200 research outputs found
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
A planetary companion around the K giant eps Corona Borealis
Aims. Our aim is to search for and study the origin of the low-amplitude and
long-periodic radial velocity (RV) variations in K giants. Methods. We present
high-resolution RV measurements of K2 giant epsilon CrB from February 2005 to
January 2012 using the fiber-fed Bohyunsan Observatory Echelle Spectrograph
(BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Results. We find
that the RV measurements for epsilon CrB exhibit a periodic variation of 417.9
+/- 0.5 days with a semi-amplitude of 129.4 +/- 2.0 m/s. There is no
correlation between RV measurements and chromospheric activity in the Ca II H
region, the Hipparcos photometry, or bisector velocity span. Conclusions.
Keplerian motion is the most likely explanation, with the RV variations arising
from an orbital motion. Assuming a possible stellar mass of 1.7 +/- 0.1 M_Sun
for epsilon CrB, we obtain a minimum mass for the planetary companion of 6.7
+/- 0.3 M_Jup with an orbital semi-major axis of 1.3 AU and eccentricity of
0.11. We also discuss the implications of our observations for stellar
metallicity versus planet occurrence rate and stellar mass versus planetary
mass relations.Comment: 5 pages, 7 figures, 3 tables, accepted for publisation in Astronomy &
Astrophysic
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
Submillisievert chest CT in patients with COVID-19 - experiences of a German Level-I center
Purpose:
Computed tomography (CT) is used for initial diagnosis and therapy monitoring of patients with coronavirus disease 2019 (COVID-19). As patients of all ages are affected, radiation dose is a concern. While follow-up CT examinations lead to high cumulative radiation doses, the ALARA principle states that the applied dose should be as low as possible while maintaining adequate image quality. The aim of this study was to evaluate parameter settings for two commonly used CT scanners to ensure sufficient image quality/diagnostic confidence at a submillisievert dose.
Materials and methods:
We retrospectively analyzed 36 proven COVID-19 cases examined on two different scanners. Image quality was evaluated objectively as signal-to-noise ratio (SNR)/contrast-to-noise ratio (CNR) measurement and subjectively by two experienced, independent readers using 3-point Likert scales. CT dose index volume (CTDIvol) and dose-length product (DLP) were extracted from dose reports, and effective dose was calculated.
Results:
With the tested parameter settings we achieved effective doses below 1 mSv (median 0.5 mSv, IQR: 0.2 mSv, range: 0.3−0.9 mSv) in all 36 patients. Thirty-four patients had typical COVID-19 findings. Both readers were confident regarding the typical COVID-19 CT-characteristics in all cases (3 ± 0). Objective image quality parameters were: SNRnormal lung: 17.0 ± 5.9, CNRGGO/normal lung: 7.5 ± 5.0, and CNRconsolidation/normal lung: 15.3 ± 6.1.
Conclusion:
With the tested parameters, we achieved applied doses in the submillisievert range, on two different CT scanners without sacrificing diagnostic confidence regarding COVID-19 findings
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
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
Three-dimensional hydrodynamical simulations of red giant stars: semi-global models for the interpretation of interferometric observations
Context. Theoretical predictions from models of red giant branch stars are a
valuable tool for various applications in astrophysics ranging from galactic
chemical evolution to studies of exoplanetary systems. Aims. We use the
radiative transfer code OPTIM3D and realistic 3D radiative-hydrodynamical (RHD)
surface convection simulations of red giants to explore the impact of
granulation on interferometric observables. Methods. We compute intensity maps
for the 3D simulation snapshots in two filters: in the optical at 5000 \pm 300
{\AA} and in the K band 2.14 0.26 {\mu}m FLUOR filter, corresponding to
the wavelength-range of instruments mounted on the CHARA interferometer. From
the intensity maps, we construct images of the stellar disks, accounting for
center-to-limb variations. We then derive interferometric visibility amplitudes
and phases. We study their behavior with position angle and wavelength.
Results. We provide average limb-darkening coefficients for different
metallicities and wavelength-ranges. We detail the prospects for the detection
and characterization of granulation and center-to-limb variations of red giant
stars with today's interferometers. We find that the effect of
convective-related surface structures depends on metallicity and surface
gravity. We provided theoretical closure phases that should be incorporated
into the analysis of red giant planet companion closure phase signals. We
estimate 3D-1D corrections to stellar radii determination: 3D models are ~ 3.5%
smaller to ~ 1% larger in the optical with respect to 1D, and roughly 0.5 to
1.5% smaller in the infrared. Even if these corrections are small, they are
important to properly set the zero point of effective temperature scale derived
by interferometry and to strengthen the confidence of existing red giant
catalogues of calibrating stars for interferometry.Comment: Accepted for publication on Astronomy & Astrophysics, 14 pages, 13
figure
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
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&
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&
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