arXiv subjects

Samuel Halverson

Publications and source records attributed to Samuel Halverson.

At least 19 recordsLinked to original sources

arXiv2026

Shadowing the Sun I: Testing Synthetic Granulation Spectra with NEID Solar Eclipse Observations

The subtle Doppler radial velocity (RV) signature of an Earth-mass exoplanet in the Habitable Zone around a nearby Sun-like star is difficult to detect due to stellar variability. While multiple techniques aim to mitigate rotationally-modulated stellar activity, granulation, introducing 0.1-1 m s$^{-1}$ RV shifts, currently sets the floor for detecting Earth-mass RV signals for low-activity stars. Observations of the Rossiter-McLaughlin (RM) effect during a solar eclipse, with a peak-to-trough amplitude of $\sim$1.4 km s$^{-1}$, are a rigorous test of granulation and convective blueshift in the solar spectrum. Using NEID solar feed data of the October 2023 solar eclipse and GRASS-E (a geometry-modified version of GRASS), we compare the observed RVs for 22 lines with the simulated RVs from the synthetic granulation-driven spectra. Residuals of the GRASS-E Fe I 5434 angstrom model show an RMS of 13.47 m s$^{-1}$ over the RM signal. We find that a model with an optimized convective blueshift polynomial for Fe I 5434 significantly outperforms GRASS-E with an RMS of only 2.90 m s$^{-1}$, suggesting that GRASS-E is currently limited by sparse center-to-limb observations. By combining a set of spectral lines, we achieve a weighted RMS of 5.2 m s$^{-1}$ over the RM signal ($\sim$0.37\%), a level of accuracy well beyond the reach of current exoplanet transit observations. In all, we demonstrate that solar eclipse observations allow us to recover the center-to-limb variation of the convective blueshift for individual spectral lines.

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arXiv2026

Exposure-averaged Gaussian Processes for Combining Overlapping Datasets

Physically motivated Gaussian process (GP) kernels for stellar variability, like the commonly used damped, driven simple harmonic oscillators that model stellar granulation and p-mode oscillations, quantify the instantaneous covariance between any two points. For kernels whose timescales are significantly longer than the typical exposure times, such GP kernels are sufficient. For time series where the exposure time is comparable to the kernel timescale, the observed signal represents an exposure-averaged version of the true underlying signal. This distinction is important in the context of recent data streams from Extreme Precision Radial Velocity (EPRV) spectrographs like fast readout stellar data of asteroseismology targets and solar data to monitor the Sun's variability during daytime observations. Current solar EPRV facilities have significantly different exposure times per-site, owing to the different design choices made. Consequently, each instrument traces different binned versions of the same "latent" signal. Here we present a GP framework that accounts for exposure times by computing integrated forms of the instantaneous kernels typically used. These functions allow one to predict the true latent oscillation signals and the exposure-binned version expected by each instrument. We extend the framework to work for instruments with significant time overlap (i.e., similar longitude) by including relative instrumental drift components that can be predicted and separated from the stellar variability components. We use Sun-as-a-star EPRV datasets as our primary example, but present these approaches in a generalized way for application to any dataset where exposure times are a relevant factor or combining instruments with significant overlap.

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arXiv2026

A Headwind-like Helium Outflow from TOI-3884b

Constraining atmospheric loss is crucial for understanding the evolution of planetary atmospheres. We report the detection of an evaporating exosphere from TOI-3884b, a super-Neptune in a polar orbit around an M4 dwarf star. We infer this evaporation from a 6.8$\pm$0.27\% excess transit depth in the He I 10830 Å\, line, observed during five transits of TOI-3884b with the Habitable Zone Planet Finder spectrometer. While we observe outflow signatures starting as early as 5 hours before the onset of the planet's optical transit, no significant post-transit tail is detected. Hitherto, such a pre-transit headwind without a significant post-transit tail has only been detected from HAT-P-67b, a hot Saturn orbiting an F-type star. This asymmetric outflow morphology supports irradiation-driven mass loss over core-heated mass loss, and we explore whether this could be caused by sub-Alfvén star-planet interaction. The detection of such complex outflow morphologies is now opening a new window, enabling us to test theories of star-planet interactions and planetary magnetic fields with observations.

