178 research outputs found
Modeling of Closure Phase Measurements with AMBER/VLTI - Towards Characterization of Exoplanetary Atmospheres
Differential phase observations with a near-IR interferometer offer a way to
obtain spectra of extrasolar planets. The method makes use of the wavelength
dependence of the interferometer phase of the planet/star system, which depends
both on the interferometer geometry and on the brightness ratio between the
planet and the star. The differential phase is strongly affected by
instrumental and atmospheric dispersion effects. Difficulties in calibrating
these effects might prevent the application of the differential phase method to
systems with a very high contrast, such as extrasolar planets. A promising
alternative is the use of spectrally resolved closure phases, which are immune
to many of the systematic and random errors affecting the single-baseline
phases. We have modeled the response of the AMBER instrument at the VLTI to
realistic models of known extrasolar planetary systems, taking into account
their theoretical spectra as well as the geometry of the VLTI. We present a
strategy to determine the geometry of the planetary system and the spectrum of
the extrasolar planet from closure phase observations in two steps. We show
that there is a close relation between the nulls in the closure phase and the
nulls in the corresponding single-baseline phases: every second null of a
single-baseline phase is also a null in the closure phase. In particular, the
nulls in the closure phase do not depend on the spectrum but only on the
geometry. Therefore the geometry of the system can be determined by measuring
the nulls in the closure phase, and braking the remaining ambiguity due to the
unknown system orientation by means of observations at different hour angles.
Based on the known geometry, the planet spectrum can then be directly
synthesized from the closure phases.Comment: replaced version with corrected Fig.5; 9 pages, 6 figures, Proceeding
of the SPIE conference, Glasgow, 2004, Proc. SPIE 5491, in pres
Precise radial velocities of giant stars. X. Bayesian stellar parameters and evolutionary stages for 372 giant stars from the Lick planet search
The determination of accurate stellar parameters of giant stars is essential
for our understanding of such stars in general and as exoplanet host stars in
particular. Precise stellar masses are vital for determining the lower mass
limit of potential substellar companions with the radial velocity method. Our
goal is to determine stellar parameters, including mass, radius, age, surface
gravity, effective temperature and luminosity, for the sample of giants
observed by the Lick planet search. Furthermore, we want to derive the
probability of these stars being on the horizontal branch (HB) or red giant
branch (RGB), respectively. We compare spectroscopic, photometric and
astrometric observables to grids of stellar evolutionary models using Bayesian
inference. We provide tables of stellar parameters, probabilities for the
current post-main sequence evolutionary stage, and probability density
functions for 372 giants from the Lick planet search. We find that of
the stars in our sample are more probably on the HB. In particular, this is the
case for 15 of the 16 planet host stars in the sample. We tested the
reliability of our methodology by comparing our stellar parameters to
literature values and find very good agreement. Furthermore, we created a small
test sample of 26 giants with available asteroseismic masses and evolutionary
stages and compared these to our estimates. The mean difference of the stellar
masses for the 24 stars with the same evolutionary stages by both methods is
only . We do not find any
evidence for large systematic differences between our results and estimates of
stellar parameters based on other methods. In particular we find no significant
systematic offset between stellar masses provided by asteroseismology to our
Bayesian estimates based on evolutionary models.Comment: 15 pages, 7 figures, accepted for publication in A&
Precise Radial Velocities of Giant Stars VII. Occurrence Rate of Giant Extrasolar Planets as a Function of Mass and Metallicity
(abridged) We have obtained precise radial velocities for a sample of 373 G
and K type giants at Lick Observatory regularly over more than 12 years.
Planets have been identified around 15 giant stars; an additional 20 giant
stars host planet candidates. We investigate the occurrence rate of substellar
companions around giant stars as a function of stellar mass and metallicity. We
probe the stellar mass range from about 1 to beyond 3 M_Sun, which is not being
explored by main-sequence samples. We fit the giant planet occurrence rate as a
function of stellar mass and metallicity with a Gaussian and an exponential
distribution, respectively. We find strong evidence for a planet-metallicity
correlation among the secure planet hosts of our giant star sample, in
agreement with the one for main-sequence stars. However, the planet-metallicity
correlation is absent for our sample of planet candidates, raising the
suspicion that a good fraction of them might indeed not be planets. Consistent
with the results obtained by Johnson for subgiants, the giant planet occurrence
rate increases in the stellar mass interval from 1 to 1.9 M_Sun. However, there
is a maximum at a stellar mass of 1.9 +0.1/-0.5 M_Sun, and the occurrence rate
drops rapidly for masses larger than 2.5-3.0 M_Sun. We do not find any planets
around stars more massive than 2.7 M_Sun, although there are 113 stars with
masses between 2.7 and 5 M_Sun in our sample (corresponding to a giant planet
occurrence rate < 1.6% at 68.3% confidence in that stellar mass bin). We also
show that this result is not a selection effect related to the planet
detectability being a function of the stellar mass. We conclude that giant
planet formation or inward migration is suppressed around higher mass stars,
possibly because of faster disk depletion coupled with a longer migration
timescale.Comment: 13 pages plus long table appendix, accepted by A&
Dynamical analysis of the circumprimary planet in the eccentric binary system HD59686
We present a detailed orbital and stability analysis of the HD~59686
binary-star planet system. HD~59686 is a single-lined moderately close (AU) eccentric () binary, where the primary is an evolved
