366 research outputs found
Towards a new full-sky list of radial velocity standard stars
The calibration of the Radial Velocity Spectrometer (RVS) onboard the ESA
Gaia satellite (to be launched in 2012) requires a list of standard stars with
a radial velocity (RV) known with an accuracy of at least 300 m/s. The IAU
Commission 30 lists of RV standard stars are too bright and not dense enough.
We describe the selection criteria due to the RVS constraints for building an
adequate full-sky list of at least 1000 RV standards from catalogues already
published in the literature. A preliminary list of 1420 candidate standard
stars is built and its properties are shown. An important re-observation
programme has been set up in order to ensure within it the selection of objects
with a good stability until the end of the Gaia mission (around 2018). The
present list of candidate standards is available at CDS and usable for many
other projects.Comment: Astronomy & Astrophysics, in press, 8 pages, 8 figure
The catalog of radial velocity standard stars for the Gaia RVS: status and progress of the observations
A new full-sky catalog of Radial Velocity standard stars is being built for
the determination of the Radial Velocity Zero Point of the RVS on board of
Gaia. After a careful selection of 1420 candidates matching well defined
criteria, we are now observing all of them to verify that they are stable
enough over several years to be qualified as reference stars. We present the
status of this long-term observing programme on three spectrographs : SOPHIE,
NARVAL and CORALIE, complemented by the ELODIE and HARPS archives. Because each
instrument has its own zero-point, we observe intensively IAU RV standards and
asteroids to homogenize the radial velocity measurements. We can already
estimate that ~8% of the candidates have to be rejected because of variations
larger than the requested level of 300 m/s.Comment: Proceedings of SF2A2010, S. Boissier, M. Heydari-Malayeri, R. Samadi
and D. Valls-Gabaud (eds), 3 pages, 2 figure
DENIS-P J104814.7-395606.1: An M9 dwarfs at about 4 pc
We present the discovery of a previously unknown member of the immediate
solar neighbourhood, DENIS-P J104814.7-395606.1 (hereafter DENIS 1048-39),
identified while mining the DENIS database for new nearby stars. A HIRES
echelle spectrum obtained with the 10-m Keck telescope shows that it is an M9
dwarf. DENIS 1048-39 has a very bright apparent magnitude (I=12.67) for its
spectral type and colour (I-J=3.07), and is therefore very nearby. If it is
single its distance is only 4.1 +- 0.6pc, ranking it as between our twelfth and
fortyth closest neighbour. It is also the closest star or brown dwarf with a
spectral type later than M7V. Its proper motion was determined through
comparison of Sky atlas Schmidt plates, scanned by the MAMA microdensitometer,
with the DENIS images. At 1.53"/yr it further attests the closeness of DENIS
1048-39, and hence its dwarf status. These characteristics make it an obvious
target for further detailed studies.Comment: In press in A&A Letter
Calibration of the Gaia RVS from ground-based observations of candidate standard stars
International audienceThe Radial Velocity Spectrometer (RVS) on board of Gaia will perform a large spectroscopic survey to determine the radial velocities of some 1.5 Ă— 10^8 stars. We present the status of ground-based observations of a sample of 1420 candidate standard stars designed to calibrate the RVS. Each candidate star has to be observed several times before Gaia launch (and at least once during the mission) to ensure that its radial velocity remains stable during the whole mission. Observations are performed with the high-resolution spectrographs SOPHIE, NARVAL and CORALIE, completed with archival data of the ELODIE and HARPS instruments. The analysis shows that about 7% of the current catalogue exhibits variations larger than the adopted threshold of 300 m s^{-1}. Consequently, those stars should be rejected as reference targets, due to the expected accuracy of the Gaia RVS. Emphasis is also put here on our observations of bright asteroids to calibrate the ground-based velocities by a direct comparison with celestial mechanics. It is shown that the radial velocity zero points of SOPHIE, NARVAL and CORALIE are consistent with each other, within the uncertainties. Despite some scatter, their temporal variations remain small with respect to our adopted stability criterion
The Inner Rings of Beta Pictoris
We present Keck images of the dust disk around Beta Pictoris at 17.9 microns
that reveal new structure in its morphology. Within 1" (19 AU) of the star, the
long axis of the dust emission is rotated by more than 10 degrees with respect
to that of the overall disk. This angular offset is more pronounced than the
warp detected at 3.5" by HST, and in the opposite direction. By contrast, the
long axis of the emission contours at ~ 1.5" from the star is aligned with the
HST warp. Emission peaks between 1.5" and 4" from the star hint at the presence
of rings similar to those observed in the outer disk at ~ 25" with HST/STIS. A
deconvolved image strongly suggests that the newly detected features arise from
