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1. The California Legacy Survey. V. Chromospheric Activity Cycles in Main-sequence Stars

   https://doi.org/10.3847/1538-4365/ad676c  

   Isaacson, Howard; Howard, Andrew W; Fulton, Benjamin; Petigura, Erik A; Weiss, Lauren M; Kane, Stephen R; Carter, Brad; Beard, Corey; Giacalone, Steven; Van_Zandt, Judah; et al (September 2024, The Astrophysical Journal Supplement Series)

   Abstract We present optical spectroscopy of 710 solar neighborhood stars collected over 20 years to catalog chromospheric activity and search for stellar activity cycles. The California Legacy Survey stars are amenable to exoplanet detection using precise radial velocities, and we present their CaiiH and K time series as a proxy for stellar and chromospheric activity. Using the High Resolution Echelle Spectrometer at Keck Observatory, we measured stellar flux in the cores of the CaiiH and K lines to determineS-values on the Mount Wilson scale and the $\log \left(R_{\mathrm{HK}}^{\prime }\right)$metric, which is comparable across a wide range of spectral types. From the 710 stars, with 52,372 observations, 285 stars were sufficiently sampled to search for stellar activity cycles with periods of 2–25 yr, and 138 stars showed stellar cycles of varying length and amplitude.S-values can be used to mitigate stellar activity in the detection and characterization of exoplanets. We used them to probe stellar dynamos and to place the Sun's magnetic activity into context among solar neighborhood stars. Using precise stellar parameters and time-averaged activity measurements, we found tightly constrained cycle periods as a function of stellar temperature between $\log \left(R_{\mathrm{HK}}^{\prime }\right)$of −4.7 and −4.9, a range of activity in which nearly every star has a periodic cycle. These observations present the largest sample of spectroscopically determined stellar activity cycles to date. 

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2. Efficient Ionizers with Low H β + [O iii ] Equivalent Widths: JADES Spectroscopy of a Peculiar High-redshift Population

   https://doi.org/10.3847/1538-4357/adddb5  

   Laseter, Isaac H; Maseda, Michael V; Simmonds, Charlotte; Endsley, Ryan; Stark, Daniel; Bunker, Andrew J; Bhatawdekar, Rachana; Boyett, Kristan; Cameron, Alex J; Carniani, Stefano; et al (July 2025, The Astrophysical Journal)

   Abstract Early JWST photometric studies discovered a population of UV-faint ( $<{\mathit{L}}_{\mathrm{UV}}^{*}$)z ∼ 6.5–8 Lyman break galaxies with spectral energy distributions implying young ages (∼10 Myr) yet relatively weak Hβ+ [Oiii] equivalent widths (EW_{Hβ+ [Oiii]} ≈ 400 Å). These galaxies seemingly contradict the implicit understanding that young star-forming galaxies are ubiquitously strong Hβ+ [Oiii] emitters, i.e., extreme emission line galaxies (EW ≳750 Å). Low metallicities, high Lyman continuum escape fractions, and rapidly declining star formation histories have been proposed as primary drivers behind low Hβ+ [Oiii] EWs, but the blend of Hβ+ [Oiii] in photometric studies makes proving one of these scenarios difficult. We aim to characterize this peculiar population with deep spectroscopy from the JWST Advanced Deep Extragalactic Survey. We find that a significant subset of these galaxies atz ≳ 2 with modest Hβ+ [Oiii] EWs (≈300–600 Å) have high ionization efficiencies ( $\log {\xi }_{\mathrm{ion}}\gtrsim 25.5\mathrm{Hz}{\mathrm{erg}}^{-1}$). Suppressed [Oiii] EW values yet elevated Hαand HβEW values imply that the level of chemical enrichment is the primary culprit, supported by spectroscopic measurements of metallicities below 12 + log(O/H) ≈ 7.70 (0.1Z_⊙). We demonstrate that integrated Hβ+ [Oiii] selections (e.g., Hβ+ [Oiii] EW > 700 Å) exclude the most metal-poor efficient ionizers and favor (1) more chemically enriched systems with comparable extreme radiation fields and (2) older starbursting systems. In contrast, metallicity degeneracies are reduced in Hαspace, enabling the identification of these metal-poor efficient ionizers by their specific star formation rate. 

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3. Absolute Dimensions of the Interferometric Binary HD 174881: A Test of Stellar Evolution Models for Evolved Stars

   https://doi.org/10.3847/1538-4357/ad8dcc  

   Torres, Guillermo; Boden, Andrew F; Monnier, John D; van_Belle, Gerard T (December 2024, The Astrophysical Journal)

   Abstract We report high-resolution spectroscopic monitoring and long-baseline interferometric observations with the Palomar Testbed Interferometer (PTI) of the 215 day binary system HD 174881 (K1 II-III), composed of two giant stars. The system is spatially resolved with the PTI, as well as in archival measurements with the CHARA Array. Our analysis of these observations, along with an analysis of the spectral energy distribution, have allowed us to infer accurate values for the absolute masses ( ${3.367}_{-0.041}^{+0.045}$and ${3.476}_{-0.043}^{+0.043}{M}_{☉}$), radii (34.0 ± 1.3 and 22.7 ± 1.8R_☉), effective temperatures (4620 ± 100 and 4880 ± 150 K), and bolometric luminosities of both components, as well as other properties including the orbital parallax (distance). These provide valuable tests of stellar evolution models for evolved stars, which are still relatively uncommon compared to the situation for main-sequence stars. We find generally good agreement of all of these properties of HD 174881 with two sets of recent models (MIST and PARSEC) at compositions near solar, for ages of 255–273 Myr. We also find evidence of an infrared excess, based largely on the flux measurements from IRAS at 60 and 100μm. 

