The lower main sequence and the atmospheres of M dwarfs

Abstract

A study of the atmospheric parameters of a sample of M dwarfs of different kinematic populations clarifies the separation of the lower main sequence into three groups of different age or composition. These arc: (1) the young, contracting stars, superluminous in the HR diagram and distinguished by their lower gravity, (2) a reference Zero Age Main Sequence of solar composition, (3) the metal deficient stars, subluminous in the HR diagram and distinguished by their metal abundance. These populations are separated by JHK infrared photometry through the sensitivity of the color J-H to the gas pressure in the photosphere of M dwarfs. Supporting spectrophotometric indicators of gravity (CaH bands) and composition (TiO bands) are also calibrated by line blanketed theoretical model atmospheres. It is suggested that this atmospheric basis for the separation of populations is preferable to the corresponding, but more statistical, kinematic basis. The structure of the lower main sequence in the theoretical HR diagram (derived from RIJHKL photometry) is in qualitative agreement with the results of stellar interior calculations. The quantitative predictions are also compared with observations. Finally, a test of the abundance calibration of the TiO bands is made by high dispersion spectroscopy of the nearby M subdwarf, Kapteyn's star. The metal deficiency apparent from lines of Fe I and Ti I concurs with the estimate from the TiO bands. The strength of the Ca II IR triplet is somewhat discrepant, however. Interpretation of the spectra of M dwarfs is severely complicated by the effects of pressure broadening. Howeve r, the TiO bands (and Ca II lines) suggest a wide range of metal abundances among the nearest halo stars . Show more