Herschel/HIFI observations of ionised carbon in the β Pictoris debris disk
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.
Origin : Publication funded by an institution
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