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Context. The atmospheres of phosphorus-rich (P-rich) stars have been shown to contain between 10 and 100 times more P than our Sun. Given its crucial role as an essential element for life, it is especially necessary to uncover the origin of P-rich stars to gain insights into the still unknown nucleosynthetic formation pathways of P in our Galaxy. Aims. Our objective is to obtain the extensive chemical abundance inventory of four P-rich stars, covering a large range of heavy (Z > 30) elements. This characterization will serve as a milestone for the nuclear astrophysics community to uncover the processes that form the unique chemical fingerprint of P-rich stars. Methods. We performed a detailed 1D local thermodynamic equilibrium abundance analysis on the optical UVES spectra of four P-rich stars. The abundance measurements, complemented with upper-limit estimates, included 48 light and heavy elements. Our focus lay on the neutron-capture elements (Z > 30), in particular, on the elements between Sr and Ba, as well as on Pb, as they provide valuable constraints to nucleosynthesis calculations. In past works, we showed that the heavy-element observations from the first P-rich stars are not compatible with either classical s-process or r-process abundance patterns. In this work, we compare the obtained abundances with three different nucleosynthetic scenarios: a single i-process, a double i-process, and a combination of s- and i-processes. Results. We have performed the most extensive abundance analysis of P-rich stars to date, including the elements between Sr and Ba, such as Ag, which are rarely measured in any type of stars. We also estimated constraining upper limits for Cd I, In I, and Sn I. We found overabundances with respect to solar in the s-process peak elements, accompanied by an extremely high Ba abundance and slight enhancements in some elements between Rb and Sn. No global solution explaining all four stars could be found for the nucleosynthetic origin of the pattern. The model that produces the least number of discrepancies in three of the four stars is a combination of s- and i-processes, but the current lack of extensive multidimensional hydrodynamic simulations to follow the occurrence of the i-process in different types of stars makes this scenario highly uncertain. [ABSTRACT FROM AUTHOR]

Dusty post-red giant branch (post-RGB) stars are low- and intermediate-mass stars where the RGB evolution was prematurely terminated by a poorly understood binary interaction. These binary stars are considered to be low-luminosity analogues of post-asymptotic giant branch (post-AGB) binary stars. In this study, we investigated the chemical composition of two dusty post-RGB binary stars, SZ Mon and DF Cyg, using multiwavelength spectroscopic data from HERMES/Mercator (optical) and the APOGEE survey (near-infrared). Owing to challenges posed by existing spectral analysis tools for the study of evolved stars with complex atmospheres, we developed E-iSpec: a dedicated spectral analysis tool for evolved stars, to consistently determine atmospheric parameters, elemental abundances, and carbon isotopic ratios. Our abundance analysis revealed that observed depletion patterns and estimated depletion efficiencies resemble those found in post-AGB binary stars. However, the onset of chemical depletion in post-RGB targets occurs at higher condensation temperatures (⁠|$T_{\rm turn-off,~post-RGB}\approx 1\, 400$|  K), than in most post-AGB stars (⁠|$T_{\rm turn-off,~post-AGB}\approx 1\, 100$|  K). Additionally, our study resulted in the first estimates of carbon isotopic ratios for post-RGB stars (12C/13CSZ Mon = 8 ± 4, 12C/13CDF Cyg = 12 ± 3). We found that the observationally derived CNO abundances and the carbon isotopic ratios of our post-RGB binary targets are in good agreement with theoretical predictions from the ATON single star evolutionary models involving first dredge-up and moderately deep extra mixing. This agreement emphasizes that in post-RGB binary targets, the observed CNO abundances reflect the chemical composition expected from single star nucleosynthesis (i.e. convective and non-convective mixing processes) occurring during the RGB phase before it is terminated. [ABSTRACT FROM AUTHOR]

Context. Hydrodynamical (or 3D) and non-local thermodynamic equilibrium (NLTE) effects are known to affect abundance analyses. However, there are very few observational abundance tests of 3D and NLTE models. Aims. We developed a new way of testing the abundance predictions of 3D and NLTE models, taking advantage of large spectroscopic survey data. Methods. We use a line-by-line analysis of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra (H band) with the Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS). We compute line-by-line abundances of Mg, Si, Ca, and Fe for a large number of globular cluster K giants in the APOGEE survey. We compare this line-by-line analysis against NLTE and 3D predictions. Results. While the 1D–NLTE models provide corrections in the right direction, there are quantitative discrepancies between different models. We observe a better agreement with the data for the models including reliable collisional cross-sections. The agreement between data and models is not always satisfactory when the 3D spectra are computed in LTE. However, we note that for a fair comparison, 3D corrections should be computed with self-consistently derived stellar parameters, and not on 1D models with identical stellar parameters. Finally, we focus on 3D and NLTE effects on Fe lines in the H band, where we observe a systematic difference in abundance relative to the value from the optical. Our results suggest that the metallicities obtained from the H band are more accurate in metal-poor giants. Conclusions. Current 1D–NLTE models provide reliable abundance corrections, but only when the atom data and collisional cross-sections are accurate and complete. Therefore, we call for more atomic data for NLTE calculations. In contrast, we show that 3D corrections in LTE conditions are often not accurate enough, thus confirming that 3D abundance corrections are only valid when NLTE is taken into account. Consequently, more extended self-consistent 3D–NLTE computations need to be made. The method we have developed for testing 3D and NLTE models could be extended to other lines and elements, and is particularly suited for large spectroscopic surveys. [ABSTRACT FROM AUTHOR]

