Accretion from Circumstellar Discs and the λ Boo PhenomenonMonthly Notices of the Royal Astronomical Society
AbstractThis paper examines the suggestion by Venn & Lambert that the λ 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 λ 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 λ Boo entries in the catalogue and finite circumstellar disc lifetimes. Additionally, it is argued that the amount of accreted depleted gas required to cause λ 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-λ Boo stars showing infrared excesses differ at the 84 per cent confidence level, which is oniy 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 λ Boo stars. These values are, however, oniy weakly constrained. The accretion rates are also consistent with Charbonneau×s independent estimate.
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