There is No Place Like Home -- Finding Birth Radii of Stars in the Milky Way

Abstract

Stars move away from their birth places over time via a process known as radial migration, which blurs chemo-kinematic relations used for reconstructing the Milky Way formation history. One of the ultimate goals of Galactic Archaeology, therefore, is to find stars' birth aggregates in the disk via chemical tagging. Here we show that stellar birth radii can be derived directly from the data with minimum prior assumptions on the Galactic enrichment history. We recover the time evolution of the stellar birth metallicity gradient, $d$[Fe/H]($R$, $\tau$)/$dR$, through its inverse relation to the metallicity range as a function of age today, allowing us to place any star with age and metallicity measurements back to its birthplace, $R_b$. Applying our method to a high-precision large data set of Milky Way disk subgiant stars, we find a steepening of the birth metallicity gradient from 11 to 8 Gyr ago, which coincides with the time of the last major merger, Gaia-Sausage-Enceladus (GSE). This transition appears to play a major role in shaping both the age-metallicity relation and the bimodality in the [$\alpha$/Fe]-[Fe/H] plane. By dissecting the disk into mono-$R_b$ populations, clumps in the low-[$\alpha$/Fe] sequence appear, which are not seen in the total sample and coincide in time with known star-formation bursts. We estimated that the Sun was born at $4.5 \pm 0.4$ kpc from the Galactic center. Our $R_b$ estimates provide the missing piece needed to recover the Milky Way formation history, while the by-product,[Fe/H]$(R$, $\tau)$, can be used as the thus-far missing prior for chemical evolution modeling.Comment: Under review at Nature Letters, submitted Oct. 19, 202