93 research outputs found
Testing planet formation theories with Giant stars
Planet searches around evolved giant stars are bringing new insights to
planet formation theories by virtue of the broader stellar mass range of the
host stars compared to the solar-type stars that have been the subject of most
current planet searches programs. These searches among giant stars are
producing extremely interesting results. Contrary to main sequence stars
planet-hosting giants do not show a tendency of being more metal rich. Even if
limited, the statistics also suggest a higher frequency of giant planets (at
least 10 %) that are more massive compared to solar-type main sequence stars.
The interpretation of these results is not straightforward. We propose that the
lack of a metallicity-planet connection among giant stars is due to pollution
of the star while on the main sequence, followed by dilution during the giant
phase. We also suggest that the higher mass and frequency of the planets are
due to the higher stellar mass. Even if these results do not favor a specific
formation scenario, they suggest that planetary formation might be more complex
than what has been proposed so far, perhaps with two mechanisms at work and one
or the other dominating according to the stellar mass. We finally stress as the
detailed study of the host stars and of the parent sample is essential to
derive firm conclusions.Comment: IAU 249: Exoplanets: Detection, Formation and Dynamics J.L. Zhou,
Y.S. Sun & S. Ferraz-Mello, eds. in pres
Angular Diameters and Effective Temperatures of Twenty-five K Giant Stars from the CHARA Array
Using Georgia State University's CHARA Array interferometer, we measured
angular diameters for 25 giant stars, six of which host exoplanets. The
combination of these measurements and Hipparcos parallaxes produce physical
linear radii for the sample. Except for two outliers, our values match angular
diameters and physical radii estimated using photometric methods to within the
associated errors with the advantage that our uncertainties are significantly
lower. We also calculated the effective temperatures for the stars using the
newly-measured diameters. Our values do not match those derived from
spectroscopic observations as well, perhaps due to the inherent properties of
the methods used or because of a missing source of extinction in the stellar
models that would affect the spectroscopic temperatures
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
Basic physical parameters of a selected sample of evolved stars
We present the detailed spectroscopic analysis of 72 evolved stars, including
the [Fe/H] determination for the whole sample. These metallicities, together
with the Teff values and the absolute V magnitude derived from Hipparcos
parallaxes, are used to estimate basic stellar parameters (ages, masses, radii,
(B-V)o and log g using theoretical isochrones and a Bayesian estimation method.
The (B-V)o values so estimated turn out to be in excellent agreement with the
observed (B-V), confirming the reliability of the (Teff,(B-V)o) relation used
in the isochrones. The estimated diameters have been compared with limb
darkening-corrected ones measured with independent methods, finding an
agreement better than 0.3 mas within the 1-10 mas interval. We derive the
age-metallicity relation for the solar neighborhood; for the first time such a
relation has been derived from observations of field giants rather than from
open clusters and field dwarfs and subdwarfs. The age-metallicity relation is
characterized by close-to-solar metallicities for stars younger than ~4 Gyr,
and by a large [Fe/H] spread with a trend towards lower metallicities for
higher ages. We find that the [Fe/H] dispersion of young stars (less than 1
Gyr) is comparable to the observational errors, indicating that stars in the
solar neighbourhood are formed from interstellar matter of quite homogeneous
chemical composition. The three giants of our sample which have been proposed
to host planets are not metal rich, what is at odds with those for main
sequence stars. However, two of these stars have masses much larger than a
solar mass so we may be sampling a different stellar population from most
radial velocity searches for extrasolar planets. We also confirm that the
radial velocity variability tends to increase along the RGB.Comment: 17 pgs, 19 fig
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
A new interferometric study of four exoplanet host stars : {\theta} Cygni, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis
Studying exoplanet host stars is of the utmost importance to establish the
link between the presence of exoplanets around various types of stars and to
understand the respective evolution of stars and exoplanets.
