Abstract
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 ± 120 days. We examine the short-term photometric signal in the available All-Sky Automated Survey and Microvariability and Oscillations of STars (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 High-Accuracy Radial velocity Planet Searcher and High Resolution Echelle Spectrometer 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 starspot scenarios and find it challenging to simultaneously explain the rapidly varying photometric signal and the stable radial velocity signal as being caused by starspots corotating 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 indicates an inclination angle of 15.°5 ± 2.°5 and a planetary companion mass of 0.237 ± 0.047 M Jup.
Highlights
The Doppler spectroscopy technique has been a successful method for the detection of planets orbiting nearby stars by enabling observers to measure the changes in stellar radial velocities caused by planets orbiting them on Keplerian orbits
We have presented analyses of All-Sky Automated Survey (ASAS) V-band photometry, Microvariability and Oscillations of STars (MOST) photometry, and High-Accuracy Radial velocity Planet Searcher (HARPS) and High Resolution Echelle Spectrometer (HIRES) radial velocities of AD Leo
AD Leo is a rapidly rotating star with a rotation period of approximately 2.23 days, we only see evidence for a photometric rotation period of the star in ASAS-N and MOST photometry (Figure 4) when the star is at a brightness minimum of its activity cycle
Summary
The Doppler spectroscopy technique has been a successful method for the detection of planets orbiting nearby stars by enabling observers to measure the changes in stellar radial velocities caused by planets orbiting them on Keplerian orbits. Both Bonfils et al (2013) and Reiners et al (2013) interpreted the 2.23 day periodicity as a signal originating from the corotation of starspots on the stellar surface because the spectra showed line asymmetries that were correlated with the velocity variations This means that the radial velocities of AD Leo might provide a benchmark case for examining the differences between Doppler signals caused by stellar rotation and planets on Keplerian orbits. Teitler & Königl (2014) ran numerical simulations to investigate “why there is a dearth of close-orbiting planets around fast-orbiting stars” and found that this can be attributed to tidal ingestion of close-in planets by their host stars Finding examples of such stars in the solar neighborhood would enable studying this mechanism in detail. We compare the results to other known rapidly rotating nearby M dwarfs in order to see what, if any, connections there are between photometric rotation periods and radial velocity variations
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