The assembly history of the central regions of disc galaxies is regulated by dynamical processes that trigger gas infall events, leading to active star formation in nuclear stellar discs (NSD) and in nuclear stellar clusters (NSC). In the Milky Way, recent studies of its nuclear regions have revealed a complex star formation history (SFH), with an initial burst associated to the formation of the Galactic bar, followed by a non-constant star formation rate. In this work, we aim to study the formation and evolution of nuclear structures and their link with the formation of large-scale structures. Our goal is to investigate the effects of the bar and spiral arms on the gas dynamics and, as a result on the SFH of NSDs and NSCs. We run a simulation of an isolated Milky Way-like galaxy with the SWIFT N-Body+hydro simulation code, including star formation and stellar feedback from SNIa SNII. We start from a live DM halo and a pre-existing stellar gaseous disc with 20 We find that the SFH of the nuclear regions display a main burst at bar formation time, due to bar-driven gas inflows. After bar formation, we find secondary periodic formation bursts, that do not appear in the disc SFH. These bursts occur when the spiral arms and the bar, rotating at different pattern speeds, reconnect, triggering secondary gas inflow events. The interaction of spiral arms and the galactic bar can enhance non-axisymmetric features in the disc, triggering bar-driven gas infall even after the bar has formed. These bar-spiral reconnection events are imprinted into the SFH of the NSCs and NSDs as episodic star formation bursts.

An important step in understanding the formation and evolution of the nuclear star cluster (NSC) is to investigate its chemistry and chemical evolution. Additionally, exploring the NSC’s relationship to the other structures in the Galactic center and the Milky Way disks is of great interest. Extreme optical extinction has previously prevented optical studies, but near-IR high-resolution spectroscopy is now possible. Here, we present a detailed chemical abundance analysis of 19 elements—more than 4 times as many as previously published—for nine stars in the NSC of the Milky Way, observed with the Immersion GRating INfrared Spectrometer on the Gemini South telescope. This study provides new, crucial observational evidence to shed light on the origin of the NSC. We demonstrate that it is possible to probe a variety of nucleosynthetic channels, reflecting different chemical evolution timescales. Our findings reveal that the NSC trends for the elements F, Mg, Al, Si, S, K, Ca, Ti, Cr, Mn, Co, Ni, Cu, and Zn, as well as for the s -process elements Ba, Ce, Nd, and Yb, generally follow the inner-bulge trends within uncertainties. This suggests a likely shared evolutionary history, and our results indicate that the NSC population is consistent with the chemical sequence observed in the inner Galaxy (the inner-disk sequence). However, we identify a significant and unexplained difference in the form of higher Na abundances in the NSC compared to the inner bulge. This is also observed in few Galactic globular clusters and may suggest a common enrichment process at work in all these systems.

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.

The Galactic center and inner disk of the Milky Way contain complex stellar populations obscured by heavy dust extinction. To study their chemical composition, high-resolution near-infrared (near-IR) spectroscopy is necessary. Expanding the set of elements measurable in the near-IR, especially neutron-capture elements, improves our ability to trace nucleosynthesis and Galactic chemical evolution. This work aims to identify and characterize a spectral line suitable for determining rubidium (Rb) abundances. Rubidium is produced in roughly equal parts by the r-and s-processes. We analyze high-resolution (R = 45,000) Immersion GRating INfrared Spectrograph (or IGRINS) near-IR spectra of 40 M giants in the solar neighborhood, most observed with Gemini South. We perform spectral synthesis of the Rb I line at 15289.48 & Aring;, using new log gf values and including an astrophysical calibration of the blending Fe I lines. The resulting [Rb/Fe] ratios are compared to other neutron-capture elements and interpreted with chemical evolution models. We demonstrate that the used Rb line is a reliable abundance indicator in M giants and the coolest K giants, but becomes too weak at higher temperatures. [Rb/Fe] shows a decreasing trend with metallicity, mirroring that of ytterbium (Yb), another mixed r-/s-process element. Our results agree with optical studies, validating the use of this near-IR line. Comparisons with chemical evolution models confirm that both s-and r-process sources are needed to explain the Rb trend. This work adds Rb to the list of elements measurable in high-resolution H-and K-band spectra, enabling studies of one more neutron-capture element in dust-obscured regions like the Galactic center and inner disk.

