The environment of the fast rotating star Achernar - High-resolution thermal infrared imaging with VISIR in BURST mode

Context: The geometry of the circumstellar envelopes (CSE) surrounding Be stars is still uncertain, although it is often assumed that they are formed by a disk around the stellar equator and a hot polar wind. Achernar (Alpha Eri) is the nearest Be star, and we recently detected a CSE using near-IR interferometry. Aims: Our initial goal was to constrain the geometry and flux contribution of the CSE of Achernar at distances of a few tens of AU from the star, in the thermal IR domain to complement our near-IR interferometric observations. Methods: We obtained diffraction-limited images of Achernar in the thermal infrared using VISIR at the VLT. In order to freeze the turbulence, we used the BURST mode of this instrument. Results: The images obtained in the PAH1 band show a point-like source located 0.280" north-west of Achernar (projected linear separation of 12.3 AU). Its emission is 1.8 % of the flux of Achernar in this band, but is not detected in the PAH2, SiC and NeII bands. We also provide new thermal IR photometry of Achernar in four bands. Conclusions: Being aligned with the expected azimuth of the equatorial plane of Achernar, the detected source could be a main sequence stellar companion. In this case, it apparent brightness would correspond to an A7V spectral type.Comment: 4 pages, 4 figure

The close-in companion of the fast rotating Be star Achernar

Accepted for publication as an A&A LetterContext: Be stars are massive dwarf or subgiant stars that present temporary emission lines in their spectrum, and particularly in the Halpha line. The mechanism triggering these Be episodes is currently unknown, but binarity could play an important role. Aims: Previous observations with the VLT/VISIR instrument (Kervella & Domiciano de Souza 2007) revealed a faint companion to Achernar, the brightest Be star in the sky. The present observations are intended to characterize the physical nature of this object. Methods: We obtained near-IR images and an H-band spectrum of Achernar B using the VLT/NACO adaptive optics systems. Results: Our images clearly show the displacement of Achernar B over a portion of its orbit around Achernar A. Although these data are insuficient to derive the orbital parameters, they indicate a period of the order of 15 yr. The projected angular separation of the two objects in December 2007 was smaller than 0.15 arcsec, or 6.7 AU at the distance of Achernar. Conclusions: From its flux distribution in the near- and thermal-IR, Achernar B is most likely an A1V-A3V star. Its orbital period appears similar to the observed pseudo-periodicity of the Be phenomenon of Achernar. This indicates that an interaction between A and B at periastron could be the trigger of the Be episodes

Hot Stars Mass-loss studied with Spectro-Polarimetric INterferometry (SPIN)

We present a prospective work undertaken on Spectro-Polarimetric INterferometry (SPIN). Our theoretical studies suggest that SPIN is a powerful tool for studying the mass loss from early type stars. Based on Monte Carlo simulations, we computed the expected SPIN signal for numerous hot star spectral types covering a broad range of geometries and optical depths. The SPIN technique is based on the detection and comparison of the fringe characteristics (complex visibility) between two perpendicular directions of polarization. In particular, we demonstrate that the SPIN technique is very sensitive to the beta parameter from the so-called 'beta velocity law' for optically thin winds. Moreover, the location where the bulk of polarization is generated can be defined accurately. The required sensitivity for studying main sequence OB star winds is still very demanding (inferior to 0.5%), but the signal expected from denser winds or extended atmospheres is well within the capabilities of existing interferometers. The visibility curves obtained in two perpendicular polarizations for LBVs or WR stars can differ by more than 15%, and their corresponding limb-darkened radii obtained by the fit of these curves by more than 35%. The signal expected from the extended circumstellar environment of Be stars and B[e] appears also to be easy to detect, relaxing the required instrumental accuracy to 1%. For these spectral types, the SPIN technique provide a good tool to extract the highly polarized and spatially confined envelope contribution from the bright star emission.Comment: 15 pages, 21 figures, accepted in A&

ACHERNAR CAN BE A DIFFERENTIAL ROTATOR

We take advantage of interferometric measurements of Achernar to inquire on its internal rotational law. The reinterpretation of interferome-tric data and the use of fundamental parameters corrected for gravitational darkening effects and models of 2D-models of internal stellar structures, lead us to the conclusion that the star could not be a rigid, near critical, rotator but a differential rotator with the core rotating $\sim3$ times faster than the surface

