Kappa Coronae Borealis
April 10, 2013

Herschel Eyes Retired Star Surrounded By Dust Belt

Lee Rannals for redOrbit.com - Your Universe Online

The European Space Agency's (ESA) Herschel space observatory has snapped images of a dust belt orbiting a subgiant star that hosts a planetary system.

The space observatory's sensitive far-infrared detection helped astronomers resolve bright emission around Kappa Coronae Borealis, indicating the presence of a dusty debris disc. The star is a little heavier than our own Sun at 1.5 solar masses, is around 2.5 billion years old, and sits about 100 light years away from us.

Kappa Coronae Borealis is known to host one giant planet about twice the mass of Jupiter orbiting at a distance equivalent to the asteroid belt in our own Solar System. Astronomers suspect that a second planet is orbiting the star, but not much is known about it yet.

Herschel's observations provide rare insight into the life of planetary systems orbiting subgiant stars, and enables scientists to perform a detailed study of the architecture of its planet and disc system.

“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.”

A "retired" star is a star that has exhausted its central fuel reserve and starts to burn shells around the core. During this subgiant phase, planets, asteroids and comet belts around these stars are expected to survive.

The team used models to propose three possible configurations for the disc and planets that fit Herschel's observations. The first model has just one continuous dust belt extending from 20 Astronomical Units (AU) and 220 AU. An AU is the distance between Earth and Sun.

“It is a mysterious and intriguing system: is there a planet or even two planets sculpting one wide disc, or does the star have a brown dwarf companion that has split the disc in two?” says Dr Bonsor.

ESA said this is the first known example of a subgiant star with planets and a debris disc orbiting it.

“Thanks to Herschel´s sensitive far-infrared capabilities and its rich dataset, we already have hints of other subgiant stars that may also have dusty discs. More work will be needed to see if they also have planets,” says Göran Pilbratt, ESA´s Herschel project scientist.

Herschel doesn't just fix its sights on old stars, but it has been known to seek out some younger stars as well. A team of astronomers just recently said they used Herschel, along with NASA´s Spitzer Space Telescope and the Atacama Pathfinder Experiment (APEX) telescope in Chile, to reveal some of the youngest stars ever detected before.

REFERENCE: “Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant κ CrB”, by A. Bonsor et al. is published in the Monthly Notices of the Royal Astronomical Society, April 2013.