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1. The Open Cluster Chemical Abundances and Mapping Survey. VII. APOGEE DR17 [C/N]–Age Calibration

   https://doi.org/10.3847/1538-3881/ac5d53  

   Spoo, Taylor; Tayar, Jamie; Frinchaboy, Peter M.; Cunha, Katia; Myers, Natalie; Donor, John; Majewski, Steven R.; Bizyaev, Dmitry; García-Hernández, D. A.; Jönsson, Henrik; et al (April 2022, The Astronomical Journal)

   Abstract Large-scale surveys open the possibility to investigate Galactic evolution both chemically and kinematically; however, reliable stellar ages remain a major challenge. Detailed chemical information provided by high-resolution spectroscopic surveys of the stars in clusters can be used as a means to calibrate recently developed chemical tools for age-dating field stars. Using data from the Open Cluster Abundances and Mapping survey, based on the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment 2 survey, we derive a new empirical relationship between open cluster stellar ages and the carbon-to-nitrogen ([C/N]) abundance ratios for evolved stars, primarily those on the red giant branch. With this calibration, [C/N] can be used as a chemical clock for evolved field stars to investigate the formation and evolution of different parts of our Galaxy. We explore how mixing effects at different stellar evolutionary phases, like the red clump, affect the derived calibration. We have established the [C/N]–age calibration for APOGEE Data Release 17 (DR17) giant star abundances to be $\log {\left[\mathrm{Age}\right(\mathrm{yr}\left)\right]}_{\mathrm{DR}17}=10.14(\pm 0.08)+2.23(\pm 0.19)\left[\mathrm{C}/\mathrm{N}\right]$, usable for $8.62\le \log \left(\mathrm{Age}\right[\mathrm{yr}\left]\right)\le 9.82$, derived from a uniform sample of 49 clusters observed as part of APOGEE DR17 applicable primarily to metal-rich, thin- and thick-disk giant stars. This measured [C/N]–age APOGEE DR17 calibration is also shown to be consistent with asteroseismic ages derived from Kepler photometry. 

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2. DESI Survey Validation Spectra Reveal an Increasing Fraction of Recently Quenched Galaxies at z ∼ 1

   https://doi.org/10.3847/2041-8213/acc9b5  

   Setton, David J.; Dey, Biprateep; Khullar, Gourav; Bezanson, Rachel; Newman, Jeffrey A.; Aguilar, Jessica N.; Ahlen, Steven; Andrews, Brett H.; Brooks, David; de la Macorra, Axel; et al (April 2023, The Astrophysical Journal Letters)

   Abstract We utilize ∼17,000 bright luminous red galaxies (LRGs) from the novel Dark Energy Spectroscopic Instrument Survey Validation spectroscopic sample, leveraging its deep (∼2.5 hr galaxy⁻¹exposure time) spectra to characterize the contribution of recently quenched galaxies to the massive galaxy population at 0.4 <z< 1.3. We useProspectorto infer nonparametric star formation histories and identify a significant population of recently quenched galaxies that have joined the quiescent population within the past ∼1 Gyr. The highest-redshift subset (277 atz> 1) of our sample of recently quenched galaxies represents the largest spectroscopic sample of post-starburst galaxies at that epoch. At 0.4 <z< 0.8, we measure the number density of quiescent LRGs, finding that recently quenched galaxies constitute a growing fraction of the massive galaxy population with increasing look-back time. Finally, we quantify the importance of this population among massive ( $\log \left(M_{\star }/M_{\odot }\right)$> 11.2) LRGs by measuring the fraction of stellar mass each galaxy formed in the gigayear before observation,f_{1 Gyr}. Although galaxies withf_{1 Gyr}> 0.1 are rare atz∼ 0.4 (≲0.5% of the population), byz∼ 0.8, they constitute ∼3% of massive galaxies. Relaxing this threshold, we find that galaxies withf_{1 Gyr}> 5% constitute ∼10% of the massive galaxy population atz∼ 0.8. We also identify a small but significant sample of galaxies atz= 1.1–1.3 that formed withf_{1 Gyr}> 50%, implying that they may be analogs to high-redshift quiescent galaxies that formed on similar timescales. Future analysis of this unprecedented sample promises to illuminate the physical mechanisms that drive the quenching of massive galaxies after cosmic noon. 

