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1. Elemental Abundances in M31: Properties of the Inner Stellar Halo*

   https://doi.org/10.3847/1538-4357/abb474  

   Escala, Ivanna ; Kirby, Evan N. ; Gilbert, Karoline M. ; Wojno, Jennifer ; Cunningham, Emily C. ; Guhathakurta, Puragra ( October 2020 , The Astrophysical Journal)

   We present measurements of [Fe/H] and [α/Fe] for 128 individual red giant branch stars (RGB) in the stellar halo of M31, including its Giant Stellar Stream (GSS), obtained using spectral synthesis of low- and medium-resolution Keck/DEIMOS spectroscopy (and 6000, respectively). We observed four fields in M31's stellar halo (at projected radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc). In combination with existing literature measurements, we have increased the sample size of [Fe/H] and [α/Fe] measurements from 101 to a total of 229 individual M31 RGB stars. From this sample, we investigate the chemical abundance properties of M31's inner halo, findingand. Between 8 and 34 kpc, the inner halo has a steep [Fe/H] gradient (−0.025 ± 0.002 dex kpc⁻¹) and negligible [α/Fe] gradient, where substructure in the inner halo is systematically more metal-rich than the smooth component of the halo at a given projected distance. Although the chemical abundances of the inner stellar halo are largely inconsistent with that of present-day dwarf spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars kinematically associated with the smooth component of the stellar halomore »that have chemical abundance patterns similar to M31 dSphs. We discuss formation scenarios for M31's halo, concluding that these dSph-like stars may have been accreted from galaxies of similar stellar mass and star formation history, or of higher stellar mass and similar star formation efficiency.

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2. Kinematics and Metallicity of Red Giant Branch Stars in the Northeast Shelf of M31*

   https://doi.org/10.3847/1538-3881/ac7146  

   Escala, Ivanna ; Gilbert, Karoline M. ; Fardal, Mark ; Guhathakurta, Puragra ; Sanderson, Robyn E. ; Kalirai, Jason S. ; Mobasher, Bahram ( June 2022 , The Astronomical Journal)

   We obtained Keck/DEIMOS spectra of 556 individual red giant branch stars in four spectroscopic fields spanning 13−31 projected kpc along the northeast (NE) shelf of M31. We present the first detection of a complete wedge pattern in the space of projected M31-centric radial distance versus line-of-sight velocity for this feature, which includes the returning stream component of the shelf. This wedge pattern agrees with expectations of a tidal shell formed in a radial merger and provides strong evidence in favor of predictions of Giant Stellar Stream (GSS) formation models in which the NE shelf originates from the second orbital wrap of the tidal debris. The observed concentric wedge patterns of the NE, west (W), and southeast (SE) shelves corroborate this interpretation independently of the models. We do not detect a kinematical signature in the NE shelf region corresponding to an intact progenitor core, favoring GSS formation models in which the progenitor is completely disrupted. The shelf’s photometric metallicity ([Fe/H]_phot) distribution implies that it is dominated by tidal material, as opposed to the phase-mixed stellar halo or the disk. The metallicity distribution ([Fe/H]_phot= −0.42 ± 0.01) also matches the GSS, and consequently the W and SE shelves, further supporting amore »direct physical association between the tidal features.

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3. The Open Cluster Chemical Abundances and Mapping Survey. VI. Galactic Chemical Gradient Analysis from APOGEE DR17

   https://doi.org/10.3847/1538-3881/ac7ce5  

   Myers, Natalie ; Donor, John ; Spoo, Taylor ; Frinchaboy, Peter M. ; Cunha, Katia ; Price-Whelan, Adrian M. ; Majewski, Steven R. ; Beaton, Rachael L. ; Zasowski, Gail ; O’Connell, Julia ; et al ( August 2022 , The Astronomical Journal)

