arXiv subjects

Andrew McWilliam

Publications and source records attributed to Andrew McWilliam.

At least 19 recordsLinked to original sources

arXiv2026

WINERED Detects a Strong Atmospheric Outflow on the Sub-Neptune GJ 3090b

Sub-Neptunes are the most common short-period ($P<100$ d) planets known. A major goal for the field is to build a comprehensive understanding of their compositions and evolutionary histories, and an important tool for achieving this goal is metastable helium (He$^*$) transmission spectroscopy, which traces atmospheric escape. In this paper, we report the detection of He$^*$ in the atmosphere of the sub-Neptune GJ 3090b at high spectral resolution. We observed two consecutive transits of this planet with Magellan II/WINERED, and detected peak He$^*$ absorptions during transit of $2.033\pm0.086\%$ (23.6$σ$) and $1.619\pm0.122\%$ (13.3$σ$), respectively. The first transit was affected by a stellar flare near egress while the second transit was unaffected. The signal is variable in amplitude and deeper than anticipated by the JWST/NIRISS observations of Ahrer et al. (2025). We do not observe a significant Doppler shift, and the signal has a FWHM of $25$ km s$^{-1}$, implicating photoevaporation. Our outflow modeling suggests that $Z_\mathrm{atm}\lesssim100\times$ solar, as the observed He$^*$ amplitudes are challenging to achieve in a metal-rich atmosphere. To match the muted transmission spectra from JWST and VLT/CRIRES+, GJ 3090b's lower atmosphere is likely covered by aerosols. Finally, we found that GJ 3090b has an atmospheric lifetime of $\sim300$ Myr, much shorter than previously reported. This adolescent ($\sim1$ Gyr) sub-Neptune is currently in a transformative phase of photoevaporative evolution.

astro-ph.EP↗

arXiv2026

Chemodynamical evidence of the HR 1614 moving group as a bar resonance

Moving groups (MGs) are ensembles of disk stars that clump in velocity and action space. A multitude of explanations as to their origin has been attempted, from dissolved star clusters to resonances with the Galactic bar, or other non-axisymmetric perturbations. The object of this work, the old HR 1614 group, has in the past already been shown to not be a disrupted cluster in terms of its broad age and metallicity distribution. Thus its stars have more likely been trapped by the Milky Way bar at corotation. Here, we present a new study that aims to chemically and dynamically characterize this group in the context of such a resonance. To this end, we used MIKE/Magellan high-resolution, high signal-to-noise spectra of six member stars of the HR 1614 MG to determine the chemical abundance ratios for 24 elements, several for the first time in those stars. The Fe-abundances of our sample range from +0.13 to +0.38 dex and the majority of abundance ratios is fully in line with those of metal-rich Milky Way disk stars, with few exceptions. In particular, the group's eponym, HR 1614, is enhanced in essentially all abundances and coincides with the higher-[X/Fe] trends of the Milky Way's thick disk. All elements (but S and Ca) show significant intrinsic abundance scatter that argue against this MG having formed in a contained environment such as a dissolved cluster. Isochrone ages, tailored to the measured metallicities support earlier findings of a broad age mix in this group, ranging from 1 Gyr to as old as 8 Gyr. We performed an orbital frequency analysis in a Galactic potential that includes a rotating bar. Indeed, our sample stars fall onto thin resonances on corotation. Hence, our results support the idea that the HR 1614 MG is a resonantly perturbed disk feature, mustering a melange of stars from different parent populations in the inner Galaxy.

astro-ph.GA↗

arXiv2026

Helium escaping from the atmosphere of a nearby rocky exoplanet orbiting in a habitable zone

Observations of highly irradiated gas giant exoplanets have shown helium escaping from their atmospheres. There is limited evidence for atmospheres on rocky exoplanets, perhaps because they have already escaped. We report spectroscopic observations of LHS 1140b, a rocky exoplanet that orbits in the habitable zone of a nearby low-mass star. The near-infrared transit spectra show absorption by helium escaping from the planet's atmosphere. Helium absorption is detected in 2024 but not in 2025, indicating time-variable atmospheric escape. We interpret these results as indicating an upper atmosphere dominated by helium and depleted in hydrogen, with other volatile species trapped at lower altitudes, consistent with atmospheric fractionation models. No helium absorption is detected for LHS 1140c, a smaller and more heavily irradiated exoplanet in the same system.

astro-ph.EP↗

arXiv2025

Searching for GEMS: The Occurrence of Giant Planets orbiting M-dwarfs within 100 pc

