arXiv subjects

Sergio Cristallo

Publications and source records attributed to Sergio Cristallo.

At least 19 recordsLinked to original sources

arXiv2026

HRMOS: A High-Resolution Multi-Object Spectrograph for the VLT

This White Paper presents the scientific rationale and instrument concept for HRMOS (High-Resolution Multi-Object Spectrograph), a next-generation instrument proposed for the ESO Very Large Telescope within the VLT 2030 roadmap. Current and planned facilities offer either multi-object spectroscopy or ultra-high spectral resolution, but not both. HRMOS fills this gap by combining very high spectral resolution, multi-object capability, and radial-velocity stability, enabling transformative studies in Galactic and extragalactic astrophysics. The baseline design provides a resolving power of R = 80000, radial-velocity precision of 10 m s-1 (goal: 5 m s-1), simultaneous observations of 50-60 targets, and broad optical coverage down to 385 nm. These capabilities enable precise measurements of elemental abundances, isotopic ratios, line profiles, and radial velocities for large stellar samples, including crowded fields, star clusters, the Galactic bulge, and nearby dwarf galaxies. HRMOS will address key questions on the age of the oldest stellar populations through nucleocosmochronology, the formation and survival of planetary systems, the assembly history of the Milky Way and satellites, the origin of the heaviest elements, stellar evolution, and the chemical and dynamical properties of the interstellar and circumgalactic medium. It will bridge large spectroscopic surveys and the next generation of extremely large telescopes, with strong synergies with 4MOST, Gaia, TESS, PLATO, the proposed Haydn mission, and future ELT instruments. Building on VLT/FLAMES heritage, HRMOS represents a strategic investment for European astronomy in the 2030s.

astro-ph.IM↗

arXiv2026

The nucleosynthesis of Ba in the Early Universe. Constraints from elemental abundances and isotopic ratios

The [Ba/Eu] abundance ratio is commonly adopted as a tracer of the relative contributions of the slow (s) and rapid (r) neutron-capture processes. However, at [Fe/H] < -2 dex, Ba can be produced efficiently by both processes, rendering [Ba/Eu] non-deterministic. We propose to use barium isotopic ratio from the fitting of resonance Ba line profiles affected by hyperfine splitting. This approach requires precise atomic and stellar parameters, together with advanced spectral modelling, which, so far, remained insufficiently validated. We aim to provide a robust prescription of line-profile modelling for a reliable determination of the s- and r-processes fractions of barium in ordinary and peculiar stars. We assess the performance of 1D LTE and 1D non-LTE synthesis, and 3D non-LTE abundance corrections to model Ba lines. Alongside barium abundances and its isotopic ratios, we determine Eu and other neutron-capture element abundances to validate the method in the Titans metal-poor benchmark stars. The observational results are compared with the predictions of stochastic Galactic chemical evolution models that account the inhomogeneous mixing in the early times. We find that 1D LTE and 3D non-LTE Ba abundance determinations are equivalent, whereas the 1D non-LTE approach leads to systematic underestimations. These underestimations bias isotopic fractions toward higher r-process contributions. The inferred s- and r-process fractions demonstrate that [Ba/Eu] alone is an ambiguous tracer for ordinary stars within the range -0.8 < [Ba/Eu] < 0 dex. The comparison of our set of models, both for the proto-Milky Way halo and for Gaia-Enceladus galaxy is used to put constraints on the production of Ba at low [Fe/H], especially evaluating the role of rotating massive stars. The method here developed can be applied with confidence to both ordinary stars and peculiar stars enhanced in barium.

astro-ph.SR↗

arXiv2026

Observational Signatures and Constraints on the Intermediate Neutron-Capture Process. The Case of the CEMP star TYC 6044-714-1 (RAVE J094921.8-161722)

Observational abundances of CEMP stars with patterns in between those produced by the rapid and slow nucleosynthesis processes (CEMP-rs stars) are currently invoked as evidence of synthesis via the intermediate process in the early AGB evolutionary phase of metal-poor low mass stars. Nevertheless, discriminating between r+s- and i-process hypotheses requires high-precision abundances obtained through advanced spectral modelling techniques. Theoretical models of the i-process have become more robust, incorporating refined stellar modelling and nuclear reaction physics, providing ranges of probable elemental abundances and isotopic ratios predictions to be confronted with observational determinations. We performed a new analysis of a high resolution and high S/N UVES spectrum of TYC 6044-714-1. We derived accurate effective temperature and highly precise atmospheric parameters, element abundances, and isotopic ratios using state-of-the-art 1D non-LTE and 3D non-LTE spectral line modelling. Using the latest AGB nucleosynthesis models, we assessed the possibility of the i-process to act aside the s-process. We find that TYC~6044-714-1 was likely born as a normal in-situ halo star about 13 Gyr ago, pre-enriched by the r-process through a standard Galactic chemical-evolution pathway. The s+r model provides the best overall reproduction of the observed heavy-element abundance pattern and Ba isotopic ratios, yielding excellent agreement across all three s-process peaks. While i+s+r models with increasing overshooting efficiency improve the fit for specific elements, they do not consistently reproduce the full abundance pattern. The i+s+r models require extreme and physically implausible conditions, and predict s-process Ba fractions inconsistent with those inferred from isotopic ratios of the 4934 \AA\ resonance line. We conclude that the pure s+r scenario is the most plausible explanation.