   The goal of the Open Cluster Chemical Abundances and Mapping (OCCAM) survey is to constrain key Galactic dynamic and chemical evolution parameters by the construction and analysis of a large, comprehensive, uniform data set of infrared spectra for stars in hundreds of open clusters. This sixth contribution from the OCCAM survey presents analysis of SDSS/APOGEE Data Release 17 (DR17) results for a sample of stars in 150 open clusters, 94 of which we designate to be “high-quality” based on the appearance of their color–magnitude diagram. We find the APOGEE DR17-derived [Fe/H] values to be in good agreement with those from previous high-resolution spectroscopic open cluster abundance studies. Using a subset of the high-quality sample, the Galactic abundance gradients were measured for 16 chemical elements, including [Fe/H], for both Galactocentric radius (R_GC) and guiding center radius (R_guide). We find an overall Galactic [Fe/H] versusR_GCgradient of −0.073 ± 0.002 dex kpc⁻¹over the range of 6 >R_GC< 11.5 kpc, and a similar gradient is found for [Fe/H] versusR_guide. Significant Galactic abundance gradients are also noted for O, Mg, S, Ca, Mn, Na, Al, K, and Ce. Our large sample additionally allows us to explore the evolution of the gradients in four agemore »bins for the remaining 15 elements.

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4. Exploring the S-process History in the Galactic Disk: Cerium Abundances and Gradients in Open Clusters from the OCCAM/APOGEE Sample

   https://doi.org/10.3847/1538-4357/ac4254  

   Sales-Silva, J. V. ; Daflon, S. ; Cunha, K. ; Souto, D. ; Smith, V. V. ; Chiappini, C. ; Donor, J. ; Frinchaboy, P. M. ; García-Hernández, D. A. ; Hayes, C. ; et al ( February 2022 , The Astrophysical Journal)

   Abstract The APOGEE Open Cluster Chemical Abundances and Mapping survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce ii lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for ages < 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/ α -element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]–age plane (where X can be H, Fe, or the α -elements O, Mg, Si, or Ca). These metallicity-dependent relations result in [Ce/Fe] and [Ce/ α ] ratios with ages that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d [Ce/H]/ d R GC being negative, while d [Ce/Fe]/ d R GC is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by ∼+0.009 dex kpc −1 Gyr −1 . Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] andmore »[Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from asymptotic giant branch (AGB) stars, gigayear time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.« less
5. Tracing Birth Properties of Stars with Abundance Clustering

   https://doi.org/10.3847/1538-4357/ac3481  

   Ratcliffe, Bridget L. ; Ness, Melissa K. ; Buck, Tobias ; Johnston, Kathryn V. ; Sen, Bodhisattva ; Beraldo e Silva, Leandro ; Debattista, Victor P. ( January 2022 , The Astrophysical Journal)

   To understand the formation and evolution of the Milky Way disk, we must connect its current properties to its past. We explore hydrodynamical cosmological simulations to investigate how the chemical abundances of stars might be linked to their origins. Using hierarchical clustering of abundance measurements in two Milky Way–like simulations with distributed and steady star formation histories, we find that groups of chemically similar stars comprise different groups in birth place (R_birth) and time (age). Simulating observational abundance errors (0.05 dex), we find that to trace distinct groups of (R_birth, age) requires a large vector of abundances. Using 15 element abundances (Fe, O, Mg, S, Si, C, P, Mn, Ne, Al, N, V, Ba, Cr, Co), up to ≈10 groups can be defined with ≈25% overlap in (R_birth, age). We build a simple model to show that in the context of these simulations, it is possible to infer a star’s age andR_birthfrom abundances with precisions of ±0.06 Gyr and ±1.17 kpc, respectively. We find that abundance clustering is ineffective for a third simulation, where low-αstars form distributed in the disk and early high-αstars form more rapidly in clumps that sink toward the Galactic center as their constituent stars evolvemore »to enrich the interstellar medium. However, this formation path leads to large age dispersions across the [α/Fe]–[Fe/H] plane, which is inconsistent with the Milky Way’s observed properties. We conclude that abundance clustering is a promising approach toward charting the history of our Galaxy.

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