Red Clump stars from LAMOST II: the outer disc of the Milky Way

Abstract: We present stellar density maps of the Galactic outer disc with red clump stars from the LAMOST data. These samples are separated into younger (mean age ~ 2.7 Gyr) and older (mean age ~ 4.6 Gyr) populations so that they can trace the variation of the structures with ages in the range of the Galactocentric radius R from 9 to 13.5 kpc. We show that both the scale heights for the two populations increase with R and display radial gradients of 48 +/- 6 and 40 +/- 4 pc/kpc for the older and younger populations, respectively. This is evident that the flaring occurs in the thin disc populations with a wide range of ages. Moreover, the intensity of flaring seems not significantly related to the age of the thin disc populations. On the other hand, the scale lengths of the radial surface density profiles are 4.7 +/- 0.5 kpc for the younger and 3.4 +/- 0.2 kpc for the older population, meaning that the younger disc population is more radially extended than the older one. Although the fraction of the younger population mildly increases from 28% at R ~ 9 to about 35% at R ~ 13 kpc, the older population is prominent with the fraction no less than 65% in the outer disc.