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Abstract Giant planets and brown dwarfs are thought to form via a combination of pathways, including bottom-up mechanisms in which gas is accreted onto a solid core and top-down mechanisms in which gas collapses directly into a gravitationally bound object. One can distinguish the prevalence of these mechanisms using host star metallicities. Bottom-up formation thrives in metal-rich environments, whereas top-down formation is weakly dependent on ambient metal content. Using a hierarchical Bayesian model and the results of the California Legacy Survey (CLS), a low-bias and homogeneously analyzed radial velocity survey, we find evidence for a transition in the stellar metallicity distribution at a companion mass of $\gamma =27_{-8}^{+12}{M}_{\mathrm{Jup}}$ for companions with orbital separations between 1 and 50 au. Companions below and above this threshold tend to orbit stars with higher ([Fe/H] = 0.17 ± 0.12 dex) and lower ([Fe/H] = −0.03 ± 0.10 dex) metallicities, respectively. Previous studies of relatively close-in companions reported evidence of a lower transition mass of ≤10M_Jup. When applied to the CLS sample, our model predicts the probability of a transition in the stellar metallicity distribution at or below 10M_Jupto be <1%. We compare our results to estimates ofγgleaned from other observational metrics and discuss implications for planet-formation theory. 

Abstract We present a 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 consists of two stars, CWW 89A (EPIC 219388192) and CWW 89B, with the primary hosting a transiting brown dwarf, CWW 89Ab. We use in-transit, highly precise radial velocity measurements with the Keck Planet Finder to characterize the Rossiter–McLaughlin (RM) effect and measure the projected spin–orbit obliquity ∣λ∣ = 1 $\stackrel{\text{°}}{.}$ 4 ± 2 $\stackrel{\text{°}}{.}$ 5 and the full 3D spin–orbit obliquity of the brown dwarf to be $\psi =15.1_{-10.9}^{+15.0^{\circ }}$ . This value ofλimplies 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 spin–orbit alignment (λ ≈ 0°) regardless of the stellar effective temperature. This contrast with the transiting giant planet population, whose spin–orbit alignments depend on hostT_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 influenced by CWW 89B. 

Interaction in close binary systems is common in massive stars. Typically, the mass donor is stripped of its hydrogen envelope and evolves to become a hot helium star, while the accretor gains mass and angular momentum, spinning up in the process. However, the small number of well-constrained post-interaction binary systems currently limits detailed comparisons with binary evolution models. We have identified a new post-interaction binary, HIP 15429, consisting of a stripped star and a recently formed rapidly rotating Be star companion (v_rotsini ≈ 270 km/s) that shares many similarities with recently identified bloated stripped stars. Based on the orbital fitting of multi-epoch radial velocities, we find a 221-day binary period. We also find an eccentricity ofe = 0.52, which is unexpectedly high, as tides are expected to have circularised the orbit efficiently during the presumed recent mass transfer. The formation of a circumbinary disc during the mass-transfer phase or the presence of an unseen tertiary companion might explain the orbit’s high eccentricity. We determined the physical parameters for both stars in HIP 15429 by fitting the spectra of the disentangled binary components and multi-band photometry. The stripped nature of the donor star is affirmed by its high luminosity at a low inferred mass (≲1 M_⊙) and the imprints of CNO-processed material on the surface abundances. The donor’s large radius and cool temperature (T_eff = 13.5 ± 0.5 kK) suggest that it has only recently ceased mass transfer. Evolutionary models assuming a 5–6 M_⊙progenitor can reproduce these parameters, and they imply that the binary is currently evolving towards a stage where the donor becomes a subdwarf orbiting a Be star. The remarkably high eccentricity of HIP 15429 challenges standard tidal evolution models, suggesting either inefficient tidal dissipation or external influences, such as a tertiary companion or circumbinary disc. This underscores the need to identify and characterise more post-mass transfer binaries to benchmark and refine theoretical models of binary evolution. 

