The sensitivity of the color J - H to the gas pressure in the photo sphere of M dwarfs permits a rather clear separation of the component populations of the lower main sequence. These are the young contracting stars, the "zero-age" stars, and the oldest metal-deficient halo stars. The separation of these populations in color-magnitude diagrams is not a result of differential line blanketing, as has been suggested, but truly extends to the (Mbol, log Te) plane. Subject headings: infrared: general - stars: late-type - stars: pre-main-sequence - stars: sub dwarfs
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Mould et al. (1976) studied this question.