We investigate the Fe, C, N, O, Mg, Al, Si, K, Ca, Ce, and Nd abundances of 2283 red giant stars in 31 globular clusters from high-resolution spectra observed in both the Northern and Southern hemisphere by the SDSS-IV APOGEE-2 survey. This unprecedented homogeneous data set, largest to date, allows us to discuss the intrinsic Fe spread, the shape, and statistics of Al-Mg and N-C anti-correlations as a function of cluster mass, luminosity, age, and metallicity for all 31 clusters. We find that the Fe spread does not depend on these parameters within our uncertainties including cluster metallicity, contradicting earlier observations. We do not confirm the metallicity variations previously observed in M22 and NGC 1851. Some clusters show a bimodal Al distribution, while others exhibit a continuous distribution as has been previously reported in the literature. We confirm more than two populations in ω Cen and NGC 6752, and find new ones in M79. We discuss the scatter of Al by implementing a correction to the standard chemical evolution of Al in the Milky Way. After correction, its dependence on cluster mass is increased suggesting that the extent of Al enrichment as a function of mass was suppressed before the correction. We observe a turnover in the Mg-Al anticorrelation at very low Mg in ω Cen, similar to the pattern previously reported in M15 and M92. ω Cen may also have a weak K-Mg anticorrelation, and if confirmed, it would be only the third cluster known to show such a pattern. [ABSTRACT FROM AUTHOR]

We present a novel approach to address dust production by low- and intermediate-mass stars. We study the asymptotic giant branch (AGB) phase, during which the formation of dust takes place, from the perspective of post-AGB and planetary nebula (PN) evolutionary stage. Using results from stellar evolution and dust formation modelling, we interpret the spectral energy distribution of carbon-dust-rich sources currently evolving through different evolutionary phases, believed to descend from progenitors of similar mass and chemical composition. Comparing the results of different stages along the AGB to PNe transition, we can provide distinct insights on the amount of dust and gas released during the very late AGB phases. While the post-AGB traces the history of dust production back to the tip of the AGB phase, investigating the PNe is important to reconstruct the mass-loss process experienced after the last thermal pulse. The dust surrounding the post-AGB was formed soon after the tip of the AGB. The PNe dust-to-gas ratio is ∼10−3, 2.5 times smaller than what expected for the same initial mass star during the last AGB interpulse, possibly suggesting that dust might be destroyed during the PN phase. Measuring the amount of dust present in the nebula can constrain the capacity of the dust to survive the central star heating. [ABSTRACT FROM AUTHOR]

We study a group of evolved M-stars in the Large Magellanic Cloud, characterized by a peculiar spectral energy distribution. While the 9.7  |$\mu$| m feature arises from silicate particles, the whole infrared data seem to suggest the presence of an additional featureless dust species. We propose that the circumstellar envelopes of these sources are characterized by a dual dust chemistry, with an internal region, harbouring carbonaceous particles, and an external zone, populated by silicate, iron, and alumina dust grains. Based on the comparison with results from stellar modelling that describe the dust formation process, we deduce that these stars descend from low-mass (M < 2 M⊙) objects, formed 1–4 Gyr ago, currently evolving either in the post-AGB phase or through an after-pulse phase, when the shell CNO nuclear activity is temporarily extinguished. Possible observations able to confirm or disregard the present hypothesis are discussed. [ABSTRACT FROM AUTHOR]

Context. We previously explored the circumstellar effects on Rb and Zr abundances in a sample (21) of massive Galactic O-rich asymptotic giant branch (AGB) stars. Here we are interested in clarifying the role of the extended atmosphere in the case of Li and Ca. Li is an important indicator of hot bottom burning while the total Ca abundances in these stars could be affected by neutron captures. Aims. We report new Li and Ca abundances in a larger sample (30) of massive Galactic O-rich AGB stars by using more-realistic extended model atmospheres. Li abundances had previously studied with hydrostatic models, while the Ca abundances have been determined here for the first time. Methods. We used a modified version of the spectral synthesis code Turbospectrum and consider the presence of a gaseous circumstellar envelope and radial wind in the modelling of the spectra of these massive AGB stars. The Li and Ca abundances were obtained from the 6708 Å Li I and 6463 Å Ca I resonance lines, respectively. In addition, we studied the sensitivity of the pseudo-dynamical models to variations of the stellar and wind parameters. Results. The Li abundances derived with the pseudo-dynamical models are very similar to those obtained from hydrostatic models (the average difference is 0.18 dex, σ2 = 0.02), with no difference for Ca. This indicates that the Li and Ca content in these stars is only slightly affected by the presence of a circumstellar envelope. We also found that the Li I and Ca I line profiles are not very sensitive to variations of the model wind parameters. Conclusions. The new Li abundances confirm the Li-rich (and super Li-rich, in some cases) nature of the sample stars, supporting the activation of hot bottom burning in massive Galactic AGB stars. This is in good agreement with the theoretical predictions for solar metallicity AGB models from ATON, Monash, and NuGrid/MESA but is at odds with the FRUITY database, which predicts no hot bottom burning leading to the production of Li. Most (20) sample stars display nearly solar (within the estimated errors and considering possible non-local thermodynamic equilibrium effects) Ca abundances that are consistent with the available s-process nucleosynthesis models for solar metallicity massive AGB stars, which predict overproduction of 46Ca relatively to the other Ca isotope and the creation of the radioactive isotope 41Ca (half life of 0.1 Myr) but no change in the total Ca abundance. A minority (five) of the sample stars seem to show a significant Ca depletion (by up to 1.0 dex). Possible explanations are offered to explain their apparent and unexpected Ca depletion. [ABSTRACT FROM AUTHOR]