a system of four non-coplanar rings. Bayesian estimates based on the primary
image lead to ring radii of 14+/-1 AU, 28+/-3 AU, 52+/-2 AU and 82+/-2 AU, with
orbital inclinations that alternate in orientation relative to the overall disk
and decrease in magnitude with increasing radius. We believe these new results
make a strong case for the existence of a nascent planetary system around Beta
Pic.Comment: 5 pages, 2 figures, PDF format. Published in ApJL, December 20,200
Radial velocities for the Hipparcos-Gaia Hundred-Thousand-Proper-Motion project
(abridged) The Hundred-Thousand-Proper-Motion (HTPM) project will determine
the proper motions of ~113500 stars using a 23-year baseline. The proper
motions will use the Hipparcos data, with epoch 1991.25, as first epoch and the
first intermediate-release Gaia astrometry, with epoch ~2014.5, as second
epoch. The expected HTPM proper-motion standard errors are 30-190 muas/yr,
depending on stellar magnitude. Depending on the characteristics of an object,
in particular its distance and velocity, its radial velocity can have a
significant impact on the determination of its proper motion. The impact of
this perspective acceleration is largest for fast-moving, nearby stars. Our
goal is to determine, for each star in the Hipparcos catalogue, the
radial-velocity standard error that is required to guarantee a negligible
contribution of perspective acceleration to the HTPM proper-motion precision.
We employ two evaluation criteria, both based on Monte-Carlo simulations, with
which we determine which stars need to be spectroscopically (re-)measured. Both
criteria take the Hipparcos measurement errors into account. For each star in
the Hipparcos catalogue, we determine the confidence level with which the
available radial velocity and its standard error, taken from the XHIP
compilation catalogue, are acceptable. We find that for 97 stars, the radial
velocities available in the literature are insufficiently precise for a 68.27%
confidence level. We also identify 109 stars for which radial velocities are
currently unknown yet need to be acquired to meet the 68.27% confidence level.
To satisfy the radial-velocity requirements coming from our study will be a
daunting task consuming a significant amount of spectroscopic telescope time.
Fortunately, the follow-up spectroscopy is not time-critical since the HTPM
proper motions can be corrected a posteriori once (improved) radial velocities
become available.Comment: Accepted in A&
Planetary embryos and planetesimals residing in thin debris disks
We consider constraints on the planetesimal population residing in the disks
of AU Microscopii, Beta Pictoris and Fomalhaut taking into account their
observed thicknesses and normal disk opacities. We estimate that bodies of
radius 5, 180 and 70 km are responsible for initiating the collisional cascade
accounting for the dust production for AU-Mic, Beta-Pic and Fomalhaut's disks,
respectively, at break radii from the star where their surface brightness
profiles change slope. Larger bodies, of radius 1000km and with surface density
of order 0.01 g/cm^2, are required to explain the thickness of these disks
assuming that they are heated by gravitational stirring. A comparison between
the densities of the two sizes suggests the size distribution in the largest
bodies is flatter than that observed in the Kuiper belt. AU Mic's disk requires
the shallowest size distribution for bodies with radius greater than 10km
suggesting that the disk contains planetary embryos experiencing a stage of
runaway growth.Comment: submitted to MNRA
The rotation and coma profiles of comet C/2004 Q2 (Machholz)
Aims. Rotation periods of cometary nuclei are scarce, though important when
studying the nature and origin of these objects. Our aim is to derive a
rotation period for the nucleus of comet C/2004 Q2 (Machholz). Methods. C/2004
Q2 (Machholz) was monitored using the Merope CCD camera on the Mercator
telescope at La Palma, Spain, in January 2005, during its closest approach to
Earth, implying a high spatial resolution (50km per pixel). One hundred seventy
images were recorded in three different photometric broadband filters, two blue
ones (Geneva U and B) and one red (Cousins I). Magnitudes for the comet's
optocentre were derived with very small apertures to isolate the contribution
of the nucleus to the bright coma, including correction for the seeing. Our CCD
photometry also permitted us to study the coma profile of the inner coma in the
different bands. Results. A rotation period for the nucleus of P = 9.1 +/- 0.2
h was derived. The period is on the short side compared to published periods of
other comets, but still shorter periods are known. Nevertheless, comparing our
results with images obtained in the narrowband CN filter, the possibility that
our method sampled P/2 instead of P cannot be excluded. Coma profiles are also
presented, and a terminal ejection velocity of the grains v_gr = 1609 +/- 48
m/s is found from the continuum profile in the I band.Comment: 11 pages, 9 figures, accepted by A&
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