arXiv subjects

N. Ryde

Publications and source records attributed to N. Ryde.

At least 19 recordsLinked to original sources

arXiv2026

X-shooter spectroscopy of giant stars in the nuclear star cluster

Near-IR spectra of a homogeneous sample of 15 red giants in the Galactic nuclear star cluster, within 1.5 pc of SgrA*, have been acquired with X-shooter at the Very Large Telescope. From these spectra line-of-sight radial velocities and chemical abundances of iron, carbon, oxygen and other alpha and light elements have been derived. By combining radial velocities from this study and proper motions from the literature we computed the orbits of these stars, finding that they have been confined within 12 pc from SgrA* for their entire lifetime, thus strongly suggesting an in-situ formation and evolution. Iron abundances between half and twice solar, about solar-scaled values within +/-0.1 dex for {\alpha} elements, Ti and V, some enhancement of [Na/Fe], [K/Fe] and [Al/Fe] and some depletion of [C/Fe] with respect to the solar ratios have been measured. The inferred chemical abundance distributions are consistent with a formation from a gas enriched by both type II and type I SNe over a prolonged timescale, closely matching those of the metal-rich populations of the inner bulge field and other complex stellar systems like Terzan 5 and Liller 1.

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arXiv2026

Reassessing the low-$\alpha$ massive sequence stars in Gaia RVS

Recently, a chemically depleted young massive stellar population was identified using the spectroscopic catalogue of Gaia DR3. To explain its characteristics, a recent enhanced star formation event, via a third infall occurring within the last 2 Gyr, has been evoked. In this paper we reassess the low alpha sequence of massive stars identified in the Gaia spectroscopic catalog and investigate their presence in other Milky Way spectroscopic survey catalogs. We select massive sequence stars and RGB stars from the Gaia DR3 catalogue using the same filtering strategy adopted in previous chemical cartography studies. These samples are then cross matched with APOGEE DR17, GALAH DR4, and Gaia CNN to enable a detailed comparison of stellar parameters and alpha abundances. Stellar masses are estimated by projecting their atmospheric parameters and infrared magnitudes onto PARSEC isochrones. For the massive star sample, we find large discrepancies in stellar parameters and calcium abundances between Gaia DR3 and the three external surveys. The external catalogues do not show a low ca sequence but rather resemble those of thin disc RGB stars. Other alpha elements (si in APOGEE and GALAH, and mg in GALAH) also do not show depleted values. In APOGEE, however, massive sequence stars with metallicities above -0.5 dex display lower mg abundances. We attribute this to APOGEE's use of macroturbulence velocities calibrated solely on metallicity. Our analysis does not show any evidence for alpha element depletion in massive sequence stars. Alpha abundances of massive sequence stars derived from the Gaia RVS spectra should therefore be used with caution. Nevertheless, the previously proposed three infall chemical evolution models remain plausible: even without a chemically depleted young massive population, scenarios involving only mild dilution could still account for recent star formation episodes.

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arXiv2026

OH/IR stars in the inner Galactic bulge: I. Colour, CO line, and stellar light curve characteristics

Stars on the asymptotic giant branch (AGB) play important roles in a number of astronomical contexts. To quantify these roles, it is necessary to establish the mass-loss characteristics of stars as they evolve up and beyond the AGB. We used an equidistant sample of 77 AGB stars in the inner Galactic bulge, selected on the existence and strength of OH1612\,MHz maser emission, to obtain information on the mass loss of O-rich AGB stars, and on its transformations in geometry and kinematics that occur at the tip of the AGB. Observations of circumstellar lines from several rotational transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O were performed with the Atacama Large Millimeter/submillimeter Array (ALMA), and, for a sub-sample, with the Atacama Pathfinder EXperiment telescope (APEX). The ALMA observations also provide continuum data. Existing infrared photometry was used to estimate colours and produce stellar light curves. Based on mid-infrared colour, CO line, and near-infrared variability characteristics, the objects were divided into four categories of distinct stellar and/or circumstellar properties. Various circumstellar CO line characteristics are presented and compared between the categories, such as morphologies and extents of brightness distributions (BDs), line profiles (LPs), line intensity ratios, and kinematics. A majority of the objects form a homogenous group with sharply, centrally peaked BDs and LPs of the soft-parabola type, while the rest show extended and complex BDs and/or LPs with high-velocity wings. The C$^{18}$O line and ALMA continuum detection rates vary significantly between the categories. CO line emission is also detected from interaction zones where the expanding circumstellar gas meets the interstellar medium.