We map the distribution and properties of the Milky Way's interstellar medium as traced by diffuse interstellar bands (DIBs) detected in near-infrared stellar spectra from the SDSS-III/APOGEE survey. Focusing exclusively on the strongest DIB in the H band, at λ ∼ 1.527 μm, we present a projected map of the DIB absorption field in the Galactic plane, using a set of about 60,000 sightlines that reach up to 15 kpc from the Sun and probe up to 30 mag of visual extinction. The strength of this DIB is linearly correlated with dust reddening over three orders of magnitude in both DIB equivalent width (WDIB) and extinction, with a power law index of 1.01 ± 0.01, a mean relationship of WDIB/AV = 0.1 Å mag–1 and a dispersion of ∼0.05 Å mag–1 at extinctions characteristic of the Galactic midplane. These properties establish this DIB as a powerful, independent probe of dust extinction over a wide range of AV values. The subset of about 14,000 robustly detected DIB features have a WDIB distribution that follows an exponential trend. We empirically determine the intrinsic rest wavelength of this transition to be λ0 = 15 272.42 Å and use it to calculate absolute radial velocities of the carrier, which display the kinematical signature of the rotating Galactic disk. We probe the DIB carrier distribution in three dimensions and show that it can be characterized by an exponential disk model with a scale height of about 100 pc and a scale length of about 5 kpc. Finally, we show that the DIB distribution also traces large-scale Galactic structures, including the Galactic long bar and the warp of the outer disk. [ABSTRACT FROM AUTHOR]

Stellar spectral classification has been highly useful in the study of stars. While there is a currently accepted spectral classification system for carbon stars, the subset of hydrogen-deficient carbon (HdC) stars has not been well described by such a system, due predominantly to their rarity and their variability. Here we present the first system for the classification of HdCs based on their spectra, which is made wholly on their observable appearance. We use a combination of dimensionality reduction and clustering algorithms with human classification to create such a system with eight total classes corresponding to temperature, and an additional second axis corresponding to the carbon molecular band strength. We classify over half of the known sample of HdC stars using this, and roughly calibrate the temperatures of each class using their colours. Additionally, we express trends in the occurrence of certain spectral peculiarities such as the presence of hydrogen and lithium lines. We also present three previously unpublished spectra, report the discovery of two new Galactic dustless HdC stars, and additionally discuss one especially unique star that appears to border between the hottest HdCs and the coolest extreme helium stars. [ABSTRACT FROM AUTHOR]

Galactic haloes in a Λ-CDM universe are predicted to host today a swarm of debris resulting from cannibalized dwarf galaxies. The chemodynamical information recorded in their stellar populations helps elucidate their nature, constraining the assembly history of the Galaxy. Using data from APOGEE and Gaia , we examine the chemical properties of various halo substructures, considering elements that sample various nucleosynthetic pathways. The systems studied are Heracles, Gaia -Enceladus/Sausage (GES), the Helmi stream, Sequoia, Thamnos, Aleph, LMS-1, Arjuna, I'itoi, Nyx, Icarus, and Pontus. Abundance patterns of all substructures are cross-compared in a statistically robust fashion. Our main findings include: (i) the chemical properties of most substructures studied match qualitatively those of dwarf Milky Way satellites, such as the Sagittarius dSph. Exceptions are Nyx and Aleph, which are chemically similar to disc stars, implying that these substructures were likely formed in situ ; (ii) Heracles differs chemically from in situ populations such as Aurora and its inner halo counterparts in a statistically significant way. The differences suggest that the star formation rate was lower in Heracles than in the early Milky Way; (iii) the chemistry of Arjuna, LMS-1, and I'itoi is indistinguishable from that of GES, suggesting a possible common origin; (iv) all three Sequoia samples studied are qualitatively similar. However, only two of those samples present chemistry that is consistent with GES in a statistically significant fashion; (v) the abundance patterns of the Helmi stream and Thamnos are different from all other halo substructures. [ABSTRACT FROM AUTHOR]

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]/dRGC being negative, while d[Ce/Fe]/dRGC 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] and [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. [ABSTRACT FROM AUTHOR]

