Stephen T. Ridgway

Circumstellar material in the Vega inner system revealed by CHARA/FLUOR

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 beamcombiner 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 (θUD = 3.202 ± 0.005 mas), we show that a significant deficit in squared visibility (∆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.

Imaging the spotty surface of Betelgeuse in the H band

Aims. This paper reports on H-band interferometric observations of Betelgeuse made at the three-telescope interferometer IOTA. We image Betelgeuse and its asymmetries to understand the spatial variation of the photosphere, including its diameter, limb darkening, effective temperature, surrounding brightness, and bright (or dark) star spots. Methods. We used different theoretical simulations of the photosphere and dusty environment to model the visibility data. We made images with parametric modeling and two image reconstruction algorithms: MIRA and WISARD. Results. We measure an average limb-darkened diameter of 44.28 ± 0.15 mas with linear and quadratic models and a Rosseland diameter of 45.03 ± 0.12 mas with a MARCS model. These measurements lead us to derive an updated effective temperature of 3600 ± 66 K. We detect a fully-resolved environment to which the silicate dust shell is likely to contribute. By using two imaging reconstruction algorithms, we unveiled two bright spots on the surface of Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5% of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of the total flux. Conclusions. Resolved images of Betelgeuse in the H band are asymmetric at the level of a few percent. The MOLsphere is not detected in this wavelength range. The amount of measured limb-darkening is in good agreement with model predictions. The two spots imaged at the surface of the star are potential signatures of convective cells.

A near-infrared interferometric survey of debris disk stars. I. Probing the hot dust content around eps Eridani and tau Ceti with CHARA/FLUOR

Context. The quest for hot dust in the central region of debris disks re quires high resolution and high dynamic range imaging. Nearinfrared interferometry is a powerful means to directly det ect faint emission from hot grains. Aims. We probed the first 3 AU aroundτ Ceti andǫ Eridani with the CHARA array (Mt Wilson, USA) in order to gauge the 2� m excess flux emanating from possible hot dust grains in the debris dis ks and to also resolve the stellar photospheres. Methods. High precision visibility amplitude measurements were performed with the FLUOR single mode fiber instrument and telescope pairs on baselines ranging from 22 to 241 m of projected length. The short baseline observations allow us to disentangle the contribution of an extended structure from the photospheric emission, while the long baselines constrain the stellar d iameter. Results. We have detected a resolved emission aroundτ Cet, corresponding to a spatially integrated, fractional e xcess flux of 0.98± 0.21× 10 −2 with respect to the photospheric flux in the K ′ ‐band. Aroundǫ Eri, our measurements can exclude a fractional excess of greater than 0.6× 10 −2 (3σ). We interpret the photometric excess aroundτ Cet as a possible signature of hot grains in the inner debris disk and demonstrate that a faint, physical or background, companion can be safely excluded. In addition, we measured both stellar

VLBA observations of SiO masers towards Mira variable stars

We present new total intensity and linear polarization VLBA observations of the ν = 2a ndν = 1 J = 1−0 maser transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over a substantial fraction of their pulsation periods. These observations were part of an observing program that also includes interferometric measurements at 2.2 and 3.6 micron (Mennesson et al. 2002); comparison of the results from different wavelengths allows studying the envelope independently of the poorly known distances to these stars. Nine stars were observed at from one to four epochs during 2001. The SiO emission is largely confined to rings which are smaller than the inner radius of the dust shells reported by Danchi et al. (1994). Two stars (U Orionis, R Aquarii) have maser rings with diameters corresponding to the size of the hot molecular layer as measured at 3.6 micron; in the other cases, the SiO rings are substantially larger. Variations of ring diameter for most, but not all stars, had an rms amplitude in agreement with the models of Humphreys et al. (2002) although the expected relationship between the diameter and pulsation phase was not seen. The ring diameter in U Orionis shows remarkably small variation. A correlation between the 2.2/3.6 µm diameter ratio with that of the SiO/3.6 µm diameter ratio is likely due to differences in the opacities at 2.2 and 3.6 µm in a molecular layer. A further correlation with the inner size of the dust shell reported by Danchi et al. (1994) suggest some differences in the temperature structure. Clear evidence is seen in R Aquarii for an equatorial disk similar to that reported by Hollis et al. (2001); rotation is possibly also detected in S Coronae Boralis.

