Resolving the dusty circumstellar environment of the A[e] supergiant HD 62623 with the VLTI/MIDI
by A. Meilland, S. Kanaan, M. Borges Fernandes, O. Chesneau, F. Millour, Ph. Stee, and B. Lopez 2010, A&A, 512, 73
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 studied the geometry of its circumstellar envelope in the mid-infrared using long-baseline interferometry, which is the only observing technique able to spatially resolve objects smaller than a few tens of milliarcseconds.
We obtained nine calibrated visibility measurements between October 2006 and January 2008 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 (PA). 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 (a/b∼1.3±0.1) and can be separated into two components: a compact one whose extension grows from 17 mas at 8µm to 30 mas at 9.6µm and stays almost constant up to 13µm, 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°, and a mass of 2.10−7 M_sun.
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.
Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD 62623
by F. Millour, A. Meilland, O. Chesneau, Ph. Stee, S. Kanaan, R. Petrov, D. Mourard and S. Kraus 2011, A&A, 526, A107
To progress in the understanding of evolution of massive stars one needs to constrain the mass-loss and determine the phenomenon responsible for the ejection of matter an its reorganization in the circumstellar environment
In order to test various mass-ejection processes, we probed the geometry and kinematics of the dust and gas surrounding the A[e] supergiant HD 62623.
We used the combined high spectral and spatial resolution offered by the VLTI/AMBER instrument. Thanks to a new multi-wavelength optical/IR interferometry imaging technique, we reconstructed the first velocity-resolved images with a milliarcsecond resolution in the infrared domain.
We managed to disentangle the dust and gas emission in the HD 62623 circumstellar disc. We measured the dusty disc inner rim, i.e. 6 mas, constrained the inclination angle and the position angle of the major-axis of the disc. We also measured the inner gaseous disc extension (2 mas) and probed its velocity field thanks to AMBER high spectral resolution. We find that the expansion velocity is negligible, and that Keplerian rotation is a favoured velocity field. Such a velocity field is unexpected if fast rotation of the central star alone is the main mechanism of matter ejection.
As the star itself seems to rotate below its breakup-up velocity, rotation cannot explain the formation of the dense equatorial disc. Moreover, as the expansion velocity is negligible, radiatively driven wind is also not a suitable explanation to explain the disc formation. Consequently, the most probable hypothesis is that the accumulation of matter in the equatorial plane is due to the presence of the spectroscopic low mass companion.
The press releases can be browsed by following these links:
- CNRS (French) http://www2.cnrs.fr/presse/communique/2088.htm
- Max-Planck (English) http://www.mpg.de/1054436/Supergiant_star_imaging
- Max-Planck (German)http://www.mpg.de/1051794/Riesenstern_Staubscheibe
- ESO (English) http://www.eso.org/public/announcements/ann11002/
The first B[e] star observed with the VLTI using both MIDI and AMBER instruments: CPD -57 2874
We have obtained the first high spatial and spectral resolution observations of the circumstellar envelope (CSE) of a B[e] supergiant (CPD −57? 2874), performed with the Very Large Telescope Interferometer (VLTI). Spectra, visibilities and closure phase were obtained using the beam-combiner instruments AMBER (near-IR interferometry with three 8.3 m Unit Telescopes or UTs) and MIDI (mid-IR interferometry with two UTs). The interferometric observations of the CSE are well fitted by an elliptical Gaussian model with FWHM diameters varying linearly with wavelength.
Typical diameters measured are ~ 1.8 × 3.4 mas or ~ 4.5 × 8.5 AU (adopting a distance of 2.5 kpc) at 2.2 μm, and ~12 × 15 mas or ~ 30 × 38 AU at 12 μm. The size of the region emitting the Brγ flux is ~2.8 × 5.2 mas or ~7.0 × 13.0 AU. The major-axis position angle of the elongated CSE in the mid-IR (~144? ) agrees well with previous polarimetric data, hinting that the hot-dust emission originates in a disk-like structure.
In addition to the interferometric observations we also present new optical (U BV Rc Ic ) and near-IR (JHKL) broadband photometric observations of CPD −57? 2874. Our spectro-interferometric VLTI observations and data analysis support the non-spherical CSE paradigm for B[e] supergiants. These results are described in A&A, 464, 81
The following image was done by my PhD student Anthony Meilland and can be found on the ESO press release HERE.
Fast ray-tracing algorithm for circumstellar structures (FRACS)
II. Disc parameters of the B[e] supergiant CPD-57°,2874 from VLTI/MIDI data
by A. Domiciano de Souza, P. Bendjoya, G. Niccolini, O. Chesneau, M. Borges Fernandes, A. C. Carciofi, A. Spang, P. Stee and T. Driebe, 2011, A&A, 525, A22
B[e] supergiants are luminous, massive post-main sequence stars exhibiting non-spherical winds, forbidden lines, and hot dust in a disc-like structure. The physical properties of their rich and complex circumstellar environment (CSE) are not well understood, partly because these CSE cannot be easily resolved at the large distances found for B[e] supergiants (typically 1 kpc).
From mid-IR spectro-interferometric observations obtained with VLTI/MIDI we seek to resolve and study the CSE of the Galactic B[e] supergiant CPD-57° 2874.
For a physical interpretation of the observables (visibilities and spectrum) we use our ray-tracing radiative transfer code (FRACS), which is optimised for thermal spectro-interferometric observations.
Thanks to the short computing time required by FRACS (<10 s per monochromatic model), best-fit parameters and uncertainties for several physical quantities of CPD-57° 2874 were obtained, such as inner dust radius, relative flux contribution of the central source and of the dusty CSE, dust temperature profile, and disc inclination.
The analysis of VLTI/MIDI data with FRACS allowed one of the first direct determinations of physical parameters of the dusty CSE of a B[e] supergiant based on interferometric data and using a full model-fitting approach. In a larger context, the study of B[e] supergiants is important for a deeper understanding of the complex structure and evolution of hot, massive stars.