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Optical interferometry is entering a new age with several ground- based long-baseline observatories now making observations of unprecedented spatial resolution. Based on a great leap forward in the quality and quantity of interferometric data, the astrophysical applications are not limited anymore to classical subjects, such as determination of fundamental properties of stars; namely, their effective temperatures, radii, luminosities and masses, but the present rapid development in this field allowed to move to a situation where optical interferometry is a general tool in studies of many astrophysical phenomena. Particularly, the advent of long-baseline interferometers making use of very large pupils has opened the way to faint objects science and first results on extragalactic objects have made it a reality. The first decade of XXI century is also remarkable for aperture synthesis in the visual and near-infrared wavelength regimes, which provided image reconstructions from stellar surfaces to Active Galactic Nuclei. Here I review the numerous astrophysical results obtained up to date, except for binary and multiple stars milliarcsecond astrometry, which should be a subject of an independent detailed review, taking into account its importance and expected results at microarcsecond precision level. To the results obtained with currently available interferometers, I associate the adopted instrumental settings in order to provide a guide for potential users concerning the appropriate instruments which can be used to obtain the desired astrophysical information.

Aims. We study the geometry and kinematics of the circumstellar environment of the Be star Kappa CMa in the Br gamma emission line and its nearby continuum. Methods. We use the VLTI/AMBER instrument operating in the K band which provides a spatial resolution of about 6 mas with a spectral resolution of 1500 to study the kinematics within the disk and to infer its rotation law. In order to obtain more kinematical constraints we also use an high spectral resolution Pa beta line profile obtain in December 2005 at the Observatorio do Pico do Dios, Brazil and we compile V/R line profile variations and spectral energy distribution data points from the literature. Results. Using differential visibilities and differential phases across the Br gamma line we detect an asymmetry in the disk. Moreover, we found that kappa CMa seems difficult to fit within the classical scenario for Be stars, illustrated recently by alpha Arae observations, i.e. a fast rotating B star close to its breakup velocity surrounded by a Keplerian circumstellar disk with an enhanced polar wind. Finally we discuss the possibility for kappa CMa to be a critical rotator with a Keplerian rotating disk and try to see if the detected asymmetry can be interpreted within the "one-armed" viscous disk framework.

By means of numerical simulations, we investigate the ability of optical interferometry, via the fringe phase observ- able, to address stellar activity and magnetism. To derive abundance maps and stellar rotation axes, we use color differential interferometry which couples high angular resolution to high spectral resolution. To constrain magnetic field topologies, we add to this spectro-interferometer a polarimetric mode. Two cases of well-known Chemically Peculiar (CP) stars (βCrB and α 2 CVn) are simulated to derive instrumental requirements to obtain 2D-maps of abundance inhomogeneities and magnetic fields. We conclude that the near-infrared instrument AMBER of the VLTI will allow us to locate abundance inhomogeneities of CP stars larger than a fraction of milliarcsecond whereas the polarimetric mode of the French GI2T/REGAIN interferometer would permit one to disentangle various magnetic field topologies on CP stars. We emphasize the crucial need for developing and validating inversion algorithms so that future instruments on optical aperture synthesis arrays can be optimally used.

Ori is a B2IIIe star for which rotational modulation and non-radial pulsations (NRP) have been recently investi- gated from two independent observational campaigns in 1998 and 1999. Putting the data of these 2 campaigns together, and adding data obtained in 2001, we search for multiperiodicity in the line profile variations and evidence for outbursts. From new spectropolarimetric data obtained at the Telescope Bernard Lyot (TBL, Pic du Midi, France) in 2001 we also measure the Stokes V parameter in the polarised light. We find evidence for the presence of a weak magnetic field in ! Ori sinusoidally varying with a period of 1.29 d. The equivalent widths (EW) of the wind sensitive UV resonance lines also show a variation with the same period, which we identify as the rotational period of the star. We propose an oblique rotator model and derive Bpol = 530 230 G to explain the observations. Moreover, we carry out an abundance analysis and find the star to be N- enriched, a property which is shared with other magnetic stars. We propose! Ori as the first known classical Be star hosting a magnetic field.

