arXiv subjects

Amanda I. Karakas

Publications and source records attributed to Amanda I. Karakas.

At least 37 records · Page 2Linked to original sources

arXiv2022

Peeking beneath the precision floor I: metallicity spreads and multiple elemental dispersions in the globular clusters NGC 288 and NGC 362

The view of globular clusters (GCs) as simple systems continues to unravel, revealing complex objects hosting multiple chemical peculiarities. Using differential abundance analysis, we probe the chemistry of the Type I GC, NGC 288 and the Type II GC, NGC 362 at the 2\% level for the first time. We measure 20 elements and find differential measurement uncertainties on the order 0.01-0.02 dex in both clusters. The smallest uncertainties are measured for Fe I in both clusters, with an average uncertainty of $\sim$0.013 dex. Dispersion in the abundances of Na, Al, Ti I, Ni, Fe I, Y, Zr, Ba and Nd are recovered in NGC 288, none of which can be explained by a spread in He. This is the first time, to our knowledge, a statistically significant spread in $s$-process elements and a potential spread in metallicity has been detected in NGC 288. In NGC 362, we find significant dispersion in the same elements as NGC 288, with the addition of Co, Cu, Zn, Sr, La, Ce, and Eu. Two distinct groups are recovered in NGC 362, separated by 0.3 dex in average differential $s$-process abundances. Given strong correlations between Al and several $s$-process elements, and a significant correlation between Mg and Si, we propose that the $s$-process rich group is younger. This agrees with asymptotic giant branch star (AGB) enrichment between generations, if there is overlap between low- and intermediate-mass AGBs. In our scenario, the older population is dominated by the $r$-process with a $Δ^{\mathrm{La}}-Δ^{\mathrm{Eu}}$ ratio of $-0.16\pm0.06$. We propose that the $r$-process dominance and dispersion found in NGC 362 are primordial.

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arXiv2022

Bridging the Gap between Intermediate and Massive Stars I: Validation of MESA against the State-of-the-Art Monash Stellar Evolution Program for a 2$M_{\odot}$ AGB Star

One--dimensional stellar structure and evolution programs are built using different physical prescriptions and algorithms, which means there can be variations between models' predictions even when using identical input physics. This leads to questions about whether such deviations are physical or numerical; code validation studies are important and necessary tools for studying these questions. We provide the first direct comparison between the Monash stellar evolution program and MESA for a $2M_{\odot}$ model evolved from the zero-age main sequence to the tip of the thermally pulsing asymptotic giant branch. We compare the internal structure of the two models at six critical evolutionary points and find that they are in excellent agreement in characteristics like central temperature, central density and the temperature at the base of the convective envelope during the thermally pulsing asymptotic giant branch. The hydrogen-exhausted core mass between the models differs by less than 4.2% throughout the entire evolution, the final values vary only by 1.5%. Surface quantities such as luminosity and radius vary by less than 0.2% prior to the asymptotic giant branch. During thermal pulses, the difference extends to 3.4%, largely due to uncertainties in mixing and the treatment of atmospheric boundary conditions. Given that the veteran Monash code is closed source, the present work provides the first fully open-source computational analog. This increases accessibility to precision modeling on the asymptotic giant branch and lays the groundwork for higher-mass calculations that are performed with MESA but preserve the standards of the Monash code during the AGB.

