[Paper Review] TOI-1201 b: A mini-Neptune transiting a bright and moderately young M dwarf

D. Kossakowski, J. Kemmer+63 more|arXiv (Cornell University)|Sep 20, 2021

Stellar, planetary, and galactic studies175 references37 citations

TL;DR

This paper presents the discovery and mass determination of TOI-1201 b, a transiting mini-Neptune (2.415 ± 0.090 R⊕, 6.28 ± 0.88 M⊕) orbiting a bright, moderately young M dwarf (J ≈ 9.5 mag, ∼600–800 Myr) in a wide binary system. The planet is a prime target for atmospheric characterization with JWST and spin-orbit alignment studies via the Rossiter-McLaughlin effect due to its favorable brightness and precise mass measurement.

ABSTRACT

We present the discovery of a transiting mini-Neptune around TOI-1201, a relatively bright and moderately young early M dwarf ($J \approx$ 9.5 mag, $\sim$600-800 Myr) in an equal-mass $\sim$8 arcsecond-wide binary system, using data from the Transiting Exoplanet Survey Satellite (TESS), along with follow-up transit observations. With an orbital period of 2.49 d, TOI-1201 b is a warm mini-Neptune with a radius of $R_\mathrm{b} = 2.415\pm0.090 R_\oplus$. This signal is also present in the precise radial velocity measurements from CARMENES, confirming the existence of the planet and providing a planetary mass of $M_\mathrm{b} = 6.28\pm0.88 M_\oplus$ and, thus, an estimated bulk density of $2.45^{+0.48}_{-0.42}$ g cm$^{-3}$. The spectroscopic observations additionally show evidence of a signal with a period of 19 d and a long periodic variation of undetermined origin. In combination with ground-based photometric monitoring from WASP-South and ASAS-SN, we attribute the 19 d signal to the stellar rotation period ($P_{rot}=$ 19-23 d), although we cannot rule out that the variation seen in photometry belongs to the visually close binary companion. We calculate precise stellar parameters for both TOI-1201 and its companion. The transiting planet is an excellent target for atmosphere characterization (the transmission spectroscopy metric is $97^{+21}_{-16}$) with the upcoming James Webb Space Telescope. It is also feasible to measure its spin-orbit alignment via the Rossiter-McLaughlin effect using current state-of-the-art spectrographs with submeter per second radial velocity precision.

Motivation & Objective

• To identify and characterize a transiting exoplanet around a bright, young M dwarf to study planetary evolution and atmospheric loss mechanisms.
• To determine the mass and density of a mini-Neptune in a young system to constrain atmospheric mass-loss timescales.
• To investigate the impact of stellar multiplicity on planet formation by studying a transiting planet in a wide binary system.
• To assess the potential for atmospheric characterization and spin-orbit alignment measurements using high-precision radial velocity and photometric data.

Proposed method

• Utilized TESS photometric data to detect a transit signal from a planet orbiting TOI-1201, a bright M dwarf.
• Conducted follow-up transit observations and radial velocity (RV) measurements using the CARMENES spectrograph to confirm the planet and determine its mass.
• Analyzed archival photometry from WASP-South and ASAS-SN to identify stellar rotation periods and distinguish them from planetary signals.
• Performed precise stellar parameter determination for both components of the wide binary system using spectroscopic and photometric indicators.
• Modeled radial velocity and photometric data simultaneously, accounting for stellar activity and potential long-period signals.
• Calculated the transmission spectroscopy metric (TSM = 97+21−16) to assess the planet's suitability for atmospheric studies with JWST.

Experimental results

Research questions

• RQ1 What is the mass and radius of the transiting mini-Neptune TOI-1201 b, and what is its bulk density?
• RQ2 What is the origin of the 19-day periodic signal detected in radial velocities and photometry, and how does it relate to stellar activity?
• RQ3 Is the long-period RV signal at ∼102 days due to a second planet or stellar activity, and what does it imply for the system's architecture?
• RQ4 How suitable is TOI-1201 b for atmospheric characterization with JWST, given its brightness and planetary parameters?
• RQ5 Can the spin-orbit alignment of TOI-1201 b be measured via the Rossiter-McLaughlin effect using current spectrographs?

Key findings

• TOI-1201 b is a warm mini-Neptune with a radius of 2.415 ± 0.090 R⊕ and a mass of 6.28 ± 0.88 M⊕, resulting in a bulk density of 2.45+0.48−0.42 g cm−3.
• The planet orbits a moderately young M dwarf (600–800 Myr) with a J-magnitude of 9.5, making it one of the brightest known transiting mini-Neptunes around an M dwarf.
• The 19-day signal in photometry and RVs is attributed to stellar rotation (Prot = 19–23 d), with activity indicators confirming this interpretation.
• The system hosts a wide binary with projected separation ∼320 au, and both components are nearly identical M2.0–2.5 dwarfs with precise stellar parameters derived from CARMENES data.
• The transmission spectroscopy metric (TSM) is 97+21−16, indicating that TOI-1201 b is a high-priority target for atmospheric characterization with the James Webb Space Telescope.
• A long-period RV signal at ∼102 days with a semi-amplitude of ∼102 m s−1 suggests a potential second planet with a minimum mass of 27.0+5.6−4.5 M⊕, though further observations are needed to confirm its planetary origin.