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HD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and dust. Its spectrum shows characteristics which may indicate either a pre- or a post-main-sequence system. Mapping the kinematics of the gas in the inner few au of the system contributes to a better understanding of its physical nature. We present the first high spatial and spectral resolution interferometric observations of the Br line of HD 50138, obtained with VLTI/AMBER. The line emission originates in a region more compact (up to 3 au) than the continuum-emitting region. Blue- and red-shifted emission originates from the two different hemispheres of an elongated structure perpendicular to the polarization angle. The velocity of the emitting medium decreases radially. An overall offset along the NW direction between the line- and continuum-emitting regions is observed. We compare the data with a geometric model of a thin Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the data are well reproduced by this model, except for the variability, the global offset and the visibility at the systemic velocity. The evolutionary state of the system is discussed; most diagnostics are ambiguous and may point either to a post-main-sequence or a pre-main-sequence nature. [ABSTRACT FROM AUTHOR]

We present the first high angular resolution observations in the near-infrared H band (1.6µm) of the luminous blue variable star P Cygni. We obtained six-telescope interferometric observations with the CHARA Array and the MIRC beam combiner. These show that the spatial flux distribution is larger than expected for the stellar photosphere. A two-component model for the star (uniform disk) plus a halo (two-dimensional Gaussian) yieldsan excellent fit of the observations, and we suggest that the halo corresponds to flux emitted from the base ofthe stellar wind. This wind component contributes about 45% of the H-band flux and has an angular FWHM =0.96 mas, compared to the predicted stellar diameter of 0.41 mas. We show several images reconstructed from the interferometric visibilities and closure phases, and they indicate a generally spherical geometry for the wind. We also obtained near-infrared spectrophotometry of P Cygni from which we derive the flux excess comparedto a purely photospheric spectral energy distribution. The H-band flux excess matches that from the wind flux fraction derived from the two-component fits to the interferometry. We find evidence of significant near-infrared flux variability over the period from 2006 to 2010 that appears similar to the variations in the Ha emission flux from the wind. [ABSTRACT FROM AUTHOR]

In 2005 we obtained very precise interferometric measurements of the pole-on rapid rotator Vega (A0 V) with the longest baselines of the Center for High Angular Angular Resolution (CHARA) Array and the Fiber Linked Unit for Optical Recombination (FLUOR). For the analysis of these data, we developed a code for mapping sophisticated PHOENIX model atmospheres on to the surface of rotationally distorted stars described by a Roche-von Zeipel formalism. Given a set of input parameters for a star or binary pair, this code predicts the interferometric visibility, spectral energy distribution and high-resolution line spectrum expected for the system. For the gravity-darkened Vega, our model provides a very good match to the K-band interferometric data, a good match to the spectral energy distribution – except below 160 nm – and a rather poor match to weak lines in the high dispersion spectrum where the model appears overly gravity darkened. In 2006, we used the CHARA Array and FLUOR to obtain high precision measurements of the massive, non-eclipsing, double-line spectroscopic binary Spica, a 4-day period system where both components are gravity darkened rapid rotators. These data supplement recent data obtained with the Sydney University Stellar Interferometer (SUSI). Our study follows the classic 1971 study by Herbison-Evans et al. who resolved Spica as a binary with the Narrabri Stellar Intensity Interferometer (NSII). We will report on our progress modeling the new interferometric and archival spectroscopic data, with the goal towards better constraining the apsidal constant. [ABSTRACT FROM PUBLISHER]

Context. Red giant branch (RGB) stars are very bright objects in galaxies and are often used as standard candles. Interferometry is the ideal tool to characterize the dynamics and morphology of their atmospheres. Aims. We aim at precisely characterising the surface dynamics of a sample of RGB stars. Methods. We obtained interferometric observations for three RGB stars with the MIRC instrument mounted at the CHARA interferometer. We looked for asymmetries on the stellar surfaces using limb-darkening models. Results. We measured the apparent diameters of HD197989 (ε Cyg) = 4.61 ± 0.02 mas, HD189276 (HR 7633) = 2.95 ± 0.01 mas, and HD161096 (β Oph) = 4.43 ± 0.01 mas. We detected departures from the centrosymmetric case for all three stars with the tendency of a greater effect for lower logg of the sample. We explored the causes of this signal and conclude that a possible explanation to the interferometric signal is the convection-related and/or the magnetic-related surface activity. However, it is necessary to monitor these stars with new observations, possibly coupled with spectroscopy, in order to firmly establish the cause. [ABSTRACT FROM AUTHOR]