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arXiv2022

Viability of novae as sources of Galactic lithium

Of all the light elements, the evolution of lithium (Li) in the Milky Way is perhaps the most difficult to explain. Li is difficult to synthesize and is easily destroyed, making most stellar sites unsuitable for producing Li in sufficient quantities to account for the proto-solar abundance. For decades, novae have been proposed as a potential explanation to this 'Galactic Li problem', and the recent detection of 7Be in the ejecta of multiple nova eruptions has breathed new life into this theory. In this work, we assess the viability of novae as dominant producers of Li in the Milky Way. We present the most comprehensive treatment of novae in a galactic chemical evolution code to date, testing theoretical- and observationally-derived nova Li yields by integrating metallicity-dependent nova ejecta profiles computed using the binary population synthesis code binary c with the galactic chemical evolution code OMEGA+. We find that our galactic chemical evolution models which use observationally-derived Li yields account for the proto-solar Li abundance very well, while models relying on theoretical nova yields cannot reproduce the proto-solar observation. A brief exploration of physical uncertainties including single-stellar yields, the metallicity resolution of our nova treatment, common-envelope physics, and nova accretion efficiencies indicates that this result is robust to physical assumptions. Scatter within the observationally-derived Li yields in novae is identified as the primary source of uncertainty, motivating further observations of 7Be in nova ejecta.

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arXiv2021

The most metal-rich stars in the universe: chemical contributions of low and intermediate mass asymptotic giant branch stars with metallicities between $0.04 \leq Z \leq 0.10.$

Low and intermediate mass stars with super solar metallicities comprise a known portion of the universe. Yet yields for asymptotic giant branch (AGB) stars with metallicities greater than $Z=0.04$ do not exist in the literature. This contributes a significant uncertainty to galactic chemical evolution simulations. We present stellar yields of AGB stars for $M=1-8$$M_\odot$ and $Z=0.04-0.10$. We also weight these yields to represent the chemical contribution of a metal-rich stellar population. We find that as metallicity increases, the efficiency of the mixing episodes (known as the third dredge up) on the thermally pulsing AGB (TP-AGB) decrease significantly. Consequently, much of the nucleosynthesis that occurs on the TP-AGB is not represented on the surface of very metal-rich stars. It instead remains locked inside the white dwarf remnant. The temperatures at the base of the convective envelope also decrease with increasing metallicity. For the intermediate mass models, this results in the occurrence of only partial hydrogen burning at this location, if any burning at all. We also investigate heavy element production via the slow neutron capture process (s-process) for three 6$M_\odot$ models: $Z=0.04, 0.05$ and $0.06$. There is minor production at the first s-process peak at strontium, which decreases sharply with increasing metallicity. We find the chemical contributions of our models are dominated by proton capture nucleosynthesis, mixed to the surface during first and second dredge up events. This conclusion is mirrored in our stellar population yields, weighted towards the lower mass regime to reflect the mass distribution within a respective galaxy.

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arXiv2021

The impact of metallicity on nova populations

The metallicity of a star affects its evolution in a variety of ways, changing stellar radii, luminosities, lifetimes, and remnant properties. In this work, we use the population synthesis code binary_c to study how metallicity affects novae in the context of binary stellar evolution. We compute a 16-point grid of metallicities ranging from $Z=10^{-4}$ to 0.03, presenting distributions of nova white dwarf masses, accretion rates, delay-times, and initial system properties at the two extremes of our 16-point metallicity grid. We find a clear anti-correlation between metallicity and the number of novae produced, with the number of novae at $Z=0.03$ roughly half that at $Z=10^{-4}$. The white dwarf mass distribution has a strong systematic variation with metallicity, while the shape of the accretion rate distribution is relatively insensitive. We compute a current nova rate of approximately 33 novae per year for the Milky Way, a result consistent with observational estimates relying on extra-Galactic novae but an under-prediction relative to observational estimates relying on Galactic novae. However, the shape of our predicted Galactic white dwarf mass distribution differs significantly to existing observationally derived distributions, likely due to our underlying physical assumptions. In M31, we compute a current nova rate of approximately 36 novae per year, under-predicting the most recent observational estimate of $65^{+15}_{-16}$. Finally, we conclude that when making predictions about currently observable nova rates in spiral galaxies, or stellar environments where star formation has ceased in the distant past, metallicity can likely be considered of secondary importance compared to uncertainties in binary stellar evolution.

