5 resultados para VLTI


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Mira and R Coronae Borealis (R CrB) variable stars are evolved objects surrounded by circumstellar envelopes (CSE) composed of the ejected stellar material. We present a detailed high-spatial resolution morfological study of the CSE of three stars: IRC+10216, the closest and more studied Carbon-Rich Mira; o Ceti, the prototype of the Mira class; and RY Sagitarii (RY Sgr), the brightest R CrB variable of the south hemisphere. JHKL near-infrared adaptive optics images of IRC+10216 with high dynamic range and Vband images with high angular resolution and high depth, collected with the VLT/NACO and VLT/FORS1 instruments, were analyzed. NACO images of o Ceti were also analyzed. Interferometric observations of RY Sgr collected with the VLTI/MIDI instrument allowed us to explore its CSE innermost regions (»20 40 mas). The CSE of IRC+10216 exhibit, in near-infrared, clumps with more complex relative displacements than proposed in previous studies. In V-band, the majority of the non-concentric shells, located in the outer CSE layers, seem to be composed of thinner elongated shells. In a global view, the morphological connection between the shells and the bipolar core of the nebulae, located in the outer layers, together with the clumps, located in the innermost regions, has a difficult interpretation. In the CSE of o Ceti, preliminar results would be indicating the presence of possible clumps. In the innermost regions (.110 UA) of the CSE of RY Sgr, two clouds were detected in different epochs, embedded in a variable gaussian envelope. Based on a rigorous verification, the first cloud was located at »100 R¤ (or »30 AU) from the centre, toward the east-north-east direction (modulo 180o) and the second one was almost at a perpendicular direction, having aproximately 2£ the distance of the first cloud. This study introduces new constraints to the mass-loss history of these kind of variables and to the morphology of their innermost CSE regions

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While the presence of discs around classical Be stars is well established, their origin is still uncertain. To understand what processes result in the creation of these discs and how angular momentum is transported within them, their physical properties must be constrained. This requires comparing high spatial and spectral resolution data with detailed radiative transfer modelling. We present a high spectral resolution, R similar to 80 000, sub-milliarcsecond precision, spectroastrometric study of the circumstellar disc around the Be star beta CMi. The data are confronted with 3D, non-local thermodynamic equilibrium radiative transfer calculations to directly constrain the properties of the disc. Furthermore, we compare the data to disc models featuring two velocity laws: Keplerian, the prediction of the viscous disc model, and angular momentum conserving rotation. It is shown that the observations of beta CMi can only be reproduced using Keplerian rotation. The agreement between the model and the observed spectral energy distribution, polarization and spectroastrometric signature of beta CMi confirms that the discs around Be stars are well modelled as viscous decretion discs.

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Context. Spectrally resolved long-baseline optical/IR interferometry of rotating stars opens perspectives to investigate their fundamental parameters and the physical mechanisms that govern their interior, photosphere, and circumstellar envelope structures. Aims. Based on the signatures of stellar rotation on observed interferometric wavelength-differential phases, we aim to measure angular diameters, rotation velocities, and orientation of stellar rotation axes. Methods. We used the AMBER focal instrument at ESO-VLTI in its high-spectral resolution mode to record interferometric data on the fast rotator Achernar. Differential phases centered on the hydrogen Br gamma line (K band) were obtained during four almost consecutive nights with a continuous Earth-rotation synthesis during similar to 5h/night, corresponding to similar to 60 degrees position angle coverage per baseline. These observations were interpreted with our numerical code dedicated to long-baseline interferometry of rotating stars. Results. By fitting our model to Achernar's differential phases from AMBER, we could measure its equatorial radius R-eq = 11.6 +/- 0.3 R-circle dot, equatorial rotation velocity V-eq = 298 +/- 9 km s(-1), rotation axis inclination angle i = 101.5 +/- 5.2 degrees, and rotation axis position angle (from North to East) PA(rot) = 34.9 +/- 1.6 degrees. From these parameters and the stellar distance, the equatorial angular diameter circle divide(eq) of Achernar is found to be 2.45 +/- 0.09 mas, which is compatible with previous values derived from the commonly used visibility amplitude. In particular, circle divide(eq) and PA(rot) measured in this work with VLTI/AMBER are compatible with the values previously obtained with VLTI/VINCI. Conclusions. The present paper, based on real data, demonstrates the super-resolution potential of differential interferometry for measuring sizes, rotation velocities, and orientation of rotating stars in cases where visibility amplitudes are unavailable and/or when the star is partially or poorly resolved. In particular, we showed that differential phases allow the measurement of sizes up to similar to 4 times smaller than the diffraction-limited angular resolution of the interferometer.

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Aims. Spectroscopic, polarimetric, and high spectral resolution interferometric data covering the period 1995-2011 are analyzed to document the transition into a new phase of circumstellar disk activity in the classical Be-shell star 48 Lib. The objective is to use this broad data set to additionally test disk oscillations as the basic underlying dynamical process. Methods. The long-term disk evolution is described using the V/R ratio of the violet and red emission components of H alpha and Br gamma, radial velocities and profiles of He I and optical metal shell lines, as well as multi-band BVRI polarimetry. Single-epoch broad-band and high-resolution interferometric visibilities and phases are discussed with respect to a classical disk model and the given baseline orientations. Results. Spectroscopic signatures of disk asymmetries in 48 Lib vanished in the late nineties but recovered some time between 2004 and 2007, as shown by a new large-amplitude and long-duration V/R cycle. Variations in the radial velocity and line profile of conventional shell lines correlate with the V/R behavior. They are shared by narrow absorption cores superimposed on otherwise seemingly photospheric He I lines, which may form in high-density gas at the inner disk close to the photosphere. Large radial velocity variations continued also during the V/R-quiet years, suggesting that V/R may not always be a good indicator of global density waves in the disk. The comparison of the polarization after the recovery of the V/R activity shows a slight increase, while the polarization angle has been constant for more than 20 years, placing tight limits on any 3-D precession or warping of the disk. The broad H-band interferometry gives a disk diameter of (1.72 +/- 0.2) mas (equivalent to 15 stellar radii), position angle of the disk (50 +/- 9)degrees and a relatively low disk flattening of 1.66 +/- 0.3. Within the errors the same disk position angle is derived from polarimetric observations and from photocenter shifts across Br gamma. The high-resolution interferometric visibility and phase profiles show a double or even multiple-component structure. A preliminary estimate based on the size of the Br gamma emitting region indicates a large diameter for the disk (tens of stellar radii). Overall, no serious contradiction between the observations and the disk-oscillation model could be construed.

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