The SkyMapper search for extremely metal-poor stars in the Large Magellanic Cloud

Abstract

ABSTRACT We present results of a search for extremely metal-poor stars in the Large Magellanic Cloud (LMC), which can provide crucial information about the properties of the first stars as well as on the formation conditions prevalent during the earliest stages of star formation in dwarf galaxies. Our search utilized SkyMapper photometry, together with parallax and proper motion cuts (from Gaia), colour–magnitude cuts (by selecting the red giant branch region), and finally a metallicity-sensitive cut. Low-resolution spectra of a sample of photometric candidates were taken using the ANU 2.3m telescope/WiFeS spectrograph, from which seven stars with $\rm [Fe/H] \le -2.75$ were identified, two of which have $\rm [Fe/H] \le -3$. Radial velocities, derived from the Ca ii triplet lines, closely match the outer rotation curve of the LMC for the majority of the candidates in our sample. Therefore, our targets are robustly members of the LMC based on their 6D phase-space information (coordinates, spectrophotometric distance, proper motions, and radial velocities), and they constitute the most metal-poor stars so far discovered in this galaxy.