Chemical abundances and kinematics of two very metal-poor stars: HE 1421+0022 and HE 2150−1800,

Abstract We present, for the first time, a detailed high-resolution spectroscopic analysis of two VMP stars, HE 1421+0022 and HE 2150−1800. Both stars exhibit significant carbon enhancement ([C/Fe] ≥+2.0)and enhanced nitrogen abundances ([N/Fe] ~+2.17 and +1.37 respectively). The derived metallicities, [Fe/H] ~−2.79 and −2.48, respectively, place both stars firmly in the VMP regime. We determined the abundances of the light and iron-peak elements C, N, Na, Mg, Ca, Sc, Ti, Cr, Mn, and Ni, as well as the neutron-capture elements Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu. Based on their locations both stars are classified as Group I objects in the Yoon-Beers diagram of A(C) vs [Fe/H]. Applying established classification criteria for carbon-enhanced metal-poor stars, HE 1421+0022 is identified as a CEMP-r/s star and HE 2150−1800 as a CEMP-s star. Our kinematic analysis shows that HE 1421+0022 is a high-energy, prograde star associated with the outer Galactic halo, while HE 2150−1800 is a high-energy, retrograde star associated with the inner halo. The combined chemodynamical properties suggest that HE 1421+0022 may be associated with either the Sagittarius (Sgr) or Gaia-Sausage/Enceladus (GSE) accretion event, whereas HE 2150−1800 is likely associated with the I’itoi structure. These results provide further evidence that metal-poor stars in the Galactic halo preserve signatures of accretion events involving dwarf satellite galaxies, offering valuable insights into the hierarchical assembly and chemical evolution of the Galaxy.