arXiv subjects

T. Battich

Publications and source records attributed to T. Battich.

7 recordsLinked to original sources

arXiv2026

Evidence for neutron capture in heavy-metal hot subdwarfs: Far-UV spectroscopy of EC22536-5304 and LSIV-14 116

Most hot subdwarfs (sdO/B) are low-mass core-helium-burning stars formed through binary interaction. A subgroup of intermediate He-rich sdOBs shows extreme heavy-metal (Z>30) enrichments exceeding $10^4$ times solar, especially in Zr or Pb. We analyse the first ultraviolet spectra of the "heavy metal" subdwarfs LSIV-14 116 (Zr-rich) and EC22536-5304 (Pb-rich) to determine their abundance patterns and test nucleosynthesis models. Both stars show exceptionally rich heavy-element spectra dominated by ions in stages III-VI, many absent from standard line lists. We compiled literature energy levels, wavelengths, and oscillator strengths and implemented them in the SYNSPEC code. In addition, we computed new oscillator strengths for As III, Se III, Hf IV, and Tl IV. New photoionisation cross-sections for Pb III-VI enabled the first non-LTE models of multiply ionised Pb. In LSIV-14 116 we detect 16 light and 24 heavy metals (Ga-Bi); Br, Nb, Mo, Pd, In, Sb, Te, and Xe are measured in an sdO/B star for the first time. In EC22536-5304 13 light and 26 heavy metals are detected, including first detections of La, Ce, Pr, Nd, Er, Yb, Lu, Hf, Ta, W, Os, Pt, Hg, Tl, and Bi. LSIV-14 116 peaks at ~4.3 dex for Sr-Sn relative to solar, declining to 3.1 dex at Pb and 2.3 dex at Bi, whereas EC22536-5304 reaches 6.2 dex for Pb and 5.4 dex for Bi. Both stars are Fe-poor. The abundance patterns cannot be explained by atomic diffusion alone and retain a clear nucleosynthetic signature. EC22536-5304 closely matches predictions of i-process nucleosynthesis, providing strong evidence for i-process self-enrichment in hot subdwarfs. EC22536-5304 likely formed via Roche-lobe overflow, whereas LSIV-14 116 likely originated from the merger of two low-mass white dwarfs, which may explain differences in its enrichment pattern. These results suggest that heavy metals in other He-sdO/Bs may also be self-synthesised.

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arXiv2025

The $i$-processes nucleosynthesis during the formation of He-rich hot-subdwarf stars

It has been shown that proton ingestion episodes can happen in the formation of hot-subdwarf stars, and that neutron-capture processes are possible in those cases. Moreover, some helium-rich hot subdwarfs display extraordinarily high abundances of heavy elements such as Zr, Yr and Pb on their surfaces. We explore under which conditions neutron-capture processes can occur in late helium core flashes, i.e. those occurring in the cores of stripped red-giant stars. We compute evolutionary models through the helium core flash and the subsequent hydrogen ingestion episode in stripped red giant stars. Stellar structure models are then used in post-processing to compute the detailed evolution of neutron-capture elements. We find that for metallicities of $10^{-3}$ and below, neutron densities can be as high as $10^{15}\,$cm$^{-3}$ and intermediate neutron capture processes occur in some of our models. The results depend very strongly on the H-envelope mass that survives after the stripping. Interestingly, we find that computed abundances in some of our models closely match the element abundances up to tin observed for EC 22536-5304, the only well-studied star for which the hot-flasher scenario assumed in our models is the most likely evolutionary path. Intermediate neutron capture processes can occur in the He-core flash experienced by the cores of some stripped red giants, and might be connected to the abundances of heavy elements observed in some helium-rich hot-subdwarf stars. The agreement between the observed abundances in EC 22536-5304 and those of our models offers support to our nucleosynthesis calculations. Moreover, if confirmed, the idea that heavy element abundances retain signatures of the different evolutionary channels opens the possibility that heavy element abundances in iHe-sdOB stars can be used to infer their evolutionary origin.

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arXiv2023

A self-synthesized origin of heavy metals in hot subdwarf stars?