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arXiv2026

The LEGARE Project. I. Chemical evolution model of the Nuclear Stellar Disc in a Bayesian framework

The Nuclear Stellar Disc (NSD) of the Milky Way is a dense, rotating stellar system in the central 200 pc. The NSD is thought to be primarily fuelled by bar-driven gas inflows from the inner Galactic disc. As part of the LEGARE project, we construct the first chemical evolution models for the NSD using a Bayesian approach tailored to reproduce the observed metallicity distribution functions (MDFs) and compared with the available abundance ratios for Mg, Si, Ca relative to Fe. We adopt a state-of-the-art chemical evolution model in which the gas responsible for the formation of the NSD is assumed to be driven by the Galactic bar-induced inflows. The chemical composition of the accreted material is assumed to reflect that of the Galactic disc at a radius of 4 kpc. A Bayesian MCMC framework is used to fit the MDFs of different samples of NSD stars. If we take the NSD data at face value, without considering a possible contamination from bulge stars, we find that a formation scenario based on the inner disc flowing gas is inconsistent with the low metallicity tail of the observed MDF. This is because the inner disc metallicity, at the epoch of bar formation, was already near solar. On the other hand, models invoking dilution from additional metal-poor inflows successfully reproduce the observations. The best-fit model requires inflow metallicity 5 times lower than the inner disc and a moderate star formation efficiency. The same model successfully reproduces the observed [$\alpha$/Fe] vs. [Fe/H] ratios and predicts a star formation history consistent with the most recent estimates. However, if we assume that the MDF is contaminated by metal poor bulge stars and restricted to [Fe/H] > -0.3 dex, gas dilution is no longer required. In this case, the best-fit model has a very low star formation efficiency and a mild galactic wind.

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arXiv2025

The Nephele ecosystem: stars, globular clusters, and stellar streams associated with the progenitor galaxy of $\omega$ Centauri

Globular clusters (GCs) and their associated stellar streams are key tracers of the hierarchical assembly history of the Milky Way. $\omega$ Centauri, the most massive and chemically complex GC in the Galaxy, is widely believed to be the remnant nucleus of an accreted dwarf galaxy. Identifying its associated debris and that of chemically similar clusters can provide important constraints on the nature of this progenitor system. We aim to identify field stars that are chemically and kinematically linked to $\omega$ Cen and to a group of globular clusters associated with the Nephele accretion event. We analyse APOGEE DR17 data using a Gaussian Mixture Model (GMM) in a 8-dimensional chemical space to identify field stars whose abundances match those of $\omega$ Cen. We then compute the orbital energy and angular momentum of these stars and apply a second GMM, calibrated on simulations from the e-TidalGCs project, to determine kinematic compatibility with the predicted streams of $\omega$ Cen and the associated Nephele GCs. We identify 470 stars chemically compatible with $\omega$ Cen, of which 58 are also Al-rich, consistent with second-generation stars found in GCs. Of these, 6 stars show kinematics consistent with the predicted $\omega$ Cen stream, and additional stars are linked to the tidal streams of NGC 6205, NGC 6254, NGC 6273, NGC 6656, and NGC 6809. We also find overlap in chemical and kinematic properties between Nephele stars and the Gaia Sausage-Enceladus population. Our findings indicate stellar debris linked to $\omega$ Cen and its candidate globular cluster family, consistent with a shared, now-disrupted galactic progenitor. Despite residual uncertainties from disc contamination and limited sky coverage, the results demonstrate the effectiveness of combined chemical and dynamical analyses in uncovering relics of past accretion events in the inner Galaxy.

