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Context. Carbon is one of the most abundant components in the Universe. While silicates have been the main focus of solid phase studies in protoplanetary discs (PPDs), little is known about the solid carbon content especially in the planet-forming regions (~0.1–10 au). Fortunately, several refractory carbonaceous species present C-H bonds (such as hydrogenated nano-diamond and amorphous carbon as well as polycyclic aromatic hydrocarbons), which generate infrared (IR) features that can be used to trace the solid carbon reservoirs. The new mid-IR instrument MATISSE, installed at the Very Large Telescope Interferometer (VLTI), can spatially resolve the inner regions (~1–10 au) of PPDs and locate, down to the au-scale, the emission coming from carbon grains. Aims. Our aim is to provide a consistent view on the radial structure, down to the au-scale, as well as basic physical properties and the nature of the material responsible for the IR continuum emission in the inner disk region around HD 179218. Methods. We implemented a temperature-gradient model to interpret the disk IR continuum emission, based on a multiwavelength dataset comprising a broadband spectral energy distribution and VLTI H-, L-, and N-bands interferometric data obtained in low spectral resolution. Then, we added a ring-like component, representing the carbonaceous L-band features-emitting region, to assess its detectability in future higher spectral resolution observations employing mid-IR interferometry. Results. Our temperature-gradient model can consistently reproduce our dataset. We confirmed a spatially extended inner 10 au emission in H- and L-bands, with a homogeneously high temperature (~1700 K), which we associate with the presence of stochastically heated nano-grains. On the other hand, the N-band emitting region presents a ring-like geometry that starts at about 10 au with a temperature of 400 K. Moreover, the existing low resolution MATISSE data exclude the presence of aromatic carbon grains (i.e., producing the 3.3 μm feature) in close proximity tothe star (≲1 au). Future medium spectral resolution MATISSE data will confirm their presence at larger distances. Conclusions. Our best-fit model demonstrates the presence of two separated dust populations: nano-grains that dominate the near- to mid-IR emission in the inner 10 au region and larger grains that dominate the emission outward. The presence of such nano-grains in the highly irradiated inner 10 au region of HD 179218 requires a replenishment process. Considering the expected lifetime of carbon nano-grains from The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS model), the estimated disk accretion inflow of HD 179218 could significantly contribute to feed the inner 10 au region in nano-grains.Moreover, we also expect a local regeneration of those nano-grains by the photo-fragmentation of larger aggregates.

In this paper, we summarize the basic properties of radiative stellar winds from the theoretical and observational point of views. We illustrate two examples of a radiative code applied to stellar physics: the SIMECA code successfully used to constrain the physics of the circumstellar environment of the Be star α Arae constrained by VLTI-AMBER spectrally resolved measurements and the CMFGEN code applied to the BA supergiants Deneb and Rigel constrained by CHARA-VEGA measurements.

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. We used the VEGA visible beam combiner installed on the CHARA array that reaches a sub milliarcsecond resolution. We derived, for the first time, the extension and flattening of 51 Oph photosphere. We found an elongated ratio of 1.45 ± 0.12.

Aims. We use spectral energy distributions (SEDs), Hα line profiles, and visibilities available in the literature to study Achernar's envelope geometry and to propose a possible scenario for its circumstellar disk formation and dissipation.Methods. We use the SIMECA code to investigate possible geometries of the circumstellar environment by comparing our synthetic results with spectroscopic and high angular resolution data from the VLTI/VINCI instrument. We compute three different kinds of models: an equatorial disk, a polar wind, and a disk+wind model.Results. We develop a 2D axial symmetric kinematic model to study the variation of the observed Hα line profiles, which provides clear evidence of Achernar's equatorial disk formation and dissipation between 1991 and 2002. Our model can reproduce the polar-wind extension greater than 10 and a possible equatorial disk (≤ 5 ) but we were unable to estimate the wind opening angle. We reproduce the Hα line-profile variations using an outburst scenario, but the disk final contraction requieres an additional physical effect to be taken into account. The polar stellar wind does not appear to be linked to the presence of a disk or a ring around the star. We test the possibility of a binary companion to Achernar, as found by Kervella & Domiciano de Souza (2007, A&A, 474, L49), but conclude that the VLTI/VINCI visibilities cannot be explained by a rotationally-distorted Be star and a companion alone. The presence of a polar jet provides an important component to reproduce the observations even if it is not excluded that the companion could partially influence the observations. New interferometric observations at short baselines (5 ≤ B ≤ 20 m) are mandatory to constrain Achernar's circumstellar envelope, as well as spectroscopic long-term follow-up observational programs to link Achernar's mass-loss episodes with its circumstellar disk formation.

