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Imaging the surface of massive stars

Red supergiant stars are among the brightest stars known. They are a primary target for interferometers today because of their large diameter, proximity, and high infrared luminosity. A group of international astronomers lead by Andrea Chiavassa (MPA) has provided a new way to detect and characterize the granulation pattern on red supergiants using three-dimensional simulations of surface convection. Moreover, they unveil for the first time the photosphere of the very cool late-type star VX Sgr using interferometric observations with the Very Large Telescope Interferometer. Theses studies bring a step forward to understand the mass-loss mechanism of red supergiant stars, which contribute extensively to the chemical enrichment of our Galaxy.

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.

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.

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, linkPfeilExtern.gifarXiv:0911.4422


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