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Double trouble
for a dusty star

KEITH COOPER
ASTRONOMY NOW

Posted: August 05, 2009


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These dramatic new pictures taken by telescopes at the European Southern Observatory (ESO) in Chile show a special type of giant, dust-emitting star in the Southern Hemisphere constellation of Carina the Keel. The unprecedented clarity of the images has revealed the star in question to not be just one object, but two.

The wide-angle image of HD 87643, and its surrounding nebula. Note the dark nebula on the left of HD 87643. Image: ESO/F Millour et al.

The star is called HD 87643, and we call its kind B[e] stars. These are spectral type B stars on the Hertzsprung–Russell diagram, which categorises stars based on their temperature (indicated by their colour) and their luminosity. B-type stars have surface temperatures in the region of 20,000 degrees Celsius, so they are about four times as hot as the Sun. In the case of HD 87643 and its kind, the addition of an [e] in the name tells us that the star exhibits ‘forbidden’ emission lines of hydrogen. They’re called forbidden lines because they do not occur on Earth, but in space conditions are different.

A close-up of HD 87643Ős nebula, indicating the arcs produced by frequent outbursts of material. Image: ESO/F Millour et al.

With regards to B[e] stars like HD 87643, the emission lines are indicative of some kind of shroud of gas and dust encasing the star, probably ejected by the giant star itself during many outbursts. In particular, HD 87643 is notable for the most extreme infrared radiation emanating from any B[e] star, in the parts of the nebula where the temperature is cool enough for dust grains to form. HD 87643 has also been fading since 1980, from magnitude +8.5 to magnitude +9.4 today, while it also undergoes short term variations of half a magnitude or so every few months.

A team of European astronomers led by Florentin Millour of the Max Planck Institute in Germany appropriated the Wide Field Imaging instrument on the 2.2 metre telescope at ESO’s La Silla Observatory to produce one of the best images ever taken of a B[e] star’s glorious, cavernous nebula of gas and surrounding dust, as you can see here. It shows details in the morphology of the nebula, where arcs and streamers of gas and dust emitted by outbursts from the star, and powerful stellar radiation sculpts the inner walls of the nebula like a desert wind sculpting rock. But explaining the presence of these arcs is problematic – what causes their distinct shape and regular spacing? On closer inspection, the team found evidence for regular outbursts of matter from the star. Some arcs of material emanating from the star showed a frequency of every 15 years, whilst a second series of streamers of gas and dust imply a period with an upper limit of 50 years.

The resolved binary star system of HD 87643. Image: ESO/F Millour et al.

In order to get to the bottom of this mystery Millour’s team had to go to another of ESO’s observatories at Paranal in Chile to use the Very Large Telescope Interferometer. An interferometer combines the light from more than one telescope, giving a total resolving power equivalent to a telescope many times the size of the individual optics. In addition to the adaptive optics on the Very Large Telescope, which compensates for the blurring effect of Earth’s atmosphere, the team were able to take images of HD 87643 with the same clarity as if they had been in space. They were able to resolve HD 87643 into two stars of a binary system – at least one of them being a giant star – separated by 51 astronomical units (i.e 51 times the distance between Earth and the Sun, about 7.6 billion kilometres). The periods of the outbursts from the larger B[e] star may be linked to the period of the orbit of the binary system, which must be somewhere between 15 and 50 years. Every time the companion moves in close to the main star, it stirs up activity on its surface, causing an outburst.

The new images also show additional detail in the surrounding, large scale nebula. On the left of HD 87643 is a dark nebula blotting out the light from some of the stars behind, with only a few foreground stars scattered in front of it. This dust may have been emitted by the binary system, and there are hints of further detail in the shape of the far side of the nebula, possibly with bipolar lobe. The question now is, do all B[e] stars have undetected companion stars?

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