We report new CHARA/Michigan InfraRed Combiner interferometric observations of the Cepheid archetype δ Cep, which aimed at detecting the newly discovered spectroscopic companion. We reached a maximum dynamic range ΔH = 6.4, 5.8 and 5.2 mag, respectively, within the relative distance to the Cepheid r < 25 mas, 25 < r < 50 mas and 50 < r < 100 mas. Our observations did not show strong evidence of a companion. We have a marginal detection at 3s with a flux ratio of 0.21 per cent, but nothing convincing as we found other possible probable locations. We ruled out the presence of companion with a spectral type earlier than F0V, A1V and B9V, respectively for the previously cited ranges r. From our estimated sensitivity limits and the Cepheid light curve, we derived lower limit magnitudes in the H band for this possible companion to be Hcomp > 9.15, 8.31 and 7.77 mag, respectively, for r < 25 mas, 25 < r < 50 mas and 50 < r < 100 mas. We also found that to be consistent with the predicted orbital period (Anderson et al.), the companion has to be located at a projected separation <24 mas with a spectral type later than an F0V star. [ABSTRACT FROM AUTHOR]

Context. Determining the effective temperature of Ap stars, including the roAp stellar pulsators, is a difficult task owing to their strong magnetic field and their related spotted surfaces. It is, however, an important step towards constraining models of their complex atmosphere and testing proposed pulsation excitation mechanisms. Aims. Using the unique angular resolution provided by long-baseline visible interferometry, we aim at deriving accurate angular diameters of a number of Ap targets, so as to determine their unbiased effective temperature (Teff) and their accurate position in the Hertzsprung-Russell diagram, to estimate their mass and age, and to test non-adiabatic pulsation models. Interferometric results on four Ap stars have been published in earlier works. Here we report the results on a fifth, significantly hotter star. Methods. We observed 78 Vir with the visible spectrograph VEGA installed at the combined focus of the CHARA long-baseline optical array. We derived the limb-darkened diameter of this Ap star from our interferometric measurements. Based on photometric and spectroscopic data available in the literature, we estimated the star's bolometric flux and used it, in combination with its parallax and angular diameter, to determine the star's luminosity and effective temperature. We then used the derived fundamental parameters to perform a non-adiabatic pulsation analysis. Results. We determined a limb-darkened angular diameter of 0.346 ± 0.006 mas and deduced a linear radius of R = 2.11 ± 0.04 R☉. Considering a bolometric flux of 2.73 ± 0.20 10-7 erg/cm2/s we obtained a luminosity of L/L☉ = 27 ± 2 and an effective temperature of Teff = 9100 ± 190 K. The non-adiabatic pulsation modeling allows us to predict that high overtone pulsations could be excited in 78 Vir at frequencies ranging from 1.2 to 1.9 mHz, provided that the magnetic field is capable of suppressing envelope convection in the polar regions. Conclusions. Visible long-baseline interferometry is a unique means of deriving accurate fundamental parameters of Ap stars. The Ap star 78 Vir is found to be a promising roAp-star candidate and one that would allow us to extend recent tests on the roAp stars' excitation mechanism towards the blue edge of the instability strip. Asteroseismic data of this star would, therefore, be of strong interest. [ABSTRACT FROM AUTHOR]

Context. Long-baseline interferometry is an important technique to spatially resolve binary or multiple systems in close orbits. By combining several telescopes together and spectrally dispersing the light, it is possible to detect faint components around bright stars in a few hours of observations. Aims. We provide a rigorous and detailed method to search for high-contrast companions around stars, determine the detection level, and estimate the dynamic range from interferometric observations. Methods. We developed the code CANDID (Companion Analysis and Non-Detection in Interferometric Data), a set of Python tools that allows us to search systematically for point-source, high-contrast companions and estimate the detection limit using all interferometric observables, i.e., the squared visibilities, closure phases and bispectrum amplitudes. The search procedure is made on a N × N grid of fit, whose minimum needed resolution is estimated a posteriori. It includes a tool to estimate the detection level of the companion in the number of sigmas. The code CANDID also incorporates a robust method to set a 3σ detection limit on the flux ratio, which is based on an analytical injection of a fake companion at each point in the grid. Our injection method also allows us to analytically remove a detected component to 1) search for a second companion; and 2) set an unbiased detection limit. Results.We used CANDID to search for the companions around the binary Cepheids V1334 Cyg, AX Cir, RT Aur, AWPer, SU Cas, and T Vul. First, we showed that our previous discoveries of the components orbiting V1334 Cyg and AX Cir were detected at >25σ and >13σ, respectively. The astrometric positions and flux ratios provided by CANDID for these two stars are in good agreement with our previously published values. The companion around AW Per is detected at more than 15σ with a flux ratio of f = 1:22 ± 0:30%, and it is located at ρ = 32:16 ± 0:29 mas and PA = 67:1 ± 0:3°. We made a possible detection of the companion orbiting RT Aur with f = 0:22 ± 0:11%, and at ρ = 2:10 ± 0:23 mas and PA = -136 ± 6°. It was detected at 3.8σ using the closure phases only, and so more observations are needed to confirm the dectection. No companions were detected around SU Cas and T Vul. We also set the detection limit for possible undetected companions around these stars. We found that there is no companion with a spectral type earlier than B7V, A5V, F0V, B9V, A0V, and B9V orbiting the Cepheids V1334 Cyg, AX Cir, RT Aur, AW Per, SU Cas, and T Vul, respectively. This work also demonstrates the capabilities of the MIRC and PIONIER instruments, which can reach a dynamic range of 1:200, depending on the angular distance of the companion and the (u, v) plane coverage. In the future, we plan to work on improving the sensitivity limits for realistic data through better handling of the correlations. [ABSTRACT FROM AUTHOR]