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arXiv2025

Supergiant GCIRS 22 in the Milky Way Nuclear Star Cluster: Revised alpha abundances

Context: The chemical abundances of alpha-elements in Galactic Centre (GC) supergiants provide key insights into the chemical enrichment and star formation history of the Milky Way's Nuclear Star Cluster. Previous studies have reported enhanced alpha-element abundances, raising questions about the chemical evolution of this unique region. Aims: We aim to reassess the alpha-element abundances in the GC supergiant GCIRS 22 using updated spectral modelling and non-local thermodynamic equilibrium (NLTE) corrections to resolve discrepancies from earlier abundance analyses. Methods: High-resolution near-infrared spectra of GCIRS 22 were analysed using contemporary line lists and precise stellar parameters derived from scandium line diagnostics. We applied comprehensive NLTE corrections to accurately determine the abundances of silicon and calcium. Results: Our analysis reveals solar-scale alpha abundances ([Ca/Fe] = 0.06 $\pm$ 0.07; [Si/Fe] = $-$0.08 $\pm$ 0.20) for GCIRS 22, significantly lower than previous LTE-based findings. NLTE corrections reduce the calcium abundance by approximately 0.3,dex compared to LTE estimates, aligning our results with recent studies and highlighting the importance of accurate NLTE modelling. Conclusions: The solar-scale alpha-element abundances observed in GCIRS 22 suggest that recent star formation in the region has not been dominated by Type II supernovae, such as those expected from a recent starburst. Our findings support a scenario of episodic star formation, characterized by intermittent bursts separated by extended quiescent phases, or potentially driven by gas inflows from the inner disk, funnelled by the Galactic bar. Future comprehensive NLTE studies of additional GC stars will be essential for refining our understanding of the region's chemical evolution and star formation history.

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arXiv2025

Chemical Abundances in the Milky Way's Nuclear Stellar Disc

The Nuclear Stellar Disc (NSD) is a rotating, disc-like structure in the Galactic Center, believed to have a distinct star formation. However, its formation history and evolutionary links to other structures in the Galactic Center remain uncertain. This study aims to present the first comprehensive chemical census of the NSD by deriving abundance trends for 18 elements in 9 M giants in the metallicity range of -1.0 <[Fe/H]< 0.5. By comparing these trends with those of other Galactic populations we seek to understand the chemical relationships between these structures. We obtained high-resolution H- and K-band spectra of NSD stars using the IGRINS spectrometer mounted on the Gemini South telescope. The giants were analyzed consistently with stars from a comparison populations to minimize systematic uncertainties. The abundance trends of NSD stars exhibit similarities with those of the inner-bulge and Nuclear Star Cluster (NSC) populations across a broad range of elements with different chemical evolution histories. The trends for 17 elements align closely with the local thick-disc behaviour at subsolar metallicities. At super-solar metallicities, most elements follow the NSC and inner-bulge trends. Sodium is the only element exhibiting a distinct trend, with enhanced abundances in the NSD and NSC compared to both thin-disc and inner-bulge stars. The chemical similarity suggests that the NSD likely shares an evolutionary history with the NSC and possibly the inner-disc sequence. Further studies are required to determine potential evolutionary links to Liller 1 and metal-rich globular clusters. We find no evidence of typical globular cluster abundance signatures in our NSD stars with subsolar metallicities. Our study demonstrates the feasibility of obtaining high-quality abundance data even in highly dust-obscured regions of the Milky Way, paving the way for future surveys.

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arXiv2024

Chemical Abundances in the Nuclear Star Cluster of the Milky Way: alpha-Element Trends and Their Similarities with the Inner Bulge

A chemical characterization of the Galactic Center is essential for understanding its formation and structural evolution. Trends of alpha-elements, such as Mg, Si, and Ca, serve as powerful diagnostic tools, offering insights into star-formation rates and gas-infall history. However, high extinction has previously hindered such studies. In this study, we present a detailed chemical abundance analysis of M giants in the Milky Way's Nuclear Star Cluster (NSC), focusing on alpha-element trends with metallicity. High-resolution, near-infrared spectra were obtained using the IGRINS spectrograph on the Gemini South telescope for nine M giants. Careful selection of spectral lines, based on a solar-neighborhood control sample of 50 M giants, was implemented to minimize systematic uncertainties. Our findings show enhanced alpha-element abundances in the predominantly metal-rich NSC stars, consistent with trends in the inner bulge. The NSC stars follow the high-[alpha/Fe] envelope seen in the solar vicinity's metal-rich population, indicating a high star-formation rate. The alpha-element trends decrease with increasing metallicity, also at the highest metallicities. Our results suggest the NSC population likely shares a similar evolutionary history with the inner bulge, challenging the idea of a recent dominant star formation burst. This connection between the NSC and the inner-disk sequence suggests that the chemical properties of extragalactic NSCs of Milky Way type galaxies could serve as a proxy for understanding the host galaxies' evolutionary processes.

