Virtual Telescope Sees Stars

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.

Achieving this requires that components of the Very Large Telescope Interferometer (VLTI) system be positioned to an accuracy of a fraction of a micron over about 100 m and maintained so throughout the observations – a formidable technical challenge.

This image from ESO’s Very Large Telescope Interferometer is one of the sharpest color images ever made. It shows the Mira-like star T Leporis in great detail. The central disk is the surface of the star, which is surrounded by a spherical shell of molecular material expelled from the star. To appreciate the feat of such measurement, one should realize that, on the sky, the star appears as small as a two-story house on the moon. The resolution of the image is about 4 milli-arcseconds. In this image, obtained by combining hundreds of interferometric measurements, the blue channel includes IR light from 1.4 to 1.6 µm, the green, from 1.6 to 1.75 µm, and the red, from 1.75 to 1.9 µm. In the green channel, the molecular envelope is thinner and appears as a thin ring around the star. (Image: ESO/J.-B. Le Bouquin)

Apparently, the challenge has been met. According to Jean-Baptiste Le Bouquin of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), “This is one of the first images made using near-infrared interferometry.”

When doing interferometry, astronomers often must content themselves with fringes, the characteristic pattern of dark and bright lines produced when two beams of light combine, from which they can model the physical properties of the object studied. 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-m 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,” said Antoine Mérand, a 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 × 15 pixels across, the reconstructed image shows an extreme close-up 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. Because 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,” Le Bouquin said.

To create this image with the VLTI, astronomers had to observe the star for several consecutive nights, using all the four movable 1.8-m VLT auxiliary telescopes (ATs). The ATs were combined in various groups of three and also were moved to different positions, creating more new interferometric configurations, so that astronomers could emulate a virtual telescope approximately 100 m across and build up an image.

“Obtaining images like these was one of the main motivations for building the VLTI. We have now truly entered the era of stellar imaging,” Mérand said.

A perfect illustration of this is another VLTI image showing the double star system Theta1 Orionis C in the Orion Nebula Trapezium. This image, which was the first constructed from VLTI data, separates clearly the two young, massive stars from this system. The observations themselves have a spatial resolution of about 2 milli-arcseconds. From these and several other observations, the team of astronomers, led by Stefan Kraus and Gerd Weigelt from Max-Planck Institute in Bonn, could derive the properties of the orbit of this binary system, including the total mass of the two stars (47 solar masses) and their distance from us (1350 light-years).

For more information, visit: www.eso.org