Abstract
ABSTRACT We develop a Bayesian Machine Learning framework called BINGO (Bayesian INference for Galactic archaeOlogy) centred around a Bayesian neural network. After being trained on the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Kepler asteroseismic age data, BINGO is used to obtain precise relative stellar age estimates with uncertainties for the APOGEE stars. We carefully construct a training set to minimize bias and apply BINGO to a stellar population that is similar to our training set. We then select the 17 305 stars with ages from BINGO and reliable kinematic properties obtained from Gaia DR2. By combining the age and chemo-kinematical information, we dissect the Galactic disc stars into three components, namely the thick disc (old, high-[α/Fe], [α/Fe] ≳ 0.12), the thin disc (young, low-[α/Fe]), and the Bridge, which is a region between the thick and thin discs. Our results indicate that the thick disc formed at an early epoch only in the inner region, and the inner disc smoothly transforms to the thin disc. We found that the outer disc follows a different chemical evolution pathway from the inner disc. The outer metal-poor stars only start forming after the compact thick disc phase has completed and the star-forming gas disc extended outwardly with metal-poor gas accretion. We found that in the Bridge region the range of [Fe/H] becomes wider with decreasing age, which suggests that the Bridge region corresponds to the transition phase from the smaller chemically well-mixed thick to a larger thin disc with a metallicity gradient.
Highlights
The Galactic disc is traditionally separated into the geometric thick and thin disc after Gilmore & Reid (1983) found from star counts that the vertical density profile of the Milky Way was better characterized by a superposition of two exponential profiles rather than one
We introduce BINGO that is a Bayesian Machine Learning framework to obtain stellar ages of evolved stars using photometric information from the second data release of the European Space Agency’s (ESA) Gaia mission (Gaia DR2; Gaia Collaboration 2018) and the stellar parameter information from the fourteenth data release of the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE)-2 (Majewski et al 2017)
For a selected type of stars with a G-band magnitude between about 4 and 13 magnitudes, the mean line-of-sight velocities measured with Gaia Radial Velocity Spectrometer (RVS), line-ofsight velocities have been provided in Gaia DR2 (Cropper et al 2018; Sartoretti et al 2018; Katz et al 2019)
Summary
The Galactic disc is traditionally separated into the geometric thick and thin disc after Gilmore & Reid (1983) found from star counts that the vertical density profile of the Milky Way was better characterized by a superposition of two exponential profiles rather than one. Chiappini et al (2001) suggested that the high-[α/Fe], chemically homogenous disc forms early during an intense star formation period dominated by Type II supernovae (SNe II) following a rapid infall of primordial gas. Bekki & Tsujimoto (2011) follow a semi-analytical approach to explain the existence of two distinct populations. In their continuous star formation model, the high-[α/Fe] sequence up to around solar [Fe/H], i.e. the thick disc, forms early during a rapid, intense star formation period. A sequence of increasing [Fe/H] and decreasing [α/Fe] builds up gradually Still, this sequence is lower in [α/Fe] as Type Ia SNe can already enrich the environment at this time. Once star formation reaches its peak and starts decreasing, a sequence with decreasing [α/Fe] and increasing in [Fe/H] follows along with the same low[α/Fe] sequence
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