arXiv subjects

D. A. García-Hernández

Publications and source records attributed to D. A. García-Hernández.

At least 19 recordsLinked to original sources

arXiv2026

The JWST Proto-PAH Project: Detection of the methyl radical CH$_{3}$ in the highly evolved C-rich object SMP LMC 011

We report the first detection of the neutral methyl radical CH$_3$ in a highly evolved C-rich object, SMP LMC 011, using JWST MIRI/MRS. CH$_3$ is well fitted by an excitation temperature of $T_{\rm ex} \simeq$~190\,K and a column density of $N_{\rm tot}(\mathrm{CH_3}) \simeq 5.6\times10^{17}$ cm$^{-2}$. We also report the non-detection of ethane (C$_2$H$_6$), consistent with CH$_3$ reacting preferentially with unsaturated radicals rather than self-recombination, which may be inefficient in SMP LMC 011. Combined with the exceptionally large benzene column density of SMP LMC 011, this supports a methyl-addition route from benzene towards alkyl-substituted aromatics. We propose that the high CH$_3$ abundance is driven by the erosion of hydrogenated amorphous carbon (HAC) dust grains in the dense warm torus by UV photons from the central star and/or by shocks, which release CH$_3$ directly into the gas phase. These results establish CH$_3$ as a key reactive intermediate in the formation of complex hydrocarbons in C-rich circumstellar environments. Chemical models that incorporate methyl-addition reactions are needed for a better understanding of PAH formation pathways in evolved stars. The potential detection of alkyl-substituted aromatics such as toluene (C$_7$H$_8$) and ethylbenzene (C$_8$H$_{10}$) in C-rich evolved stars would provide direct confirmation of CH$_3$-driven aromatic growth in circumstellar environments.

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arXiv2026

Stellar nucleosynthesis in the era of large surveys: S-process polluted binaries in GALAH DR4

Binary interactions during the AGB phase can lead to the formation of chemically peculiar stars with overabundances of s-process elements. Only a few hundreds of these stars have been subject to detailed chemical or dynamical studies. This work aims at compiling a systematic sample of s-process-polluted candidates using GALAH DR4. We also want to compare their properties with those of confirmed s-polluted stars to have stronger evidence of their nature. GALAH DR4 uses neural networks and automatic spectral analysis methods as well as data of a lower spectral resolution than normally used to characterise these objects. Because of this, we built a validation sample, for which we obtained UVES@VLT and HERMES@Mercator high-resolution spectra. We compare our stellar parameters and abundances with those of the survey and use this validation to define the thresholds that a star in GALAH DR4 must pass to be flagged as a good s-process-rich candidate. Based on our comparisons, we define thresholds on [s/Fe], [Y/Fe], [Zr/Fe], [Ba/Fe], and [La/Fe]. We identified 1059 stars in GALAH DR4 that are good candidates to be s-process polluted stars, covering a broad parameter space. They share many similarities with the samples of confirmed s-rich stars, especially their ratios of heavy over light s-elements ([hs/ls]), which strengthen our confidence in the purity of the sample. We find that only 7% of the candidates have measured orbital periods and eccentricities, limiting for now a full comparison with confirmed Ba and related stars. However, their binary fraction is, as expected, higher than the one we found for the full GALAH DR4 catalogue. Our sample of candidates is almost five times larger than the number of currently confirmed polluted stars. This and the fact that it has been homogeneously treated by GALAH open very interesting avenues to confront nucleosynthesis and binary evolution models.

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arXiv2026

Detection of C$_{60}$ Combination Bands in the Near-IR Spectrum of Tc 1

We report the detection of a set of new near-infrared emission features between 3.5 and 5.2 $μ$m in JWST/NIRSpec observations of Tc 1, the planetary nebula known for displaying the cleanest and most prominent mid-infrared cosmic fullerene spectrum. These broad features share the same spatial distribution as the well-known C$_{60}$ and C$_{70}$ mid-infrared emission bands, peaking in an asymmetric ring approximately 5-6" from the central star. Through comparison with new anharmonic quantum chemical calculations, we demonstrate that these features arise from C$_{60}$ combination bands, marking their first detection in an astrophysical environment. The total energy radiated in the combination bands amounts to ~17% of the total energy emitted from all C$_{60}$ modes, with direct implications for fullerene cooling models. These near-infrared combination bands offer a promising new window for identifying and studying the molecular astrophysics of C$_{60}$ in sources where mid-infrared spectra are more complex.

