The transition disc frequency in M stars

Abstract

Abstract We re-examine the recent suggestion of a high fraction of transition discs (i.e. those with a cleared inner hole) in M stars, motivated by the fact that we expect that, for M stars, even discs without inner holes should exhibit very weak excess shortwards of around 10 μm. Our analysis of spectral energy distribution models suggests that this indeed means that M stars where a detectable excess begins at around 6 μm may be misclassified as transition discs when in fact they have optically thick dust extending in to the dust sublimation radius. Consequently, we estimate that the transition disc fraction among M stars in the Coronet cluster is ∼15 ± 10 per cent (rather than the recently claimed value of 50 per cent). This revised figure would imply that the transition disc fraction is not after all markedly higher in later type stars. We suggest that, for M stars, transition discs can only be readily identified if they have emission that is close to photospheric out to >10 μm.