789 research outputs found
Hydrogen column density evaluations toward Capella: consequences on the interstellar deuterium abundance
The deuterium abundance evaluation in the direction of Capella has for a long
time been used as a reference for the local interstellar medium (ISM) within
our Galaxy. We show here that broad and weak HI components could be present on
the Capella line of sight, leading to a large new additional systematic
uncertainty on the N(HI) evaluation.
The D/H ratio toward Capella is found to be equal to 1.67 (+/-0.3)x10^-5 with
almost identical chi^2 for all the fits (this range includes only the
systematic error; the 2 sigma statistical one is almost negligible in
comparison). It is concluded that D/H evaluations over HI column densities
below 10^19 cm^-2 (even perhaps below 10^20 cm^-2 if demonstrated by additional
observations) may present larger uncertainties than previously anticipated. It
is mentionned that the D/O ratio might be a better tracer for DI variations in
the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer
(FUSE).Comment: Accepted for publication in the Astrophysical Journal Letter
Atmospheric Escape from Hot Jupiters
The extra-solar planet HD209458b has been found to have an extended
atmosphere of escaping atomic hydrogen (Vidal-Madjar et al. 2003), suggesting
that ``hot Jupiters'' closer to their parent stars could evaporate. Here we
estimate the atmospheric escape (so called evaporation rate) from hot Jupiters
and their corresponding life time against evaporation. The calculated
evaporation rate of HD209458b is in excellent agreement with the HI Lyman-alpha
observations. We find that the tidal forces and high temperatures in the upper
atmosphere must be taken into account to obtain reliable estimate of the
atmospheric escape. Because of the tidal forces, we show that there is a new
escape mechanism at intermediate temperatures at which the exobase reaches the
Roche lobe. From an energy balance, we can estimate plausible values for the
planetary exospheric temperatures, and thus obtain typical life times of
planets as a function of their mass and orbital distance.Comment: A&A Letters, in pres
Corrigendum to "The upper atmosphere of the exoplanet HD209458b revealed by the sodium D lines: Temperature-pressure profile, ionization layer and thermosphere" [2011, A&A, 527, A110]
An error was detected in the code used for the analysis of the HD209458b
sodium profile (Vidal-Madjar et al. 2011). Here we present an updated T-P
profile and briefly discuss the consequences.Comment: Published in Astronomy & Astrophysics, 533, C
New observations of the extended hydrogen exosphere of the extrasolar planet HD209458b
Atomic hydrogen escaping from the planet HD209458b provides the largest
observational signature ever detected for an extrasolar planet atmosphere.
However, the Space Telescope Imaging Spectrograph (STIS) used in previous
observational studies is no longer available, whereas additional observations
are still needed to better constrain the mechanisms subtending the evaporation
process, and determine the evaporation state of other `hot Jupiters'. Here, we
aim to detect the extended hydrogen exosphere of HD209458b with the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find
evidence for a hydrogen comet-like tail trailing the planet, which size would
depend on the escape rate and the amount of ionizing radiation emitted by the
star. These observations also provide a benchmark for other transiting planets,
in the frame of a comparative study of the evaporation state of close-in giant
planets. Eight HST orbits are used to observe two transits of HD209458b.
Transit light curves are obtained by performing photometry of the unresolved
stellar Lyman-alpha emission line during both transits. Absorption signatures
of exospheric hydrogen during the transit are compared to light curve models
predicting a hydrogen tail. Transit depths of (9.6 +/- 7.0)% and (5.3 +/-
10.0)% are measured on the whole Lyman-alpha line in visits 1 and 2,
respectively. Averaging data from both visits, we find an absorption depth of
(8.0 +/- 5.7)%, in good agreement with previous studies. The extended size of
the exosphere confirms that the planet is likely loosing hydrogen to space.
