Population and dynamical state of the η Chamaeleontis sparse young open cluster

We report new results in our continuing study of the unique compact (1 pc extent), nearby (d = 97 pc), young (t = 9 Myr) stellar cluster dominated by the B9 star η Chamaeleontis. An optical photometric survey spanning 1.3 × 1.3 pc adds two M5-M5.5 weak-lined T Tauri (WTT) stars to the cluster inventory which is likely to be significantly complete for primaries with masses M > 0.15 M_⊙. The cluster now consists of 17 primaries and ≃9 secondaries lying within 100 au of their primaries. The apparent distribution of 9:7:1 single : binary : triple systems shows 2-4 times higher multiplicity than in the field main-sequence stars, and is comparable to that seen in other pre-main-sequence populations. The initial mass function (IMF) is consistent with that of rich young clusters and field stars. By extending the cluster IMF to lower masses, we predict 10-14 additional low-mass stars with 0.08 < M < 0.15 M_⊙ and 10-15 brown dwarfs with 0.025 < M < 0.08 M_⊙ remain to be discovered. The η Cha cluster extends the established stellar density and richness relationship for young open clusters. The radial distribution of stars is consistent with an isothermal sphere, but mass segregation is present with >50 per cent of the stellar mass residing in the inner 6 arcmin (0.17 pc). Considering that the η Cha cluster is sparse, diffuse and young, the cluster may be an ideal laboratory for distinguishing between mass segregation that is primordial in nature, or arising from dynamical interaction processes.