A. Meilland

Publications (38)9.17 Total impact

• Article: Evidence of an asymmetrical Keplerian disk in the Br{\gamma} and He I emission lines around the Be star HD 110432

  Philippe Stee, A. Meilland, Ph. Bendjoya, F. Millour, M. Smith, A. Spang, G. Duvert, K. -H. Hofmann, F. Massi
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  ABSTRACT: Context. HD 110432 was classified as a "\gamma Cas X-ray analog" since it has similar peculiar X-ray and optical characteristics, i.e. a hard-thermal X-ray variable emission and an optical spectrum affected by an extensive disk. Lopes de Oliveira et al. (2007) suggest that it might be a Be star harboring an accreting white dwarf or that the X-rays may come from an interaction between the surface of the star and its disk. Aims. To investigate the disk around this Be star we used the VLTI/AMBER instrument, which combines high spectral (R=12000) and high spatial (\theta min =4 mas) resolutions. Methods. We constrain the geometry and kinematics of its circumstellar disk from the highest spatial resolution ever achieved on this star. Results. We obtain a disk extension in the Br{\gamma} line of 10.2 D\ast and 7.8 D\ast in the He I line at 2.05 \mu m assuming a Gaussian disk model. The disk is clearly following a Keplerian rotation. We obtained an inclination angle of 55\degree, and the star is a nearly critical rotator with Vrot /Vc =1.00$\pm$0.2. This inclination is greater than the value found for \gamma Cas (about 42\degree, Stee et al. 2012), and is consistent with the inference from optical Fe II emission profiles by Smith & Balona (2006) that the inclination should be more than the \gamma Cas value. In the near-IR continuum, the disk of HD 110432 is 3 times larger than \gamma Cas's disk. We have no direct evidence of a companion around HD 110432, but it seems that we have a clear signature for disk inhomogeneities as detected for {\zeta} Tau. This asymmetrical disk detection may be interpreted within the one-armed oscillation viscous disk framework. Another finding is that the disk size in the near-IR is similar to other Be stars with different spectral types and thus may be independent of the stellar parameters, as found for classical Be stars.
  01/2013;
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  Available from: ArXiv

  Article: Spatially resolving the outer atmosphere of the M giant BK Vir in the CO first overtone lines with VLTI/AMBER

  K. Ohnaka, K. -H. Hofmann, D. Schertl, G. Weigelt, F. Malbet, F. Massi, A. Meilland, Ph. Stee
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  ABSTRACT: 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.
  11/2011;
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  Available from: Alain Spang

  Article: The 2011 outburst of the recurrent nova T Pyxidis . Evidence for a face-on bipolar ejection

  O Chesneau, A Meilland, D ~P ~K Banerjee, J -B Le Bouquin, H McAlister, F Millour, S ~T Ridgway, A Spang, T Ten Brummelaar, M Wittkowski, [......], T Boyajian, C Farrington, P ~J Goldfinger, A Merand, N Nardetto, R Petrov, T Rivinius, G Schaefer, Y Touhami, G Zins
  åp. 10/2011; 534:L11.
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  Available from: Alain Spang

  Article: The 2011 outburst of the recurrent novaT Pyx. Evidence for a face-on bipolar ejection

  Olivier Chesneau, A. Meilland, D. P. K. Banerjee, Jean-Baptiste Le Bouquin, H. A. McAlister, F. Millour, S. T. Ridgway, A. Spang, T. A. ten Brummelaar, M. Wittkowski, [......], J. -P. Berger, T. S. Boyajian, C. Farrington, P. J. Goldfinger, A. Mérand, N. Nardetto, R. Petrov, T. Rivinius, G. Schaefer, G. Zins
  [show abstract] [hide abstract]
  ABSTRACT: We report on near-IR interferometric observations of the outburst of the recurrent nova T Pyx. We obtained near-IR observations of T Pyx at dates ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC recombiner, located at the CHARA array, and with the PIONIER and AMBER recombiners, located at the VLTI array. These data are supplemented with near-IR photometry and spectra obtained at Mount Abu, India. Slow expansion velocities were measured (<300km/s) before t=20d (assuming D=3.5kpc). From t=28d on, the AMBER and PIONIER continuum visibilities (K and H band, respectively) are best simulated with a two component model consisting of an unresolved source plus an extended source whose expansion velocity onto the sky plane is lower than 700km/s. The expansion of the Brgamma line forming region, as inferred at t=28d and t=35d is slightly larger, implying velocities in the range 500-800km/s, still strikingly lower than the velocities of 1300-1600km/s inferred from the Doppler width of the line. Moreover, a remarkable pattern was observed in the Brgamma differential phases. A semi-quantitative model using a bipolar flow with a contrast of 2 between the pole and equator velocities, an inclination of i=15^{\circ} and a position angle P.A.=110^{\circ} provides a good match to the AMBER observables (spectra, differential visibilities and phases). At t=48d, a PIONIER dataset confirms the two component nature of the H band emission, consisting of an unresolved stellar source and an extended region whose appearance is circular and symmetric within error bars.These observations are most simply interpreted within the frame of a bipolar model, oriented nearly face-on. This finding has profound implications for the interpretation of past, current and future observations of the expanding nebula.
  09/2011;
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  Available from: Alain Spang

