ESPRESSO observations of HE 0107−5240 and other CEMP-no stars with [Fe/H] ≤ –4.5

• Published in: Astronomy and astrophysics (Berlin) 2022-12, Vol.668, p.A86
• Main Authors: Aguado, D. S., Molaro, P., Caffau, E., González Hernández, J. I., Zapatero Osorio, M. R., Bonifacio, P., Allende Prieto, C., Rebolo, R., Damasso, M., Suárez Mascareño, A., Howell, S. B., Furlan, E., Cristiani, S., Cupani, G., Di Marcantonio, P., D’Odorico, V., Lovis, C., Martins, C. J. A. P., Milakovi, D., Murphy, M. T., Nunes, N. J., Pepe, F., Santos, N. C., Schmidt, T. M., Sozzetti, A.
• Format: Article
• Language: English
• Subjects: Sciences of the Universe
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/202244627

Context. HE 0107−5240 is a hyper metal-poor star with [Fe/H] = −5.39, one of the lowest-metallicity stars known. Its stellar atmosphere is enhanced in carbon, with [C/Fe] = +4.0, without a detectable presence of neutron-capture elements. Therefore, it belongs to the carbon-enhanced metal-poor (CEMP−no) group, along with the majority of the most metal-poor stars known to date. Recent studies have revealed variations in the line-of-sight velocity of HE 0107−5240, suggesting it belongs to a binary system. CEMP-no stars are the closest descendants of the very first Pop III stars, and binarity holds important clues for the poorly known mechanism that leads to their formation. Aims. We performed high-resolution observations with the E SPRESSO spectrograph at the VLT to constrain the kinematical properties of the binary system HE 0107−5240 and to probe the binarity of the sample of the eight most metal-poor stars with [Fe/H] ≤ −4.5. Methods. Radial velocities are obtained by using a cross-correlation function in the interval 4200−4315 Å , which contains the relatively strong CH band, against a template that could be either a synthetic spectrum or a combined observed spectrum in an iterative process. A Bayesian method is applied to calculate the orbit using the E SPRESSO measurements and others from the literature. Chemical analysis has also been performed for HE 0107−5240, employing spectral synthesis with the SYNTHE and ATLAS codes. Results. Observations of HE 0107−5240 spanning more than 3 years show a monotonic decreasing trend in radial velocity at a rate of approximately 0.5 m s −1  d −1 . A maximum v rad was reached between March 13, 2012, and December 8, 2014. The period is constrained at P orb  = 13009 −1370 +1496 d. New, more stringent upper limits have been found for several elements: (a) [Sr/Fe] and [Ba/Fe] are lower than −0.76 and +0.2, respectively, confirming the star is a CEMP-no; (b) A(Li)