The Keele-Exeter young cluster survey - I. Low-mass pre-main-sequence stars in NGC 2169

We have used R_CI_C CCD photometry from the Isaac Newton telescope and intermediate-resolution spectroscopy from the Gemini North telescope to identify and characterize low-mass (0.15 < M/M_⊙ < 1.3) pre-main-sequence stars in the young open cluster NGC 2169. Isochrone fitting to the high- and low-mass populations yields an intrinsic distance modulus of 10.13^+0.06_-0.09 mag and a model-dependent age of 9 ± 2 Myr. Compared to the nearby, kinematically defined groups of a similar age, NGC 2169 has a large low-mass population which potentially offers a more precise statistical investigation of several aspects of star formation and early stellar evolution. By modelling the distribution of low-mass stars in the I_C versus R_C-I_C diagram, we find that any age spread among cluster members has a Gaussian full width at half-maximum (FWHM) ≤ 2.5 Myr. A young age and a small age spread (<10 Myr) are supported by the lack of significant lithium depletion in the vast majority of cluster members. There is no clear evidence for accretion or warm circumstellar dust in the low-mass members of NGC 2169, bolstering the idea that strong accretion has ceased and inner discs have dispersed in almost all low-mass stars by ages of 10 Myr.