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arXiv2026

POSEIDON III: The Aligned Orbit of the Hot Neptune Around the Hot Star WASP-195

Stellar obliquities provide important clues as to the formation and migration histories of planetary systems, but measurements remain scarce for Neptune-mass planets, especially those orbiting hot stars (above the Kraft break). Here we present observations of the Rossiter-McLaughlin effect in the hot-star/hot-Neptune system WASP-195 ($T_{\rm eff}=6470\pm100$ K, $v\sin{i_\star}=10.5\pm1.1$ km s$^{-1}$) obtained with the Keck Planet Finder and NEID spectrographs. A joint analysis of these observations, archival photometry, and archival radial velocities yields a sky-projected stellar obliquity of $λ=-10\pm7^\circ$, consistent with spin-orbit alignment. This makes WASP-195 one of the few hot-star/hot-Neptune systems with a measured obliquity. Archival radial velocities from SOPHIE exclude Jupiter-mass planets within approximately 3 au at $5σ$ confidence. The aligned and nearly circular orbit is naturally consistent with a history of disk-driven migration, although coplanar high-eccentricity migration or Roche-lobe overflow cannot be ruled out. We also investigate why so few Neptunes around hot stars have measured obliquities. Their scarcity likely reflects a combination of the lower intrinsic occurrence of short-period Neptunes around hot stars and the difficulty of confirming planet candidates in this regime, where rapid stellar rotation broadens spectral lines and hampers conventional radial-velocity confirmation. Rapid rotation also increases the detectability of the Rossiter-McLaughlin effect, a feature that could help to widen the planet confirmation bottleneck while expanding the obliquity census of small planets around hot stars.

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arXiv2026

The NEID Earth Twin Survey. VI. Discovery of a Low-Mass Planet Orbiting HD 126053 and RV Signals Near the Rotation Periods of HD 168009 and HD 10780

Extreme precision radial velocity (EPRV) spectrographs are making strides in extending the limits of radial velocity exoplanet detection. Perhaps the best evidence for this is how readily EPRV instruments are facilitating the discovery and detailed characterization of small-amplitude planetary signals around stars that have already been intensively monitored for decades with less stable instruments. In this paper, we use data from the ongoing NEID Earth Twin Survey (NETS) to detect and confirm the planet HD 126053 b (P=10.91 d), which is among the least massive planets (m sin i = 3.60 m M_\oplus) around solar-type stars within 25 pc. We then jointly characterize the known planet HD 168009 b (P=15.14 d; m sin i = 8.64 M_\oplus) orbiting a solar analog along with a previously unreported signal at 27.8 d which we tentatively attribute to rotational modulation of stellar activity (P_rot = 30.1 d). We also analyze and discuss periodic RV variations in HD 10780 arising from a combination of rotationally-modulated activity and a long-term activity cycle. We discuss HD 126053 b and HD 168009 b in the context of the broader small planet population and our sensitivity to long-period planets in these systems, and we comment on our capacity to disambiguate planet-induced and activity-induced radial velocity signals. HD 126053 is exceptionally RV-quiet, making it a promising target for continued RV monitoring in search of low-mass exoplanets in the habitable zone.

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arXiv2026

Predicting Macroturbulence in F/G/K Dwarfs to 100 m/s Precision

We leverage the high resolution and spectral stability of the Keck Planet Finder (KPF) spectrograph to investigate macroturbulence broadening via the stellar cross-correlation function (CCF). As our calibration sample, we use main sequence benchmark slow-rotators v$\sin$i < 4 km/s where rotation rates were derived from the most robust asteroseismic mode splitting measurements, independent from spectral line broadening. We fit a linear relationship for macroturbulence as a function of derived $ν_{max}$, the frequency of maximum power due to stellar oscillations, with an RMS scatter of 90 m/s. Previous studies have calibrated macroturbulence against T$_{eff}$, but we find $ν_{max}$ to be a lower dispersion predictor by a factor of 4 indicating a deeper relationship between $ν_{max}$and convective motions.

