AD Leonis: Radial velocity signal of stellar rotation or spin-orbit resonance?

TL;DR

This study investigates whether the 2.23-day radial velocity signal of AD Leonis is caused by stellar rotation or a planet in spin-orbit resonance, analyzing photometric and spectroscopic data to distinguish between these scenarios.

Contribution

It provides evidence supporting the possibility of a planetary companion in spin-orbit resonance as the source of the radial velocity signal, challenging starspot explanations.

Findings

Radial velocity signal is stable over 4700 days.

Photometric signals vary considerably and are inconsistent.

A planet in spin-orbit resonance could explain the observations.

Abstract

AD Leonis is a nearby magnetically active M dwarf. We find Doppler variability with a period of 2.23 days as well as photometric signals: (1) a short period signal which is similar to the radial velocity signal albeit with considerable variability; and (2) a long term activity cycle of 4070$\pm$120 days. We examine the short-term photometric signal in the available ASAS and MOST photometry and find that the signal is not consistently present and varies considerably as a function of time. This signal undergoes a phase change of roughly 0.8 rad when considering the first and second halves of the MOST data set which are separated in median time by 3.38 days. In contrast, the Doppler signal is stable in the combined HARPS and HIRES radial velocities for over 4700 days and does not appear to vary in time in amplitude, phase, period or as a function of extracted wavelength. We consider a variety of star-spot scenarios and find it challenging to simultaneously explain the rapidly varying photometric signal and the stable radial velocity signal as being caused by starspots co-rotating on the stellar surface. This suggests that the origin of the Doppler periodicity might be the gravitational tug of a planet orbiting the star in spin-orbit resonance. For such a scenario and no spin-orbit misalignment, the measured $v \sin i$ indicates an inclination angle of 15.5$\pm$2.5 deg and a planetary companion mass of 0.237$\pm$0.047 M$_{\rm Jup}$.