5,299 research outputs found
Evaluating the stability of atmospheric lines with HARPS
Context: In the search for extrasolar systems by radial velocity technique, a
precise wavelength calibration is necessary for high-precision measurements.
The choice of the calibrator is a particularly important question in the
infra-red domain, where the precision and exploits still fall behind the
achievements of the optical.
Aims: We investigate the long-term stability of atmospheric lines as a
precise wavelength reference and analyze their sensitivity to different
atmospheric and observing conditions.
Methods: We use HARPS archive data on three bright stars, Tau Ceti, Mu Arae
and Epsilon Eri, spanning 6 years and containing high-cadence measurements over
several nights. We cross-correlate this data with an O2 mask and evaluate both
radial velocity and bisector variations down to a photon noise of 1 m/s.
Results: We find that the telluric lines in the three data-sets are stable
down to 10 m/s (r.m.s.) over the 6 years. We also show that the radial velocity
variations can be accounted for by simple atmospheric models, yielding a final
precision of 1-2 m/s.
Conclusions: The long-term stability of atmospheric lines was measured as
being of 10 m/s over six years, in spite of atmospheric phenomena. Atmospheric
lines can be used as a wavelength reference for short-time-scales programs,
yielding a precision of 5 m/s "out-of-the box". A higher precision, down to 2
m/s can be reached if the atmospheric phenomena are corrected for by the simple
atmospheric model described, making it a very competitive method even on long
time-scales.Comment: 7 pages, accepted for publication in A&
The Stellar Dynamics of Omega Centauri
The stellar dynamics of Omega Centauri are inferred from the radial
velocities of 469 stars measured with CORAVEL (Mayor et al. 1997). Rather than
fit the data to a family of models, we generate estimates of all dynamical
functions nonparametrically, by direct operation on the data. The cluster is
assumed to be oblate and edge-on but mass is not assumed to follow light. The
mean motions are consistent with axisymmetry but the rotation is not
cylindrical. The peak rotational velocity is 7.9 km/s at 11 pc from the center.
The apparent rotation of Omega Centauri is attributable in part to its proper
motion. We reconstruct the stellar velocity ellipsoid as a function of
position, assuming isotropy in the meridional plane. We find no significant
evidence for a difference between the velocity dispersions parallel and
perpendicular to the meridional plane. The mass distribution inferred from the
kinematics is slightly more extended than, though not strongly inconsistent
with, the luminosity distribution. We also derive the two-integral distribution
function f(E,Lz) implied by the velocity data.Comment: 25 Latex pages, 12 Postscript figures, uses aastex, epsf.sty.
Submitted to The Astronomical Journal, December 199
Extrasolar planets and brown dwarfs around A-F type stars V. A planetary system found with HARPS around the F6IV-V star HD 60532
Aims: In the frame of the search for extrasolar planets and brown dwarfs
around early-type stars, we present the results obtained for the F-type
main-sequence star HD 60532 (F6V) with HARPS.
Methods: Using 147 spectra obtained with HARPS at La Silla on a time baseline
of two years, we study the radial velocities of this star.
Results: HD 60532 radial velocities are periodically variable, and the
variations have a Keplerian origin. This star is surrounded by a planetary
system of two planets with minimum masses of 1 and 2.5 Mjup and orbital
separations of 0.76 and 1.58 AU respectively. We also detect high-frequency,
low-amplitude (10 m/s peak-to-peak) pulsations. Dynamical studies of the system
point toward a possible 3:1 mean-motion resonance which should be confirmed
within the next decade.Comment: 7 pages, 11 figures, accepted for publication in A&
Signal buffering in random networks of spiking neurons: microscopic vs. macroscopic phenomena
In randomly connected networks of pulse-coupled elements a time-dependent
input signal can be buffered over a short time. We studied the signal buffering
properties in simulated networks as a function of the networks state,
characterized by both the Lyapunov exponent of the microscopic dynamics and the
macroscopic activity derived from mean-field theory. If all network elements
receive the same signal, signal buffering over delays comparable to the
intrinsic time constant of the network elements can be explained by macroscopic
properties and works best at the phase transition to chaos. However, if only 20
percent of the network units receive a common time-dependent signal, signal
buffering properties improve and can no longer be attributed to the macroscopic
dynamics.Comment: 5 pages, 3 figure
A CORAVEL radial-velocity monitoring of giant Ba and S stars: spectroscopic orbits and intrinsic variations
This paper provides orbital parameters for 38 barium stars and 10 extrinsic S
stars derived from a decade-long CORAVEL monitoring. Lower bounds on the
orbital period (generally exceeding 10 y) have been obtained for 10 more
systems. Mira S, SC and (Tc-poor) C stars have also been monitored and show
intrinsic radial-velocity variations due to atmospheric phenomena. Tentative
orbital solutions are proposed for 3 stars (S UMa, X Cnc, BD-08:1900) where the
velocity and photometric periods are different. Three stars (RZ Peg, SS Vir and
R CMi) exhibit radial-velocity variations synchronous with the light
variations. Pseudo-orbital solutions have been derived for those stars. In the
case of RZ Peg, a line-doubling phenomenon is observed near maximum light, and
probably reflects the shock wave propagating through the photosphere.Comment: Astronomy & Astrophysics Supplements, 20 pages, 8 figures, 8 tables
(LaTeX). Also available at:
http://obswww.unige.ch/~udry/cine/barium/barium.htm
New constrains on Gliese 86 B
We present the results of multi epochs imaging observations of the companion
to the planetary host Gliese 86. Associated to radial velocity measurements,
this study aimed at characterizing dynamically the orbital properties and the
mass of this companion (here after Gliese 86 B), but also at investigating the
possible history of this particular system. We used the adaptive optics
instrument NACO at the ESO Very Large Telescope to obtain deep coronographic
imaging in order to determine new photometric and astrometric measurements of
Gliese 86 B. Part of the orbit is resolved. The photometry of Gliese B
indicates colors compatible with a ~70 Jupiter mass brown dwarf or a white
dwarf. Both types of objects allow to fit the available, still limited
astrometric data. Besides, if we attribute the long term radial velocity
residual drift observed for Gliese A to B, then the mass of the latter object
is ~0.5 Msun. We analyse both astrometric and radial velocity data to propose
first orbital parameters for Gliese B. Assuming Gliese B is a ~0.5 Msun white
dwarf, we explore the constraints induced by this hypothesis and refine the
parameters of the system.Comment: 10 pages, 18 figures, accepted in A&
Comparing HARPS and Kepler surveys: The alignment of multiple-planet systems
Aims. We study a subset of the planetary population characterized both by
HARPS and Kepler surveys. We compare the statistical properties of planets in
systems with m.sin i >5-10 M_Earth and R>2 R_Earth. If we assume that the
underlying population has the same characteristics, the different detection
sensitivity to the orbital inclination relative to the line of sight allows us
to probe the planets' mutual inclination.
