Chemical Evolution Models Research Articles

DIISC-IV. DIISCovery of Anomalously Low Metallicity H ii Regions in NGC 99: Indirect Evidence of Gas Inflows

As a part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey, we investigate indirect evidence of gas inflow into the disk of the galaxy NGC 99. We combine optical spectra from the Binospec spectrograph on the MMT telescope with optical imaging data from the Vatican Advanced Technology Telescope, radio H i 21 cm emission images from the NSF Karl G. Jansky’s Very Large Array, and UV spectroscopy from the Cosmic Origins Spectrograph on the Hubble Space Telescope. We measure emission lines (Hα, Hβ, [O iii]λ5007, [N ii]λ6583, and [S ii]λ6717, 31) in 26 H ii regions scattered about the galaxy and estimate a radial metallicity gradient of −0.017 dex kpc−1 using the N2 metallicity indicator. Two regions in the sample exhibit an anomalously low metallicity (ALM) of 12 + log(O/H) = 8.36 dex, which is ∼0.16 dex lower than other regions at that galactocentric radius. They also show a high difference between their H i and Hα line of sight velocities on the order of 35 km s−1. Chemical evolution modeling indicates gas accretion as the cause of the ALM regions. We find evidence for corotation between the interstellar medium of NGC 99 and Lyα clouds in its circumgalactic medium, which suggests a possible pathway for low metallicity gas accretion. We also calculate the resolved Fundamental Metallicity Relation (rFMR) on subkiloparsec scales using localized gas-phase metallicity, stellar mass surface density, and star formation rate surface density. The rFMR shows a similar trend as that found by previous localized and global FMR relations.

Reactivity of chondritic meteorites under H2-rich atmospheres: Formation of H2S.

Abstract Current models of chemical evolution during star and planetary formation rely on the presence of dust grains to act as a third body. However, they generally ignore the reactivity of the dust grains themselves. Dust grains present in the protoplanetary phase will evolve as the solar system forms and, after protoplanets have appeared, they will be constantly delivered to their surfaces in the form of large aggregates or meteorites. Chondritic meteorites are mostly unaltered samples of the dust present in the first stages of the Solar System formation, that still arrive nowadays to the surface of Earth and allow us to study the properties of the materials forming the early Solar System. These materials contain, amongst others, transition metals that can potentially act as catalysts, as well as other phases that can potentially react in different astrophysical conditions, such as FeS. In this work, we present the reactivity of chondritic meteorites under H2-rich atmospheres, particularly towards the reduction of FeS for the formation of H2S and metallic Fe during the early phases of the planetary formation. We present the obtained results on the reaction rates and the percentage of FeS available to react in the materials. Additionally, we include a computational study of the reaction mechanism and the energetics. Finally, we discuss the implications of an early formation of H2S in planetary surfaces.

The earliest phases of CNO enrichment in galaxies

Context. The recent detection of super-solar carbon-to-oxygen and nitrogen-to-oxygen abundance ratios in a group of metal-poor galaxies at high redshift by the James Webb Space Telescope has sparked renewed interest in exploring the chemical evolution of carbon, nitrogen, and oxygen (the CNO elements) at early times and prompted fresh inquiries into their origins. Aims. The main goal of this paper is to shed light onto the early evolution of the main CNO isotopes in the Galaxy and in young distant systems, such as GN-z11 at ɀ = 10.6 and GS-zl2 at ɀ = 12.5. Methods. To this aim, we incorporated a stochastic star formation component into a chemical evolution model calibrated with high-quality Milky Way (MW) data while focusing on the contribution of Population III (Pop III) stars to the early chemical enrichment. Results. By comparing the model predictions with CNO abundance measurements from high-resolution spectroscopy of an homogeneous sample of Galactic halo stars, we first demonstrate that the scatter observed in the metallicity range −4.5 ≤ [Fe/H] ≤ −1.5 can be explained by pre-enrichment from Pop III stars that explode as supernovae (SNe) with different initial masses and energies. Then, by exploiting the chemical evolution model, we provide testable predictions for log(C/N), log(N/O), and log(C/O) versus log(O/H)+12 in MW-like galaxies observed at different cosmic epochs (redshifts). Finally, by calibrating the chemical evolution model to replicate the observed properties of GN-z11 and GS-z12, we provide an alternative interpretation of their high N/O and C/O abundance ratios, demonstrating that an anomalously high N or C content can be reproduced through enrichment from faint Pop III SNe. Conclusions. Stochastic chemical enrichment from primordial stars explains both the observed scatter in CNO abundances in MW halo stars and the exceptionally high C/O and N/O ratios in some distant galaxies. These findings emphasize the critical role of Pop III stars in shaping early chemical evolution.

