The scale height of low mass stars in the galactic disc: evidence for a young population

Samples of M stars have been obtained in the direction of the SGP and at galactic latitude b=-470 from R and I band UK Schmidt plates. The absolute magnitudes MR were obtained from (R-I) colour; and the space density evaluated as a function of height above the galactic plane, for a range of luminosities MR=9-14. This was then used to obtain the relation between scale height and absolute magnitude. Combining the data with previously published results for more massive stars shows a steady decline in the scale height from a maximum around 300 pc for early M stars down to 80 pc for the least luminous stars in the sample. This result is interpreted as implying a small age of 5 x 108 yr for these low luminosity stars, an age which is supported by the small velocity dispersion found in earlier studies. (author)