Abstract The “Neptunian ridge” is a recently identified peak in the frequency of planets with sizes between that of Neptune and Saturn orbiting their host stars with periods between 3 and 6 days. These planets may have formed similarly to their larger, hot Jupiter counterparts in the “3 day pileup,” through a dynamically excited migration pathway. The distribution of stellar obliquities in hot Neptune systems may therefore provide a vital clue as to their origin. We report a new stellar obliquity measurement for TOI-2374b, a planet in the Neptunian ridge (P= 4.31 days,R_p = 7.5R_⊕). We observed a spectroscopic transit of TOI-2374b with the Keck Planet Finder, detecting the Rossiter–McLaughlin (RM) anomaly with an amplitude of 3 m s⁻¹, and measured a sky-projected obliquity of $\lambda ={81°}_{-22^{\circ }}^{+23^{\circ }}$ , indicating an orbit significantly misaligned with the spin axis of its host star. A reloaded RM analysis of the cross-correlation functions confirms this misalignment, measuring $\lambda ={65°}_{-24^{\circ }}^{+32^{\circ }}$ . Additionally, we measured a stellar rotation period of $P_{\mathrm{rot}}=26.4_{-0.8}^{+0.9}$ days with photometry from theTierrasobservatory, allowing us to deduce the three-dimensional stellar obliquity of $\psi ={85\stackrel{°}{.}9}_{-9\stackrel{°}{.}2}^{+8\stackrel{°}{.}6}$ . TOI-2374b joins a growing number of hot Neptunes on polar orbits. The high frequency of misaligned orbits for Neptunian ridge and desert planets, compared with their longer period counterparts, is reminiscent of patterns seen for the giant planets and may suggest a similar formation mechanism. 

Abstract We measure the true obliquity of TOI-2364, a K dwarf with a sub-Saturn-mass (M_p = 0.18M_J) transiting planet on the upper edge of the hot-Neptune desert. We used new Rossiter–McLaughlin observations gathered with the Keck Planet Finder to measure the sky-projected obliquityλ = 7° + 10°–11°. Combined with a stellar rotation period of 23.47 ± 0.29 days measured with photometry from theTierrasObservatory, this yields a stellar inclination of 90° ± 13° and a true obliquityψ = 15 $\stackrel{\text{°}}{.}$ 6 + 7 $\stackrel{\text{°}}{.}$ 7–7 $\stackrel{\text{°}}{.}$ 3, indicating that the planet’s orbit is well aligned with the rotation axis of its host star. The determination ofψis important for investigating a potential bimodality in the orbits of short-period sub-Saturns around cool stars, which tend to be either aligned with or perpendicular to their host stars’ spin axes. 

Abstract We report the discovery of a sub-Neptune and a Neptune-like planet (R ${}_b=2.48_{-0.10}^{+0.14}$ R_⊕,R ${}_c=4.03_{-0.15}^{+0.23}$ R_⊕) orbiting the F-type star TOI-4495. The planets have orbital periods of 2.567 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 toM_b = 7.7 ± 1.4M_⊕and M_c = 23.2 ± 4.7M_⊕. 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 $\lambda =-2.3_{-7.8}^{+8.3}°$ . Combined with the low mutual inclination constrained by the photodynamical analysis (ΔI < 8.7°), 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 ( $e_b=0.078_{-0.013}^{+0.02}$ ). 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 in turn can explain the observed eccentricity by converting resonantly excited eccentricity into free eccentricity. However, additional mechanisms (e.g., planetesimal scattering) may be required to further excite the eccentricity by ∼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 $Q^{\prime }\gtrsim 10^5$ , consistent with the presence of a thick envelope on the planet. 

