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1. A North–South Metallicity Asymmetry in the Outer Galactic Disk—Evidence for the Pericentric Passage of the Sagittarius Dwarf Galaxy

   https://doi.org/10.3847/2041-8213/adf18a  

   Wang, Chun; Huang, Yang; Beers, Timothy C; Das, Payel; Yuan, Haibo; Xie, Lizhi; Shao, Shi (July 2025, The Astrophysical Journal Letters)

   Abstract We present maps of the mean metallicity distributions on the GalactocentricR–Zplane at different azimuthal angles using red clump stars selected from the LAMOST and APOGEE surveys. In the inner disk (R < 11 kpc), the metallicity distribution is symmetric between the upper and lower disk. However, we find a north–south metallicity asymmetry in the outer disk (R > 11 kpc), especially toward the anti-Galactic center (−5^∘ < Φ < 15°) direction. By further dissecting the map in age space, we detect this asymmetry across all mono-age stellar populations. However, the asymmetry is less pronounced in older populations (τ > 8 Gyr) compared to younger ones (τ < 6 Gyr). This reduced significance likely stems from three factors: larger age uncertainties, fewer stars in the outer disk, and the kinematically hotter nature of older populations. The observed metallicity asymmetry may be the consequence of the perturbation of the recent pericentric passage through the Galactic disk and tidal force of the well-known Sagittarius dwarf galaxy. 

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2. Evolution of Flare Activity in GKM Stars Younger Than 300 Myr over Five Years of TESS Observations

   https://doi.org/10.3847/1538-3881/ad4edf  

   Feinstein, Adina D; Seligman, Darryl Z; France, Kevin; Gagné, Jonathan; Kowalski, Adam (July 2024, The Astronomical Journal)

   Abstract Stellar flares are short-duration (< hours) bursts of radiation associated with surface magnetic reconnection events. Stellar magnetic activity generally decreases as a function of both the age and Rossby number,R₀, a measure of the relative importance of the convective and rotational dynamos. Young stars (<300 Myr) have typically been overlooked in population-level flare studies due to challenges with flare-detection methods. Here, we select a sample of stars that are members of 26 nearby moving groups, clusters, or associations with ages <300 Myr that have been observed by the Transiting Exoplanet Survey Satellite at 2 minute cadence. We identified 26,355 flares originating from 3160 stars and robustly measured the rotation periods of 1847 stars. We measure and find the flare frequency distribution slope,α, saturates for all spectral types atα∼ −0.5 and is constant over 300 Myr. Additionally, we find that flare rates for starst_age= 50–250 Myr are saturated belowR₀< 0.14, which is consistent with other indicators of magnetic activity. We find evidence of annual flare rate variability in eleven stars, potentially correlated with long-term stellar activity cycles. Additionally, we crossmatch our entire sample with the Galaxy Evolution Explorer and find no correlation between flare rate and far- and near-ultraviolet flux. Finally, we find the flare rates of planet-hosting stars are relatively lower than comparable, larger samples of stars, which may have ramifications for the atmospheric evolution of short-period exoplanets. 

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3. A Galactic Self-portrait: Density Structure and Integrated Properties of the Milky Way Disk

   https://doi.org/10.3847/1538-4357/adf723  

   Imig, Julie; Holtzman, Jon A; Zasowski, Gail; Lian, Jianhui; Boardman, Nicholas F; Stone-Martinez, Alexander; Mackereth, J Ted; Prescott, Moire_K M; Beaton, Rachael L; Beers, Timothy C; et al (September 2025, The Astrophysical Journal)

   Abstract The evolutionary history of the Milky Way disk is imprinted in the ages, positions, and chemical compositions of individual stars. In this study, we derive the intrinsic density distribution of different stellar populations using the final data release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. A total of 203,197 red giant branch stars are used to sort the stellar disk (R≤ 20 kpc) into subpopulations of metallicity (Δ[M/H]  = 0.1 dex), age ( $\mathrm{\Delta }\log \left(\frac{\mathrm{age}}{\mathrm{yr}}\right)=0.1$), andα-element abundances ([α/M]). We fit the present-day structural parameters and density distribution of each stellar subpopulation after correcting for the survey selection function. The low-αdisk is characterized by longer scale lengths and shorter scale heights, and is best fit by a broken exponential radial profile for each population. The high-αdisk is characterized by shorter scale lengths and larger scale heights, and is generally well-approximated by a single exponential radial profile. These results are applied to produce new estimates of the integrated properties of the Milky Way from early times to the present day. We measure the total stellar mass of the disk to be $5.27_{-1.5}^{+0.2}\times 10^{10}$M_⊙, and the average mass-weighted scale length isR_d = 2.37 ± 0.2 kpc. The Milky Way’s present-day color of (g − r) = 0.72 ± 0.02 is consistent with the classification of a red spiral galaxy, although it has only been in the “green valley” region of the galaxy color–mass diagram for the last ∼3 Gyr. 