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arXiv2026

Detection of unidentified molecular pure rotational lines in C-rich PNe I. The fullerene-containing PN IC 418

Molecular emission is observed in a wide variety of astrophysical environments, yet a substantial fraction of spectral features detected at mm wavelengths remains unidentified. Identifying these features is essential for constraining the inventory of interstellar and circumstellar molecules and for understanding the chemical pathways operating in evolved stars. In this study, we investigate a set of weak unidentified molecular features detected in the C-rich PN IC 418. We aim to constrain the nature of their molecular carriers and assess their possible connection to fullerene-related chemistry. High-sensitivity observations at 2, 3, and 7 mm were carried out using the RT40m and IRAM 30m radio telescopes. The spectral features found in these data sets were compared with public molecular spectroscopic databases. Spectral-pattern searches and line fits were performed under linear, symmetric, and asymmetric rotor approximations to estimate rotational constants of potential molecular carriers. We report the detection of 20 weak (SNR~2-13) unidentified features (UFs), none of which can be attributed to mm radio recombination lines, instrumental artifacts, or known molecular species. The observed features are inconsistent with the regular line spacing expected from linear molecules or symmetric rotors, although a recurrent doublet-like pattern is identified. The estimated rotational constants (B~2 500-3 660 MHz) suggest carriers with 4-13 atoms and are incompatible with those expected from C60 derivatives (B~100 MHz). Non-planar carbonaceous molecules produced during the destruction/processing of hydrogenated amorphous carbon (HAC) grains are suggested as promising candidates. The catalog of UFs presented here is made publicly available to facilitate future comparisons with laboratory measurements and theoretical calculations aimed at identifying their molecular carriers.

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arXiv2025

Peculiarities in the infrared emission of PAH-C$_{60}$ adducts

The coexistence of PAHs and the C$_{60}$ fullerene in different astrophysical environments can give rise to the formation of new complex species denoted as PAH-C$_{60}$ adducts, which may contribute to the infrared (IR) emission observed. These PAH-C$_{60}$ adducts have been previously reported experimentally due to the high reactivity between PAHs and C$_{60}$. From the astrophysical point of view, however, they have not been considered in detail yet. Here we have performed a combined experimental and theoretical study in order to characterize the IR spectra of PAH-C$_{60}$ adducts, including multiple adducts. By using new advanced experimental techniques, we have been able to synthesize some specific PAH-C$_{60}$ adduct isomers, and measured their IR spectra. These experimental data are used to correct their harmonic scaled spectra, as obtained from quantum-chemistry calculations performed at the DFT level under the B3LYP-GD3/6-31+G(d) approach. This way, we simulate the IR ($\sim$3$-$25 $μ$m) spectra of multiple PAH-C$_{60}$ adducts, composed by a different number of PAH units: mostly one or two units. In addition, the chemical kinetics data available in the literature are used to tentatively estimate the possible order of magnitude of the abundances of these PAH-C$_{60}$ adducts using the available observational data. Essentially, our results reveal a possible strong modification of the IR spectra when astronomically estimated abundances are considered. Several spectral peculiarities are observed, such as a broad $\sim$3.4-3.6 $μ$m feature, and important modifications in the 6-10 and 12-16 $μ$m spectral regions together with contributions to the C$_{60}$ features at 7.0 and 18.9 $μ$m. Interestingly, these PAH-C$_{60}$ adducts lack aliphatic CH bonds, but they display IR features around 3.4 $μ$m, challenging previous interpretations of this astronomical feature.

