ESA's Herschel space observatory has provided the first images of a dust belt - produced by colliding comets or asteroids - orbiting a subgiant star known to host a planetary system.
After billions of years steadily burning hydrogen in their cores, stars like our Sun exhaust this central fuel reserve and start burning it in shells around the core. They swell to become subgiant stars, before later becoming red giants.
At least during the subgiant phase, planets, asteroids and comet belts around these 'retired' stars are expected to survive, but observations are needed to measure their properties. One approach is to search for discs of dust around the stars, generated by collisions between populations of asteroids or comets.
Thanks to the sensitive far-infrared detection capabilities of the Herschel space observatory, astronomers have been able to resolve bright emission around Kappa Coronae Borealis (K CrB, or Kappa Cor Bor), indicating the presence of a dusty debris disc.
Kappa Coronae Borealis, based on Herschel PACS observations at 100 µm. North is up and east is left. The star is in the centre of the frame (not visible in this graphic) with an excess of infrared emission detected around it, interpreted as a dusty debris disc containing asteroids and/or comets. The inclination of the planetary system is constrained at an angle of 60° from face-on. Copyright: ESA/Bonsor et al (2013)
"This is the first 'retired' star that we have found with a debris disc and one or more planets," says Amy Bonsor of the Institute de Planétologie et d'Astrophysique de Grenoble, and lead author of the study.
"The disc has survived the star's entire lifetime without being destroyed. That's very different to our own Solar System, where most of the debris was cleared away in a phase called the Late Heavy Bombardment era, around 600 million years after the Sun formed."
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"Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant K CrB", par A. Bonsor et al. publiée dans Monthly Notices of the Royal Astronomical Society, Avril 2013.