We present recent results on active hot stars using the first VLTI/MIDI and VLTI/AMBER observations, and the SIMECA code developed by Stee (1994). � Arae was the first classical Be star ob- served with MIDI (June 2003) and AMBER (February 2005). The size of its circumstellar envelope was measured and thanks to the spectrally- resolved interferometric AMBER observations, we were able, for the first time to evidence the Keplerian rotation of the circumstellar disc. MWC297 is one of the brightest Herbig Be star, and it was then ob- served during the first commissioning run of AMBER in May 2004. The data obtained were good enough to allow us to infer the accretion disc extension, as well as to put strong constraints on the stellar wind geometry and kinematics.

We report new observations of the peculiar star HD 62623 obtained with aperture masking interferometry performed on the Keck I telescope at λ = 1.24, 1.65, 2.26 and 3.08 $\rm \mu m$. The envelope around this star appears partially resolved in the near infrared except at 1.65 $\rm \mu m$. Radiative transfer modeling of the dust shell of this star has been performed in spherical geometry. This modeling has two goals: it provides a framework for understanding the partially resolved object shown by the visibility curves, and it allows investigation of the envelope of HD 62623 by discrimination between proposed models. We show that reasonable fits to photometric observations can be obtained with a simple spherically symmetric silicate dust shell. Nevertheless, our high angular resolution measurements bring some important constraints on the modeling of the circumstellar environment of HD 62623. More realistic model involving the presence of a non spherically distributed dust envelope including the gas component in the circumstellar environment is required.

We report theoretical HI visible and near-IR line profiles, i.e. Hα (6562 A), Hβ (4861 A) and Brγ (21 656 A), and intensity maps for a large set of parameters (density, temperature, envelope geometry, inclination angle), representative of early to late Be spectral types. We have computed the size of the emitting region in the Brγ line and its nearby continuum which both originate from a very extended region, i.e. at least 40 stellar radii which is twice the size of the Hα emitting region. We predict the relative fluxes from the central star, the envelope contribution in the given lines and in the continuum for a wide range of parameters characterizing the disk models. For a density ρ = 5 10−13 g cm−3 at the base of the stellar photosphere, we obtain the largest probability of HI IR lines in emission, which is a factor of 100 lower than typical values found for Be stars. We have also studied the effect of changing the spectral type on our results and we obtain a clear correlation between the luminosity in Hα and in the infrared. We found that for a density ρ = 5 10−12 g cm−3, the probability of detecting HI IR lines in emission must be stronger for late-B spectral type stars. If no IR lines are detected for late types, it may indicate that the density in the disc is very high (∼10−11 g cm−3). On the other hand, we found that around ρ = 5 10−13 g cm−3, it is possible to have a large envelope contribution in the Brγ line and a similar or even smaller emission in the Balmer lines. Even if Brγ is formed in an extended region, it is possible to obtain a FWHM and a V/R that agree well with observed profiles. Finally, it seems that the contribution in the Brγ line increases when the envelope becomes more and more “disk-like”, contrary to the Hα and Hβ lines.

We report on the results of an observing run of Cassiopeiae using the optical long baseline interfer- ometer Grand Interf erom etre a 2T elescopes (GI2T) in the southern France equipped with lm-sheet polarizers. Four interferometric baselines ranging from 20 to 40 m were obtained on this star during 1994 on September 5th and 6th, with Cephei as a calibrator. We aimed at detecting possible variations of the angular diameter of Cas as a function of polarization directions parallel and orthogonal to the north-south baseline of the GI2T. We do not detect any variation of the visibility as a function of Hour Angle on Cephei, neither on Cas. We nd an upper limit of 1.3 on the ratio of the angular diameters of the envelope of Cas seen through polarizers parallel and perpendicular to the North-South direction of the GI2T's baseline. We discuss how this limit could constrain density laws in the wind of this Be star. We clearly resolve Cas for increas- ing baselines in the red continuum whatever the polar- ization. Assuming the continuum flux originating from a photosphere surrounded by an elliptical gaussian envelope we estimate the extent of the latter as (2:9 0:5) mas. This diameter agrees well with the H emitting envelope as obtained by other interferometers.

