Abundances Of Stars Research Articles

Public Data Release of the FIRE-2 Cosmological Zoom-in Simulations of Galaxy Formation

We describe a public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation (available at http://flathub.flatironinstitute.org/fire) from the Feedback In Realistic Environments (FIRE) project. FIRE-2 simulations achieve parsec-scale resolution to explicitly model the multiphase interstellar medium while implementing direct models for stellar evolution and feedback, including stellar winds, core-collapse and Type Ia supernovae, radiation pressure, photoionization, and photoelectric heating. We release complete snapshots from three suites of simulations. The first comprises 20 simulations that zoom in on 14 Milky Way (MW)–mass galaxies, five SMC/LMC-mass galaxies, and four lower-mass galaxies including one ultrafaint; we release 39 snapshots across z = 0–10. The second comprises four massive galaxies, with 19 snapshots across z = 1–10. Finally, a high-redshift suite comprises 22 simulations, with 11 snapshots across z = 5–10. Each simulation also includes dozens of resolved lower-mass (satellite) galaxies in its zoom-in region. Snapshots include all stored properties for all dark matter, gas, and star particles, including 11 elemental abundances for stars and gas, and formation times (ages) of star particles. We also release accompanying (sub)halo catalogs, which include galaxy properties and member star particles. For the simulations to z = 0, including all MW-mass galaxies, we release the formation coordinates and an “ex situ” flag for all star particles, pointers to track particles across snapshots, catalogs of stellar streams, and multipole basis expansions for the halo mass distributions. We describe publicly available python packages for reading and analyzing these simulations.

Atomic Transition Probabilities for Transitions of Si i and Si ii and the Silicon Abundances of Several Very Metal-poor Stars ∗ ∗ Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Other data

We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Si i) originating in the 3s 23p4s 3Po 1,2,1Po 1, and 3s3p 3 1Do 1,2 upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the 4P1/2–2Po 1/2,3/2 intercombination lines in the spectrum of singly ionized silicon (Si ii). The weak spin-forbidden lines of Si i and Si ii provide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log(gf) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars.

Detailed α abundance trends in the inner Galactic bulge

Context. Until now, heavy interstellar extinction has meant that only a few studies of chemical abundances have been possible in the inner Galactic bulge. However, it is crucial to learn more about this structure in order to better understand the formation and evolution of the centre of the Galaxy and galaxies in general. Aims. In this paper, we aim to derive high-precision α-element abundances using CRIRES high-resolution IR spectra of 72 cool M giants of the inner Galactic bulge. Methods. Silicon, magnesium, and calcium abundances were determined by fitting a synthetic spectrum for each star. We also incorporated recent theoretical data into our spectroscopic analysis (i.e. updated K-band line list, better broadening parameter estimation, non-local thermodynamic equilibrium (NLTE) corrections). We compare these inner bulge α abundance trends with those of solar neighbourhood stars observed with IGRINS using the same line list and analysis technique; we also compare our sample to APOGEE DR17 abundances for inner bulge stars. We investigate bulge membership using spectro-photometric distances and orbital simulations. We construct a chemical-evolution model that fits our metallicity distribution function (MDF) and our α-element trends. Results. Among our 72 stars, we find four that are not bulge members. [Si/Fe] and [Mg/Fe] versus [Fe/H] trends show a typical thick disc α-element behaviour, except that we do not see any plateau at supersolar metallicities as seen in other works. The NLTE analysis lowers [Mg/Fe] typically by ∼0.1 dex, resulting in a noticeably lower trend of [Mg/Fe] versus [Fe/H]. The derived [Ca/Fe] versus [Fe/H] trend has a larger scatter than those for Si and Mg, but is in excellent agreement with local thin and thick disc trends. With our updated analysis, we constructed one of the most detailed studies of the α abundance trends of cool M giants in the inner Galactic bulge. We modelled these abundances by adopting a two-infall chemical-evolution model with two distinct gas-infall episodes with timescales of 0.4 Gyr and 2 Gyr, respectively. Conclusions. Based on a very meticulous spectral analysis, we have constructed detailed and precise chemical abundances of Mg, Si, and Ca for cool M giants. The present study can be used as a benchmark for future spectroscopic surveys.

