Context. The dusty debris disk around the ~20 Myr old main-sequence A-star beta 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 mum from the beta 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 mum 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 mum.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.