Context. The eclipsing binary method for determining distance in the local group is based on the surface brightness-color relation (SBCR), and early-type stars are preferred targets because of their intrinsic brightness. However, this type of star exhibits wind, mass-loss, pulsation, and rotation, which may generate bias on the angular diameter determination. An accurate calibration of the SBCR relation thus requires careful analysis.Aims. In this paper we aim to quantify the impact of stellar rotation on the SBCR when the calibration of the relation is based on interferometric measurements of angular diameters.Methods. Six stars with V − K color indices ranging between –1 and 0.5 were modeled using the code for high angular resolution of rotating objects in nature (CHARRON) with various rotational velocities (0, 25, 50, 75, and 95% of the critical rotational velocity) and inclination (0, 25, 50, 75, and 90 degrees). All these models have their equatorial axis aligned in an east-west orientation in the sky. We then simulated interferometric observations of these theoretical stars using three representative sets of the CHARA baseline configurations. The simulated data were then interpreted as if the stars were non-rotating to determine an angular diameter and estimate the surface-brightness relation. The V − K color of the rotating star was calculated directly from the CHARRON code. This provides an estimate of the intrinsic dispersion of the SBCR relation when the rotation effects of flattening and gravity darkening are not considered in the analysis of interferometric data.Results. We find a clear relation between the rotational velocity and (1) the shift in zero point (Δa 0 ) of the SBCR (compared to the static relation) and (2) its dispersion (σ ). When considering stars rotating at less than 50% of their critical velocity, Δa 0 and σ have about 0.01 mag, while these quantities can reach 0.08 and 0.04 mag, respectively, when the rotation is larger than 75% of the critical velocity. Besides this, the inclination angle mostly has an impact on the V − K color: i > 50°) makes the star redder (resp. bluer). When considering the 150 models, Δa 0 and σ have 0.03 and 0.04 mag, respectively. These values are slightly but not significantly modified (about 0.03 and 0.01 mag in Δa 0 and σ , respectively) when considering different CHARA configurations. Interestingly, these 150 models, regardless of the interferometric configuration, are consistent with the empirical SBCR, which is within its dispersion of 0.16 mag. In addition, if one only considers projected rotational velocity V rot sini lower than 100 km s-1 , then Δa 0 and σ have 0.02 and 0.03 mag, respectively.Conclusions. To calibrate the SBCR interferometrically at the 0.02 mag precision (or lower), one should consider (1) a baseline configuration covering all directions of the (u , v ) plan; (2) a sample of stars with rotational velocity lower than 50% of their critical velocity or, alternatively, stars with V rot sini lower than 100 km s-1 ; (3) homogeneous visible and infrared photometry precisely at the 0.02 mag level or lower.

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. We wished to spatially resolve the photosphere and gaseous environment of 51 Oph, a peculiar star with a very high vsin(i) of 267km s$^{-1}$ and an evolutionary status that remains unsettled. It has been classified by different authors as a Herbig, a $\beta$ Pic, or a classical Be star. We used the VEGA visible beam combiner installed on the CHARA array that reaches a submilliarcsecond resolution. Observation were centered on the H$\alpha$ emission line. We derived, for the first time, the extension and flattening of 51 Oph photosphere. We found a major axis of $\theta_{{\mathrm{eq}}}$=8.08$\pm$0.70$R_\odot$ and a minor axis of $\theta_{{\mathrm{pol}}}$=5.66$\pm$0.23$R_\odot$ . This high photosphere distortion shows that the star is rotating close to its critical velocity. Finally, using spectro-interferometric measurements in the H$ \alpha$ line, we constrained the circumstellar environment geometry and kinematics and showed that the emission is produced in a 5.2$\pm$2R$_{*}$ disk in Keplerian rotation. From the visible point of view, 51 Oph presents all the features of a classical Be star: near critical-rotation and double-peaked H$\alpha $ 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., Comment: Published in A&A Letter. 5 pages, 3 figures, 3 tables