The EBLM project X. Benchmark masses, radii, and temperatures for two fully convective M-dwarfs using K2

ABSTRACT M-dwarfs are the most abundant stars in the galaxy and popular targets for exoplanet searches. However, their intrinsic faintness and complex spectra inhibit precise characterization. We only know of dozens of M-dwarfs with fundamental parameters of mass, radius, and effective temperature characterized to better than a few per cent. Eclipsing binaries remain the most robust means of stellar characterization. Here we present two targets from the Eclipsing Binary Low Mass (EBLM) survey that were observed with K2: EBLM J0055-00 and EBLM J2217-04. Combined with HARPS and CORALIE spectroscopy, we measure M-dwarf masses with precisions better than 5 per cent, radii better than 3 per cent, and effective temperatures on order 1 per cent. However, our fits require invoking a model to derive parameters for the primary star and fitting the M-dwarf using the transit and radial velocity observations. By investigating three popular stellar models, we determine that the model uncertainty in the primary star is of similar magnitude to the statistical uncertainty in the model fits of the secondary M-dwarf. Therefore, whilst these can be considered benchmark M-dwarfs, we caution the community to consider model uncertainty when pushing the limits of precise stellar characterization.

Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

The study of Li phenomena in red clump (RC) stars can give us a deeper understanding of the structure and evolution of stars. Chanamé et al. explained the RC Li abundance distributions naturally using only standard post-main-sequence (MS) Li evolution models when the distribution of progenitor masses and the depletion of Li during the MS observed in MS stars were considered, thus neither an extra Li depletion nor Li creation mechanism is required. Nevertheless, it is interesting to consider the effects of mixing caused by some extra mechanisms. By constructing different models, we find that the mixing caused by internal gravity waves can explain the observed Li abundances of RC stars with low-mass progenitors. To explain this, we rely on the extra mixing induced by internal gravity waves that are excited at the bottom of the convective envelope during the red giant branch (RGB) stage. During the RGB stage, introducing internal gravity waves can improve the diffusion coefficient and strengthen the mixing effect. The effective enrichment of Li occurs during the late RGB stage and requires the diffusion coefficient of the H-burning shell to reach ∼108 cm2 s−1. Our models predict that the Li abundance decreases from ∼1.5 to ∼0.0 dex at the end of the core He-burning stage, thereby revealing ∼99% of the observed Li abundance distribution. Thermohaline mixing regulates the Li abundance of RGB stars, which combined with internal gravity waves can explain the Li abundances of most giants.

The Chemical Enrichment of the Milky Way Disk Evaluated Using Conditional Abundances

Chemical abundances of Milky Way disk stars are empirical tracers of its enrichment history. However, they capture joint-information that is valuable to disentangle. In this work, we quantify how individual abundances evolve across the present-day Galactic radius, at fixed supernovae contribution ([Fe/H], [Mg/Fe]). We use 18,135 Apache Point Observatory Galactic Evolution Experiment Data Release 17 red clump stars and 7943 GALactic Archaeology with HERMES Data Release 3 main-sequence stars to compare the abundance distributions conditioned on ([Fe/H], [Mg/Fe]) across 3–13 kpc and 6.5–9.5 kpc, respectively. We examine 15 elements: C, N, Al, K (light), O, Si, S, Ca, (α), Mn, Ni, Cr, Cu, (iron-peak) Ce, Ba (s-process) and Eu (r-process). We find that the conditional neutron-capture and light elements most significantly trace variations in the disk’s enrichment history, with absolute conditional radial gradients ≤0.03 dex kpc−1. The other elements studied have absolute conditional gradients ≲0.01 dex kpc−1. We uncover structured conditional abundance variations with [Fe/H] for the low-α, but not the high-α , sequence. The average scatter between the mean conditional abundances at different radii is σ intrinsic ≈ 0.02 dex (Ce, Eu, Ba σ intrinsic > 0.05 dex). These results serve as a measure of the magnitude via which different elements trace Galactic radial enrichment history once fiducial supernovae correlations are accounted for. Furthermore, we uncover subtle systematic variations in moments of the conditional abundance distributions and bimodal differences in [Al/Fe]. These suggest a nonuniform enrichment of each chemical cell, and will presumably constrain chemical evolution models of the Galaxy.

