Chemical Properties of the Local Galactic Disk and Halo. I. Fundamental
  Properties of 1,544 Nearby, High Proper-Motion M dwarfs and subdwarfs
  Large numbers of low-to-medium resolution spectra of M-type dwarf stars from
both the local Galactic disk and halo are available from various surveys. In
order to fully exploit these data, we develop a template-fit method using a set
of empirically assembled M dwarf/subdwarf classification templates, based on
the measurements of the TiO and CaH molecular bands, to classify M
dwarfs/subdwarfs by spectral type and metallicity class. We further present a
pipeline to automatically determine the effective temperature, metallicity
([M/H]), alpha-element to iron abundance ratio ([alpha/Fe]), and surface
gravity of M dwarfs/subdwarfs using the latest version of BT-Settl model
atmospheres. We apply these methods to a set of low-to-medium resolution
optical spectra of 1,544 high proper-motion (> 0.4"/yr) M dwarfs/subdwarfs. Our
metallicity estimates appear to be consistent with the expected color-magnitude
variation of stars relative to atmospheric composition, as our sample shows a
clear stratification with respect to metallicity in the Gaia H-R diagram.
Furthermore, the measured chemical parameters of the two components in 48
binary systems are in a good agreement with each other, which suggest a
precision of 0.22 dex in [M/H], 0.08 dex in [alpha/Fe], and 0.16 dex in the
combined index [alpha/Fe]+[M/H]. There is also a good consistency between
metallicity class, obtained from the empirical classification templates, and
the index [alpha/Fe]+[M/H] (~ [alpha/H]), obtained from model-fitting, which
means that the more easily measured metallicity class can be used as a
relatively reliable indicator of absolute alpha-element abundance, [alpha/H],
in low-mass stars. Finally, we examine the distribution of our stars in the
[alpha/Fe] vs. [M/H] diagram, which shows evidence of clustering in chemical
abundance makeup, suggestive of discrete populations among the local disk and
halo stars.
