1,234 research outputs found
Periodic Halpha variations in GL 581: Further evidence for an activity origin to GL 581d
Radial velocity measurements showed evidence that the M dwarf GL 581 might
host a planet, GL 581d, in the so-called "habitable zone" of the star. A study
of Halpha in GL 581 demonstrated that changes in this activity indicator
correlated with radial velocity variations attributed to GL 581d. An exopplanet
that was important for studies of planet habitability may be an artifact of
stellar activity. Previous investigations analyzing the same activity data have
reached different conclusions regarding the existence of GL 581d. We therfore
investigated the Halpha variations for GL 581 to assess the nature of the
radial velocity variations attributed to the possible planet GL 581d. We
performed a Fourier analysis of the published Halpha measurements for GL 581d
using a so-called pre-whitening process to isolate the variations at the
orbital frequency of GL 581d. The frequency analysis yields five significant
frequencies, one of which is associated with the 66.7 d orbital period of the
presumed planet Gl 581d. The Halpha variations at this period show sine-like
variations that are 180 degrees out-of-phase with the radial velocity
variations of GL 581d. This is seen in thefull data set that spans almost 7
years, as well as a subset of the data that had good temporal sampling over 230
days. Furthermore, No significant temporal variations are found in the ratio of
the amplitudes of the Halpha index and radial velocity variations. This
provides additional evidence that the radial velocity signal attributed to GL
581d is in fact due to stellar activity.Comment: 5 pages, 6 figures, accepted by Astronomy and Astrophysic
Planets around evolved intermediate-mass stars. I. Two substellar companions in the open clusters NGC 2423 and NGC 4349
Context. Many efforts are being made to characterize extrasolar planetary
systems and unveil the fundamental mechanisms of planet formation. An important
aspect of the problem, which remains largely unknown, is to understand how the
planet formation process depends on the mass of the parent star. In particular,
as most planets discovered to date orbit a solar-mass primary, little is known
about planet formation around more massive stars. Aims. To investigate this
point, we present first results from a radial velocity planet search around red
giants in the clump of intermediate-age open clusters. We choose clusters
harbouring red giants with masses between 1.5 and 4 M_sun, using the well-known
cluster parameters to accurately determine the stellar masses. We are therefore
exploring a poorly-known domain of primary masses, which will bring new
insights into the properties of extrasolar planetary systems. Methods. We are
following a sample of about 115 red giants with the Coralie and HARPS
spectrographs to obtain high-precision radial velocity (RV) measurements and
detect giant planets around these stars. We use bisector and activity index
diagnostics to distinguish between planetary-induced RV variations and stellar
photospheric jitter. Results. We present the discoveries of a giant planet and
a brown dwarf in the open clusters NGC 2423 and NGC 4349, orbiting the 2.4
M_sun-star NGC2423 No3 (TYC 5409-2156-1) and the 3.9 M_sun-star NGC4349 No127
(TYC 8975-2606-1). These low-mass companions have orbital periods of 714 and
678 days and minimum masses of 10.6 and 19.8 M_jup, respectively. Combined with
the other known planetary systems, these detections indicate that the frequency
of massive planets is higher around intermediate-mass stars, and therefore
probably scales with the mass of the protoplanetary disk.Comment: 9 pages, 11 figures, accepted for publication in A&
Planets around active stars
We present the results of radial velocity measurements of two samples of
active stars. The first sample contains field G and K giants across the Red
Giant Branch, whereas the second sample consists of nearby young stars (d < 150
pc) with ages between 10 - 300 Myrs. The radial velocity monitoring program has
been carried out with FEROS at 1.52 m ESO telescope (1999 - 2002) and continued
since 2003 at 2.2 m MPG/ESO telescope. We observed stellar radial velocity
variations which originate either from the stellar activity or the presence of
stellar/substellar companions. By means of a bisector technique we are able to
distinguish the sources of the radial velocity variation. Among them we found
few candidates of planetary companions, both of young stars and G-K giants
sample.Comment: 4 pages, 5 figures, to appear in the Proceedings of the ESO Workshop
"Precision Spectroscopy in Astrophysics", eds. L. Pasquini, M. Romaniello,
N.C. Santos, A. Correi
On the Nature of the Radial Velocity Variability of Aldebaran: A Search for Spectral Line Bisector Variations
The shape of the Ti I 6303.8 A spectral line of Aldebaran as measured by the
line bisector was investigated using high signal-to-noise, high resolution
data. The goal of this study was to understand the nature of the 643-day period
in the radial velocity for this star reported by Hatzes and Cochran. Variations
in the line bisector with the radial velocity period would provide strong
evidence in support of rotational modulation or stellar pulsations as the cause
