arXiv subjects

Howard Isaacson

Publications and source records attributed to Howard Isaacson.

At least 19 recordsLinked to original sources

arXiv2026

V347 Aurigae: from Peak to Quiescence

We present an analysis of eight epochs of high-resolution optical spectra of the YSO (young stellar object) V347 Aur. The source exhibits periodic brightness variations spanning $\sim$2 mag over several months, interpreted as cyclic accretion-burst and quiescent phases, making V347 Aur a useful probe of the broad range of accretion and outflow variability in YSOs. When faint, V347 Aur shows an M-type photospheric absorption spectrum; when bright, it exhibits a strong emission-line spectrum. Our Keck/HIRES spectroscopy was obtained during the decay of an accretion burst. The emission spectrum includes the strong H I, Ca II, and He I transitions commonly seen in YSOs, as well as numerous weaker metal lines, for which we provide a complete inventory. As the star fades, photospheric absorption lines deepen, consistent with decreasing veiling from the accretion continuum. H I and He I emission, which trace accretion, strengthen relative to the continuum as it fades, but show the expected decline in strength with decreasing accretion after correcting for the strong early veiling. Ca II behaves similarly, with line profiles that alternate between red- and blueshifted dominance, which we interpret as the rotation of the magnetospheric accretion flows. Narrow, slightly blueshifted absorption in Ca II, Na I, and K I indicates disk winds with localized density enhancements. Forbidden [O I] and [S II] emission is blueshifted, with broad shoulders and low- and high-velocity narrow peaks, consistent with a disk geometry that obscures much of the receding outflow. Their behavior as the continuum fades is similar to that of the broad accretion lines. The V347 Aur system is a useful proxy both for the ubiquitous short-term accretion variability observed in YSOs, and for their secular evolution on longer time scales.

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arXiv2026

Spectroscopic follow-up of compact object binary candidates from Gaia DR3: White dwarfs, neutron stars, black holes, and the parallax zeropoint

Astrometry and radial velocities (RVs) from Gaia DR3 yielded orbits for hundreds of thousands of binary systems, including several samples proposed to contain black holes (BHs), neutron stars (NSs), and white dwarfs (WDs). We present results of a systematic spectroscopic follow-up program targeting these objects. Beginning with a sample of 227 sources, we used a combination of archival data and many-epoch spectroscopic follow-up to characterize more than 200. We obtained 1292 high-quality RVs over a period of four years using the TRES and FEROS spectrographs, achieving a typical precision of 50 m/s and at least 10 RVs for 60 sources. We use these data to test the Gaia orbital solutions and tighten constraints on orbital parameters and component masses. Joint fitting of astrometry and RVs allows us to directly constrain flux ratios, verifying that undetected companions are genuinely dark. We find that ~60% of the astrometric candidates indeed host compact objects, including the two known Gaia BHs, 27 NS candidates, and a dozen massive WDs. We show that tight WD+WD binaries may masquerade as NSs within this sample. The spectroscopic candidates have lower purity: ~50% have spurious solutions, and a majority of the rest are post-mass-transfer binaries or hierarchical triples. Joint astrometry+RV fits of binaries with dark companions yield direct, parallax-independent distance measurements. Using 40 such systems, we measure the Gaia DR3 parallax zeropoint for astrometric orbital solutions. We find $Z=-0.0362\pm0.0053$ mas, consistent with the single-star zeropoint for sources of similar color and magnitude. These results will guide the selection of cleaner candidate samples from Gaia DR4, where a longer observing baseline will enable discovery of many more compact object binaries. RV follow-up will remain important for confirming individual systems, particularly those with extreme parameters.

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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

Evidence for an Extended Hydrogen Outflow on WASP-12 b

Recent observations of atmospheric escape from planets orbiting early-type stars indicate that planets with higher Roche filling factors have significantly higher mass loss rates. For three of these planets, full-orbit observations have revealed the presence of large leading and trailing tails of escaping planetary material. These 3D outflow geometries encode information about the underlying outflow physics, and can also be used to constrain their stellar wind properties and to predict their long-term orbital evolution. We present new evidence for extended H$α$ and H$β$ absorption from escaping hydrogen for a fourth planet, WASP-12 b. We observed WASP-12 b with the Keck Planet Finder on Keck I over a period of approximately eight hours centered on the transit. We find a H$α$ and H$β$ absorption signal in the stellar rest frame that is strongest during the transit (H$α$ amplitude: $2.66\pm0.25\%$, H$β$ amplitude: $2.88\pm0.43\%$), with evidence of both pre- and post-transit absorption. This measurement is consistent with outflow models where the escaping gas overflows the planet's Roche lobe and is advected into the stellar rest frame before being tidally sheared into extended tails. This outflow behavior is qualitatively similar to previous H$α$ observations of this planet as well as observations of extended outflows from other gas giants orbiting early-type stars.

