AD Leonis: Radial Velocity Signal of Stellar Rotation or Spin–Orbit Resonance?

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 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±2.5 deg and a planetary companion mass of 0.237±0.047 M Jup .

The GAPS Programme at TNG

“…Considering planets with periods between 1 and 100 d, we are sensitive to planets of minimum masses M p sini > 15.5 M ⊕ , for 1 < P < 10 d, and M p sini > 29.5 M ⊕ for 10 < P < 100 d. It is worth noting that the shortperiod detection threshold is well below the mass of the candidate planet M p = 75.4 ± 14.7 M ⊕ proposed by Tuomi et al (2018). Tuomi et al (2018) investigated the available photometric time series finding a modulation at the rotation period with an amplitude of 9.3 mmag in the V passband data before about JD 2455000, while no rotational modulation exceeding 6.7 mmag was observed after that date. With an amplitude of the optical photometric modulation of 10 mmag and a vsini of 3 km s −1 , using Eq.…”

“…In this Section we present the RV measurements of AD Leo (Gl 388, M4.5Ve, V=9.5, H=4.8, Reiners et al 2013 Leo has an inclination of the rotation axis of ∼20°according to Morin et al (2008) and ∼15°according to Tuomi et al (2018), so it is seen nearly pole-on. Given its brightness and proximity, AD Leo is a very well studied star.…”

“…We employed multi-wavelength spectroscopy to investigate the presence of a hot Jupiter around the very active star AD Leo, as proposed by Tuomi et al (2018). Our GIARPS data show that the modulation of the VIS RVs is not reproduced by the simul-Article number, page 8 of 18 I.…”

“…In this paper, we present the first results of the GAPS (Global Architecture of Planetary Systems) Young Objects Project, based on the observations of stars with claimed planets. We report the RV analysis for two previously announced hot-Jupiter planets orbiting around HD 285507 (Quinn et al 2014) and AD Leo (Tuomi et al 2018). These observations were performed in GIARPS mode (Claudi et al 2017), which simultaneously uses HARPS-N (Cosentino et al 2012) in the VIS range (0.39 -0.68 µm) and GIANO-B (the NIR high-resolution spectrograph at Telescopio Nazionale Galileo, TNG, Oliva et al 2006) in the NIR range (0.97 -2.45 µm).…”

“…GLS of AD Leo for HARPS-N data (HT1). The black vertical dashed line is the rotational period at 2.22791 days, while the red vertical dashed line is the proposed planet period at 2.22579 days (both periods byTuomi et al 2018). The two lines overlap each other.…”