astro-ph.SR↗

arXiv2026

The MARTINI Platform (I): Se I-X atomic calculation and expansion opacity for early-stage kilonova spectral analysis

Kilonovae represent key sites of r-process nucleosynthesis, making opacity estimation and spectral analysis crucial for constraining their composition. Since light r-process elements shape the early ($\sim$0.5-1.5$\mathrm{d}$) ejecta opacity, a detailed study of the selenium element with a focus on atomic data calculation, expansion opacity estimation and spectral analysis is presented. Se atomic data are calculated from Se I to Se X using the GRASP2018 code. A systematic analysis and evaluation of their precision is performed through detailed comparison with the NIST ASD, and other works available in the literature. These atomic data are then used to estimate expansion opacity at different temperatures (e.g., T=5000 K, 10000 K, 20000 K, 100000 K) and densities (e.g., $\rho = 10^{-13}\,\mathrm{g\,cm^{-3}},\,3\times10^{-12}\,\mathrm{g\,cm^{-3}}$). Spectral analysis has been performed with radiative transfer code POSSIS with a pre-computed opacity grid calculated with new densities and temperatures, ranging from -19.5 to -4.5 $\mathrm{g\,cm^{-3}}$ in log-scale and from 1 000 to 51 000 K, respectively. Two scenarios are considered: one in which the opacity contribution comes from 100\% Se ejecta, and another in which Se contributes only partially to the total opacity ($\sim$ 10\% of the total mass). Se atomic calculations show a good agreement with NIST ASD, with accurate energy levels and transitions determined alongside atomic data for higher ionisation stages not fully covered by NIST. The expansion opacities calculated with these new Se data exhibit differences in comparison to existing literature works. Se spectral features can only be observed in the KN scenario consisting of 100\% Se. When Se accounts for about 10\% of the total KN mass, these features become undetectable. All Se results are now available in the new open-source MARTINI platform dedicated to element nucleosynthesis.

astro-ph.HE↗

arXiv2026

Impact of newly measured 26Al(n, p)26Mg and 26Al(n, {\alpha})23Na reaction rates on the nucleosynthesis of 26Al in stars

The cosmic production of the short-lived radioactive nuclide 26Al is crucial for our understanding of the evolution of stars and galaxies. However, simulations of the stellar sites producing 26Al are still weakened by significant nuclear uncertainties. We re-evaluate the 26Al(n, p)26Mg, and 26Al(n, {\alpha})23Na ground state reactivities from 0.01 GK to 10 GK, based on the recent n TOF measurement combined with theoretical predictions and a previous measurement at higher energies, and test their impact on stellar nucleosynthesis. We computed the nucleosynthesis of low- and high-mass stars using the Monash nucleosynthesis code, the NuGrid mppnp code, and the FUNS stellar evolutionary code. Our low-mass stellar models cover the 2-3 Msun mass range with metallicities between Z = 0.01 and 0.02, their predicted 26Al/27Al ratios are compared to 62 meteoritic SiC grains. For high-mass stars, we test our reactivities on two 15 Msun models with Z = 0.006 and 0.02. The new reactivities allow low-mass AGB stars to reproduce the full range of 26Al/27Al ratios measured in SiC grains. The final 26Al abundance in high-mass stars, at the point of highest production, varies by a factor of 2.4 when adopting the upper, or lower limit of our rates. However, stellar uncertainties still play an important role in both mass regimes. The new reactivities visibly impact both low- and high-mass stars nucleosynthesis and allow a general improvement in the comparison between stardust SiC grains and low-mass star models. Concerning explosive nucleosynthesis, an improvement of the current uncertainties between T9~0.3 and 2.5 is needed for future studies.

astro-ph.SR↗

arXiv2025

Mapping the Ages of Stars with Chemistry

Chemical clocks, based on age-sensitive stellar abundance ratios, offer a powerful and scalable approach to reconstruct the formation history of the Milky Way. This white paper outlines how wide-field, high-resolution spectroscopy can transform chemical clocks into precise and broadly applicable stellar age estimators when combined with astrometry and asteroseismology. We summarize the current limitations, including calibration across Galactic environments and the impact of internal stellar evolution, and define the observational requirements needed to overcome them. The Wide-field Spectroscopic Telescope (WST), with its large field of view, high multiplex, and broad wavelength coverage at high spectral resolution, is uniquely suited to deliver the homogeneous datasets required to map the age structure of the Galaxy at unprecedented scale. Such a capability will enable decisive progress in Galactic archaeology and stellar evolution studies.