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arXiv2025

Planetary Nebulae of the Large Magellanic Cloud II: the connection with the progenitors' properties

The study of planetary nebulae (PNe) offers the opportunity of evaluating the efficiency of the dust production mechanism during the very late asymptotic giant branch (AGB) phases. We study the relationship between the properties of PNe, particularly the gas and dust content, with the mass and metallicity of the progenitor stars, to understand how dust production works in the late AGB phases, and to shed new light on the physical processes occurring to the stars and the material in their surroundings since the departure from the AGB until the PN phase. We consider a sample of 9 PNe in the Large Magellanic Cloud, 7 out of which characterized by the presence of carbonaceous dust, the remaining 2 with silicates. For these stars the masses and the metallicity of the progenitor stars were estimated. We combine results from stellar evolution and dust formation modelling with those coming from the analysis of the spectral energy distribution, to find the relation between the dust and gas mass of the PNe considered and the The physical properties of carbon-rich PNe are influenced by the mass of the progenitor star. Specifically, the dust-to-gas ratio in the nebula increases from 5x10^{-4}to 6x10^{-3} as the progenitor star's mass increases from approximately 0.9Msun to 2Msun. This change is partly influenced by the effective temperature of the PNe, and it occurs because higher-mass carbon stars are more efficient at producing dust. Consequently, as the progenitor's mass increases, the gas mass of the PNe decreases, since the larger amounts of dust lead to greater effects from radiation pressure, which pushes the gas outwards. No meaningful conclusions can be drawn by the study of the PNe with silicate-type dust, because the sub-sample is made up of 2 PNe only, one of which is almost dust-free.

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arXiv2024

Unveiling the chemical fingerprint of phosphorus-rich stars II. Heavy-element abundances from UVES/VLT spectra

The atmospheres of phosphorus-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. Our objective is to obtain the extensive chemical abundance inventory of four P-rich stars, covering a large range of heavy 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. We performed a detailed 1D local thermodynamic equilibrium abundance analysis on the optical UVES spectra of four P-rich stars. Our focus lay on the neutron-capture elements, in particular, on the elements between Sr and Ba, as well as on Pb, as they provide valuable constraints to nucleosynthesis calculations. 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. 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 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.

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arXiv2024

Carbon enrichment in APOGEE disk stars as evidence of mass transfer in binaries

Carbon abundances in first-ascent giant stars are usually lower than those of their main-sequence counterparts. At moderate metallicities, stellar evolution of single stars cannot account for the existence of red-giant branch stars with enhanced carbon abundances. The phenomenon is usually interpreted as resulting from past mass transfer from an evolved binary companion now in the white dwarf evolutionary stage. Aims: We aim to confirm the links between [C/O] enhancement, s-process element enhancement and binary fraction using large-scale catalogues of stellar abundances and probable binary stars. Methods: We use a large data set from the 17 data release of the SDSS-IV/APOGEE~2 survey to identify carbon-enhanced stars in the Galactic disk. We identify a continuum of carbon enrichment throughout three different sub-populations of disk stars and explore links between the degree of carbon enrichment and binary frequency, metallicity and chemical compositions. Results: We verify a clear correlation between binary frequency and enhancement in the abundances of both carbon and cerium, lending support to the scenario whereby carbon-enhanced stars are the result of mass transfer by an evolved binary companion. In addition, we identify clustering in the carbon abundances of high-$α$ disk stars, suggesting that those on the high metallicity end are likely younger, in agreement with theoretical predictions for the presence of a starburst population following the gas-rich merger of the Gaia-Enceladus/Sausage system.