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4. GLIMPSE: An Ultrafaint ≃10 ⁵ M _⊙ Pop III Galaxy Candidate and First Constraints on the Pop III UV Luminosity Function at z  ≃  6–7

   https://doi.org/10.3847/1538-4357/ade9a1  

   Fujimoto, Seiji; Naidu, Rohan P; Chisholm, John; Atek, Hakim; Endsley, Ryan; Kokorev, Vasily; Furtak, Lukas J; Pan, Richard; Liu, Boyuan; Bromm, Volker; et al (August 2025, The Astrophysical Journal)

   Abstract Detecting the first generation of stars, Population III (Pop III), has been a long-standing goal in astrophysics, yet they remain elusive even in the JWST era. Here we present a novel NIRCam-based selection method for Pop III galaxies, and carefully validate it through completeness and contamination simulations. We systematically search ≃ 500 arcmin²across JWST legacy fields for Pop III candidates, including GLIMPSE, which, assisted by gravitational lensing, has produced JWST’s deepest NIRCam imaging thus far. We discover one promising Pop III galaxy candidate (GLIMPSE-16043) at $z=6.50_{-0.24}^{+0.03}$, a moderately lensed galaxy ( $\mu =2.9_{-0.2}^{+0.1}$) with an intrinsic UV magnitude of $M_{\mathrm{UV}}=-15.89_{-0.14}^{+0.12}$. It exhibits key Pop III features: strong Hαemission (rest-frame EW 2810 ± 550 Å); a Balmer jump; no dust (UV slopeβ = −2.34 ± 0.36); and undetectable metal lines (e.g., [Oiii]; [Oiii]/Hβ < 0.44), implying a gas-phase metallicity ofZ_gas/Z_⊙ < 0.5%. These properties indicate the presence of a nascent, metal-deficient young stellar population (<5 Myr) with a stellar mass of ≃10⁵M_⊙. Intriguingly, this source deviates significantly from the extrapolated UV–metallicity relation derived from recent JWST observations atz= 4–10, consistent with UV enhancement by a top-heavy Pop III initial mass function or the presence of an extremely metal-poor active galactic nucleus. We also derive the first observational constraints on the Pop III UV luminosity function atz ≃ 6–7. The volume density of GLIMPSE-16043 (≈10⁻⁴cMpc⁻³) is in excellent agreement with theoretical predictions, independently reinforcing its plausibility. This study demonstrates the power of our novel NIRCam method to finally reveal distant galaxies even more pristine than the Milky Way’s most metal-poor satellites, thereby promising to bring us closer to the first generation of stars than we have ever been before. 

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5. The Epoch of Giant Planet Migration Planet Search Program. III. The Occurrence Rate of Young Giant Planets inside the Water Ice Line ^*

   https://doi.org/10.3847/1538-3881/ade230  

   Tran, Quang H; Bowler, Brendan P; Cochran, William D; Bender, Chad F; Halverson, Samuel; Mahadevan, Suvrath; Ninan, Joe P; Robertson, Paul; Roy, Arpita; Stefánsson, Guðmundur; et al (July 2025, The Astronomical Journal)

   Abstract We present statistical results from the Epoch of Giant Planet Migration RV planet search program. This survey was designed to measure the occurrence rate of giant planets interior to the water ice line of young Sun-like stars, compare this to the prevalence of giant planets at older ages, and provide constraints on the timescale and dominant inward migration mechanism of giant planets. Our final sample amounts to 85 single young (20–200 Myr) G and K dwarfs that we target across a 4 yr time baseline with the near-infrared Habitable-zone Planet Finder spectrograph at McDonald Observatory’s Hobby-Eberly Telescope. As part of this survey, we discovered the young hot Jupiter HS Psc b. We characterize survey detection completeness with realistic injection-recovery tests and measure an occurrence rate of $1.9_{-1.4}^{+2.6}$% for intermediate-age giant planets ( $0.3M_{\mathrm{J}}<m\sin i<13M_{\mathrm{J}}$) within 2.5 au. This is lower than the field age occurrence rate for the same planet masses and separations and favors an increase in the prevalence of giant planets over time from ∼100 Myr to several Gyr, although our results cannot rule out a constant rate. A decaying planet occurrence rate is, however, strongly excluded. This suggests that giant planets located inside the water ice line originate from a combination of in situ formation or early migration coupled with longer-term inward scattering. The completeness-corrected prevalence of young hot Jupiters in our sample is $1.5_{-1.1}^{+2.2}$% —similar to the rate for field stars—and the 95% upper limit for young brown dwarfs within 5000 days is <3.6% . 

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