330 research outputs found
Dependence of Gas Phase Abundances in the ISM on Column Density
Sightlines through high- and intermediate-velocity clouds allow measurements
of ionic gas phase abundances, A, at very low values of HI column density,
N(HI). Present observations cover over 4 orders of magnitude in N(HI).
Remarkably, for several ions we find that the A vs N(HI) relation is the same
at high and low column density and that the abundances have a relatively low
dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have
slightly higher values of A than disk gas at the same N(HI), suggesting that
part of the dispersion may be attributed to the environment. We note that the
dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to
large errors. Important implications of the low dispersions regarding the
physical nature of the ISM are: (a) because of clumping, over sufficiently long
pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the
H atoms along the sight line; (b) the destruction of grains does not mainly
take place in catastrophic events such as strong shocks, but is a continuous
function of the mean density; (c) the cycling of the ions becoming attached to
grains and being detached must be rapid, and the two rates must be roughly
equal under a wide variety of conditions; (d) in gas that has a low average
density the attachment should occur within denser concentrations
Detection of deuterium Balmer lines in the Orion Nebula
The detection and first identification of the deuterium Balmer emission
lines, D-alpha and D-beta, in the core of the Orion Nebula is reported.
Observations were conducted at the 3.6m Canada-France-Hawaii Telescope, using
the Echelle spectrograph Gecko. These lines are very narrow and have identical
11 km/s velocity shifts with respect to H-alpha and H-beta. They are probably
excited by UV continuum fluorescence from the Lyman (DI) lines and arise from
the interface between the HII region and the molecular cloud.Comment: 4 pages, latex, 1 figure, 1 table, accepted for publication in
Astronomy & Astrophysics, 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
Consequences of spectrograph illumination for the accuracy of radial-velocimetry
For fiber-fed spectrographs with a stable external wavelength source,
scrambling properties of optical fibers and, homogeneity and stability of the
instrument illumination are important for the accuracy of radial-velocimetry.
Optical cylindric fibers are known to have good azimuthal scrambling. In
contrast, the radial one is not perfect. In order to improve the scrambling
ability of the fiber and to stabilize the illumination, optical double
scrambler are usually coupled to the fibers. Despite that, our experience on
SOPHIE and HARPS has lead to identified remaining radial-velocity limitations
due to the non-uniform illumination of the spectrograph. We conducted tests on
SOPHIE with telescope vignetting, seeing variation and centering errors on the
fiber entrance. We simulated the light path through the instrument in order to
explain the radial velocity variation obtained with our tests. We then
identified the illumination stability and uniformity has a critical point for
the extremely high-precision radial velocity instruments (ESPRESSO@VLT,
CODEX@E-ELT). Tests on square and octagonal section fibers are now under
development and SOPHIE will be used as a bench test to validate these new feed
optics.Comment: to appear in the Proceedings conference "New Technologies for Probing
the Diversity of Brown Dwarfs and Exoplanets", Shanghai, 200
Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
Far Ultraviolet Spectroscopic Explorer observations are presented for
WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a
distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40
per 20 km/s resolution element and cover the wavelength range 905-1187 \AA.
LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II,
N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an
ionized nitrogen fraction of > 0.23. We determine the ratio (2). Assuming a standard interstellar
oxygen abundance, we derive . Using the
value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I
ratio is (2).Comment: accepted for publication in the ApJ
Exploring Interstellar Titanium and Deuterium Abundances and Other Correlations
The origin of the observed variability of the gas-phase D/H ratio in the
local interstellar medium is still debated, and in particular the role of
deuterium depletion onto dust grains. Here we extend the study of the
relationship between deuterium and titanium, a refractory species and tracer of
elemental depletion, and explore other relationships. We have acquired high
resolution spectra for nine early-type stars using the VLT/UVES spectrograph,
and detected the absorption lines of interstellar TiII. Using a weighted
orthogonal distance regression (ODR) code and a special method to treat non
symmetric errors, we compare the TiII columns with the corresponding HI, DI and
also OI columns. We perform in parallel the same comparisons for available FeII
data. We find a significant correlation between TiII/HI and D/H in our data
set, and, when combined with published results, we confirm and better constrain
a previously published trend and extend it to low HI columns. We exclude
uncertainties in HI and OI columns as the main contributor to the derived
metals-deuterium correlations by showing that the TiII/HI ratio is positively
correlated with DI/OI. We find a similar correlation between FeII/HI and
DI/OI.The TiII gradients are similar or slightly smaller than for FeII, while
one would expect larger variations on the basis of the higher condensation
temperature of titanium. However we argue that ionisation effects introduce
biases that affect iron and not titanium and may explain the gradient
similarity. We find a less significant negative correlation between the TiII/DI
ratio and the hydrogen column, possibly a sign of different evaporation of D
and metals according to the cloud properties
Exoplanet HD 209458b : Evaporation strengthened
Following re-analysis of Hubble Space Telescope observations of primary
transits of the extrasolar planet HD209458b at Lyman-alpha, Ben-Jaffel (2007,
BJ007) claims that no sign of evaporation is observed. Here we show that, in
fact, this new analysis is consistent with the one of Vidal-Madjar et al.
(2003, VM003) and supports the detection of evaporation. The apparent
disagreement is mainly due to the disparate wavelength ranges that are used to
derive the transit absorption depth. VM003 derives a (15+/-4)% absorption depth
during transit over the core of the stellar Lyman-alpha line (from -130 km/s to
+100 km/s), and this result agrees with the (8.9+/-2.1)% absorption depth
reported by BJ007 from a slightly expanded dataset but over a larger wavelength
range (+/-200 km/s). These measurements agree also with the (5+/-2)% absorption
reported by Vidal-Madjar et al. (2004) over the whole Lyman-alpha line from
independent, lower-resolution data. We show that stellar Lyman-alpha
variability is unlikely to significantly affect those detections. The HI atoms
must necessarily have velocities above the escape velocities and/or be outside
the Roche lobe, given the lobe shape and orientation. Absorption by HI in
HD209458b's atmosphere has thus been detected with different datasets, and now
with independent analyses. All these results strengthen the concept of
evaporating hot-Jupiters, as well as the modelization of this phenomenon.Comment: To be published in ApJ
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