Constraints on the gas content of the Fomalhaut debris belt. Can gas-dust interactions explain the belt's morphology?

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 CII 158 $\mu$m and OI 63 $\mu$m 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 $\beta$ Pictoris debris disc. Results: The gas mass is shown to be below the millimetre dust mass by a factor of at least $\sim$3 (for solar abundances) respectively $\sim$300 (for $\beta$ 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.

5 pages, 3 figures, published in A&A; versions 2 and 3: language editing

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Cited by in corpus (5)

• Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO${}_2$ ice abundance in exocomets and Solar System comets
• Simulations of the Fomalhaut System Within Its Local Galactic Environment
• Tidal disruption versus planetesimal collisions as possible origins for the dispersing dust cloud around Fomalhaut
• ALMA imaging of the M-dwarf Fomalhaut C's debris disc
• 94 Ceti: a triple star with a planet and dust disc