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arXiv2021

The stellar beta-decay rate of 134Cs and its impact on the Barium nucleosynthesis in the s process

We have calculated the stellar $β$-decay rate of the important s-process branching point ${}^{134}$Cs based on the state of the art shell model calculations. At typical $s$-process temperatures ($T\sim$ 0.2-0.3 GK), our new rate is one order of magnitude lower than the widely-used rate from Takahashi and Yokoi (hereafter TY87). The impact on the nucleosynthesis in AGB stars is investigated with various masses and metallicities. Our new decay rate leads to an overall decrease in the ${}^{134}$Ba/${}^{136}$Ba ratio, and well explains the measured ratio in meteorities without introducing the $i$ process. We also derive the elapsed time from the last AGB nucleosynthetic event that polluted the early Solar System to be $>$28 Myr based on the ${}^{135}$Cs/${}^{133}$Cs ratio, which is consistent with the elapsed times derived from ${}^{107}$Pd and ${}^{182}$Hf. The $s$-process abundance sum of ${}^{135}$Ba and ${}^{135}$Cs is found to increase, resulting in a smaller $r$-process contribution of ${}^{135}$Ba in the Solar System.

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arXiv2021

Population synthesis of accreting white dwarfs: Rates and evolutionary pathways of H and He novae

Novae are some of the most commonly detected optical transients and have the potential to provide valuable information about binary evolution. Binary population synthesis codes have emerged as the most effective tool for modelling populations of binary systems, but such codes have traditionally employed greatly simplified nova physics, precluding detailed study. In this work, we implement a model treating H and He novae as individual events into the binary population synthesis code \binaryc. This treatment of novae represents a significant improvement on the `averaging' treatment currently employed in modern population synthesis codes. We discuss the evolutionary pathways leading to these phenomena and present nova event rates and distributions of several important physical parameters. Most novae are produced on massive white dwarfs, with approximately 70 and 55 per cent of nova events occurring on O/Ne white dwarfs for H and He novae respectively. Only 15 per cent of H-nova systems undergo a common-envelope phase, but these systems are responsible for the majority of H nova events. All He-accreting He-nova systems are considered post-common-envelope systems, and almost all will merge with their donor star in a gravitational-wave driven inspiral. We estimate the current annual rate of novae in M31 (Andromeda) to be approximately $41 \pm 4$ for H novae, underpredicting the current observational estimate of $65^{+15}_{-16}$, and $0.14\pm0.015$ for He novae. When varying common-envelope parameters, the H nova rate varies between 20 and 80 events per year.

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arXiv2021

The GALAH survey: tracing the Galactic disk with Open Clusters

Open clusters are unique tracers of the history of our own Galaxy's disk. According to our membership analysis based on \textit{Gaia} astrometry, out of the 226 potential clusters falling in the footprint of GALAH or APOGEE, we find that 205 have secure members that were observed by at least one of the survey. Furthermore, members of 134 clusters have high-quality spectroscopic data that we use to determine their chemical composition. We leverage this information to study the chemical distribution throughout the Galactic disk of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is $-$0.076$\pm$0.009 dex kpc$^{-1}$, which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the [Fe/H] - guiding radius (r$_{\rm guid}$) plane is $-$0.073$\pm$0.008 dex kpc$^{-1}$. We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disk differently than field stars. In particular, at given radius, open clusters show an age-metallicity relation that has less scatter than field stars. As such scatter is often interpreted as an effect of radial migration, we suggest that these differences are due to the physical selection effect imposed by our Galaxy: clusters that would have migrated significantly also had higher chances to get destroyed. Finally, our results reveal trends in the [X/Fe]$-$r$_{\rm guid}$$-$age space, which are important to understand production rates of different elements as a function of space and time.

