Browsing by Author "Aguilera-Gomez, Claudia"

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    Are lithium-rich giants binaries? A radial velocity variability analysis of 1400 giants
    (2024) Castro-Tapia, Matias; Aguilera-Gomez, Claudia; Chaname, Julio
    Context. The existence of low-mass giants with large amounts of lithium (Li) in their surfaces has challenged stellar evolution for decades. One of the possibilities usually discussed in the literature to explain these Li-rich giants involves the interaction with a close binary companion, a scenario that predicts that, when compared against their non-enriched counterparts, Li-rich giants should preferentially be found as part of binary systems. Aims. We aim to assemble the largest possible sample of low-mass giants with well-measured Li abundances, to determine with high statistical significance the close binary fractions of Li-rich and Li-normal giants, and thus test the binary interaction scenario for the emergence of Li-rich giants. Methods. We developed a method that uses radial velocities (RVs) at three different epochs to quantify the degree of RV variability, which we used as a proxy for the presence of a close binary companion. The method was tested and calibrated against samples of known RV standard stars and known spectroscopic binaries. We then assembled a sample of 1418 giants with available RVs from RAVE, GALAH, and Gaia, as well as stellar parameters and Li abundances from GALAH, to which we applied our variability classification. We could determine an evolutionary state for 1030 of these giants. We also compared the results of our RV variability analysis with binarity indicators from the Gaia mission. Results. When applying our methodology to the control samples, we found that the accuracy of the classification is controlled by the precision of the RVs used in the analysis. For the set of RVs available for the giants, this accuracy is 80-85%. Consistent with seismic studies, the resulting sample of giants contains a fraction of Li-rich objects in the red clump (RC) that is twice as large as that in the first ascent red giant branch (RGB). Among RC giants, the fractions of Li-rich objects with a high RV variability and with no RV variability are the same as those for Li-normal objects, but we find some evidence that these fractions may be different for giants in the first-ascent RGB. Analysis of binary indicators in Gaia DR3 shows a smaller fraction of binary giants than our criteria, but no relation can be seen between Li enrichment and binarity either. Conclusions. Our RV variability analysis indicates that there is no preference for Li-rich giants in the RC to be part of binary systems, thus arguing against a binary interaction scenario for the genesis of the bulk of Li-rich giants at that evolutionary stage. On the other hand, Li-rich giants in the RGB appear to have a small but measurable preference for having close companions, something that deserves further scrutiny with more and better data. Additional measurements of the RVs of these giants at a higher RV precision would greatly help in confirming and more robustly quantifying these results.
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    Assembling a high-precision abundance catalogue of solar twins in GALAH for phylogenetic studies
    (2024) Walsen, Kurt; Jofre, Paula; Buder, Sven; Yaxley, Keaghan; Das, Payel; Yates, Robert M.; Hua, Xia; Signor, Theosamuele; Eldridge, Camilla; Rojas-Arriagada, Alvaro; Tissera, Patricia B.; Johnston, Evelyn; Aguilera-Gomez, Claudia; Zoccali, Manuela; Gilmore, Gerry; Foley, Robert
    Stellar chemical abundances have proved themselves a key source of information for understanding the evolution of the Milky Way, and the scale of major stellar surveys such as GALAH have massively increased the amount of chemical data available. However, progress is hampered by the level of precision in chemical abundance data as well as the visualization methods for comparing the multidimensional outputs of chemical evolution models to stellar abundance data. Machine learning methods have greatly improved the former; while the application of tree-building or phylogenetic methods borrowed from biology are beginning to show promise with the latter. Here, we analyse a sample of GALAH solar twins to address these issues. We apply The Cannon algorithm to generate a catalogue of about 40 000 solar twins with 14 high precision abundances which we use to perform a phylogenetic analysis on a selection of stars that have two different ranges of eccentricities. From our analyses, we are able to find a group with mostly stars on circular orbits and some old stars with eccentric orbits whose age-[Y/Mg] relation agrees remarkably well with the chemical clocks published by previous high precision abundance studies. Our results show the power of combining survey data with machine learning and phylogenetics to reconstruct the history of the Milky Way.
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    Evidence of extra mixing in field giants as traced by the lithium and carbon isotope ratio
    (2023) Aguilera-Gomez, Claudia; Jones, Matias I.; Chaname, Julio
    Context. Although not predicted by standard stellar evolution, the surface abundance of light elements, such as lithium (Li), carbon, and nitrogen, changes during the red giant branch (RGB) as a result of extra mixing. This is usually associated with thermohaline mixing acting after the RGB bump. Peculiar Li-enriched RGB stars might also be related to either enhanced mixing or pollution from external sources.Aims. We measure the Li abundance and carbon isotopic ratio C-12/C-13 in a sample of 166 field red giants with -0.3 <= [Fe/H] <= 0.2, targeted by the EXPRESS radial velocity program to analyze the effects of extra mixing.Methods. We measured the abundances with spectral synthesis using high-quality spectra. Multiple-epoch observations needed for exoplanet detection were used to decrease the effects of telluric contamination in C-12/C-13 measurements.Results. Due to the prevalence of upper limits, the Li abundance pattern is complicated to interpret, but the comparison between RGB and core He-burning giants shows effects of mixing consistent with thermohaline. The most Li-enriched giant in the sample, classified as a RGB star close to the RGB bump, has low C-12/C-13. Given that the C-12/C-13 should not be affected by planet engulfment, this does not seem to be the source of the high Li. There is a decreasing correlation between mass and C-12/C-13 in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also show a correlation between C-12/C-13 and [Fe/H]. There is no evident impact of binarity either on Li or on C-12/C-13.Conclusions. Our sample shows behavior consistent with additional mixing acting after the RGB bump. The C-12/C-13 adds new clues which can be used to describe extra mixing, and it could well be the best tool to study mixing in giants. Additional measurements of C-12/C-13 in field stars would greatly improve our ability to compare data with models and understand mixing mechanisms.