Optical long baseline interferometry is a powerful tool to study detailed stellar structures. In particular, rapid rotation induces interferometric signatures requiring a detailed modelling for correctly interpreting high angular resolution data. In the present paper we study the effects of uniform stellar rotation on interferometric observables using a physically coherent model that includes gravity darkening and geometrical deformation, as well as a radiation transfer code. We investigate the use of multi-baseline and/or multi-channel observations, both in the continuum and spectral lines, in order to obtain the unique solutions for relevant model parameters. We show that this is possible and we provide a guide for observers in order to perform this task. However, this turns out to be quite demanding in terms of sensitivity requiring the use of new generation interferometers.

w Ori (HD 37490, HR 1934) is a Be star known to have presented variations. In order to investigate the nature and origin of its short-term and mid-term variability, a study is performed of several spectral lines (Ha, Hδ, HeI 4471, 4713, 4921, 5876, 6678, CII 4267, 6578, 6583, MgII 4481, SiIII 4553 and Sill 6347), based on 249 high signal-to-noise high-resolution spectra taken with 8 telescopes over 22 consecutive nights during the MuSiCoS (Multi Site Continuous Spectroscopy) campaign in November-December 1998. The stellar parameters are revisited and the projected rotational velocity (v sin i = 179 kms - 1 ) is redetermined using several methods. With the MuSiCoS 98 dataset, a time series analysis of line-profile variations (LPVs) is performed using the Restricted Local Cleanest (RLC) algorithm and a least squares method. The behaviour of the velocity of the centroid of the lines, the equivalent widths and the apparent vsini for several lines, as well as Violet and Red components of photospheric lines affected by emission (red He I lines, Sill 6347, CII 6578, 6583) are analyzed. The non-radial pulsation (NRP) model is examined using phase diagrams and the Fourier-Doppler Imaging (FDI) method. The LPVs are consistent with a NRP mode with l = 2 or 3, ‖m‖ = 2 with frequency 1.03 cd - 1 . It is shown that an emission line outburst occurred in the middle of the campaign. Two scenarios are proposed to explain the behaviour of a dense cloud, temporarily orbiting around the star with a frequency 0.46 cd - 1 , in relation to the outburst.

In this paper we discuss the combination of two basic approaches which should potentially generate images of spatially unresolved stars: differential interferometry and classical spectroscopy. Doppler Imaging provides indirect observational information on stellar surface structures by modeling the rotational modulation of the observed flux distribution across spectral lines. Similarly, differential interferometry makes it possible to measure the disturbances of photocentroid location of an unresolved star as a function of wavelength and to deduce the corresponding stellar map. We present the general formalism to reconstruct images from spectroscopy and differential interferometry data for sources with spatially unresolved structures, and we discuss how their combination improves the image reconstructions. This technique, that we call Interferometric-Doppler Imaging (IDI), leads to significant progress in solving some long-standing problems of Doppler Imaging, such as latitude smearing and bias as well as the non-uniqueness of the solution in the special case of an equator-on star. We treat explicitly the most delicate case of non-radial stellar pulsations, for which the cancellation of opposite sign temperature or velocity fields introduces additional difficulties. The performance of the method is demonstrated, using the indirect imaging code built on the basis of the developed approach to reconstruct an input image from a series of generated noisy spectra. The problem of image reconstruction from two-aperture interferometry data has been particularly addressed since it represents the case of most presently operating interferometers.

In addition to the common applications (temperature, magnetic field, abundance distributions) the Doppler Imaging technique can also be applied to non-radial stellar pulsations (NRP). Due to their rapid rotation, the Be stars are ideal candidates for Doppler Imaging. However, for the stars for which the periodic temporal line profile variability is produced by multiple mode NRP, the classical Doppler image in not an appropriate representation. When the corotating frequencies of existing pulsation modes are non-zero, the reference frame in which the resulting NRP is quasi-stationary cannot be defined. The technique can be applied “mode-by-mode”, which is illustrated in time-wavelength and Fourier frequency domain, using the time-resolved sequence of high resolution, high signal-to-noise ratio spectra of the Be star ζ Oph.

We present photometric data and 209 high- resolution, high signal-to-noise ratio optical spectra of the Be starCentauri obtained in 1993 and 1995, re- spectively. Time series analysis of these and other data, performed using the CLEAN, CLEANEST and Fourier Doppler Imaging techniques, show the presence of multi- periodic variations. They are interpreted in terms of low and high order non-radial pulsation modes. A strong fre- quency of 1.29 c/d is detected in line prole and photo- metric variations. It is attributed to a mode with l =2 . Other high amplitude signals present in spectroscopic data are 1.78 c/d, 3.82 c/d and 4.51 c/d. A frequency of 1.48 c/d appearing in spectroscopic data is compatible with the 1.56 c/d periodicity largely dominant in observa- tions taken prior to 1993. The corresponding \superperi- ods" for four of the frequencies are commensurate at the 8% level. The star showed period and non-radial pulsa- tion degree variations in timescales as short as 1.5 hour. The dominant mode apparently alternates between l =2 and l = 4 every other day during the eight day time span. This could be a modulation linked to the superperiod or to the stellar rotation. The main periodicities detected in our analysis are compatible with theoretical unstable g modes in SPB variables. Short time scale variability (1 3 hr), typical of high-order p modes is also systemat- ically present.

