We have measured the angular diameters of six M dwarfs with the CHARA Array, a long-baseline optical interferometer located at Mount Wilson Observatory. Spectral types range from M1.0 V to M3.0 V and ...linear radii from 0.38 to 0.69 R sub( ). These results are consistent with the seven other M dwarf radii measurements from optical interferometry and with those for 14 stars in eclipsing binary systems. We compare all directly measured M dwarf radii to model predictions and find that current models underestimate the true stellar radii by up to 15%-20%. The differences are small among the metal-poor stars but become significantly larger with increasing metallicity. This suggests that theoretical models for low-mass stars may be missing some opacity source that alters the computed stellar radii.
The CHARA Array is a six 1 m telescope optical/IR interferometric array located on Mount Wilson, California, designed and built by the Center for High Angular Resolution Astronomy of Georgia State ...University. In this paper we describe the main elements of the Array hardware and software control systems, as well as the data reduction methods currently being used. Our plans for upgrades in the near future are also described.
We present interferometric measurements obtained with the CHARA Array of 13 adolescent-age stars in nearby moving groups. The motivation was to spatially resolve the largest stars and to search for ...binary companions. Nine stars have diameters smaller than the resolution limit and no evidence for companions within 0.5-50 mas and ΔH < 2.0 mag. The diameters of three stars were spatially resolved: GJ 159 (0.582 0.016 mas) and GJ 393 (0.564 0.021 mas) in the AB Dor moving group, and former member HD 89744 (0.556 0.032 mas). Combining the angular diameters with their distances and bolometric fluxes, we measured radii and effective temperatures. The temperatures of GJ 159 (6286 123 K) and GJ 393 (3515 68 K) are consistent with spectroscopic measurements. Comparisons with evolutionary models show that HD 89744 has evolved off the main sequence. GJ 159 and GJ 393 lie within 1.5 of the zero-age main sequence, complicating their age estimates because it is unclear whether the stars are contracting or expanding. GJ 159 has a mass of 1.2 0.1 with an age spanning 0.021-3.0 Gyr. Its debris disk and lithium abundance favor a young age. GJ 393 has a mass of 0.42 0.03 and a lower limit on its age 0.06 Gyr. This overlaps with the age of the moving group; however, an older age would be more consistent with its slow rotation, low activity, and luminosity, suggesting that GJ 393 is a kinematic interloper.
Dust is expected to be ubiquitous in extrasolar planetary systems owing to the dynamical activity of minor bodies. Inner dust populations are, however, still poorly known because of the high contrast ...and small angular separation with respect to their host star, and yet, a proper characterisation of exozodiacal dust is mandatory for the design of future Earth-like planet imaging missions. We aim to determine the level of near-infrared exozodiacal dust emission around a sample of 42 nearby main sequence stars with spectral types ranging from A to K and to investigate its correlation with various stellar parameters and with the presence of cold dust belts. This study provides new insight into the phenomenon of bright exozodiacal discs, showing that hot dust populations are probably linked to outer dust reservoirs in the case of solar-type stars. For A-type stars, no clear conclusion can be made regarding the origin of the detected near-infrared excesses.