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    Lithium in Kepler Red Giants: Defining Normal and Anomalous
    (2023) Tayar, Jamie; Carlberg, Joleen K.; Aguilera-Gomez, Claudia; Sayeed, Maryum
    The orders-of-magnitude variations in the lithium abundances of evolved stars have long been a puzzle. Diluted signals, ambiguous evolutionary states, and unknown masses have made it challenging to both map the expected lithium signals and explain the anomalously lithium-rich stars. Using a set of asteroseismically characterized evolved stars, we show here that the base lithium abundance in red giant stars is mass-dependent, with higher-mass stars having higher "normal" lithium abundances, while highly lithium-enhanced stars may cluster around 0.8 or 1.8 M (☉). We confirm previous studies that have shown that lithium enhancement and rapid rotation are often coincident but find that the actual correlation between lithium abundance and rotation rate, whether surface, internal, or radial differential rotation, is weak. Our data support previous assertions that most lithium-rich giants are in the core-helium-burning phase. We also note a tentative correlation between the highest lithium abundances and unusual carbon-to-nitrogen ratios, which is suggestive of binary interactions, though we find no simple correlation between lithium richness and indicators of binarity.
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    Mass Matters: No Evidence for Ubiquitous Lithium Production in Low-mass Clump Giants
    (2022) Chaname, Julio; Pinsonneault, Marc H.; Aguilera-Gomez, Claudia; Zinn, Joel C.
    Known sources of lithium (Li) in the universe include the Big Bang, novae, asymptotic giant branch stars, and cosmic-ray spallation. During their longer-lived evolutionary phases, stars are not expected to add to the Li budget of the Galaxy, but to largely deplete it. In this context, recent analyses of Li data from GALAH and LAMOST for field red clump (RC) stars have concluded that there is the need for a new production channel of Li, ubiquitous among low-mass stars, and that would be triggered on the upper red giant branch (RGB) or at helium ignition. This is distinct from the Li-rich giant problem and reflects bulk RC star properties. We provide an analysis of the GALAH Li data that accounts for the distribution of progenitor masses of field RC stars observed today. Such progenitors are different than today's field RGB stars. Using standard post-main-sequence stellar evolution, we show that the distribution of Li among field RC giants as observed by GALAH is consistent with standard model predictions, and does not require new Li production mechanisms. Our model predicts a large fraction of very low Li abundances from low-mass progenitors, with higher abundances from higher mass ones. Moreover, there should be a large number of upper limits for RC giants, and higher abundances should correspond to higher masses. The most recent GALAH data indeed confirm the presence of large numbers of upper limits, and a much lower mean Li abundance in RC stars, in concordance with our interpretation.
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    On the Evolutionary History of a Simulated Disk Galaxy as Seen by Phylogenetic Trees
    (2024) Silva, Danielle de Brito; Jofre, Paula; Tissera, Patricia B.; Yaxley, Keaghan J.; Jara, Jenny Gonzalez; Eldridge, Camilla J. L.; Sillero, Emanuel; Yates, Robert M.; Hua, Xia; Das, Payel; Aguilera-Gomez, Claudia; Johnston, Evelyn J.; Rojas-Arriagada, Alvaro; Foley, Robert; Gilmore, Gerard
    Phylogenetic methods have long been used in biology and more recently have been extended to other fields-for example, linguistics and technology-to study evolutionary histories. Galaxies also have an evolutionary history and fall within this broad phylogenetic framework. Under the hypothesis that chemical abundances can be used as a proxy for the interstellar medium's DNA, phylogenetic methods allow us to reconstruct hierarchical similarities and differences among stars-essentially, a tree of evolutionary relationships and thus history. In this work, we apply phylogenetic methods to a simulated disk galaxy obtained with a chemodynamical code to test the approach. We found that at least 100 stellar particles are required to reliably portray the evolutionary history of a selected stellar population in this simulation, and that the overall evolutionary history is reliably preserved when the typical uncertainties in the chemical abundances are smaller than 0.08 dex. The results show that the shapes of the trees are strongly affected by the age-metallicity relation, as well as the star formation history of the galaxy. We found that regions with low star formation rates produce shorter trees than regions with high star formation rates. Our analysis demonstrates that phylogenetic methods can shed light on the process of galaxy evolution.