Herschel Observes Dust Belt around Kappa Coronae Borealis

Astronomers using ESA’s Herschel Space Observatory have captured the first images of a dust belt orbiting a subgiant star known to host a planetary system.

Kappa Coronae Borealis, based on Herschel observations at 100 μm (ESA / Bonsor et al)

Kappa Coronae Borealis, based on Herschel observations at 100 μm (ESA / Bonsor et al)

“This is the first ‘retired’ star that we have found with a debris disc and one or more planets,” said Dr Amy Bonsor of the Institute de Planétologie et d’Astrophysique de Grenoble, lead author of the study accepted for publication in the Monthly Notices of the Royal Astronomical Society (arXiv.org version).

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, indicating the presence of a dusty debris disc.

Kappa Coronae Borealis is a little heavier than our own Sun at 1.5 solar masses, is around 2.5 billion years old and lies at a distance of about 100 light years.

From ground-based observations, it is known to host one giant planet roughly twice the mass of Jupiter orbiting at a distance equivalent to the Asteroid Belt in our own Solar System. A second planet is suspected, but its mass is not well constrained.

Herschel’s detection provides rare insight into the life of planetary systems orbiting subgiant stars, and enables a detailed study of the architecture of its planet and disc system.

“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,” Dr Bonsor said.

Possible configurations of the Kappa Coronae Borealis planetary system. The top panel illustrates the possibility that the second companion lies interior to the observed dusty material, that lies either in a single wide belt or is split into two narrow belts. The middle panel illustrates the possibility that the outer companion is in fact a binary and orbits between the two narrow dust belts. In the latter scenario the outer dusty belt would be a circumbinary debris disc. The lower panel illustrates the stirring model, in which the rate of dust production is maximum at 80AU (Bonsor et al)

Possible configurations of the Kappa Coronae Borealis planetary system. The top panel illustrates the possibility that the second companion lies interior to the observed dusty material, that lies either in a single wide belt or is split into two narrow belts. The middle panel illustrates the possibility that the outer companion is in fact a binary and orbits between the two narrow dust belts. In the latter scenario the outer dusty belt would be a circumbinary debris disc. The lower panel illustrates the stirring model, in which the rate of dust production is maximum at 80AU (Bonsor et al)

The team used models to propose three possible configurations for the disc and planets that fit Herschel’s observations of Kappa Coronae Borealis.

The first model has just one continuous dust belt extending from 20 AU to 220 AU (Astronomical Units). In this model, one of the planets orbits at a distance of greater than 7 AU from the star, and its gravitational influence may sculpt the inner edge of the disc.

A variation on this model has the disc being stirred by the gravitational influence of both companions, mixing it up such that the rate of dust production in the disc peaks at around 70 – 80 AU from the star.

In another interesting scenario, the dust disc is divided into two narrow belts, centered on 40 AU and 165 AU, respectively. Here, the outermost companion may orbit between the two belts between a distance of about 7 AU and 70 AU, opening the possibility of it being rather more massive than a planet, possibly a substellar brown dwarf.

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Bibliographic information: Amy Bonsor et al. 2013. Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant Kappa CrB. Mon. Not. R. Astron. Soc., in press; arXiv: 1302.7000