Chemical Evolution of R-process Elements in Stars (CERES)

Context. Carbon, nitrogen, and oxygen are the most abundant elements throughout the universe, after hydrogen and helium. Studying these elements in low-metallicity stars can provide crucial information on the chemical composition in the early Galaxy and possible internal mixing processes that can alter the surface composition of the stars. Aims. This work aims to investigate the chemical abundance patterns for CNO elements and Li in a homogeneously analyzed sample of 52 metal-poor halo giant stars. From these results, we have been able to determine whether internal mixing processes have taken place in these stars. Methods. We used high-resolution spectra with a high signal-to-noise ratio (S/N) to carry out a spectral synthesis to derive detailed C, N, O, and Li abundances for a sample of stars with metallicities in the range of −3.58 ≤ [Fe/H] ≤ −1.79 dex. Our study was based on the assumption of one-dimensional (1D) local thermodynamic equilibrium (LTE) atmospheres. Results. Based on carbon and nitrogen abundances, we investigated the deep mixing taking place within stars along the red giant branch (RGB). The individual abundances of carbon decrease towards the upper RGB while nitrogen shows an increasing trend, indicating that carbon has been converted into nitrogen. No signatures of ON-cycle processed material were found for the stars in our sample. We computed a set of galactic chemical evolution (GCE) models, implementing different sets of massive star yields, both with and without including the effects of stellar rotation on nucleosynthesis. We confirm that stellar rotation is necessary to explain the highest [N/Fe] and [N/O] ratios observed in unmixed halo stars. The predicted level of N enhancement varies sensibly in dependence of the specific set of yields that are adopted. For stars with stellar parameters similar to those of our sample, heavy elements such as Sr, Y, and Zr appear to have unchanged abundances despite the stellar evolution mixing processes. Conclusions. The unmixed RGB stars provide very useful constraints on chemical evolution models of the Galaxy. As they are more luminous than unevolved (main sequence and turnoff) stars, they also allow for stars to be probed at greater distances. The stellar CN-cycle clearly changes the atmospheric abundances of the lighter elements, but no changes were detected with respect to the heavy elements.

Evolution of lithium in the disc of the Galaxy and the role of novae

Context. Lithium plays a crucial role in probing stellar physics, stellar and primordial nucleosynthesis, and the chemical evolution of the Galaxy. Stars are considered to be the main source of Li, yet the identity of its primary stellar producer has long been a matter of debate. Aims. In light of recent theoretical and observational results, we investigate in this study the role of two candidate sources of Li enrichment in the Milky Way, namely asymptotic giant branch (AGB) stars and, in particular, novae. Methods. We utilised a one-zone Galactic chemical evolution (GCE) model to assess the viability of AGB stars and novae as stellar sources of Li. We used recent theoretical Li yields for AGB stars, while for novae we adopted observationally inferred Li yields and recently derived delay time distributions (DTDs). Subsequently, we extended our analysis by using a multi-zone model with radial migration to investigate spatial variations in the evolution of Li across the Milky Way disc and compared the results with observational data for field stars and open clusters. Results. Our analysis shows that AGB stars clearly fail to reproduce the meteoritic Li abundance. In contrast, novae appear as promising candidates within the adopted framework, allowing us to quantify the contribution of each Li source at the Sun’s formation and today. Our multi-zone model reveals the role of the differences in the DTDs of Type Ia supernovae and novae in shaping the evolution of Li in the various galactic zones. Its results are in fair agreement with the observational data for most open clusters, but small discrepancies appear in the outer disc.