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arXiv2026

Precise and Accurate Mass and Radius Measurements of Thirty Galactic Metal-Poor Stars in Detached Eclipsing Binaries

Detached eclipsing binaries have long served as fundamental benchmarks for making direct comparisons with stellar models. By combining the light curve and radial velocity orbit of a detached eclipsing binary, precise masses and radii can be measured for both components. However, catalogs of eclipsing binaries are dominated by solar-metallicity stars. Here we characterize fifteen eclipsing binaries selected to have $[\rm{M/H}]\lesssim -0.5$ using TESS photometry, Gaia XP spectra, large spectroscopic surveys, and Galactic kinematics. We use three spectrographs, PEPSI on the Large Binocular Telescope, the Levy spectrograph on the Automated Planet Finder telescope, and CHIRON on the SMARTS 1.5m telescope. We jointly model the light curves and radial velocities and measure the stellar masses and radii with median fractional uncertainties of < 1%. We use spectral disentangling to extract the spectra of both stars and model them using the spectral synthesis code Korg. We find three systems with $[\rm{M/H}] < -2.0$, two of which are kinematically consistent with the Milky Way's halo. J0001+2156 has $[\rm{M/H}]=-2.51$, making it the most metal-poor detached eclipsing binary characterized to date. Eleven of the twelve other systems have $[\rm{M/H}] \lesssim -0.5$ and are split between being highly ($[α/\rm{M}] > 0.25$) and moderately ($[α/\rm{M}] < 0.25$) $α$-enhanced. We compare our measured masses and radii to predictions from theoretical isochrones and estimate the ages of the binaries. We estimate ages with typical uncertainties of $\lesssim 0.5$ Gyr, and three of the binaries have estimated ages greater than 10 Gyr, up to $13.8^{+0.8}_{-0.7}$ Gyr. Finally, we update the empirical relations for stellar masses and radii on the main sequence using a sample of 621 stars.

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arXiv2026

Does HD 3167 Have Planets with Perpendicular Orbits?

The two transiting planets of HD 3167 were reported to follow nearly perpendicular paths, with the inner orbit aligned with the stellar equator and the outer orbit nearly polar. This interpretation depends critically on a challenging single-transit detection of the Rossiter-McLaughlin effect of the inner planet. We observed three additional transits of the inner planet with the Keck Planet Finder and analyzed the new data together with two archival ESPRESSO transit datasets. We do not confirm the previously reported low obliquity. Our analysis favors a projected obliquity of $-66^{+14}_{-12}$ degrees, consistent with coplanar orbits. However, because the best-fit projected rotation velocity is higher than expected, and because the obliquity uncertainty grows substantially when the most discrepant of the five transit datasets is omitted, we regard the geometry of the HD 3167 system as still unsettled.

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arXiv2026

The $γ$ Cephei System: Updated Orbits, Dynamical Architecture, and Limits on Additional Companions

The $γ$ Cephei system hosts one of the first exoplanets discovered and is orbited by one of the closest known stellar companions to a planet-hosting star. Here, we derive updated orbital fits for $γ$ Cep AB, the stellar binary, and Ab, the planet, by combining literature data with \textit{Hipparcos-Gaia} astrometry, new radial velocities (RVs), and adaptive optics imaging. We acquired 328 RVs of $γ$ Cep A with Keck/HIRES, AFP/Levy, McDonald/Tull, and Whipple/TRES, and eight adaptive optics imaging epochs with Keck/NIRC2, including the earliest spatially resolved image of $γ$ Cep B in 2003. These observations extend the precision RV baseline of $γ$ Cep to 45 years and the direct imaging baseline to 23 years, improving inferred orbital parameter precisions by a factor of 2--10 compared to previous work. For $γ$ Cep B, we derive a semi-major axis of $a_B=20.07 \pm 0.06$ AU, a mass of $M_B=415 \pm 2$ $M_{Jup}$ ($0.396 \pm 0.002$ $M_{\odot}$), an eccentricity of $e_B=0.422 \pm 0.002$, and an inclination of $i_B=119.8^{\circ}\pm0.1^{\circ}$. For $γ$ Cep Ab, we find a separation of $a_{Ab}=1.978 \pm 0.007$ AU, a minimum mass of $M_{Ab} \sin i = 1.62 \pm 0.04$ $M_{Jup}$, and an eccentricity of $e_{Ab}=0.07 \pm0.03$. Using the RV residuals and dynamical constraints, we rule out additional Jovians between 2.5--20 AU, and companions more massive than Neptune for $a<1$ AU, both at $>90\%$ confidence. The absence of additional giant planets over a broad range of orbital separations is consistent with a dynamically sculpted system in which the close stellar companion limited the formation or long-term survival of other distant companions.