  Article: Kinematics and geometrical study of the Be stars 48 Per and ψ Per with the VEGA/CHARA interferometer

  O Delaa, Ph Stee, A Meilland, J Zorec, D Mourard, Ph Bério, D Bonneau, O Chesneau, J M Clausse, P Cruzalebes, [......], A Marcotto, A Roussel, A Spang, H Mcalister, T Ten Brummelaar, J Sturmann, L Sturmann, N Turner, C Farrington, P J Goldfinger
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  ABSTRACT: Context. Five different physical processes might be responsible for the formation of decretion disks around Be stars: fast rotation of the star, stellar pulsations, binarity, stellar winds and magnetic fields. Our observations indicate that fast rotation seems to produce a disk in Keplerian rotation, at least in the specific case of the two stars observed. We do not know if this observational result is a generality or not. Aims. We measure the size, orientation, shape, and kinematics of the disks around 2 Be stars, namely 48 Per and ψ Per. Methods. We used the VEGA/CHARA interferometer with a spectral resolution of 5000 to obtain spectrally dispersed visibility modulus and phases within the Hα emission line. Results. We were able to estimate the disk extension in the continuum and in the Hα line, as well as flattening, for both stars. Both stars rotate at nearly a critical rotation, but while the disk of 48 Per seems to be in Keplerian rotation, our preliminary data suggest that the disk of ψ Per is possibly faster than Keplerian, similarly to what has been found for κ CMa with observations carried out in the near-IR. However, more data is needed to confirm the fast rotation of the disk. Conclusions. Assuming a simple uniform disk model for the stellar photosphere in the continuum and a Gaussian brightness distribution in the line emission region, we obtain a ratio of the disk diameter over the photospheric diameter of 8 48 Per and 11 for and ψ Per. We also found that the major axis of 48 Per is parallel to the polarization angle and not perpendicular to it as previously observed for many Be stars, including ψ Per. This might be due to the optical thickness of the disk, which is also responsible for the incoherent scattering of a non negligible part of the Hα line emission. To our knowledge, this is the first time that this effect has been measured in a Be star.
  03/2011;
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  Available from: A. S. Miroshnichenko

  Article: VLTI/AMBER spectro-interferometry of the Herbig Be star MWC 297 with spectral resolution 12 000

  G. Weigelt, V. P. Grinin, J. H. Groh, K. -H. Hofmann, S. Kraus, A. S. Miroshnichenko, D. Schertl, L. V. Tambovtseva, M. Benisty, T. Driebe, S. Lagarde, F. Malbet, A. Meilland, R. Petrov, E. Tatulli
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  ABSTRACT: Circumstellar disks and outflows play a fundamental role in star formation. Infrared spectro-interferometry allows the inner accretion-ejection region to be resolved. We measured interferometric visibilities, wavelength-differential phases, and closure phases of MWC 297 with a spectral resolution of 12000. To interpret our MWC 297 observations, we employed disk-wind models. The measured continuum visibilities confirm previous results that the continuum-emitting region of MWC 297 is remarkably compact. We derive a continuum ring-fit radius of ~2.2 mas (~0.56 AU at a distance of 250 pc), which is ~5.4 times smaller than the 3 AU dust sublimation radius expected for silicate grains (in the absence of radiation-shielding material). The strongly wavelength-dependent and asymmetric Br gamma-emitting region is more extended (~2.7 times) than the continuum-emitting region. At the center of the Br gamma line, we derive a Gaussian fit radius of ~6.3 mas HWHM (~1.6 AU). To interpret the observations, we employ a magneto-centrifugally driven disk-wind model consisting of an accretion disk, which emits the observed continuum radiation, and a disk wind, which emits the Br gamma line. The calculated wavelength-dependent model intensity distributions and Br gamma line profiles are compared with the observations (i.e., K-band spectrum, visibilities, differential phases, and closure phases). The closest fitting model predicts a continuum-emitting disk with an inner radius of ~0.3 AU and a disk wind ejection region with an inner radius of ~0.5 AU (~17.5 stellar radii). We obtain a disk-wind half-opening angle (the angle between the rotation axis and the innermost streamline of the disk wind) of ~80 degrees, which is larger than in T Tau models, and a disk inclination angle of ~20 degrees (i.e., almost pole-on).
  01/2011;
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  Available from: archives-ouvertes.fr

