Infrared study of the η Chamaeleontis cluster and the longevity of circumstellar discs

We have analysed JHKL observations of the stellar population of the ≈ 9 Myr-old η Chamaeleontis cluster. Using infrared (IR) colour-colour and colour-excess diagrams, we find that the fraction of stellar systems with near-IR excess emission is 0.60 ± 0.13 (2σ). This result implies considerably longer disc lifetimes than found in some recent studies of other young stellar clusters. For the classical T Tauri (CTT) and weak-lined T Tauri (WTT) star population, we also find a strong correlation between the IR excess and Hα emission. The IR excesses of these stars indicate a wide range of star-disc activity: from a CTT star showing high levels of accretion to CTT-WTT transition objects with evidence for some on-going accretion and WTT stars with weak or absent IR excesses. Of the 15 known cluster members, four stars with IR excesses Δ(AT - L) > 0.4 mag are likely experiencing ongoing accretion owing to strong or variable optical emission. The resulting accretion fraction (0.27 ± 0.13; 2σ) shows that the accretion phase, in addition to the discs themselves, can endure for at least ∼10 Myr.