Abstract
Context. The vast majority of the known stars of ultra low metallicity ([Fe/H] < −4.5) are known to be enhanced in carbon, and belong to the “low-carbon band” (A(C) = log(C/H)+12 ≤ 7.6). It is generally, although not universally, accepted that this peculiar chemical composition reflects the chemical composition of the gas cloud out of which these stars were formed. The first ultra-metal-poor star discovered, HE 0107−5240, is also enhanced in carbon and belongs to the “low-carbon band”. It has recently been claimed to be a long-period binary, based on radial velocity measurements. It has also been claimed that this binarity may explain its peculiar composition as being due to mass transfer from a former AGB companion. Theoretically, low-mass ratios in binary systems are much more favoured amongst Pop III stars than they are amongst solar-metallicity stars. Any constraint on the mass ratio of a system of such low metallicity would shed light on the star formation mechanisms in this metallicity regime. Aims. We acquired one high precision spectrum with ESPRESSO in order to check the reality of the radial velocity variations. In addition we analysed all the spectra of this star in the ESO archive obtained with UVES to have a set of homogenously measured radial velocities. Methods. The radial velocities were measured using cross correlation against a synthetic spectrum template. Due to the weakness of metallic lines in this star, the signal comes only from the CH molecular lines of the G-band. Results. The measurement obtained in 2018 from an ESPRESSO spectrum demonstrates unambiguously that the radial velocity of HE 0107−5240 has increased from 2001 to 2018. Closer inspection of the measurements based on UVES spectra in the interval 2001–2006 show that there is a 96% probability that the radial velocity correlates with time, hence the radial velocity variations can already be suspected from the UVES spectra alone. Conclusions. We confirm the earlier claims of radial velocity variations in HE 0107−5240. The simplest explanation of such variations is that the star is indeed in a binary system with a long period. The nature of the companion is unconstrained and we consider it is equally probable that it is an unevolved companion or a white dwarf. Continued monitoring of the radial velocities of this star is strongly encouraged.
Highlights
The ultra-metal-poor star HE 0107−5240 was the first star that was discovered to have [Fe/H]1 < −5.0 (Christlieb et al 2002)
Closer inspection of the measurements based on Ultraviolet and Visual Echelle Spectrograph (UVES) spectra in the interval 2001– 2006 show that there is a 96% probability that the radial velocity correlates with time, the radial velocity variations can already be suspected from the UVES spectra alone
At the time most such stars appeared to be enhanced in neutron capture elements and their peculiar chemical composition was thought to be the result of mass transfer from an Asymptotic Giant Branch (AGB) companion, being the extremelymetal-poor counterpart of CH stars (Vanture 1992a,b; McClure 1997)
Summary
The ultra-metal-poor star HE 0107−5240 was the first star that was discovered to have [Fe/H]1 < −5.0 (Christlieb et al 2002). In spite of the fact that the majority of the community accepted the chemical composition of HE 0107−5240, and its siblings, as being the same as that of the gas clouds out of which they formed, dissenting opinions that invoke mass transfer from a companion exist (Suda et al 2004; Lau et al 2007; Cruz et al 2013) Such models are compatible with the observed lack of enhancement in n-capture elements. In spite of the large errors on their four measurements, the indication was clear that the radial velocity of HE 0107−5240 was almost 4 km s−1 larger than it was in the time interval 2001–2006 On this basis, Arentsen et al (2019) suggested that the scenario of mass transfer in this star becomes highly likely. We reanalysed the Ultraviolet and Visual Echelle Spectrograph (UVES; Dekker et al 2000) spectra retrieved from the ESO archive
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call