Context. The peculiar Galactic globular cluster omega Centauri (NGC 5139) has drawn attention for its unique features, such as an unusually high stellar mass compared to other Galactic globular clusters and a broad distribution of chemical elements. These features have led to the hypothesis that omega Centauri might be the nuclear remnant of an ancient dwarf galaxy accreted by the Milky Way, potentially bringing along its own globular cluster system. Aims. In this work, we adopt an innovative approach by examining the individual chemical abundances of Galactic globular clusters to identify shared patterns with omega Centauri. Methods. Applying Gaussian mixture models to globular cluster stars, whose membership is based on the analysis of the Gaia EDR3 release, and whose chemical abundances have been obtained from APOGEE DR17, we depart from traditional kinematic-based procedures and search for globular clusters that are chemically compatible with omega Centauri in an eight-dimensional space defined by [Fe/H], alpha-elements such as [Mg/Fe], [Si/Fe], and [Ca/Fe], light+odd-Z elements such as [C/Fe], [Al/Fe], and [K/Fe], and an iron-peak element as [Mn/Fe]. With this approach, clusters that are chemically compatible with omega Centauri are clusters whose chemical patterns are contained in the abundance domain defined by omega Centauri stars. Results. Our analysis leads to the identification of six globular clusters - NGC 6752, NGC 6656, NGC 6809, NGC 6273, NGC 6205, and NGC 6254 - that exhibit strong chemical similarities with omega Centauri, and that have metallicities that coincide with those of the two known peaks (primary and secondary) of omega Centauri's metallicity distribution. They all exhibit non-null intrinsic [Fe/H] dispersions, ranging between 0.07 and 0.12 dex, unless the ASPCAP uncertainties had been severely underestimated, and three of them have statistically significant skewed [Fe/H] distributions. Furthermore, the chemical patterns of these clusters lead to the exclusion that they were formed in progenitor galaxies with chemical enrichment histories similar to those of the Large and Small Magellanic Clouds, Sagittarius, and Fornax. Once placed in kinematic spaces such as the energy - angular momentum plane, these clusters result scatter across an extended region, which is predicted by N-body simulations if their common progenitor was sufficiently massive compared to the Milky Way. Conclusions. Our novel approach suggests a common origin for NGC 6752, NGC 6656, NGC 6809, NGC 6273, NGC 6205, NGC 6254, and omega Centauri, indicating that Nephele, as we propose to call the progenitor in which all these clusters formed, played a substantial role in the Galaxy's history. The finding that a set of globular clusters can be associated with omega Centauri reinforces the hypothesis that this system is the remnant of a galaxy, and not simply an unusual globular cluster. This study also shows that the spectroscopic data at our disposal have reached the quality needed to compare chemical patterns of stellar systems, to reveal their common origins or exclude their association with specific progenitor galaxies.

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.

Abstract The Galactic center is a unique region in the center of the Milky Way, with the Nuclear Star Cluster (NSC) and the Nuclear Stellar Disk (NSD) embedded in the Central Molecular Zone. A promising approach to study these regions, is the analysis of abundance trends of a range of elements with different evolutionary timescales. Using high-resolution near-infrared spectroscopy, accurate elemental abundances can now be derived in the NSC and NSD. A powerful tool is to compare the trends from these populations with those from inner-bulge and thick-disk stars analyzed with the same methodology. In a few papers we have pioneered this approach finding enhanced alpha-element trends, indicating a high star formation rate, and similarities between the NSC and inner bulge, challenging previous models of recent dominant starbursts. These findings provide new insights into the formation history of the Milky Way’s central regions and emphasize the need for further high-resolution spectroscopic studies.

