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1. 3D gas-phase elemental abundances across the formation histories of Milky Way-mass galaxies in the FIRE simulations: initial conditions for chemical tagging

   https://doi.org/10.1093/mnras/stab1606  

   Bellardini, Matthew A ; Wetzel, Andrew ; Loebman, Sarah R ; Faucher-Giguère, Claude-André ; Ma, Xiangcheng ; Feldmann, Robert ( June 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We use FIRE-2 simulations to examine 3D variations of gas-phase elemental abundances of [O/H], [Fe/H], and [N/H] in 11 MW and M31-mass galaxies across their formation histories at z ≤ 1.5 ($t_{\rm lookback} \le 9.4 \, \rm {Gyr}$), motivated by characterizing the initial conditions of stars for chemical tagging. Gas within $1 \, \rm {kpc}$ of the disc mid-plane is vertically homogeneous to $\lesssim 0.008 \, \rm {dex}$ at all z ≤ 1.5. We find negative radial gradients (metallicity decreases with galactocentric radius) at all times, which steepen over time from $\approx \! -0.01 \, \rm {dex}\, \rm {kpc}^{-1}$ at z = 1 ($t_{\rm lookback} = 7.8 \, \rm {Gyr}$) to $\approx \! -0.03 \, \rm {dex}\, \rm {kpc}^{-1}$ at z = 0, and which broadly agree with observations of the MW, M31, and nearby MW/M31-mass galaxies. Azimuthal variations at fixed radius are typically $0.14 \, \rm {dex}$ at z = 1, reducing to $0.05 \, \rm {dex}$ at z = 0. Thus, over time radial gradients become steeper while azimuthal variations become weaker (more homogeneous). As a result, azimuthal variations were larger than radial variations at z ≳ 0.8 ($t_{\rm lookback} \gtrsim 6.9 \, \rm {Gyr}$). Furthermore, elemental abundances are measurably homogeneous (to ≲0.05 dex) across a radial range of $\Delta R \approx 3.5 \, \rm {kpc}$ at z ≳ 1 and $\Delta R \approx 1.7 \, \rm {kpc}$ at z = 0. We also measure full distributions of elemental abundances, finding typically negatively skewed normal distributions at z ≳ 1 that evolve to typically Gaussian distributions by z = 0. Our results on gas abundances inform the initial conditions for stars, including the spatial and temporal scales for applying chemical tagging to understand stellar birth in the MW. 

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2. High-resolution spectroscopic analysis of four new chemically peculiar stars

   https://doi.org/10.1093/mnras/stac3378  

   Roriz, M. P. ; Pereira, C. B. ; Junqueira, S. ; Lugaro, M. ; Drake, N. A. ; Sneden, C. ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   We present detailed chemical compositions of four stars on the first-ascent red giant branch that are classified as chemically peculiar, but lack comprehensive analyses at high spectral resolution. For BD+03°2688, HE 0457−1805, HE 1255−2324, and HE 2207−1746, we derived metallicities [Fe/H] = −1.21, −0.19, −0.31, and −0.55, respectively, indicating a range in Galactic population membership. In addition to atmospheric parameters, we extracted elemental abundances for 28 elements, including the evolutionary-sensitive CNO group and 12C/13C ratios. Novel results are also presented for the heavy elements tungsten and thallium. All four stars have very large enhancements of neutron-capture elements, with high [La/Eu] ratios indicating enrichments from the slow neutron capture (s-process). To interpret these abundances, all indicative of [s/Fe] >1.0, we compared our results with data from literature, as well as with predictions from the Monash and fruity s-process nucleosynthesis models. BD+03°2688, HE 1255−2324, and HE 2207−1746 show C/O >1, while HE 0457−1805 has C/O <1. Since HE 0457−1805 and HE 1255−2324 are binary stars, their peculiarities are attributable to mass transfer. We identified HE 0457−1805 as a new barium giant star, and HE 1255−2324 as a new CH star, in fact a higher metallicity analogue CEMP-r/s star; the single object reported in literature so far with similar characteristics is the barium star HD 100503 ([Fe/H] = −0.72). A systematic monitoring is needed to confirm the binary nature of BD+03°2688 and HE 2207−1746, which are probably CH stars.

    

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3. 3D elemental abundances of stars at formation across the histories of Milky Way-mass galaxies in the FIRE simulations

   https://doi.org/10.1093/mnras/stac1637  

   Bellardini, Matthew A. ; Wetzel, Andrew ; Loebman, Sarah R. ; Bailin, Jeremy ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   We characterize the 3D spatial variations of [Fe/H], [Mg/H], and [Mg/Fe] in stars at the time of their formation, across 11 simulated Milky Way (MW)- and M31-mass galaxies in the FIRE-2 simulations, to inform initial conditions for chemical tagging. The overall scatter in [Fe/H] within a galaxy decreased with time until $\approx 7 \, \rm {Gyr}$ ago, after which it increased to today: this arises from a competition between a reduction of azimuthal scatter and a steepening of the radial gradient in abundance over time. The radial gradient is generally negative, and it steepened over time from an initially flat gradient $\gtrsim 12 \, \rm {Gyr}$ ago. The strength of the present-day abundance gradient does not correlate with when the disc ‘settled’; instead, it best correlates with the radial velocity dispersion within the galaxy. The strength of azimuthal variation is nearly independent of radius, and the 360 deg scatter decreased over time, from $\lesssim 0.17 \, \rm {dex}$ at $t_{\rm lb} = 11.6 \, \rm {Gyr}$ to $\sim 0.04 \, \rm {dex}$ at present-day. Consequently, stars at $t_{\rm lb} \gtrsim 8 \, \rm {Gyr}$ formed in a disc with primarily azimuthal scatter in abundances. All stars formed in a vertically homogeneous disc, Δ[Fe/H]$\le 0.02 \, \rm {dex}$ within $1 \, \rm {kpc}$ of the galactic mid-plane, with the exception of the young stars in the inner $\approx 4 \, \rm {kpc}$ at z ∼ 0. These results generally agree with our previous analysis of gas-phase elemental abundances, which reinforces the importance of cosmological disc evolution and azimuthal scatter in the context of stellar chemical tagging. We provide analytic fits to our results for use in chemical-tagging analyses.