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arXiv2026

Investigating the differential limb coupling effect for diffraction-limited spectrographs with PARVI

A promising new architecture for extreme-precision radial velocity (EPRV) spectrographs, hunting for small-amplitude stellar Doppler shifts induced by orbiting planets, is to build diffraction-limited instruments by using single-mode fibers fed by an adaptive optics system. However, the target stars are partially resolved when observing at the diffraction limit, and the resulting RVs are expected to be affected by differential limb coupling (DLC), an effect where the red- and blue-shifted sides of the stellar disk are coupled unequally into the spectrograph, producing an RV error term on the order of m/s for nearby EPRV target stars for the upcoming HISPEC spectrograph for Keck II. We present our efforts to directly measure the RV shifts resulting from DLC for the first time, using the diffraction-limited spectrograph PARVI, in order to verify the expected behavior of DLC and subsequently develop mitigation strategies for HISPEC and other future instruments. We outline an observing strategy designed to produce DLC-induced RV shifts of hundreds of m/s, and describe the execution of this experiment with PARVI. We use these data to characterize the PARVI tip-tilt guide camera and its performance, and have begun analysis of the derived RVs, though this investigation has proven complicated since the RVs are deeply entangled with other instrumental and algorithmic effects.

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arXiv2026

Searching for GEMS: Three warm Saturns and a super-Jupiter orbiting four early M-dwarfs

We report the confirmation and characterization of four transiting giant planets orbiting early-M dwarfs discovered by the Searching for Giant Exoplanets around M-dwarf Stars (GEMS) survey: TOI-7189 b, TOI-7265B b, TOI-7393 b, and TOI-7394B b. Joint modeling of TESS and ground-based photometry with precision radial velocities from the Habitable-zone Planet Finder and NEID spectrographs yields self-consistent orbital and physical parameters for all systems. The planets have short orbital periods ($P = 1.25-4.17$ days), masses spanning from $0.5\,M_{\rm J}$ to $2.1\,M_{\rm J}$, and radii comparable to Jupiter ($0.95\,R_{\rm J} < R_p < 1.02\,R_{\rm J}$). TOI-7189 b ($0.50\,M_{\rm J}$), TOI-7265B b ($0.71\,M_{\rm J}$), and TOI-7393 b ($0.61\,M_{\rm J}$) are Saturn-like in mass and density, whereas TOI-7394B b is a dense super-Jupiter ($2.10\,M_{\rm J}$, $ρ_p \approx 2.4$ g cm$^{-3}$) on a 1.25-day orbit. All hosts are early-M dwarfs with a narrow range of stellar properties, enabling a controlled comparison of giant-planet outcomes around low-mass stars. Three systems orbit super-solar metallicity stars, while TOI-7393 ($\mathrm{[Fe/H]} = -0.35 \pm 0.16$) is the most metal-poor GEMS host identified to date, and exhibits kinematics approaching the thin/thick-disk transition, suggestive of an older stellar population. Together, these systems reveal substantial diversity in the masses and bulk properties of short-period giant planets orbiting early-M dwarfs, demonstrating that markedly different planetary outcomes can arise around stars with otherwise similar fundamental properties.

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arXiv2026

The KPF SURFS-UP Survey I: Transmission Spectroscopy of WASP-76 b

We introduce the KPF SURFS-UP (Spectroscopy of the Upper-atmospheres and ReFractory Species in Ultra-hot Planets) Survey, a high-resolution survey to investigate the atmospheric composition and dynamics of a sample of ultra-hot Jupiters with the Keck Planet Finder (KPF). Due to the unique design of KPF, we developed a publicly available pipeline for KPF that performs blaze removal, continuum normalization, order stitching, science spectra combination, telluric correction, and atmospheric detection via cross-correlation. As a first demonstration, we applied this pipeline to a transit of WASP-76 b and achieved some of the highest signal-to-noise detections of refractory species in WASP-76 b to date (e.g., Fe I is detected at a SNR of 14.5). We confirm previous observations of an asymmetry in Fe I absorption, but find no measurable ingress-egress asymmetry in Na I and Ca II. Together, these results suggest variations within different layers of the atmosphere of WASP-76 b: neutral metals such as Fe I trace deeper regions with stronger asymmetries, while Na I and Ca II probe regions higher in the atmosphere where the ingress-egress asymmetries are weaker. Unlike some other ultra-hot Jupiters, our results are qualitatively consistent with GCM predictions of decreasing velocity asymmetry with altitude and do not require a high-altitude super-rotating jet that has been invoked for other planets (e.g., WASP-121 b). These results suggest that atmospheric circulation patterns in ultra-hot Jupiters may be more diverse than previously thought, highlighting the need for broader surveys to study how atmospheric dynamics depend on planetary and stellar properties.