Aims. We study the inner sub-AU region of the circumstellar environment of the UX Ori-type star KK Oph with near-infrared VLTI/AMBER interferometry. We are particularly interested in the inclination of the star-disk system, and we use this information to test the current standard picture for UX Ori stars. Methods. We recorded spectrally dispersed (R 35) interferograms in the near-infraredH andK bands with the VLTI/AMBER instrument. The derived visibilities, closure phases, and the spectral energy distribution of KK Oph were compared with two-dimensional geometric and radiative transfer models (RADMC). Results. We obtained visibilities at four di erent position angles. Using two-dimensional geometric models, we derive an axis ratio 3:0 corresponding to an inclination of 70 . A fitted inclined ring model leads to a ring radius of 2:8 0:2 mas, corresponding to 0:44 0:03 AU at a distance of 160 pc, which is larger than the dust sublimation radius of 0:1 AU predicted for a dust sublimation temperature of 1500 K. Our derived two-dimensional RADMC model consists of a circumstellar disk with an inclination angle of 70 and an additional dust envelope. Conclusions. The finding of an 70 inclined disk around KK Oph is consistent with the prediction that UX Ori objects are seen under large inclination angles, and orbiting clouds in the line of sight cause the observed variability. Furthermore, our results suggest that the orbit of the companion KK Oph B and the disk plane are coplanar.

\gamma Cas is the prototypical classical Be star and is best known for its variable hard X-ray emission. To elucidate the reasons for this emission, we mounted a multiwavelength campaign in 2010 centered around 4 XMM observations. The observational techniques included long baseline optical interferometry (LBOI), monitoring by an Automated Photometric Telescope and Halpha observations. Because gamma Cas is also known to be in a binary, we measured Halpha radial velocities and redetermined its period as 203.55+/-0.2 days and an eccentricity near zero. The LBOI observations suggest that the star's decretion disk was axisymmetric in 2010, has an inclination angle near 45^o, and a larger radius than previously reported. The Be star began an "outburst" at the beginning of our campaign, made visible by a disk brightening and reddening during our campaign. Our analyses of the new high resolution spectra disclosed many attributes found from spectra obtained in 2001 (Chandra) and 2004 (XMM). As well as a dominant hot 14 keV thermal component, these familiar ones included: (i) a fluorescent feature of Fe K stronger than observed at previous times, (ii) strong lines of N VII and Ne XI lines indicative of overabundances, and (iii) a subsolar Fe abundance from K-shell lines but a solar abundance from L-shell ions. We also found that 2 absorption columns are required to fit the continuum. While the first one maintained its historical average of 1X10^21 cm^-2, the second was very large and doubled to 7.4X10^23 cm^-2 during our X-ray observations. Although we found no clear relation between this column density and orbital phase, it correlates well with the disk brightening and reddening both in the 2010 and earlier observations. Thus, the inference from this study is that much (perhaps all?) of the X-ray emission from this source originates behind matter ejected by gamma Cas into our line of sight., Comment: 17 pages, 9 figures, accepted by Astronomy & Astrophysics

The mass-loss mechanism in normal K--M giant stars with small variability amplitudes is not yet understood, although they are the majority among red giant stars. We present high-spatial and high-spectral resolution observations of the 2.3 micron CO lines in the M7 giant BK Vir with a spatial resolution of 9.8 mas and a spectral resolution of 12000, using AMBER at the Very Large Telescope Interferometer (VLTI). The angular diameters observed in the CO lines are 12--31% larger than those measured in the continuum. We also detected asymmetry in the CO line-forming region. The data taken 1.5 months apart show possible time variation on a spatial scale of 30 mas (corresponding to 3 x stellar diameter) at the CO band head. Comparison of the observed data with the MARCS photospheric model shows that whereas the observed CO line spectrum can be well reproduced by the model, the angular sizes observed in the CO lines are much larger than predicted by the model. Our model with two additional CO layers above the MARCS photosphere reproduces the observed spectrum and interferometric data in the CO lines simultaneously. This model suggests that the inner CO layer at ~1.2 stellar radii is very dense and warm with a CO column density of ~10^{22} cm^{-2} and temperatures of 1900--2100K, while the outer CO layer at 2.5--3.0 stellar radii is characterized by column densities of 10^{19}--10^{20} cm^{-2} and temperatures of 1500--2100K. Our AMBER observations of BK Vir have spatially resolved the extended molecular outer atmosphere of a normal M giant in the individual CO lines for the first time. The temperatures derived for the CO layers are higher than or equal to the uppermost layer of the MARCS photospheric model, implying the operation of some heating mechanism in the outer atmosphere., 10 pages, 9 figures, accepted for publication in Astronomy and Astrophysics

