Using stellar interferometry, a team of French astronomers has captured one of the sharpest color images ever made. They observed the star T Leporis with the European Southern Observatory's Very Large Telescope Interferometer (VLTI; Cerro Paranal, Chile), which emulates a virtual telescope about 100 meters across, and which revealed a spherical molecular shell around the aged star.
"This is one of the first images made using near-IR interferometry," said lead author Jean-Baptiste Le Bouquin, from the European Southern Observatory (ESO; Garching, Germany). In the technique, light from several telescopes is captured and combined coherently, resulting in an image with as much detail as that of a telescope with a diameter equal to the largest separation between the telescopes used. Achieving this requires the VLTI system components to be positioned to an accuracy of a fraction of a micrometer over about 100 meters and maintained so throughout the observations.
If doing interferometry with two telescopes, astronomers must content themselves with fringes. But, 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 now been done with ESO's VLTI, using the 1.8-metre Auxiliary Telescopes.
"We were able to construct an amazing image, and reveal the onionlike structure of the atmosphere of a giant star at a late stage of its life for the first time," says Antoine Mérand, member of the team. "Numerical models and indirect data have allowed us to imagine the appearance of the star before, but it is quite astounding that we can now see it, and in color."
Although it is only 15 by 15 pixels across, the reconstructed image shows an extreme closeup of a star 100 times larger than the Sun, a diameter corresponding roughly to the distance between the Earth and the Sun. This star is, in turn, surrounded by a sphere of molecular gas, which is about three times as large again.
T Leporis, in the constellation of Lepus (the Hare), is located 500 light-years away. It belongs to the family of Mira stars, well known to amateur astronomers. These are giant variable stars that have almost extinguished their nuclear fuel and are losing mass. They are nearing the end of their lives as stars, and will soon die, becoming white dwarfs. The Sun will become a Mira star in a few billion years, engulfing the Earth in the dust and gas expelled in its final throes.
Mira stars are among the biggest factories of molecules and dust in the Universe, and T Leporis is no exception. It pulsates with a period of 380 days and loses the equivalent of the Earth's mass every year. Since the molecules and dust are formed in the layers of atmosphere surrounding the central star, astronomers would like to be able to see these layers. But this is no easy task, given that the stars themselves are so far away — despite their huge intrinsic size, their apparent radius on the sky can be just half a millionth that of the Sun.
"T Leporis looks so small from the Earth that only an interferometric facility, such as the VLTI at Paranal, can take an image of it. VLTI can resolve stars 15 times smaller than those resolved by the Hubble Space Telescope," says Le Bouquin.
To create this image with the VLTI, astronomers had to observe the star for several consecutive nights, using all the four movable 1.8-metre VLT Auxiliary Telescopes (ATs). The ATs were combined in different groups of three, and were also moved to different positions, creating more new interferometric configurations, so that astronomers could emulate a virtual telescope approximately 100 meters across.
These results are to appear in a Letter to the Editor in Astronomy and Astrophysics (J.-B. Le Bouquin et al., Pre-maximum spectro-imaging of the Mira star T Lep with AMBER/VLTI).
The team is composed of Jean-Baptiste Le Bouquin and Antoine Mérand (ESO), Sylvestre Lacour and Stéphanie Renard (LAOG, CNRS, Grenoble, France), and Eric Thiébaut (AIRI, Observatoire de Lyon, France).
