The age-chemical abundance structure of the Galactic disc II:
  $\alpha$-dichotomy and thick disc formation
  We extend our previous work on the age-chemical abundance structure of the
Galactic outer disc to the inner disc (4 < r < 8 kpc) based on the SDSS/APOGEE
survey. Different from the outer disc, the inner disc stars exhibit a clear
bimodal distribution in the [Mg/Fe]-[Fe/H] plane. While a number of scenarios
have been proposed in the literature, it remains challenging to recover this
bimodal distribution with theoretical models. To this end, we present a
chemical evolution model embedding a complex multi-phase inner disc formation
scenario that matches the observed bimodal [Mg/Fe]-[Fe/H] distribution. In this
scenario, the formation of the inner disc is dominated by two main starburst
episodes 6 Gyr apart with secular, low-level star formation activity in
between. In our model, the first starburst occurs at early cosmic times (t~1
Gyr) and the second one 6 Gyr later at a cosmic time of t~7 Gyr. Both these
starburst episodes are associated with gas accretion events in our model, and
are quenched rapidly. The first starburst leads to the formation of the
high-$\alpha$ sequence, and the second starburst leads to the formation of the
metal-poor low-$\alpha$ sequence. The metal-rich low-$\alpha$ stars, instead,
form during the secular evolution phase between the two bursts. Our model shows
that the $\alpha$-dichotomy originates from the rapid suppression of star
formation after the first starburst. The two starburst episodes are likely to
be responsible for the formation of the geometric thick disc (z >1 kpc), with
the old inner thick disc and the young outer thick disc forming during the
first and the second starbursts, respectively.
