[meteorite-list] Vega: The Star With Comets?

[Ron Baalke]

Observatoire de Paris
Paris, France

Contact:
Vincent Coudé du Foresto
Observatoire de Paris, LESIA
Tél: 33 1 45 07 79 61
Fax: 33 1 45 07 71 02

11 April 2006

Vega: the star with comets?

The observation of the immediate vicinity of a star other than the Sun has 
just been carried out for the first time. A debris disc made up of hot 
(1300 degrees) dust grains, residues of comet evaporation and collisions 
between asteroids, was indeed detected for the first time around Vega. 
This discovery is the work of an international team, which includes 
researchers of Paris Observatory (LESIA).

Vega is an important star in astronomy in more than one way: the fifth 
brightest star of the night sky, one of the three "beauties of summer" 
(with Deneb and Altair) which forms a large triangle at the zenith of our 
latitudes on estival evenings; it was a long time considered as a 
reference star, to which is compared the brightness of all the other 
stars. Located at 25 light-years, therefore relatively near the Sun, it is 
approximately three times larger and more massive, and 60 times more 
luminous than the Sun, and much younger (350 million years against 4.5 
billion).

An international team [1] detected in the vicinity of Vega a weak 
infra-red flux (78 times less important than that of the star) which would 
come from particles heated by the star to temperatures close to 1300 
degrees C. It seems that these particles have a chemical composition 
different from those of the solar system, with a dominant carbonaceous 
material (like graphite), whereas our zodiacal cloud contains essentially 
silicates. They would be also on average smaller (of a diameter lower than 
the micron, equivalent to particles constituting cigarette smoke). Such 
small grains should normally be driven out by the pressure created by the 
intense radiation of Vega. Their abundance thus proves that they are 
produced in permanence, probably in a phase of intense meteoritic and 
cometary bombardment like those experienced by the Earth at the origins of 
the solar system. The dust production rate would correspond in the daily 
passing of 13 large comets in the environment of Vega.

The presence of cold dust around Vega (-190 C), located at a distance 
three times larger than the orbit of Pluto, was known for a long time. 
This phenomenon is found also around a large number on stars similar to 
Vega. However, nothing was known on the internal part of these debris 
discs, where planets similar to the Earth are supposed to be formed. The 
observation of the internal part of the disc of Vega was made possible 
thanks to the array of CHARA (Center for High Angular Resolution 
Astronomy) of the State University of Georgia, which allows, from six 
telescopes of 1 meter distributed on Mount Wilson in California, to 
simulate a giant telescope of almost 330 m, and thus to distinguish 
details of only 200 microseconds of arc, hardly larger than a soccer ball 
seen from the Moon! The light collected by CHARA is recombined by the 
instrument FLUOR (Fiber Linked Unit for Optical Recombination), developed 
by the LESIA (CNRS, Observatoire of Paris, Université of Paris VI and 
VII).

This device also allowed to observe the atmosphere of Vega, and to confirm 
some astonishing properties of the star: its high speed of rotation on 
itself (12.5 hours) indeed confers it a lenticular form, flattened at the 
poles, the latter being brighter and hotter by 2300 C than the equator.

[1] The team comprises O. Absil (Université de Liège); E. di Folco 
(Observatoire de Genève); J.-C. Augereau (Laboratoire d'Astrophysique de 
Grenoble, UMR CNRS, Université Joseph Fourier); A. Mérand, V. Coudé du 
Foresto and P. Kervella (LESIA, UMR CNRS, Observatoire de Paris, 
Universités Paris VI et VII); J.-P. Aufdenberg et S. Ridgway (NOAO); D. 
Berger, T. ten Brummelaar, J. Sturmann, L. Sturmann, N. Turner, and H. 
McAlister (CHARA, Georgia State University).

See also the Press Release du CNRS [in French],
     http://www.insu.cnrs.fr/web/article/art.php?art=1738

Reference

Article published in Astronomy & Astrophysics, April 2006

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/apr06/vega-f1.jpg (76KB)]
Zoom on the internal part of the debris disk around Vega (image 
reconstituted from observations by CHARA/FLUOR). © Olivier Absil, 
Université de Liège.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/apr06/vega-f2.jpg (182KB)]
The CHARA array. © Center for High Angular Resolution Astronomy - Georgia 
State University.