1,741 research outputs found
Fundamental Properties of Cool Stars with Interferometry
We present measurements of fundamental astrophysical properties of nearby,
low-mass, K- and M-dwarfs from our DISCOS survey (DIameterS of COol Stars). The
principal goal of our study is the determination of linear radii and effective
temperatures for these stars. We calculate their radii from angular diameter
measurements using the CHARA Array and Hipparcos distances. Combined with
bolometric flux measurements based on literature photometry, we use our angular
diameter results to calculate their effective surface temperatures. We present
preliminary results established on an assortment of empirical relations to the
stellar effective temperature and radius that are based upon these
measurements. We elaborate on the discrepancy seen between theoretical and
observed stellar radii, previously claimed to be related to stellar activity
and/or metallicity. Our preliminary conclusion, however, is that convection
plays a larger role in the determination of radii of these late-type stars.
Understanding the source of the radius disagreement is likely to impact other
areas of study for low-mass stars, such as the detection and characterization
of extrasolar planets in the habitable zones.Comment: Contribution to Proceedings of Cool Stars 16 Workshop; 8 pages in ASP
format; 9 figure
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&
The close circumstellar environment of the semi-regular S-type star Pi^1 Gruis
We study the close circumstellar environment of the nearby S-type star Pi^1
Gruis using high spatial-resolution, mid-infrared observations from the
ESO/VLTI. Spectra and visibilities were obtained with the MIDI interferometer
on the VLT Auxiliary Telescopes. The cool M5III giant Beta Gruis was used as
bright primary calibrator, and a dedicated spectro-interferometric study was
undertaken to determine its angular diameter accurately. The MIDI measurements
were fitted with the 1D numerical radiative transfer code DUSTY to determine
the dust shell parameters of Pi^1 Gruis. Taking into account the low spatial
extension of the model in the 8-9 m spectral band for the smallest
projected baselines, we consider the possibility of a supplementary molecular
shell. The MIDI visibility and phase data are mostly dominated by the spherical
21 mas (694 Rsol) central star, while the extended dusty environment is
over-resolved even with the shortest baselines. No obvious departure from
spherical symmetry is found on the milliarcsecond scale. The
spectro-interferometric observations are well-fitted by an optically thin
(tau(dust)<0.01 in the band) dust shell that is located at about 14 stellar
radii with a typical temperature of 700 K and composed of 70% silicate and 30%
of amorphous alumina grains. An optically thin (tau(mol)<0.1 in the N band)
H2O+SiO molecular shell extending from the photosphere of the star up to 4.4
stellar radii with a typical temperature of 1000 K is added to the model to
improve the fit in the 8-9 m spectral band. We discuss the probable binary
origin of asymmetries as revealed by millimetric observations
Young Low-Mass Stars and Brown Dwarfs in IC 348
I present new results from a continuing program to identify and characterize
the low-mass stellar and substellar populations in the young cluster IC 348
(1-10~Myr). Optical spectroscopy has revealed young objects with spectral types
as late as M8.25. The intrinsic J-H and H-K colors of these sources are
dwarf-like, whereas the R-I and I-J colors appear intermediate between the
colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are
reproduced well with averages of standard dwarf and giant spectra, suggesting
that such averages should be used in the classification of young late-type
sources. An H-R diagram is constructed for the low-mass population in IC 348
(K6-M8). The presumably coeval components of the young quadruple system GG~Tau
(White et al.) and the locus of stars in IC 348 are used as empirical
isochrones to test the theoretical evolutionary models. For the models of
Baraffe et al., an adjustment of the temperature scale to progressively warmer
temperatures at later M types, intermediate between dwarfs and giants, brings
all components of GG~Tau onto the same model isochrone and gives the population
of IC 348 a constant age and age spread as a function of mass. When other
observational constraints are considered, such as the dynamical masses of
GM~Aur, DM~Tau, and GG~Tau~A, the models of Baraffe et al. are the most
consistent with observations of young systems. With compatible temperature
scales, the models of both D'Antona & Mazzitelli and Baraffe et al. suggest
that the hydrogen burning mass limit occurs near M6 at ages of <10 Myr. Thus,
several likely brown dwarfs are discovered in this study of IC 348, with masses
down to ~20-30 M_J.Comment: 23 pages, 9 figures, accepted to Ap
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