Gas-phase metallicity gradients in galaxies at z ∼ 6–8

The study of gas-phase metallicity and its spatial distribution at high redshift is crucial to understand the processes that shaped the growth and evolution of galaxies in the early Universe. Here we study the spatially resolved metallicity in three systems at z ∼ 6 − 8, namely A2744-YD4, BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution (R ∼ 100) spectroscopic observations. These are among the highest-z sources in which metallicity gradients have been probed so far. Each of these systems hosts several spatial components in the process of merging within a few kiloparsecs, identified from the rest-frame UV and optical stellar continuum and ionised gas emission line maps. The sources have heterogeneous properties, with stellar masses log(M*/M⊙) ∼7.6–9.3, star formation rates (SFRs) ∼1–15 M⊙ yr−1, and gas-phase metallicities 12+log(O/H) ∼7.7–8.3, which exhibit a large scatter within each system. Their properties are generally consistent with those of the highest-redshift samples to date (z ∼ 3 − 10), though the sources in A2744-YD4 and COSMOS24108 are at the high end of the mass-metallicity relation (MZR) defined by the z ∼ 3 − 10 sources. Moreover, the targets in this work follow the predicted slope of the MZR at z ∼ 6 − 8 from most cosmological simulations. The gas-phase metallicity gradients are consistent with being flat in the main sources of each system. Flat metallicity gradients are thought to arise from gas mixing processes on galaxy scales, such as mergers or galactic outflows and supernova winds driven by intense stellar feedback, which wash out any gradient formed in the galaxy. The existence of flat gradients at z ∼ 6 − 8 sets also important constraints on future cosmological simulations and chemical evolution models, whose predictions on the cosmic evolution of metallicity gradients often differ significantly, especially at high redshift, but are mostly limited to z ≲ 3 so far.

(Re)mind the gap: A hiatus in star formation history unveiled by APOGEE DR17

Context. Analysis of several spectroscopic surveys indicates the presence of a bimodality between the disc stars in the abundance ratio space of [α/Fe] versus [Fe/H]. The two stellar groups are commonly referred to as the high-α and low-α sequences. Some models capable of reproducing such a bimodality invoke the presence of a hiatus in the star formation history in our Galaxy, whereas other models explain the two sequences by means of stellar migration. Aims. Our aim is to show that the existence of the gap in the star formation rate between high-α and low-α is evident in the stars of APOGEE DR17, if one plots [Fe/α] versus [α/H], confirming previous suggestions. We then try to interpret the data by means of detailed chemical models. Methods. We compare the APOGEE DR17 red giant stars with the predictions of a detailed chemical evolution model based on the two-infall paradigm, taking into account also the possible accretion of dwarf satellites. Results. The APOGEE DR17 abundance ratios [Fe/α] versus [α/H] exhibit a sharp increase in [Fe/α] at a nearly constant [α/H] (where α elements considered are Mg, Si, O) during the transition between the two disc phases. This observation strongly supports the hypothesis that a hiatus in star formation occurred during this evolutionary phase. Notably, the most pronounced growth in the [Fe/α] versus [α/H] relation is observed for oxygen, as this element is exclusively synthesised in core-collapse supernovae. The revised version of the two-infall chemical evolution model proposed in this study reproduces the APOGEE DR17 abundance ratios better than before. Particularly noteworthy is the model’s ability to predict the hiatus in the star formation between the two infalls of gas, which form the thick and thin disc, respectively, and thus generate abundance ratios compatible with APOGEE DR17 data. Conclusions. We show that the signature of a hiatus in the star formation is imprinted in the APOGEE DR17 abundance ratios. A chemical model predicting a pause in the star formation of a duration of roughly 3.5 Gyr, and in which the high-α disc starts forming from pre-enriched gas by a previous encounter with a dwarf galaxy, could well explain the observations

