Issue |
A&A
Volume 503, Number 1, August III 2009
|
|
---|---|---|
Page(s) | 183 - 195 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200912247 | |
Published online | 22 July 2009 |
Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER *
1
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: kohnaka@mpifr-bonn.mpg.de
2
INAF – Osservatorio Astrofisico di Arcetri, Instituto Nazionale di Astrofisica, Largo E. Fermi 5, 50125 Firenze, Italy
3
Laboratoire d'Astrophysique de Grenoble, UMR 5571, Université Joseph Fourier/CNRS, BP 53, 38041 Grenoble Cedex 9, France
4
Lab. H. Fizeau, CNRS UMR 6525, Univ. de Nice-Sophia Antipolis, Obs. de la Côte d'Azur, Parc Valrose, 06108 Nice, France
5
Lab. H. Fizeau, CNRS UMR 6525, Univ. de Nice-Sophia Antipolis, Obs. de la Côte d'Azur, Avenue Copernic, 06130 Grasse, France
Received:
1
April
2009
Accepted:
22
June
2009
Aims. We present spatially resolved, high-spectral resolution K-band observations of the red supergiant Betelgeuse (α Ori) using AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to probe inhomogeneous structures in the dynamical atmosphere of Betelgeuse.
Methods. Betelgeuse was observed in the wavelength range between 2.28 and 2.31 μm with VLTI/AMBER using baselines of 16, 32, and 48 m. The spectral resolutions of 4800-12 000 allow us to study inhomogeneities seen in the individual CO first overtone lines.
Results.
Spectrally dispersed interferograms have been successfully obtained
in the second, third, and fifth lobes, which
represents the highest spatial resolution (9 mas) achieved
for Betelgeuse. This corresponds to five resolution elements over its
stellar disk.
The AMBER visibilities and closure phases in the K-band continuum
can be reasonably fitted by a uniform disk
with a diameter of 43.19 ± 0.03 mas or a limb-darkening disk with
43.56 ± 0.06 mas and a limb-darkening parameter of
(1.2 ± 0.07) 10-1.
These AMBER data and the previous K-band
interferometric data taken
at various epochs suggest that Betelgeuse seen in the K-band continuum
shows much smaller deviations from the above uniform disk or limb-darkened
disk than predicted by recent 3-D convection simulations for red supergiants.
On the other hand, our AMBER data in the CO lines reveal
salient inhomogeneous structures.
The visibilities and phases (closure phases, as well as differential phases
representing asymmetry in lines with respect to the continuum)
measured within the CO lines
show that the blue and red wings originate in spatially
distinct regions over the stellar disk, indicating an inhomogeneous
velocity field that makes the star appear different in the blue and red
wings.
Our AMBER data in the CO lines can be roughly explained by a simple
model, in which a patch of CO gas is moving outward or inward with velocities
of 10-15 km s-1, while the CO gas in the remaining region in the atmosphere
is moving in the opposite direction at the same velocities.
Also, the AMBER data are consistent with the presence
of warm molecular layers (so-called MOLsphere)
extending to ~1.4-1.5
with a CO column density
of ~1
1020 cm-2.
Conclusions. Our AMBER observations of Betelgeuse are the first spatially resolved study of the so-called macroturbulence in a stellar atmosphere (photosphere and possibly MOLsphere as well) other than the Sun. The spatially resolved CO gas motion is likely to be related to convective motion in the upper atmosphere or intermittent mass ejections in clumps or arcs.
Key words: infrared: stars / techniques: interferometric / stars: supergiants / stars: late-type / stars: atmospheres / stars: individual: Betelgeuse
© ESO, 2009
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