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Long-term activity cycles in planetary M stars observed with SOPHIE

C. G. Oviedo, A. P. Buccino, R. F. Díaz, R. Petrucci, E. Jofré, I. Boisse, P. D. Colombo, X. Delfosse

Abstract

M dwarfs are prime targets for exoplanet searches due to their low masses and radii, which enable the detection of small planets in their habitable zones (HZs). However, the magnetic activity of M dwarfs can introduce signals in radial velocity measure- ments that may be mistaken for planetary signatures, making the understanding of stellar activity cycles crucial for accurate planet detection and characterisation. We aim to identify and characterise long-term magnetic activity cycles in M dwarfs using a homogeneous and extensive spectroscopic dataset in order to better understand their magnetic variability and its implications for exoplanet detection. We analysed 13 years of high-resolution spectra obtained with the SOPHIE spectrograph for two early M dwarfs known to host exoplanets. We simultaneously monitored chromospheric activity using two indicators, the Hα index and the Mount Wilson S -index. Long-term trends were modelled using both sinusoidal and low-order polynomial fits to robustly identify stellar activity cycles. For GJ 617A, we report a cycle of approximately 4.8 years, while for GJ 411, we find several characteristic timescales of variability of about 4.9 years. In addition, TESS photometric data reveal signs of short-term variability in GJ617A. The periods of the long-term variability detected for GJ 617A and GJ 411 do not coincide with any of the planetary signals previously reported, which reinforces the hypothesis that they are of magnetic origin. If indeed the variability is due to activity, the cycles detected would not be driven by the same mechanism: The cycle in GJ 617A is consistent with a solar-like dynamo, while the rotation seems to play a different role in the long-term cycles detected in GJ 411.

Long-term activity cycles in planetary M stars observed with SOPHIE

Abstract

M dwarfs are prime targets for exoplanet searches due to their low masses and radii, which enable the detection of small planets in their habitable zones (HZs). However, the magnetic activity of M dwarfs can introduce signals in radial velocity measure- ments that may be mistaken for planetary signatures, making the understanding of stellar activity cycles crucial for accurate planet detection and characterisation. We aim to identify and characterise long-term magnetic activity cycles in M dwarfs using a homogeneous and extensive spectroscopic dataset in order to better understand their magnetic variability and its implications for exoplanet detection. We analysed 13 years of high-resolution spectra obtained with the SOPHIE spectrograph for two early M dwarfs known to host exoplanets. We simultaneously monitored chromospheric activity using two indicators, the Hα index and the Mount Wilson S -index. Long-term trends were modelled using both sinusoidal and low-order polynomial fits to robustly identify stellar activity cycles. For GJ 617A, we report a cycle of approximately 4.8 years, while for GJ 411, we find several characteristic timescales of variability of about 4.9 years. In addition, TESS photometric data reveal signs of short-term variability in GJ617A. The periods of the long-term variability detected for GJ 617A and GJ 411 do not coincide with any of the planetary signals previously reported, which reinforces the hypothesis that they are of magnetic origin. If indeed the variability is due to activity, the cycles detected would not be driven by the same mechanism: The cycle in GJ 617A is consistent with a solar-like dynamo, while the rotation seems to play a different role in the long-term cycles detected in GJ 411.
Paper Structure (13 sections, 4 equations, 16 figures, 3 tables)

This paper contains 13 sections, 4 equations, 16 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations and stellar sample
  3. High-resolution spectroscopy with SOPHIE
  4. Data selection
  5. Long-term activity analysis
  6. Magnetic activity cycles in GJ 617A and GJ 411
  7. GJ 617A - HD147379A - HIP 79755
  8. GJ 411 - HD 95735 - Lalande 21185
  9. Stellar population membership for GJ 411 and GJ 617A
  10. Searching for hints of short-term photometric activity in TESS data
  11. Rotation period
  12. Flares
  13. Summary and conclusions

Figures (16)

  • Figure 1: Spectrum from the SOPHIE database for GJ 617A. A zoomed-in view highlights the Ca ii K (in emission) with a dashed blue line and the H$\alpha$ lines with a dashed red line, along with the H$\alpha$ region.
  • Figure 2: Time series of the $S$-index for GJ 617A. Each point represents a SOPHIE spectrum.
  • Figure 3: Time series of the H$\alpha$ index for GJ 617A. Each point represents the spectra (data) obtained with the SOPHIE spectrograph.
  • Figure 4: Generalised Lomb--Scargle (blue) and LinGLS (red) periodograms of GJ 617A using the $S$-index. The main peaks are at (1831.67 $\pm$ 838.75) d (GLS) and (1752.03 $\pm$ 767.40) d (LinGLS).
  • Figure 5: Generalised Lomb--Scargle (blue) and LinGLS (red) periodograms of GJ 617A using the H$\alpha$ index. The main peaks are at (1918.93 $\pm$ 920.57) d (GLS) and (1752.07 $\pm$ 767.43) d (LinGLS).
  • ...and 11 more figures