The death of Vulcan: NEID reveals the planet candidate orbiting HD 26965 is stellar activity

TL;DR

Recent NEID observations of HD 26965 show that the previously claimed 45-day planet candidate is actually caused by stellar activity, specifically rotational modulation and convective blueshift suppression, not an orbiting planet.

Contribution

This study demonstrates how combining classical activity indicators with line-by-line RV analysis can distinguish stellar activity signals from planetary signals in RV data.

Findings

The 45-day RV signal is linked to stellar activity, not a planet.

Line-by-line analysis reveals depth-dependent RV correlations indicative of activity.

A 42-day rotational activity period explains the observed RV variability.

Abstract

We revisit the long-studied radial velocity (RV) target HD26965 using recent observations from the NASA-NSF 'NEID' precision Doppler facility. Leveraging a suite of classical activity indicators, combined with line-by-line RV analyses, we demonstrate that the claimed 45-day signal previously identified as a planet candidate is most likely an activity-induced signal. Correlating the bulk (spectrally-averaged) RV with canonical line activity indicators confirms a multi-day 'lag' between the observed activity indicator time series and the measured RV. When accounting for this lag, we show that much of the observed RV signal can be removed by a linear detrending of the data. Investigating activity at the line-by-line level, we find a depth-dependent correlation between individual line RVs and the bulk RVs, further indicative of periodic suppression of convective blueshift causing the observed RV variability, rather than an orbiting planet. We conclude that the combined evidence of the activity correlations and depth dependence is consistent with a radial velocity signature dominated by a rotationally-modulated activity signal at a period of $\sim$42 days. We hypothesize that this activity signature is due to a combination of spots and convective blueshift suppression. The tools applied in our analysis are broadly applicable to other stars, and could help paint a more comprehensive picture of the manifestations of stellar activity in future Doppler RV surveys.