EVIDENCE FOR VERY EXTENDED GASEOUS LAYERS AROUND O-RICH MIRA VARIABLES AND M GIANTS

Nine bright O-rich Mira stars and five semiregular variable cool M giants have been observed with the Infrared and Optical Telescope Array (IOTA) interferometer in both K 0 (� 2.15 lm) and L 0 (� 3.8 lm) broadband filters, in most cases at very close variability phases. All of the sample Mira stars and four of the semiregular M giants show strong increases, from ’20% to ’100%, in measured uniform-disk (UD) diameters between the K 0 and L 0 bands. (A selection of hotter M stars does not show such a large increase.) There is no evidence that K 0 and L 0 broadband visibility measurements should be dominated by strong molecular bands, and cool expanding dust shells already detected around some of these objects are also found to be poor candidates for producing these large apparent diameter increases. Therefore, we propose that this must be a continuum or pseudocontinuum opacity effect. Such an apparent enlargement can be reproduced using a simple two-component model consisting of a warm (1500–2000 K), extended (up to ’3 stellar radii), optically thin (� ’ 0:5) layer located above the classical photosphere. The Planck weighting of the continuum emission from the two layers will suffice to make the L 0 UD diameter appear larger than the K 0 UD diameter. This twolayer scenario could also explain the observed variation of Mira UD diameters versus infrared wavelength— outside of strong absorption bands—as already measured inside the H, K, L, and N atmospheric windows. This interpretation is consistent with the extended molecular gas layers (H2O, CO, etc.) inferred around some of these objects from previous IOTA K 0 -band interferometric observations obtained with the Fiber Linked Unit for Optical Recombination (FLUOR) and from Infrared Space Observatory and high-resolution ground-based FTS infrared spectra. The two-component model has immediate implications. For example, the Mira photosphere diameters are smaller than previously recognized—this certainly implies higher effective temperatures, and it may favor fundamental mode pulsation. Also, the UD model fails generally to represent the brightness distribution and has very limited applicability for Mira stars. The presence of a very extended gas layer extending up to ’3 stellar radii seems now well established on a fair sample of asymptotic giant branch stars ranging from late-type giants to long-period variables, with some probable impact on stellar model atmospheres and mass-loss mechanisms. Subject headings: circumstellar matter — instrumentation: interferometers — stars: atmospheres — stars: variables: other — techniques: interferometric

CHARA ARRAY MEASUREMENTS OF THE ANGULAR DIAMETERS OF EXOPLANET HOST STARS

Wehavemeasuredtheangulardiametersforasampleof 24exoplanethoststarsusingGeorgiaStateUniversity’s CHARA Array interferometer. We use these improved angular diameters together with Hipparcos parallax measurements to derive linear radii and to estimate the stars’ evolutionary states. Subjectheadingginfrared:stars — planetarysystems — stars:fundamentalparameters — techniques:interferometric Online material: extended figure, machine-readable table

The projection factor of δ Cephei A calibration of the Baade-Wesselink method using the CHARA Array

Cepheids play a key role in astronomy as standard candles for measuring intergalactic distances. Their distance is usually inferred from the period-luminosity relationship, calibrated using the semi-empirical Baade-Wesselink method. Using this method, the distance is known to a multiplicative factor, called the projection factor. Presently, this factor is computed using numerical models - it has hitherto never been measured directly. Based on our new interferometric measurements obtained with the CHARA Array and the already published parallax, we present a geometrical measurement of the projection factor of a Cepheid, δ Cep. The value we determined, p = 1.27 ± 0.06, confirms the generally adopted value of p = 1.36 within 1.5 sigmas. Our value is in line with recent theoretical predictions of Nardetto et al. (2004, A&A, 428, 131).

