183 research outputs found
The relevance of prior inclination determination for direct imaging of Earth-like planets
Direct imaging and characterization of extrasolar Earth-like planets is
strongly impacted by the orbital inclination of the planet to be studied, as a
combination of pure geometrical effects and the impact of exozodiacal dust.
Here, we perform simulations to quantify the impact of a priori knowledge of
inclination for the efficiency of a typical coronagraphic or occulter-based
mission. The relative impact and complementarity with prior knowledge of
exozodiacal brightness down to achievable levels is examined and discussed. It
is found that inclination has an even greater impact than the exozodiacal
brightness, though the two have excellent complementarity. We also discuss
different methods for inclination determination, and their respective
applicability to the context of precursor science to an imaging mission. It is
found that if technologically achievable, a combined effort to determine
inclinations and exozodiacal brightnesses with ground-based facilities would
substantially increase the efficiency of a space-based dedicated mission to
image and characterize Earth-like planets.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Direction of light propagation to order G^2 in static, spherically symmetric spacetimes: a new derivation
A procedure avoiding any integration of the null geodesic equations is used
to derive the direction of light propagation in a three-parameter family of
static, spherically symmetric spacetimes within the post-post-Minkowskian
approximation. Quasi-Cartesian isotropic coordinates adapted to the symmetries
of spacetime are systematically used. It is found that the expression of the
angle formed by two light rays as measured by a static observer staying at a
given point is remarkably simple in these coordinates. The attention is mainly
focused on the null geodesic paths that we call the "quasi-Minkowskian light
rays". The vector-like functions characterizing the direction of propagation of
such light rays at their points of emission and reception are firstly obtained
in the generic case where these points are both located at a finite distance
from the centre of symmetry. The direction of propagation of the
quasi-Minkowskian light rays emitted at infinity is then straightforwardly
deduced. An intrinsic definition of the gravitational deflection angle relative
to a static observer located at a finite distance is proposed for these rays.
The expression inferred from this definition extends the formula currently used
in VLBI astrometry up to the second order in the gravitational constant G.Comment: 19 pages; revised introduction; added references for introduction;
corrected typos; published in Class. Quantum Gra
Atomic hydrogen in AGB circumstellar environments. A case study: X Her
We report the detection of the HI line at 21 cm from the circumstellar shell
around the AGB star X Her using the position-switching technique with the
Nancay Radio Telescope. At the star position the line shows 2 components: (i) a
broad one (FWHM ~ 13 km/s) centered at -72.2 km/s, and (ii) a narrow one (FWHM
\~ 4 km/s) centered at ~ -70.6 km/s. Our map shows that the source associated
to the broad component is asymmetric with material flowing preferentially
towards the North-East. This source extends to ~ 10 arcmin. (~ 0.4 pc) from the
star in that direction. On the other hand, the narrow component is detected
only at the star position and indicates material flowing away from the
observer. The total mass of atomic hydrogen is ~ 6.5 10^{-3} solar mass which,
within a factor 2, agrees with the estimate obtained from IRAS data at 60
microns.Comment: accepted for publication in MNRA
Carbon-Rich Mira Variables: Radial Velocities and Distances
Optical radial velocities have been measured for 38 C-type Mira variables.
These data together with others in the literature are used to study the
differences between optical and CO mm observations for C-Miras and the
necessary corrections to the optical velocities are derived in order to obtain
the true radial velocities of the variables. The difference between absorption
and emission line velocities is also examined. A particularly large difference
(+30 km\s) is found in the case of the H-alpha emission line. A catalogue is
given of 177 C-Miras with estimated distances and radial velocities. The
distances are based on bolometric magnitudes derived in Paper I using SAAO
observations or (for 60 of the stars) using non-SAAO photometry. In the latter
case the necessary transformations to the SAAO system are derived. These data
will be used in paper III to study the kinematics of the C-Miras.Comment: 11 pages, accepted for publication in MNRA
Kinematic parameters and membership probabilities of open clusters in the Bordeaux PM2000 catalogue
We derive lists of proper-motions and kinematic membership probabilities for
49 open clusters and possible open clusters in the zone of the Bordeaux PM2000
proper motion catalogue (). We test
different parametrisations of the proper motion and position distribution
functions and select the most successful one. In the light of those results, we
analyse some objects individually. The segregation between cluster and field
member stars, and the assignment of membership probabilities, is accomplished
by applying a new and fully automated method based on both parametrisations of
the proper motion and position distribution functions, and genetic algorithm
optimization heuristics associated with a derivative-based hill climbing
algorithm for the likelihood optimization. We present a catalogue comprising
kinematic parameters and associated membership probability lists for 49 open
clusters and possible open clusters in the Bordeaux PM2000 catalogue region. We
note that this is the first determination of proper motions for five open
clusters. We confirm the non-existence of two kinematic populations in the
region of 15 previously suspected non-existent objects.Comment: 14 pages, 6 figures, 4 tables. Accepted for publication in Astronomy
& Astrophysic
Pinning down the ram-pressure-induced halt of star formation in the Virgo cluster spiral galaxy NGC 4388. A joint inversion of spectroscopic and photometric data
In a galaxy cluster, the evolution of spiral galaxies depends on their
cluster environment. Ram pressure due to the rapid motion of a spiral galaxy
within the hot intracluster medium removes the galaxy's interstellar medium
from the outer disk. Once the gas has left the disk, star formation stops. The
passive evolution of the stellar populations should be detectable in optical
spectroscopy and multi-wavelength photometry. The goal of our study is to