Context. 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 centre and along the Galactic plane, abundance analyses based on near-infrared (NIR) 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. Aims. Due to the scarcity of spectral lines from neutron-capture elements in the NIR, 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. Methods. We observed the NIR spectra of 37 M giants in the solar neighbourhood at high spectral resolution and with a high signal-to-noise ratio using the IGRINS spectrometer on the GEMINI South telescope. The full H- and K-bands were recorded simultaneously at R = 45 000. Using a manual spectral synthesis method, we determined the fundamental stellar parameters for these stars and derived the barium abundance from the Ba line (6s5d D-3(2) -> 6s6p P-3(2)o) at lambda(air) = 23 253.56 & Aring; in the K band. Results. We demonstrate that the Ba line in the K band at 2.33 mu m (lambda 23 253.56) is useful for abundance analyses from the 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. Conclusions. 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. Thus, these elements can be studied for heavily dust-obscured regions of the Galaxy, such as the Galactic centre.

We report the first high-resolution, detailed abundances of 21 elements for giants in the Galactic bulge/bar within 1 degrees of the Galactic plane, where high extinction has rendered such studies challenging. Our high-signal-to-noise-ratio and high-resolution, near-infrared spectra of seven M giants in the inner bulge, located at (l, b) = (0 degrees, +1 degrees), are observed using the IGRINS spectrograph. We report the first multichemical study of the inner Galactic bulge by investigating, relative to a robust new solar neighborhood sample, the abundance trends of 21 elements, including the relatively difficult to study heavy elements. The elements studied are: F, Mg, Si, S, Ca, Na, Al, K, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Ce, Nd, and Yb. We investigate bulge membership of all seven stars using distances and orbital simulations, and we find that the most metal-poor star may be a halo interloper. Our investigation also shows that the inner bulge as close as 1 degrees north of the Galactic Center displays a similarity to the inner disk sequence, following the high-[alpha/Fe] envelope of the solar vicinity metal-rich population, though no firm conclusions for a different enrichment history are evident from this sample. We find a small fraction of metal-poor stars ([Fe/H] > -0.5), but most of our stars are mainly of supersolar metallicity. Fluorine is found to be enhanced at high metallicity compared to the solar neighborhood, but confirmation with a larger sample is required. We will apply this approach to explore the populations of the nuclear stellar disk and the nuclear star cluster.

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.

Context. Cool stars, such as M giants, can only be analyzed in the near-infrared (NIR) regime due to the ubiquitous titanium oxide features in optical spectra of stars with T-eff < 4000 K. In dust-obscured regions, the inner bulge and Galactic center region, the intrinsically bright M giants observed in the NIR are an optimal option for studying stellar abundances and the chemical evolution of stellar populations. Because of the uncertainties in photometric methods, a method for determining the stellar parameters for M giants from the NIR spectra themselves is needed. Aims. We develop a method for determining the stellar parameters for M giants from the NIR spectra. We validate the method by deriving the stellar parameters for nearby well-studied M giants with spectra from the spectral library of the Immersion GRating INfrared Spectrograph (IGRINS). We demonstrate the accuracy and precision of our method by determining the stellar parameters and alpha-element trends versus metallicity for solar neighborhood M giants. Methods. We carried out new observations of 44M giant stars with IGRINS mounted on the Gemini South telescope. We also obtained the full H and K band IGRINS spectra of six nearby well-studied M giants at a spectral resolving power of R = 45 000 from the IGRINS spectral library. We used the tool called spectroscopy made easy in combination with one-dimensional (1D) model atmospheres in a radiative and convective scheme (MARCS) stellar atmosphere models to model the synthetic spectrum that fits the observed spectrum best. Results. The e ffective temperatures that we derive from our new method (tested for 3400 less than or similar to T-eff less than or similar to 4000K here) agree excellently with those of the six nearby well-studied M giants, which indicates that the accuracy is indeed high. For the 43 solar neighborhood M giants, our T-eff, log g, [Fe/H], xi(micro), [C/Fe], [N/Fe], and [O/Fe] agree with APOGEE with mean differences and a scatter (our method - APOGEE) of -67 +/- 33 K, -0.31 +/- 0.15 dex, 0.02 +/- 0.05 dex, 0.22 +/- 0.13 km s(-1), -0.05 +/- 0.06 dex, 0.06 +/- 0.06 dex, and 0.02 +/- 0.09 dex, respectively. Furthermore, the tight offset with a small dispersion compared to the APOGEE T-eff indicates a high precision in our derived temperatures and those derived from the APOGEE pipeline. The typical uncertainties in the stellar parameters are found to be +/- 100K in T-eff, +/- 0.2 dex in log g, +/- 0.1 dex in [Fe/H], and +/- 0.1 km s(-1) in xi(micro). The alpha-element trends versus metallicity for Mg, Si, Ca, and Ti are consistent with the APOGEE DR17 trends for the same stars and with the GILD optical trends. We also find a clear enhancement in the abundances for thick-disk stars.