The Rise of the r-process in the Gaia-Sausage/Enceladus Dwarf Galaxy ∗

Neutron star mergers (NSMs) produce r-process elements after a time-delayed inspiral process. Once a significant number of NSMs are present in a galaxy, r-process elements, such as Eu, are expected to significantly increase with time. Yet, there have been limited observational data in support of Eu increasing within Local Group galaxies. We have obtained high-resolution Magellan/MIKE observations of 43 metal-poor stars in the Gaia-Sausage/Enceladus (GSE) tidally disrupted galaxy with −2.5 < [Fe/H] < −1. For the first time, we find a clear rise in [Eu/Mg] with increasing [Mg/H] within one galaxy. We use a simple chemical evolution model to study how such a rise can result from the interplay of prompt and delayed r-process enrichment events. Delayed r-process sources are required to explain the rise and subsequent leveling off of [Eu/Mg] in this disrupted galaxy. However, the rise may be explained by delayed r-process sources with either short (∼10 Myr) or long (∼500 Myr) minimum delay times. Future studies on the nature of r-process sources and their enrichment processes in the GSE will require additional stars in the GSE at even lower metallicities than the present study.

The AMBRE Project: Lead abundance in Galactic stars

Context. The chemical evolution of neutron capture elements in the Milky Way is still a matter of debate. Although more and more studies investigate their chemical behaviour, there is still a lack of a significant large sample of abundances of a key heavy element: lead. Aims. Lead is the final product of the s-process nucleosynthesis channel and is one of the most stable heavy elements. The goal of this article is to present the largest catalogue of homogeneous Pb abundances, in particular for metallicities higher than −1.0 dex, and then to study the lead content of the Milky Way. Methods. We analysed high-resolution spectra from the ESO UVES and FEROS archives. Atmospheric parameters were taken from the AMBRE parametrisation. We used the automated abundance method GAUGUIN to derive lead abundances in 653 slow-rotating FGK-type stars from the 368.34 nm Pb I line. Results. We present the largest catalogue of Local Thermodynamic Equilibrium (LTE) and non-LTE lead abundances ever published with metallicities ranging from −2.9 to 0.6 dex and [Pb/Fe] from −0.7 to 3.3 dex. Within this sample, no lead-enhanced Asymptotic Giant Branch (AGB) stars were found, but nine lead-enhanced metal-poor stars ([Pb/Fe] &gt; 1.5) were detected. Most of them were already identified as carbon-enhanced metal-poor stars with enrichments in other s-process species. The lead abundance of 13 Gaia Benchmark Stars are also provided. We then investigated the Pb content of the Milky Way disc by computing vertical and radial gradients and found a slightly decreasing [Pb/Fe] radial trend with metallicity. This trend together with other related ratios ([Pb/Eu], [Pb/Ba], and [Pb/α]) are interpreted thanks to chemical evolution models. The two-infall model closely reproduces the observed trends with respect to the metallicity. It is also found that the AGB contribution to the Pb Galactic enrichment has to be strongly reduced. Moreover, the contribution of massive stars with rather high rotational velocities should be favoured in the low-metallicity regime.

The Scale of Stellar Yields: Implications of the Measured Mean Iron Yield of Core Collapse Supernovae

The scale of α-element yields is difficult to predict from theory because of uncertainties in massive star evolution, supernova physics, and black hole formation, and it is difficult to constrain empirically because the impact of higher yields can be compensated by greater metal loss in galactic winds. We use a recent measurement of the mean iron yield of core collapse supernovae (CCSN) by Rodriguez et al., y¯Fecc=0.058±0.007M⊙ , to infer the scale of α-element yields by assuming that the plateau of [α/Fe] abundance ratios observed in low-metallicity stars represents the yield ratio of CCSN. For a plateau at [α/Fe]cc = 0.45, we find that the population-averaged yields of O and Mg are about equal to the solar abundance of these elements, logyOcc/ZO,⊙=logyMgcc/ZMg,⊙=−0.01±0.1 , where yXcc is the mass of element X produced by massive stars per unit mass of star formation. The inferred O and Fe yields agree with predictions of the Sukhbold et al. CCSN models assuming their Z9.6+N20 neutrino-driven engine, a scenario in which many progenitors with M < 40M ⊙ implode to black holes rather than exploding. The yields are lower than assumed in many models of the galaxy mass–metallicity relation, reducing the level of outflows needed to match observed abundances. Our one-zone chemical evolution models with η=Ṁout/Ṁ*≈0.6 evolve to solar metallicity at late times. By further requiring that models reach [α/Fe] ≈ 0 at late times, we infer a Hubble-time integrated Type Ia supernova rate of 1.1×10−3M⊙−1 , compatible with estimates from supernova surveys.

