The pristine nature of SMSS 1605$-$1443 revealed by ESPRESSO

TL;DR

This study uses high-resolution spectroscopy to analyze the chemical composition and binarity of the extremely metal-poor star SMSS 1605$-$1443, confirming its pristine nature and CEMP-no classification by measuring a high $^{12}$C/$^{13}$C ratio and detecting no mass transfer evidence.

Contribution

First high-resolution ESPRESSO observations combined with archival data to determine the isotopic ratio and binarity status of SMSS 1605$-$1443, confirming its pristine, unmixed, and CEMP-no nature.

Findings

Confirmed binarity in SMSS 1605$-$1443.

Measured $^{12}$C/$^{13}$C$>60$, indicating no mixing.

Established the star's pristine chemical composition.

Abstract

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