Star caught smoking

Aug 03, 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: Chandra X-ray Observatory finds planet that makes star act deceptively old

add to favorites email to friend print save as pdf

Related Stories

Solar system simulation reveals planetary mystery

Sep 08, 2014

When we look at the Solar System, what clues show us how it formed? We can see pieces of its formation in asteroids, comets and other small bodies that cluster on the fringes of our neighborhood (and sometimes, ...

Image: Rosetta comet observed with Very Large Telescope

Sep 08, 2014

( —Since early August 2014, Rosetta has been enjoying a close-up view of comet 67P/Churyumov–Gerasimenko. Meanwhile, astronomers on Earth have been busy following the comet with ground-based ...

Cosmic forecast: Dark clouds will give way to sunshine

Sep 03, 2014

Lupus 4, a spider-shaped blob of gas and dust, blots out background stars like a dark cloud on a moonless night in this intriguing new image. Although gloomy for now, dense pockets of material within clouds ...

Mysteries of space dust revealed

Aug 29, 2014

The first analysis of space dust collected by a special collector onboard NASA's Stardust mission and sent back to Earth for study in 2006 suggests the tiny specks open a door to studying the origins of the ...

Spitzer telescope witnesses asteroid smashup

Aug 28, 2014

( —NASA's Spitzer Space Telescope has spotted an eruption of dust around a young star, possibly the result of a smashup between large asteroids. This type of collision can eventually lead to the ...

Recommended for you

Mystery of rare five-hour space explosion explained

2 hours ago

Next week in St. Petersburg, Russia, scientists on an international team that includes Penn State University astronomers will present a paper that provides a simple explanation for mysterious ultra-long gamma-ray ...

Glowing galaxies in telescopic timelapse

2 hours ago

We often speak of the discoveries and data flowing from astronomical observatories, which makes it easy to forget the cool factor. Think of it—huge telescopes are probing the universe under crystal-clear ...

Violent origins of disc galaxies probed by ALMA

9 hours ago

For decades scientists have believed that galaxy mergers usually result in the formation of elliptical galaxies. Now, for the the first time, researchers using the Atacama Large Millimeter/sub-millimeter ...

User comments : 0