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3. The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich

   https://doi.org/10.3847/1538-4357/addbe5  

   Johnson, James W; Weinberg, David H; Blanc, Guillermo A; Bonaca, Ana; Rudie, Gwen C; Lu, Yuxi Lucy; Reichardt_Chu, Bronwyn; Griffith, Emily J; Sit, Tawny; Johnson, Jennifer A; et al (July 2025, The Astrophysical Journal)

   Abstract Metallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to ages of ∼9 Gyr follow a similar relation between metallicity and Galactocentric radius. This constancy challenges current models of Galactic chemical evolution, which typically predict lower metallicities for older stellar populations. Our results favor anequilibrium scenario, in which the gas-phase gradient reaches a nearly constant normalization early in the disk lifetime. Using a fiducial choice of parameters, we demonstrate that one possible origin of this behavior is an outflow that more readily ejects gas from the interstellar medium (ISM) with increasing Galactocentric radius. A direct effect of the outflow is that baryons do not remain in the ISM for long, which causes the ratio of star formation to accretion, ${\dot{\Sigma }}_{\star }/{\dot{\Sigma }}_{\text{in}}$, to quickly become constant. This ratio is closely related to the local equilibrium metallicity, since its numerator and denominator set the rates of metal production by stars and hydrogen gained through accretion, respectively. Building in a merger event results in a perturbation that evolves back toward the equilibrium state on ∼Gyr timescales. Under the equilibrium scenario, the radial metallicity gradient is not a consequence of the inside-out growth of the disk but instead reflects a trend of declining ${\dot{\Sigma }}_{\star }/{\dot{\Sigma }}_{\text{in}}$with increasing Galactocentric radius. 

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4. Resolving an Asteroseismic Catastrophe: Structural Diagnostics from p -mode Phase Functions off the Main Sequence

   https://doi.org/10.3847/1538-4357/ada949  

   Ong_王, J_M_Joel 加冕; Lindsay, Christopher J; Reyes, Claudia; Stello, Dennis; Roxburgh, Ian W (February 2025, The Astrophysical Journal)

   Abstract On the main sequence, the asteroseismic small frequency separationδν₀₂between radial and quadrupolep-modes is customarily interpreted to be a direct diagnostic of internal structure. Such an interpretation is based on a well-known integral estimator relatingδν₀₂to a radially averaged sound-speed gradient. However, this estimator fails, catastrophically, when evaluated on structural models of red giants: their small separations must therefore be interpreted differently. We derive a single expression that both reduces to the classical estimator when applied to main-sequence stellar models and reproduces the qualitative features of the small separation for stellar models of very evolved red giants. This expression indicates that the small separations of red giants scale primarily with their global seismic properties as $\delta {\nu }_{02}\propto \Delta {\nu }^2/{\nu }_{\max }$, rather than being in any way sensitive to their internal structure. Departures from this asymptotic behavior, during the transition from the main-sequence to red giant regimes, have been recently reported in open-cluster Christensen–Dalsgaard (C-D) diagrams from K2 mission data. Investigating them in detail, we demonstrate that they occur when the convective envelope boundary passes a specific acoustic distance—roughly one-third of a wavelength at ${\nu }_{\max }$—from the center of the star, at which point radial modes become maximally sensitive to the position of the boundary. The shape of the corresponding features onϵ_pand C-D (orr₀₂) diagrams may be useful in constraining the nature of convective boundary mixing in the context of undershooting beneath a convective envelope. 

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5. A Ghost in Boötes: The Least-Luminous Disrupted Dwarf Galaxy

   https://doi.org/10.3847/1538-4357/ac9b4b  

   Chandra, Vedant; Conroy, Charlie; Caldwell, Nelson; Bonaca, Ana; Naidu, Rohan P.; Zaritsky, Dennis; Cargile, Phillip A.; Han, Jiwon Jesse; Johnson, Benjamin D.; Speagle 沈, Joshua S. 佳士; et al (November 2022, The Astrophysical Journal)

   Abstract We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy revealed by the H3 Spectroscopic Survey. We detected this structure via a pair of comoving metal-poor stars at a distance of 12.5 kpc, and further characterized it with Gaia astrometry and follow-up spectroscopy. Specter is a 25° × 1° stream of stars that is entirely invisible until strict kinematic cuts are applied to remove the Galactic foreground. The spectroscopic members suggest a stellar ageτ≳ 12 Gyr and a mean metallicity $〈\left[\mathrm{Fe}/\mathrm{H}\right]〉=-{1.84}_{-0.18}^{+0.16}$, with a significant intrinsic metallicity dispersion ${\sigma }_{\left[\mathrm{Fe}/\mathrm{H}\right]}={0.37}_{-0.13}^{+0.21}$. We therefore argue that Specter is the disrupted remnant of an ancient dwarf galaxy. With an integrated luminosityM_V≈ −2.6, Specter is by far the least-luminous dwarf galaxy stream known. We estimate that dozens of similar streams are lurking below the detection threshold of current search techniques, and conclude that spectroscopic surveys offer a novel means to identify extremely low surface brightness structures. 

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