We aim to constrain the geometrical extent of the chromosphere of non-binary K giant stars and detect any spatial structures in the chromosphere. We performed observations with the CHARA interferometer and the VEGA beam combiner at optical wavelengths. We observed seven non-binary K giant stars. We measured the ratio of the radii of the photosphere to the chromosphere using the interferometric measurements in the Halpha and the Ca II infrared triplet line cores. For beta Ceti, spectro-interferometric observations are compared to an non-local thermal equilibrium (NLTE) semi-empirical model atmosphere including a chromosphere. The NLTE computations provide line intensities and contribution functions that indicate the relative locations where the line cores are formed and can constrain the size of the limb-darkened disk of the stars with chromospheres. We measured the angular diameter of seven K giant stars and deduced their fundamental parameters: effective temperatures, radii, luminosities, and masses. We determined the geometrical extent of the chromosphere for four giant stars. The chromosphere extents obtained range between 16% to 47% of the stellar radius. The NLTE computations confirm that the Ca II/849 nm line core is deeper in the chromosphere of ? Cet than either of the Ca II/854 nm and Ca II/866 nm line cores. We present a modified version of a semi-empirical model atmosphere derived by fitting the Ca II triplet line cores of this star. In four of our targets, we also detect the signature of a differential signal showing the presence of asymmetries in the chromospheres. Conclusions. It is the first time that geometrical extents and structure in the chromospheres of non-binary K giant stars are determined by interferometry. These observations provide strong constrains on stellar atmosphere models., 10 pages, 12 figures

Aims. This study aims at constraining the properties of two interacting binary systems by measuring their continuum-forming region in the visible and the forming regions of some emission lines, in particular Hα, using optical interferometry. Methods. We have obtained visible medium (R ∼ 1000) spectral resolution interferometric observations of β Lyr and of υ Sgr using the VEGA instrument of the CHARA array. For both systems, visible continuum (520/640 nm) visibilities were estimated and differential interferometry data were obtained in the Hα emission line at several epochs of their orbital period. For β Lyr, dispersed visibilities and phases were also obtained in the Hβ and the HeI 6678 A lines. Results. As expected, for baselines shorter than 60 m, the system of β Lyr is unresolved in the visible continuum, but the source associated with the Hα ,t he Hβ and the HeI 6678 A lines appears to be well resolved at any orbital phase. The differential visibilities through these lines are lower during eclipses, indicating that significant emission originates close to the stars. The Hα line forming region appears to be made up of a compact source located near the orbital plane (possibly linked with the “hot point”) and an extended source (i.e. ≥ 2m as, i.e. 125R� ) out of the orbital plane (possibly associated to the “jet-like feature”). The υ Sgr continuum visibilities are at a similar level for short (20–25 m) and long (90–110 m) baselines. This is interpreted as the presence of an extended structure surrounding a compact bright source. No binary signal was detected, excluding a flux ratio between the stellar components of the

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. delta Sco is a binary system whose primary suddently began to exhibit the Be phenomenon at the last periastron in 2000. We want to constrain the geometry and kinematics of its circumstellar environment. We observed the star between 2007 and 2010 using spectrally-resolved interferometry with the VLTI/AMBER and CHARA/VEGA instruments. We found orbital elements that are compatible with previous estimates. The next periastron should take place around July 5, 2011 (+- 4,days). We resolved the circumstellar disk in the HAlpha (FWHM = 4.8+-1.5mas), BrGamma (FWHM = 2.9 0.,mas), and the 2.06$ \mu$m HeI (FWHM = 2.4+-0.3mas) lines as well as in the K band continuum (FWHM ~2.4mas). The disk kinematics are dominated by the rotation, with a disk expansion velocity on the order of 0.2km/s. The rotation law within the disk is compatible with Keplerian rotation. As the star probably rotates at about 70% of its critical velocity the ejection of matter doesn't seems to be dominated by rotation. However, the disk geometry and kinematics are similar to that of the previously studied quasi-critically rotating Be stars, namely Alpha Ara, Psi Per and 48 Per., Comment: A\&A accepted paper

