Star caught smoking

August 3, 2007
Star caught smoking
Artist rendering of the surroundings of a R Coronae Borealis star, as inferred from the observations obtained with ESO's Very Large Telescope. Such stars show erratic variability that is thought to arise from the presence of large clouds of dust in their envelope. Credit: ESO

Using ESO's Very Large Telescope Interferometer, astronomers from France and Brazil have detected a huge cloud of dust around a star. This observation is further evidence for the theory that such stellar puffs are the cause of the repeated extreme dimming of the star.

R Coronae Borealis stars are supergiants exhibiting erratic variability. Named after the first star that showed such behaviour, they are more than 50 times larger than our Sun. R Coronae Borealis stars can see their apparent brightness 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 (or R CrB for short) 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 our line-of-sight, it eclipses the star. As the dust is blown away by the star's strong light, the 'curtain' vanishes and the star reappears.

RY Sagittarii is 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.

In 2004, near-infrared adaptive optics observations made with NACO on ESO's Very Large Telescope allowed astronomers Patrick de Laverny and Djamel Mékarnia to clearly detect the presence of clouds around RY Sagittarii. This was the first direct confirmation of the standard scenario explaining the light variations of R CrB stars by the presence of heterogeneities in their envelope surrounding the star.

However, the precise place where such dust clouds would form was still unclear. The brightest cloud detected was several hundred stellar radii from the centre, but it had certainly formed much closer. But how much closer?

To probe the vicinity of the star, the astronomers then turned to ESO's Very Large Telescope Interferometer. Combining two different pairs of the 8.2-m Unit Telescopes, the astronomers explored the inner 110 astronomical units around the star. Given the remoteness of RY Sagittarii, this corresponds to looking at details on a one-euro coin that is about 75 km away!

The astronomers found that a huge envelope, about 120 times as big as RY Sagittarii itself, surrounds the supergiant star. But 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," says Patrick de Laverny, leader of the team. "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."

If the cloud moves at the speed of 300 km/s, as one can conservatively assume, it was probably ejected more than 6 months before its discovery from deeper inside the envelope. The astronomers are now planning to monitor RY Sagittarii more carefully to shed more light on the evolution of the dusty clouds surrounding it.

"Two hundred years after the discovery of the variable nature of R CrB, many aspects of the R CrB phenomenon remain mysterious," concludes de Laverny.

Citation: "A snapshot of the inner dusty regions of a R CrB-type variable", by I.C. Leão et al., Astronomy and Astrophysics 466, L1-L4, 2007

Source: European Southern Observatory

Explore further: ALMA finds massive primordial galaxies swimming in vast ocean of dark matter

Related Stories

Next generation astronomical survey to map the entire sky

November 16, 2017

The next generation of the Sloan Digital Sky Survey (SDSS-V), directed by Juna Kollmeier of the Carnegie Institution for Science, will move forward with mapping the entire sky following a $16 million grant from the Alfred ...

Recommended for you

Eclipse 2017: Science from the moon's shadow

December 11, 2017

On Dec. 11, 2017, six researchers discussed initial findings based on observations of the Sun and on Earth gathered during the solar eclipse that stretched across North America on Aug. 21, 2017. Ranging from new information ...

Unravelling the mysteries of extragalactic jets

December 11, 2017

University of Leeds researchers have mathematically examined plasma jets from supermassive black holes to determine why certain types of jets disintegrate into huge plumes.

The initial mass function

December 11, 2017

The gas and dust in giant molecular clouds gradually come together under the influence of gravity to form stars. Precisely how this occurs, however, is incompletely understood. The mass of a star, for example, is by far the ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.