Astronomers observe for the first time how dust forms around an erupting star

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Munich, July 25 : Using the ESO's (European Southern Observatory's) Very Large Telescope Interferometer, astronomers were able for the first time to witness the appearance of a shell of dusty gas around a star that had just erupted, and follow its evolution for more than 100 days. This provides the astronomers with a new way to estimate the distance of this object and obtain invaluable information on the operating mode of dense stars that suck material from a companion.

The star in question is the Nova Scorpii 2007a (or V1280 Scorpii), which was discovered by Japanese amateur astronomers on 4 February 2007 towards the constellation Scorpius ("the Scorpion").

For a few days, it became brighter and brighter, reaching its maximum on 17 February, to become one of the brightest novae of the last 35 years.

At that time, it was easily visible with the unaided eye.

Eleven days after reaching its maximum, astronomers witnessed the formation of dust around the object.

Dust was present for more than 200 days, as the nova only slowly emerged from the smoke between October and November 2007. During these 200 days, the erupting source was screened out efficiently, becoming more than 10,000 times dimmer in the visual.

An unprecedented high spatial resolution monitoring of the dust formation event was carried out with the Very Large Telescope Interferometer (VLTI), extending over more than 5 months following the discovery.

The first observations, secured 23 days after the discovery, showed that the source was very compact, less than 1 thousandth of an arcsecond (1 milli-arcsecond or mas), which is a size comparable to viewing one grain of sand from about 100 kilometres away.

A few days later, after the detection of the major dust formation event, the source measured 13 mas.

"It is most likely that the latter size corresponds to the diameter of the dust shell in expansion, while the size previously measured was an upper limit of the erupting source," explained lead author Olivier Chesneau.

Over the following months, the dusty shell expanded regularly, at a rate close to 2 million km/h.

According to astronomer Dipankar Banerjee from India, "This is the first time that the dust shell of a nova is spatially resolved and its evolution traced starting from the onset of its formation up to the point that it becomes too diluted to be seen."

The measurement of the angular expansion rate, together with the knowledge of the expansion velocity, enables the astronomer to derive the distance of the object, in this case about 5500 light-years.


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