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 values of T eff and $\mathrm{log}g$ that are very similar to some red giant branch (RGB) stars. Especially for low-resolution spectroscopic studies where T eff, $\mathrm{log}g$, and [Fe/H] can only be estimated with limited precision, separating RC stars from RGB through established methods can incur ~20% contamination. Recently, Hawkins et al. 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 92249 stars with a contamination of only ~3%, by restricting the combined analysis to LAMOST stars with S/Npix ≥ 75. This demonstrates that high-signal-to-noise ratio (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 sample of RC stars to reveal the three-dimensional (3D) structure of the Milky Way. ; Y.S.T. is grateful to be supported by the Martin A. and Helen Chooljian Membership from the Institute for Advanced Study in Princeton, the joint Carnegie-Princeton Fellowship from Princeton University and Carnegie Observatories and the Australian Research Council Discovery Program DP160103747. K.H. is funded by the Simons Foundation Society of Fellows and the Flatiron Institute Center for Computational Astrophysics in New York City. H.W.R.'s research contribution is supported by the European Research Council under the European Union's Seventh Framework Programme (FP 7) ERC Grant Agreement n., [321035] and by the DFG's SFB-881 (A3) Program