Abstract Stellar spin down is a critical yet poorly understood component of stellar evolution. In particular, results from the Kepler Mission imply that mature age, solar-type stars have inefficient magnetic braking, resulting in a stalled spin-down rate. However, a large number of precise asteroseismic ages are needed for mature (≥3 Gyr) stars in order to probe the regime where traditional and stalled spin-down models differ. In this paper, we present a new asteroseismic benchmark star for gyrochronology discovered using reprocessed Kepler short cadence data. KIC 11029516 (Papayu) is a bright (Kp= 9.6 mag) solar-type star with a well-measured rotation period (21.1 ± 0.8 days) from spot modulation using 4 yr of Kepler long-cadence data. We combine asteroseismology and spectroscopy to obtainT_eff= 5888 ± 100 K, [Fe/H] = 0.30 ± 0.06 dex,M= 1.24 ± 0.05M_⊙,R= 1.34 ± 0.02R_⊙, and age of 4.0 ± 0.4 Gyr, making Papayu one of the most similar stars to the Sun in terms of temperature and radius with an asteroseismic age and a rotation period measured from spot modulation. We find that Papayu sits at the transition of where traditional and weakened spin-down models diverge. A comparison with stars of similar zero-age main-sequence temperatures supports previous findings that weakened spin-down models are required to explain the ages and rotation periods of old solar-type stars. 

Abstract We report an observation of a transit of the hot Jupiter (HJ) KELT-23A b with the Keck Planet Finder spectrograph and a measurement of the sky-projected obliquity (λ) of its Sun-like (T_eff ≈ 5900 K) host star. We measured a projected stellar obliquity ofλ ≈ 180°, indicating that the orbit of the HJ is retrograde relative to the direction of the stellar spin. Due to the slow sky-projected rotational velocity of the host star ( $v\sin i_{\star }\approx 0.5$ km s⁻¹), the true orbit of the HJ could be closer to polar. HJs around stars with effective temperatures below the Kraft break—such as KELT-23A—are generally found to have prograde orbits that are well-aligned with the equatorial planes of their host stars (i.e.,λ ∼ 0°), most likely due to spin–orbit realignment driven by stellar tidal dissipation. This system is therefore a unique outlier that strains migration and tidal theories. The fact that the HJ has a highly misaligned orbit may suggest that the planet arrived at its close-in orbit relatively recently, possibly via interactions with the wide-separation (570 au) M-dwarf companion in the system, or that it has stalled near an antialigned or polar orientation while realigning. Using Gaia DR3, we determined the orbit of the stellar companion to be moderately face-on (γ = 60° ± 4°). We show that the distribution of observed systems in theγ–λplane can be broadly reproduced using a toy model in which the orbits of the planetary and stellar companions begin aligned with the equatorial plane of the primary star and, upon migrating inwards, the planet preferentially obtains either an aligned or polar orbit. 

Abstract Extreme precision radial velocity (EPRV) measurements contend with internal noise (instrumental systematics) and external noise (intrinsic stellar variability) on the road to 10 cm s⁻¹“exo-Earth” sensitivity. Both of these noise sources are well-probed using “Sun-as-a-star” RVs and cross-instrument comparisons. We built the Solar Calibrator (SoCal), an autonomous system that feeds stable, disk-integrated sunlight to the recently commissioned Keck Planet Finder (KPF) at the W. M. Keck Observatory. With SoCal, KPF acquires signal-to-noise ratio (S/N) ∼ 1200,R= 98,000 optical (445–870 nm) spectra of the Sun in 5 s exposures at unprecedented cadence for an EPRV facility using KPF’s fast readout mode (<16 s between exposures). Daily autonomous operation is achieved by defining an operations loop using state machine logic. Data affected by clouds are automatically flagged using a reliable quality control metric derived from simultaneous irradiance measurements. Comparing solar data across the growing global network of EPRV spectrographs with solar feeds will allow EPRV teams to disentangle internal and external noise sources and benchmark spectrograph performance. To facilitate this, all SoCal data products are immediately available to the public on the Keck Observatory Archive. We compared SoCal RVs to contemporaneous RVs from NEID, the only other immediately public EPRV solar data set. We find agreement at the 30–40 cm s⁻¹level on timescales of several hours, which is comparable to the combined photon-limited precision. Data from SoCal were also used to assess a detector problem and wavelength calibration inaccuracies associated with KPF during early operations. Long-term SoCal operations will collect upwards of 1000 solar spectra per six-hour day using KPF’s fast readout mode, enabling stellar activity studies at high S/N on our nearest solar-type star.