First detection of oscillations in the Halo giant HD 122563: Validation of seismic scaling relations and new parameters^⋆

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
e-mail: orlagh.creevey@oca.eu
² Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁴ Instituto de Astrofisica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Universidad de La Laguna (ULL), Departamento de Astrofisica, 38206 La Laguna, Tenerife, Spain
⁶ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
⁷ Université Paris Diderot, AIM, Sorbonne Paris Cité, CEA, CNRS, 91191 Gif-sur-Yvette, France

Received: 26 November 2018
Accepted: 4 February 2019

Abstract

Aims. The nearby metal-poor giant HD 122563 is an important astrophysical laboratory in which to test stellar atmospheric and interior physics. It is also a benchmark star for which to calibrate methods to apply to large scale surveys. Recently it has been remeasured using various methodologies given the new high precision instruments at our disposal. However, inconsistencies in the observations and models have been found.

Methods. In order to better characterise this star using complementary techniques we have been measuring its radial velocities since 2016 using the Hertzsprung telescope (SONG network node) in order to detect oscillations.

Results. In this work we report the first detections of sun-like oscillations in this star, and to our knowledge, a detection in the most metal-poor giant to date. We applied the classical seismic scaling relation to derive a new surface gravity for HD 122563 of log g_ν = 1.39 ± 0.01. Reasonable constraints on the mass imposed by its PopII giant classification then yields a radius of 30.8 ± 1.0 ℛ_⊙. By coupling this new radius with recent interferometric measurements we infer a distance to the star of 306 ± 9 pc. This result places it further away than was previously thought and is inconsistent with the HIPPARCOS parallax. Independent data from the Gaia mission corroborate the distance hypothesis (d_GDR2 = 290 ± 5 pc), and thus the updated fundamental parameters.

Conclusions. We confirm the validity of the classical seismic scaling relation for surface gravity in metal-poor and evolved star regimes. The remaining discrepancy of 0.04 dex between log g_GDR2 (= 1.43 ± 0.03) reduces to 0.02 dex by applying corrections to the scaling relations based on the mean molecular weight and adiabatic exponent. The new constraints on the Hertzsprung–Russell diagram (L_⋆ν = 381 ± 26 ℒ_⊙) significantly reduce the disagreement between the stellar parameters and evolution models, however, a discrepancy of the order of 150 K still exists. Fine-tuned stellar evolution calculations show that this discrepancy can be reconciled by changing the mixing-length parameter by an amount (−0.35) that is in agreement with predictions from recent 3D simulations and empirical results. Asteroseismic measurements are continuing, and analysis of the full frequency data complemented by a distance estimate promises to bring important constraints on our understanding of this star and of the accurate calibration of the seismic scaling relations in this regime.