Masses and Ages for 230,000 LAMOST Giants, via Their Carbon and Nitrogen
  Abundances
  We measure carbon and nitrogen abundances to $\lesssim$ 0.1 dex for 450,000
giant stars from their low-resolution (R$\sim$1800) LAMOST DR2 survey spectra.
We use these [C/M] and [N/M] measurements, together with empirical relations
based on the APOKASC sample, to infer stellar masses and implied ages for
230,000 of these objects to 0.08 dex and 0.2 dex respectively. We use The
Cannon, a data-driven approach to spectral modeling, to construct a predictive
model for LAMOST spectra. Our reference set comprises 8125 stars observed in
common between the APOGEE and LAMOST surveys, taking seven APOGEE DR12 labels
(parameters) as ground truth: Teff, logg, [M/H], [$\alpha$/M], [C/M], [N/M],
and Ak. We add seven colors to the Cannon model, based on the g, r, i, J, H, K,
W1, and W2 magnitudes from APASS, 2MASS & WISE, which improves our constraints
on Teff and logg by up to 20% and on Ak by up to 70%. Cross-validation of the
model demonstrates that, for high-SNR objects, our inferred labels agree with
the APOGEE values to within 50 K in temperature, 0.04 magnitudes in Ak, and <
0.1 dex in logg, [M/H], [C/M], [N/M], and [$\alpha$/M]. We apply the model to
450,000 giants in LAMOST DR2 that have not been observed by APOGEE. This
demonstrates that precise individual abundances can be measured from
low-resolution spectra, and represents the largest catalog of [C/M], [N/M],
masses and ages to date. As as result, we greatly increase the number and sky
coverage of stars with mass and age estimates.