  Article: Imaging “Pinwheel” nebulae with optical long-baseline interferometry

  F Millour, T Driebe, J H Groh, O Chesneau, G Weigelt, A Liermann, A Meilland
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  ABSTRACT: Dusty Wolf-Rayet stars are few but remarkable in terms of dust production rates (up to ˙ M = 10 −6 M⊙/yr). Infrared excesses associated to mass-loss are found in the sub-types WC8 and WC9. Few WC9d stars are hosting a "pinwheel" nebula, indirect evidence of a companion star around the primary. While few other WC9d stars have a dust shell which has been barely resolved so far, the available angular resolution offered by single telescopes is insufficient to confirm if they also host "pinwheel" nebulae or not. In this article, we present the possible de-tection of such nebula around the star WR 118. We discuss about the potential of interferometry to image more "pinwheel" nebulae around other WC9d stars.
  01/2010;
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  Available from: ArXiv

  Article: Resolving the dusty circumstellar environment of the A[e] supergiant HD 62623 with the VLTI/MIDI

  A. Meilland, S. Kanaan, M. Borges Fernandes, O. Chesneau, F. Millour, Ph. Stee, B. Lopez
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  ABSTRACT: B[e] stars are hot stars surrounded by circumstellar gas and dust responsible for the presence of emission lines and IR-excess in their spectra. How dust can be formed in this highly illuminated and diluted environment remains an open issue. HD 62623 is one of the very few A-type supergiants showing the B[e] phenomenon. We obtained nine calibrated visibility measurements using the VLTI/MIDI instrument in SCI-PHOT mode and PRISM spectral dispersion mode with projected baselines ranging from 13 to 71 m and with various position angles. We used geometrical models and physical modeling with a radiative transfer code to analyze these data. The dusty circumstellar environment of HD 62623 is partially resolved by the VLTI/MIDI even with the shortest baselines. The environment is flattened and can be separated into two components: a compact one whose extension grows from 17 mas at 8 microns to 30 mas at 9.6 microns and stays almost constant up to 13 microns, and a more extended one that is over-resolved even with the shortest baselines. Using the radiative transfer code MC3D, we managed to model HD 62623's circumstellar environment as a dusty disk with an inner radius of 3.85+-0.6 AU, an inclination angle of 60+-10 deg, and a mass of 2x10^-7Mo. It is the first time that the dusty disk inner rim of a supergiant star exhibiting the B[e] phenomenon is significantly constrained. The inner gaseous envelope likely contributes up to 20% to the total N band flux and acts like a reprocessing disk. Finally, the hypothesis of a stellar wind deceleration by the companion's gravitational effects remains the most probable case since the bi-stability mechanism does not seem to be efficient for this star. Comment: 13 pages, 11 figures. A&A accepted paper
  12/2009;
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  Available from: ArXiv

  Article: Revealing the sub-AU asymmetries of the inner dust rim in the disk around the Herbig Ae star R CrA

