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Aims. This paper reports on H-band interferometric observations of Betelgeusemade at the three-telescope interferometer IOTA. We image Betelgeuseand 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.

High-resolution $R\sim$50 000 long-slit spectroscopy of the inner knots of the highly symmetrical protostellar outflow HH 212 was obtained in the 1-0 S(1) line of H2at 2.12 μm with a spatial resolution of ~0$\farcs$45. At the resulting velocity resolution of ~6 km s-1, multiple slit oriented observations of the northern first knot NK1 clearly show double-peaked line profiles consistent with either a radiative bow shock or dual (forward and reverse) shocks. In contrast, the velocity distribution of the southern first knot SK1 remains single-peaked, suggesting a significantly lower jet velocity and possibly a different density variation in the jet pulses in the southern flow compared to the northern flow. Comparison with a semi-empirical analytical model of bow shock emission allows us to constrain parameters such as the bow inclination to the line of sight, the bow shock and jet velocities for each flow. Although a few features are not reproduced by this model, it confirms the presence of several dynamical and kinematical asymmetries between opposite sides of the HH 212 bipolar jet. The position-velocity diagrams of both knots exhibit complex dynamics that are broadly consistent with emission from a bow shock and/or jet shock, which does not exclude jet rotation, although a clear signature of jet rotation in HH 212 is missing. Alternative interpretations of the variation of radial velocity across these knots, such as a variation in the jet orientation, as well as for the velocity asymmetries between the flows, are also considered. The presence of a correlation between flow velocity and collimation in each flow is suggested.

Context. Betelgeuse is one the largest stars in the sky in terms of angular diameter. Structures on the stellar photosphere have been detected in the visible and near-infrared as well as a compact molecular environment called the MOLsphere. Mid-infrared observations have revealed the nature of some of the molecules in the MOLsphere, some being the precursor of dust.

The GIII red giant star ϵOph has been found to exhibit several modes of oscillation by the MOST mission. We interpret the observed frequencies of oscillation in terms of theoretical radial p-mode frequencies of stellar models. Evolutionary models of this star, in both shell H-burning and core He-burning phases of evolution, are constructed using as constraints a combination of measurements from classical ground-based observations (for luminosity, temperature, and chemical composition) and seismic observations from MOST. Radial frequencies of models in either evolutionary phase can reproduce the observed frequency spectrum of ϵOph almost equally well. The best-fit models indicate a mass in the range of 1.85 ±0.05 ${M_\odot}$with radius of 10.55 ±0.15${R_\odot}$. We also obtain an independent estimate of the radius of ϵOph with highly accurate interferometric observations in the infrared K'band, using the CHARA/FLUOR instrument. The measured limb-darkened disk angular diameter of ϵOph is 2.961 ±0.007 mas. Together with the Hipparcosparallax, this translates into a photospheric radius of R= 10.39 ±0.07 ${R_\odot}$. The radius obtained from the asteroseismic analysis matches the interferometric value quite closely even though the radius was not constrained during the modelling.

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.

Context.The study of the atmosphere of red supergiant stars in general and of Betelgeuse(αOrionis) in particular is of prime importance to understand dust formation and how mass is lost to the interstellar medium in evolved massive stars.

Context.The quest for hot dust in the central region of debris disks requires high resolution and high dynamic range imaging. Near-infrared interferometry is a powerful means to directly detect faint emission from hot grains.

We present the results of long-baseline interferometric observations of the classical Cepheids Polaris and δCep in the near infrared K'band (1.9-2.3 μm), using the FLUOR instrument of the CHARA Array. Following our previous detection of a circumstellar envelope (CSE) around $\ell$Car (Kervella et al. 2006), we report similar detections around Polaris and δCep. Owing to the large data set acquired on Polaris, in both the first and second lobes of visibility function, we have detected the presence of a circum-stellar envelope (CSE), located at $2.4\pm0.1$stellar radii, accounting for $1.5\pm0.4$% of the stellar flux in the Kband. A similar model is applied to the δCep data, which shows improved agreement compared to a model without CSE. Finally, we find that the bias in estimating the angular diameter of δCep in the framework of the Baade-Wesselink method (Mérand et al 2005b) is of the order of $1\%$or less in the Kband. A complete study of the influence of the CSE is proposed in this context, showing that at the optimum baseline for angular diameter variation detection, the bias is of the order of the formal precision in the determination of the δCep pulsation amplitude ($1.6\%$).

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.

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, δCep. 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).

