Some anomalies in the occurrence of debris discs around main-sequence A and G stars

Debris discs consist of large dust grains that are generated by collisions of comets or asteroids around main-sequence stars, and the quantity and distribution of debris may be used to detect the presence of perturbing planets akin to Neptune. We use stellar and disc surveys to compare the material seen around A- and G-type main-sequence stars. Debris is detected much more commonly towards A stars, even when a comparison is made only with G stars of comparable age. Detection rates are consistent with disc durations of similar to0.5 Gyr, which may occur at any time during the main sequence. The higher detection rate for A stars can result from this duration being a larger fraction of the main-sequence lifetime, possibly boosted by a globally slightly larger disc mass than for the G-type counterparts. The disc mass range at any given age is a factor of at least similar to100 and any systematic decline with time is slow, with a power law estimated to not be steeper than t(-1/2). Comparison with models shows that dust can be expected as late as a few Gyr when perturbing planetesimals form slowly at large orbital radii. Currently, the Solar system has little dust because the radius of the Kuiper Belt is small and hence the time-scale to produce planetesimals was less than 1 Gyr. However, the apparently constant duration of similar to0.5 Gyr when dust is visible is not predicted by the models.