Gas and dust around A-type stars at tens of Myr: Signatures of cometary breakup

Discs of dusty debris around main-sequence stars indicate fragmentation of orbiting planetesimals,and for a few A-type stars, a gas component is also seen that may come fromcollisionally released volatiles. Here we find the sixth example of a CO-hosting disc, aroundthe ~30 Myr-old A0-star HD 32997. Two more of these CO-hosting stars, HD 21997 and 49Cet, have also been imaged in dust with SCUBA-2 within the SCUBA-2 Survey of NearbyStars project. A census of 27 A-type debris hosts within 125 pc now shows 7/16 detections ofcarbon-bearing gas within the 5-50 Myr epoch, with no detections in 11 older systems. Such aprolonged period of high fragmentation rates corresponds quite well to the epoch when most ofthe Earth was assembled from planetesimal collisions. Recent models propose that collisionalproducts can be spatially asymmetric if they originate at one location in the disc, with COparticularly exhibiting this behaviour as it can photodissociate in less than an orbital period. Of the six CO-hosting systems, only β Pic is in clear support of this hypothesis. However,radiative transfer modelling with the ProDiMo code shows that the CO is also hard to explainin a proto-planetary disc context.