Chemical evolution of a young super star cluster at the Sunburst Arc

ABSTRACT Recent observations of high-redshift galaxies have revealed starburst galaxies with excessive amounts of nitrogen, well above that expected in standard evolutionary models. The Sunburst Arc galaxy, particularly its young and massive star cluster, represents the closest ($z=2.4$) and brightest of these as a strongly lensed object. In this work, we study the chemical history of this star cluster to determine the origin of the elevated gas-phase nitrogen using a chemical evolution model. Our model includes the enrichment of OB stars through stellar winds and core-collapse supernovae assuming that massive stars ($M\gt 25$ $\mathrm{ M}_\odot$) collapse directly into black holes at the end of their lives. We fit the model parameters to the observed chemical abundances of the Sunburst Arc cluster: O/H, C/O, and N/O. We find that the observed chemical abundances can be explained by models featuring intense star formation events, characterized by rapid gas accretion and high star formation efficiencies. Additionally, the stellar population contributing to the gas enrichment must exclude Wolf–Rayet stars. These conditions might be present in other nitrogen-rich objects as their similar chemical abundances suggest a common history. As previous studies have proposed the presence of Wolf–Rayet stars in the new nitrogen-rich objects, further research using chemodynamic modeling is necessary to ascertain the true nature of these objects.

Mapping radial abundance gradients with Gaia -ESO open clusters. Evidence of recent gas accretion in the Milky Way disk

Recent evidence from spectroscopic surveys points towards the presence of a metal-poor, young stellar population in the low-alpha , chemically thin disk. In this context, the investigation of the spatial distribution and time evolution of precise, unbiased abundances is fundamental to disentangle the scenarios of formation and evolution of the Galaxy. We study the evolution of abundance gradients in the Milky Way by taking advantage of a large sample of open star clusters, which are among the best tracers for this purpose. In particular, we used data from the last release of the Gaia -ESO survey. We performed a careful selection of open cluster member stars, excluding those members that may be affected by biases in spectral analysis. We compared the cleaned open cluster sample with detailed chemical evolution models for the Milky Way, using well-tested stellar yields and prescription for radial migration. We tested different scenarios of Galaxy evolution to explain the data, namely, the two-infall and the three-infall frameworks, which suggest the chemical thin disk is formed by one or two subsequent gas accretion episodes, respectively. With the performed selection in cluster member stars, we still find a metallicity decrease between intermediate-age ($1&lt;$ Age/Gyr $&lt;3$) and young (Age $&lt;1$ Gyr) open clusters. This decrease cannot be explained in the context of the two-infall scenario, even by accounting for the effect of migration and yield prescriptions. The three-infall framework, with its late gas accretion in the last 3 Gyr, is able to explain the low metallic content in young clusters. However, we have invoked a milder metal dilution for this gas infall episode relative to previous findings. To explain the observed low metallic content in young clusters, we propose that a late gas accretion episode triggering a metal dilution would have taken place, extending the framework of the three-infall model for the first time to the entire Galactic disk.