LIN 358 and SMC N73 are two symbiotic binaries in the halo of the Small Magellanic Cloud, each composed of a hot white dwarf accreting from a cool giant companion. In this work, we characterize these systems using a combination of spectral energy distribution (SED)-fitting to the extant photometric data spanning a broad wavelength range (X-ray/ultraviolet to near-infrared), detailed analysis of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra for the giant stars, and orbit fitting to high quality radial velocities from the APOGEE database. Using the calculated Roche lobe radius for the giant component and the mass ratio for each system, it is found that LIN 358 is likely undergoing mass transfer via wind Roche lobe overflow, while the accretion mechanism for SMC N73 remains uncertain. This work presents the first orbital characterization for both of these systems (yielding periods of >270 and >980 days, respectively, for SMC N73 and LIN 358) and the first global SED fitting for SMC N73. In addition, variability was identified in APOGEE spectra of LIN 358 spanning 17 epochs over two years that may point to a time variable accretion rate as the product of an eccentric orbit. [ABSTRACT FROM AUTHOR]

We study the multiple populations of ω Cen by using the abundances of Fe, C, N, O, Mg, Al, Si, K, Ca, and Ce from the high-resolution, high signal-to-noise (S/N > 70) spectra of 982 red giant stars observed by the SDSS-IV/APOGEE-2 survey. We find that the shape of the Al–Mg and N–C anticorrelations changes as a function of metallicity, continuous for the metal-poor groups, but bimodal (or unimodal) at high metallicities. There are four Fe populations, similarly to previous literature findings, but we find seven populations based on Fe, Al, and Mg abundances. The evolution of Al in ω Cen is compared to its evolution in the Milky Way and in five representative globular clusters. We find that the distribution of Al in metal-rich stars of ω Cen closely follows what is observed in the Galaxy. Other α-elements and C, N, O, and Ce are also compared to the Milky Way, and significantly elevated abundances are observed over what is found in the thick disc for almost all elements. However, we also find some stars with high metallicity and low [Al/Fe], suggesting that ω Cen could be the remnant core of a dwarf galaxy, but the existence of these peculiar stars needs an independent confirmation. We also confirm the increase in the sum of CNO as a function of metallicity previously reported in the literature and find that the [C/N] ratio appears to show opposite correlations between Al-poor and Al-rich stars as a function of metallicity. [ABSTRACT FROM AUTHOR]

The second phase of the APOGEE survey is providing near-infrared (near-IR), high-resolution, high signal-to-noise spectra of stars in the halo, disc, bar, and bulge of the Milky Way. The near-IR spectral window is especially important in the study of the Galactic bulge, where stars are obscured by the dust and gas of the disc in its line of sight. We present a chemical characterization of the globular cluster NGC 6544 with high-resolution spectroscopy. The characterization of the cluster chemical fingerprint, given its status of 'interloper' towards the Galactic bulge and clear signatures of tidal disruption in its core is crucial for future chemical tagging efforts. Cluster members were selected from the DR16 of the APOGEE survey, using chemodynamical criteria of individual stars. A sample of 23 members of the cluster was selected. An analysis considering the intracluster abundance variations, known as anticorrelations is given. According to the red giant branch (RGB) content of the cluster, the iron content and α-enhancement are [Fe/H] = −1.44 ± 0.04 dex and [α/Fe] = 0.20 ± 0.04 dex, respectively. Cluster members show a significant spread in [Fe/H] and [Al/Fe] that is larger than expected based on measurement errors. An [Al/Fe] spread, signal of an Mg–Al anticorrelation is observed and used to constrain the cluster mass budget, along with C, N, Mg, Si, K, Ca, and Ce element variations discussed. Across all the analysed evolutionary stages (RGB and asymptotic giant branch), about ∼2/3 (14 out of 23) show distinct chemical patterns, possibly associated with second-generation stars. [ABSTRACT FROM AUTHOR]

Given their location on the Hertzsprung-Russell (H-R) diagram, thoroughly characterized subgiant stars can place stringent constraints on a wide range of astrophysical problems. Accordingly, they are prime asteroseismic targets for the Transiting Exoplanet Survey Satellite (TESS) mission. In this work, we infer stellar properties for a sample of 347 subgiants located in the TESS Continuous Viewing Zones, which we select based on their likelihood of showing asteroseismic oscillations. We investigate how well they can be characterized using classical constraints (photometry, astrometry) and validate our results using spectroscopic values. We derive luminosities, effective temperatures, and radii with mean 1σ random (systematic) uncertainties of 4.5% (2%), 33 K (60 K), and 2.2% (2%), as well as more model-dependent quantities such as surface gravities, masses, and ages. We use our sample to demonstrate that subgiants are ideal targets for mass and age determination based on H-R diagram location alone, discuss the advantages of stellar parameters derived from a detailed characterization over widely available catalogs, show that the generally used 3D extinction maps tend to overestimate the extinction for nearby stars (distance ≲500 pc), and find a correlation that supports the rotation–activity connection in post-main-sequence stars. The complementary roles played by classical and asteroseismic data sets will open a window to unprecedented astrophysical studies using subgiant stars. [ABSTRACT FROM AUTHOR]