Infrared interferometry of supergiant and Mira stars has recently been reinterpreted as revealing the presence of deep molecular layers. Empirical models for a photosphere surrounded by a simple molecular layer or envelope have led to a consistent interpretation of previously inconsistent data. The stellar photospheres are found to be smaller than previously understood, and the molecular layer is much higher and denser than predicted by hydrostatic equilibrium. However, the analysis was based on spatial observations with medium-band optical filters, which mixed the visibilities of different spatial structures. This paper reports spatial interferometry with narrow spectral bands, isolating near-continuum and strong molecular features, obtained for the supergiant μCep. The measurements confirm strong variation of apparent diameter across the K-band. A layer model shows that a stellar photosphere of angular diameter $14.11\pm0.60$mas is surrounded by a molecular layer of diameter $18.56\pm0.26$mas, with an optical thickness varying from nearly zero at 2.15 μm to >1 at 2.39 μm. Although μCep and αOri have a similar spectral type, interferometry shows that they differ in their radiative properties. Comparison with previous broad-band measurements shows the importance of narrow spectral bands. The molecular layer or envelope appears to be a common feature of cool supergiants.

We present near-IR interferometric measurements of the K1.5 giant Arcturus(αBootis), obtained at the IOTA interferometer with the FLUOR instrument, in four narrow filters with central wavelengths ranging from 2.03 μm to 2.39 μm. These observations were expected to allow us to quantify the wavelength dependence of the diameter of a typical K giant. They are compared to predictions from both plane-parallel and spherical model atmospheres. Unexpectedly, neither can explain the observed visibilities. We show that these data suggest the presence of a companion, in accordance with the Hipparcos data on this star, and discuss this solution with respect to Arcturus' single star status.

We have observed Mira stars with the FLUOR beamcombiner on the IOTA interferometer in narrow bands around $2.2\,\mu$m wavelength. We find systematically larger diameters in bands contaminated by water vapor and CO. The visibility measurements can be interpreted with a model comprising a photosphere surrounded by a thin spherical molecular layer. The high quality of the fits we obtain demonstrates that this simple model accounts for most of the star's spatial structure. For each star and each period we were able to derive the radius and temperature of the star and of the molecular layer as well as the optical depth of the layer in absorption and continuum bands. The typical radius of the molecular layer is $2.2\,R_\star$with a temperature ranging between 1500 and 2100 K. The photospheric temperatures we find are in agreement with spectral types of Mira stars. Our photospheric diameters are found smaller than in previous studies by several tens of percent. We believe previous diameters were biased by the use of unsuited geometrical models to explain visibilities. The conclusions of this work are various. First, we offer a consistent view of Mira stars over a wide range of wavelengths. Second, the parameters of the molecular layer we find are consistent with spectroscopic studies. Third, from our diameter measurements we deduce that all Mira stars are fundamental mode pulsators and that previous studies leading to the conclusion of the first-overtone mode were biased by too large diameter estimates.

We report the observations in the Kband of the red supergiant star αOrionis and of the bright giant star αHerculis with the FLUOR beamcombiner at the IOTA interferometer. The high quality of the data allows us to estimate limb-darkening and derive precise diameters in the Kband which combined with bolometric fluxes yield effective temperatures. In the case of Betelgeuse, data collected at high spatial frequency although sparse are compatible with circular symmetry and there is no clear evidence for departure from circular symmetry. We have combined the Kband data with interferometric measurements in the Lband and at 11.15 μm. The full set of data can be explained if a 2055 K layer with optical depths $\tau_{K}=0.060\pm0.003$, $\tau_{L}=0.026\pm0.002$and $\tau_{11.15~\rm \mu m}=2.33\pm0.23$is added 0.33 $R_{\star}$above the photosphere providing a first consistent view of the star in this range of wavelengths. This layer provides a consistent explanation for at least three otherwise puzzling observations: the wavelength variation of apparent diameter, the dramatic difference in limb darkening between the two supergiant stars, and the previously noted reduced effective temperature of supergiants with respect to giants of the same spectral type. Each of these may be simply understood as an artifact due to not accounting for the presence of the upper layer in the data analysis. This consistent picture can be considered strong support for the presence of a sphere of warm water vapor, proposed by [CITE] when interpreting the spectra of strong molecular lines.

Recently available near-infrared interferometric data on late-type stars show a strong increase of diameter for asymptotic giant branch (AGB) stars between the K($2.0{-}2.4~{\rm \mu m}$) and L($3.4{-}4.1~{\rm \mu m}$) bands. Aiming at an explanation of these findings, we chose the objects αOrionis(Betelgeuse), SW Virginis, and R Leonis, which are of different spectral types and stages of evolution, and which are surrounded by circumstellar envelopes with different optical thicknesses. For these stars, we compared observations with spherically symmetric dust shell models. Photometric and $11~{\rm \mu m}$interferometric data were also taken into account to further constrain the models. – We find the following results. For all three AGB stars, the photosphere and dust shell model is consistent with the multi-wavelength photometric data. For αOrionisthe model dust shell has a very small optical depth (0.0065 at $11~{\rm \mu m}$); the visibility data and model in Kand Lare essentially entirely photospheric with no significant contribution from the dust, and the visibility data at $11~{\rm \mu m}$show a strong dust signature which agrees with the model. For SW Virginisthe model dust shell has a small optical depth (0.045 at $11~{\rm \mu m}$); in Kthe visibility data and model are essentially purely photospheric, in Lthe visibility data demand a larger object than the photosphere plus dust model allows, and at $11~{\rm \mu m}$there was no data available. For R Leonisthe model dust shell has a moderate optical depth (0.1 at $11~{\rm \mu m}$); in Kand Lthe visibility data and model situation is similar to that of SW Vir, and at $11~{\rm \mu m}$the visibility data and model are in agreement. – We conclude that AGB models comprising a photosphere and dust shell, although consistent with SED data and also interferometric data in Kand at $11~{\rm \mu m}$, cannot explain the visibility data in L; an additional source of model opacity, possibly related to a gas component, is needed in Lto be consistent with the visibility data.