We present results from a systematic search for transiting short-period Giant Exoplanets around M-dwarf Stars (GEMS; $P < 10$ days, $R_p \gtrsim 8~R_\oplus$) within a distance-limited 100 pc sample of $149,316$ M-dwarfs using TESS-Gaia Light Curve (TGLC) data. We describe the development and application of the \textit{TESS-miner} package and associated vetting procedures used in this analysis. To assess detection completeness, we conducted $\sim$72 million injection-recovery tests across $\sim$26,000 stars with an average of $\sim$3 sectors of data per star, subdivided into early-type (M0--M2.5), mid-type (M2.5--M4), and late-type (M4 or later) M-dwarfs. Our pipeline demonstrates high sensitivity across all subtypes within the injection bounds. We estimate the occurrence rates of short-period GEMS as a function of stellar mass, and combine our measured rates with those derived for FGK stars, fitting an exponential trend with stellar mass, consistent with core-accretion theory predictions. We find GEMS occurrence rates of $0.118\% \pm 0.068\%$ for early-type M-dwarfs, $0.153\% \pm 0.069\%$ for mid-type, and $0.036\% \pm 0.024\%$ for late-type M-dwarfs, with a mean rate of $0.068\%\pm0.024\%$ across the full sample. While our search spanned $1.0~\mathrm{days} < P < 10.0$ days, these rates were calculated using planets orbiting with $1.0~\mathrm{days} < P < 5.0$ days. This work establishes the basis for future occurrence rate studies of transiting GEMS.

astro-ph.EP↗

arXiv2025

Assessing robustness and bias in 1D retrievals of 3D Global Circulation Models at high spectral resolution: a WASP-76 b simulation case study in emission

High-resolution spectroscopy (HRS) of exoplanet atmospheres has successfully detected many chemical species and is quickly moving toward detailed characterization of the chemical abundances and dynamics. HRS is highly sensitive to the line shape and position, thus, it can detect three-dimensional (3D) effects such as winds, rotation, and spatial variation of atmospheric conditions. At the same time, retrieval frameworks are increasingly deployed to constrain chemical abundances, pressure-temperature (P-T) structures, orbital parameters, and rotational broadening. To explore the multidimensional parameter space, they need computationally fast models that are consequently mostly one-dimensional (1D). However, this approach risks introducing interpretation bias since the planet's true nature is 3D. We investigate the robustness of this methodology at high spectral resolution by running 1D retrievals on simulated observations in emission within an observational framework using 3D Global Circulation Models of the quintessential HJ WASP-76 b. We find that the retrieval broadly recovers conditions present in the atmosphere, but that the retrieved P-T and chemical profiles are not a homogeneous average of all spatial and phase-dependent information. Instead, they are most sensitive to spatial regions with large thermal gradients, which do not necessarily coincide with the strongest emitting regions. Our results further suggest that the choice of parameterization for the P-T and chemical profiles, as well as Doppler offsets among opacity sources, impact retrieval results. These factors should be carefully considered in future retrieval analyses.

astro-ph.EP↗

arXiv2024

Detailed abundances of the planet-hosting TOI-1173 A/B system: Possible evidence of planet engulfment in a very wide binary

Over the last decade, studies of large samples of binary systems identified chemical anomalies, and showed that they might be attributed to planet formation or planet engulfment. However, both scenarios have primarily been tested in pairs without known exoplanets. In this work, we explore these scenarios in the newly detected planet-hosting wide binary TOI-1173 A/B (projected separation $\sim 11,400$ AU) using high-resolution MAROON-X and ARCES spectra. We determined photospheric stellar parameters both by fitting stellar models and via the spectroscopic equilibrium approach. Both analyses agree and suggest that they are cool main sequence stars located in the thin disc. A line-by-line differential analysis between the components (B$-$A) displays an abundance pattern in the condensation temperature plane where the planet-hosting star TOI-1173 A is enhanced in refractory elements such as iron by more than 0.05 dex. This suggests the engulfment of $\sim$18 M$_{\oplus}$ of rocky material in star A. Our hypothesis is supported by the dynamics of the system detailed in our companion paper Yana Galarza et al. 2024, which suggest that the Super-Neptune TOI-1173 A $b$ might have been delivered to its current short period ($\sim7$ days) through circulatization and von Zeipel-Lidov-Kozai mechanisms, thereby triggering the engulfment of inner rocky exoplanets.

astro-ph.SR↗

arXiv2024

A High-Resolution Non-Detection of Escaping Helium In The Ultra-Hot Neptune LTT 9779b: Evidence for Weakened Evaporation