Isotopic abundance of carbon in the DLA towards QSO B1331+170

ABSTRACT Chemical evolution models predict a gradual build-up of 13C in the Universe, based on empirical nuclear reaction rates and assumptions on the properties of stellar populations. However, old metal-poor stars within the Galaxy contain more 13C than is predicted, suggesting that further refinements to the models are necessary. Gas at high-redshift provides important supplementary information at metallicities $-2\lesssim \left[{\rm Fe/H}\right]\lesssim -1$, for which there are only a few measurements in the Galaxy. We obtained new, high-quality, VLT/ESPRESSO observations of the QSO B1331$+$170 and used them to measure 12C/13C in the damped Lyman-$\alpha$ system (DLA) at $z_{\rm abs}=1.776$, with $\left[{\rm Fe/H}\right]$ = −1.27. ai-vpfit, an artificial intelligence tool based on genetic algorithms and guided by a spectroscopic information criterion, was used to explore different possible kinematic structures of the carbon gas. Three hundred independent ai-vpfit models of the absorption system were produced using pre-set 12C/13C values, ranging from 4 to 500. Our results show that ${\rm ^{12}C / ^{13}C}=28.5^{+51.5}_{-10.4}$, suggesting a possibility of 13C production at low metallicity.

Galactic Chemical Evolution Models Favor an Extended Type Ia Supernova Delay-time Distribution

Type Ia supernovae (SNe Ia) produce most of the Fe-peak elements in the Universe and therefore are a crucial ingredient in galactic chemical evolution models. SNe Ia do not explode immediately after star formation, and the delay-time distribution (DTD) has not been definitively determined by supernova surveys or theoretical models. Because the DTD also affects the relationship among age, [Fe/H], and [α/Fe] in chemical evolution models, comparison with observations of stars in the Milky Way is an important consistency check for any proposed DTD. We implement several popular forms of the DTD in combination with multiple star formation histories for the Milky Way in multizone chemical evolution models that include radial stellar migration. We compare our predicted interstellar medium abundance tracks, stellar abundance distributions, and stellar age distributions to the final data release of the Apache Point Observatory Galactic Evolution Experiment. We find that the DTD has the largest effect on the [α/Fe] distribution: a DTD with more prompt SNe Ia produces a stellar abundance distribution that is skewed toward a lower [α/Fe] ratio. While the DTD alone cannot explain the observed bimodality in the [α/Fe] distribution, in combination with an appropriate star formation history it affects the goodness of fit between the predicted and observed high-α sequence. Our model results favor an extended DTD with fewer prompt SNe Ia than the fiducial t −1 power law.

The ratio of [Eu/α] differentiates accreted/in-situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters

Abstract We combine stellar orbits with the abundances of the heavy, r-process element europium and the light, α-element, silicon to separate in-situ and accreted populations in the Milky Way across all metallicities. At high orbital energy, the accretion-dominated halo shows elevated values of [Eu/Si], while at lower energies, where many of the stars were born in-situ, the levels of [Eu/Si] are lower. These systematically different levels of [Eu/Si] in the MW and the accreted halo imply that the scatter in [Eu/α] within a single galaxy is smaller than previously thought. At the lowest metallicities, we find that both accreted and in-situ populations trend down in [Eu/Si], consistent with enrichment via neutron star mergers. Through compiling a large dataset of abundances for 54 globular clusters (GCs), we show that differences in [Eu/Si] extend to populations of in-situ/accreted GCs. We interpret this consistency as evidence that in r-process elements GCs trace the star formation history of their hosts, motivating their use as sub-Gyr timers of galactic evolution. Furthermore, fitting the trends in [Eu/Si] using a simple galactic chemical evolution model, we find that differences in [Eu/Si] between accreted and in-situ MW field stars cannot be explained through star formation efficiency alone. Finally, we show that the use of [Eu/Si] as a chemical tag between GCs and their host galaxies extends beyond the Local Group, to the halo of M31 - potentially offering the opportunity to do Galactic Archaeology in an external galaxy.

