ESPRESSO highlights the binary nature of the ultra-metal-poor giant HE 0107−5240^⋆

¹ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
e-mail: Piercarlo.Bonifacio@observatoiredeparis.psl.eu, Piercarlo.Bonifacio@obspm.fr
² Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
³ Institute for Fundamental Physics of the Universe, Via Beirut, 2, 34151 Grignano, Trieste, Italy
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3 0HA, UK
⁶ Physikalisches Institut, Universität Bern, Bern, Switzerland
⁷ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁸ Scuola Normale Superiore, P.zza dei Cavalieri 7, 56126 Pisa, Italy
⁹ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁰ Observatoire de l’Université de Genève, Ch. des Maillettes 51, 1290 Versoix, Switzerland
¹¹ European Southern Observatory, Casilla 19001, Santiago, Chile
¹² Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹³ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, Palermo, Italy
¹⁴ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
¹⁵ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Lisboa 1749-016, Portugal
¹⁶ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
¹⁷ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
¹⁸ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁹ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
²⁰ Centro de Astrobiología (CSIC-INTA), Carretera Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 3 May 2019
Accepted: 4 July 2019

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.