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.

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

Context. Cool stars, such as M giants, can only be analyzed in the near-infrared (NIR) regime due to the ubiquitous titanium oxide features in optical spectra of stars with Teff < 4000 K. In dust-obscured regions, the inner bulge and Galactic center region, the intrinsically bright M giants observed in the NIR are an optimal option for studying stellar abundances and the chemical evolution of stellar populations. Because of the uncertainties in photometric methods, a method for determining the stellar parameters for M giants from the NIR spectra themselves is needed. Aims. We develop a method for determining the stellar parameters for M giants from the NIR spectra. We validate the method by deriving the stellar parameters for nearby well-studied M giants with spectra from the spectral library of the Immersion GRating INfrared Spectrograph (IGRINS). We demonstrate the accuracy and precision of our method by determining the stellar parameters and α-element trends versus metallicity for solar neighborhood M giants. Methods. We carried out new observations of 44 M giant stars with IGRINS mounted on the Gemini South telescope. We also obtained the full H and K band IGRINS spectra of six nearby well-studied M giants at a spectral resolving power of R = 45 000 from the IGRINS spectral library. We used the tool called spectroscopy made easy in combination with one-dimensional (1D) model atmospheres in a radiative and convective scheme (MARCS) stellar atmosphere models to model the synthetic spectrum that fits the observed spectrum best. Results. The effective temperatures that we derive from our new method (tested for 3400 ≲ Teff ≲ 4000 K here) agree excellently with those of the six nearby well-studied M giants, which indicates that the accuracy is indeed high. For the 43 solar neighborhood M giants, our Teff, log g, [Fe/H], ξmicro, [C/Fe], [N/Fe], and [O/Fe] agree with APOGEE with mean differences and a scatter (our method – APOGEE) of −67±33 K, −0.31±0.15 dex, 0.02±0.05 dex, 0.22±0.13 km s−1, −0.05±0.06 dex, 0.06±0.06 dex, and 0.02±0.09 dex, respectively. Furthermore, the tight offset with a small dispersion compared to the APOGEE Teff indicates a high precision in our derived temperatures and those derived from the APOGEE pipeline. The typical uncertainties in the stellar parameters are found to be ±100 K in Teff, ±0.2 dex in log g, ±0.1 dex in [Fe/H], and ±0.1 km s−1 in ξmicro. The α-element trends versus metallicity for Mg, Si, Ca, and Ti are consistent with the APOGEE DR17 trends for the same stars and with the GILD optical trends. We also find a clear enhancement in the abundances for thick-disk stars.

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.

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.

We report metallicities for three ∼Gyr-old stars in the Milky Way nuclear star cluster (NSC) using high-resolution near-infrared spectroscopy. We derive effective temperatures from a calibration with Sc line strength, which yields results in good agreement with other methods, and metallicities from spectral fits to Fe i lines. Our derived metallicities range from −1.2 < [Fe/H] < + 0.5, a span of 1.7 dex. In addition we use isochrone projection to obtain masses of 1.6–4.3 M _⊙ , and ages assuming single-star evolution. The oldest of these stars is 1.5 Gyr while the youngest and most metal-rich is only 100 Myr. The wide range in metallicity poses interesting questions concerning the chemical evolution and enrichment of the NSC and adds to the evidence for the presence of a young, metal-rich population in the NSC. We suggest that the candidate intermediate-age, metal-poor ([Fe/H] = −1.2) star may be best explained as a blue straggler from an underlying old population.

