Guozhen Hu, Zhengyi Shao, Erbil Gugercinoglu, Wenyuan Cui

We explore the metal-poor regime of the Galactic disk on the distribution of stars in the [$\alpha$/M]-$V_{\phi}$ plane, to identify the most metal-poor thin disk (MPTnD) stars belonging to the low-$\alpha$ sequence. Chemical abundances and velocities of sample stars are either taken or derived from APOGEE DR17 and Gaia DR3 catalogs. We find the existence of a well-separated extension of the kinematically thin disk stars in the metallicity range of -1.2 $<$[M/H]$<$ -0.8 dex. Based on two-by-two distributions of [Mg/Mn], [Al/Fe] and [C+N/Fe], we further confirmed 56 high-possibility metal-poor thin disk (HP-MPTnD) giant stars and suggested the lower metallicity limit of the thin disk below -0.95 dex. A comparative analysis of HP-MPTnD sample with other Galactic components revealed its chemo-dynamical similarities with canonical thin disk stars. These low-$\alpha$ metal-poor stars are predominantly located in the outer disk region and formed in the early stage of the formation of thin disk. Their existence provides compelling support for the two-infall model of the Milky way's disk formation. Moreover, these stars impose observational constraints on the timing and metallicity of the second gas infall event.