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arXiv2026

A Revised Mass and Period for the Habitable Zone super-Earth GJ 3378b: A Planet Straddling the Cosmic Shoreline

The nearby ($d = 7.7$ pc) M4V star GJ~3378 is a target of our radial velocity (RV) exoplanet survey of fully convective stars in the Solar neighborhood with the near-infrared spectrograph HPF on the Hobby-Eberly Telescope (HET) at McDonald Observatory. Recently, Moutou et al.~(2024) announced the discovery of an $m\sin i = 5.26^{+0.94}_{-0.97} M_\oplus$ planet, GJ 3378b, with an orbital period of $24.73 \pm 0.06$ days, based on SPIRou RV data. Here, we present our HPF RVs for GJ 3378, as well as additional Doppler spectroscopy from the extreme precision NEID Spectrometer on the WIYN telescope at Kitt Peak National Observatory. We have analyzed the HPF+NEID RVs jointly with the published RVs from the CARMENES and SPIRou spectrometers. We present an orbital model for GJ 3378b that differs significantly from the Moutou et al.~solution. The joint RV model reduces the orbital period to $P = 21.45 \pm 0.01$d and the minimum mass to $m \sin i = 2.3 \pm 0.4 M_\oplus$. The shortened orbital distance remains within the conservative circumstellar liquid-water habitable zone (HZ), while the reduced mass increases the likelihood that the planet has a terrestrial composition. The revised planet properties place it near the ``cosmic shoreline," where planets in the HZs of M dwarfs may lose their atmospheres due to radiative stripping.

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arXiv2026

A Test of Substellar Evolutionary Models with High-Precision Ages from Asteroseismology and Gyrochronology for the Benchmark System HR 7672AB

We present high-precision measurements for HR~7672AB, composed of a Sun-like (G0V) star and an L~dwarf companion. Three nights of precise (70 cm/s) radial velocity (RV) asteroseismology with the Keck Planet Finder clearly detect 5-minute oscillations from the primary HR~7672A, and modeling of the frequency spectrum yields an asteroseismic age of $1.87\pm0.65$~Gyr. We also determine a gyrochronological age of $2.58\pm0.47$~Gyr, and we combine these two results for a final age of $2.26\pm0.40$~Gyr. In addition, we obtained new RVs for HR~7672A and new astrometry for the companion HR~7672B. From a joint orbit fit, we measured a dynamical mass of $1.111\pm0.017$~$\text{M}_\odot$ for HR~7672A and $75.39\pm0.67$~$\text{M}_{\text{Jup}}$ for HR~7672B. This places the companion near the stellar/substellar boundary and thus particularly sensitive to differences in model predictions. The joint precision in host star age (18\% uncertainty) and companion mass (0.9\% uncertainty) makes HR~7672AB an exceptional substellar benchmark. Combined with the companion's luminosity, we use these measurements to test predictions from six brown dwarf cooling models. The best agreement occurs with the Chabrier et al. (2023) models, which incorporate a new equation of state, resulting in predictions that agree within $<$0.3$σ$ with all the observations. The other 5 sets of models agree at the 1--3$σ$ level depending on the particular test, and some models struggle to predict a sufficient low luminosity for HR~7672B at any age given its dynamical mass. We also detected a weak seismic signal in near-simultaneous TESS photometry of HR~7672A, with the resulting RV-to-photometry oscillation amplitude ratio consistent with solar values.

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arXiv2026

The NEID Earth Twin Survey. IV. Confirming an 89 d, $m\sin i=10~\mathrm{M_\oplus}$ Planet Orbiting a Nearby Sun-like Star

We present the confirmation of HD 190360 d, a warm ($P=88.690^{+0.051}_{-0.049}~\mathrm{d}$), low-mass ($m\sin i=10.23^{+0.81}_{-0.80}~\mathrm{M_\oplus}$) planet orbiting the nearby ($d=16.0$ pc), Sun-like (G7) star HD 190360. We detect HD 190360 d at high statistical significance even though its radial velocity (RV) semi-amplitude is only $K=1.48\pm0.11~\mathrm{m~s^{-1}}$. Such low-amplitude signals are often challenging to confirm due to potential confusion with low-amplitude stellar signals. The HD 190360 system previously had two known planets: the $1.7~\mathrm{M_J}$ (true mass) HD 190360 b on a $7.9$ yr orbit and the $21~\mathrm{M_\oplus}$ (minimum mass) HD 190360 c on a $17.1$ d orbit. Here, we present an in-depth analysis of the HD 190360 planetary system that comprises more than 30 years of RV measurements and absolute astrometry from the Hipparcos and Gaia spacecrafts. Our analysis uses more than 1400 RVs, including nearly 100 from NEID. The proper motion anomaly as measured by these two astrometric missions solves for the dynamical mass of HD 190360 b and contributes to our understanding of the overall system architecture, while the long baseline of RVs enables the robust characterization of HD 190360 c and confirms the discovery of HD 190360 d.