Methods. We considered the frequency of systems with one, two and three
planets as dictated by HARPS data. We used Kepler's planetary period and host
mass and radii distributions (corrected from detection bias) to model planetary
systems in a simple yet physically plausible way. We then varied the mutual
inclination between planets in a system according to different prescriptions
(completely aligned, Rayleigh distributions and isotropic) and compared the
transit frequencies with one, two or three planets with those measured by
Kepler.
Results. The results show that the two datasets are compatible, a remarkable
result especially because there are no tunable knobs other than the assumed
inclination distribution. For m.sin i cutoffs of 7-10 M_Earth, which are those
expected to correspond to the radius cutoff of 2 R_Earth, we conclude that the
results are better described by a Rayleigh distribution with mode of 1 deg or
smaller. We show that the best-fit scenario only becomes a Rayleigh
distribution with mode of 5 deg if we assume a rather extreme mass-radius
relationship for the planetary population.
Conclusions. These results have important consequences for our understanding
of the role of several proposed formation and evolution mechanisms. They
confirm that planets are likely to have been formed in a disk and show that
most planetary systems evolve quietly without strong angular momentum exchanges
(abridged).Comment: 10 pages, 6 figures, 4 tables, accepted for publication in Astronomy
& Astrophysic
Detection of Planetary Transits Across a Sun-like Star
We report high precision, high cadence photometric measurements of the star
HD 209458, which is known from radial velocity measurements to have a planetary
mass companion in a close orbit. We detect two separate transit events at times
that are consistent with the radial velocity measurements. In both cases, the
detailed shape of the transit curve due to both the limb darkening of the star
and the finite size of the planet is clearly evident. Assuming stellar
parameters of 1.1 R_Sun and 1.1 M_Sun, we find that the data are best
interpreted as a gas giant with a radius of 1.27 +/- 0.02 R_Jup in an orbit
with an inclination of 87.1 +/- 0.2 degrees. We present values for the
planetary surface gravity, escape velocity, and average density, and discuss
the numerous observations that are warranted now that a planet is known to
transit the disk of its parent star.Comment: 10 pages, 3 figures, accepted by ApJ Letter
Search for exoplanets with the radial-velocity technique: quantitative diagnostics of stellar activity
Aims: Stellar activity may complicate the analysis of high-precision
radial-velocity spectroscopic data when looking for exoplanets signatures. We
aim at quantifying the impact of stellar spots on stars with various spectral
types and rotational velocities and comparing the simulations with data
obtained with the HARPS spectrograph. Methods: We have developed detailed
simulations of stellar spots and estimated their effects on a number of
observables commonly used in the analysis of radial-velocity data when looking
for extrasolar planets, such as radial-velocity curves, cross-correlation
functions, bisector spans and photometric curves. The computed stellar spectra
are then analyzed in the same way as when searching for exoplanets. Results: 1)
A first grid of simulation results is built for F-K type stars, with different
stellar and spot properties. 2) It is shown quantitatively that star spots with
typical sizes of 1% can mimic both radial-velocity curves and the bisector
behavior of short-period giant planets around G-K type stars with a vsini lower
than the spectrograph resolution. For stars with intermediate vsini, smaller
spots may produce similar features. In these cases, additional observables
(e.g., photometry, spectroscopic diagnostics) are mandatory to confirm the
presence of short-period planets. We show that, in some cases, photometric
variations may not be enough to clearly rule out spots as explanations of the
observed radial-velocity variations. This is particularly important when
searching for super-Earth planets. 3) It is also stressed that quantitative
values obtained for radial-velocity and bisector span amplitudes depend
strongly on the detailed star properties, on the spectrograph used, on the set
of lines used, and on the way they are measured.Comment: 12 pages, 16 figures, accepted for publication in A&
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