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Fig. 1:
Top panel: intensity map from a snapshot of a 3D hydrodynamical
model in the H band (the intensity range is
[0; 2.5x105] erg/cm2/s/Å).
Bottom panel: visibility curves from the above snapshot computed for 36
different angles (thin grey lines). Note the logarithm visibility scale.
The solid black curve is a uniform disk model. The dashed black line is
a partially limb darkened disk model. The dot-dashed line is a fully
limb darkened disk model. The triple-dot-dashed line is the new limb
darkening law we determined in this work.
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Fig. 2:
Reconstructed images of the very cool late type star VX Sgr for
several VLTI/AMBER spectral bins across the H and K bands. The
resolution of the interferometer is illustrated in the bottom left part
of each image by the PSF of an 88x70 metre telescope.
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Massive stars with masses between roughly 10 and 25 solar masses spend
some time as red supergiants being the largest stars in the universe.
They have a surface temperature of ~ 4000K (while the Sun is 5780K), and
are ~ 1000 times larger in size than the Sun, which makes them some of
the brightest stars known. Such extreme properties foretell the demise
of a short-lived stellar king because they are nearing the end of their
life and they are doomed to explode as a supernova.
Red supergiants still hold several unsolved mysteries: (i) the mass-loss
mechanism, shedding tremendous quantities of gas, is unidentified; (ii)
their chemical composition is largely unknown due to difficulties in
analyzing their complex spectra due to the low surface temperatures and
vigorous convection.
The solution to these mysteries relies on a theoretical approach based
on realistic three-dimensional hydrodynamical simulations of red
supergiant stars. This challenging endeavour has been pioneered with
numerical simulations of the entire gas flow of the star including the
effect of radiation.
A team of international astronomers lead by Andrea Chiavassa (MPA) and
including collaborators from Montpellier and Lyon have analyzed the
properties of these simulations in detail and found that the surface of
the stellar model is covered by a few large convective cells with some
500 solar radii in size that evolve on a timescale of years. Close to
the surface, there are short-lived (a few months to one year) small-
scale (50-100 solar radii) granules. Moreover, the authors described the
prospects for the detection and characterization of granulation (i.e.
contrast, size and time evolution) with today's interferometers, thus
providing the first solid detection of a convective pattern on the
prototypical red supergiant Betelgeuse.
Interferometry is a technique that combines the light from several
telescopes, resulting in a vision as sharp as that of a giant telescope
with a diameter equal to the largest separation between the telescopes
used. If an object is observed on several runs with different
combinations and configurations of telescopes, it is possible to put
these results together to reconstruct an image of the object. This is
what has been done with ESO’s Very Large Telescope Interferometer
(VLTI), using the 1.8-meter Auxiliary Telescopes by Andrea Chiavassa and
collaborators from Paris, Bonn, ESO, Montpellier and Heidelberg. They
unveiled for the first time the photosphere of the very cool late-type
star VX Sgr using interferometric observations with AMBER and performing
image reconstructions for different wavelengths. VX Sgr is at ~ 5000
light years from the Earth and thus appears so small that only
interferometric facilities can produce an image.
The classification of VX Sgr is uncertain: it could be a red supergiant
star because of its extremely high luminosity and radius (5.6
astronomical units, which is larger than the Jupiter orbit). However,
its very low temperature and large variations are much closer to the
typical Mira stars (evolved giant variable stars of about the mass of
the Sun that will die becoming white dwarfs), which in revenge, cannot
have such high luminosity.
The images reveal for the first time the shape of VX Sgr. The
authors, comparing them to the latest hydrodynamical simulations,
found that the surface of VX Sgr is characterized by inhomogeneities
interpreted as large convective cells and that the atmosphere rather
resembles a Mira star surrounded by molecular water layers than red
supergiant. Understanding the physical properties behind this
peculiar object is important to constrain stellar evolution and
atmosphere models and to push VLTI facilities to their limits
entering a new era of stellar imaging.
The key-point of this research is the synergy between theory and
observations: on the one hand there are highly realistic 3D
hydrodynamical simulations and on the other hand there is a large set of
excellent observations involving spectroscopy, photometry,
interferometry, and imaging.
Red supergiant stars contribute extensively to the chemical enrichment
of our Galaxy loosing enormous quantities of their mass due to an
unknown process. The vigorous convection that they experience could be
at the base of the mass-loss mechanism and only hydrodynamical
simulations help the astronomers to solve the puzzle.
Andrea Chiavassa
Further Readings
Chiavassa, A., Plez, B., Josselin, E., Freytag, B.,
"Radiative hydrodynamics simulations of red supergiant stars. I. interpretation of interferometric observations",
2009, A&A, 506, 1351-1365
Chiavassa, A.; Lacour, S.; Millour, F.; et al.,
"VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere",
2010, A&A, in press,
 arXiv:0911.4422
Websites
Astronomy and Astrophysics highlight for the paper
Andrea Chiavassa home page
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