Recent evidence based on APOGEE data for stars within a few kpc of the Galactic Centre suggests that dissolved globular clusters (GCs) contribute significantly to the stellar mass budget of the inner halo. In this paper, we enquire into the origins of tracers of GC dissolution, N-rich stars, that are located in the inner 4 kpc of the Milky Way. From an analysis of the chemical compositions of these stars, we establish that about 30 per cent of the N-rich stars previously identified in the inner Galaxy may have an accreted origin. This result is confirmed by an analysis of the kinematic properties of our sample. The specific frequency of N-rich stars is quite large in the accreted population, exceeding that of its in situ counterparts by near an order of magnitude, in disagreement with predictions from numerical simulations. We hope that our numbers provide a useful test to models of GC formation and destruction. [ABSTRACT FROM AUTHOR]

We present an infrared (IR) characterization of the born-again planetary nebulae (PNe) A 30 and A 78 using IR images and spectra. We demonstrate that the carbon-rich dust in A 30 and A 78 is spatially coincident with the H-poor ejecta and coexists with hot X-ray-emitting gas up to distances of 50 arcsec from the central stars of PNe (CSPNe). Dust forms immediately after the born-again event and survives for 1000 yr in the harsh environment around the CSPN as it is destroyed and pushed away by radiation pressure and dragged by hydrodynamical effects. Spitzer IRS spectral maps showed that the broad spectral features at 6.4 and 8.0  μ m, attributed to amorphous carbon formed in H-deficient environments, are associated with the disrupted disc around their CSPN, providing an optimal environment for charge exchange reactions with the stellar wind that produces the soft X-ray emission of these sources. Nebular and dust properties are modelled for A 30 with cloudy taking into account different carbonaceous dust species. Our models predict dust temperatures in the 40–230 K range, five times lower than predicted by previous works. Gas and dust masses for the born-again ejecta in A 30 are estimated to be |$M_\mathrm{gas}=4.41^{+0.55}_{-0.14}\times 10^{-3}$|  M⊙ and |$M_\mathrm{dust}=3.20^{+3.21}_{-2.06}\times 10^{-3}$|  M⊙, which can be used to estimate a total ejected mass and mass-loss rate for the born-again event of |$7.61^{+3.76}_{-2.20}\times 10^{-3}$|  M⊙ and |$\dot{M}=(5{\!-\!}60)\times 10^{-5}$|  M⊙ yr−1, respectively. Taking into account the carbon trapped into dust grains, we estimate that the C/O mass ratio of the H-poor ejecta of A 30 is larger than 1, which favours the very late thermal pulse model over the alternate hypothesis of a nova-like event. [ABSTRACT FROM AUTHOR]

Many approaches to galaxy dynamics assume that the gravitational potential is simple and the distribution function is time invariant. Under these assumptions there are traditional tools for inferring potential parameters given observations of stellar kinematics (e.g., Jeans models). However, spectroscopic surveys measure many stellar properties beyond kinematics. Here we present a new approach for dynamical inference, Orbital Torus Imaging, which makes use of kinematic measurements and element abundances (or other invariant labels). We exploit the fact that, in steady state, stellar labels vary systematically with orbit characteristics (actions), yet must be invariant with respect to orbital phases (conjugate angles). The orbital foliation of phase space must therefore coincide with surfaces along which all moments of all stellar label distributions are constant. Both classical-statistics and Bayesian methods can be built on this; these methods will be more robust and require fewer assumptions than traditional tools because they require no knowledge of the (spatial) survey selection function and do not involve second moments of velocity distributions. We perform a classical-statistics demonstration with red giant branch stars from the APOGEE surveys: we model the vertical orbit structure in the Milky Way disk to constrain the local disk mass, scale height, and the disk–halo mass ratio (at fixed local circular velocity). We find that the disk mass can be constrained (naďvely) at the few-percent level with Orbital Torus Imaging using only eight element-abundance ratios, demonstrating the promise of combining stellar labels with dynamical invariants. [ABSTRACT FROM AUTHOR]

We compare abundance ratio trends in a sample of ∼11,000 Milky Way bulge stars (RGC < 3 kpc) from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to those of APOGEE stars in the Galactic disk (5 kpc < RGC < 11 kpc). We divide each sample into low-Ia (high-[Mg/Fe]) and high-Ia (low-[Mg/Fe]) populations, and in each population, we examine the median trends of [X/Mg] versus [Mg/H] for elements X = Fe, O, Na, Al, Si, P, S, K, Ca, V, Cr, Mn, Co, Ni, Cu, and Ce. To remove small systematic trends of APOGEE abundances with stellar , we resample the disk stars to match the distributions of the bulge data. After doing so, we find nearly identical median trends for low-Ia disk and bulge stars for all elements. High-Ia trends are similar for most elements, with noticeable (0.05–0.1 dex) differences for Mn, Na, and Co. The close agreement of abundance trends (with typical differences ≲0.03 dex) implies that similar nucleosynthetic processes enriched bulge and disk stars despite the different star formation histories and physical conditions of these regions. For example, we infer that differences in the high-mass slope of the stellar initial mass function between disk and bulge must have been ≲0.30. This agreement, and the generally small scatter about the median sequences, means that one can predict all of a bulge star's APOGEE abundances with good accuracy knowing only its measured [Mg/Fe] and [Mg/H] and the observed trends of disk stars. [ABSTRACT FROM AUTHOR]