We present direct radii measurements of the well-known transiting exoplanet host stars HD 189733 and HD 209458 using the CHARA Array interferometer. We find the limb-darkened angular diameters to be ...θLD = 0.3848 ± 0.0055 and 0.2254 ± 0.0072 mas for HD 189733 and HD 209458, respectively. HD 189733 and HD 209458 are currently the only two transiting exoplanet systems where detection of the respective planetary companion's orbital motion from high-resolution spectroscopy has revealed absolute masses for both star and planet. We use our new measurements together with the orbital information from radial velocity and photometric time series data, Hipparcos distances, and newly measured bolometric fluxes to determine the stellar effective temperatures (T
eff = 4875 ± 43, 6092 ± 103 K), stellar linear radii (R
* = 0.805 ± 0.016, 1.203 ± 0.061 R⊙), mean stellar densities (ρ* = 1.62 ± 0.11, 0.58 ± 0.14 ρ⊙), planetary radii (R
p = 1.216 ± 0.024, 1.451 ± 0.074 R
Jup), and mean planetary densities (ρp = 0.605 ± 0.029, 0.196 ± 0.033 ρJup) for HD 189733b and HD 209458b, respectively. The stellar parameters for HD 209458, an F9 dwarf, are consistent with indirect estimates derived from spectroscopic and evolutionary modelling. However, we find that models are unable to reproduce the observational results for the K2 dwarf, HD 189733. We show that, for stellar evolutionary models to match the observed stellar properties of HD 189733, adjustments lowering the solar-calibrated mixing-length parameter to αMLT =1.34 need to be employed.
We have obtained high-precision interferometric measurements of Vega with the CHARA Array and FLUOR beam combiner in the K' band at projected baselines between 103 and 273 m. The measured visibility ...amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star characterized by a single effective temperature and surface gravity. Our measurements, when compared to synthetic visibilities and synthetic spectrophotometry from a Roche-von Zeipel gravity-darkened model atmosphere, provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole-on. Our best-fitting model indicates that Vega is rotating at 691% of its angular break-up rate with an equatorial velocity of 275 km s super(-1). Together with the measured v sin i, this velocity yields an inclination for the rotation axis of 5. For this model the pole-to-equator effective temperature difference is 62250 K, a value much larger than previously derived from spectral line analyses. A polar effective temperature of 10,150 K is derived from a fit to ultraviolet and optical spectrophotometry. The synthetic and observed spectral energy distributions are in reasonable agreement longward of 140 ran, where they agree to 5% or better. Shortward of 140 ran, the model is up to 10 times brighter than observed. The model has a luminosity of 637 L , a value 35% lower than Vega's apparent luminosity based on its bolometric flux and parallax, assuming a slowly rotating star. Our model predicts the spectral energy distribution of Vega as viewed from its equatorial plane, and it may be employed in radiative models for the surrounding debris disk.
Aims.
We present a detailed visible and near-infrared spectro-interferometric analysis of the Be-shell star
o
Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations.
Methods.
We ...analyzed spectro-interferometric data in the H
α
(VEGA) and Br
γ
(AMBER) lines using models of increasing complexity: simple geometric models, kinematic models, and radiative transfer models computed with the 3D non-LTE code HDUST.
Results.
We measured the stellar radius of
o
Aquarii in the visible with a precision of 8%: 4.0 ± 0.3
R
⊙
. We constrained the circumstellar disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The emitting disk sizes in the H
α
and Br
γ
lines were found to be similar, at ~10–12 stellar diameters, which is uncommon since most results for Be stars indicate a larger extension in H
α
than in Br
γ
. We found that the inclination angle
i
derived from H
α
is significantly lower (~15°) than the one derived from Br
γ
:
i
~ 61.2° and 75.9°, respectively. While the two lines originate from a similar region of the disk, the disk kinematics were found to be near to the Keplerian rotation (i.e.,
β
= −0.5) in Br
γ
(
β
~ −0.43), but not in H
α
(
β
~ −0.30). After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the circumstellar disk in both lines: a base disk surface density Σ
0
= 0.12 g cm
−2
and a radial density law exponent
m
= 3.0. The same kind of discrepancy, as with the kinematic model, is found in the determination of
i
using the BeAtlas grid. The stellar rotational rate was found to be very close (~96%) to the critical value. Despite being derived purely from the fit to interferometric data, our best-fit HDUST model provides a very reasonable match to non-interferometric observables of
o
Aquarii: the observed spectral energy distribution, H
α
and Br
γ
line profiles, and polarimetric quantities. Finally, our analysis of multi-epoch H
α
profiles and imaging polarimetry indicates that the disk structure has been (globally) stable for at least 20 yr.
