The SPIRou Legacy Survey: Detection of a nearby world orbiting in the habitable zone of Gl725B achieved by correcting strong telluric contamination in near-infrared radial velocities with WAPITI

TL;DR

This study tackles the challenge of measuring precise near-infrared RVs for M-dwarfs in the presence of strong telluric contamination. By applying the wapiti weighted PCA-based telluric correction to SPIRou LBL RVs of Gl 725 B and validating it with simulations and injection-recovery tests, the authors reveal a compact ordered two-planet system, including a confirmed planet Gl 725 Bc in the habitable zone and a candidate Gl 725 Bb. Multidimensional Gaussian Process activity modeling confirms the planetary interpretation by disentangling stellar activity signatures from Keplerian signals. The results demonstrate the viability of NIR RVs for discovering and characterizing rocky planets around nearby M dwarfs and highlight Gl 725 Bc as a nearby, potentially habitable world, with future atmospheric studies and high-contrast observations offering promising avenues for characterization.

Abstract

M dwarfs are prime targets in the search for exoplanets because of their prevalence and because low-mass planets can be better detected with radial velocity (RV) methods. In particular, the near-infrared (NIR) spectral domain offers an increased RV sensitivity and potentially reduced stellar activity signals. Howevern precise NIR RV measurements can be strongly affected by telluric absorption lines from the Earth's atmosphere. We searched for planets orbiting Gl 725 B, a nearby late-M dwarf at $3.5$ pc, using high-precision SPIRou RV observations. We assessed the impact of telluric contamination and evaluated the performance of the weighted principal component analysis reconstruction method (WAPITI), designed to mitigate these systematics and improve planet detectability. Using synthetic and observational SPIRou data, we simulated telluric effects on RVs under varying barycentric Earth radial velocity (BERV) conditions and applied WAPITI to correct line-by-line RVs. The method was tested through injection-recovery experiments and applied to real SPIRou observations of Gl 725 B. WAPITI efficiently corrects telluric contamination in simulated and real datasets, enhancing the detectability and accuracy of planetary signals. We identify a two-planet system around Gl 725 B composed of a candidate inner planet (Gl 725 Bb) with a period of $4.765 \pm 0.004$ days and semi-amplitude $1.4 \pm 0.3$ m.s$^{-1}$, and a confirmed outer planet (Gl 725 Bc) with a period of $37.90 \pm 0.17$ days and semi-amplitude $1.7 \pm 0.3$ m.s$^{-1}$. Their minimum masses are $1.5 \pm 0.4$ and $3.5 \pm 0.7$ M$_\oplus$, respectively, and the outer planet lies in the habitable zone. Using a multi-dimensional Gaussian process framework to model stellar activity, we also recover a stellar rotation period of $105.1 \pm 3.3$ days.

Figures (14)

• Figure 1: Simulation results when telluric residuals were injected at a $3 \pm 1$% level. Top: Resulting RV time series from the simulations in time (left) and BERV (right) spaces. Bottom: Their respective GLS periodograms. The RVs computed with a wide and narrow BERV coverage are indicated in red and green, respectively. The significance levels of FAP = $10^{-3}$ and FAP = $10^{-5}$ are indicated by the solid and dashed horizontal lines, respectively. The gray line in the periodogram corresponds to the window function.
• Figure 2: Left: Detection maps showing the percentage of recovered injected signals as a function of period and semi-amplitude for the narrow and wide BERV coverages. Middle: Same detection tests after applying the wapiti correction. Right: Difference between the recovered and injected $K$ values ($\Delta K = K_\text{out} - K_\text{in}$) at 1 m$\cdot$s$^{-1}$, with the shaded area indicating one standard deviation. The color bar indicates the detection rate in percent.
• Figure 3: Time series (top) and Bayesian periodograms (bottom) for the longitudinal magnetic field $B_\ell$ (left) and the differential effective temperature activity indicator dET (right). The dashed red line shows the significance threshold of 5. The vertical dashed lines mark the periods of the two signals in RV data at 37.9 and 4.77 days.
• Figure 4: Iterative Bayesian periodograms for the Gl 725 B RV time series under different models. From left to right: (1) Raw RVs after trend correction, (2) after removal of systematics via wapiti, (3) with the 38-day planet, and (4) residuals after modeling the 38-day and 4.77-day signals. The dashed red line marks the detection threshold at log BF = 5.
• Figure 5: Two-planet GP model for Gl 725 B. Top: Raw RVs with trend. Middle: Phase-folded RVs for the 38-day and 4.77-day signals with binned points (black). Bottom: Modeled RV and dET time series (with GP) and corresponding residuals.