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arXiv2021

Explosive nucleosynthesis of a metal-deficient star as the source of a distinct odd-even effect in the solar twin HIP 11915

The abundance patterns observed in the Sun and in metal-poor stars show a clear odd-even effect. An important question is whether the odd-even effect in solar-metallicity stars is similar to the Sun, or if there are variations that can tell us about different chemical enrichment histories. In this work, we report for the first time observational evidence of a differential odd-even effect in the solar twin HIP 11915, relative to the solar odd-even abundance pattern. The spectra of this star were obtained with high resolving power (140 000) and signal-to-noise ratio ($\sim$420) using the ESPRESSO spectrograph and the VLT telescope. Thanks to the high spectral quality, we obtained extremely precise stellar parameters ($σ(T_{\rm eff})$ = 2 K, $σ(\rm{[Fe/H]})$ = 0.003 dex, and $σ(\log g)$ = 0.008 dex). We determine the chemical abundance of 20 elements ($Z\leq39$) with high precision ($\sim$0.01 dex), which shows a strong pattern of the odd-even effect even after performing Galactic Chemical Evolution corrections. The odd-even effect is reasonably well-reproduced by a core-collapse supernova of 13 $\rm{M_{\odot}}$ and metallicity Z = 0.001 diluted into a metal-poor gas of 1 $\rm{M_{\odot}}$. Our results indicate that HIP 11915 has an odd-even effect slightly different than the Sun, thus confirming a different supernova enrichment history.

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arXiv2020

Detection of Pb II in the Ultraviolet Spectra of Three Metal-Poor Stars

We report the first detection of the Pb II line at 2203.534 Angstroms in three metal-poor stars, using ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We perform a standard abundance analysis assuming local thermodynamic equilibrium (LTE) to derive lead (Pb, Z=82) abundances. The Pb II line yields a higher abundance than Pb I lines by +0.36 +/- 0.34 dex and +0.49 +/- 0.28 dex in the stars HD 94028 and HD 196944, where Pb I lines had been detected previously. The Pb II line is likely formed in LTE, and these offsets affirm previous calculations showing that Pb I lines commonly used as abundance indicators underestimate the Pb abundance in LTE. Pb is enhanced in the s-process-enriched stars HD 94028 ([Pb/Fe] = +0.95 +/- 0.14) and HD 196944 ([Pb/Fe] = +2.28 +/- 0.23), and we show that Pb-208 is the dominant Pb isotope in these two stars. The log epsilon(Pb/Eu) ratio in the r-process-enhanced star HD 222925 is 0.76 +/- 0.14, which matches the Solar System r-process ratio and indicates that the Solar System r-process residuals for Pb are, in aggregate, correct. The Th/Pb chronometer in HD 222925 yields an age of 8.2 +/- 5.8 Gyr, and we highlight the potential of the Th/Pb chronometer as a relatively model-insensitive age indicator in r-process-enhanced stars.

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arXiv2020

The Origin of Elements from Carbon to Uranium

To reach a deeper understanding of the origin of elements in the periodic table, we construct Galactic chemical evolution (GCE) models for all stable elements from C (A=12) to U (A=238) from first principles, i.e., using theoretical nucleosynthesis yields and event rates of all chemical enrichment sources. This enables us to predict the origin of elements as a function of time and environment. In the solar neighborhood, we find that stars with initial masses of M>30M_\odot can become failed supernovae if there is a significant contribution from hypernovae (HNe) at M~20-50M_\odot. The contribution to GCE from super asymptotic giant branch (AGB) stars (with M~8-10M_\odot at solar metallicity) is negligible, unless hybrid white dwarfs from low-mass super-AGB stars explode as so-called Type Iax supernovae, or high-mass super-AGB stars explode as electron-capture supernovae (ECSNe). Among neutron-capture elements, the observed abundances of the second (Ba) and third (Pb) peak elements are well reproduced with our updated yields of the slow neutron-capture process (s-process) from AGB stars. The first peak elements, Sr, Y, and Zr, are sufficiently produced by ECSNe together with AGB stars. Neutron star mergers can produce rapid neutron-capture process (r-process) elements up to Th and U, but the timescales are too long to explain observations at low metallicities. The observed evolutionary trends, such as for Eu, can well be explained if ~3% of 25-50 M_\odot hypernovae are magneto-rotational supernovae producing r-process elements. Along with the solar neighborhood, we also predict the evolutionary trends in the halo, bulge, and thick disk for future comparison with galactic archaeology surveys.

