The pristine nature of SMSS 1605−1443 revealed by ESPRESSO

SMSS J160540.18$-$144323.1 is the carbon-enhanced metal-poor (CEMP) star with lowest iron abundance ever measured, [Fe/H]=-6.2, which was first reported SkyMapper telescope. The carbon A(C)~6.1 in low-C band, as majority of stars this metallicity range. Yet, constraining isotopic ratio key species, such carbon, sheds light on properties and origin these elusive stars. We performed high-resolution observations SMSS1605$-$1443 ESPRESSO spectrograph to look for variations radial velocity ($v_{rad}$) time. These data have been combined older MIKE UVES archival enlarge temporal baseline. $^{12}$C/$^{13}$C also studied explore possibility mass transfer from a binary companion. A cross-correlation function against natural template applied detect $v_{rad}$ variability spectral synthesis technique used derive stellar atmosphere. confirm previous indications binarity measured lower limit $^{12}$C/$^{13}$C$>60$ at more than 3$\sigma$ confidence level, proving that system chemically unmixed no unseen companion has happened so far. Thus, we CEMP-no nature show pristine chemical composition cloud it formed currently imprinted its atmosphere free contamination.