We present the determination of stellar parameters and individual elemental abundances for 6 million stars from ∼8 million low-resolution (R ∼ 1800) spectra from LAMOST DR5. This is based on a modeling approach that we dub T he Data-Driven P ayne (DD-P ayne), which inherits essential ingredients from both The Payne (Ting et al. 2019) and T he Cannon (Ness et al. 2015). It is a data-driven model that incorporates constraints from theoretical spectral models to ensure the derived abundance estimates are physically sensible. Stars in LAMOST DR5 that are in common with either GALAH DR2 or APOGEE DR14 are used to train a model that delivers stellar parameters (T eff , log g, V mic ) and abundances for 16 elements (C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Ba) when applied to LAMOST spectra. Cross-validation and repeat observations suggest that, for S/N pix ≥ 50, the typical internal abundance precision is 0.03-0.1 dex for the majority of these elements, with 0.2-0.3 dex for Cu and Ba, and the internal precision of T eff and log g is better than 30 K and 0.07 dex, respectively. Abundance systematics at the ∼0.1 dex level are present in these estimates, but are inherited from the high-resolution surveys' training labels. For some elements, GALAH provides more robust training labels, for others, APOGEE. We provide flags to guide the quality of the label determination and to identify binary/multiple stars in LAMOST DR5. The abundance catalogs are publicly accessible via http://dr5.lamost.org/doc/vac.

The rotation curve (RC) of the Milky Way out to ∼ 100 kpc has been constructed using ∼ 16, 000 primary red clump giants (PRCGs) in the outer disk selected from the LSS-GAC and the SDSS-III/APOGEE survey, combined with ∼ 5700 halo K giants (HKGs) selected from the SDSS/SEGUE survey. To derive the RC, the PRCG sample of the warm disc population and the HKG sample of halo stellar population are respectively analyzed using a kinematical model allowing for the asymmetric drift corrections and re-analyzed using the spherical Jeans equation along with measurements of the anisotropic parameter β currently available. The typical uncertainties of RC derived from the PRCG and HKG samples are respectively 5-7 km s −1 and several tens km s −1 . We determine a circular velocity at the solar position, V c (R 0 ) = 240 ± 6 km s −1 and an azimuthal peculiar speed of the Sun, V ⊙ = 12.1 ± 7.6 km s −1 , both in good agreement with the previous determinations. The newly constructed RC has a generally flat value of 240 km s −1 within a Galactocentric distance r of 25 kpc and then decreases steadily to 150 km s −1 at r ∼ 100 kpc. On top of this overall trend, the RC exhibits two prominent localized dips, one at r ∼ 11 kpc and another at r ∼ 19 kpc. From the newly constructed RC, combined with other constraints, we have built a parametrized mass model for the Galaxy, yielding a virial mass of the Milky Way's dark matter halo of 0.90 +0.07 −0.08 × 10 12 M ⊙ and a local dark matter density, ρ ⊙,dm = 0.32 +0.02 −0.02 GeV cm −3 .

We present estimates of stellar age and mass for 0.93 million Galactic disk main sequence turn-off and sub-giant stars from the LAMOST Galactic Spectroscopic Surveys. The ages and masses are determined by matching with stellar isochrones using Bayesian algorithm, utilizing effective temperature T eff , absolute magnitude M V , metallicity Gyr exhibit both the thin and thick disk sequences, while younger (older) stars show only the thin (thick) disk sequence, indicating that the thin disk became prominent 8-10 Gyr ago, while the thick disk formed earlier and almost quenched 8 Gyr ago. Stellar ages exhibit positive vertical and negative radial gradients across the disk, and the outer disk of R 9 kpc exhibits a strong flare in stellar age distribution.