AMBER/VLTI and MIDI/VLTI spectro-interferometric observations of the B[e] supergiant CPD−57° 2874. Size and geometry of the circumstellar envelope in the near- and mid-IR
Résumé
We present 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 BVRcIc ) 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.
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