The recent discovery of ``ultra-hot'' ($P < 1$ day) Neptunes has come as a surprise: some of these planets have managed to retain gaseous envelopes despite being close enough to their host stars to trigger strong photoevaporation and/or Roche lobe overflow. Here, we investigate atmospheric escape in LTT 9779b, an ultra-hot Neptune with a volatile-rich envelope. We observed two transits of this planet using the newly-commissioned WINERED spectrograph ($R\sim68,000$) on the 6.5 m Clay/Magellan II Telescope, aiming to detect an extended upper atmosphere in the He 10830 A triplet. We found no detectable planetary absorption: in a 0.75 A passband centered on the triplet, we set a 2$σ$ upper limit of 0.12% ($δR_p/H < 14$) and a 3$σ$ upper limit of 0.20% ($δR_p/H < 22$). Using a H/He isothermal Parker wind model, we found corresponding 95% and 99.7% upper limits on the planetary mass-loss rate of $\dot{M} < 10^{10.03}$ g s$^{-1}$ and $\dot{M} < 10^{11.11}$ g s$^{-1}$ respectively, smaller than predicted by outflow models even considering the weak stellar XUV emission. The low evaporation rate is plausibly explained by a metal-rich envelope, which would decrease the atmospheric scale height and increase the cooling rate of the outflow. This hypothesis is imminently testable: if metals commonly weaken planetary outflows, then we expect that \textit{JWST} will find high atmospheric metallicities for small planets that have evaded detection in He 10830 A.

astro-ph.EP↗

arXiv2023

Insight into the Formation of $β$ Pic b through the Composition of Its Parent Protoplanetary Disk as Revealed by the $β$ Pic Moving Group Member HD 181327

It has been suggested that $β$ Pic b has a supersolar metallicity and subsolar C/O ratio. Assuming solar carbon and oxygen abundances for the star $β$ Pic and therefore the planet's parent protoplanetary disk, $β$ Pic b's C/O ratio suggests that it formed via core accretion between its parent protoplanteary disk's H$_{2}$O and CO$_{2}$ ice lines. $β$ Pic b's high metallicity is difficult to reconcile with its mass $M_{\text{p}}~=~11.7~M_{\text{Jup}}$ though. Massive stars can present peculiar photospheric abundances that are unlikely to record the abundances of their former protoplanetary disks. This issue can be overcome for early-type stars in moving groups by infering the elemental abundances of the FGK stars in the same moving group that formed in the same molecular cloud and presumably share the same composition. We infer the photospheric abundances of the F dwarf HD 181327, a $β$ Pic moving group member that is the best available proxy for the composition of $β$ Pic b's parent protoplanetary disk. In parallel, we infer updated atmospheric abundances for $β$ Pic b. As expected for a planet of its mass formed via core-accretion beyond its parent protoplanetary disk's H$_{2}$O ice line, we find that $β$ Pic b's atmosphere is consistent with stellar metallicity and confirm that is has superstellar carbon and oxygen abundances with a substellar C/O ratio. We propose that the elemental abundances of FGK dwarfs in moving groups can be used as proxies for the otherwise difficult-to-infer elemental abundances of early-type and late-type members of the same moving groups.

astro-ph.SR↗

arXiv2021

NGC 1261: an $r$-process enhanced globular cluster from the Gaia-Enceladus event

Our Milky Way (MW) has witnessed a series of major accretion events. One of the later additions, Gaia-Enceladus, has contributed a considerable mass to the inner Galaxy, but also generously donated to the outer halo. So far, associations with present-day MW globular clusters (GCs) have been chiefly based on their kinematics and ages. Here, we present a chemical abundance study of the outer halo (R$_{\rm GC}$=18 kpc) GC NGC 1261, which has been suggested to be an accreted object. We measured 31 species of 29 elements in two stars from high-resolution Magellan/MIKE spectra and find that the cluster is moderately metal poor, at [Fe/H]=-1.26. NGC 1261 is moderately $α$-enhanced to the 0.3-dex level. While from the small sample alone it is difficult to assert any abundance correlations, the light elements Na,O,Mg, and Al differ significantly between the two stars in contrast to the majority of other elements with smaller scatter; this argues in favour of multiple generations of stars coexisting in this GC. Intriguingly for its metallicity, NGC 1261 shows heavy element abundances that are consistent with $r$-process nucleosynthesis and we discuss their origin in various sites. In particular the Eu overabundance quantitatively suggests that one single $r$-process event, such as a neutron-star neutron-star merger or a rare kind of supernova, can be responsible for the stellar enhancement or even the enrichment of the cluster with the excess $r$-material. Its heavy element pattern makes NGC 1261 resemble the moderately enhanced r-I stars that are commonly found in the halo and have been detected in Gaia-Enceladus as well. Therefore, combining all kinematical, age, and chemical evidence we conclude that NGC 1261 is a chemically intriguing GC that was born in Gaia-Enceladus and has been subsequently accreted into the MW halo. [abridged]