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4. The Triangulum Extended (TREX) Survey: The Stellar Disk Dynamics of M33 as a Function of Stellar Age

   https://doi.org/10.3847/1538-3881/ac5324  

   Quirk, Amanda C. N.; Guhathakurta, Puragra; Gilbert, Karoline M.; Chemin, Laurent; Dalcanton, Julianne J.; Williams, Benjamin F.; Seth, Anil; Patel, Ekta; Fung, Justin T.; Tangirala, Pujita; et al (March 2022, The Astronomical Journal)

   Abstract Triangulum (M33) is a low-mass, relatively undisturbed spiral galaxy that offers a new regime in which to test models of dynamical heating. In spite of its proximity, M33's dynamical heating history has not yet been well-constrained. In this work, we present the TREX Survey, the largest stellar spectroscopic survey across the disk of M33. We present the stellar disk kinematics as a function of age to study the past and ongoing dynamical heating of M33. We measure line-of-sight velocities for ∼4500 disk stars. Using a subset, we divide the stars into broad age bins using Hubble Space Telescope and Canada–France–Hawaii Telescope photometric catalogs: massive main-sequence stars and helium-burning stars (∼80 Myr), intermediate-mass asymptotic branch stars (∼1 Gyr), and low-mass red giant branch stars (∼4 Gyr). We compare the stellar disk dynamics to that of the gas using existing Hi, CO, and Hαkinematics. We find that the disk of M33 has relatively low-velocity dispersion (∼16 km s⁻¹), and unlike in the Milky Way and Andromeda galaxies, there is no strong trend in velocity dispersion as a function of stellar age. The youngest disk stars are as dynamically hot as the oldest disk stars and are dynamically hotter than predicted by most M33-like low-mass simulated analogs in Illustris. The velocity dispersion of the young stars is highly structured, with the large velocity dispersion fairly localized. The cause of this high-velocity dispersion is not evident from the observations and simulated analogs presented here. 

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5. Physical conditions in the warped accretion disk of a massive star: 349 GHz ALMA observations of G023.01−00.41

   https://doi.org/10.1051/0004-6361/202040000  

   Sanna, A.; Giannetti, A.; Bonfand, M.; Moscadelli, L.; Kuiper, R.; Brand, J.; Cesaroni, R.; Caratti o Garatti, A.; Pillai, T.; Menten, K. M. (November 2021, Astronomy & Astrophysics)

   Young massive stars warm up the large amount of gas and dust that condenses in their vicinity, exciting a forest of lines from different molecular species. Their line brightness is a diagnostic tool of the gas’s physical conditions locally, which we use to set constraints on the environment where massive stars form. We made use of the Atacama Large Millimeter/submillimeter Array at frequencies near 349 GHz, with an angular resolution of 0′′.1, to observe the methyl cyanide (CH 3 CN) emission which arises from the accretion disk of a young massive star. We sample the disk midplane with twelve distinct beams, where we get an independent measure of the gas’s (and dust’s) physical conditions. The accretion disk extends above the midplane, showing a double-armed spiral morphology projected onto the plane of the sky, which we sample with ten additional beams: Along these apparent spiral features, gas undergoes velocity gradients of about 1 km s −1 per 2000 au. The gas temperature ( T ) rises symmetrically along each side of the disk, from about 98 K at 3000 au to 289 K at 250 au, following a power law with radius R −0.43 . The CH 3 CN column density ( N ) increases from 9.2 × 10 15 cm −2 to 8.7 × 10 17 cm −2 at the same radii, following a power law with radius R −1.8 . In the framework of a circular gaseous disk observed approximately edge-on, we infer an H 2 volume density in excess of 4.8 ×10 9 cm −3 at a distance of 250 au from the star. We study the disk stability against fragmentation following the methodology by Kratter et al. (2010, ApJ, 708, 1585), which is appropriate under rapid accretion, and we show that the disk is marginally prone to fragmentation along its whole extent. 

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