ACCRETION FROM CIRCUMSTELLAR DISKS AND THE LAMBDA-BOO PHENOMENON

This paper examines the suggestion by Venn & Lambert that the lambda Boo phenomenon may be caused by accretion of depleted circumstellar gas. The IRAS data base was searched for sources corresponding to those in a catalogue of over 100 lambda Boo candidates. A relatively small fraction (< 20 per cent) of the catalogue stars that were detected by IRAS show infrared excesses like those seen in other early-type stars. However, this is expected, due to non-genuine lambda Boo entries in the catalogue and finite circumstellar disc lifetimes. Additionally, it is argued that the amount of accreted depleted gas required to cause lambda Boo abundance patterns is so small that any associated dust is not necessarily detectable by thermal emission in the infrared or submillimetre regions. The proportions of bona fide and non-lambda Boo stars showing infrared excesses differ at the 84 per cent confidence level, which is only suggestive evidence for accretion. A simple model, where accretion of depleted gas once in a circumstellar disc's 'central hole' occurs, gives accretion rates and masses consistent with semi-empirical values inferred from observed abundances in lambda Boo stars. These values are, however, only weakly constrained. The accretion rates are also consistent with Charbonneau's independent estimate.