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arXiv2024

Planetary Nebulae of the Large Magellanic Cloud I: A multiwavelength analysis

Planetary nebulae (PNe) have three main components: a central star (CS), ionised gas and dust in the nebula. Each of them contains critical chemical fingerprints of their evolution, serving as tracers of the evolution, nucleosynthesis and dust production that occurred during the preceding asymptotic giant branch (AGB) phase. We aim to build a bridge to link the PN phase to the evolution of their progenitors, trying to better understand the dust production and mass-loss mechanism during the final AGB phase. Here, we present a comprehensive study of nine Large Magellanic Cloud (LMC) spherical or elliptical PNe whose observations from the ultraviolet (UV) through the infrared (IR) are available in the literature. We characterize nebulae and CSs, finding information as the amount of gas that makes up the nebula and the dust that surrounds the CS, necessary to reconstruct the evolutionary history of mass-loss and dust production. We compare the observed energy distribution of the selected PNe to that obtained from photoionization modeling, taking into account the presence of dust. The physical and chemical parameters of the central stars are then compared with the predictions from the evolutionary tracks. We characterized the source, assigning to each CS a progenitor, early-AGB mass. We estimated the mass of the nebula and the dust-to-gas ratio. For 5 objects, we find evidence for the presence of a near-IR bump, which would be connected to the presence of hot dust.

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arXiv2024

The first measurements of carbon isotopic ratios in post-RGB stars: SZ Mon and DF Cyg. E-iSpec: A spectral analysis tool to derive elemental abundances and isotopic ratios for evolved stars

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 multi-wavelength 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 ($T_{\rm turn-off, post-RGB}\approx1400$ K), than in most post-AGB stars ($T_{\rm turn-off, post-AGB}\approx1100$ K). Additionally, our study resulted in the first estimates of carbon isotopic ratios for post-RGB stars ($^{12}$C/$^{13}$C$_{\rm SZ Mon}=8\pm4$, $^{12}$C/$^{13}$C$_{\rm DF Cyg}=12\pm3$). 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 emphasises 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.

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arXiv2024

Hydrogenated amorphous carbon grains as an alternative carrier of the 9$-$13$μ$m plateau feature in the fullerene planetary nebula Tc 1

Fullerenes have been observed in several astronomical objects since the discovery of C$_{60}$ in the mid-infrared (mid-IR) spectrum of the planetary nebula (PN) Tc 1. It has been suggested that the carriers of the broad unidentified infrared (UIR) plateau features, such as the 9$-$13$μ$m emission feature (12$μ$m hereafter), may be related to the formation of fullerenes. In particular, their carriers have been suggested to be mixed aromatic or aliphatic hydrocarbons such as hydrogenated amorphous carbon (HAC-like hereafter) grains. For this study, we modeled the mid-IR emission of the C$_{60}$-PN Tc 1 with a photoionization code, including for the first time the laboratory optical constants ($n$ and $k$ indices) of HAC-like dust at 300 K. Interestingly, we find that the broad 12$μ$m plateau feature in Tc 1 is well reproduced by using a distribution of canonical HAC grains, while at the same time they provide an important fraction of the IR dust continuum emission and are consistent with the other UIR features observed (e.g., the broad 6$-$9$μ$m plateau feature). This finding suggests that HAC-like grains may be possible carriers of the 12$μ$m plateau feature, being likely related to the fullerene formation mechanism in PNe. More laboratory experiments, to obtain the optical constants of HAC-like dust with several structures or a composition at different physical conditions, are strongly encouraged -- that is, in order to extend this pilot study to more fullerene PNe, and to unveil the details of fullerene formation and of the potential carriers of the elusive UIR plateau features.

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arXiv2024

An ultraviolet spectral study of fullerene-rich planetary nebulae

Several planetary nebulae (PNe) have been found to contain both polycyclic aromatic hydrocarbon (PAH-like) species and fullerenes (C$_{60}$) distinguished by their mid-infrared emission. Previous laboratory and astronomical studies suggest that the formation of both species could be related to the decomposition, by photochemical processing, of hydrogenated amorphous carbon (HAC) grains. Then, HACs and, seemingly, big-fullerene related species (e.g., carbon onions) have been suggested as potential carriers of the UV bump at 2175Å and the far-UV rise common to interstellar extinction curves. Our goal is to investigate the UV bump with the possible presence of a HAC extinction component in the International Ultraviolet Explorer (IUE) spectra of C-rich PNe; both with detected and non-detected fullerenes. The considered sample includes three C$_{60}$-PNe (Tc 1, IC 418, and IC 2501) and the non-C$_{60}$-PN Hen 2-5. Independently of the presence of C$_{60}$ in their circumstellar envelopes, we found that the UV bump in all sample PNe is well explained by interstellar extinction, suggesting that species different from those of the foreground insterstellar medium, e.g., large fullerene-related species like carbon onions, are not the carrier. Interestingly, we found that PNe Tc 1 and Hen 2-5 show an absorption in the FUV rise. Their IUE continuum spectra may be very well reproduced by including the extinction curve of HAC-like very small grains (VSG). The possible presence of both species, HAC-like grains and fullerenes (C$_{60}$), in Tc 1 could support the HAC photochemical processing scenario for the formation of fullerenes in the complex circumstellar envelopes of PNe.