Context.The interpretation of stellar pulsations in terms of internal structure depends on the knowledge of the fundamental stellar parameters. Long-base interferometers permit us to determine very accurate stellar radii, which are independent constraints for stellar models that help us to locate the star in the HR diagram.

Aims.T Pyxis the first recurrent nova ever historically studied. It was seen in outburst six times between 1890 and 1966 and then not for 45 years. We report on near-IR interferometric observations of the recent outburst of 2011.

Context.Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed IR-excess and emission lines. The influence of binarity on these phenomena remains controversial.

Context.Mass-loss occurring in red supergiants (RSGs) is a major contributor to the enrichment of the interstellar medium in dust and molecules. The physical mechanism of this mass loss is however relatively poorly known. Betelgeuse is the nearest RSG, and as such a prime object for high angular resolution observations of its surface (by interferometry) and close circumstellar environment.

Context. The radius of a star is a very important constraint to evolutionary models, particularly when combined with asteroseismology. Diameters can now be measured interferometrically with great precision (better than 1%), but the center-to-limb darkening (CLD) remains a potential source of bias. Measuring this bias is possible by completely resolving the star using long-baseline interferometry, and has only been achieved for a handful of stars.Aims. The red giant ηSer (K0III-IV) is a particularly interesting target, as asteroseismic oscillations have recently been detected in this star by spectroscopy. We aim to measure its radius with high accuracy, debiased from limb darkening, in order to bring new constraints to its models.Methods. We obtained interferometric observations of ηSer in the near-infrared using the CHARA/FLUOR instrument, in particular in the so-called second lobe of visibility in order to constrain the CLD and debias our diameter estimation.Results. The limb darkened angular diameter of ηSer is 2.944 ±0.010 mas (using spherical photosphere models PHOENIX and MARCS for the limb darkening), that converts into a radius of 5.897 ±0.028 $R_\odot$with the Hipparcosparallax. Thanks to a precise visibility measurement in the second lobe of the visibility function of ηSer and a one-parameter limb-darkened visibility profile, we were able to show that the photosphere models have the best agreement possible. Conclusions. Our limb darkening measurement of ηSer is in agreement with existing atmosphere models of this star, with a slightly better agreement for models using spherical geometry. This is a strong indication that interferometric angular diameter measurements for red giants, corrected for the CLD using models, are unbiased at a very small level (a fraction of 1%). In particular, this strengthens our confidence in the existing catalogues of calibrator stars for interferometry that are based on giant stars similar to ηSer. The high accuracy of our measurement brings a new and strong constraint for the asteroseismic modeling of this star.

Context. The prospects for using the asteroseismology of rapidly oscillating Ap (roAp) stars are hampered by the large uncertainty in fundamental stellar parameters. Results in the literature for the effective temperature (Teff) often span a range of 1000 K.Aims. Our goal is to reduce systematic errors and improve the Teffcalibration of Ap stars based on new interferometric measurements.Methods. We obtained long-baseline interferometric observations of βCrB using the CHARA/FLUOR instrument. To disentangle the flux contributions of the two components of this binary star, we obtained VLT/NACO adaptive optics images.Results. We determined limb-darkened angular diameters of 0.699±0.017 mas for βCrB A (from interferometry) and 0.415±0.017 mas for βCrB B (from surface brightness-colour relations), corresponding to radii of 2.63±0.09 ${R}_\odot$(3.4% uncertainty) and 1.56±0.07 ${R}_\odot$(4.5%). The combined bolometric flux of the A+B components was determined from satellite UV data, spectrophotometry in the visible, and broadband data in the infrared. The flux from the B component constitutes $16\pm4$% of the total flux and was determined by fitting an ATLAS9 model atmosphere to the broad-band NACO Jand Kmagnitudes. By combining the flux of the A component with its measured angular diameter, we determined the effective temperature Teff(A) = 7980±180 K ($2.3{\%}$).Conclusions. Our new interferometric and imaging data enable nearly model-independent determination of the effective temperature of βCrB A. Including our recent study of αCir, we now have direct Teffmeasurements of two of the brightest roAp stars, providing a strong benchmark for improved calibration of the Teffscale for Ap stars. This will support the use of potentially strong constraints imposed by asteroseismic studies of roAp stars.