Yet, the photometric precision achieved does not allow us to better constrain
the hydrogen mass loss rate.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 3
figure
GTC OSIRIS transiting exoplanet atmospheric survey: detection of sodium in XO-2b from differential long-slit spectroscopy
We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran
Telescopio Canarias (GTC). The time series observations were performed using
long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS
instrument, enabling differential specrophotometric transit lightcurves capable
of measuring the exoplanet's transmission spectrum. Two optical low-resolution
grisms were used to cover the optical wavelength range from 3800 to 9300{\AA}.
We find that sub-mmag level slit losses between the target and reference star
prevent full optical transmission spectra from being constructed, limiting our
analysis to differential absorption depths over ~1000{\AA} regions. Wider long
slits or multi-object grism spectroscopy with wide masks will likely prove
effective in minimising the observed slit-loss trends. During both transits, we
detect significant absorption in the planetary atmosphere of XO-2b using a
50{\AA} bandpass centred on the Na I doublet, with absorption depths of
Delta(R_pl/R_star)^2=0.049+/-0.017 % using the R500R grism and 0.047+/-0.011 %
using the R500B grism (combined 5.2-sigma significance from both transits). The
sodium feature is unresolved in our low-resolution spectra, with detailed
modelling also likely ruling out significant line-wing absorption over an
~800{\AA} region surrounding the doublet. Combined with narrowband photometric
measurements, XO-2b is the first hot Jupiter with evidence for both sodium and
potassium present in the planet's atmosphere.Comment: 9 pages, 10 figures, 1 table, accepted for publication in MNRA
Elimination of visually evoked BOLD responses during carbogen inhalation: Implications for calibrated MRI
Breathing a mixture of 10% CO2 with 90% O2 (referred to here as carbogen-10) increases blood flow due to the vasodilatory effect of CO2, and raises blood O2 saturation due to the enriched oxygen level. These effects both tend to reduce the level of deoxygenated hemoglobin in brain tissues, thereby reducing the potential for further increases in BOLD contrast. In the present study, blocks of intense visual stimulation (60 s) were presented amid longer blocks (180 s) during which subjects breathed various fractional concentrations (0â100%) of carbogen-10 diluted with medical air. When breathing undiluted carbogen-10, the BOLD response to visual stimulation was reduced below the level of noise against the background of the carbogen-10 response. At these concentrations, the total (visual+carbogen) BOLD response amplitude (7.5±1.0%, n=6) converged toward that seen with carbogen alone (7.5 ± 1.0%, n = 6). In spite of the almost complete elimination of the visual BOLD response, pseudo-continuous arterial spin-labeling on a separate cohort indicated a largely preserved perfusion response (89±34%, n=5) to the visual stimulus during inhalation of carbogen-10. The previously discussed observations suggest that venous saturation can be driven to very high levels during carbogen inhalation, a finding which has significant implications for calibrated MRI techniques. The latter methods involve estimation of the relative change in venous O2 saturation by expressing activation-induced BOLD signal increases as a fraction of the maximal BOLD signal M that would be observed as venous saturation approaches 100%. While the value of M has generally been extrapolated from much smaller BOLD responses induced using hypercapnia or hyperoxia, our results suggest that these effects could be combined through carbogen inhalation to obtain estimates of M based on larger BOLD increases. Using a hybrid BOLD calibration model taking into account changes in both blood flow and arterial oxygenation, we estimated that inhalation of carbogen-10 led to an average venous saturation of 91%, allowing us to compute an estimated M value of 9.5%
Final Calibration of the Berkeley Extreme and Far-Ultraviolet Spectrometer on the ORFEUS-SPAS I and II Missions
The Berkeley Extreme and Far-Ultraviolet Spectrometer (BEFS) flew as part of
the ORFEUS telescope on the ORFEUS-SPAS I and II space-shuttle missions in 1993
and 1996, respectively. The data obtained by this instrument have now entered
the public domain. To facilitate their use by the astronomical community, we
have re-extracted and re-calibrated both data sets, converted them into a
standard (FITS) format, and placed them in the Multimission Archive at Space
Telescope (MAST). Our final calibration yields improved wavelength scales and
effective-area curves for both data sets.Comment: To appear in the January 2002 issue of the PASP. 17 pages with 9
embedded postscript figures; uses emulateapj5.st
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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