    

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4. The Relationship between Age, Metallicity, and Abundances for Disk Stars in a Simulated Milky Way

   https://doi.org/10.3847/1538-4357/aca1c7  

   Carrillo, Andreia ; Ness, Melissa K. ; Hawkins, Keith ; Sanderson, Robyn E. ; Wang, Kaile ; Wetzel, Andrew ; Bellardini, Matthew A. ( January 2023 , The Astrophysical Journal)

   Abstract Observations of the Milky Way’s low- α disk show that several element abundances correlate with age at fixed metallicity, with unique slopes and small scatters around the age–[X/Fe] relations. In this study, we turn to simulations to explore the age–[X/Fe] relations for the elements C, N, O, Mg, Si, S, and Ca that are traced in a FIRE-2 cosmological zoom-in simulation of a Milky Way–like galaxy, m12i, and understand what physical conditions give rise to the observed age–[X/Fe] trends. We first explore the distributions of mono-age populations in their birth and current locations, [Fe/H], and [X/Fe], and find evidence for inside-out radial growth for stars with ages <7 Gyr. We then examine the age–[X/Fe] relations across m12i’s disk and find that the direction of the trends agrees with observations, apart from C, O, and Ca, with remarkably small intrinsic scatters, σ int (0.01 − 0.04 dex). This σ int measured in the simulations is also metallicity dependent, with σ int ≈ 0.025 dex at [Fe/H] = −0.25 dex versus σ int ≈ 0.015 dex at [Fe/H] = 0 dex, and a similar metallicity dependence is seen in the GALAH survey for the elements in common. Additionally, we find that σ int is higher in the inner galaxy, where stars are older and formed in less chemically homogeneous environments. The age–[X/Fe] relations and the small scatter around them indicate that simulations capture similar chemical enrichment variance as observed in the Milky Way, arising from stars sharing similar element abundances at a given birth place and time. 

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5. The Galactic chemical evolution of phosphorus observed with IGRINS

   https://doi.org/10.1051/0004-6361/202244724  

   Nandakumar, G. ; Ryde, N. ; Montelius, M. ; Thorsbro, B. ; Jönsson, H. ; Mace, G. ( December 2022 , Astronomy & Astrophysics)

   Context. Phosphorus (P) is considered to be one of the key elements for life, making it an important element to look for in the abundance analysis of spectra of stellar systems. Yet, only a select number of spectroscopic studies exist to estimate the phosphorus abundances and investigate its trend across a range of metallicities. This is due to the lack of good phosphorus lines in the optical wavelength region and the requirement of careful manual analysis of the blended phosphorus lines in near-infrared H-band spectra obtained with individual observations and surveys such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Aims. Based on a consistent and systematic analysis of high-resolution, near-infrared Immersion GRating INfrared Spectrograph (IGRINS) spectra of 38 K giant stars in the Solar neighborhood, we present and investigate the phosphorus abundance trend in the metallicity range of −1.2 dex < [Fe/H] < 0.4 dex. Furthermore, we compare this trend with the available chemical evolution models to shed some light on the origin and evolution of phosphorus. Methods. We have observed full H - and K -band spectra at a spectral resolving power of R = 45 000 with IGRINS mounted on the Gemini South telescope, the Discovery Channel Telescope, and the Harlan J Smith Telescope at McDonald Observatory. Abundances were determined from spectral lines by modeling the synthetic spectrum that best matches the observed spectrum by χ 2 minimization. For this task, we used the Spectroscopy Made Easy (SME) tool in combination with one-dimensional (1D) Model Atmospheres in a Radiative and Convective Scheme (MARCS) stellar atmosphere models. The investigated sample of stars have reliable stellar parameters estimated using optical FIber-fed Echelle Spectrograph (FIES) spectra obtained in a previous study of a set of stars called Giants in the Local Disk (GILD). In order to determine the phosphorus abundances from the 16482.92 Å phosphorus line, we needed to take special care blending the CO( v = 7−4) line. With the stellar parameters known, we thus determined the C, N, and O abundances from atomic carbon and a range of nonblended molecular lines (CO, CN, and OH) which are plentiful in the H-band region of K giant stars, assuring an appropriate modeling of the blending CO( v = 7−4) line. Results. We present the [P/Fe] versus [Fe/H] trend for K giant stars in the metallicity range of −1.2 dex < [Fe/H] < 0.4 dex and enhanced phosphorus abundances for two metal-poor s-rich stars. We find that our trend matches well with the compiled literature sample of prominently dwarf stars and the limited number of giant stars. Our trend is found to be higher by ~0.05−0.1 dex compared to the theoretical chemical evolution trend resulting from the core collapse supernova (type II) of massive stars with the phosphorus yields arbitrarily increased by a factor of 2.75. Thus the enhancement factor might need to be ~0.05−0.1 dex higher to match our trend. We also find an empirically determined primary behavior for phosphorus. Furthermore, the phosphorus abundance is found to be elevated by ~0.6−0.9 dex in the two s-enriched stars compared to the theoretical chemical evolution trend. 

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