Magnesium isotope ratios in Milky Way and dwarf galaxy stars

ABSTRACT Under the assumption of hierarchical galaxy formation, dwarf galaxies are the closest existing analogues to the high-redshift protogalaxies that merged to form the Milky Way. These low-mass systems serve as unique laboratories for studying nucleosynthetic channels given that the chemical compositions of their stars play a pivotal role in constraining their chemical enrichment history. To date, stellar abundances in dwarf galaxies have focused almost exclusively on elemental abundance ratios. While important, elemental abundances omit critical information about the isotopic composition. Here, we compute the Mg isotopic ratios of six accreted dwarf galaxy stars (low $\alpha$) and seven Milky Way stars (high $\alpha$) using a set of high-resolution (65 000 &amp;lt; R &amp;lt; 160 000) and high signal-to-noise ratio ($\rm {S/N} \gt 250$) optical spectra. We show, for the first time, that at a given [Fe/H] stars born in a dwarf galaxy differ in their Mg isotopic ratios from stars born in the Milky Way. However, when comparing isotopic ratios at a given [Mg/H] rather than [Fe/H], a powerful diagnostic emerges that suggests nucleosynthesis processes are consistent across different stellar environments. This universality of Mg isotopic abundances provides additional dimensionality for chemical evolution models and helps to constrain massive star nucleosynthesis across cosmic time.

Interstellar Nitrogen Isotope Ratios: Measurements on Tracers of C14N and C15N

The nitrogen isotope ratio 14N/15N is a powerful tool to trace Galactic stellar nucleosynthesis and constrain Galactic chemical evolution. Previous observations have found lower 14N/15N ratios in the Galactic center and higher values in the Galactic disk. This is consistent with the inside-out formation scenario of our Milky Way. However, previous studies mostly utilized double isotope ratios also including 12C/13C, which introduces additional uncertainties. Here we present observations of C14N and its rare isotopologue, C15N, toward a sample of star-forming regions measured by the IRAM 30 m and/or the ARO 12 m telescope at λ ∼ 3 mm wavelength. For those 35 sources detected in both isotopologues, physical parameters are determined. Furthermore, we have obtained nitrogen isotope ratios using the strongest hyperfine components of CN and C15N. For those sources showing small deviations from local thermodynamical equilibrium and/or self-absorption, the weakest hyperfine component, likely free of the latter effect, was used to obtain reliable 14N/15N values. Our measured 14N/15N isotope ratios from C14N and C15N measurements are compatible with those from our earlier measurements of NH3 and 15NH3 (Paper I), i.e., increasing ratios to a Galactocentric distance of ∼9 kpc. The unweighted second-order polynomial fit yields C14NC15N=(−4.85±1.89)kpc−2×RGC2+(82.11±31.93)kpc−1×RGC−(28.12±126.62) . Toward the outer galaxy, the isotope ratio tends to decrease, supporting an earlier finding by H13CN/HC15N. Galactic chemical evolution models are consistent with our measurements of the 14N/15N isotope ratio, i.e., a rising trend from the Galactic center region to approximately 9 kpc, followed by a decreasing trend with increasing R GC toward the outer Galaxy.

First Detection of CO Isotopologues in a High-redshift Main-sequence Galaxy: Evidence of a Top-heavy Stellar Initial Mass Function

Recent observations and theories have presented a strong challenge to the universality of the stellar initial mass function (IMF) in extreme environments. A notable example has been found for starburst conditions, where evidence favors a top-heavy IMF, i.e., there is a bias toward massive stars compared to the IMF that is responsible for the stellar mass function and elemental abundances observed in the Milky Way. Local starburst galaxies have star formation rates similar to those in high-redshift main-sequence galaxies, which appear to dominate the stellar mass budget at early epochs. However, the IMF of high-redshift main-sequence galaxies is yet to be probed. Since 13CO and C18O isotopologues are sensitive to the IMF, we have observed these lines toward four strongly lensed high-redshift main-sequence galaxies using the Atacama Large Millimeter/submillimeter Array. Of our four targets, SDSS J0901+1814, at z ≈ 2.26, is seen clearly in 13CO and C18O, the first detection of CO isotopologues in the high-redshift main-sequence galaxy population. The observed 13C/18O ratio, 2.4 ± 0.8, is significantly lower than that of local main-sequence galaxies. We estimate the isotope ratio, oxygen abundance, and stellar mass using a series of chemical evolution models with varying star formation histories and IMFs. All models favor an IMF that is more top-heavy than that of the Milky Way. Thus, as with starburst galaxies, main-sequence galaxies in the high-redshift Universe have a greater fraction of massive stars than a Milky Way IMF would imply.