From emblematic to problematic: The case of Astrospartus mediterraneus (Risso, 1826) (Echinodermata: Ophiuroidea) in the artisanal fishing grounds of the Cap de Creus area (NW Mediterranean Sea)

Although Astrospartus mediterraneus (Risso, 1826) is an emblematic Mediterranean species, limited information is currently available, as it has not received much attention from the marine scientific community to date. In this context, an unusually high abundance of this basket star was observed in 2018 as part of the by-catch of local artisanal fishers in the Cap de Creus area (NW Mediterranean Sea). Indeed, fishers reported that this species had increased in abundance and expanded its distribution in recent years, ultimately interfering with their fishing activity. As such, this study aimed to elucidate the abundance, distribution, size, and structure of A. mediterraneus populations, as well as to evaluate the potential impact this species has on artisanal fisheries’ performance in the study area. To such aim, this work benefited from the Local Ecological Knowledge (LEK) of the fishers, by-catch analyses of regular fishing events and video transects recorded employing Remote Operated Vehicles (ROV).Results show that A. mediterraneus has a specific distribution in the study area, mainly driven by habitat requirements. High abundances of the basket star were associated with rocky substrates with the presence of gorgonians, located between 50 and 80 m depth and preferentially occurring on sloping areas. Despite the lack of long-time monitoring data, A. mediterraneus' aggregations recorded in the studied area are the densest ones so far reported for the whole Mediterranean Sea with peak values of 18 ind. m−2 and a mean density of 0.45 ind. m−2 ± 0.71 ind. m−2. Additionally, the average size of their central disks (2.67 ± 0.97 cm), suggests that this is rather young population, which could be possibly linked with the beginning of a massive outbreak, which would be in accordance with the fishers’ own experiences, as it seems apparent that the species has been on the rise in recent years. Furthermore, they unanimously consider this proliferation to be a considerable handicap in terms of time and monetary losses.Overall, additional studies are needed to increase current knowledge on the ecology of this species. Understanding the species will be paramount to monitor and assess this phenomenon with stakeholders to avoid further ecological and economic consequences.

Spectroscopic Determination of C, N, and O Abundances of Solar-analog Stars Based on the Lines of Hydride Molecules

Photospheric C, N, and O abundances of 118 solar-analog stars were determined by applying the synthetic-fitting analysis to their spectra in the blue or near-UV region comprising lines of CH, NH, and OH molecules, with an aim of clarifying the behaviors of these abundances in comparison with [Fe/H]. It turned out that, in the range of −0.6 ≲ [Fe/H] ≲ +0.3, [C/Fe] shows a marginally increasing tendency with decreasing [Fe/H] with a slight upturn around [Fe/H] ∼ 0, [N/Fe] tends to somewhat decrease toward lower [Fe/H], and [O/Fe] systematically increases (and thus [C/O] decreases) with a decrease in [Fe/H]. While these results are qualitatively consistent with previous determinations mostly based on atomic lines, the distribution centers of these [C/Fe], [N/Fe], and [O/Fe] at the near-solar metallicity are slightly negative by several hundredths of dex, which is interpreted as due to unusual solar abundances possibly related to the planetary formation of our solar system. However, clear anomalies are not observed in the [C, N, O/Fe] ratios of planet-host stars. Three out of four very Be-deficient stars were found to show anomalous [C/Fe] or [N/Fe] which may be due to mass transfer from the evolved companion, though its relation to the Be depletion mechanism is still unclear.

Stellar initial mass function varies with metallicity and time.

Most structural and evolutionary properties of galaxies strongly rely on the stellar initial mass function (IMF), namely the distribution of the stellar mass formed in each episode of star formation1-4. The IMF shapes the stellar population in all stellar systems, and so has become one of the most fundamental concepts of modern astronomy. Both constant and variable IMFs across different environments have been claimed despite a large number of theoretical5-7 and observational efforts8-15. However, the measurement of the IMF in Galactic stellar populations has been limited by the relatively small number of photometrically observed stars, leading to high uncertainties12-16. Here we report a star-counting result based on approximately 93,000 spectroscopically observed M-dwarf stars, an order of magnitude more than previous studies, in the 100-300 parsec solar neighbourhood. We find unambiguous evidence of a variable IMF that depends on both metallicity and stellar age. Specifically, the stellar population formed at early times contains fewer low-mass stars compared with the canonical IMF, independent of stellar metallicities. In more recent times, however, the proportion of low-mass stars increases with stellar metallicity. The variable abundance of low-mass stars in our Milky Way establishes a powerful benchmark for models of star formation and can heavily affect results in Galactic chemical-enrichment modelling, mass estimation of galaxies and planet-formation efficiency.

