The Habitable Zone Planet Finder Reveals a High Mass and Low Obliquity for the Young Neptune K2-25b

Abstract

Using radial velocity data from the Habitable Zone Planet Finder, we have measured the mass of the Neptune-sized planet K2-25b, as well as the obliquity of its M4.5 dwarf host star in the 600-800 Myr Hyades cluster. This is one of the youngest planetary systems for which both of these quantities have been measured and one of the very few M dwarfs with a measured obliquity. Based on a joint analysis of the radial velocity data, time-series photometry from the K2 mission, and new transit light curves obtained with diffuser-assisted photometry, the planet's radius and mass are 3.44±0.12 R ⊕ and -+ Å M 24.5 5.2 5.7. These properties are compatible with a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass). We measure an orbital eccentricity of e=0.43±0.05. The skyprojected stellar obliquity is λ=3°±16°, compatible with spin-orbit alignment, in contrast to other "hot Neptunes" that have been studied around older stars.

Cluster Difference Imaging Photometric Survey. II. TOI 837: A Young Validated Planet in IC 2602

“…This gives σ RV,rot ≈300 m s −1 and is consistent with the scatter we observed in our RVs from FEROS. Detecting a planet's Keplerian motion in this regime is challenging and requires a significant amount of data and care in signal extraction (Barragán et al 2019;Stefansson et al 2020). RM measurement avoids the majority of this issue because the transit occurs over a much shorter duration than a single stellar rotation period.…”

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

“…It is expected that closeorbiting giant planets around such stars are aligned (Gaudi & Winn 2007;Winn et al 2010;Muñoz & Perets 2018;Louden et al 2021) because of strong tidal interactions with the stellar convective envelope. To date, there are only six other planets around M dwarfs with measured obliquities, namely GJ 436 b (Bourrier et al 2018), TRAPPIST-1 b, e, f (Hirano et al 2020a), AU Mic b (Addison et al 2021;Hirano et al 2020b;Palle et al 2020), and K2-25 b (Stefansson et al 2020). Determining the spin-orbit angle of TOI-1201 b is especially interesting in the context of the companion (320 au away) because it could hint possible interaction.…”

“…Planets found above the radius gap are in the process of losing their atmospheres through two conceivable mechanisms with corresponding timescales of ∼1 Gyr due to core-powered mass loss (Gupta & Schlichting 2020) or of ∼100 Myr due to photoevaporation (Owen & Wu 2017). Other currently known transiting mini-Neptunes planets with mass determinations in young systems orbit stellar hosts are too faint for atmospheric characterization (J > 11 mag), for example, K2-25 (Mann et al 2016;Stefansson et al 2020;Thao et al 2020), EPIC-247 589 423 (Mann et al 2018), K2-264 (Rizzuto et al 2018;Livingston et al 2019), andK2-101, K2-101, K2-103, K2-104, EPIC-211 901 114, andK2-95 (Mann et al 2017). The only suitable candidate orbiting a sufficiently bright host star for this selective target sample is K2-100 b (Barragán et al 2019;Gaidos et al 2020).…”

A Mini-Neptune and a Radius Valley Planet Orbiting the Nearby M2 Dwarf TOI-1266 in Its Venus Zone: Validation with the Habitable-zone Planet Finder

“…Figure 5 shows generalized LS periodograms of the HPF RVs, along with a number of activity indicators measured from the HPF spectra, including the dLW, the chromatic index (CRX), and the line indices of the three Ca II IRT lines. To calculate the activity indicators, we follow the definition and procedures in the SERVAL pipeline (Zechmeister et al 2018), and we note that their use of HPF spectra, including listing the exact wavelength ranges used to calculate the Ca II IRT indices, is further discussed in Stefansson et al (2020b). We calculate the generalized LS periodograms using the astropy.timeseries package, and we calculate the EXOFASTv2 derived values using MIST isochrones with the Gaia parallax and spectroscopic parameters in and using the spectral matching parameters derived from the HPF spectral matching analysis as priors.…”

“…For the ground-based data sets, we follow Stefansson et al (2020b) and use the approximate Matern 3/2 kernel multiplied by an exponential kernel available in juliet. 29 This kernel has covariance properties that are better matched to shorterterm instrumental and/or atmospheric red-noise structures often seen in ground-based data sets (see, e.g., Pepper et al 2017;Espinoza et al 2019).…”