Modelling dust formation in single stars evolving through the carbon-star stage of the asymptotic giant branch (AGB) reproduces well the mid-infrared colours and magnitudes of most of the C-rich sources in the Large Magellanic Cloud (LMC), apart from a small subset of extremely red objects (EROs). An analysis of the spectral energy distributions of EROs suggests the presence of large quantities of dust, which demand gas densities in the outflow significantly higher than expected from theoretical modelling. We propose that binary interaction mechanisms that involve common envelope (CE) evolution could be a possible explanation for these peculiar stars; the CE phase is favoured by the rapid growth of the stellar radius occurring after C/O overcomes unity. Our modelling of the dust provides results consistent with the observations for mass-loss rates |$\dot{M} \sim 5\times 10^{-4}\,{\rm M}_{\odot }$|  yr−1, a lower limit to the rapid loss of the envelope experienced in the CE phase. We propose that EROs could possibly hide binaries with orbital periods of about days and are likely to be responsible for a large fraction of the dust production rate in galaxies. [ABSTRACT FROM AUTHOR]

We use data of ∼13 000 stars from the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment survey to study the shape of the bulge metallicity distribution function (MDF) within the region |ℓ| ≤ 11° and | b | ≤ 13°, and spatially constrained to R GC ≤ 3.5 kpc. We apply Gaussian mixture modelling and non-negative matrix factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find that the shape and spatial variations of the MDF (at [Fe/H] ≥ −1 dex) are well represented as a smoothly varying contribution of three overlapping components located at [Fe/H] = +0.32, −0.17, and −0.66 dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disc into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help us to link a present-day structure with those observed in formation in the centre of high-redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc. [ABSTRACT FROM AUTHOR]

The chemical homogeneity of surviving stellar clusters contains important clues about interstellar medium (ISM) mixing efficiency, star formation, and the enrichment history of the Galaxy. Existing measurements in a handful of open clusters suggest homogeneity in several elements at the 0.03 dex level. Here we present (i) a new cluster member catalog based only on APOGEE radial velocities and Gaia-DR2 proper motions, (ii) improved abundance uncertainties for APOGEE cluster members, and (iii) the dependence of cluster homogeneity on Galactic and cluster properties, using abundances of eight elements from the APOGEE survey for 10 high-quality clusters. We find that cluster homogeneity is uncorrelated with Galactocentric distance, , age, and metallicity. However, velocity dispersion, which is a proxy for cluster mass, is positively correlated with intrinsic scatter at relatively high levels of significance for [Ca/Fe] and [Mg/Fe]. We also see a possible positive correlation at a low level of significance for [Ni/Fe], [Si/Fe], [Al/Fe], and [Fe/H], while [Cr/Fe] and [Mn/Fe] are uncorrelated. The elements that show a correlation with velocity dispersion are those that are predominantly produced by core-collapse supernovae (CCSNe). However, the small sample size and relatively low correlation significance highlight the need for follow-up studies. If borne out by future studies, these findings would suggest a quantitative difference between the correlation lengths of elements produced predominantly by Type Ia SNe versus CCSNe, which would have implications for Galactic chemical evolution models and the feasibility of chemical tagging. [ABSTRACT FROM AUTHOR]

The Galactic center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), allow the measurement of metallicities in the inner region of our Galaxy. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool Asymptotic Giant branch (AGB) stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the red giant branch. We studied metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ∼ +0.3 dex and a metal-poor peak around {Fe/H] = −0.5 dex, which is 0.2 dex poorer than Baade's Window. The α-elements Mg, Si, Ca, and O show a similar trend to the Galactic bulge. The metal-poor component is enhanced in the α-elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high-velocity stars with vgal >  300 km s−1; the metal-rich stars show a much higher rotation velocity (∼200 km s−1) with respect to the metal-poor stars (∼140 km s−1). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disk and the nuclear star cluster show distinct chemical signatures and might be formed differently. [ABSTRACT FROM AUTHOR]

We present stellar age distributions of the Milky Way bulge region using ages for ∼6000 high-luminosity (), metal-rich ([Fe/H] ≥ −0.5) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment. Ages are derived using The Cannon label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars (>8 Gyr). We find evidence that the planar region of the bulge (kpc) is enriched in metallicity, Z, at a faster rate (dZ/dt ∼ 0.0034 Gyr−1) than regions farther from the plane (dZ/dt ∼ 0.0013 Gyr−1 at kpc). We identify a nonnegligible fraction of younger stars (age ∼2–5 Gyr) at metallicities of +0.2 < [Fe/H] < +0.4. These stars are preferentially found in the plane (kpc) and at Rcy ≈ 2–3 kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found inside and outside the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at supersolar metallicities in a thin disk at 2 kpc ≲ Rcy ≲ 3 kpc until ∼2 Gyr ago. [ABSTRACT FROM AUTHOR]

Astronomical infrared spectral features at ∼6.6, 9.8 and 20 µm have recently been suggested as being due to the planar graphene form of C24 carbon cluster. Here, we report density functional theory and coupled cluster calculations on wavefunctions stability, relative energies and infrared spectra of four different types of C24 isomers, including the graphene and fullerene forms. The types of vibrational motions under these bands are also discussed. Among the four isomers, we find that the astronomical data are best approximated by the graphene form of C24. [ABSTRACT FROM AUTHOR]