Context. One of the main challenges of optical stellar interferometers is to increase the number of telescopes in the recombining unit to provide a larger number of measurements and an improved imaging capability. At the same time there is a need to preserve the spectroscopic capabilities, which leads to complex recombining schemes that may inhibit development. Aims. We describe the possibilities of combining the spatial and spectral encoding of fringes for the design of more compact beam combiners and for minimizing the number of pixels that must be read. Methods. We establish the formalism of the spatio-spectral fringe encoding, discuss general applications, and describe an implementation in the 3T/4T observing mode of the VEGA (Visible spEctroGraph and polArimeter) instrument installed at the coherent focus of the CHARA Array located on Mt Wilson in California. We finally present the science cases made possible by this instrumental implementation in the case of VEGA/CHARA. Results. We demonstrate the interest in implementing an optimized spatio-spectral encoding of fringes in a multi-telescope beam combiner. On-sky results, obtained with the 3T mode of the VEGA combiner are presented. At visible wavelengths and with the hectometric baselines of CHARA, sub-mas stellar diameters could be determined with a precision of a few percent with a spectral resolution of 5000. Our first estimates of closure phase show that accuracies better than 1 degree can be achieved. Conclusions. The first on-sky results obtained with the 3T-4T VEGA instrument using spatio-spectral fringe encoding show the validity of using this principle in the design of future complex beam combiners.

Physical processes working in the stellar interiors as well as the evolution of stars depend on some fundamental stellar properties, such as mass, radius, luminosity, and chemical abundances. A classical way to test stellar interior models is to compare the predicted and observed location of a star on theoretical evolutionary tracks in a H-R diagram. This requires the best possible determinations of stellar mass, radius, luminosity and abundances. To derive its fundamental parameters, we observed the well-known rapidly oscillating Ap star, $\gamma$ Equ, using the visible spectro-interferometer VEGA installed on the optical CHARA array. We computed the calibrated squared visibility and derived the limb-darkened diameter. We used the whole energy flux distribution, the parallax and this angular diameter to determine the luminosity and the effective temperature of the star. We obtained a limb-darkened angular diameter of 0.564~$\pm$~0.017~mas and deduced a radius of $R$~=~2.20~$\pm$~0.12~${\rm R_{\odot}}$. Without considering the multiple nature of the system, we derived a bolometric flux of $(3.12\pm 0.21)\times 10^{-7}$ erg~cm$^{-2}$~s$^{-1}$ and an effective temperature of 7364~$\pm$~235~K, which is below the effective temperature that has been previously determined. Under the same conditions we found a luminosity of $L$~=~12.8~$\pm$~1.4~${\rm L_{\odot}}$. When the contribution of the closest companion to the bolometric flux is considered, we found that the effective temperature and luminosity of the primary star can be, respectively, up to $\sim$~100~K and up to $\sim$~0.8~L$_\odot$ smaller than the values mentioned above.These new values of the radius and effective temperature should bring further constraints on the asteroseismic modelling of the star.

Interferometry in the visible now provides milliarcsecond spatial resolution and thus has been intensely used for studying the circumstellar environment of evolved and pre-sequence stars. Up to now, most of the scientific results were obtained using only the modulus of the visibility as a function of baseline, time or projected baseline (the so-called "super-synthesis" effect). In this paper we will try to show that even without images, the phase of the visibility is able to put very strong constraints on stellar modeling since it is possible to achieve μarsecond measurements using this technique. We will illustrate through two examples from the GI2T interferometer how the modulus and the phase of the visibility were used to understand the physics of Be disks. We will stress on the importance and the potential of coupling high angular resolution with high spectral resolution for the study of stellar physics. Finally, We will present a possible study on the phase signatures due to stellar rapid rotation that can be done using the VLTI interferometer.

Using the SIMECA code developped by Stee & Araujo (1994), we report theoretical HI visible and near-IR line profiles, i.e. Hα (6562 A), Hβ (4861 A) and Brγ (21 656 A), and intensity maps for a large set of parameters representative of early to late Be spectral types. We have computed the size of the emitting region in the Br γ line and its nearby continuum which both originate from a very extended region, i.e . at least 40 stellar radii which is twice the size of the Hα emitting region. We predict the relative fluxes from the central star, the envelope contribution in the given lines and in the continuum for a wide range of parameters characterizing the disk models. Finally, we have also studied the effect of changing the spectral type on our results and we obtain a clear correlation between the luminosity in Hα and in the infrared.