Detailed Chemical Abundances of Stars in the Outskirts of the Tucana II Ultrafaint Dwarf Galaxy* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

We present chemical abundances and velocities of five stars between 0.3 and 1.1 kpc from the center of the Tucana II ultrafaint dwarf galaxy (UFD) from high-resolution Magellan/MIKE spectroscopy. We find that every star is deficient in metals (−3.6 < [Fe/H] < −1.9) and in neutron-capture elements as is characteristic of UFD stars, unambiguously confirming their association with Tucana II. Other chemical abundances (e.g., C, iron peak) largely follow UFD trends and suggest that faint core-collapse supernovae (SNe) dominated the early evolution of Tucana II. We see a downturn in [α/Fe] at [Fe/H] ≈ −2.8, indicating the onset of Type Ia SN enrichment and somewhat extended chemical evolution. The most metal-rich star has strikingly low [Sc/Fe] = −1.29 ± 0.48 and [Mn/Fe] = −1.33 ± 0.33, implying significant enrichment by a sub-Chandrasekhar mass Type Ia SN. We do not detect a radial velocity gradient in Tucana II ( km s−1 kpc−1), reflecting a lack of evidence for tidal disruption, and derive a dynamical mass of M ⊙. We revisit formation scenarios of the extended component of Tucana II in light of its stellar chemical abundances. We find no evidence that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in situ stellar halo formation, then other UFDs should show similar such features. Although not a unique explanation, the decline in [α/Fe] is consistent with an early galactic merger triggering later star formation. Future observations may disentangle such formation channels of UFD outskirts.

Impact of newly measured 26Al(n, p)26Mg and 26Al(n, α)23Na reaction rates on the nucleosynthesis of 26Al in stars

ABSTRACT The cosmic production of the short-lived radioactive nuclide 26Al is crucial for our understanding of the evolution of stars and galaxies. However, simulations of the stellar sites producing 26Al are still weakened by significant nuclear uncertainties. We re-evaluate the 26Al(n, p)26Mg, and 26Al(n, α)23Na ground state reactivities from 0.01 GK to 10 GK, based on the recent n_TOF measurement combined with theoretical predictions and a previous measurement at higher energies, and test their impact on stellar nucleosynthesis. We computed the nucleosynthesis of low- and high-mass stars using the Monash nucleosynthesis code, the NuGrid mppnp code, and the FUNS stellar evolutionary code. Our low-mass stellar models cover the 2–3 M⊙ mass range with metallicities between Z = 0.01 and 0.02, their predicted 26Al/27Al ratios are compared to 62 meteoritic SiC grains. For high-mass stars, we test our reactivities on two 15 M⊙ models with Z = 0.006 and 0.02. The new reactivities allow low-mass AGB stars to reproduce the full range of 26Al/27Al ratios measured in SiC grains. The final 26Al abundance in high-mass stars, at the point of highest production, varies by a factor of 2.4 when adopting the upper, or lower limit of our rates. However, stellar uncertainties still play an important role in both mass regimes. The new reactivities visibly impact both low- and high-mass stars nucleosynthesis and allow a general improvement in the comparison between stardust SiC grains and low-mass star models. Concerning explosive nucleosynthesis, an improvement of the current uncertainties between T9∼0.3 and 2.5 is needed for future studies.

Non-LTE radiative transfer with Turbospectrum

Physically realistic models of stellar spectra are needed in a variety of astronomical studies, from the analysis of fundamental stellar parameters, to studies of exoplanets and stellar populations in galaxies. Here we present a new version of the widely used radiative transfer code Turbospectrum, which we update so that it is able to perform spectrum synthesis for lines of multiple chemical elements in non-local thermodynamic equilibrium (NLTE). We use the code in the analysis of metallicites and abundances of the Gaia FGK benchmark stars, using 1D MARCS atmospheric models and the averages of 3D radiation-hydrodynamics simulations of stellar surface convection. We show that the new more physically realistic models offer a better description of the observed data, and we make the program and the associated microphysics data publicly available, including grids of NLTE departure coefficients for H, O, Na, Mg, Si, Ca, Ti, Mn, Fe, Co, Ni, Sr, and Ba.

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

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.

