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

TL;DR

This study identifies and analyzes gas and dust signatures around A-type stars aged 5-50 million years, highlighting cometary breakup as a key process in debris disc evolution and planet formation.

Contribution

It presents new observations of CO gas around A-type stars and discusses the implications for debris disc dynamics and planetesimal collisions.

Findings

7 out of 16 young A-type stars show carbon-bearing gas detections.

Gas detections are absent in systems older than 50 Myr.

CO gas may originate from cometary breakup rather than protoplanetary discs.

Abstract

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.