299 research outputs found
High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk
High resolution FUV echelle spectra showing absorption features arising from
CI and CO gas in the Beta Pictoris circumstellar (CS) disk were obtained on
1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS).
An unsaturated spin-forbidden line of CI at 1613.376 A not previously seen in
spectra of Beta Pictoris was detected, allowing for an improved determination
of the column density of CI at zero velocity relative to the star (the stable
component), N = (2-4) x 10^{16} cm^{-2}. Variable components with multiple
velocities, which are the signatures of infalling bodies in the Beta Pictoris
CS disk, are observed in the CI 1561 A and 1657 A multiplets. Also seen for the
first time were two lines arising from the metastable singlet D level of
carbon, at 1931 A and 1463 A The results of analysis of the CO A-X (0-0),
(1-0), and (2-0) bands are presented, including the bands arising from {13}^CO,
with much better precision than has previously been possible, due to the very
high resolution provided by the STIS echelle gratings. Only stable CO gas is
observed, with a column density N(CO) = (6.3 +/- 0.3) x 10^{14} cm{-2}. An
unusual ratio of the column densities of {12}^CO to {13}^CO is found (R = 15
+/- 2). The large difference between the column densities of CI and CO
indicates that photodissociation of CO is not the primary source of CI gas in
the disk, contrary to previous suggestion.Comment: 13 pages, including 6 figures. LaTex2e (emulateapj5.sty). Accepted
for publication in Ap
A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
Exoplanets orbiting close to their parent stars could lose some fraction of
their atmospheres because of the extreme irradiation. Atmospheric mass loss
primarily affects low-mass exoplanets, leading to suggest that hot rocky
planets might have begun as Neptune-like, but subsequently lost all of their
atmospheres; however, no confident measurements have hitherto been available.
The signature of this loss could be observed in the ultraviolet spectrum, when
the planet and its escaping atmosphere transit the star, giving rise to deeper
and longer transit signatures than in the optical spectrum. Here we report that
in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese
436b) has transit depths of 56.3 +/- 3.5% (1 sigma), far beyond the 0.69%
optical transit depth. The ultraviolet transits repeatedly start ~2 h before,
and end >3 h after the ~1 h optical transit, which is substantially different
from one previous claim (based on an inaccurate ephemeris). We infer from this
that the planet is surrounded and trailed by a large exospheric cloud composed
mainly of hydrogen atoms. We estimate a mass-loss rate in the range of
~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of a
Neptune-like planet in the lifetime of the parent star, but would have been
much greater in the past.Comment: Published in Nature on 25 June 2015. Preprint is 28 pages, 12
figures, 2 table
FUSE Observations of Possible Infalling Planetesimals in the 51 Ophiuchi Circumstellar Disk
We present the first observations of the circumstellar (CS) disk system 51
Ophiuchi with the Far Ultraviolet Spectroscopic Explorer. We detect several
absorption lines arising from the unusual metastable atomic species NI (2D), NI
(2P), and SII (2D). These levels lie 1.8 - 3.6 eV above the ground level and
have radiative decay lifetimes of 2 days or less, indicating that the lines
arise from warm CS gas. The high S/N FUSE spectra, obtained six days apart,
also show time-variable absorption features arising from NI, NII, OI (1D), and
FeIII, which are redshifted with respect to the stellar velocity. The resolved
redshifted absorption extends over many tens of km/s (40 for NI, 100 for NII,
65 for OI, and 84 for FeIII). We calculate column densities for all the
variable infalling CS gasses, using the apparent optical depth method. The
FeIII and NII infalling gasses must be produced through collisional ionization,
and the ionization fraction of nitrogen suggests a gas temperature between
20000 and 34000 K. The infalling gas shows a peculiar, non-solar composition,
with nitrogen and iron more abundant than carbon. We also set upper limits on
the line-of-sight column densities of molecular hydrogen and carbon monoxide.
These observations strengthen the connection between 51 Oph and the older
debris-disk system Beta Pictoris, and indicate that there may be infalling
planetesimals in the 51 Oph system.Comment: 11 pages, 4 color figures, LaTex2e, accepted for publication in Ap
Transmission spectrum of Venus as a transiting exoplanet
On 5-6 June 2012, Venus will be transiting the Sun for the last time before
2117. This event is an unique opportunity to assess the feasibility of the
atmospheric characterisation of Earth-size exoplanets near the habitable zone
with the transmission spectroscopy technique and provide an invaluable proxy
for the atmosphere of such a planet. In this letter, we provide a theoretical
transmission spectrum of the atmosphere of Venus that could be tested with
spectroscopic observations during the 2012 transit. This is done using
radiative transfer across Venus' atmosphere, with inputs from in-situ missions
such as Venus Express and theoretical models. The transmission spectrum covers
a range of 0.1-5 {\mu}m and probes the limb between 70 and 150 km in altitude.