Near-infrared chemical abundances of stars in the Sculptor dwarf galaxy

Owing to the recent identification of major substructures in our Milky Way (MW), the astronomical community has started to reevaluate the importance of dissolved and existing dwarf galaxies. In this work, we investigate up to 13 elements in 43 giant stars of the Sculptor dwarf galaxy (Scl) using high-signal-to-noise-ratio near-infrared (NIR) APOGEE spectra. Thanks to the strong feature lines in the NIR, we were able to determine high-resolution O, Si, and Al abundances for a large group of sample stars for the first time in Scl. By comparing the [α/Fe] (i.e., O, Mg, Si, Ca, and Ti) of the stars in Scl, Sagittarius, and the MW, we confirm the general trend that less massive galaxies tend to show lower [α/Fe]. The low [Al/Fe] (∼ − 0.5) in Scl demonstrates the value of this ratio as a discriminator with which to identify stars born in dwarf galaxies (from MW field stars). A chemical-evolution model suggests that Scl has a top-light initial mass function (IMF), with a high-mass IMF power index of ∼ − 2.7, and a minimum Type Ia supernovae delay time of ∼100 Myr. Furthermore, a linear regression analysis indicates a negative radial metallicity gradient and positive radial gradients for [Mg/Fe] and [Ca/Fe], in qualitative agreement with the outside-in formation scenario.

High-precision abundances of first-population stars in NGC 2808: confirmation of a metallicity spread

Photometric investigations have revealed that Galactic globular clusters (GCs) exhibit internal metallicity variations amongst the so-called first-population stars, which until now were considered to have a homogeneous initial chemical composition. This is not fully supported by the sparse spectroscopic evidence, which so far gives conflicting results. Here, we present a high-resolution re-analysis of five stars in the Galactic GC NGC 2808 taken from the literature. Target stars are bright red giants with nearly identical atmospheric parameters belonging to the first population according to their identification in the chromosome map of the cluster, and we measured precise differential abundances for Fe, Si, Ca, Ti, and Ni to the ∼0.03 dex level. Thanks to the very small uncertainties associated with the differential atmospheric parameters and abundance measurements, we find that target stars span a range of iron abundance equal to 0.25 ± 0.06 dex. The individual elemental abundances are highly correlated with the positions of the stars along the extended sequence described by first-population objects in the cluster chromosome map: bluer stars have a lower iron content. This agrees with inferences from the photometric analysis. The differential abundances of all other elements also show statistically significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti, and Ni variations are highly correlated with iron variations and the total abundance spreads for all elements are consistent within the error bars. This suggests a scenario in which short-lived massive stars exploding as supernovae contributed to the self-enrichment of the gas in the natal cloud while star formation was still ongoing.

The Cosmic Hunt for members in the outskirts of ultra-faint dwarf galaxies: Ursa Major I, Coma Berenices, and Boötes I

ABSTRACT Gaia EDR3 data were used to identify potential members in the outskirts of three ultra-faint dwarf (UFD) galaxies: Coma Berenices (&amp;gt;2Rh), Ursa Major I (∼4Rh), and Boötes I (∼4Rh), as well as a new member in the central region of Ursa Major I. These targets were observed with the Gemini GRACES spectrograph, which was used to determine precision radial velocities and metallicities that confirm their associations with the UFD galaxies. The spectra were also used to measure absorption lines for 10 elements (Na, Mg, K, Ca, Sc, Ti, Cr, Fe, Ni, and Ba), which confirm that the chemical abundances of the outermost stars are in good agreement with stars in the central regions. The abundance ratios and chemical patterns of the stars in Coma Berenices are consistent with contributions from SN Ia, which is unusual for its star formation history and in conflict with previous suggestions that this system evolved chemically from a single core collapse supernova event. The chemistries for all three galaxies are consistent with the outermost stars forming in the central regions, then moving to their current locations through tidal stripping and/or supernova feedback. In Boötes I, however, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger.

Metal production in the early Universe: what chemical abundances in old stellar populations in the Milky Way tell us

AbstractOld stellar populations in our Galaxy provide fossil records of the metal enrichment in the first few billion years of the cosmic history. Growing elemental abundance data of individual stars combined with stellar ages and kinematics allow us to make constraints on characteristic properties of the metal-enrichment sources in the early Universe, such as the first stars. In order to interpret observed chemical abundances in the oldest stellar populations in terms of metal-enrichment sources, stellar and supernova yield models are crucial. In this article, we review how we can interpret observed chemical abundances in old stars in terms of the nature of metal enrichment sources. We discuss the limitations and the prospects of empirically constraining supernova yield models based on a large sample of extremely metal-poor stars. At the same time, we emphasize the importance of hunting old stars beyond the Solar neighborhood, which can be achieved with the next-generation multi-object spectrographs at large telescopes.

