  Discs of dusty debris around main-sequence star indicate fragmentation of
orbiting planetesimals, and for a few A-type stars, a gas component is also
seen that may come from collisionally-released volatiles. Here we find the
sixth example of a CO-hosting disc, around the 30Myr old A0-star HD 32297. Two
more of these CO-hosting stars, HD 21997 and 49 Cet, have also been imaged in
dust with SCUBA-2 within the SONS project. A census of 27 A-type debris hosts
within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5-50
Myr epoch, with no detections in 11 older systems. Such a prolonged period of
high fragmentation rates corresponds quite well to the epoch when most of the
Earth was assembled from planetesimal collisions. Recent models propose that
collisional products can be spatially asymmetric if they originate at one
location in the disc, with CO particularly exhibiting this behaviour as it can
photodissociate in less than an orbital period. Of the six CO-hosting systems,
only beta Pic is in clear support of this hypothesis. However, radiative
transfer modelling with the ProDiMo code shows that the CO is also hard to
explain in a proto-planetary disc context.
