Surface Imaging of Betelgeuse
Surface Imaging of Beletegeuse with COAST and the WHT
We present here results from COAST that were recently published in Monthly Notices of the Royal Astronomical Society (Young et al. 2000, MNRAS 315, 635). An electronic offprint of the paper is available from this site.
Betelgeuse is a nearby red supergiant star, about 500 times larger than the Sun. The parametric images below were secured within a few days of each other in November 1997. The two images on the right were reconstructed using data from COAST, and the left-hand image is a result of an aperture-masking experiment performed in La Palma using the William Herschel Telescope. The resolution of all three images is similar (20-30 milliarcseconds), and each shows an area 0.1 arc-second across.
Each image was taken through a different colour filter. From left to right, the images show how Betelgeuse appears in red (a wavelength of 700 nanometres), very near-infrared (905 nm), and near-infrared (1290 nm) light. To show this, we have coloured the images different shades of orange and red.
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The images are strikingly different from one other. Three bright features (“hotspots”) are visible on the surface of the Betelgeuse at 700 nm, but only one feature is discernible at 905 nm, and at 1290 nm the star presents a featureless disk. The disk is also smaller at 1290 nm than at 905 nm, and its intensity falls off more sharply towards the edge.
Most of the earlier images of Betelgeuse, made at wavelengths shorter than 800 nm, have exhibited a small number of bright regions, as in the left-hand image. Astronomers have explained the bright regions as the tops of convection cells – bubbles of hot gas welling up from the interior of the star. But why don’t we see the hotspots in the near-infrared?
As we look at different wavelengths of light, we see to different depths in cool stars like Betelgeuse. Consider the diagram below. The outer layers of the star (red in the diagram) are transparent to infrared light. Thus if we look in the infrared, we see a small, featureless star (the orange sphere). The outer layers are not transparent to red light, because of absorption by titanium oxide molecules. However, if hot gas rising from below disturbs the outer layers, they become transparent to red as well as infrared light, and bright features are seen on the star, where hotter gas is visible through the “holes” in the outer layers.