Near-infrared imaging of the Becklin-Neugebauer-IRc2 region in Orion with subarcsecond resolution

The Becklin-Neugebauer source has been partially resolved by speckle imagery at 3.6 μm with an angular resolution of 0.″2. The images, fluxes, and additional constraints are consistent with a spherical dust shell of inner diameter 0.″1, surrounding a hot star. The surrounding region, including the vicinity of IRc2, has been imaged with short-exposure/deconvolution methods at several wavelengths, at a resolution of 0.″5. IRc2 is resolved into four components, and the fluxes of the detected sources were determined at 2.8 and 4 μm. Accurate astronomy of the infrared sources with respect to the SiO maser emission has been obtained. The centroid of the SiO maser is definitely not located at the center of symmetry between lobes A and B, but may correspond to IRc2 component A

THE CHARA ARRAY ANGULAR DIAMETER OF HR 8799 FAVORS PLANETARY MASSES FOR ITS IMAGED COMPANIONS

HR 8799 is an hF0 mA5 γ Doradus-, λ Bootis-, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799s limb-darkened angular diameter to be 0.342 ± 0.008 mas (an error of only 2%). By combining our measurement with the stars parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 ± 0.06 R_☉), effective temperature (7193 ± 87 K, consistent with F0), luminosity (5.05 ± 0.29 L_☉), and the extent of the habitable zone (HZ; 1.62-3.32 AU). These improved stellar properties permit much more precise comparisons with stellar evolutionary models, from which a mass and age can be determined, once the metallicity of the star is known. Considering the observational properties of other λ Bootis stars and the indirect evidence for youth of HR 8799, we argue that the internal abundance, and what we refer to as the effective abundance, is most likely near solar. Finally, using the Yonsei-Yale evolutionary models with uniformly scaled solar-like abundances, we estimate HR 8799s mass and age considering two possibilities: 1.516^(+0.038)_(–0.024) M_☉ and 33^(+7)_(–13.2) Myr if the star is contracting toward the zero-age main sequence or 1.513^(+0.023)_(–0.024_ M_☉ and 90^(+381)_(–50) Myr if it is expanding from it. This improved estimate of HR 8799s age with realistic uncertainties provides the best constraints to date on the masses of its orbiting companions, and strongly suggests they are indeed planets. They nevertheless all appear to orbit well outside the HZ of this young star.

The abundances of CH4, CH3D, NH3, and PH3 in the troposphere of Jupiter derived from high-resolution 1100-1200/cm spectra

High-resolution (..delta..v = 1 cm/sup -1/) spectra of the 1110--1200 cm/sup -1/ region of the central part of Jupiter were obtained in 1980 March and 1981 April. The best fit NH/sub 3/ distribution curve shows a higher than solar mixing ratio, (NH/sub 3/)/(H/sub 2/) = (3.3 +- 1.7) x 10/sup -4/, below the 147 K layer (>0.6 atmosphere). If we introduce NH/sub 3/ ice particles as an opacity source, the NH/sub 3/ mixing ratio below the 147 K layer can be lowered, but the fit is worse than that given by the model excluding NH/sub 3/ ice particles. The best fit PH/sub 3/ distribution curve has a (PH/sub 3/)/(H/sub 2/) mixing ratio of (8.3 +- 2.0) x 10/sup -7/ in the troposphere. We also found a (CH/sub 4/)/(H/sub 2/) mixing ratio of (2.5 +- 0.4) x 10/sup -3/ in the troposphere. The derived D/H ratio is 3.0/sup +1.1//sub -0.8/ x 10/sup -5/. Most of the flux in this spectral region comes from layers above the 170 K level (<1 atmosphere).