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arXiv2024

Metallicity-dependent kinematics and orbits in the Milky Way's nuclear stellar disc

The nuclear stellar disc (NSD) is a flat and dense stellar structure at the centre of the Milky Way. Previous work has identified the presence of metal-rich and metal-poor stars in the NSD, suggesting that they have different origins. The recent publication of photometric, metallicity, proper motion, and orbital catalogues allows the NSD stellar population to be characterised with unprecedented detail. We aim to explore the proper motions and orbits of NSD stars with different metallicities to assess whether they have different origins and to better understand the metallicity distribution in the NSD. We distinguished between metal-rich and metal-poor stars by applying a Gaussian mixture model, as done in previous work, and analysed the proper motions, orbits, and spatial distribution of stars with different metallicities. We find that metal-rich stars exhibit a lower velocity dispersion, suggesting that they trace a kinematically cooler component compared to metal-poor ones. Furthermore, z-tube orbits are predominant among metal-rich stars, while chaotic/box orbits are more common among metal-poor ones. We also find that metal-rich and metal-poor stars show a similar extinction and are present throughout the analysed regions. As a secondary result, we detected a metallicity gradient in the metal-rich population with higher metallicity towards the centre of the NSD and a tentative gradient for the metal-poor stars, which is consistent with previous studies that did not distinguish between the two populations. Our results suggest that metal-rich stars trace the NSD, whereas metal-poor ones are related to the Galactic bar and probably constitute Galactic bar interlopers and/or are NSD stars that originated from accreted clusters. The detected metallicity gradients aligns with the currently accepted inside-out formation of the NSD.

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arXiv2024

M giants with IGRINS IV. Identification and characterization of a near-IR line of the s-element Barium

Neutron-capture elements represent an important nucleosynthetic channel in the study of the Galactic Chemical Evolution of stellar populations. For stellar populations behind significant extinction, such as those in the Galactic Center and along the Galactic plane, abundance analyses based on near-IR spectra are necessary. Previously, spectral lines from the neutron-capture elements such as copper (Cu), cerium (Ce), neodymium (Nd), and ytterbium (Yb) have been identified in the H band, while yttrium (Y) lines have been identified in the K band. Due to the scarcity of spectral lines from neutron-capture elements in the near-IR, the addition of useful spectral lines from other neutron-capture elements is highly desirable. The aim of this work is to identify and characterise a spectral line suitable for abundance determination from the most commonly used s-process element, namely barium. We observed near-IR spectra of 37 M giants in the solar neighbourhood at high S/N and high spectral resolution using the IGRINS spectrometer on the GEMINI South telescope. Using a manual spectral synthesis method, we determined the stellar parameters for these stars and derived the barium abundance from the Ba line (6s5d $^3$D$_2 \rightarrow$ 6s6p $^3$P$^o_2$) at $\lambda_\mathrm{air}=23\,253.56\,$\AA\ in the K band. We demonstrate that the Ba line in the K band at 2.33\,\mic\ ($\lambda$23253.56) is useful for abundance analysis from spectra of M giants. The line becomes progressively weaker at higher temperatures and is only useful in M giants and the coolest K giants at supersolar metallicities. We can now add Ba to the trends of the heavy elements Cu, Zn, Y, Ce, Nd, and Yb, which can be retrieved from high-resolution H- and K-band spectra. This opens up the study of nucleosynthetic channels, including the s-process and the r-process, in dust-obscured populations such as the Galactic Center.

