Icarus Revisited: An Ancient, Metal-poor Accreted Stellar Stream in the Disk of the Milky Way

The search for accreted satellites in the Galactic disk is a challenging task, to which Gaia plays a crucial role in synergy with ground-based spectroscopic surveys. In 2021, P. Re Fiorentin et al. discovered five substructures with disk kinematics including Icarus. To gain more insight into the origin of Icarus as a remnant of a dwarf galaxy rather than a signature of secular processes of disk formation, we complement astrometric Gaia DR3 data with spectroscopy from APOGEE DR17 and GALAH DR3, and explore the chemodynamical distributions within 3 kpc of the Sun. We select 622 stars in the accreted/unevolved regions of [Mg/Mn]-[Al/Fe] and [Mg/Fe]-[Fe/H], where we identify 81 and 376 stars with −2 < [Fe/H] < −0.7 belonging to Icarus and Gaia-Sausage-Enceladus (GSE), respectively. The revised properties of Icarus are: 〈V + V LSR〉 ≃ 171 km s−1, σ V ≃ 37 km s−1, 〈e〉 ≃ 0.36, 〈[Fe/H]〉 ≃ −1.35, 〈[Mg/Fe]〉 ≃ +0.27, 〈[Al/Fe]〉 ≃ −0.13, and 〈[Mn/Fe]〉 ≃ −0.39. From the color-magnitude diagram of its members, Icarus appears older than 12 Gyr. Such age and dynamical properties are reminiscent of the metal-weak thick disk. However, detailed chemical analysis in the diagnostic spaces [Ni/Fe]-[(C+N)/O], [Y/Eu]-[Fe/H], [Eu/Mg]-[Fe/H], [Ba/Y]-[Fe/H], and [Ba/Mg]-[Mg/H] evidences that Icarus and GSE occupy the accreted region, well separated from the bulk of in situ disk stars. Updated comparisons with N-body simulations confirm that Icarus’s stars are consistent with the debris of a dwarf galaxy with a stellar mass of ∼109 M ☉ accreted onto a primordial disk on an initial prograde low-inclination orbit.