Herschel/HIFI observations of ionised carbon in the β Pictoris debris disk^⋆

¹ AlbaNova University Centre, Stockholm UniversityDepartment of Astronomy, 106 91 Stockholm, Sweden
e-mail: gianni.cataldi@astro.su.se
² Stockholm University Astrobiology Centre, 106 91 Stockholm, Sweden
³ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 124 89 Berlin, Germany
⁶ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁸ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁹ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁰ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹¹ Department of Astronomy and Astrophysics, University of Toronto, Toronto ON M5S 3H4, Canada

Received: 25 November 2013
Accepted: 29 January 2014

Abstract

Context. The dusty debris disk around the ~20 Myr old main-sequence A-star β Pictoris is known to contain gas. Evidence points towards a secondary origin of the gas as opposed to being a direct remnant from the initial protoplanetary disk, although the dominant gas production mechanism is so far not identified. The origin of the observed overabundance of C and O compared with solar abundances of metallic elements such as Na and Fe is also unclear.

Aims. Our goal is to constrain the spatial distribution of C in the disk, and thereby the gas origin and its abundance pattern.

Methods. We used the HIFI instrument on board the Herschel Space Observatory to observe and spectrally resolve C ii emission at 158 μm from the β Pic debris disk. Assuming a disk in Keplerian rotation and a model for the line emission from the disk, we used the spectrally resolved line profile to constrain the spatial distribution of the gas.

Results. We detect the C ii 158 μm emission. Modelling the shape of the emission line shows that most of the gas is located at about ~100 AU or beyond. We estimate a total C gas mass of 1.3_-0.5^+1.3 × 10^-2 M_⊕ (central 90% confidence interval). The data suggest that more gas is located on the south-west side of the disk than on the north-east side. The shape of the emission line is consistent with the hypothesis of a well mixed gas (constant C/Fe ratio throughout the disk). Assuming instead a spatial profile expected from a simplified accretion disk model, we found it to give a significantly poorer fit to the observations.

Conclusions. Since the bulk of the gas is found outside 30 AU, we argue that the cometary objects known as “falling evaporating bodies” are probably not the dominant source of gas; production from grain-grain collisions or photodesorption seems more likely. The incompatibility of the observations with a simplified accretion disk model might favour a preferential depletion explanation for the overabundance of C and O, although it is unclear how much this conclusion is affected by the simplifications made. More stringent constraints on the spatial distribution will be available from ALMA observations of C i emission at 609 μm.