Conclusions.
Looking at the visible continuum and Br
γ
emission line only,
o
Aquarii fits in the global scheme of Be stars and their circumstellar disk: a (nearly) Keplerian rotating disk well described by the viscous decretion disk (VDD) model. However, the data in the H
α
line shows a substantially different picture that cannot fully be understood using the current generation of physical models of Be star disks. The Be star
o
Aquarii presents a stable disk (close to the steady-state), but, as in previous analyses, the measured
m
is lower than the standard value in the VDD model for the steady-state regime (
m
= 3.5). This suggests that some assumptions of this model should be reconsidered. Also, such long-term disk stability could be understood in terms of the high rotational rate that we measured for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.
Context. The main sequence binary star 61 Cyg (K5V+K7V) is our nearest stellar neighbour in the northern hemisphere. This proximity makes it a particularly well suited system for very high accuracy ...interferometric radius measurements. Aims. Our goal is to constrain the poorly known evolutionary status and age of this bright binary star. Methods. We obtained high accuracy interferometric observations in the infrared K logical or prime band, using the CHARA/FLUOR instrument. We then computed evolutionary models of 61 Cyg A & B with the CESAM2k code. As model constraints, we used a combination of observational parameters from classical observation methods (photometry, spectroscopy) as well as our new interferometric radii. Results. The measured limb darkened disk angular diameters are theta_{\rm LD}({\rm A}) = 1.775 pm 0.013 mas and theta_{\rm LD}({\rm B}) = 1.581 pm 0.022 mas, respectively for 61 Cyg A and B. Considering the high accuracy parallaxes available, these values translate into photospheric radii of R({\rm A}) = 0.665 pm 0.005 R_{\odot} and R({\rm B}) = 0.595 pm 0.008 R_{\odot}. The new radii constrain efficiently the physical parameters adopted for the modeling of both stars, allowing us to predict asteroseismic frequencies based on our best-fit models. Conclusions. The CESAM2k evolutionary models indicate an age around 6 Gyr and are compatible with small values of the mixing length parameter. The measurement of asteroseismic oscillation frequencies in 61 Cyg A & B would be of great value to improve the modeling of this important fiducial stellar system, in particular to better constrain the masses.
We present the results of long-baseline optical interferometry observations using the Precision Astronomical Visual Observations (PAVO) beam combiner at the Center for High Angular Resolution ...Astronomy (CHARA) Array to measure the angular sizes of three bright Kepler stars: θ Cygni, and both components of the binary system 16 Cygni. Supporting infrared observations were made with the Michigan Infrared Combiner (MIRC) and Classic beam combiner, also at the CHARA Array. We find limb-darkened angular diameters of 0.753 ± 0.009 mas for θ Cyg, 0.539 ± 0.007 mas for 16 Cyg A and 0.490 ± 0.006 mas for 16 Cyg B. The Kepler Mission has observed these stars with outstanding photometric precision, revealing the presence of solar-like oscillations. Due to the brightness of these stars the oscillations have exceptional signal-to-noise, allowing for detailed study through asteroseismology, and are well constrained by other observations. We have combined our interferometric diameters with Hipparcos parallaxes, spectrophotometric bolometric fluxes and the asteroseismic large frequency separation to measure linear radii (θ Cyg: 1.48 ± 0.02 R, 16 Cyg A: 1.22 ± 0.02 R, 16 Cyg B: 1.12 ± 0.02 R), effective temperatures (θ Cyg: 6749 ± 44 K, 16 Cyg A: 5839 ± 42 K, 16 Cyg B: 5809 ± 39 K) and masses (θ Cyg: 1.37 ± 0.04 M, 16 Cyg A: 1.07 ± 0.05 M, 16 Cyg B: 1.05 ± 0.04 M) for each star with very little model dependence. The measurements presented here will provide strong constraints for future stellar modelling efforts.
PDF
