2014A&A...563A..66C

The dusty debris disk around the ∼20Myr 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. Our goal is to constrain the spatial distribution of C in the disk, and thereby the gas origin and its abundance pattern. We used the HIFI instrument on board the Herschel Space Observatory to observe and spectrally resolve CII emission at 158µm 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. We detect the CII 158µm emission. Modelling the shape of the emission line shows that most of the gas is located at about ∼100AU or beyond. We estimate a total C gas mass of 1.3_–0.5^+1.3x10^–2M_⊕ (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. 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 CI emission at 609µm.