A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host $ u$ Octantis A

The single-lined spectroscopic binary $ u$ Octantis provided evidence of the first conjectured circumstellar planet demanding an orbit retrograde to the stellar orbits. The planet-like behaviour is now based on 1437 radial velocities (RVs) acquired from 2001 to 2013. $ u$ Octs semimajor axis is only 2.6 AU with the candidate planet orbiting $ u$ Oct A about midway between. These details seriously challenge our understanding of planet formation and our decisive modelling of orbit reconfiguration and stability scenarios. However, all non-planetary explanations are also inconsistent with numerous qualitative and quantitative tests including previous spectroscopic studies of bisectors and line-depth ratios, photometry from Hipparcos and the more recent space missions TESS and GAIA (whose increased parallax classifies $ u$ Oct A closer still to a subgiant ~ K1 IV). We conducted the first large survey of $ u$ Oct As chromosphere: 198 Ca II H-line and 1160 H $alpha$ indices using spectra from a previous RV campaign (2009-2013). We also acquired 135 spectra (2018-2020) primarily used for additional line-depth ratios, which are extremely sensitive to the photospheres temperature. We found no significant RV-correlated variability. Our line-depth ratios indicate temperature variations of only $pm$ 4 K, as achieved previously. Our atypical Ca II analysis models the indices in terms of S/N and includes covariance significantly in their errors. The H $alpha$ indices have a quasi-periodic variability which we demonstrate is due to telluric lines. Our new evidence provides further multiple arguments realistically only in favor of the planet.