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

Abstract

Aims. The nearby metal-poor giant HD 122563 is an important astrophysical laboratory for 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. 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. Methods.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 apply the classical seismic scaling relation to derive a new surface gravity for HD 122563 of log g ν = 1.39 ± 0.01 dex. 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, which places it further away than previously thought and inconsistent with the Hipparcos parallax. Independent data from the Gaia mission corroborates the distance hypothesis (d GDR2 = 290 ± 5 pc), and thus the updated fundamental parameters. Conclusions. We confirm the validity of the seismic scaling relation without corrections 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 HR diagram (L ⋆ν = 381 ± 26) significantly reduce the disagreement between the stellar parameters and evolution models, however, a discrepancy on the order of 150 K still exists. Fine-tuned stellar evolution calculations show that this 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.

Observational constraints on the origin of the elements

“…These estimates are fully consistent with the values we adopted in Bergemann et al (2012). Creevey et al (2019) propose a new asteroseismic surface gravity for HD 122563, log g = 1.39 ± 0.01 dex. This is a substantial downward revision of this parameter.…”

“…Decreasing the log g might help to increase the abundance from the resonance lines, but a change to log g < 0.5 would be necessary. All these changes in stellar parameters can be ruled out, given the robust constraints from asteroseismology and interferometry (Creevey et al 2012;Karovicova et al 2018;Creevey et al 2019). Also micro-turbulence, despite being a free parameter, is relatively well-constrained in the literature (Bergemann et al 2012;Afşar et al 2016).…”

Acoustic oscillations and dynamo action in the G8 sub-giant EK Eridani,

“…3.3. The power spectrum of the velocity time-series was calculated as both an un-weighted and weighted least-squares fit of sinusoidal components following Corsaro et al (2012) (see also Frandsen et al 1995;Arentoft et al 1998;Bedding et al 2004;Kjeldsen et al 2005;Creevey et al 2019). The resulting power was then converted into power spectral density (PSD) by normalizing for the spectral resolution, which is calculated as the integral of the spectral window of the observing set, here corresponding to ∼ 4.87 µHz.…”

“…Similarly to Creevey et al (2019) we used the Diamonds code (Corsaro & De Ridder 2014) for the Bayesian inference of the PSD of EK Eri. The model adopted consists in a simple flat noise component and a Gaussian envelope to model the oscillation power excess (Corsaro & De Ridder 2014).…”

Contribution of Type Ia Supernovae to the Chemical Enrichment of the Ultra-Faint Dwarf Galaxy Boötes I

“…If the distance modulus is assumed to be 19.07 mag Okamoto et al (2012), then log g will decrease only by 0.07 dex. In the literature there are reports about a similar inconsistency of the positions of giant stars on the HR diagram with their evolutionary tracks, for example, for the halo giant HD 122563 with reliably measured atmospheric parameters (Creevey et al 2019). This may be related to uncertainties in the calculations of metal-poor tracks in the region of red giants.…”