  S. Kraus, K. -H. Hofmann, F. Malbet, A. Meilland, A. Natta, D. Schertl, P. Stee, G. Weigelt
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  ABSTRACT: Models predict that in the innermost AU of the disk around Herbig Ae/Be star, the dust disk forms a "puffed-up" inner rim, which should result in a strongly asymmetric brightness distribution for disks seen under intermediate inclination. Using the VLTI/AMBER long-baseline interferometer, we obtained 24 near-infrared (H- and K-band) spectro-interferometric observations on the Herbig Ae star R CrA. In the derived visibility function, we detect the signatures of an extended (25 mas) and a compact component (5.8 mas), with the compact component contributing about 2/3 of the total flux. The brightness distribution is highly asymmetric, as indicated by the strong closure phases (up to 40 deg) and the detected position angle dependence of the visibilities and closure phases. To interpret these asymmetries, we employ geometric as well as physical models, including a binary model, a skewed ring model, and a puffed-up inner rim model with a vertical or curved rim shape. Our curved puffed-up rim model can reasonably well reproduce the interferometric observables and the SED simultaneously and suggests a luminosity of 29 L_sun and the presence of relatively large (> 1.2 micron) Silicate dust grains. Perpendicular to the disk, two bow shock-like structures appear in the associated reflection nebula NGC 6729, suggesting that the resolved sub-AU size disk is the driving engine of a large-scale outflow. Detecting, for the first time, strong non-localized asymmetries in the inner regions of a Herbig Ae disk, our study supports the existence of a puffed-up inner rim in YSO disks. Comment: 17 pages; 15 figures; Accepted by A&A
  11/2009;
• Chapter: Is the Critical Rotation of Be Stars Really Critical for the Be Phenomenon?

  Ph. Stee, A. Meilland
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  ABSTRACT: We aim to study the effect of the fast rotation, stellar wind and circumstellar disks around active hot stars and their effects on the formation and evolution of these massive stars. For that purpose, we obtained, for the first time, interferometric measurements of three active hot stars, namely α Arae, κ CMa and Achernar, using the VLTI /AMBER and VLTI/MIDI instruments which allow us to study the kinematics of the central star and its surrounding circumstellar matter. These data coupled with our numerical code SIMECA (SIMulation pour Etoiles Chaudes Actives) seem to indicate that the presence of equatorial disks and polar stellar wind around Be stars are not correlated. A polar stellar wind was detected for α Arae and Achernar whereas κ CMa seems to exhibit no stellar wind. On the other hand, these two first Be stars are certainly nearly critical rotators whereas the last one seems to be far from the critical rotation. Thus a polar stellar wind may be due to the nearly critical rotation which induces a local effective temperature change following the von Zeipel theorem, producing a hotter polar region triggering a polar stellar wind. This critical rotation may also explain the formation of a circumstellar disk which is formed by the centrifugal force balancing the equatorial effective gravity of the central star. Following these results we try to investigate if critical rotation may be the clue for the Be phenomenon.
  11/2008: pages 195-205;
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  Available from: ArXiv

  Article: The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

  S. Kraus, K. -H. Hofmann, M. Benisty, J. -P. Berger, O. Chesneau, A. Isella, F. Malbet, A. Meilland, N. Nardetto, A. Natta, T. Preibisch, D. Schertl, M. Smith, P. Stee, E. Tatulli, L. Testi, G. Weigelt
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  ABSTRACT: To trace the accretion and outflow processes around YSOs, diagnostic spectral lines such as the BrG 2.166 micron line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the BrG line emission with mass infall or mass outflow processes. Using the VLTI/AMBER instrument, we spatially and spectrally (R=1500) resolved the inner environment of five Herbig Ae/Be stars (HD163296, HD104237, HD98922, MWC297, V921Sco) in the BrG emission line as well as in the adjacent continuum. All objects (except MWC297) show an increase of visibility within the BrG emission line, indicating that the BrG-emitting region in these objects is more compact than the dust sublimation radius. For HD98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD163296, HD104237, MWC297, and V921Sco we identify a stellar wind or a disk wind as the most likely line-emitting mechanism. We search for general trends and find that the size of the BrG-emitting region does not seem to depend on the basic stellar parameters, but correlates with the H-alpha line profile shape. We find evidence for at least two distinct BrG line-formation mechanisms. Stars with a P-Cygni H-alpha line profile and a high mass-accretion rate seem to show particularly compact BrG-emitting regions (R_BrG/R_cont<0.2), while stars with a double-peaked or single-peaked H-alpha-line profile show a significantly more extended BrG-emitting region (0.6<R_BrG/R_cont<1.4), possibly tracing a stellar wind or a disk wind. Comment: 20 pages; 11 figures; Accepted by A&A; a high quality version of the paper can be obtained at http://www.skraus.eu/papers/kraus.HAeBe-BrGsurvey.pdf
  07/2008;
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  Available from: Alain Blazit