We report on H -band spectra of chemically peculiar Mercury–Manganese (HgMn) stars obtained via the SDSS/APOGEE survey. As opposed to other varieties of chemically peculiar stars such as classical Ap/Bp stars, HgMn stars lack strong magnetic fields and are defined by extreme overabundances of Mn, Hg, and other heavy elements. A satisfactory explanation for the abundance patterns remains to be determined, but low rotational velocity is a requirement and involvement in binary/multiple systems may be as well. The APOGEE HgMn sample currently consists of 269 stars that were identified among the telluric standard stars as those whose metallic absorption content is limited to or dominated by the H -band Mn  ii lines. Due to the fainter magnitudes probed by the APOGEE survey as compared to past studies, only 9/269 stars in the sample were previously known as HgMn types. The 260 newly identified HgMn stars represents a more than doubling of the known sample. At least 32 per cent of the APOGEE sample are found to be binary or multiple systems, and from multi-epoch spectroscopy, we were able to determine orbital solutions for at least one component in 32 binaries. Many of the multilined systems include chemically peculiar companions, with noteworthy examples being the HgMn+Ap/Bp binary HD 5429, the HgMn+HgMn binary HD 298641, and the HgMn+Bp Mn + Am triple system HD 231263. As a further peculiarity, roughly half of the sample produces narrow emission in the C  i  16895 Å line, with widths and radial velocities that match those of the Mn  ii lines. [ABSTRACT FROM AUTHOR]

We have detected CH+ and CH molecular absorption lines from the youngcompact planetary nebula IC 4997 from high resolution optical spectra. A high-resolution infra-red (H and K bands) spectrum provides detection of H2 emission lines among many other lines. The H2 lines provide an excitation temperature of 2100 K which may result from UV fluorescence in the envelope or from shocks formed at the interface between an expanding outflow of ionized gas and the neutral envelope ejected when the star was on the AGB. It is suggested that the CH+ may result from the endothermic reaction C + H2 → CH+ + H. Intriguingly, CH+ and also CH show a higher expansion velocity than H2 emission suggesting they may be part of the post-shocked gas. [ABSTRACT FROM AUTHOR]

Many problems in contemporary astrophysics—from understanding the formation of black holes to untangling the chemical evolution of galaxies—rely on knowledge about binary stars. This, in turn, depends on the discovery and characterization of binary companions for large numbers of different kinds of stars in different chemical and dynamical environments. Current stellar spectroscopic surveys observe hundreds of thousands to millions of stars with (typically) few observational epochs, which allows for binary discovery but makes orbital characterization challenging. We use a custom Monte Carlo sampler (The Joker) to perform discovery and characterization of binary systems through radial velocities, in the regime of sparse, noisy, and poorly sampled multi-epoch data. We use it to generate posterior samplings in Keplerian parameters for 232,495 sources released in APOGEE Data Release 16. Our final catalog contains 19,635 high-confidence close-binary (P ≲ few years, a ≲ few) systems that show interesting relationships between binary occurrence rate and location in the color–magnitude diagram. We find notable faint companions at high masses (black hole candidates), at low masses (substellar candidates), and at very close separations (mass-transfer candidates). We also use the posterior samplings in a (toy) hierarchical inference to measure the long-period binary-star eccentricity distribution. We release the full set of posterior samplings for the entire parent sample of 232,495 stars. This set of samplings involves no heuristic "discovery" threshold and therefore can be used for myriad statistical purposes, including hierarchical inferences about binary-star populations and subthreshold searches. [ABSTRACT FROM AUTHOR]

We present a semi-empirical spectral classification scheme for normal B-type stars using near-infrared (NIR) spectra (1.5–1.7 μm) from the Sloan Digital Sky Survey Apache Point Observatory Galaxy Evolution Experiment (APOGEE2)-N data release 14 (DR14) database. The main motivation for working with B-type stars is their importance in the evolution of young stellar clusters; however, we also take advantage of having a numerous sample (316 stars) of B-type star candidates in APOGEE2-N, for which we also have optical (3600–9100 Å) counterparts from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey. By first obtaining an accurate spectral classification of the sources using the LAMOST DR3 spectra and the canonical spectral classification scheme, we found a linear relation between optical spectral types and the equivalent widths of the hydrogen lines of the Brackett series in the APOGEE2-N NIR spectra. This relation extends smoothly from a similar relation for O and early B stars found by Roman-Lopes et al. This way, we obtain a catalog of B-type sources with features in both the optical and NIR and a classification scheme refined down to one spectral subclass. [ABSTRACT FROM AUTHOR]