astro-ph.GA↗

arXiv2020

The Massive M31 Cluster G1: Detailed Chemical Abundances from Integrated Light Spectroscopy

G1, also known as Mayall II, is one of the most massive star clusters in M31. Its mass, ellipticity, and location in the outer halo make it a compelling candidate for a former nuclear star cluster. This paper presents an integrated light abundance analysis of G1, based on a moderately high-resolution (R=15,000) spectrum obtained with the High Resolution Spectrograph on the Hobby-Eberly Telescope in 2007 and 2008. To independently determine the metallicity, a moderate resolution (R~4,000) spectrum of the calcium-II triplet lines in the near-infrared was also obtained with the Astrophysical Research Consortium's 3.5-m telescope at Apache Point Observatory. From the high-resolution spectrum, G1 is found to be a moderately metal-poor cluster, with [Fe/H]=-0.98+/-0.05. G1 also shows signs of alpha-enhancement (based on Mg, Ca, and Ti) and lacks the s-process enhancements seen in dwarf galaxies (based on comparisons of Y, Ba, and Eu), indicating that it originated in a fairly massive galaxy. Intriguingly, G1 also exhibits signs of Na and Al enhancement, a unique signature of GCs -- this suggests that G1's formation is intimately connected with GC formation. G1's high [Na/Fe] also extends previous trends with cluster velocity dispersion to an even higher mass regime, implying that higher mass clusters are more able to retain Na-enhanced ejecta. The effects of intracluster abundance spreads are discussed in a subsequent paper. Ultimately, G1's chemical properties are found to resemble other M31 GCs, though it also shares some similarities with extragalactic nuclear star clusters.

astro-ph.GA↗

arXiv2020

A "blue ring nebula" surrounding a thousands of years old stellar merger

Stellar mergers are a brief-lived but common phase in the evolution of binary star systems. Among the many astrophysical implications of these events include the creation of atypical stars (e.g. magnetic stars, blue stragglers, rapid rotators), interpretation of stellar populations, and formation channels of LIGO-detected compact object mergers. Although stellar mergers are thus commonly invoked phenomena, observations of these events and details of their evolution remain elusive. While a handful of stellar mergers have been directly observed in recent years, the central remnants of these events remain shrouded by an opaque shell of dust and molecules, making it impossible to observe their final state (e.g. as a single merged star or a tighter surviving binary). Here we report observations of an unusual, ring-shaped ultraviolet nebula and the star at its center, TYC 2597-735-1. The nebula shows two opposing fronts, suggesting a bipolar outflow from TYC 2597-735-1. TYC 2597-735-1's spectrum and proximity above the Galactic plane suggest it is an old star, yet it shows abnormally low surface gravity and a detectable long-term luminosity decay, uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits H-alpha emission, radial velocity variations, enhanced ultraviolet radiation, and excess infrared emission, common signposts of dusty circumstellar disks, stellar activity, and accretion. The combined observations, paired with stellar evolution models, suggests TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger found at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, directly inferring how two stars merge into a single star.

astro-ph.SR↗

arXiv2020

A high-precision abundance analysis of the nuclear benchmark star HD 20

We present our chemical abundance investigation of the metal-poor ([Fe/H]=-1.60 dex), r-process-enriched ([Eu/Fe]=0.73 dex) halo star HD 20 using novel and archival high-resolution spectra at outstanding signal-to-noise ratios. By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with non-LTE analyses of iron lines, we derive a set of highly accurate and precise stellar parameters. These allow us to delineate a chemical pattern comprised of solid detections of 48 elements, including 28 neutron-capture elements, which establishes HD 20 among the few benchmark stars that have almost complete patterns with low systematic dependencies on the stellar parameters. Our light-element (Z<30) abundances are representative of other, similarly metal-poor stars in the Galactic halo with contributions from core-collapse supernovae of type II. A comparison to the scaled solar r-pattern shows that the lighter neutron-capture elements (37<Z<60) are poorly matched. In particular, we find imprints of the weak r-process acting at low metallicities. Nonetheless, by comparing our detailed abundances to the observed metal-poor star BD +17 3248, we find a persistent residual pattern that is indicative of enrichment contributions from the s-process. We show that mixing with material from predicted yields of massive, rotating AGB stars at low metallicity considerably improves the fit. Based on a solar ratio of heavy- to light-s elements -- at odds with model predictions for the i-process -- and a missing clear residual pattern with respect to other stars with claimed contributions from this process, we refute (strong) contributions from such astrophysical sites providing intermediate neutron densities. Finally, nuclear cosmochronology is used to tie our detection of the radioactive element Th to an age estimate for HD 20 of $11.0\pm3.8$ Gyr. [abridged]

