More Like this

1. Age-dating Red Giant Stars Associated with Galactic Disk and Halo Substructures

   https://doi.org/10.3847/1538-4357/ac0532  

   Grunblatt, Samuel K.; Zinn, Joel C.; Price-Whelan, Adrian M.; Angus, Ruth; Saunders, Nicholas; Hon, Marc; Stokholm, Amalie; Bellinger, Earl P.; Martell, Sarah L.; Mosser, Benoit; et al (August 2021, The Astrophysical Journal)

   The vast majority of Milky Way stellar halo stars were likely accreted from a small number (<~3) of relatively large dwarf galaxy accretion events. However, the timing of these events is poorly constrained and predominantly relies on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be within the stellar halo, a subset of which are associated with the Gaia–Enceladus–Sausage halo substructure, and compare their ages to 3 red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined here, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age determination packages, we measure a weighted average age for the Gaia–Enceladus–Sausage stars in our sample of 8+/-3 (stat.)+/-1 (sys.) Gyr. We also determine hierarchical ages using isochrones for the populations of Gaia–Enceladus–Sausage, in situ halo and disk stars, finding a Gaia–Enceladus–Sausage population age of 8.0+2.3-3.2 Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited data and sample of stars, this framework should allow a distinct characterization of Galactic substructures using larger stellar samples and additional data available in the near future 

   more » « less
2. The Galactic Distribution of Phosphorus: A Survey of 163 Disk and Halo Stars*

   https://doi.org/10.3847/1538-3881/ac77f8  

   Maas, Zachary G.; Hawkins, Keith; Hinkel, Natalie R.; Cargile, Phillip; Janowiecki, Steven; Nelson, Tyler (July 2022, The Astronomical Journal)

   Abstract Phosphorus (P) is a critical element for life on Earth, yet the cosmic production sites of P are relatively uncertain. To understand how P has evolved in the solar neighborhood, we measured abundances for 163 FGK stars over a range of –1.09 < [Fe/H] < 0.47 using observations from the Habitable-zone Planet Finder instrument on the Hobby–Eberly Telescope. Atmospheric parameters were calculated by fitting a combination of astrometry, photometry, and Fe I line equivalent widths. Phosphorus abundances were measured by matching synthetic spectra to a P I feature at 10529.52 Å. Our [P/Fe] ratios show that chemical evolution models generally underpredict P over the observed metallicity range. Additionally, we find that the [P/Fe] differs by ∼0.1 dex between thin disk and thick disk stars that were identified with kinematics. The P abundances were compared withα-elements, iron-peak, odd-Z, and s-process elements, and we found that the evolution of P in the disk most strongly resembles that of theα-elements. We also find that molar P/C and N/C ratios for our sample match the scatter seen from other abundance studies. Finally, we measure a [P/Fe] = 0.09 ± 0.1 ratio in one low-αhalo star and probable Gaia–Sausage–Enceladus member, an abundance ratio ∼0.3–0.5 dex lower than the other Milky Way disk and halo stars at similar metallicities. Overall, we find that P is likely most significantly produced by massive stars in core-collapse supernovae, based on the largest P abundance survey to date. 

   more » « less
3. Metallicity Mapping of the Ionized Diffuse Gas at the Milky Way Disk–Halo Interface

   https://doi.org/10.3847/1538-4357/ad84f8  

   Choi, Bo-Eun; Werk, Jessica_K; Tchernyshyov, Kirill; Prochaska, J_Xavier; Zheng, Yong; Putman, Mary_E; Fielding, Drummond_B; Strader, Jay (November 2024, The Astrophysical Journal)

   Abstract Metals in the diffuse, ionized gas at the boundary between the Milky Way’s interstellar medium (ISM) and circumgalactic medium, known as the disk–halo interface (DHI), are valuable tracers of the feedback processes that drive the Galactic fountain. However, metallicity measurements in this region are challenging due to obscuration by the Milky Way ISM and uncertain ionization corrections that affect the total hydrogen column density. In this work, we constrain ionization corrections to neutral hydrogen column densities using precisely measured electron column densities from the dispersion measures of pulsars that lie in the same globular clusters as UV-bright targets with high-resolution absorption spectroscopy. We address the blending of absorption lines with the ISM by jointly fitting Voigt profiles to all absorption components. We present our metallicity estimates for the DHI of the Milky Way based on detailed photoionization modeling of the absorption from ionized metal lines and ionization-corrected total hydrogen columns. Generally, the gas clouds show a large scatter in metallicity, ranging between 0.04 and 3.2Z_⊙, implying that the DHI consists of a mixture of gaseous structures having multiple origins. We estimate the inflow and outflow timescales of the DHI ionized clouds to be 6–35 Myr. We report the detection of an infalling cloud with supersolar metallicity that suggests a Galactic fountain mechanism, whereas at least one low-metallicity outflowing cloud (Z< 0.1Z_⊙) poses a challenge for Galactic fountain and feedback models. 

   more » « less
4. Existence of the Metal-rich Stellar Halo and High-velocity Thick Disk in the Galaxy

   https://doi.org/10.3847/1538-4357/abbd3d  

   Yan, Yepeng; Du, Cuihua; Li, Hefan; Shi, Jianrong; Ma, Jun; Newberg, Heidi Jo (November 2020, The Astrophysical Journal)

   null (Ed.)
5. The TREX Survey: Kinematical Complexity Throughout M33's Stellar Disk and Evidence for a Stellar Halo*

   https://doi.org/10.3847/1538-4357/ac3480  

   Gilbert, Karoline M.; Quirk, Amanda C. N.; Guhathakurta, Puragra; Tollerud, Erik; Wojno, Jennifer; Dalcanton, Julianne J.; Durbin, Meredith J.; Seth, Anil; Williams, Benjamin F.; Fung, Justin T.; et al (January 2022, The Astrophysical Journal)

   Abstract We present initial results from a large spectroscopic survey of stars throughout M33's stellar disk. We analyze a sample of 1667 red giant branch (RGB) stars extending to projected distances of ∼11 kpc from M33's center (∼18 kpc, or ∼10 scale lengths, in the plane of the disk). The line-of-sight velocities of RGB stars show the presence of two kinematical components. One component is consistent with rotation in the plane of M33's Hidisk and has a velocity dispersion (∼19 km s⁻¹), consistent with that observed in a comparison sample of younger stars, while the second component has a significantly higher velocity dispersion. A two-component fit to the RGB velocity distribution finds that the high-dispersion component has a velocity dispersion of ${59.3}_{-2.5}^{+2.6}$km s⁻¹and rotates very slowly in the plane of the disk (consistent with no rotation at the <1.5σlevel), which favors interpreting it as a stellar halo rather than a thick disk population. A spatial analysis indicates that the fraction of RGB stars in the high-velocity-dispersion component decreases with increasing radius over the range covered by the spectroscopic sample. Our spectroscopic sample establishes that a significant high-velocity-dispersion component is present in M33's RGB population from near M33's center to at least the radius where M33's Hidisk begins to warp at 30′ (∼7.5 kpc) in the plane of the disk. This is the first detection and spatial characterization of a kinematically hot stellar component throughout M33's inner regions. 

   more » « less