recover the stripping age of the Virgo spiral galaxy NGC 4388, i.e. the time
elapsed since the halt of star formation in the outer galactic disk using a
combined analysis of optical spectra and photometry. We performed VLT FORS2
long-slit spectroscopy of the inner star-forming and outer gas-free disk of NGC
4388. We developed a non-parametric inversion tool that allows us to
reconstruct the star formation history of a galaxy from spectroscopy and
photometry. The tool was tested on a series of mock data using Monte Carlo
simulations. The results from the non-parametric inversion were refined by
applying a parametric inversion method. The star formation history of the
unperturbed galactic disk is flat. The non-parametric method yields a rapid
decline of star formation < 200 Myr ago in the outer disk. The parametric
method is not able to distinguish between an instantaneous and a long-lasting
star formation truncation. The time since the star formation has dropped by a
factor of two from its pre-stripping value is 190 +- 30 Myr. We are able to
give a precise stripping age that is consistent with revised dynamical models.Comment: 12 pages, 10 figures, accepted for publication in A&
Discovery of a detached HI gas shell surrounding alpha Orionis
We report the detection of the HI line at 21 cm in the direction of alpha Ori
with the Nancay Radiotelescope and with the Very Large Array. The observations
confirm the previous detection of HI emission centered on alpha Ori, but
additionally reveal for the first time a quasi-stationary detached shell of
neutral atomic hydrogen ~4 arcmin. in diameter (0.24 pc at a distance of 200
pc). The detached shell appears elongated in a direction opposite to the star's
space motion. A simple model shows that this detached atomic gas shell can
result from the collision of the stellar wind from alpha Ori with the local
interstellar medium (ISM). It implies that alpha Ori has been losing matter at
a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In
addition, we report the detection of atomic hydrogen associated with the
far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been
suggested to trace the bow shock resulting from the motion of the star through
the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX)
of a far-UV counterpart to this arc.Comment: Accepted for publication in the Monthly Notices; version with full
resolution figures available at
http://aramis.obspm.fr/~lebertre/paper-alphaOri_MNRAS.pd
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Remote sensing of vegetation fires and its contribution to a fire management information system
In the last decade, research has proven that remote sensing can provide very useful support to fire managers. This chapter provides an overview of the type of information remote sensing can provide to the fire community. It considers first fire management information needs in the context of fire management information system. An introduction to remote sensing then precedes the description of fire information obtainable from remote sensing data (such as vegetation status, active fire detection and burned areas assessment). Finally, operational examples in five African countries illustrate how the information can be used in practice
Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame
The ICRF, currently based on the position of 717 extragalactic radio sources
observed by VLBI, is the fundamental celestial reference frame adopted by the
IAU in 1997. Within the next 10 years, the European space astrometry mission
Gaia, to be launched by 2011, will permit determination of the extragalactic
reference frame directly in the visible for the first time. Aligning these two
frames with the highest accuracy will therefore be very important in the future
for ensuring consistency between the measured radio and optical positions. This
paper is aimed at evaluating the current astrometric suitability of the
individual ICRF radio sources which are considered appropriate for the
alignment with the future Gaia frame. To this purpose, we cross-identified the
ICRF and the optical catalog V\'eron-Cetty and V\'eron (2006), to identify the
optically-bright ICRF sources that will be positioned with the highest accuracy
with Gaia. Then we investigated the astrometric suitability of these sources by
examining their VLBI brightness distribution. We identified 243 candidate ICRF
sources for the alignment with the Gaia frame (with an optical counterpart
brighter than the apparent magnitude 18), but only 70 of these (10% of the ICRF
sources) are found to have the necessary high astrometric quality (i.e. a
brightness distribution that is compact enough) for this link. Additionally, it
was found that the QSOs that will have the most accurate positions in the Gaia
frame tend to have less-accurate VLBI positions, most probably because of their
physical structures. Altogether, this indicates that identifying other
high-quality VLBI radio sources suitable for the alignment with the future Gaia
frame is mandatory.Comment: To be published in A&A (2008)
The Pulsation of Chi Cygni Imaged by Optical Interferometry; a Novel Technique to Derive Distance and Mass of Mira Stars
We present infrared interferometric imaging of the S-type Mira star Chi
Cygni. The object was observed at four different epochs in 2005-2006 with the
IOTA optical interferometer (H band). Images show up to 40% variation in the
stellar diameter, as well as significant changes in the limb darkening and
stellar inhomogeneities. Model fitting gave precise time-dependent values of
the stellar diameter, and reveals presence and displacement of a warm molecular
layer. The star radius, corrected for limb darkening, has a mean value of 12.1
mas and shows a 5.1mas amplitude pulsation. Minimum diameter was observed at
phase 0.94+/-0.01. Maximum temperature was observed several days later at phase
1.02+/-0.02. We also show that combining the angular acceleration of the
molecular layer with CO (Delta v = 3) radial velocity measurements yields a
5.9+/-1.5 mas parallax. The constant acceleration of the CO molecules -- during
80% of the pulsation cycle -- lead us to argument for a free-falling layer. The
acceleration is compatible with a gravitational field produced by a
2.1(+1.5/-0.7) solar mass star. This last value is in agreement with
fundamental mode pulsator models. We foresee increased development of
techniques consisting in combining radial velocity with interferometric angular
measurements, ultimately allowing total mapping of the speed, density, and
position of the diverse species in pulsation driven atmospheres.Comment: 36 pages, accepted in Ap
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