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 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.
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.
Context.Only a handful of debris disks have been imaged up to now. Due to the need for high dynamic range and high angular resolution, very little is known about the inner planetary region, where ...small amounts of warm dust are expected to be found. Aims.We investigate the close neighbourhood of Vega with the help of infrared stellar interferometry and estimate the integrated K-band flux originating from the central 8 AU of the debris disk. Methods.We performed precise visibility measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to separately resolve the emissions from the extended debris disk (short baselines) and from the stellar photosphere (long baselines). Results.After revising Vega's K-band angular diameter ($\theta_{\rm UD} = 3.202$ ± 0.005 mas), we show that a significant deficit in squared visibility ($\Delta V^2 = 1.88$ ± 0.34%) is detected at short baselines with respect to the best-fit uniform disk stellar model. This deficit can be either attributed to the presence of a low-mass stellar companion around Vega, or as the signature of the thermal and scattered emissions from the debris disk. We show that the presence of a close companion is highly unlikely, as well as other possible perturbations (stellar morphology, calibration), and deduce that we have most probably detected the presence of dust in the close neighbourhood of Vega. The resulting flux ratio between the stellar photosphere and the debris disk amounts to 1.29 ± 0.19% within the FLUOR field-of-view (~7.8 AU). Finally, we complement our K-band study with archival photometric and interferometric data in order to evaluate the main physical properties of the inner dust disk. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.
We present results of a long-baseline interferometry campaign using the PAVO beam combiner at the CHARA Array to measure the angular sizes of five main-sequence stars, one subgiant and four red giant ...stars for which solar-like oscillations have been detected by either Kepler or CoRoT. By combining interferomettic angular diameters, Hipparcos parallaxes, asteroseismic densities, bolometric fluxes, and high-resolution spectroscopy, we derive a full set of near-model-independent fundamental properties for the sample. We first use these properties to test asteroseismic scaling relations for the frequency of maximum power (v sub(max)) and the large frequency separation ( Delta v). We find excellent agreement within the observational uncertainties, and empirically show that simple estimates of asteroseismic radii for main-sequence stars are accurate to lap4%. We furthermore find good agreement of our measured effective temperatures with spectroscopic and photometric estimates with mean deviations for stars between T sub(eff) = 4600-6200 K of -22 + or - 32 K (with a scatter of 97 K) and -58 + or - 31 K (with a scatter of 93 K), respectively. Finally, we present a first comparison with evolutionary models, and find differences between observed and theoretical properties for the metal-rich main-sequence star HD 173701. We conclude that the constraints presented in this study will have strong potential for testing stellar model physics, in particular when combined with detailed modeling of individual oscillation frequencies.
Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby ...companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.
Algol ( beta Per) is an extensively studied hierarchical triple system whose inner pair is a prototype semi-detached binary with mass transfer occurring from the sub-giant secondary to the ...main-sequence primary. We present here the results of our Algol observations made between 2006 and 2010 at the CHARA interferometer with the Michigan Infrared Combiner in the H-band. The use of four telescopes with long baselines allows us to achieve better than 0.5 mas resolution and to unambiguously resolve the three stars. The inner and outer orbital elements, as well as the angular sizes and mass ratios for the three components, are determined independently from previous studies. We report a significantly improved orbit for the inner stellar pair with the consequence of a 15% change in the primary mass compared with previous studies. We also determine the mutual inclination of the orbits to be much closer to perpendicularity than previously established. State-of-the-art image reconstruction algorithms are used to image the full triple system. In particular an image sequence of 55 distinct phases of the inner pair orbit is reconstructed, clearly showing the Roche-lobe-filling secondary revolving around the primary, with several epochs corresponding to the primary and secondary eclipses.
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.
A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages ...could result from interactions with the companion during the common-envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 ± 0.59 kiloparsecs from the Sun.
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