It is dominated in UV by carbon dioxide absorption producing a broad transit
signal of ~20 ppm as seen from Earth, and from 0.2 to 2.7 {\mu}m by Mie
extinction (~5 ppm at 0.8 {\mu}m) caused by droplets of sulfuric acid composing
an upper haze layer above the main deck of clouds. These features are not
expected for a terrestrial exoplanet and could help discriminating an
Earth-like habitable world from a cytherean planet.Comment: 4 pages, 3 figures, 1 table. Figure 3 and Table 1 will be only
available on-line. Table 1 will be fully available at the CDS. Accepted for
publication in Astronomy and Astrophysics (Letter
FUSE Observations of the HD Molecule toward HD 73882
The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur
in the far UV range below 1200 A. The high sensitivity of the FUSE mission can
give access, at moderate resolution, to hot stars shining through translucent
clouds, in the hope of observing molecular cores in which deuterium is
essentially in the form of HD. Thus, the measurement of the HD/H2 ratio may
become a new powerful tool to evaluate the deuterium abundance, D/H, in the
interstellar medium. We report here on the detection of HD toward the high
extinction star HD 73882 [E(B-V)=0.72]. A preliminary analysis is presented.Comment: 4 pages + 4 .ps figures. This paper will appear in a special issue of
Astrophysical Journal Letters devoted to the first scientific results from
the FUSE missio
Dust Migration and Morphology in Optically Thin Circumstellar Gas Disks
We analyze the dynamics of gas-dust coupling in the presence of stellar
radiation pressure in circumstellar gas disks, which are in a transitional
stage between the gas-dominated, optically thick, primordial nebulae, and the
dust-dominated, optically thin Vega-type disks. Dust undergo radial migration,
seeking a stable equilibrium orbit in corotation with gas. The migration of
dust gives rise to radial fractionation of dust and creates a variety of
possible observed disk morphologies, which we compute by considering the
equilibrium between the dust production and the dust-dust collisions removing
particles from their equilibrium orbits. Sand-sized and larger grains are
distributed throughout most of the gas disk, with concentration near the gas
pressure maximum in the inner disk. Smaller grains (typically in the range of
10 to 200 micron) concentrate in a prominent ring structure in the outer region
of the gas disk (presumably at radius 100 AU), where gas density is rapidly
declining with radius. The width and density, as well as density contrast of
the dust ring with respect to the inner dust disk depend on the distribution of
gas. Our results open the prospect for deducing the distribution of gas in
circumstellar disks by observing their dust. We have qualitatively compared our
models with two observed transitional disks around HR 4796A and HD 141569A.
Dust migration can result in observation of a ring or a bimodal radial dust
distribution, possibly very similar to the ones produced by gap-opening
planet(s) embedded in the disk, or shepherding it from inside or outside. We
conclude that a convincing planet detection via dust imaging should include
specific non-axisymmetric structure following from the dynamical simulations of
perturbed disks.Comment: 27 pages, 16 figures, submitted to Ap
Prehistory of Transit Searches
Nowadays the more powerful method to detect extrasolar planets is the transit
method. We review the planet transits which were anticipated, searched, and the
first ones which were observed all through history. Indeed transits of planets
in front of their star were first investigated and studied in the solar system.
The first observations of sunspots were sometimes mistaken for transits of
unknown planets. The first scientific observation and study of a transit in the
solar system was the observation of Mercury transit by Pierre Gassendi in 1631.
Because observations of Venus transits could give a way to determine the
distance Sun-Earth, transits of Venus were overwhelmingly observed. Some
objects which actually do not exist were searched by their hypothetical
transits on the Sun, as some examples a Venus satellite and an infra-mercurial
planet. We evoke the possibly first use of the hypothesis of an exoplanet
transit to explain some periodic variations of the luminosity of a star, namely
the star Algol, during the eighteen century. Then we review the predictions of
detection of exoplanets by their transits, those predictions being sometimes
ancient, and made by astronomers as well as popular science writers. However,
these very interesting predictions were never published in peer-reviewed
journals specialized in astronomical discoveries and results. A possible
transit of the planet beta Pic b was observed in 1981. Shall we see another
transit expected for the same planet during 2018? Today, some studies of
transits which are connected to hypothetical extraterrestrial civilisations are
published in astronomical refereed journals. Some studies which would be
classified not long ago as science fiction are now considered as scientific
ones.Comment: Submiited to Handbook of Exoplanets (Springer
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
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