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

We have used RCIC 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-circle dot < 1.3) pre-main-sequence stars in the young open cluster NGC 2169. Isochrone(-0.09)(+0.06) fitting to the high- and low-mass populations yields an intrinsic distance modulus of 10.13 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 IC 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.