of the 643-day period. A lack of any bisector variability at this period would
support the planet hypothesis. Variations in the line asymmetries are found
with a period of 49.93 days. These variations are uncorrelated with 643-day
period found previously in the radial velocity measurements. It is demonstrated
that this 50-day period is consistent with an m=4 nonradial sectoral g-mode
oscillation. The lack of spectral variability with the radial velocity period
of 643 days may provide strong evidence in support of the hypothesis that this
variability stems from the reflex motion of the central star due to a planetary
companion having a mass of 11 Jupiter masses. However, this long-period
variability may still be due to a low order (m=2) pulsation mode since these
would cause bisector variations less than the error measurement.Comment: LaTeX, 8 pages, 10 figures. Accepted in Monthly Notices of the Royal
Astronomical Societ
The way forward
For the last few decades the study of disks around stars young and old and of
different types have progressed significantly. During the same time a
completely new discipline, the study of exoplanets, planets orbiting stars
other than our Sun, have emerged. Both these fields, which are interconnected,
have benefited from the development of new instrumentation, and especially by
telescopes and detectors deployed in space. In this chapter we are describing
the state of the art of such instruments and make an inventory of what is being
currently developed. We also state some of the requirements of the next steps
and what type of instruments will lead the way forward.Comment: 25 pages, 4 figure
Planetary companions around the K giant stars 11 UMi and HD 32518
11 UMi and HD 32518 belong to a sample of 62 K giant stars that has been
observed since February 2004 using the 2m Alfred Jensch telescope of the
Th\"uringer Landessternwarte (TLS) to measure precise radial velocities (RVs).
The aim of this survey is to investigate the dependence of planet formation on
the mass of the host star by searching for planetary companions around
intermediate-mass giants. An iodine absorption cell was used to obtain accurate
RVs for this study. Our measurements reveal that the RVs of 11 UMi show a
periodic variation of 516.22 days. The RV curve of HD 32518 shows sinusoidal
variations with a period of 157.54 days. The HIPPARCOS photometry as well as
our H\alpha core flux measurements reveal no variability with the RV period.
Thus, Keplerian motion is the most likely explanation for the observed RV
variations for both giant stars. An exoplanet with a minimum mass of 10.5
Jupiter masses orbits the K giant 11 UMi. The K1 III giant HD 32518 hosts a
planetary companion with a minimum mass of 3.0 Jupiter masses in a nearly
circular orbit. These are the 4th and 5th planets published from this TLS
survey.Comment: 11 pages, 16 figure
Coefficient of restitution for viscoelastic disks
The dissipative collision of two identical viscoelastic disks is studied. By
using a known law for the elastic part of the interaction force and the
viscoelastic damping model an analytical solution for the coefficient of
restitution shall be given. The coefficient of restitution depends
significantly on the impact velocity. It approaches one for small velocities
and decreases for increasing velocities.Comment: 11 pages, 3 figure
The Mass of the Planet-hosting Giant Star Beta Geminorum Determined from its p-mode Oscillation Spectrum
We use precise radial velocity measurements and photometric data to derive
the frequency spacing of the p-mode oscillation spectrum of the planet-hosting
star Beta Gem. This spacing along with the interferometric radius for this star
is used to derive an accurate stellar mass. A long time series of over 60 hours
of precise stellar radial velocity measurements of Beta Gem were taken with an
iodine absorption cell and the echelle spectrograph mounted on the 2m Alfred
Jensch Telescope. Complementary photometric data for this star were also taken
with the MOST microsatellite spanning 3.6 d. A Fourier analysis of the radial
velocity data reveals the presence of up to 17 significant pulsation modes in
the frequency interval 10-250 micro-Hz. Most of these fall on a grid of
equally-spaced frequencies having a separation of 7.14 +/- 0.12 micro-Hz. An
analysis of 3.6 days of high precision photometry taken with the MOST space
telescope shows the presence of up to 16 modes, six of which are consistent
with modes found in the spectral (radial velocity) data. This frequency spacing
is consistent with high overtone radial pulsations; however, until the
pulsation modes are identified we cannot be sure if some of these are nonradial
modes or even mixed modes. The radial velocity frequency spacing along with
angular diameter measurements of Beta Gem via interferometry results in a
stellar mass of M = 1.91 +/- 0.09 solar masses. This value confirms the
intermediate mass of the star determined using stellar evolutionary tracks.
Beta Gem is confirmed to be an intermediate mass star. Stellar pulsations in
giant stars along with interferometric radius measurements can provide accurate
determinations of the stellar mass of planet hosting giant stars. These can
also be used to calibrate stellar evolutionary tracks.Comment: Accepted by Astronomy and Astrophysic
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