Context. Stellar rotation is a key in our understanding of both mass-loss and evolution of intermediate and massive stars. It can lead to anisotropic mass-loss in the form of radiative wind or an excretion disk. Aims. We wished to spatially resolve the photosphere and gaseous environment of 51 Oph, a peculiar star with a very high vsini of 267 km s-1 and an evolutionary status that remains unsettled. It has been classified by different authors as a Herbig, a β Pic, or a classical Be star. Methods. We used the VEGA visible beam combiner installed on the CHARA array that reaches a submilliarcsecond resolution. Observation were centered on the Hα emission line. Results. We derived, for the first time, the extension and flattening of 51 Oph photosphere. We found a major axis of θeq = 8.08 ± 0.70 R☉ and a minor axis of θpol = 5.66 ± 0.23 R☉. This high photosphere distortion shows that the star is rotating close to its critical velocity. Finally, using spectro-interferometric measurements in the Hα line, we constrained the circumstellar environment geometry and kinematics and showed that the emission is produced in a 5.2 ± 2 R* disk in Keplerian rotation. Conclusions. From the visible point of view, 51 Oph presents all the features of a classical Be star: near critical-rotation and double-peaked Hα line in emission produced in a gaseous disk in Keplerian rotation. However, this does not explain the presence of dust as seen in the mid-infrared and millimeter spectra, and the evolutionary status of 51 Oph remains unsettled. [ABSTRACT FROM AUTHOR]

Context. The method of distance determination of eclipsing binaries consists in combining the radii of both components determined from spectro-photometric observations with their respective angular diameters derived from the surface brightness-color relation (SBC). However, the largest limitation of the method comes from the uncertainty on the SBC relation: about 2% for late-type stars (or 0.04 magnitude) and more than 10% for early-type stars (or 0.2 mag). Aims. The aim of this work is to improve the SBC relation for early-type stars in the -1 ⩽ V - K ⩽ 0 color domain, using optical interferometry. Methods. Observations of eight B- and A-type stars were secured with the VEGA/CHARA instrument in the visible. The derived uniform disk angular diameters were converted into limb darkened angular diameters and included in a larger sample of 24 stars, already observed by interferometry, in order to derive a revised empirical relation for O, B, A spectral type stars with a V - K color index ranging from -1 to 0. We also took the opportunity to check the consistency of the SBC relation up to V - K ≃ 4 using 100 additional measurements. Results. We determined the uniform disk angular diameter for the eight following stars: γ Ori, ζ Per, 8 Cyg, ι Her, λ Aql, ζ Peg, γ Lyr, and δ Cyg with V - K color ranging from -0.70 to 0.02 and typical precision of about 1.5%. Using our total sample of 132 stars with V - K colors index ranging from about -1 to 4, we provide a revised SBC relation. For late-type stars (0 ⩽ V - K ⩽ 4), the results are consistent with previous studies. For early-type stars (-1 ⩽ V - K ⩽ 0), our new VEGA/CHARA measurements combined with a careful selection of the stars (rejecting stars with environment or stars with a strong variability), allows us to reach an unprecedented precision of about 0.16 magnitude or ≃7% in terms of angular diameter. Conclusions. We derive for the first time a SBC relation for stars between O9 and A3, which provides a new and reliable tool for the distance scale calibration. [ABSTRACT FROM AUTHOR]