  Chapter: VEGA: A Visible Spectrograph and Polarimeter for the VLTI

  D. Mourard, P. Antonelli, A. Blazit, D. Bonneau, Y. Bresson, J. M. Clausse, A. Domiciano, M. Dugué, R. Foy, P. Harmanec, [......], N. Nardetto, R. Petrov, K. Rousselet-Perraut, D. Schertl, Ph. Stee, I. Tallon-Bosc, M. Tallon, E. Thiébaut, F. Vakili, G. Weigelt
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  ABSTRACT: The ESO/VLTI has now clearly a position of world leader in the domain of ground-based optical interferometry. With four 8.2 m telescopes and two (four) 1.8 m telescopes, the Paranal Observatory is without any doubt the best optical interferometric facility in the world. Since many years, it has attracted the major part of the European interferometric community and with the opening of MIDI and AMBER, the astronomers have now access to ’general user’ interferometric instruments in the thermal and near infrared. This paper describes a project for a second generation focal instrument of the VLTI, named VEGAfor Visible spEctroGraph and polArimeter. The goal is to give access to the visible wavelength region, with spectroscopic and polarimetric capabilities, taking advantage of the coherent field of view of the VLTI. It is a unique scientific field for the VLTI. For example, a 200m interferometer operating in the visible will be able to resolve structures of the order of 0.5 mas or 0.1 AU at the distance of the Ophiuchus cloud.
  12/2007: pages 395-406;
• Chapter: First MIDI Observations of a Be Star: α Ara

  A. Meilland, O Chesneau, T. Rivinius, Ph. Stee
  10/2007: pages 493-494;
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  Available from: adsabs.harvard.edu

  Article: First Observations of the Be Star α Ara with the VLTI

  P. Stee, A. Meilland
  [show abstract] [hide abstract]
  ABSTRACT: We aim to study the geometry and kinematics of the disk around the Be star α Arae as a function of wavelength, especially across the Brγ emission line. We use both the VLTI/MIDI and VLTI/AMBER instruments operating respectively in the N and K bands. Moreover, we have combined the high angular resolution provided with the (medium) spectral resolution of AMBER to study the kinematics of the inner part of the disk and to infer its rotation law. We obtain for the first time the clear evidence that the disk is in Keplerian rotation, closing a debate that occurs since the discovery of the first Be star γ Cas by father Secci in 1866. We found that the disk around α Arae is compatible with a dense equatorial matter confined in the central region whereas a polar wind is contributing to the global flux along the rotational axis of the central star.
  02/2007; 361:300.
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  Available from: Carla Susana Gil

  Article: An asymmetry detected in the disk of $\kappa$ Canis Majoris with AMBER/VLTI

  A. Meilland, F. Millour, P. Stee, A. Domiciano de Souza, R. G. Petrov, D. Mourard, S. Jankov, S. Robbe-Dubois, A. Spang, E. Aristidi, [......], P. Stefanini, M. Tallon, I. Tallon-Bosc, D. Tasso, L. Testi, F. Vakili, O. von der Lühe, J.-C. Valtier, M. Vannier, N. Ventura
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  ABSTRACT: 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 AMBER/VLTI 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. To obtain more kinematical constraints we also use a 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. We discuss the possibility that $\kappa$ CMa is a critical rotator with a Keplerian rotating disk and examine whether if the detected asymmetry can be interpreted within the “one-armed” viscous disk framework.
  Astronomy and Astrophysics 01/2007; · 4.59 Impact Factor
• Article: First direct detection of a Keplerian rotating disk around the Be star α Arae using AMBER/VLTI

  A. Meilland, P. Stee, M. Vannier, F. Millour, A. Domiciano de Souza, F. Malbet, C. Martayan, F. Paresce, R. G. Petrov, A. Richichi, A. Spang
  [show abstract] [hide abstract]
  ABSTRACT: Aims. We aim to study the geometry and kinematics of the disk around the Be star α Arae as a function of wavelength, especially across the Brγ emission line. The main purpose of this paper is to understand the nature of the disk rotation around Be stars. Methods. We use the AMBER/VLTI instrument operating in the K-band, which provides a gain by a factor of 5 in spatial resolution compared to previous MIDI/VLTI observations. Moreover, it is possible to combine the high angular resolution provided with the (medium) spectral resolution of AMBER to study the kinematics of the inner part of the disk and to infer its rotation law. Results. For the first time, we obtain direct evidence that the disk is in Keplerian rotation, answering a question that has existed since the discovery of the first Be star γ Cas by Father Secchi in 1866. We also present the global geometry of the disk, showing that it is compatible with a thin disk and polar enhanced winds modeled with the SIMECA code. We found that the disk around α Arae is compatible with a dense equatorial matter confined to the central region, whereas a polar wind is contributing along the rotational axis of the central star. Between these two regions, the density must be low enough to reproduce the large visibility modulus (small extension) obtained for two of the four VLTI baselines. Moreover, we obtain that α Arae is rotating very close to its critical rotation. This scenario is also compatible with the previous MIDI measurements.
  Astronomy & Astrophysics - ASTRON ASTROPHYS. 01/2007; 464(1):59-71.
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  Available from: adsabs.harvard.edu

