ALMA imaging of the M-dwarf Fomalhaut C's debris disc

FomalhautC (LP876-10) is a low mass M4V star in the intriguing Fomalhaut triple system and, like FomalhautA, possesses a debris disc. It is one of very few nearby M-dwarfs known to host a debris disc and of these has by far the lowest stellar mass. We present new resolved observations of the debris disc around Fomalhaut C with the Atacama Large Millimetre Array that allow us to model its properties and investigate the system's unique history. The ring has a radius of 26 au and a narrow full width at half-maximum of at most 4.2 au. We find a 3 sigma upper limit on the eccentricity of 0.14, neither confirming nor ruling out previous dynamic interactions with FomalhautA that could have affected Fomalhaut C's disc. We detect no (CO)-C-12 J = 3-2 emission in the system and do not detect the disc in scattered light with HST/STIS or VLT/SPHERE. We find the original Herschel detection to be consistent with our ALMA model's radial size. We place the disc in the context of the wider debris disc population and find that its radius is as expected from previous disc radius-host luminosity trends. Higher signal-to-noise observations of the system would be required to further constrain the disc properties and provide further insight to the history of the Fomalhaut triple system as a whole.

Automated summary

FomalhautC, a low mass M4V star in the Fomalhaut triple system, has a debris disc like FomalhautA. A new study using observations from ALMA has revealed the properties of the debris disc, including its radius and width. The study did not confirm nor rule out previous dynamic interactions with FomalhautA that could have affected the disc around FomalhautC. Further observations with higher signal-to-noise ratio are needed to better understand the history of the Fomalhaut triple system.