Titanium abundances in late-type stars

Context. The titanium abundances of late-type stars are important tracers of Galactic formation history. However, abundances inferred from Ti I and Ti II lines can be in stark disagreement in very metal-poor giants. Departures from local thermodynamic equilibrium (LTE) have a large impact on the minority neutral species and thus influence the ionisation imbalance, but satisfactory non-LTE modelling for both dwarfs and giants has not been achieved in the literature. Aims. The reliability of titanium abundances is reassessed in benchmark dwarfs and giants using a new non-LTE model 1D model atmospheres. Methods. A comprehensive model atom was compiled with a more extended level structure and newly published data for inelastic collisions between Ti I and neutral hydrogen. Results. In 1D LTE, the Ti I and Ti II lines agree to within 0.06 dex for the Sun, Arcturus, and the very metal-poor stars HD 84937 and HD 140283. For the very metal-poor giant HD 122563, the Ti I lines give an abundance that is 0.47 dex lower than that from Ti II . The 1D non-LTE corrections can reach +0.4 dex for individual Ti I lines and +0.1 dex for individual Ti II lines, and they reduce the overall ionisation imbalance to −0.17 dex for HD 122563. However, the corrections also increase the imbalance for the very metal-poor dwarf and sub-giant to around 0.2 dex. Conclusions. Using 1D non-LTE reduces the ionisation imbalance in very metal-poor giants but breaks the balance of other very metal-poor stars, consistent with conclusions drawn in the literature. To make further progress, consistent 3D non-LTE models are needed.

Comparing simulated Milky Way satellite galaxies with observations using unsupervised clustering

ABSTRACT We develop a new analysis method that allows us to compare multidimensional observables to a theoretical model. The method is based on unsupervised clustering algorithms which assign the observational and simulated data to clusters in high dimensionality. From the clustering result, a goodness of fit (the p-value) is determined with the Fisher–Freeman–Halton test. We first show that this approach is robust for 2D Gaussian distributions. We then apply the method to the observed MW satellites and simulated satellites from the fiducial model of our semi-analytic code a-sloth . We use the following five observables of the galaxies in the analysis: stellar mass, virial mass, heliocentric distance, mean stellar metallicity [Fe/H], and stellar metallicity dispersion σ[Fe/H]. A low p-value returned from the analysis tells us that our a-sloth fiducial model does not reproduce the mean stellar metallicity of the observed MW satellites well. We implement an ad hoc improvement to the physical model and show that the number of dark matter merger trees which have a p-value &amp;gt; 0.01 increases from 3 to 6. This method can be extended to data with higher dimensionality easily. We plan to further improve the physical model in a-sloth using this method to study elemental abundances of stars in the observed MW satellites.

Globular cluster formation with multiple stellar populations: a single-binary composite scenario

ABSTRACT We discuss a GC formation scenario in which the first generation (1G) of single asymptotic giant branch (AGB) stars and intermediate-mass close binaries (IMCBs) eject gas, from which the second generation (2G) of stars can be formed. The two key parameters in the scenario are the fractions of binary stars (fb) and the slopes (α) of the stellar initial mass functions (IMFs) for 1G stars. Principle results derived by analytic and one-zone models of GC formation are as follows. The mass fraction of 2G stars (f2g) can be higher than ≈0.4 for α &amp;lt; 1.8 and is not so dependent on fb. The ratio of the initial mass of a GC to the present-day mass (Mgc) ranges from 2 to 7 depending on α for 0.5 ≤ fb ≤ 0.9. The differences in [Na/Fe] between 1G and 2G stars can be as large as 0.7 for a wide range of model parameters. The Li abundances of 2G stars can be as high as those of 1G even if the pristine gas from IMCBs is assumed to be Li-free. Formation histories of 2G stars show at least two peaks owing to two peaks in the total ejection rate of gas from IMCB populations. The observed correlation between f2g and Mgc can be due to α depending on Mgc. The hypothetical long duration of 2G formation (≈108 yr) is possible, because massive star formation can be suppressed through frequent dynamical interaction between 1G stars and gas clouds.