  Article: Interferometric View of Massive Active Hot and Dusty Stars

  Ph. Stee, A. Meilland
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  ABSTRACT: We present in the following recent results from the VLTI MIDI and AMBER instruments operating respectively at 10 and 2 μm. Up to now most of the interferometric results were obtained using the modulus of the visibility in order to estimate the global size and geometry of the circumstellar environments. Thanks to the spectrally resolved measurements possible with the VLTI it is also possible to follow the phase of the fringes and thus, to study the shape of the disks as a function of wavelength in order to infer the gas kinematics within the circumstellar envelopes.
  11/2006; 355:233.
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  Available from: adsabs.harvard.edu

  Article: Interferometric Studies of Hot Star Disks

  Ph. Stee, A. Meilland, D. Berger, D. Gies
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  ABSTRACT: Interferometry in the visible and in the IR now provides milliarcsecond spatial resolution and thus can be used for studying the circumstellar environment of hot star disks. After a brief introduction on the interferometric technique we will illustrate how the modulus and the phase of the visibility can be used to constrain the physics of hot star disks (Be, B[e]) and stellar surfaces studies through recent results from the GI2T, Keck, MkIII, NPOI, VLA and VLTI interferometers.
  10/2005; 337:211.
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  Available from: W.J. de Wit

  Article: Disk and wind interaction in the young stellar object MWC 297 spatially resolved with VLTI/AMBER

  F. Malbet, M. Benisty, W. J. de Wit, S. Kraus, A. Meilland, F. Millour, E. Tatulli, J. -P. Berger, O. Chesneau, K. -H. Hofmann, A. Isella, A. Natta, R. Petrov, T. Preibisch, P. Stee, L. Testi, G. Weigelt, AMBER Collaboration
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  ABSTRACT: The young stellar object MWC 297 has been observed with the VLT interferometer equipped with the AMBER instrument. MWC 297 has been spatially resolved in the continuum with a visibility of 0.50 as well as in the Br gamma emission line where the visibility decrease to a lower value of 0.33. This change in the visibility with the wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by Br gamma and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind which has a much higher velocity in the polar region than at the equator. [abridged] Comment: 13 pages; to be published in A&A; Acceptance day: 11/10/2005
  10/2005;
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  Available from: Carla Susana Gil

  Article: Disk and wind interaction in the young stellar object MWC 297 spatially resolved with VLTI/AMBER

  F. Malbet, M. Benisty, W. J. de Wit, S. Kraus, A. Meilland, F. Millour, E. Tatulli, J. -P. Berger, O. Chesneau, K. -H. Hofmann, [......], P. Stefanini, Michel Tallon, Isabelle Tallon-Bosc, D. Tasso, J.-C. Valtier, M. Vannier, N. Ventura, M. Kiekebusch, F. Rantakyrö, M. Schöller
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  ABSTRACT: The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT interferometer equipped with the AMBER instrument during its first commissioning run. VLTI/AMBER is currently the only near infrared interferometer which can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of $0.50^{+0.08}_{-0.10}$ as well as in the Brgamma emission line where the visibility decrease to a lower value of $0.33\pm0.06$. This change in the visibility with the wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by the Brgamma emission line and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. We also reproduce the shape of the visibilities in the Brgamma line as well as the profile of this line obtained at an higher spectral resolution with the VLT/ISAAC spectrograph, and those of the Halpha and Hbeta lines. The disk and wind models yield a consistent inclination of the system of approximately 20 degrees. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind which has a much higher velocity in the polar region than at the equator. The VLTI/AMBER unique capability to measure spectral visibilities therefore allows us for the first time to compare the apparent geometry of a wind with the disk structure in a young stellar system.
  Astronomy and Astrophysics 10/2005; · 4.59 Impact Factor