Studies of the kinematics and chemical compositions of Galactic globular clusters (GCs) enable the reconstruction of the history of star formation, chemical evolution, and mass assembly of the Galaxy. Using the latest data release (DR16) of the SDSS/APOGEE survey, we identify 3090 stars associated with 46 GCs. Using a previously defined kinematic association, we break the sample down into eight separate groups and examine how the kinematics-based classification maps into chemical composition space, considering only α (mostly Si and Mg) elements and Fe. Our results show that (i) the loci of both in situ and accreted subgroups in chemical space match those of their field counterparts; (ii) GCs from different individual accreted subgroups occupy the same locus in chemical space. This could either mean that they share a similar origin or that they are associated with distinct satellites which underwent similar chemical enrichment histories; (iii) the chemical compositions of the GCs associated with the low orbital energy subgroup defined by Massari and collaborators is broadly consistent with an in situ origin. However, at the low-metallicity end, the distinction between accreted and in situ populations is blurred; (iv) regarding the status of GCs whose origin is ambiguous, we conclude the following: the position in Si–Fe plane suggests an in situ origin for Liller 1 and a likely accreted origin for NGC 5904 and NGC 6388. The case of NGC 288 is unclear, as its orbital properties suggest an accretion origin, its chemical composition suggests it may have formed in situ. [ABSTRACT FROM AUTHOR]

We study the M-type asymptotic giant branch (AGB) population of the Large Magellanic Cloud (LMC) by characterizing the individual sources in terms of the main properties of the progenitors and of the dust present in the circumstellar envelope. To this aim we compare the combination of the spectroscopic and photometric data collected by Spitzer , complemented by additional photometric results available in the literature, with results from AGB modelling that include the description of dust formation in the wind. To allow the interpretation of a paucity stars likely evolving through the post-AGB phase, we extended the available evolutionary sequences to reach the PN phase. The main motivation of the present analysis is to prepare the future observations of the evolved stellar populations of Local Group galaxies that will be done by the James Webb Space Telescope (JWST), by identifying the combination of filters that will maximize the possibilities of characterizing the observed sources. The present results show that for the M-star case the best planes to be used for this purpose are the colour magnitude ([F770W]–[F2550W], [F770W]) and (KS –[F770W], [F770W]) planes. In these observational diagrams the sequences of low-mass stars evolving in the AGB phases before the achievement of the C-star stage and of massive AGBs experiencing hot bottom burning are clearly separated and peculiar sources, such as post-AGB, dual-dust chemistry, and iron-dust stars can be easily identified. [ABSTRACT FROM AUTHOR]

Stellar ages are a crucial component to studying the evolution of the Milky Way. Using Gaia DR2 distance estimates, it is now possible to estimate stellar ages for a larger volume of evolved stars through isochrone matching. This work presents [M/H]–age and [α/M]–age relations derived for different spatial locations in the Milky Way disc. These relations are derived by hierarchically modelling the star formation history of stars within a given chemical abundance bin. For the first time, we directly observe that significant variation is apparent in the [M/H]–age relation as a function of both Galactocentric radius and distance from the disc mid-plane. The [M/H]–age relations support claims that radial migration has a significant effect in the plane of the disc. Using the [M/H] bin with the youngest mean age at each radial zone in the plane of the disc, the present-day metallicity gradient is measured to be −0.059 ± 0.010 dex kpc−1, in agreement with Cepheids and young field stars. We find a vertically flared distribution of young stars in the outer disc, confirming predictions of models and previous observations. The mean age of the [M/H]–[α/M] distribution of the solar neighbourhood suggests that the high-[M/H] stars are not an evolutionary extension of the low-α sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution. [ABSTRACT FROM AUTHOR]

We present an elemental abundance analysis of high-resolution spectra for five giant stars spatially located within the innermost regions of the bulge globular cluster NGC 6522 and derive Fe, Mg, Al, C, N, O, Si, and Ce abundances based on H-band spectra taken with the multi-object APOGEE-north spectrograph from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. Of the five cluster candidates, two previously unremarked stars are confirmed to have second-generation (SG) abundance patterns, with the basic pattern of depletion in C and Mg simultaneous with enrichment in N and Al as seen in other SG globular cluster populations at similar metallicity. In agreement with the most recent optical studies, the NGC 6522 stars analyzed exhibit (when available) only mild overabundances of the s-process element Ce, contradicting the idea that NGC 6522 stars are formed from gas enriched by spinstars and indicating that other stellar sources such as massive AGB stars could be the primary polluters of intra-cluster medium. The peculiar abundance signatures of SG stars have been observed in our data, confirming the presence of multiple generations of stars in NGC 6522. [ABSTRACT FROM AUTHOR]

Almost all chemical elements have been made by nucleosynthetic reactions in various kind of stars and have been accumulated along our cosmic history. Among those elements, the origin of phosphorus is of extreme interest because it is known to be essential for life such as we know on Earth. However, current models of (Galactic) chemical evolution under-predict the phosphorus we observe in our Solar System. Here we report the discovery of 15 phosphorus-rich stars with unusual overabundances of O, Mg, Si, Al, and Ce. Phosphorus-rich stars likely inherit their peculiar chemistry from another nearby stellar source but their intriguing chemical abundance pattern challenge the present stellar nucleosynthesis theoretical predictions. Specific effects such as rotation or advanced nucleosynthesis in convective-reactive regions in massive stars represent the most promising alternatives to explain the existence of phosphorus-rich stars. The phosphorus-rich stars progenitors may significantly contribute to the phosphorus present on Earth today.