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arXiv2020

Origin of large meteoritic SiC stardust grains in metal-rich AGB stars

Stardust grains that originated in ancient stars and supernovae are recovered from meteorites and carry the detailed composition of their astronomical sites of origin. We present evidence that the majority of large ($μ$m-sized) meteoritic silicon carbide (SiC) grains formed in C-rich asymptotic giant branch (AGB) stars that were more metal-rich than the Sun. In the framework of the slow neutron-captures (the s process) that occurs in AGB stars the lower-than-solar 88Sr/86Sr isotopic ratios measured in the large SiC grains can only be accompanied by Ce/Y elemental ratios that are also lower than solar, and predominately observed in metal-rich barium stars - the binary companions of AGB stars. Such an origin suggests that these large grains represent the material from high-metallicity AGB stars needed to explain the s-process nucleosynthesis variations observed in bulk meteorites (Ek et al. 2020). In the outflows of metal-rich, C-rich AGB stars SiC grains are predicted to be small ($\simeq$ 0.2 $μ$m-sized); large ($\simeq$ $μ$m-sized) SiC grains can grow if the number of dust seeds is two to three orders of magnitude lower than the standard value of $10^{-13}$ times the number of H atoms. We therefore predict that with increasing metallicity the number of dust seeds might decrease, resulting in the production of larger SiC grains.

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arXiv2020

The Chemical Evolution of Iron-Peak Elements with Hypernovae

We calculate the mean evolution of the iron-peak abundance ratios [(Cr,Mn,Co,Zn)/Fe] in the Galaxy, using modern supernova and hypernova chemical yields and a Galactic Chemical Evolution code that assumes homogeneous chemical evolution. We investigate a range of hypernova occurrence rates and are able to produce a chemical composition that is a reasonable fit to the observed values in metal-poor stars. This requires a hypernova occurence rate that is large (50%) in the early Universe, decreasing throughout evolution to a value that is within present day observational constraints (>~ 1%). A large hypernova occurence rate is beneficial to matching the high [Zn/Fe] observed in the most metal-poor stars, although including hypernovae with progenitor mass >= 60 solar masses is detrimental to matching the observed [(Mn,Co)/Fe] evolution at low [Fe/H]. A significant contribution from HNe seems to be critical for producing supersolar [(Co,Zn)/Fe] at low metallicity, though more work will need to be done in order to match the most extreme values. We also emphasise the need to update models for the enrichment sources at higher metallicity, as the satisfactory recovery of the solar values of [(Cr,Mn,Co,Zn)/Fe] still presents a challenge.

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arXiv2020

How magnetic activity alters what we learn from stellar spectra

Magnetic fields and stellar spots can alter the equivalent widths of absorption lines in stellar spectra, varying during the activity cycle. This also influences the information that we derive through spectroscopic analysis. In this study we analyse high-resolution spectra of 211 Sun-like stars observed at different phases of their activity cycles, in order to investigate how stellar activity affects the spectroscopic determination of stellar parameters and chemical abundances. We observe that equivalent widths of lines can increase as a function of the activity index log R$^\prime_{\rm HK}$ during the stellar cycle, which also produces an artificial growth of the stellar microturbulence and a decrease in effective temperature and metallicity. This effect is visible for stars with activity indexes log R$^\prime_{\rm HK}$$\geq$$-$5.0 (i.e., younger than 4-5 Gyr) and it is more significant at higher activity levels. These results have fundamental implications on several topics in astrophysics that are discussed in the paper, including stellar nucleosynthesis, chemical tagging, the study of Galactic chemical evolution, chemically anomalous stars, the structure of the Milky Way disk, stellar formation rates, photoevaporation of circumstellar disks, and planet hunting.