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arXiv2026

TOI-4495: A Pair of Aligned, Near-Resonant Sub-Neptunes that Likely Experienced Overstable Migration

We report the discovery of a sub-Neptune and a Neptune-like planet ($R_b = 2.48^{+0.14}_{-0.10}\,R_\oplus$, $R_c = 4.03^{+0.23}_{-0.15}\,R_\oplus$) orbiting the F-type star TOI-4495. The planets have orbital periods of 2.567 days and 5.185 days, lying close to a 2:1 mean-motion resonance (MMR). Our photodynamical analysis of the TESS light curves constrains the planetary masses to $M_b = 7.7 \pm 1.4\,M_\oplus$ and $M_c = 23.2 \pm 4.7\,M_\oplus$. The measured masses and radii indicate the presence of volatile-rich gaseous envelopes on both planets. The Rossiter-McLaughlin effect and the Doppler shadow of TOI-4495 c reveal a well-aligned orbit with a projected stellar obliquity of $λ= -2.3^{+8.3}_{-7.8}\,\mathrm{deg}$. Combined with the low mutual inclination constrained by the photodynamical analysis ($ΔI < 8.7\,\mathrm{deg}$), the planetary orbits are likely coplanar and aligned with the host star's spin axis. We show that the planets are near, but not in, the 2:1 MMR, with a circulating resonant angle. We also find substantial free eccentricity for the inner planet, TOI-4495 b ($e_b = 0.078^{+0.020}_{-0.013}$). Given the observed proximity to the 2:1 resonance and the more massive outer planet, TOI-4495 b and c are particularly susceptible to resonant overstability, which can convert resonantly excited eccentricity into free eccentricity. However, additional mechanisms (e.g., planetesimal scattering) may be required to further excite the eccentricity by $\sim 4\%$. To prevent tidal damping from reducing the eccentricity below the observed level over the star's lifetime (1.9 Gyr), the reduced tidal quality factor of TOI-4495 b must be $Q' \gtrsim 10^5$, consistent with the presence of a thick envelope on the planet.

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arXiv2025

Searching for GEMS: The Occurrence of Giant Planets orbiting M-dwarfs within 100 pc

We present results from a systematic search for transiting short-period Giant Exoplanets around M-dwarf Stars (GEMS; $P < 10$ days, $R_p \gtrsim 8~R_\oplus$) within a distance-limited 100 pc sample of $149,316$ M-dwarfs using TESS-Gaia Light Curve (TGLC) data. We describe the development and application of the \textit{TESS-miner} package and associated vetting procedures used in this analysis. To assess detection completeness, we conducted $\sim$72 million injection-recovery tests across $\sim$26,000 stars with an average of $\sim$3 sectors of data per star, subdivided into early-type (M0--M2.5), mid-type (M2.5--M4), and late-type (M4 or later) M-dwarfs. Our pipeline demonstrates high sensitivity across all subtypes within the injection bounds. We estimate the occurrence rates of short-period GEMS as a function of stellar mass, and combine our measured rates with those derived for FGK stars, fitting an exponential trend with stellar mass, consistent with core-accretion theory predictions. We find GEMS occurrence rates of $0.118\% \pm 0.068\%$ for early-type M-dwarfs, $0.153\% \pm 0.069\%$ for mid-type, and $0.036\% \pm 0.024\%$ for late-type M-dwarfs, with a mean rate of $0.068\%\pm0.024\%$ across the full sample. While our search spanned $1.0~\mathrm{days} < P < 10.0$ days, these rates were calculated using planets orbiting with $1.0~\mathrm{days} < P < 5.0$ days. This work establishes the basis for future occurrence rate studies of transiting GEMS.