Some He-rich hot subdwarf stars (He-sdOBs) present high abundances of trans-iron elements, such as Sr, Y, Zr and Pb. Diffusion processes are important in hot subdwarf stars, and it is thought that the high abundances of heavy elements in these stars are due to the action of radiative levitation. However, during the formation of He-sdOBs, hydrogen can be ingested into the convective zone driven by the He-core flash. It is known that episodes in which protons are being ingested into He-burning convective zones can lead to neutron-capture processes and the formation of heavy elements. In this work we aim to explore for the first time if neutron-capture processes can occur in late He-core flashes happening in the cores of the progenitors of He-sdOBs. We compute a detailed evolutionary model of a stripped red-giant star with a stellar evolution code with a nuclear network comprising 32 isotopes. Then we post-process the stellar models in the phase of He and H burning with a post-processing nucleosynthesis code with a nuclear network of 1190 species that allows us to follow the neutron-capture processes in detail. We find the occurrence of neutron-capture processes in our model, with neutron densities reaching a value of $\sim5\times10^{12}\,{\rm cm}^{-3}$. We find that the trans-iron elements are enhanced in the surface by 1 to 2 dex as compared to initial compositions. Moreover, the relative abundance pattern $[{\rm X}_i/\rm{Fe}]$ produced by neutron-capture processes closely resembles those observed in some He-sdOBs, hinting at a possible self-synthesized origin of the heavy elements in these stars. We conclude that intermediate neutron-capture processes can occur during a proton ingestion event in the He-core flash of stripped red-giant stars. This mechanism offers a natural channel to produce the heavy elements observed in some of the He-sdOBs.

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arXiv2023

Uncertainties in the 12C+12C reaction rate and their impact on the composition of ultra-massive WDs

Stars with initial masses 7 Msun . MZAMS . 9 Msun reach temperatures high enough to ignite C under degenerate conditions after the end of He-core burning (Garcia-Berro & Iben 1994). These isolated stars are expected to evolve into the so-called super AGB (SAGB) phase and may end their lives as ultra-massive ONe WDs (see Siess 2006,2007, 2010; Camisassa et al. 2019, and references therein). The exact proportions of O and Ne found in the core at the end of the SAGB phase will determine the cooling times and pulsational properties of these WDs. Uncertainties affecting the rates of nuclear reactions occurring during the C burning phase should have a measurable impact on the distribution of 16O, 20Ne, 23Na and 24Mg and, consequently, on the evolution of the WD. Here we present a study of the impact of uncertainties in the 12C(12C, α)20Ne and 12C(12C, p)23Na nuclear reaction rates (and their branching ratios) on the chemical structure of intermediate- to high-mass progenitors at the end of the C-burning phase. Using the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA) we computed evolutionary sequences for stars with initial masses 7.25<= MZAMS /Msun <=8.25, from the ZAMS to the SAGB phase, adopting different prescriptions for the 12C+12C burning rates. We found that adopting lower reaction rates for the 12C+12C burning delays C-ignition by at most 2700 yrs, and the ignition takes place in a position further from the center. Our results shows that differences in the 20Ne central abundances remain modest, below 14%.

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arXiv2022

Structure and evolution of ultra-massive white dwarfs in general relativity

We present the first set of constant rest-mass ultra-massive oxygen/neon white dwarf cooling tracks with masses larger than 1.29 Msun which fully take into account the effects of general relativity on their structural and evolutionary properties. We have computed the full evolution sequences of 1.29, 1.31, 1.33, 1.35, and 1.369 Msun white dwarfs with the La Plata stellar evolution code, LPCODE. For this work, the standard equations of stellar structure and evolution have been modified to include the full effects of general relativity. For comparison purposes, the same sequences have been computed but for the Newtonian case. According to our calculations, the evolutionary properties of the most massive white dwarfs are strongly modified by general relativity effects. In particular, the resulting stellar radius is markedly smaller in the general relativistic case, being up to 25% smaller than predicted by the Newtonian treatment for the more massive ones. We find that oxygen/neon white dwarfs more massive than 1.369 Msun become gravitationally unstable with respect to general relativity effects. When core chemical distribution due to phase separation on crystallization is considered, such instability occurs at somewhat lower stellar masses, greater than 1.36 Msun. In addition, cooling times for the most massive white dwarf sequences result in about a factor of two smaller than in the Newtonian case at advanced stages of evolution. Finally, a sample of white dwarfs has been identified as ideal candidates to test these general relativistic effects. We conclude that the general relativity effects should be taken into account for an accurate assessment of the structural and evolutionary properties of the most massive white dwarfs.

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arXiv2020

The Aarhus red giants challenge II. Stellar oscillations in the red giant branch phase

Context. The large quantity of high-quality asteroseismic data that obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties. Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies. Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences. Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties, in particular for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I; here we illustrate particular examples of this. Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential of the mixed modes of red-giant stars for the diagnostics of stellar interiors.

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arXiv2019

The Aarhus Red Giants Challenge I: Stellar structures in the red giant branch phase

(Abridged). We introduce the Aarhus Red Giants Challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes aiming at establishing the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. Using 9 state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at fixed radius along the red-giant branch for masses of 1.0 M$_\odot$, 1.5 M$_\odot$, 2.0 M$_\odot$, and 2.5 M$_\odot$, and compared the predicted stellar properties. Once models have been calibrated on the main sequence we find a residual spread in the predicted effective temperatures across all codes of ~20 K at solar radius and ~30-40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2-5% (increasing with stellar mass) which we track down to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ~0.1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and found a spread of 0.1 dex or more for all considered masses. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and call for further investigations on these matters in the context of using properties of red giants as benchmarks for astrophysical studies.

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