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arXiv2023

Dust from evolved stars: a pilot analysis of the AGB to PN transition

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 $\sim10^{-3}$, 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 constrains the capacity of the dust to survive the central star heating.

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arXiv2023

Unveiling the chemical fingerprint of phosphorus-rich stars I. In the infrared region of APOGEE-2

The origin of phosphorus, one of the essential elements for life on Earth, is currently unknown. Prevalent models of Galactic chemical evolution (GCE) underestimate the amount of P compared to observations. The recently discovered P-rich ([P/Fe] > 1 dex) and metal-poor giants further challenge current theories on stellar nucleosynthesis. Since the observed stars are low-mass giants, our primary goal is to find clues on their progenitor. By increasing the number of known P-rich stars, we aim to narrow down a reliable chemical abundance pattern and to place robust constraints on the responsible nucleosynthetic mechanism. In the long term, identifying the progenitor of the P-rich stars may contribute to the search for the source of P in our Galaxy. We performed a detailed chemical abundance analysis based on the H-band spectra from APOGEE-2 (DR17). Employing the BACCHUS code, we measured the abundances of 13 elements in the sample, which is mainly composed of a recent collection of Si-enhanced giants. We also analyzed the orbital motions and compared the abundance results to possible nucleosynthetic formation scenarios, and also to detailed GCE models. We enlarged the sample of confirmed P-rich stars from 16 to 78 giants, which represents the largest sample of P-rich stars to date. Significant enhancements in O, Al, Si and Ce, as well as systematic correlations among the elements, unveil the chemical fingerprint of the P-rich stars. The high Mg and C+N found in some of the P-rich stars with respect to P-normal stars is not confirmed over the full sample. Strikingly, the strong over-abundance in the $α$-element Si is accompanied by normal Ca and S abundances. Our analysis of the orbital motion showed that the P-rich stars do not belong to a specific sub-population. In addition, we confirm that the majority of the sample stars are not part of binary systems.

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arXiv2023

The intense production of silicates during the final AGB phases of intermediate mass stars

The formation of silicates in circumstellar envelopes of stars evolving through the AGB is still debated given the uncertainties affecting stellar evolution modelling, the description of the dust formation process, and the capability of silicate grains to accelerate stellar outflows via radiation pressure. We study the formation of dust in the winds of intermediate mass (M $\geq 4 M_{\odot}$) stars of solar metallicity while evolving through the AGB phase. We tested the different treatments of the mass-loss mechanism by this class of stars, with the aim of assessing their contribution to the general enrichment of silicates of the interstellar medium of galaxies. We consider a sub-sample of AGB stars, whose SED is characterised by deep absorption features at $10$ and $18μ$m, which can be regarded as the class of stars providing the most relevant contribution to the silicates' production across the Universe. Results from stellar evolution and dust formation modelling were used to fit the observed SED and to reproduce, at the same time, the detected pulsation periods and the derived surface chemical composition. This analysis leads to the derivation of tight constraints on the silicates' production rates experienced by these sources during the final AGB stages. Two out of the four sources investigated are interpreted as stars currently undergoing HBB, evolving through phases close to the stage when the mass-loss rate is largest. The remaining two stars are likely evolving through the very final AGB phases, after HBB was turned off by the gradual consumption of the convective mantle. Mass-loss rates of the order of $1-2\times 10^{-4} M_{\odot}/$yr are required when looking for consistency with the observational evidence. These results indicate the need for a revision of the silicate yields by intermediate mass stars, which are found to be $\sim 3$ times higher than previously determined.