Using the limb-darkened diameter (LDD) obtained from interferometric data, we
determine the fundamental parameters of four exoplanet host stars. We are
particularly interested in the F4 main-sequence star, {\theta} Cyg, for which
Kepler has recently revealed solar-like oscillations that are unexpected for
this type of star. Furthermore, recent photometric and spectroscopic
measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic
radial velocity of \sim150 days. Models of this periodic change in radial
velocity predict either a complex planetary system orbiting the star, or a new
and unidentified stellar pulsation mode.
We performed interferometric observations of {\theta} Cyg, 14 Andromedae,
{\upsilon} Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount
Wilson, California) in several three-telescope configurations. We measured
accurate limb darkened diameters and derived their radius, mass and temperature
using empirical laws.
We obtain new accurate fundamental parameters for stars 14 And, {\upsilon}
And and 42 Dra. We also obtained limb darkened diameters with a minimum
precision of \sim 1.3%, leading to minimum planet masses of Msini=5.33\pm 0.57,
0.62 \pm 0.09 and 3.79\pm0.29 MJup for 14 And b, {\upsilon} And b and 42 Dra b,
respectively. The interferometric measurements of {\theta} Cyg show a
significant diameter variability that remains unexplained up to now. We propose
that the presence of these discrepancies in the interferometric data is caused
by either an intrinsic variation of the star or an unknown close companion
orbiting around it.Comment: 10 pages + 2 pages appendix, 16 figures, accepted for publication in
A&
A search for solar-like oscillations in K giants in the globular cluster M4
To expand the range in the colour-magnitude diagram where asteroseismology
can be applied, we organized a photometry campaign to find evidence for
solar-like oscillations in giant stars in the globular cluster M4. The aim was
to detect the comb-like p-mode structure characteristic for solar-like
oscillations in the amplitude spectra. The two dozen main target stars are in
the region of the bump stars and have luminosities in the range 50-140 Lsun. We
collected 6160 CCD frames and light curves for about 14000 stars were
extracted. We obtain high quality light curves for the K giants, but no clear
oscillation signal is detected. High precision differential photometry is
possible even in very crowded regions like the core of M4. Solar-like
oscillations are probably present in K giants, but the amplitudes are lower
than classical scaling laws predict.Comment: 14 pages, 16 figures, accepted for publication in A&
Precision Astrometry of the Exoplanet Host Candidate GD 66
The potential existence of a giant planet orbiting within a few AU of a
stellar remnant has profound implications for both the survival and possible
regeneration of planets during post-main sequence stellar evolution. This paper
reports Hubble Space Telescope Fine Guidance Sensor and U.S. Naval Observatory
relative astrometry of GD 66, a white dwarf thought to harbor a giant planet
between 2 and 3 AU based on stellar pulsation arrival times. Combined with
existing infrared data, the precision measurements here rule out all
stellar-mass and brown dwarf companions, implying that only a planet remains
plausible, if orbital motion is indeed the cause of the variations in pulsation
timing.Comment: 6 pages, 3 figures, 1 table, accepted to MNRA
O Tema da liberdade religiosa na polĂtica brasileira do sĂ©culo XIX: uma via para a compreensĂŁo da secularização da esfera polĂtica
Universal mechanisms of sound production and control in birds and mammals
As animals vocalize, their vocal organ transforms motor commands into vocalizations for social communication. In birds, the physical mechanisms by which vocalizations are produced and controlled remain unresolved because of the extreme difficulty in obtaining in vivo measurements. Here, we introduce an ex vivo preparation of the avian vocal organ that allows simultaneous high-speed imaging, muscle stimulation and kinematic and acoustic analyses to reveal the mechanisms of vocal production in birds across a wide range of taxa. Remarkably, we show that all species tested employ the myoelastic-aerodynamic (MEAD) mechanism, the same mechanism used to produce human speech. Furthermore, we show substantial redundancy in the control of key vocal parameters ex vivo, suggesting that in vivo vocalizations may also not be specified by unique motor commands. We propose that such motor redundancy can aid vocal learning and is common to MEAD sound production across birds and mammals, including humans
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