astro-ph.SR↗

arXiv2019

Evidence for Sub-Chandrasekhar Type Ia Supernovae from Stellar Abundances in Dwarf Galaxies

There is no consensus on the progenitors of Type Ia supernovae (SNe Ia) despite their importance for cosmology and chemical evolution. We address this question by using our previously published catalogs of Mg, Si, Ca, Cr, Fe, Co, and Ni abundances in dwarf galaxy satellites of the Milky Way to constrain the mass at which the white dwarf explodes during a typical SN Ia. We fit a simple bi-linear model to the evolution of [X/Fe] with [Fe/H], where X represents each of the elements mentioned above. We use the evolution of [Mg/Fe] coupled with theoretical supernova yields to isolate what fraction of the elements originated in SNe Ia. Then, we infer the [X/Fe] yield of SNe Ia for all of the elements except Mg. We compare these observationally inferred yields to recent theoretical predictions for two classes of Chandrasekhar-mass (M_Ch) SN Ia as well as sub-M_Ch SNe Ia. Most of the inferred SN Ia yields are consistent with all of the theoretical models, but [Ni/Fe] is consistent only with sub-M_Ch models. We conclude that the dominant type of SN Ia in ancient dwarf galaxies is the explosion of a sub-M_Ch white dwarf. The Milky Way and dwarf galaxies with extended star formation histories have higher [Ni/Fe] abundances, which could indicate that the dominant class of SN Ia is different for galaxies where star formation lasted for at least several Gyr.

astro-ph.SR↗

arXiv2017

Evidence for a Sub-Chandrasekhar Mass Type~Ia Supernova in the Ursa Minor Dwarf Galaxy

A longstanding problem is identifying the elusive progenitors of Type Ia supernovae (SNe Ia), which can roughly be split into Chandraksekhar and sub-Chandrasekhar mass events. An important difference between these two cases is the nucleosynthetic yield, which is altered by the increased neutron excess in Chandrasekhar progenitors due to their pre-explosion simmering and high central density. From comparison with theoretical nucleosynthesis yields, we show that the chemical composition of the most metal-rich star in the Ursa Minor dwarf galaxy, COS 171, is dominated by nucleosynthesis from a low-metallicity, low-mass, sub-Chandrasekhar mass SN Ia. Key diagnostic abundance ratios include C/Fe, α/Fe, Mn/Fe and Ni/Fe ratios, which could not have been produced by Chandrasekhar-mass SNe Ia, Core-Collapse Type II supernovae or Pair-Instsability supernovae. Strong deficiencies of Ni/Fe, Cu/Fe and Zn/Fe also suggest the absence of alpha-rich freeze-out nucleosynthesis, favoring low-mass WD progenitor SNe Ia. Based on comparisons of the measured Mn/Fe and Si/Fe ratios with detonation models, we estimate a WD mass near 0.95Msun. We also compare Mn/Fe and Ni/Fe ratios to the recent theoretical yields predicted by Shen et al., finding consistent results. To explain the COS 171 [Fe/H], at -1.35 dex, requires dilution of the ejecta from a single SNIa event with ~10^4 Msun of material; this is expected for a SN Ia remnant expanding into a warm interstellar medium with n~1 /cm^3. In the future, finding more stars with the unique chemical signatures we highlight here will be important for constraining the rate and environments of sub-Chandrasekhar SNe Ia.

