Stellar Activity Manifesting at a One-year Alias Explains Barnard b as a False Positive

Lubin, Jack; Robertson, Paul; Stefánsson, Guðmundur; Ninan, Joe P.; Mahadevan, Suvrath; Endl, Michael; Wright, Jason T.; Beard, Corey; Bender, Chad F.; Cochran, William D.; Diddams, Scott A.; Fredrick, Connor; Halverson, Samuel; Kanodia, Shubham; Metcalf, Andrew J.; Ramsey, Lawrence W.; Roy, Arpita; Schwab, Christian; Terrien, Ryan C.

doi:10.3847/1538-3881/ac0057

“…Results suggest that this is a quiet star, though modestly strong peaks at ∼ 5, 19, and 60 days in the Ca infrared triplet and line width periodograms are suggestive of possible rotation periods. It is often possible for activity present in RVs, however, to have no clear signal in one or more activity indicators (Robertson et al 2014;Lubin et al 2021), and so we proceed forward with our investigation despite the lack of clear detections.…”

Section: Rv Analysis Of Toi-2136mentioning

confidence: 99%

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with HPF

Beard,

Robertson,

Kanodia

et al. 2022

Preprint

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We present the validation of two planets orbiting M dwarfs, TOI-1696b and TOI-2136b. Both planets are mini-Neptunes orbiting nearby stars, making them promising prospects for atmospheric characterization with the James Webb Space Telescope. We validated the planetary nature of both candidates using high contrast imaging, ground-based photometry, and near-infrared radial velocities. Adaptive Optics images were taken using the ShARCS camera on the 3 m Shane Telescope. Speckle images were taken using the NN-Explore Exoplanet Stellar Speckle Imager on the WIYN 3.5 m telescope. Radii and orbital ephemerides were refined using a combination of TESS, the diffuser-assisted ARC-TIC imager on the 3.5m ARC telescope at Apache Point Observatory, and the 0.6 m telescope at Red Buttes Observatory. We obtained radial velocities using the Habitable-Zone Planet Finder on the 10

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“…Results suggest that this is a quiet star, though modestly strong peaks at ∼5, 19, and 60 days in the Ca infrared triplet and line width periodograms are suggestive of possible rotation periods. It is often possible for activity present in RVs, however, to have no clear signal in one or more activity indicators (Robertson et al 2014;Lubin et al 2021), and so we proceed with our investigation despite the lack of clear detections.…”

Section: Rv Analysis Of Toi-2136mentioning

confidence: 99%

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with the Habitable-Zone Planet Finder

Beard

¹

,

Robertson

²

,

Kanodia

³

et al. 2022

Self Cite

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We present the validation of two planets orbiting M dwarfs, TOI-1696b and TOI-2136b. Both planets are mini-Neptunes orbiting nearby stars, making them promising prospects for atmospheric characterization with the James Webb Space Telescope (JWST). We validated the planetary nature of both candidates using high-contrast imaging, ground-based photometry, and near-infrared radial velocities. Adaptive optics images were taken using the ShARCS camera on the 3 m Shane Telescope. Speckle images were taken using the NN-Explore Exoplanet Stellar Speckle Imager on the WIYN 3.5 m telescope. Radii and orbital ephemerides were refined using a combination of the Transiting Exoplanet Survey Satellite, the diffuser-assisted Astrophysical Research Consortium (ARC) Telescope Imaging Camera (ARCTIC) imager on the 3.5 m ARC telescope at Apache Point Observatory, and the 0.6 m telescope at Red Buttes Observatory. We obtained radial velocities using the Habitable-Zone Planet Finder on the 10 m Hobby–Eberly Telescope, which enabled us to place upper limits on the masses of both transiting planets. TOI-1696b (P = 2.5 days; R p = 3.24 R ⊕; M p < 56.6 M ⊕) falls into a sparsely populated region of parameter space considering its host star’s temperature (T eff = 3168 K, M4.5), as planets of its size are quite rare around mid- to late-M dwarfs. On the other hand, TOI-2136b (P = 7.85 days; R p = 2.09 R ⊕; M p < 15.0 M ⊕) is an excellent candidate for atmospheric follow-up with the JWST.

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“…In such cases, the connection between stellar activity and RV might be overlooked (e.g. Robertson et al 2015;Bortle et al 2021;Lubin et al 2021). Furthermore, aliasing, small sample size, and red noise can cause bootstrap false alarm level calculations to fail (Baluev 2008;Chernick 2008;Littlefair et al 2016), making it difficult to correctly assess the significance of activity signals.…”

Section: Introductionmentioning

confidence: 99%

Magnitude-squared coherence: A powerful tool for disentangling Doppler planet discoveries from stellar activity

Dodson-Robinson,

Delgado,

Harrell

et al. 2022

Preprint

0

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If Doppler searches for earth-mass, habitable planets are to succeed, observers must be able to identify and model out stellar activity signals. Here we demonstrate how to diagnose activity signals by calculating the magnitude-squared coherence Ĉ2xy (f ) between an activity indicator time series x t and the radial velocity (RV) time series y t . Since planets only cause modulation in RV, not in activity indicators, a high value of Ĉ2xy (f ) indicates that the signal at frequency f has a stellar origin. We use Welch's method to measure coherence between activity indicators and RVs in archival observations of GJ 581, α Cen B, and GJ 3998. High RV-Hα coherence at the frequency of GJ 3998 b, and high RV-S index coherence at the frequency of GJ 3998 c, indicate that the planets may actually be stellar signals. We also replicate previous results showing that GJ 581 d and g are rotation harmonics and demonstrate that α Cen B has activity signals that are not associated with rotation. Welch's power spectrum estimates have cleaner spectral windows than Lomb-Scargle periodograms, improving our ability to estimate rotation periods. We find that the rotation period of GJ 581 is 132 days, with no evidence of differential rotation. Welch's method may yield unacceptably large bias for datasets with N < 75 observations and works best on datasets with N > 100. Tapering the time-domain data can reduce the bias of the Welch's power spectrum estimator, but observers should not apply tapers to datasets with extremely uneven observing cadence. A software package for calculating magnitudesquared coherence and Welch's power spectrum estimates is available on github.

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Stellar Activity Manifesting at a One-year Alias Explains Barnard b as a False Positive

Abstract: Barnard’s star is among the most studied stars given its proximity to the Sun. It is often considered the radial velocity (RV) standard for fully convective stars due to its RV stability and equatorial decl. Recently, an M sin i = 3.3 M … Show more

Cited by 32 publications

References 84 publications

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with HPF

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with HPF

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with the Habitable-Zone Planet Finder

Magnitude-squared coherence: A powerful tool for disentangling Doppler planet discoveries from stellar activity

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