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arXiv2025

The OATMEAL Survey. II. The 3D spin-orbit obliquity of an eccentric transiting brown dwarf in the Ruprecht 147 open cluster

We present new analysis of the CWW 89 system as part of the Orbital Architectures of Transiting Massive Exoplanets And Low-mass stars (OATMEAL) survey. The CWW 89 system is a member of the 2.8 Gyr old Ruprecht 147 (NGC 6774) cluster and features two stars, CWW 89A (EPIC 219388192) and CWW 89B, with the primary hosting a transiting brown dwarf. We use in-transit, highly precise radial velocity measurements with the Keck Planet Finder (KPF) to characterize the Rossiter-McLaughlin (RM) effect and measure the projected spin-orbit obliquity $|λ|=1.4\pm2.5^\circ$ and the full 3D spin-orbit obliquity of the brown dwarf to be $ψ=15.1^{+15.0^\circ}_{-10.9}$. This value of $λ$ implies that the brown dwarf's orbit is prograde and well-aligned with the equator of the host star, continuing the trend of transiting brown dwarfs showing a preference for alignment ($λ\approx 0^\circ$) regardless of the stellar effective temperature. We find that this contrast with the transiting giant planet population, whose spin-orbit alignments depend on host $T_{\rm eff}$, shows an increasingly clear distinction in the formation and orbital migration mechanisms between transiting giant planets and transiting brown dwarfs like CWW 89Ab. For this system in particular, we find it plausible that the brown dwarf may have undergone coplanar high-eccentricity migration influence by CWW 89B.

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arXiv2025

Order-by-order Modeling of Exoplanet Radial Velocity Data

Precise radial velocity (RV) measurements are a crucial tool for exoplanet discovery and characterization. Today, the majority of these measurements are derived from Echelle spectra in the optical wavelength region using cross-correlation techniques. Although for certain stars these approaches can produce RVs with sub-1 m~s$^{-1}$ measurement errors, for many others, we are now in a regime where instrumental precision is fundamentally below the intrinsic RV variations of the star that result from astrophysical processes that can be correlated in both time and wavelength. We explore new methods for measuring exoplanet orbital parameters that take advantage of the fact that RV data sets are fundamentally multi-wavelength. By analyzing NEID extremely precise radial velocity (EPRV) data of three known exoplanet systems, we show that fitting a single Keplerian model to multi-wavelength RVs can produce a factor of 1.5 -- 6.8 better $M_p \sin i$ uncertainties compared to fitting RVs that are derived from a weighted average across wavelength.

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arXiv2025

Precise Radial Velocities

Precise measurements of a star's radial velocity (RV) made using extremely stable, high resolution, optical or near infrared spectrographs can be used to determine the masses and orbital parameters of gravitationally-bound extra-solar planets (exoplanets). Indeed, RV surveys and follow up efforts have provided the vast majority of published exoplanet mass measurements and in doing so have enabled studies into exoplanet interior and atmospheric compositions. Here we review the current state of the RV field, with particular attention paid to: -The evolution of precise RV methodologies over the past two decades -Modern RV spectrograph designs that can be calibrated to a stability level of better than 50 cm/s over timescales of years -RV data reduction and post-processing techniques that minimize the impact of instrument systematics and stellar variability -Techniques for detecting exoplanets in RV data and disentangling planetary signals from stellar variability

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arXiv2025

Discovery of a Nearby Habitable Zone Super-Earth Candidate Amenable to Direct Imaging

We present the discovery of GJ 251 c, a candidate super-Earth orbiting in the Habitable Zone (HZ) of its M dwarf host star. Using high-precision Habitable-zone Planet Finder (HPF) and NEID RVs, in conjunction with archival RVs from the Keck I High Resolution Echelle Spectrometer (HIRES), the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrograph (CARMENES), and the SPectropolarimètre InfraROUge (SPIRou), we improve the measured parameters of the known planet, GJ 251 b ($P_{b}$ = 14.2370 days; $m \sin(i)$ = 3.85$^{+0.35}_{-0.33}$ M$_{\oplus}$), and we significantly constrain the minimum mass of GJ 251 c, placing it in a plausibly terrestrial regime (P$_{c}$ = 53.647 $\pm$ 0.044 days; $ m \sin i_{c}$ = 3.84 $\pm$ 0.75 M$_{\oplus}$). Using activity mitigation techniques that leverage chromatic information content, we perform a color-dependent analysis of the system and a detailed comparison of more than 50 models that describe the nature of the planets and stellar activity in the system. Due to GJ 251's proximity to Earth (5.5 pc), next generation, thirty meter class telescopes will likely be able to image terrestrial planets in GJ 251's HZ. In fact, GJ 251 c is currently the best candidate for terrestrial, HZ planet imaging in the Northern Sky.

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