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arXiv2026

A Century of Radial Velocity and Astrometric Monitoring of 70 Oph AB: New PFS Data and Constraints on Planetary Companions

At a distance of 5.1 pc, the 70 Oph AB binary star system is one of the most favorable targets for future direct imaging and astrometry missions surveying mature, terrestrial planets. We present new radial velocities (RVs) obtained with the Planet Finder Spectrograph (PFS) on the 6.5\,m Magellan II Clay Telescope in Chile. We collected 499 measurements of 70 Oph A and 334 measurements of 70 Oph B during 2023--2025. Combining these data with decades of archival RVs and astrometry, we derive an updated orbital solution for the binary and dynamical masses of $0.88 \pm 0.004\,M_\odot$ and $0.73 \pm 0.003\,M_\odot$ for the primary and secondary components, respectively. We find that the long-term RV variability of both components is consistent with stellar activity modulated by rotation periods, and we detect no coherent planetary signals in either component. We place upper limits on any planets orbiting in the plane of the binary. The 27 yr RV baseline for 70 Oph A excludes Jupiter-mass planets interior to 5 au and reaches a sensitivity of $0.3\,M_{\rm Jup}$ at 1 au or $0.5\,M_{\rm Jup}$ at 2 au. For 70 Oph B, with PFS data we rule out planets more massive than $0.25$--$0.3\,M_{\rm Jup}$ inside 0.5 au. We show that stable S-type orbits around 70 Oph A extend to $\sim2.5$ au, covering the habitable zone. Thus, Saturn-mass planets or smaller on stable orbits in the habitable zone of 70 Oph A are allowed. Overall, our results provide important guidance for future planet searches around this stellar system.

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arXiv2026

Conspicuous Gas, Cryptic Dust: Spectroscopy and Chromaticity of Complex Periodic Variables

Complex periodic variables (CPVs) are young low-mass stars whose light curves show periodic dips indicative of transiting corotating material. The origin and composition of this material are unclear. Here we present new optical and near-infrared spectroscopy and photometry of four CPVs from Magellan, Keck, Hale, MuSCAT1, MuSCAT2, Tierras, KeplerCam, and TESS. The spectra imply that CPVs host magnetically bound circumstellar plasma clumps, on the basis of sinusoidal-in-time Balmer emission out of transit, and Balmer dimming during transit. Yet large night-to-night changes in circumstellar hydrogen emissivity occur without clear changes in light curve morphology, suggesting that the sharp flux dips are caused not by circumstellar plasma but by dust. Optical chromaticities (depth proportional to $λ^{-β}$, with $β$ = 0.79 $\pm$ 0.16) support this, but the power law breaks in the near-infrared, where a single power law under-predicts the depths observed at 2.1 microns. We therefore favor dips caused by dusty plasma clumps with opaque cores and optically thin halos, though we cannot rule out models in which the dust properties vary per-star or per-epoch. Observations at wavelengths greater than 2 microns and less than 0.4 microns would test this interpretation and clarify the dust's origin.