Abundances of neutron-capture elements in selected solar-type stars

The primary objective of this study is to accurately determine the abundances of Cu, Sr, Y, Zr, Ba, La, and Ce in selected solar-type stars. This will allow us to establish observational abundance--metallicity and abundance--age relations and to explore the reasons for the excess of Ba compared to other $s$-elements in younger solar-type stars. The chosen $s$-process elements are critical diagnostics for understanding the chemical evolution of our Galaxy. We analysed HARPS spectra with a high resolution ($R$ = 115\,000) and high signal-to-noise ratio (close to 100) of main-sequence solar-type FGK stars with metallicities from $-0.15$ to +0.35 dex and ages from 2 to 14 Gyr using one-dimensional (1D) local thermodynamic equilibrium (LTE) synthesis and MARCS atmospheric models. In the procedure of fitting synthetic to observed line profiles, the free parameters included abundance and microturbulent and macroturbulent velocity. The macroturbulent velocity can substantially compensate for non-local thermodynamic equilibrium (NLTE) effects in the line core. The resulting elemental abundance X/H increases with metallicity and age for solar-type stars. The ratio of the abundances of $s$-process elements s/Fe increases with decreasing metallicity and age, while the Cu/Fe ratio increases with both metallicity and age. These observed trends agree well with published observational data and with predictions from Galactic chemical evolution (GCE) models. A small Ba/Fe enhancement of 0.08 ± 0.08 dex has been detected in seven younger stars with an average age of $2.8 0.6$ Gyr. Compared to the abundances of other $s$-process elements Ba/Fe is 0.07 and 0.08 dex higher than La and Ce on average, respectively. Furthermore, we find that the Ba/Fe ratio increases with increasing chromospheric activity. The average Ba/Fe for the three most active stars is $0.15 0.10$ dex higher than that of the other stars. Chromospheric activity, characterised by stronger magnetic fields found in active regions such as pores, spots, plages, and networks, can significantly alter the physical conditions in the formation layers of the Ba lines. Our primary conclusion is that to account for the observed excess of Ba/Fe abundance in younger stars, it is essential to use more complex atmospheric models that incorporate magnetic structures.

Exploring fluorine chemical evolution in the Galactic disk: The open cluster perspective

Open clusters are ideal tools for tracing the abundances of different elements because their stars are expected to have the same age, distance, and metallicity. Therefore, they serve as powerful tracers for investigating the cosmic origins of elements. This paper expands on a recent study by us, in which the element fluorine was studied in seven open clusters; here we add six open clusters and eight field stars. The primary objective is to determine the abundance of fluorine (F) to gain insight into its production and evolution. The magnesium (Mg) abundances were derived to categorize the field stars into high and low alpha disk populations. Additionally, cerium (Ce) abundances were determined to better understand the interplay between F and s-process elements. Our goal is to analyze the trend of F abundances across the Galactic disk based on metallicity and age. By comparing observational data with Galactic chemical evolution models, the origin of F can be better understood. The spectra were obtained from the high-resolution near-infrared GIANO-B instrument at the Telescopio Nazionale Galileo (TNG). For the derivation of the stellar parameters and abundances, the Python version of Spectroscopy Made Easy ( PySME ) was used. OH, CN, and CO molecular lines and band heads along with Fe I lines were used to determine the stellar parameters in the H-band region. Two HF lines in the K band ($ 2.28, and 2.33 mu m), three K-band Mg I lines ($ 2.10, 2.11, and 2.15 mu m), and two Ce II lines in the H band ($ 1.66, and 1.71 mu m) were used to derive the abundances of F, Mg, and Ce, respectively. F, Mg, and Ce abundances were derived for 14 stars from 6 OCs, as well as for 8 field stars. The F and Ce abundances were investigated as a function of metallicity, age, and galactocentric distance. We also compared our findings with different Galactic chemical evolution models. Our results indicate that asymptotic giant branch stars and massive stars, including a subset of fast rotators (whose rotation speed likely increases as metallicity decreases), are necessary to explain the cosmic origin of F. This finding is consistent with and, with the large sample size, reinforces the conclusion of our previous study.