Constraints on the gas content of the Fomalhaut debris belt

Can gas-dust interactions explain the belt’s morphology?^⋆,⋆⋆

¹ AlbaNova University Centre, Stockholm University, Department of Astronomy, 106 91 Stockholm, Sweden
e-mail: gianni.cataldi@astro.su.se
² Stockholm University Astrobiology Centre, 106 91 Stockholm, Sweden
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ ALMA SCO, Alonso de Cordova 3107, Vitacura, Casilla 763 0355, Santiago, Chile
⁵ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁶ Goddard Center for Astrobiology, Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Institute of Astronomy KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 12 November 2014
Accepted: 18 December 2014

Abstract

Context. The 440 Myr old main-sequence A-star Fomalhaut is surrounded by an eccentric debris belt with sharp edges. This sort of a morphology is usually attributed to planetary perturbations, but the orbit of the only planetary candidate detected so far, Fomalhaut b, is too eccentric to efficiently shape the belt. Alternative models that could account for the morphology without invoking a planet are stellar encounters and gas-dust interactions.

Aims. We aim to test the possibility of gas-dust interactions as the origin of the observed morphology by putting upper limits on the total gas content of the Fomalhaut belt.

Methods. We derive upper limits on the C ii 158 μm and O i 63 μm emission by using non-detections from the Photodetector Array Camera and Spectrometer (PACS) onboard the Herschel Space Observatory. Line fluxes are converted into total gas mass using the non-local thermodynamic equilibrium (non-LTE) code radex. We consider two different cases for the elemental abundances of the gas: solar abundances and abundances similar to those observed for the gas in the β Pictoris debris disc.

Results. The gas mass is shown to be below the millimetre dust mass by a factor of at least ~3 (for solar abundances) respectively ~300 (for β Pic-like abundances).

Conclusions. The lack of gas co-spatial with the dust implies that gas-dust interactions cannot efficiently shape the Fomalhaut debris belt. The morphology is therefore more likely due to a yet unseen planet (Fomalhaut c) or stellar encounters.