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arXiv2026

Spin-Orbit Alignment of Two Neptune-size Planets Younger than 500 Myr: TOI-560 b and TOI-5082 b

Stellar obliquity measurements provide a direct probe of planetary system dynamics, but remain sparse for Neptune-size planets, particularly at young ages. We present Rossiter-McLaughlin measurements for two young Neptune-size planets, TOI-560 b and TOI-5082 b, using time-resolved Keck Planet Finder (KPF) spectroscopy and joint modeling with TESS transit photometry. We measure sky-projected obliquities of $λ_b = -25 \pm 16^\circ$ for TOI-560 b and $λ_b = 19^{+17}_{-13}{}^\circ$ for TOI-5082 b. Combining these constraints with stellar rotation periods and spectroscopic estimates of $v\sin i_\star$, we obtain 95% upper limits of $ψ< 69.7^\circ$ and $ψ< 50.5^\circ$, respectively. Both systems are therefore consistent with low-to-moderate true obliquities and show no evidence of strong spin-orbit misalignment. With ages of $480 \pm 190$ Myr for TOI-560 b and $180 \pm 9$ Myr for TOI-5082 b (a likely member of the CRIUS197 stellar association), the systems are in a key evolutionary phase when post-disk dynamical processes such as secular interactions may begin to manifest. Nevertheless, both systems remain consistent with low obliquity. In the broader context of young systems with existing measurements, these results support an emerging picture in which Neptune-size planets at $\lesssim 1$ Gyr are commonly found in low-obliquity configurations.

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arXiv2026

PANTERA. I. Hunting for the Most Metal-Rich Stars in the Solar Neighborhood with High-Resolution Spectroscopy

We present PANTERA (Project for Astrophysical Nucleosynthesis and Targeted Exploration of metal-Rich Abundances), a high-resolution spectroscopic survey of the most metal-rich stars in the solar neighborhood. In this first paper, we report iron abundances for 56 metal-rich stars, selected from Gaia DR3 XP spectrophotometric metallicities. These targets were observed with the PEPSI spectrograph on the Large Binocular Telescope (LBT), the Levy spectrograph on the Automated Planet Finder (APF) telescope, and the HIRES spectrograph on the Keck telescope. We measure [Fe/H] from an equivalent-width analysis of iron lines, with the effective temperature taken from photometry and the surface gravity from the Gaia parallax. We verify our measurement using Gaia benchmark stars, observing some sources with more than one spectrograph, using a second synthesis code, and an independent equivalent-width measurement. We find that the Gaia-XP metallicities over-predict [Fe/H] for part of our sample: they are consistent for the dwarfs, though a selection bias limits what the dwarf agreement can show, and reach $0.16\pm0.02$ dex for the cool giants. We attribute the over-prediction in part to the strong blue line blanketing of the cool metal-rich giants and to the survey labels on which the XP metallicities were trained. We also found 25 of the 56 stars to be ultra-metal-rich ([Fe/H] $> 0.4$), with the most iron-rich stars reaching [Fe/H]$=+0.58$. We discuss the implications of our result on studying metal-rich populations.

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arXiv2026

Gaia Exoplanet Orbits, Demographics, and Evolution Survey (GEODES): Characteristics of Three Long-Period Companions Accelerating their Host Stars

The upcoming release of $Gaia$ DR4 will yield thousands of giant planet candidates, eventually enabling studies of giant planet eccentricities, masses, and occurrence rates across a broad range of stellar host masses, metallicities, and ages. However, some of these planet candidates are expected to be false positives, and even genuine detections will require additional observations to precisely determine their orbits and masses. We present here the first results of the $Gaia$ Exoplanet Orbits, Demographics, and Evolution Survey (GEODES), an observational campaign to identify the most promising planet candidate hosts for pre-DR4 vetting and post-DR4 validation and characterization. In this paper we showcase three systems from our broader sample exhibiting both tangential and radial accelerations, each representing a distinct outcome of our survey strategy. We combine $Hipparcos$, $Hipparcos$-$Gaia$, $Gaia$ DR2, and $Gaia$ DR3 absolute astrometry with adaptive optics (AO) imaging and precision RVs to constrain companion masses and orbits. HIP 18512, a nearby (15.3 pc) K4V dwarf, hosts a low-mass stellar companion at 10.87" $\pm$ 0.07" (166 AU) which produces significant RV and astrometric accelerations on its host star. The RV trend and astrometric acceleration of the nearby (24.2 pc) K4V star HIP 45839, together with an AO imaging non-detection, constrain the companion to $a$ = 17.9^{+4.8}_{-2.7} AU ($P$ = 70--127 years) and $M$ = 45.2^{+10.5}_{-12.7} $M_{Jup}$. In the case of HIP 81991 (43.8 pc, G5V), the astrometric and RV data indicate that the companion has a separation of 6.4^{+0.6}_{-0.3} AU ($P$ = 14.4--17.7 years) and a mass of 9.5^{+5.4}_{-2.2} $M_{Jup}$, and is more likely a planet (65%) than a brown dwarf (35%).

