A Blueprint for the Milky Way's Stellar Populations. IV. Spectrum-based Empirical Corrections and the Galactic Starburst Sequence 后印本

摘要: We continue our series of papers on phase-space distributions of stars in the Milky Way based on photometrically derived metallicities and Gaia astrometry, with a focus on the halo-disk interface in the local volume. To exploit various photometric databases, we develop a method of empirically calibrating synthetic stellar spectra based on a comparison with observations of stellar sequences and individual stars in SDSS, SMSS, and PS1, overcoming band-specific corrections employed in our previous work. In addition, photometric zero-point corrections are derived to provide an internally consistent photometric system with a spatially uniform metallicity zero point. Based on our phase-space diagrams, we find a strikingly narrow sequence in the rotational velocity ($v_\phi$) versus metallicity ([Fe/H]) space for a sample of high proper-motion stars ($>25$ mas yr$^{-1}$), which runs along the Gaia Sausage/Enceladus (GSE) and the Splash sub-structures, and is connected to the disk. Notably, a rapid increase of $v_\phi$ from a nearly zero net rotation to $200$ km s$^{-1}$ within a narrow metallicity interval ($-0.8 < {\rm [Fe/H]} < -0.2$) suggests that these stars were formed on a short gas-depletion time scale. Based on measurements of a scale height and length, we argue that they are distinct from stars dynamically heated by mergers, and are the relics formed during the starburst when the young Milky Way encountered the gas-rich GSE merger. The chain of high proper-motion stars, which we dub the Galactic Starburst Sequence (GSS), provides evidence that the post-merger metal-enriched gas settled onto the disk.