The RS CVn system HR1099 has been monitored over its orbital period to study spectroscopic variability in photospheric lines and in Ha. The observations were made at the CAT 1.4m ESO telescope with the Coude CES spectrometer and reticon, which permit a resolution of 105 and high S/N. The information provided by the variability of profiles, together with the limits of the Doppler imaging methods are also presented. Profiles are calculated with rotational broadening of immaculate and spotted stars. A line synthesis code including a model for the inhomogeneous temperature distribution of the primary star allowed us to calculate a series of spectra at different orbital phases. Different inversion algorithms were applied to test the image reconstruction of HR1099, showing the role of such constraints as chi-square adjustment of the spectra and maximum entropy of the input image.

The intrinsic polarisation parameters P, θ, Q and U of the star µ Cep, in the V spectral region during the period 1979–1989, are presented. The data were obtained from the Belgrade polarimetric observational program. The polarization percentage of the star exhibits change is of about 2.5% during this interval and the position angles change by about 90 degrees. These very large long-term changes of polarization parameters are discussed briefly.

We present results of a spectroscopic study of 37 southern (candidate) gamma Doradus stars based on echelle spectra. The observed spectra were cross-correlated with the standard template spectrum of an F0-type star for an easier detection of binary and intrinsic variations. We identified 15 objects as spectroscopic binaries, including 7 new ones, and another 3 objects are binary suspects. At least 12 objects show composite spectra. We could determine the orbital parameters for 9 binaries, of which 4 turn out to be ellipsoidal variables. For 6 binaries, we estimated the expected time-base of the orbital variations. Clear profile variations are observed for 17 objects, pointing towards stellar pulsation. For 8 of them, we have evidence that the main spectroscopic and photometric periods coincide. Our results, in combination with prior knowledge from the literature, lead to the classification of 10 objects as new bona-fide gamma Doradus stars, 1 object as new bona-fide delta Scuti star, and 8 objects as constant stars. Finally, we determined the projected rotational velocity with two independent methods. The resulting vsini values range from 3 to 135 km/s. For the bona-fide gamma Doradus stars, the majority has vsini below 60 km/s., 13 pages (+ 10 pages online material), 10 (+16) figures. Accepted for publication by A&A

We report here the first observations of a rapidly rotating Be star, $\alpha$ Eridani, using Earth-rotation synthesis on the Very Large Telescope (VLT) Interferometer. Our measures correspond to a $2a/2b = 1.56\pm0.05$ apparent oblate star, $2a$ and $2b$ being the equivalent uniform disc angular diameters in the equatorial and polar direction. Considering the presence of a circumstellar envelope (CSE) we argue that our measurement corresponds to a truly distorted star since $\alpha$ Eridani exhibited negligible H$\alpha$ emission during the interferometric observations. In this framework we conclude that the commonly adopted Roche approximation (uniform rotation and centrally condensed mass) should not apply to $\alpha$ Eridani. This result opens new perspectives to basic astrophysical problems, such as rotationally enhanced mass loss and internal angular momentum distribution. In addition to its intimate relation with magnetism and pulsation, rapid rotation thus provides a key to the Be phenomenon: one of the outstanding non-resolved problems in stellar physics., Comment: 5 pages, 3 figures, Accepted for Publication in A&A Letter

We describe results from the December 1989 multi-site continuous observing campaign (MUSICOS 89 with 17 telescopes around the globe) dedicated to the study of active structures and flares on the RS CVn system HR1099=V711 Tau. Two exceptional white-light flares were detected on 14 and 15 December with remarkable photometric and spectral signatures during rise and decay. Equivalent colours, temperature excesses, projected areas (0.55 and 0.89 solar disc areas), radiative and kinetic energy budgets were derived for the two white-light flares. At IAU Colloq. 153, we presented the relation between Doppler imaged spots, extended emission and these giant flares, and discussed them in the context of models of magnetic interconnection and filament energy storage between the stars.

v sin i was determined by applying the Fourier transform method to the line profiles of two classical Be Stars. A variation is observed in the apparent v sin i which corresponds to the main frequencies associated to nrp modes. Rotational modulation is observed in wind sensitive UV lines of the Be star ω Ori and is associated with an oblique magnetic dipole which is discovered for the first time in a classical Be star.

The long-term changes of the intrinsic polarization percentage are evident in the stars o And, γ Cas, 88 Her (during the period 1974–1992), к Dra (1979–1992) and BU Tau (1986–1992). The amplitudes of the polarization percentage variations are not more than a half of the percent. The changes of the position angles are within an interval of about 30 degrees.