31,292 research outputs found
A dynamical transition from atomic to molecular intermediate-velocity clouds
Towards the high galactic latitude sky, the far-infrared (FIR) intensity is
tightly correlated to the total hydrogen column density which is made up of
atomic (HI) and molecular hydrogen (H. Above a certain column density
threshold, atomic hydrogen turns molecular. We analyse gas and dust properties
of intermediate-velocity clouds (IVCs) in the lower galactic halo to explore
their transition from the atomic to the molecular phase. Driven by
observations, we investigate the physical processes that transform a purely
atomic IVC into a molecular one. Data from the Effelsberg-Bonn HI-Survey
(EBHIS) are correlated to FIR wavebands of the Planck satellite and IRIS.
Modified black-body emission spectra are fitted to deduce dust optical depths
and grain temperatures. We remove the contribution of atomic hydrogen to the
FIR intensity to estimate molecular hydrogen column densities. Two IVCs show
different FIR properties, despite their similarity in HI, such as narrow
spectral lines and large column densities. One FIR bright IVC is associated
with H, confirmed by CO emission; the other IVC
is FIR dim and shows no FIR excess, which indicates the absence of molecular
hydrogen. We propose that the FIR dim and bright IVCs probe the transition
between the atomic and molecular gas phase. Triggered by dynamical processes,
this transition happens during the descent of IVCs onto the galactic disk. The
most natural driver is ram pressure exerted onto the cloud by the increasing
halo density. Because of the enhanced pressure, the formation timescale of
H is reduced, allowing the formation of large amounts of H within a
few Myr.Comment: 13 pages, 14 figures, accepted for publication by A&
Heating efficiency in hydrogen-dominated upper atmospheres
Context. The heating efficiency is defined as the ratio of the net local
gas-heating rate to the rate of stellar radiative energy absorption. It plays
an important role in thermal-escape processes from the upper atmospheres of
planets that are exposed to stellar soft X-rays and extreme ultraviolet
radiation (XUV). Aims. We model the thermal-escape-related heating efficiency
of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the
extrasolar gas giant HD 209458b. The model result is then compared with
previous thermal-hydrogen-escape studies which assumed heating efficiency
values between 10-100%. Methods. The photolytic and electron impact processes
in the thermosphere were studied by solving the kinetic Boltzmann equation and
applying a Direct Simulation Monte Carlo model. We calculated the energy
deposition rates of the stellar XUV flux and that of the accompanying primary
photoelectrons that are caused by electron impact processes in the H2 to H
transition region in the upper atmosphere. Results. The heating by XUV
radiation of hydrogen-dominated upper atmospheres does not reach higher than
20% above the main thermosphere altitude, if the participation of photoelectron
impact processes is included. Conclusions. Hydrogen-escape studies from
exoplanets that assume heating efficiency values that are >= 20 % probably
overestimate the thermal escape or mass-loss rates, while those who assumed
values that are < 20% probably produce more realistic atmospheric-escape rates.Comment: 7 pages, 4 figures, accepted to A&
The GTC exoplanet transit spectroscopy survey. VII. An optical transmission spectrum of WASP-48b
We obtained long-slit optical spectroscopy of one transit of WASP-48b with
the Optical System for Imaging and low-Intermediate-Resolution Integrated
Spectroscopy (OSIRIS) spectrograph at the 10.4 m Gran Telescopio Canarias
(GTC). We integrated the spectrum of WASP-48 and one reference star in several
channels with different wavelength ranges, creating numerous color light curves
of the transit. We fit analytic transit curves to the data taking into account
the systematic effects present in the time series in an effort to measure the
change of the planet-to-star radius ratio () across wavelength. After
removing the transit model and systematic trends to the curves we reached
precisions between 261 ppm and 455-755 ppm for the white and spectroscopic
light curves, respectively. We obtained uncertainty values between
and for all the curves analyzed in
this work. The measured transit depth for the curves made by integrating the
wavelength range between 530 nm and 905 nm is in agreement with previous
studies. We report a relatively flat transmission spectrum for WASP-48b with no
statistical significant detection of atmospheric species, although the
theoretical models that fit the data more closely include of TiO and VO.Comment: 8 pages, 8 figures. Accepted for publication in Astronomy and
Astrophysic
On Reading Theodor Strehlow's 'Aranda Regular and Irregular Marriages'
Comparaison des mariages et normes décrits par Strehlow pour les Aranda du désert du centre australiensd avec les pratiques et idéaux exprimés par les Ngaatjatjarra dans le Désert de l'Oues
Modeling the Dust Spectral Energy Distributions of Dwarf Galaxies
Recent efforts on the modeling of the infrared spectral energy distributions
(SEDs) of dwarf galaxies are summarised here. The characterisation of the dust
properties in these low metallicity environments is just unfolding, as a result
of recently available mid-infrared to millimetre observations. From the limited
cases we know to date, it appears that the hard radiation fields that are
present in these star-bursting dwarf galaxies, as well as the rampent
energetics of supernovae shocks and winds have modified the dust properties, in
comparison with those in the Galaxy, or other gas and dust rich galaxies. The
sophistication of the SED models is limited by the availability of detailed
data in the mid infrared and particularly in the submillimetre to millimetre
regime, which will open up in the near future with space-based missions, such
as Herschel.Comment: 8 pages presented at "The Spectral Energy Distribution of Gas-Rich
Galaxies: Confronting Models with Data" Heidelberg (Germany), October 2004.
