Stellar mass distribution and star formation history of the Galactic
  disk revealed by mono-age stellar populations from LAMOST
  We present a detailed determination and analysis of 3D stellar mass
distribution of the Galactic disk for mono-age populations using a sample of
0.93 million main-sequence turn-off and subgiant stars from the LAMOST Galactic
Surveys. Our results show (1) all stellar populations younger than 10\,Gyr
exhibit strong disk flaring, which is accompanied with a dumpy vertical density
profile that is best described by a $sech^n$ function with index depending on
both radius and age; (2) Asymmetries and wave-like oscillations are presented
in both the radial and vertical direction, with strength varying with stellar
populations; (3) As a contribution by the Local spiral arm, the mid-plane
stellar mass density at solar radius but 400--800\,pc (3--6$^\circ$) away from
the Sun in the azimuthal direction has a value of
$0.0594\pm0.0008$\,$M_\odot$/pc$^3$, which is 0.0164\,$M_\odot$/pc$^3$ higher
than previous estimates at the solar neighborhood. The result causes doubts on
the current estimate of local dark matter density; (4) The radial distribution
of surface mass density yields a disk scale length evolving from $\sim$4\,kpc
for the young to $\sim$2\,kpc for the old populations. The overall population
exhibits a disk scale length of $2.48\pm0.05$\,kpc, and a total stellar mass of
$3.6(\pm0.1)\times10^{10}$\,$M_\odot$ assuming $R_{\odot}=8.0$\,kpc, and the
value becomes $4.1(\pm0.1)\times10^{10}$\,$M_\odot$ if $R_{\odot}=8.3$\,kpc;
(5) The disk has a peak star formation rate ({\rm SFR}) changing from 6--8\,Gyr
at the inner to 4--6\,Gyr ago at the outer part, indicating an inside-out
assemblage history. The 0--1\,Gyr population yields a recent disk total {\rm
SFR} of $1.96\pm0.12$\,$M_\odot$/yr.
