“That is where my dearest and brightest dreams have ranged — to hear for the duration of a heartbeat the universe and the totality of life in its mysterious, innate harmony.” Hermann Hesse

MRI in debris discs

The magneto-rotational instability (MRI) has been proposed to be active in debris-disc gas in our recent paper Kral & Latter (2016). As explained in the gas in planetary system section, a growing number of debris disc host stars are observed to be swathed in gas. The gas detected around beta Pic is highly depleted in hydrogen and mainly composed of atomic carbon and oxygen. As a consequence the gas is highly ionised, suggesting that magnetohydrodynamic (MHD) processes may be important. In the paper shown below, we study whether the MRI could be active in such a debris disc environment. We expected that it may be the case owing to the high ionisation fraction in these gas discs and that it may explain the high alpha required (see here) to explain beta Pic observations. However, the gas is dilute and non-ideal MHD effects could be at play (mainly ambipolar diffusion in this regime).

 

In Kral & Latter (2016), we showed that the MRI could be active in a debris disc like environment such as in the beta Pic system. However, owing to the low gas density in this system, the outer parts of the disc could be stabilised by a weak magnetic field. Even if it is the case, a magnetocentrifugal wind may be launched instead and this could also lead to equivalent (non-turbulent) transport. We find that dust-gas interactions and non-ideal MHD play a subdominant role. This work will soon be complemented by large numerical simulations of the vertically stratified MRI in the debris disc regime to establish the specific behaviour at different radii for a variety of different setups. Stay tuned.