A large and pristine sample of standard candles across the Milky Way:
  ~100,000 red clump stars with 3% contamination
  Core helium-burning red clump (RC) stars are excellent standard candles in
the Milky Way. These stars may have more precise distance estimates from
spectrophotometry than from Gaia parallaxes beyond 3 kpc. However, RC stars
have $T_{\rm eff}$ and log g very similar to some red giant branch (RGB) stars.
Especially for low-resolution spectroscopic studies where $T_{\rm eff}$, log g,
and [Fe/H] can only be estimated with limited precision, separating RC stars
from RGB through established method can incur ~20% contamination. Recently,
Hawkins et al. (2018) demonstrated that the additional information in
single-epoch spectra, such as the C/N ratio, can be exploited to cleanly
differentiate RC and RGB stars. In this second paper of the series, we
establish a data-driven mapping from spectral flux space to independently
determined asteroseismic parameters, the frequency and the period spacing. From
this, we identify 210,371 RC stars from the publicly available LAMOST DR3 and
APOGEE DR14 data, with ~9% of contamination. We provide an RC sample of 92,249
stars with a contamination of only ~3%, by restricting the combined analysis to
LAMOST stars with ${\rm S/N}_{\rm pix}$ > 75. This demonstrates that high-S/N,
low-resolution spectra covering a broad wavelength range can identify RC
samples at least as pristine as their high-resolution counterparts. As coming
and ongoing surveys such as TESS, DESI, and LAMOST will continue to improve the
overlapping training spectroscopic-asteroseismic sample, the method presented
in this study provides an efficient and straightforward way to derive a vast
yet pristine RC stars to reveal the 3D structure of the Milky Way.
