873 research outputs found
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
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
Experimental studies on the impact properties of water ice
Experimental studies on the impact of ice particles at very low velocity were continued. These measurements have applications in the dynamics of Saturn's rings. Initially data were obtained on the coefficient of restitution for ice spheres of one radius of curvature. The type of measurements were expanded to include restitution data for balls with a variety of surfaces as well as sticking forces between ice particles. Significant improvements were made to this experiment, the most important being the construction of a new apparatus. The new apparatus consists of a smaller version of the disk pendulum and a stainless steel, double-walled cryostat. The apparatus has proved to be a significant improvement over the old one. Measurements can now be made at temperatures near 90 K, comparable to the temperature of the environment of Saturn's rings, and with much greater temperature stability. It was found that a roughened contact surface or the presence of frost can cause a much larger change in the restitution measure than the geometrical effect of the radius of curvature
Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars
Exo-planets are preferentially found around high metallicity main sequence
stars. We aim at investigating whether evolved stars share this property, and
what this tells about planet formation. Statistical tools and the basic
concepts of stellar evolution theory are applied to published results as well
as our own radial velocity and chemical analyses of evolved stars. We show that
the metal distributions of planet-hosting (P-H) dwarfs and giants are
different, and that the latter do not favor metal-rich systems. Rather, these
stars follow the same age-metallicity relation as the giants without planets in
our sample. The straightforward explanation is to attribute the difference
between dwarfs and giants to the much larger masses of giants' convective
envelopes. If the metal excess on the main sequence is due to pollution, the
effects of dilution naturally explains why it is not observed among evolved
stars. Although we cannot exclude other explanations, the lack of any
preference for metal-rich systems among P-H giants could be a strong indication
of the accretion of metal-rich material. We discuss further tests, as well as
some predictions and consequences of this hypothesis.Comment: A&A, in pres
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&
Discovery of a planet around the K giant star 4 UMa
Context: For the past 3 years we have been monitoring a sample of 62 K giant
stars using precise stellar radial velocity measurements taken at the
Thueringer Landessternwarte Tautenburg. Aims: To search for sub-stellar
companions to giant stars and to understand the nature of the diverse radial
velocity variations exhibited by K giant stars. Methods: We present precise
stellar radial velocity measurements of the K1III giant star 4 UMa (HD 73108).
These were obtained using the coude echelle spectrograph of 2-m Alfred Jensch
Telescope. The wavelength reference for the radial velocity measurements was
provided by an iodine absorption cell. Results: Our measurements reveal that
the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a
semiamplitude K = 216.8 m/s. A Keplerian orbit with an eccentricity, e = 0.43
+/- 0.02 is the most reasonable explanation for the radial velocity variations.
The orbit yields a mass function, f(m) = (2.05 +/- 0.24) x 10^(- 7) M_sun. From
our high resolution spectra we calculate a metallicity of -0.25 +/- 0.05 and
derive a stellar mass of 1.23 M_sun +/- 0.15 for the host star. Conclusions:
The K giant star 4 UMa hosts a substellar companion with minimum mass m sin i =
7.1 +/- 1.6 M_Jupiter.Comment: 6 pages, 5 figures, 2 tables, accepted in A&
Radial Velocity Variations in Pulsating Ap Stars. II. 33 Librae
We present precise relative radial velocity (RV) measurements for the rapidly
oscillating Ap (roAp) star 33 Librae measured from high resolution data
spanning the wavelength interval 5000--6200 A. We find that pulsational radial
velocity amplitude determined over a broad wavelength range (~100 A) depends on
the spectral region that is examined and can be as high as 60 m/s at 5600 A and
as low as 7 m/s in the 5900 A region. RV measurements of individual spectral
lines can show even higher RV amplitudes. The acoustic cross-sections of the
atmosphere, i.e. the phase and amplitude of the pulsations, as a function of
optical depth is found for spectral lines of Ca, Cr, Fe, La, Ce, Gd, Er and Nd.
This analysis shows that pulsation phase is variable through the atmosphere and
that Nd III lines pulsate almost 180 degrees out-of-phase with those of Nd II
features and are formed significantly higher in the stellar atmosphere. This
conclusively establishes the presence of at least one radial node to the
pulsations in the upper stellar atmosphere. The histogram of pulsational phases
for all individual spectral feature shows a bi-modal Gaussian distribution with
17% of the lines having a pulsational phase approximatels 165 degrees
out-of-phase with most other spectral lines. This is also consistent with the
presence of a radial node in the stellar atmosphere. The accumulation of phase
due to a running wave component can explain the 165 degree phase difference as
well as the broader width (by a factor of two) of one of the Gaussian
components of the phase distribution.Comment: 18 pages, 12 Figures, accepted by MNRA
Coefficient of Restitution as a Fluctuating Quantity
The coefficient of restitution of a spherical particle in contact with a flat
plate is investigated as a function of the impact velocity. As an experimental
observation we notice non-trivial (non-Gaussian) fluctuations of the measured
values. For a fixed impact velocity, the probability density of the coefficient
of restitution, , is formed by two exponential functions (one
increasing, one decreasing) of different slope. This behavior may be explained
by a certain roughness of the particle which leads to energy transfer between
the linear and rotational degrees of freedom.Comment: 4 pages, 4 figure
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