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arXiv2024

M Giants with IGRINS III. Abundance Trends for 21 Elements in the Solar Neighborhood from High-Resolution, Near-Infrared Spectra

In order to investigate the chemical history of the entire MilkyWay, it is imperative to also study the dust-obscured regions, where most of the mass lies. The Galactic Center is an example of such a region of interest, where due to the intervening dust along the line-of-sight, near-infrared spectroscopic investigations are necessary. We demonstrate that M giants observed at high spectral resolution in the H and K bands (1.5-2.4 {\mu}m) can yield useful abundance-ratio trends versus metallicity for 21 elements. These elements can therefore be studied also for heavily dust-obscured regions of the Galaxy, such as the Galactic Center, and will be important for the further investigation of the Galactic chemical evolution in these regions. We have observed near-infrared spectra of 50 M giants in the solar neighbourhood at high SNR and at a high spectral resolution (R = 45, 000) with the IGRINS spectrometer on the GEMINI South telescope. We adopted the fundamental stellar parameters for these stars from Nandakumar et al. (2023a), with Teff ranging from 3400 to 3800 K. With a manual spectral synthesis method, we have derived stellar abundances for 21 elements, namely F, Mg, Si, S, Ca, Na, Al, K, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Ce, Nd, and Yb. We demonstrate what elements can be analysed from H- and K-band high-resolution spectra, and we show which spectral lines can be used for abundance analysis, showing them line by line. We discuss the 21 abundance-ratio trends and compared them with those determined from APOGEE and from the optical GILD sample. Especially, we determine the trends of the heavy elements Cu, Zn, Y, Ce, Nd, and Yb. This opens up these nucleosynthetic channels, including both the s- and the r-process, in dust-obscured populations. The [Mn/Fe] versus [Fe/H] trend is shown to be more or less flat at low metallicities, implying that existing NLTE correction are relevant.

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arXiv2023

M Giants with IGRINS II. Chemical Evolution of Fluorine at High Metallicities

The origin and evolution of fluorine in the Milky Way galaxy is still in debate. In particular, the increase of the [F/Fe] in metal-rich stars found from near-IR HF-lines is challenging to explain theoretically. We determine the fluorine abundances from 50 M giants in the solar neighbourhood spanning a broad range of metallicities (-0.9<[Fe/H]<0.25 dex). These stars are cool enough to have an array of HF lines in the K band. We observed the stars with the IGRINS and investigate each of ten HF molecular lines in detail. Based on a detailed line-by-line analysis of ten HF lines, we find that the R19, R18 and R16 lines should primarily be used for abundance analysis. The R15, R14 and R13 lines can also be used, but the trends based on these lines show increasing dependencies with the stellar parameters. The strongest HF lines, namely R12, R11, R9 and R7 should be avoided since the abundances from them show significant trends with the stellar parameters, and a high sensitivity to variations in the microturbulence, especially for coolest metal-rich stars. This leads to a huge scatter and high fluorine abundances for supersolar metallicity stars, not seen in the trends from the weaker lines for the same stars. When estimating the final mean fluorine abundance trend versus metallicity, we neglect the fluorine abundances from the four strongest lines (R7, R9, R11 and R12) for all stars and use only those derived from R16, R18, and R19 for the coolest metal-rich stars. We confirm the flat trend of [F/Fe] found in other studies in the metallicity range of -1.0<[Fe/H]<0.0. We also find a slight enhancement at supersolar metallicities (0<[Fe/H]<0.15) but we cannot confirm the upward trend seen at [Fe/H]>0.25. We need more observations of M giants at super solar metallicities with a spectrometer like IGRINS to confirm if the metal-rich fluorine abundance upturn is real or not.

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arXiv2023

M giants with IGRINS I. Stellar parameters and $\alpha$-abundance trends of the solar neighborhood population