Diameter and photospheric structures of Canopus from AMBER/VLTI interferometry

International audience% Context {Direct measurements of fundamental parameters and photospheric structures of post-main-sequence intermediate-mass stars are required for a deeper understanding of their evolution. } % Aims {Based on near-IR long-baseline interferometry we aim to resolve the stellar surface of the F0 supergiant star Canopus, and to precisely measure its angular diameter and related physical parameters.} % Methods {We used the AMBER/VLTI instrument to record interferometric data on Canopus: visibilities and closure phases in the H and K bands with a spectral resolution of 35. The available baselines ($\simeq60-110$~m) and the high quality of the AMBER/VLTI observations allowed us to measure fringe visibilities as far as in the third visibility lobe.} % Results {We determined an angular diameter of \diameter=6.93\pm0.15~mas by adopting a linearly limb-darkened disk model. From this angular diameter and Hipparcos distance we derived a stellar radius R=71.4\pm4.0 R_{\sun}. Depending on bolometric fluxes existing in the literature, the measured \diameter provides two estimates of the effective temperature: $T_\mathrm{eff}= 7284\pm107$~K and $T_\mathrm{eff}= 7582\pm252$~K.} % Conclusions {In addition to providing the most precise angular diameter obtained to date, the AMBER interferometric data point towards additional photospheric structures on Canopus beyond the limb-darkened model alone. A promising explanation for such surface structures is the presence of convection cells. We checked such a hypothesis using first order star-cell models and concluded that the AMBER observations are compatible with the presence of surface convective structures. This direct detection of convective cells on Canopus from interferometry can provide strong constraints to radiation-hydrodynamics models of photospheres of F-type supergiant

The galactic unclassified B[e] star HD 50138. I. A possible new shell phase

The observed spectral variation of HD 50138 has led different authors to classify it in a very wide range of spectral types and luminosity classes (from B5 to A0 and III to Ia) and at different evolutionary stages as either HAeBe star or classical Be. Aims: Based on new high-resolution optical spectroscopic data from 1999 and 2007 associated to a photometric analysis, the aim of this work is to provide a deep spectroscopic description and a new set of parameters for this unclassified southern B[e] star and its interstellar extinction. Methods: From our high-resolution optical spectroscopic data separated by 8 years, we perform a detailed spectral description, presenting the variations seen and discussing their possible origin. We derive the interstellar extinction to HD 50138 by taking the influences of the circumstellar matter in the form of dust and an ionized disk into account. Based on photometric data from the literature and the new Hipparcos distance, we obtain a revised set of parameters for HD 50138. Results: Because of the spectral changes, we tentatively suggest that a new shell phase could have taken place prior to our observations in 2007. We find a color excess value of E(B-V) = 0.08 mag, and from the photometric analysis, we suggest that HD 50138 is a B6-7 III-V star. A discussion of the different evolutionary scenarios is also provided.Comment: Paper accepted for publication in A&A main journal (12 pages, 16 figures and a 3 pages-table). Language corrected versio

The environment of the fast rotating star Achernar - II. Thermal infrared interferometry with VLTI/MIDI

A&A Letter, in pressContext: As is the case of several other Be stars, Achernar is surrounded by an envelope, recently detected by near-IR interferometry. Aims: We search for the signature of circumstellar emission at distances of a few stellar radii from Achernar, in the thermal IR domain. Methods: We obtained interferometric observations on three VLTI baselines in the N band (8-13 mic), using the MIDI instrument. Results: From the measured visibilities, we derive the angular extension and flux contribution of the N band circumstellar emission in the polar direction of Achernar. The interferometrically resolved polar envelope contributes 13.4 +/- 2.5 % of the photospheric flux in the N band, with a full width at half maximum of 9.9 +/- 2.3 mas (~ 6 Rstar). This flux contribution is in good agreement with the photometric IR excess of 10-20% measured by fitting the spectral energy distribution. Due to our limited azimuth coverage, we can only establish an upper limit of 5-10% for the equatorial envelope. We compare the observed properties of the envelope with an existing model of this star computed with the SIMECA code. Conclusions: The observed extended emission in the thermal IR along the polar direction of Achernar is well reproduced by the existing SIMECA model. Already detected at 2.2mic, this polar envelope is most probably an observational signature of the fast wind ejected by the hot polar caps of the star

First astrophysical results from AMBER/VLTI

The AMBER instrument installed at the Very Large Telescope (VLT) combines three beams from as many telescopes to produce spectrally dispersed fringes from milli-arcsecond angular scale in the near infrared. Two years after installation, first scientific observations have been carried out during the Science Demonstration Time and the Guaranteed Time mostly on bright sources due to some VLTI limitations. In this paper, we review these first astrophysical results and we show which types of completely new information is brought by AMBER. The first astrophysical results have been mainly focusing on stellar wind structure, kinematics, and its interaction with dust usually concentrated in a disk. Because AMBER has dramatically increased the number of measures per baseline, this instrument brings strong constraints on morphology and models despite a relatively poor (u, v) coverage for each object.Comment: SPIE 6268 proceeding

Spectroscopic and interferometric approach for differential rotation in massive fast rotators

The coupling between the convective region in the envelope and rotation can produce a surface latitudinal differential rotation that may induce changes of the stellar geometry and on the spectral line profiles that it may be scrutinized spectroscopically and by interferometry.Facultad de Ciencias Astronómicas y Geofísica