Despite being > 10 Myr , there are \sim 10 debris discs with as much CO gas as in protoplanetary discs .
Such discs have been assumed to be “ hybrid ” , i.e. , with secondary dust but primordial gas .
Here we show that both the dust and gas in such systems could instead be secondary , with the high CO content caused by accumulation of neutral carbon ( C ^ { 0 } ) that shields CO from photodissociating ; i.e. , these could be “ shielded secondary discs ” .
New ALMA observations are presented of HD131835 that detect \sim 3 \times 10 ^ { -3 } M _ { \oplus } of C ^ { 0 } , the majority 40-200au from the star , in sufficient quantity to shield the previously detected CO. A simple semi-analytic model for the evolution of CO , C and O originating in a volatile-rich planetesimal belt shows how CO shielding becomes important when the viscous evolution is slow ( low \alpha parameter ) and/or the CO production rate is high .
Shielding by C ^ { 0 } may also cause the CO content to reach levels at which CO self-shields , and the gas disc may become massive enough to affect the dust evolution .
Application to the HD 131835 observations shows these can be explained if \alpha \sim 10 ^ { -3 } ; an inner cavity in C ^ { 0 } and CO may also mean the system has yet to reach steady state .
Application to other debris discs with high CO content finds general agreement for \alpha = 10 ^ { -3 } to 0.1 .
The shielded secondary nature of these gas discs can be tested by searching for C ^ { 0 } , as well as CN , N _ { 2 } and CH ^ { + } , which are also expected to be shielded by C ^ { 0 } .