
exchanges with the stellar wind. We further report flux decreases of 15 +/- 2%
and 47 +/- 10% in the red wing of the Ly{\alpha} line and in the line of
ionised silicon (Si iii). Despite some temporal variability possibly linked
with stellar activity, these two signals occur during the exospheric transit
and could be of planetary origin. Follow-up observations will be required to
assess the possibility that the redshifted Ly{\alpha} and Si iii absorption
signatures arise from interactions between the exospheric flow and the magnetic
field of the star.Comment: 10 pages, 7 figures, published in A&
Spectropolarimetric observations of the transiting planetary system of the K dwarf HD 189733
With a Jupiter-mass planet orbiting at a distance of only 0.031 AU, the
active K2 dwarf HD 189733 is a potential candidate in which to study the
magnetospheric interactions of a cool star with its recently-discovered
close-orbiting giant planet. We decided to explore the strength and topology of
the large-scale magnetosphere of HD 189733, as a future benchmark for
quantitative studies for models of the star/planet magnetic interactions. To
this end, we used ESPaDOnS, the new generation spectropolarimeter at the
Canada-France-Hawaii 3.6m telescope, to look for Zeeman circular polarisation
signatures in the line profiles of HD 189733 in 2006 June and August. Zeeman
signatures in the line profiles of HD 189733 are clearly detected in all
spectra, demonstrating that a field is indeed present at the surface of the
star. The Zeeman signatures are not modulated with the planet's orbital period
but apparently vary with the stellar rotation cycle. The reconstructed
large-scale magnetic field, whose strength reaches a few tens of G, is
significantly more complex than that of the Sun; it involves in particular a
significant toroidal component and contributions from magnetic multipoles of
order up to 5. The CaII H & K lines clearly feature core emission, whose
intensity is apparently varying mostly with rotation phase. Our data suggest
that the photosphere and magnetic field of HD 189733 are sheared by a
significant amount of differential rotation. Our initial study confirms that HD
189733 is an optimal target for investigating activity enhancements induced by
closely orbiting planets. More data are needed, densely covering both the
orbital and rotation cycles, to investigate whether and how much the planet
contributes to the overall activity level of HD 189733.Comment: Accepted in Astronomy and Astrophysics, 12 page
The Low Velocity Wind from the Circumstellar Matter Around the B9V Star sigma Herculis
We have obtained FUSE spectra of sigma Her, a nearby binary system, with a
main sequence primary, that has a Vega-like infrared excess. We observe
absorption in the excited fine structure lines C II* at 1037 A, N II* at 1085
A, and N II** at 1086 A that are blueshifted by as much as ~30 km/sec with
respect to the star. Since these features are considerably narrower than the
stellar lines and broader than interstellar features, the C II and N II are
circumstellar. We suggest that there is a radiatively driven wind, arising from
the circumstellar matter, rather than accretion as occurs around beta Pic,
because of sigma Her's high luminosity. Assuming that the gas is liberated by
collisions between parent bodies at 20 AU, the approximate distance at which
blackbody grains are in radiative equilibrium with the star and at which 3-body
orbits become unstable, we infer dM/dt ~ 6 * 10^-12 M_{sun}/yr. This wind
depletes the minimum mass of parent bodies in less than the estimated age of
the system.Comment: 6 pages, 3 figures, ApJ in pres
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