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arXiv2022

Stellar Properties for a Comprehensive Collection of Star Forming Regions in the SDSS APOGEE-2 Survey

The Sloan Digital Sky Survey IV (SDSS-IV) APOGEE-2 primary science goal was to observe red giant stars throughout the Galaxy to study its dynamics, morphology, and chemical evolution. The APOGEE instrument, a high-resolution 300 fiber H-band (1.55-1.71 micron) spectrograph, is also ideal to study other stellar populations in the Galaxy, among which are a number of star forming regions and young open clusters. We present the results of the determination of six stellar properties ($T_{eff}$, $\log{g}$, [Fe/H], $L/L_\odot$, $M/M_\odot$, and ages) for a sample that is composed of 3360 young stars, of sub-solar to super-solar types, in sixteen Galactic star formation and young open cluster regions. Those sources were selected by using a clustering method that removes most of the field contamination. Samples were also refined by removing targets affected by various systematic effects of the parameter determination. The final samples are presented in a comprehensive catalog that includes all six estimated parameters. This overview study also includes parameter spatial distribution maps for all regions and Hertzprung-Russell ($L/L_\odot$ vs. $T_{eff}$) diagrams. This study serves as a guide for detailed studies on individual regions, and paves the way for the future studies on the global properties of stars in the pre-main sequence phase of stellar evolution using more robust samples.

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arXiv2022

The chemical characterisation of halo substructure in the Milky Way based on APOGEE

Galactic haloes in a $Λ$-CDM universe are predicted to host today a swarm of debris resulting from cannibalised dwarf galaxies. The chemo-dynamical information recorded in their stellar populations helps elucidate their nature, constraining the assembly history of the Galaxy. Using data from APOGEE and \textit{Gaia}, we examine the chemical properties of various halo substructures, considering elements that sample various nucleosynthetic pathways. The systems studied are Heracles, \textit{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: {\it 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 \textit{in situ}; {\it ii)} Heracles differs chemically from {\it 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; {\it iii)} the chemistry of Arjuna, LMS-1, and I'itoi is indistinguishable from that of GES, suggesting a possible common origin; {\it 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; {\it v)} the abundance patterns of the Helmi stream and Thamnos are different from all other halo substructures.

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arXiv2022

The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants

The contribution of dissolved globular clusters (GCs) to the stellar content of the Galactic halo is a key constraint on models for GC formation and destruction, and the mass assembly history of the Milky Way. Earlier results from APOGEE pointed to a large contribution of destroyed GCs to the stellar content of the inner halo, by as much as 25$\%$, which is an order of magnitude larger than previous estimates for more distant regions of the halo. We set out to measure the ratio between N-rich and normal halo field stars, as a function of distance, by performing density modelling of halo field populations in APOGEE DR16. Our results show that at 1.5 kpc from the Galactic Centre, N-rich stars contribute a much higher 16.8$^{+10.0}_{-7.0}$$\%$ fraction to the total stellar halo mass budget than the 2.7$^{+1.0}_{-0.8}$$\%$ ratio contributed at 10 kpc. Under the assumption that N-rich stars are former GC members that now reside in the stellar halo field, and assuming the ratio between first-and second-population GC stars being 1:2, we estimate a total contribution from disrupted GC stars of the order of 27.5$^{+15.4}_{-11.5}$$\%$ at r = 1.5 kpc and 4.2$^{+1.5}_{-1.3}$$\%$ at r = 10 kpc. Furthermore, since our methodology requires fitting a density model to the stellar halo, we integrate such density within a spherical shell from 1.5-15 kpc in radius, and find a total stellar mass arising from dissolved and/or evaporated GCs of $M_{\mathrm{GC,total}}$ = 9.6$^{+4.0}_{-2.6}$ $\times$ 10$^{7}$ M$\odot$.

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