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arXiv2019

The chemical signatures of planetary engulfment events in binary systems

Planetary engulfment events involve the chemical assimilation of a planet into a star's external layer. This can cause a change in the chemical pattern of the stellar atmosphere in a way that mirrors the composition of the rocky object engulfed, with the refractory elements being more abundant than the volatiles. Due to these stellar chemical changes, planetary engulfment events can render the process of chemical tagging potentially inaccurate. A line-by-line differential analysis of twin stars in wide binary systems allows us to test the chemical homogeneity of these associations with typical individual stellar Fe I uncertainties of 0.01 dex and eventually unveil chemical anomalies that could be attributed to planetary engulfment events. Out of the 14 systems analysed here, we report the discovery of the most chemically inhomogeneous system to date (HIP34407/HIP34426). The median difference in abundances of refractory elements within the pair is 0.19 dex and the trend between the differential abundances and condensation temperature suggests that the anomaly is likely due to a planetary engulfment event. Within our sample, five other chemically anomalous systems are found.

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arXiv2019

Learning about the intermediate neutron-capture process from lead abundances

Lead (Pb) is predominantly produced by the slow neutron-capture process (s process) in asymptotic giant branch (AGB) stars. In contrast to significantly enhanced Pb abundances predicted by low-mass, low-metallicity AGB-models, observations of Magellanic post-AGB stars show incompatibly low Pb abundances. Observations of carbon-enhanced metal-poor (CEMP) stars whose s-process enrichments are accompanied by heavy elements traditionally associated with the rapid neutron-capture process (r process) have raised the need for a neutron-capture process operating at neutron densities intermediate to the s and r process: the so-called i process. We study i-process nucleosynthesis with single-zone nuclear-network calculations. Our i-process models can explain the heavy-element abundance patterns measured in Magellanic post-AGB stars including their puzzlingly low Pb abundances. Furthermore, the heavy-element enhancements in the post-AGB and CEMP-i stars, particularly their Pb abundance, allow us to characterise the neutron densities and exposures of the i process that produced the observed abundance patterns. We find that the lower-metallicity CEMP-i stars ($\left[ \mathrm{Fe} / \mathrm{H} \right] \approx -2.5$) have heavy-element abundances best matched by models with higher neutron densities and exposures ($τ> 2.0 \, \mathrm{mbarn}^{-1}$) compared to the higher-metallicity post-AGB stars ($\left[ \mathrm{Fe} / \mathrm{H} \right] \approx -1.3$, $τ< 1.3 \, \mathrm{mbarn}^{-1}$). This offers new constraints and insights regarding the properties of i-process sites and demonstrates that the responsible process operates on time scales of the order of a few years or less.

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arXiv2019

Tidal interactions between binary stars drives lithium production in low-mass red giants

Theoretical models of stellar evolution predict that most of the lithium inside a star is destroyed as the star becomes a red giant. However, observations reveal that about 1% of red giants are peculiarly rich in lithium, often exceeding the amount in the interstellar medium or predicted from the Big Bang. With only about 150 lithium-rich giants discovered in the past four decades, and no distinguishing properties other than lithium enhancement, the origin of lithium-rich giant stars is one of the oldest problems in stellar astrophysics. Here we report the discovery of 2,330 low-mass (1 to 3$\,M_\odot$) lithium-rich giant stars, which we argue are consistent with internal lithium production that is driven by tidal spin-up by a binary companion. Our sample reveals that most lithium-rich giants have helium-burning cores ($80^{+7}_{-6}\%$), and that the frequency of lithium-rich giants rises with increasing stellar metallicity. We find that while planet accretion may explain some lithium-rich giants, it cannot account for the majority that have helium-burning cores. We rule out most other proposed explanations as the primary mechanism for lithium-rich giants, including all stages related to single star evolution. Our analysis shows that giants remain lithium-rich for only about two million years. A prediction from this lithium depletion timescale is that most lithium-rich giants with a helium-burning core have a binary companion.

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arXiv2018

Thermohaline mixing in extremely metal-poor stars

Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early Universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well-studied, they make up less than half of all EMP stars with [Fe/H] $\sim -3.0$. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offfsets are derived) is very important, however the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coeffcient. Our stellar evolution models utilise a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] $= -3$ and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.

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