Context. The origin and evolution of fluorine in the Milky Way Galaxy is still under debate. In particular, the increase in the [F/Fe] in metal-rich stars found from near-IR HF lines is challenging to explain theoretically. Chemical evolution models with current knowledge of yields from different fluorine-producing stellar sources cannot reproduce these observations. Aims. The aim of this work is to observationally study the Galactic chemical evolution of fluorine, especially for metal-rich stars. We want to investigate whether the significant rise in fluorine production at high metallicities can be corroborated. Furthermore, we want to explore the possible reasons for this upturn in [F/Fe]. Methods. We determined the fluorine abundances from 50 M giants (3300 < T eff < 3800 K) in the solar neighborhood spanning a broad range of metallicities (−0.9 < [Fe/H] < 0.25 dex). These stars are cool enough to have an array of lines from the HF molecule in the K band. We observed the stars with the Immersion GRating INfrared Spectrograph (IGRINS) spectrometer mounted on the Gemini South telescope and on the Harlan J. Smith Telescope at McDonald Observatory and investigate each of 10 HF molecular lines in detail. Results. Based on a detailed line-by-line analysis of ten HF lines, we find that the R19, R18, and R16 lines (22 699.49, 22 714.59, and 22 778.25 Å) should primarily be used for an abundance analysis. The R15, R14, and R13 lines at 22 826.86, 22 886.73, and 22 957.94 Å can also be used, but the trends based on these lines show increasing dependence on the stellar parameters. The strongest HF lines, namely R12, R11, R9, and R7 lying at 23 040.57, 23 134.76, 23 358.33, and 23 629.99 Å should be avoided. The abundances derived from these strongest lines show significant trends with the stellar parameters, as well as a high sensitivity to variations in the stellar microturbulence, especially for coolest and most 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. Conclusions. When estimating the final mean fluorine abundance trend as a function of 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 and most metal-rich stars. We confirm the flat trend of [F/Fe] found in other studies for stars in the metallicity range of −1.0 < [Fe/H] < 0.0 dex. We also find a slight enhancement at super-solar metallicities (0 < [Fe/H] < 0.15 dex) but we cannot confirm the upward trend seen at [Fe/H] > 0.25 dex. The HF line is intrinsically temperature sensitive, which calls for studies of stars with highly accurate and homogeneous stellar parameters. The spread in our trend is presumably caused by the temperature sensitivity. We need more observations of M giants at super-solar metallicities with a spectrometer that covers as many of the HF lines as possible, for instance the IGRINS spectrometer, to confirm whether the metal-rich fluorine abundance upturn is real or not.

Aims. In this work, we aim to make a differential comparison of the neutron-capture and p-process element molybdenum (Mo) in the stellar populations in the local disk(s) and the bulge, focusing on minimising possible systematic effects in the analysis. Methods. The stellar sample consists of 45 bulge and 291 local disk K-giants, observed with high-resolution optical spectra. The abundances are determined by fitting synthetic spectra using the SME-code. The disk sample is separated into thin- and thick-disk components using a combination of abundances and kinematics. The cosmic origin of Mo is investigated and discussed by comparing with previous published abundances of Mo and the neutron-capture elements cerium (Ce) and europium (Eu). Results. We determine reliable Mo abundances for 35 bulge and 282 disk giants with a typical uncertainty of [Mo/Fe]~0.2 and ~0.1 dex for the bulge and disk, respectively. Conclusions. We find that the bulge possibly is enhanced in [Mo/Fe] compared to the thick disk, which we do not observe in either [Ce/Fe] nor [Eu/Fe]. This might suggest a higher past star-formation rate in the bulge, however, since we do not observe the bulge to be enhanced in [Eu/Fe], the origin of the molybdenum enhancement is yet to be constrained. Although, the scatter is large, we may be observing evidence of the p-process contributing to the heavy element production in the chemical evolution of the bulge.

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.