K giant with mass and the secondary is a star with a
minimum mass of . Additionally, on the basis of precise
radial velocity (RV) data a Jovian planet with a minimum mass of , orbiting the primary on a nearly circular S-type orbit with
and AU, has recently been announced. We investigate
large sets of orbital fits consistent with HD 59686's radial velocity data by
applying bootstrap and systematic grid-search techniques coupled with
self-consistent dynamical fitting. We perform long-term dynamical integrations
of these fits to constrain the permitted orbital configurations. We find that
if the binary and the planet in this system have prograde and aligned coplanar
orbits, there are narrow regions of stable orbital solutions locked in a
secular apsidal alignment with the angle between the periapses, , librating about . We also test a large number of mutually
inclined dynamical models in an attempt to constrain the three-dimensional
orbital architecture. We find that for nearly coplanar and retrograde orbits
with mutual inclination , the
system is fully stable for a large range of orbital solutions.Comment: 17 pages, 11 figures, accepted for publication by A
Precise radial velocities of giant stars. XI. Two brown dwarfs in 6:1 mean motion resonance around the K giant star Ophiuchi
We present radial-velocity (RV) measurements for the K giant Oph (=
HIP88048, HD163917, HR6698), which reveal two brown dwarf companions with a
period ratio close to 6:1. For our orbital analysis we use 150 precise RV
measurements taken at Lick Observatory between 2000 and 2011, and we combine
them with RV data for this star available in the literature. Using a stellar
mass of for Oph and applying a self-consistent N-body
model we estimate the minimum dynamical companion masses to be and ,
with orbital periods d and d. We study a
large set of potential orbital configurations for this system, employing a
bootstrap analysis and a systematic grid-search coupled with our
dynamical fitting model, and we examine their long-term stability. We find that
the system is indeed locked in a 6:1 mean motion resonance (MMR), with and all six resonance angles librating
around 0. We also test a large set of coplanar inclined configurations,
and we find that the system will remain in a stable resonance for most of these
configurations. The Oph system is important for probing planetary
formation and evolution scenarios. It seems very likely that the two brown
dwarf companions of Oph formed like planets in a circumstellar disk
around the star and have been trapped in a MMR by smooth migration capture.Comment: 17 pages, 9 figures. New version with corrected number in title. No
other change
Precise radial velocities of giant stars VIII. Testing for the presence of planets with CRIRES Infrared Radial Velocities
We have been monitoring 373 very bright (V < 6 mag) G and K giants with high
precision optical Doppler spectroscopy for more than a decade at Lick
Observatory. Our goal was to discover planetary companions around those stars
and to better understand planet formation and evolution around
intermediate-mass stars. However, in principle, long-term, g-mode nonradial
stellar pulsations or rotating stellar features, such as spots, could
effectively mimic a planetary signal in the radial velocity data. Our goal is
to compare optical and infrared radial velocities for those stars with periodic
radial velocity patterns and to test for consistency of their fitted radial
velocity semiamplitudes. Thereby, we distinguish processes intrinsic to the
star from orbiting companions as reason for the radial velocity periodicity
observed in the optical. Stellar spectra with high spectral resolution have
been taken in the H-band with the CRIRES near-infrared spectrograph at ESO's
VLT for 20 stars of our Lick survey. Radial velocities are derived using many
deep and stable telluric CO2 lines for precise wavelength calibration. We find
that the optical and near-infrared radial velocities of the giant stars in our
sample are consistent. We present detailed results for eight stars in our
sample previously reported to have planets or brown dwarf companions. All eight
stars passed the infrared test. We conclude that the planet hypothesis provides
the best explanation for the periodic radial velocity patterns observed for
these giant stars.Comment: 14 pages, 6 figures, 3 tables, accepted by Astronomy & Astrophysic
Precise radial velocities of giant stars VI. A possible 2:1 resonant planet pair around the K giant star Cet
We report the discovery of a new planetary system around the K giant
Cet (HIP 5364, HD 6805) based on 118 high-precision optical radial velocities
taken at Lick Observatory since July 2000. Since October 2011 an additional
nine near-infrared Doppler measurements have been taken using the ESO CRIRES
spectrograph (VLT, UT1). The visible data set shows two clear periodicities.
Although we cannot completely rule out that the shorter period is due to
rotational modulation of stellar features, the infrared data show the same
variations as in the optical, which strongly supports that the variations are
caused by two planets. Assuming the mass of Cet to be 1.7 , the
best edge-on coplanar dynamical fit to the data is consistent with two massive
planets ( = 2.6 0.2 , = 3.3
0.2 ), with periods of = 407 3 days and
= 740 5 days and eccentricities of = 0.12 0.05 and
= 0.08 0.03. We tested a wide variety of edge-on coplanar and
inclined planetary configurations for stability, which agree with the derived
radial velocities. We find that in certain coplanar orbital configurations with
moderate eccentricity, the planets can be effectively trapped in an
anti-aligned 2:1 mean motion resonance. A much larger non-resonant stable
region exists in low-eccentricity parameter space, although it appears to be
much farther from the best fit than the 2:1 resonant region. In all other
cases, the system is categorized as unstable or chaotic. Another conclusion
from the coplanar inclined dynamical test is that the planets can be at most a
factor of 1.4 more massive than their suggested minimum masses. This
stability constraint on the inclination excludes the possibility of two brown
dwarfs, and strongly favors a planetary system.Comment: 15 pages, 11 figures, accepted for publication in A&A on June 20,
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