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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

Hydrogen airglow from an escaping ultrahot Jupiter atmosphere

Intense high-energy irradiation of close-in gaseous exoplanets drives the rapid escape of their atmospheres, fundamentally shaping planetary demographics. While atmospheric loss is routinely observed via transit absorption in atomic hydrogen, helium, and metal ions, the underlying physical properties, specifically the thermal structure, outflow dynamics, and mass-loss rate, remain poorly constrained due to inherent degeneracies in the transmission geometry. Here we report the first detection of atomic hydrogen emission from the escaping atmosphere of a gas giant. Using high-resolution spectroscopy of the ultrahot Jupiter KELT-9 b, we detect a hydrogen Balmer line (Hα 6564.6 Å) emission signature originating from the planetary dayside. The emission line profile features a distinctive double-peaked shape with 0.1-0.15% peak amplitudes at +/-30 km/s and central self-absorption. This profile breaks transmission degeneracies, providing direct observational constraints on the vertical thermal structure, excited-state hydrogen populations, and wind dynamics in the upper atmosphere of KELT-9 b. Initial modeling reveals a vigorous outflow with a mass-loss rate above 10^{13} g/s, among the highest measured to date for gaseous exoplanets. Our results establish hydrogen airglow emission as a powerful diagnostic of atmospheric escape, opening a new observational window into the evolution of worlds in extreme radiation environments.

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arXiv2026

An Outer Giant Planet or Brown Dwarf in the 51 Pegasi System?

51 Pegasi harbors the first confirmed extrasolar planet orbiting a Sun-like star. Decades of continued radial velocity (RV) observations have since uncovered signatures of an additional distant companion in the system from a shallow radial acceleration. We present new constraints on the mass and separation of a potential outer companion based on a synthesis of RVs, absolute astrometry, and new high-contrast imaging. Our analysis combines 31 years of new and previously published RV measurements from the OHP/ELODIE, Lick/Hamilton, Keck/HIRES, and APF/Levy spectrographs; a $\sim$25-year baseline of absolute astrometry from Hipparcos and Gaia; and deep imaging from Keck/NIRC2 and HST/WFPC2. We find evidence for curvature in the RVs, which when combined with non-detections from imaging and astrometry point to a super-Jupiter at $\simeq$15--100 AU or brown dwarf companion at $\approx$20--170 AU. However, the inferred radial acceleration of the host star is driven primarily by the Lick/Hamilton dataset and its slope is consistent with long-term instrument drift, calling into question the nature of the long-period signal. If an outer companion is present, it could explain the origin of the inner hot Jupiter if 51 Peg b arrived at its current location through high-eccentricity migration. On the other hand, if the signal is spurious, the exceptional baseline rules out Jovian planets within $\sim$10 AU and most brown dwarfs within several tens of AU, implying that the system is devoid of massive companions. Continued RV and astrometric monitoring together with high-contrast imaging can be used to distinguish these scenarios.

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arXiv2026

The KPF-SLOPE Survey - Small, Compact Multi-Planet Systems Appear Spin-Orbit Aligned

The angle between stellar spin axes and planetary orbits -- stellar obliquity -- probes the dynamics of planetary migration and evolution. The obliquities of giant planets have been extensively studied because they are the most easily measured. Smaller planets, while more difficult to measure, have the advantage of better reflecting the dynamics of planetary systems because they trigger negligible back-reactions onto the host star. This paper introduces a new observational campaign called the Small, Low-mass Oblique Planets Experiment (SLOPE) survey with the Keck Planet Finder (KPF) spectrograph, and presents four new obliquity measurements. The SLOPE survey focuses on planets smaller than Saturn across a variety of system architectures. The sky-projected obliquities of the four planets measured -- TOI-1386b, TOI-480b, TOI-4596b, and TOI-1823b -- are all consistent with spin-orbit alignment. We validate the planetary nature of TOI-4596b with a significant obliquity detection. Including these measurements, we conducted a statistical analysis of the obliquities of sub-Saturn size planets in different planetary system architectures. Compared to other architectures, those in compact multi-planet systems reside in orbits that appear preferentially aligned with the stellar equator with 6 sigma confidence.

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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

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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