astro-ph.HE↗

arXiv2017

APOGEE Chemical Abundances of the Sagittarius Dwarf Galaxy

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides the opportunity to measure elemental abundances for C, N, O, Na, Mg, Al, Si, P, K, Ca, V, Cr, Mn, Fe, Co, and Ni in vast numbers of stars. We analyze the chemical abundance patterns of these elements for 158 red giant stars belonging to the Sagittarius dwarf galaxy (Sgr). This is the largest sample of Sgr stars with detailed chemical abundances and the first time C, N, P, K, V, Cr, Co, and Ni have been studied at high-resolution in this galaxy. We find that the Sgr stars with [Fe/H] $\gtrsim$ -0.8 are deficient in all elemental abundance ratios (expressed as [X/Fe]) relative to the Milky Way, suggesting that Sgr stars observed today were formed from gas that was less enriched by Type II SNe than stars formed in the Milky Way. By examining the relative deficiencies of the hydrostatic (O, Na, Mg, and Al) and explosive (Si, P, K, and Mn) elements, our analysis supports the argument that previous generations of Sgr stars were formed with a top-light IMF, one lacking the most massive stars that would normally pollute the ISM with the hydrostatic elements. We use a simple chemical evolution model, flexCE to further backup our claim and conclude that recent stellar generations of Fornax and the LMC could also have formed according to a top-light IMF.

astro-ph.GA↗

arXiv2017

High Resolution Spectroscopic Abundances of Red Giant Branch Stars in NGC 6681

We obtain high resolution spectra of nine red giant branch stars in NGC 6681 and perform the first detailed abundance analysis of stars in this cluster. We confirm cluster membership for these stars based on consistent radial velocities of 214.5$\pm$3.7 km/s and find a mean [Fe/H] = -1.63$\pm$0.07 dex and [α/Fe] = 0.42$\pm$0.11 dex. Additionally, we confirm the existence of a Na-O anti-correlation in NGC 6681 and identify two populations of stars with unique abundance trends. With the use of HST photometry from Sarajedini et al. (2007) and Piotto et al. (2015) we are able to identify these two populations as discrete sequences in the cluster CMD. Although we cannot confirm the nature of the polluter stars responsible for the abundance differences in these populations, these results do help put constraints on possible polluter candidates.

astro-ph.SR↗

arXiv2017

A Differential Abundance Analysis of Very Metal-Poor Stars

We have performed a differential, line-by-line, chemical abundance analysis, ultimately relative to the Sun, of nine very metal-poor main sequence halo stars, near [Fe/H]=$-$2 dex. Our abundances range from $-2.66\leq\mathrm{[Fe/H]}\leq-1.40$ dex with conservative uncertainties of 0.07 dex. We find an average [$α$/Fe]$=0.34\pm0.09$ dex, typical of the Milky Way. While our spectroscopic atmosphere parameters provide good agreement with HST parallaxes, there is significant disagreement with temperature and gravity parameters indicated by observed colors and theoretical isochrones. Although a systematic underestimate of the stellar temperature by a few hundred degrees could explain this difference, it is not supported by current effective temperature studies and would create large uncertainties in the abundance determinations. Both 1D and $\langle$3D$\rangle$ hydrodynamical models combined with separate 1D non-LTE effects do not yet account for the atmospheres of real metal-poor MS stars, but a fully 3D non-LTE treatment may be able to explain the ionization imbalance found in this work.

astro-ph.SR↗

arXiv2017

Chemical Abundances of Two Stars in the Large Magellanic Cloud Globular Cluster NGC~1718

Detailed chemical abundances of two stars in the intermediate-age Large Magellanic Cloud (LMC) globular cluster NGC~1718 are presented, based on high resolution spectroscopic observations with the MIKE spectrograph. The detailed abundances confirm NGC~1718 to be a fairly metal-rich cluster, with an average [Fe/H] ~ -0.55+/-0.01. The two red giants appear to have primordial O, Na, Mg, and Al abundances, with no convincing signs of a composition difference between the two stars---hence, based on these two stars, NGC~1718 shows no evidence for hosting multiple populations. The Mg abundance is lower than Milky Way field stars, but is similar to LMC field stars at the same metallicity. The previous claims of very low [Mg/Fe] in NGC~1718 are therefore not supported in this study. Other abundances (Si, Ca, Ti, V, Mn, Ni, Cu, Rb, Y, Zr, La, and Eu) all follow the LMC field star trend, demonstrating yet again that (for most elements) globular clusters trace the abundances of their host galaxy's field stars. Similar to the field stars, NGC~1718 is found to be mildly deficient in explosive $α$-elements, but moderately to strongly deficient in O, Na, Mg, Al, and Cu, elements which form during hydrostatic burning in massive stars. NGC~1718 is also enhanced in La, suggesting that it was enriched in ejecta from metal-poor AGB stars.

astro-ph.GA↗