Cool stars, such as M giants, can only be analysed in the near-infrared (NIR) regime due to the ubiquitous TiO features in optical spectra of stars with Teff < 4000 K. In dust obscured regions, like the inner bulge and Galactic Center, the intrinsically bright M giants observed in the NIR is an optimal option to determine their stellar abundances. Due to uncertainties in photometric methods, a method to determine the stellar parameters for M giants from the NIR spectra themselves is needed. We have carried out new observations of 44 M giant stars (also in APOGEE DR17) with IGRINS (R=45,000) mounted on the Gemini South telescope. We also obtained HK band IGRINS spectra of six nearby well-studied M giants from the IGRINS spectral library. Using this sample, we have developed a method to determine the stellar parameters for M giants from the NIR spectra by spectral synthesis using SME. The method is validated using the six nearby well-studied M-giants. We demonstrate the accuracy and precision by determining stellar parameters and $\alpha$-element trends versus metallicity for solar neighbourhood M giants. The effective temperatures that we derive (tested for 3400$\lesssim$ Teff $\lesssim$4000\,K) agree excellently with the six nearby M giants which indicates that the accuracy is indeed high. For the 43 solar neighborhood M giants, our Teff, logg, [Fe/H], $\xi_\mathrm{micro}$, [C/Fe], [N/Fe], and [O/Fe] are in unison with APOGEE with mean differences and scatter (our method - APOGEE) of -67$\pm$33 K, -0.31$\pm$0.15 dex, 0.02$\pm$0.05 dex, 0.22$\pm$0.13 km/s, -0.05$\pm$0.06 dex, 0.06$\pm$0.06 dex, and 0.02$\pm$0.09 dex, respectively. The $\alpha$-element trends versus metallicity for Mg, Si, Ca and Ti are consistent with both APOGEE DR17 trends for the same stars as well as with the GILD optical trends. We also find clear enhancement in abundances for thick disc stars.

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arXiv2023

Detailed $\alpha$ abundance trends in the inner Galactic bulge

In this paper, we aim to derive high-precision alpha-element abundances using CRIRES high-resolution IR spectra of 72 cool M giants of the inner Galactic bulge. 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 alpha 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 alpha-element trends. 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 alpha-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 $\sim$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 alpha 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. Based on a very meticulous spectral analysis, we have constructed detailed and precise chemical abundances of Mg, Si, and Ca for cool M giants.

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arXiv2022

The first r-process enhanced star confirmed to be a member of the Galactic bulge

Aims. Stars with strong enhancements of r-process elements are rare and tend to be metal-poor, with generally [Fe/H] <-2 dex and found in the halo. In this work we aim to investigate a candidate r-process enriched bulge star with a relatively high metallicity of -0.65 dex, and compare it with a previously published r-rich candidate star in the bulge. Methods. We reconsider the abundance analysis of a high-resolution optical spectrum of the red-giant star 2MASS J18082459-2548444 and determine its europium (Eu) and molybdenum (Mo) abundance, using stellar parameters from five different previous studies. Applying 2MASS photometry, Gaia astrometry and kinematics, we estimate distance, orbits, and population membership of 2MASS J18082459-2548444 and a previously reported r-enriched star 2MASS J18174532-3353235. Results. We find that 2MASS J18082459-2548444 is a relatively metal rich enriched r-process star that is enhanced in Eu and Mo but not substantially enhanced in s-process elements. It has a high probability of membership in the Galactic bulge based on its distance and orbit. We find that both stars show r-process enhancement with elevated [Eu/Fe]-values, even though 2MASS J18174532-3353235 is 1 dex lower in metallicity. Additionally, we find that 2MASS J18174532-3353235 plausibly has a halo or thick disc origin. Conclusions. We conclude that 2MASS J18082459-2548444 represents the first example of a confirmed r-process enhanced star confined to the inner bulge, possibly a relic from a period of enrichment associated with the formation of the bar.

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arXiv2022

The Galactic Chemical Evolution of phosphorus observed with IGRINS

Phosphorus (P) is considered to be one of the key elements for life, making it an important element to look for in the abundance analysis of spectra of stellar systems. Yet, there exists only a handful of spectroscopic studies to estimate the P abundances and investigate its trend across a range of metallicities. We have observed full HK band spectra at a spectral resolving power of R=45,000 with IGRINS instrument. Abundances are determined using SME in combination with 1D MARCS stellar atmosphere models. The investigated sample of stars have reliable stellar parameters estimated using optical FIES spectra (GILD; J\"onsson et al. in prep.). In order to determine the P abundances from the 16482.92 Angstrom P line, we take special care of the CO($\nu=7-4$) blend. We determine the C, N, O abundances from atomic carbon and a range of non-blended molecular lines (CO, CN, OH) which are aplenty in the H band region of K giant stars, assuring an appropriate modelling of the blending CO($\nu=7-4$) line. We present [P/Fe] vs [Fe/H] trend for 38 K giant stars in the metallicity range of -1.2 dex $<$ [Fe/H] $<$ 0.4 dex. We find that our trend matches well with the compiled literature sample of prominently dwarf stars and limited number of giant stars. Our trend is found to be higher by $\sim$ 0.05 - 0.1 dex compared to the theoretical chemical evolution trend in Cescutti et al. 2012 resulting from core collapse supernova (type II) of massive stars with the P yields from Kobayashi et al. (2006) arbitrarily increased by a factor of 2.75. Thus the enhancement factor might need to be $\sim$ 0.05 - 0.1 dex higher to match our trend. We also find an empirically determined primary behaviour for phosphorus. Furthermore, the phosphorus abundance is found to be elevated by $\sim$ 0.6 - 0.9 dex in two metal poor s-enriched stars compared to the theoretical chemical evolution trend.

