First detection of oscillations in the Halo giant HD 122563: validation
  of seismic scaling relations and new fundamental parameters
  The nearby metal-poor giant HD122563 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 we have been measuring its radial velocities since 2016 using the
Hertzsprung telescope (SONG network node). 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 of $\log g_{\nu} = 1.39 \pm
0.01$ dex. Constraints on the mass imposed by its PopII giant classification
then yield a radius of $30.8 \pm 1.0$ R$_{\odot}$. By coupling this with recent
interferometric measurements we infer a distance to the star of 306 $\pm$ 9 pc.
Data from the Gaia mission corroborates the distance hypothesis ($d_{\rm GDR2}$
= 290 $\pm$ 5 pc), and thus the updated fundamental parameters. We confirm the
validity of the seismic scaling relation without corrections for surface
gravity in metal-poor and evolved star regimes. The small discrepancy of 0.04
dex reduces to 0.02 dex by applying corrections to the scaling relations. The
new constraints on the HR diagram ($L_{\odot} = 381 \pm 26$) reduce the
disagreement between the stellar parameters and evolution models, however, a
discrepancy 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.
