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

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Abstract
Samples of M stars have been obtained in the direction of the SGP and at galactic latitude b=−47° 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×l08 yr for these low luminosity stars, an age which is supported by the small velocity dispersion found in earlier studies. If these stars are indeed young, it is argued that they are probably ‘brown dwarfs’ which have failed to ignite hydrogen and are cooling rapidly to very low luminosities.

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