To be published in The Spectral Energy Distribution of Gas-Rich Galaxies, ed.
C. Popescu & R. Tuffs (Melville:AIP) in pres
The Diabolo photometer and the future of ground-based millimetric bolometer devices
The millimetric atmospheric windows at 1 and 2 mm are interesting targets for
cosmological studies. Two broad areas appear leading this field: 1) the search
for high redshift star-forming galaxies and 2) the measurement of
Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies at all redshifts. The
Diabolo photometer is a dual-channel photometer working at 1.2 and 2.1 mm and
dedicated to high angular resolution measurements of the Sunyaev--Zel'dovich
effect towards distant clusters. It uses 2 by 3 bolometers cooled down to 0.1 K
with a compact open dilution cryostat. The high resolution is provided by the
IRAM 30 m telescope. The result of several Winter campaigns are reported here,
including the first millimetric map of the SZ effect that was obtained by
Pointecouteau et al. (2001) on RXJ1347-1145, the non-detection of a millimetric
counterpart to the radio decrement towards PC1643+4631 and 2 mm number count
upper limits. We discuss limitations in ground-based single-dish millimetre
observations, namely sky noise and the number of detectors. We advocate the use
of fully sampled arrays of (100 to 1000) bolometers as a big step forward in
the millimetre continuum science. Efforts in France are briefly mentionned.Comment: 7 pages, 6 figures, to appear in the Proceedings of the 2K1BC
``Experimental Astronomy at millimeter wavelengths'', Breuil-Cervinia (AO)
Italy - July 9 - 13, 2001, Eds. M. De Petris et a
The dust SED in the dwarf galaxy NGC 1569: Indications for an altered dust composition?
We discuss the interpretation of the dust SED from the mid-infrared to the
millimeter range of NGC 1569. The model developed by D\'esert et al. (1990)
including three dust components (Polyaromatic Hydrocarbons, Very Small Grains
and big grains) can explain the data using a realistic interstellar radiation
field and adopting an enhanced abundance of VSGs. A simple three-temperature
model is also able to reproduce the data but requires a very low dust
temperature which is considered to be unlikely in this low-metallicity
starburst galaxy. The high abundance of Very Small Grains might be due to large
grain destruction in supernova shocks. This possibility is supported by ISO
data showing that the emission at 14.3 m, tracing VSGs, is enhanced with
respect to the emission at 6.7 m and 850 m in regions of high star
formation.Comment: 4 pages, conference proceedings paper, "The Spectral Energy
Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg,
4-8 Oct. 2004, eds. C.C. Popescu & R.J. Tuffs, AIP Conf. Ser., in pres
Ground-based optical transmission spectrum of the hot Jupiter HAT-P-1b
Time-series spectrophotometric studies of exoplanets during transit using
ground-based facilities are a promising approach to characterize their
atmospheric compositions. We aim to investigate the transit spectrum of the hot
Jupiter HAT-P-1b. We compare our results to those obtained at similar
wavelengths by previous space-based observations. We observed two transits of
HAT-P-1b with the Gemini Multi-Object Spectrograph (GMOS) instrument on the
Gemini North telescope using two instrument modes covering the 320 - 800 nm and
520 - 950 nm wavelength ranges. We used time-series spectrophotometry to
construct transit light curves in individual wavelength bins and measure the
transit depths in each bin. We accounted for systematic effects. We addressed
potential photometric variability due to magnetic spots in the planet's host
star with long-term photometric monitoring. We find that the resulting transit
spectrum is consistent with previous Hubble Space Telescope (HST) observations.
We compare our observations to transit spectroscopy models that marginally
favor a clear atmosphere. However, the observations are also consistent with a
flat spectrum, indicating high-altitude clouds. We do not detect the Na
resonance absorption line (589 nm), and our observations do not have sufficient
precision to study the resonance line of K at 770 nm. We show that even a
single Gemini/GMOS transit can provide constraining power on the properties of
the atmosphere of HAT-P-1b to a level comparable to that of HST transit studies
in the optical when the observing conditions and target and reference star
combination are suitable. Our 520 - 950 nm observations reach a precision
comparable to that of HST transit spectra in a similar wavelength range of the
same hot Jupiter, HAT-P-1b. However, our GMOS transit between 320 - 800 nm
suffers from strong systematic effects and yields larger uncertainties.Comment: A&A, accepted, 16 pages, 8 figures, 5 table
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