Astronomers spot supergiant smoking star

An international team of French and Brazilian astronomers have using the European Southern Observatorys (ESOs) Very Large Telescope Interferometer, detected a huge cloud of dust around a star.

Munich, Aug 4 : An international team of French and Brazilian astronomers have using the European Southern Observatory's (ESO's) Very Large Telescope Interferometer, detected a huge cloud of dust around a star.

Researchers say this observation is further evidence for the theory that such stellar puffs are the cause of the repeated extreme dimming of the supergiant R Coronae Borealis stars.

These supergiants exhibit erratic variability. Named after the first star that showed such behaviour, they are more than 50 times larger than our Sun. Their apparent brightness can unpredictably decline to a thousandth of their nominal value within a few weeks, with the return to normal light levels being much slower.

It has been accepted for decades that such fading could be due to obscuration of the stellar surface by newly formed dusty clouds.

This 'Dust Puff Theory' suggests that mass is lost from the R Coronae Borealis star and then moves away until the temperature is low enough for carbon dust to form. If the newly formed dust cloud is located along the line-of-sight with Earth, it eclipses the star. As the dust is blown away by the star's strong light, the 'curtain' vanishes and the star reappears again.

In 2004, Patrick de Laverny and Djamel M�karnia had been able to detect the presence of clouds around the star RY Sagittarii - the brightest member in the southern hemisphere of this family of weird stars.

Located about 6,000 light-years away towards the constellation of Sagittarius (The Archer), its peculiar nature was discovered in 1895 by famous Dutch astronomer Jacobus Cornelius Kapteyn. The team found the first direct confirmation of the standard scenario explaining the light variations of R Coronae Borealis stars by the presence of heterogeneities enveloping the star.

Further analysis with the VLT revealed a huge envelope, about 120 times as big as RY Sagittarii itself, surrounding the supergiant star.

More importantly, the astronomers also found evidence for a dusty cloud lying only about 30 astronomical units away from the star, or 100 times the radius of the star.

"This is the closest dusty cloud ever detected around a R CrB-type variable since our first direct detection in 2004. However, it is still detected too far away from the star to distinguish between the different scenarios proposed within the Dust Puff Theory for the possible locations in which the dusty clouds form," said Patrick de Laverny, leader of the team.

The astronomers are now planning to monitor RY Sagittarii more carefully to shed more light on the evolution of the dusty clouds surrounding it.

The study "A snapshot of the inner dusty regions of a R CrB-type variable", appears in the Astronomy and Astrophysics journal.

ANI