The dynamic motion of gas in the outer atmosphere of a red supergiant star has been mapped, providing clues to the mysterious mechanism that causes massive stars to lose mass through stellar winds. See Letter p.310
Stars that have more than about nine times the mass of our Sun lose considerable mass throughout their lifetimes as a result of stellar winds, a process that provides elements for the next generation of stars and for planet formation. On page 310, Ohnaka et al.1 present the first two-dimensional map to depict the velocity of gas across the surface of a red supergiant — a massive star at a late stage of its evolution. The map shows large clumps of upwelling and downdraughting gas in the star's outer atmosphere, and suggests that convection alone cannot be responsible for lifting material away from the stellar surface of massive stars as had been thought.
Ohnaka and colleagues studied Antares, the brightest star in the constellation of Scorpius (Fig. 1). It is known as the heart of the scorpion because of its red colour and location in the body of the beast depicted by the constellation. Antares has a mass about 15 times that of the Sun and is nearing the end of its life. It has therefore evolved into a red supergiant, the outer layers of which have expanded2 outward to a radius of 3.2 astronomical units (1 AU corresponds to the average distance between Earth and the Sun). If Antares were at the centre of our Solar System, its outer layers would extend well past the orbit of Mars. It is one of the nearest red supergiants, making the star's apparent size one of the largest as viewed from Earth.
Supergiant stars have large hotspots on their surfaces3. Moreover, their atmospheres are complex: some regions extend two to three times beyond the stellar radius and contain clumps of hot atomic gas (detected through ultraviolet emissions4) that coexist with much cooler molecular gas (detected from its radio5 and near-infrared emissions2). Farther out, supergiant stars are often surrounded by large envelopes of dust6 that can extend to distances of more than 100 AU.
A common explanation for these features is that large convection cells — volumes of material that move as a result of convection — lift gas away from the stellar surface7. As material moves away, dust can form in the cooler outer layers, and is subsequently blown farther out through pressure exerted by the star's electromagnetic radiation6. This outflow then carries gas away through a process known as viscous drag. However, testing this hypothesis in more detail requires techniques that can measure the motion of gas in the atmospheric layers above the stellar surface at high spatial resolution.
Obtaining such resolution for stars other than the Sun is challenging: Antares is 170 parsecs (550 light years) from Earth8, and its diameter subtends an angle of only 37.6 milliarcseconds on the sky. Ohnaka et al. overcame this problem by using the Very Large Telescope Interferometer at the European Southern Observatory in Chile. More specifically, they used an instrument called AMBER, which combined the light from three 1.8-metre-diameter telescopes to simulate a telescope that has an effective diameter of up to 82 metres, providing about seven resolution elements — parts of an image that can be discriminated from each other — across the surface of Antares.
The authors also dispersed the light to record individual spectral lines of carbon monoxide molecules in the upper atmosphere of Antares. They observed that spectral lines from different positions on the stellar surface were blueshifted or redshifted relative to lines from the spatially unresolved spectrum; this indicated that the gas was moving towards or away from the surface of the star, respectively. By calculating the gas velocity from the blue- and redshifted spectral lines at different positions, the researchers constructed a 2D 'dopplergram' of the star — a map that shows which parts of Antares' upper atmosphere are moving towards or away from the surface. This is the first dopplergram to be produced for a star other than the Sun.
Ohnaka and co-workers' analysis of the continuum emission from Antares (the part of the spectrum outside the spectral lines) reveals the star's surface to be mostly smooth. However, the images constructed from within the spectral lines reveal two large, bright spots on the stellar surface, and an irregularly shaped atmosphere that extends out to 1.7 stellar radii. The authors' velocity map indicates that these asymmetric features are flows of gas molecules that rise and fall back down to the stellar surface with speeds of up to 20 kilometres per second.
The motion of gas in the atmospheres of red supergiants had been inferred previously from spectroscopic measurements in which the surface of the star was unresolved9 and from one-dimensional, spatially resolved slices across the surface2,10. Ohnaka and colleagues' in-depth view now reveals that the motion of the gas resembles that due to convection, but the observed density and extent of the outer atmosphere are much greater than current models of convection in massive stars predict. This indicates that the scenario of material being lifted off the surface through convection is not complete, and that an additional mechanism must be at play to raise the observed amount of material to the heights seen above the stellar surface. For example, it could be that the star's radiation exerts pressure on molecules in addition to that on dust, that dust forms closer in towards the star than expected, or that the effects of rotation and of magnetic fields need to be incorporated into the convection models11.
Ohnaka and colleagues' work is just a snapshot of the velocity map of Antares. However, there is evidence2,10 that the motion of gas in the atmospheres of supergiants can change dramatically over the time span of a year. It might now be possible to follow these motions over time to observe how clumps of gas form and move outward from the star's surface. Moreover, if Ohnaka and colleagues' technique can be applied to spectral lines from atoms and molecules other than carbon monoxide, and which are found at different depths in the atmosphere of supergiant stars, it could provide a 3D view of atmospheric motions that could help to identify the driving force of the stellar winds.
Discovering the driving force is important because it would improve our understanding of how mass loss affects the evolution of massive stars12. Such stars have a major role in astronomy. The mass lost through their stellar winds and their final supernova explosions enriches the interstellar gas between the stars with chemical elements that are produced in the interiors of massive stars. These elements become the building blocks for the planets that will form around the next generation of stars. Furthermore, the high luminosity of massive stars and the extreme brightness of supernova explosions mean that they can both be used as probes with which to study the structure, environment and star-formation history in distant regions of the Universe. But to understand this, we must be able to model accurately how massive stars evolve over their lifetimes.