[Paper Review] Near-IR and optical radial velocities of the active M dwarf star Gl 388 (AD Leo) with SPIRou at CFHT and SOPHIE at OHP: A 2.23 day rotation period and no evidence for a corotating planet

A. Carmona, X. Delfosse+37 more|arXiv (Cornell University)|Mar 29, 2023

Stellar, planetary, and galactic studies64 references35 citations

TL;DR

This study uses quasi-simultaneous optical (SOPHIE) and near-infrared (SPIRou) radial velocity (RV) measurements of the active M dwarf Gl 388 (AD Leo) to test whether its 2.23-day RV signal arises from a corotating planet or stellar activity. SPIRou's high-precision near-IR RVs show no periodic signal (5 m s⁻¹ rms), while optical RVs show a 23.6 m s⁻¹ signal; the absence of a corresponding near-IR signal rules out a planet and confirms the signal is due to stellar activity, with the rotation period independently confirmed via spectropolarimetry of the longitudinal magnetic field (Bℓ).

ABSTRACT

Context: The search for extrasolar planets around the nearest M dwarfs is a crucial step towards identifying the nearest Earth-like planets. One of the main challenges in this search is that M dwarfs can be magnetically active and stellar activity can produce radial velocity (RV) signals that could mimic those of a planet. Aims: We aim to investigate whether the 2.2 day period observed in optical RVs of the nearby active M dwarf star Gl 388 (AD Leo) is due to stellar activity or to a planet that corotates with the star as suggested in the past. Methods: We obtained quasi-simultaneous optical RVs of Gl 388 from 2019 to 2021 with SOPHIE (R~75k) at the OHP in France, and near-IR RV and Stokes V measurements with SPIRou at the CFHT (R~70k). Results: The SOPHIE RV time series displays a periodic signal with a 2.23+-0.01 day period and 23.6+-0.5 m/s amplitude, which is consistent with previous HARPS observations obtained in 2005-2006. The SPIRou RV time series is flat at 5 m/s rms and displays no periodic signals. RV signals of amplitude higher than 5.3 m/s at a period of 2.23 days can be excluded with a confidence level higher than 99%. Using the modulation of the longitudinal magnetic field (Bl) measured with SPIRou, we derive a stellar rotation period of 2.2305+-0.0016 days. Conclusions: SPIRou RV measurements provide solid evidence that the periodic variability of the optical RVs of Gl 388 is due to stellar activity rather than to a corotating planet. The magnetic activity nature of the optical RV signal is further confirmed by the modulation of Bl with the same period. The SPIRou campaign on Gl 388 demonstrates the power of near-IR RV to confirm or infirm planet candidates discovered in the optical around active stars. SPIRou observations reiterate how effective spectropolarimetry is at determining the stellar rotation period.

Motivation & Objective

• To determine whether the 2.23-day radial velocity (RV) signal in the active M dwarf Gl 388 (AD Leo) is caused by a corotating planet or stellar activity.
• To test the effectiveness of near-infrared (near-IR) RV measurements in distinguishing planetary signals from stellar activity signatures.
• To use spectropolarimetric measurements of the longitudinal magnetic field (Bℓ) to independently determine the stellar rotation period.
• To assess the long-term stability of RV signals in active stars and their potential to mimic planetary signals.
• To demonstrate the utility of combining optical and near-IR RV data to resolve ambiguous planetary detections in magnetically active stars.

Proposed method

• Obtained quasi-simultaneous high-precision radial velocity (RV) measurements using the optical échelle spectrograph SOPHIE at OHP (R ~ 75,000) and the near-IR spectropolarimeter SPIRou at CFHT (R ~ 70,000) from 2019 to 2021.
• Measured the longitudinal magnetic field (Bℓ) using SPIRou's spectropolarimetric mode to track stellar activity cycles and infer rotation period.
• Compared the amplitude and phase of RV signals between optical (SOPHIE) and near-IR (SPIRou) data, leveraging the fact that planetary signals are achromatic while activity-induced signals are wavelength-dependent.
• Used Keplerian modeling to fit the optical RV data and assess the significance of a planetary signal, while testing for residual periodicity in the SPIRou data.
• Applied statistical tests to exclude planetary signals with amplitudes >5.3 m s⁻¹ at the 2.23-day period with >99% confidence.
• Cross-validated the rotation period using Bℓ variations and compared with historical optical and near-IR data from HARPS and ESPaDOnS.

Experimental results

Research questions

• RQ1 Is the 2.23-day radial velocity signal in Gl 388 caused by a corotating planet or stellar activity?
• RQ2 Can near-infrared radial velocity measurements distinguish between planetary and activity-induced signals in magnetically active M dwarfs?
• RQ3 Does the longitudinal magnetic field (Bℓ) variation from spectropolarimetry provide a reliable and independent measurement of stellar rotation period?
• RQ4 How stable is the RV signal from Gl 388 over more than a decade, and what does this imply for false positive detection rates in exoplanet surveys?
• RQ5 To what extent can near-IR RV data reduce ambiguity in planetary signal detection when optical data show periodic variability?

Key findings

• The SPIRou near-IR radial velocity time series shows no periodic signal, with a root-mean-square scatter of only 5 m s⁻¹, confirming the absence of a planetary signal at the 2.23-day period.
• A planetary signal with amplitude >5.3 m s⁻¹ at 2.23 days can be excluded with more than 99% confidence, strongly disfavoring the presence of a 0.24 MJ planet as previously suggested.
• The optical SOPHIE RV data display a stable 2.23±0.01 day period with a 23.6±0.5 m s⁻¹ amplitude, consistent with earlier HARPS observations and indicating long-term stability of the activity signal.
• The longitudinal magnetic field (Bℓ) measured with SPIRou varies with a period of 2.2305±0.0016 days, providing independent confirmation of the stellar rotation period and linking the RV signal directly to magnetic activity.
• The near-IR RV signal is insensitive to the same activity features that produce strong optical RV variations, demonstrating that near-IR RVs are less affected by temperature contrasts between spots and photosphere.
• The study confirms that spectropolarimetry is a powerful tool for measuring stellar rotation periods independently of photometry, especially in stars with complex photometric behavior.