2011MNRAS.411.2186S

We present Spitzer IRAC and MIPS 24-µm imaging of members of the 27±5 Myr old open cluster IC 4665. Models for the assembly of terrestrial planets through planetesimal collisions and mergers predict episodic dust debris discs at this epoch. We determine that 42⁺¹⁸_–13 per cent of the solar-type (F5-K5) cluster members have excess emission at 24 µm indicative of these debris discs, the highest frequency of the clusters studied with Spitzer to date. The majority of these discs have intermediate levels of excess (F₂₄/F_phot< 2), and no source is found to have extreme levels of excess indicative of a recent transient event as opposed to steady-state collisional evolution. We find no evidence of a link between multiplicity and 24-µm excess in this cluster sample. Only the early-type star TYC424-473-1 (T_eff∼ 8420 K) has significant near-infrared excess from 4.5 µm as measured with IRAC. Two solar-type targets have low-significance 8-µm excess but no significant 24-µm excess. All other targets show no evidence for near-infrared excess which could indicate the presence of an optically thick primordial disc, demonstrating that the observed 24-µm excess arises from a debris disc.