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arXiv2022

CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB

12CO and 13CO lines, as well as a mm-wave continuum, have been observed for a sample of 22 OH/IR stars in directions within 2 degrees of the Galactic Centre. Photometry data have been gathered from the literature to construct SEDs and to determine pulsational variability. Radiative transfer models have been used to interpret the data. All stars in the sample were detected in at least one CO line, and 8 objects were detected in 324 GHz continuum. Based on luminosity criteria, the sample is divided into 17 objects that most likely lie within the inner Galactic Bulge, and 5 objects that are most likely foreground objects. The median luminosity of the inner-Galactic-Bulge objects, 5600 Lsun, corresponds to an initial mass in the range 1.2-1.6 Msun, indicating that these OH/IR stars descend from solar-type stars. The objects in this sub-sample are further divided into two classes based on their SED characteristics: 11 objects have SEDs that are well matched by models invoking dust envelopes extending from a few stellar radii and outwards, while 6 objects are better modelled as having detached dust envelopes with inner radii in the range 200-600 au and warmer central stars. The former objects have periodic variability, while the latter objects are predominantly non-periodic. The median gas-mass-loss rate, gas terminal expansion velocity, gas-to-dust mass ratio, and circumstellar 12CO/13CO abundance ratio have been estimated to be 2x10{-5} Msun/yr, 18 km/s, 200 (excluding the sources with detached dust envelopes, which show markedly lower gas-to-dust ratios), and 5, respectively, for the inner-Galactic-Bulge objects. The inner-Galactic-Bulge OH/IR stars studied here constitute an excellent sample of equidistant objects for the purpose of understanding the evolution of the mass-loss-rate characteristics at the tip of the AGB.

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arXiv2022

Chemical Evolution of Ytterbium in the Galactic Disk

Measuring the abundances of neutron-capture elements in Galactic disk stars is an important part of understanding key stellar and galactic processes. In the optical wavelength regime a number of different neutron-capture elements have been measured, however from the infrared H-band only the s-process dominated element cerium has been accurately measured for a large sample of disk stars. The more r-process dominated element ytterbium has only been measured in a small subset of stars so far. In this study we aim to measure the ytterbium (Yb) abundance of local disk giants using the Yb II line at $\lambda_\text{air}$=16498\AA. We also compare the resulting abundance trend with Ce and Eu abundances for the same stars to analyse the s- and r-process contributions. We analyse 30 K-giants with high-resolution H-band spectra using spectral synthesis. The very same stars have already been analysed using high-resolution optical spectra using the same method, but the abundance of Yb was not possible to determine from those spectra due to blending issues for stars with [Fe/H]>-1. In this present analysis, we utilise the stellar parameters determined from the optical analysis. We determined the Yb abundances with an estimated uncertainty for [Yb/Fe] of 0.1 dex. From comparison, the trend of [Yb/Fe] follows closely the [Eu/Fe] trend and has clear s-process enrichment in identified s-rich stars. From the comparison, both the validity of the Yb abundances are ensured, and the theoretical prediction of a roughly 40/60 s-/r-process contribution to Yb's origin is supported. These results show that with a careful and detailed analysis of infrared spectra, reliable Yb abundances can be derived for a wider sample of cooler giants in the range -1.1<[Fe/H]<0.3. This is promising for further studies of the production of Yb and for the r-process channel, key for Galactochemical evolution, in the infrared.

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