astro-ph.GA↗

arXiv2025

Measurement of the $^{35}Cl(n, p)^{35}S$ cross-section at the CERN n\_TOF facility from subthermal energy to 120 keV

Background: The $^{35}Cl(n, p)^{35}S$ reaction is of special interest in three different applications. First, in Boron Neutron Capture Therapy due to the presence of $^{35}Cl$ in brain and skin tissue. Second, it is involved in the creation of $^{36}S$, whose astrophysical origin remains unresolved. Third, in the designing of fast nuclear reactors of new generation based on molten salts. Purpose: To measure the $^{35}Cl(n, p)^{35}S$ cross-section from thermal energy to 120 keV, determine the resonance parameters in this range and Maxwellian Averaged Cross-Section (MACS). Method: We made use of the Time-of-Flight technique with microMEGAS detectors at Experimental Area 2 (EAR-2) of n\_TOF facility at CERN. The $^{10}B(n, α)^{7}Li$ and $^{235}U(n, f)$ reactions were used as references. Rutherford Back-scattering Spectrometry technique was performed at Centro Nacional de Aceleradores (CNA) in Sevilla, in order to accurately determine the masses of the irradiated samples. Results: We obtain a thermal cross-section of $0.470 \pm 0.009$ barns. The $1/v$ energy dependence of the cross-section is observed up to the first resonance at 0.398 keV, the resonances up to 120 keV are analyzed and MACS calculated for $k_{B} T$ from 1 to 100 keV. Conclusions: The $^{35}Cl(n, p)^{35}S$ cross-section has been obtained over a wide energy range for the first time, with high accuracy across the aforementioned range. The thermal cross-section and first two resonances are in agreement with latest evaluation in ENDF/B-VIII.1, while lower resonance strength was found for high energy resonances. These data are used to calculate the MACS for different $k_{B} T$.

nucl-ex↗

arXiv2025

s-process Enriched Post-AGB Star J003643.94-723722.1 in the SMC with an Extreme C/O Ratio and the First Precise Detection of Lead

Post-asymptotic giant branch (post-AGB) stars are exquisite tracers of s-process nucleosynthesis via their surface abundances. We present a comprehensive analysis of J003643.94$-$723722.1 (J003643), a single SMC post-AGB star, using high-resolution UVES/VLT spectra analysed with E-iSpec. We find C/O = 16.21 and $[\mathrm{s}/\mathrm{Fe}]=2.09\pm0.20 \mathrm{dex}$. We also report the first direct Pb detection in a post-AGB star from the Pb II 5608.853 A line, with $[\mathrm{Pb}/\mathrm{Fe}]=3.18\,\mathrm{dex}$. Comparison with post-AGB samples in the Galaxy and Magellanic Clouds reveals that J003643 has an unusually high C/O ratio. J003643's $[\mathrm{C}/\mathrm{Fe}]=1.33\pm0.14 \mathrm{dex}$ and $[\mathrm{s}/\mathrm{Fe}]=2.09\pm0.20\,\mathrm{dex}$ are consistent with third dredge-up enrichment, but its $[\mathrm{O}/\mathrm{Fe}]=-0.08\pm0.20\,\mathrm{dex}$ is low relative to objects of similar $[\mathrm{C}/\mathrm{Fe}]$ and $[\mathrm{Fe}/\mathrm{H}]$. Together with $[α/\mathrm{Fe}]\approx0\,\mathrm{dex}$ at $[\mathrm{Fe}/\mathrm{H}]\approx-1\,\mathrm{dex}$, consistent with SMC chemical evolution, this indicates the high C/O chiefly reflects oxygen deficiency rather than exceptional carbon enrichment. Additionally, we compare the full abundance pattern with yields from ATON, MONASH and FRUITY (the latter two with post-processing nucleosynthesis). Most elements are reproduced, but Pb is strongly underpredicted, highlighting a persistent gap in models of heavy-element production in AGB stars. The photospheric chemistry of J003643 adds to the growing diversity among post-AGB stars and underscores the complexity of AGB nucleosynthesis.

astro-ph.SR↗

arXiv2025

The Science of the Einstein Telescope

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

gr-qc↗

arXiv2023

Dust survival in harsh environments -- Is photo-evaporation an important destruction mechanism?

Aims. We investigate the role of photo-evaporation of dust exposed to the radiation field from hot young stars and planetary nebulae (PNe) as a possible destruction mechanism of dust grains in the interstellar medium (ISM). Methods. We estimate photo-evaporation induced by the feedback of individual or clustered young stars, of PNe and in the presence of a variable radiation field scaled with the interstellar radiation field. For PNe we investigate dust photo-evaporation of both dust grains already present in the ISM as well as those formed in the last phases of the evolution of thermally pulsing asymptotic giant branch (TP-AGB) stars. We include dust photo-evaporation rate in models of dust evolution in galaxies for different assumptions of the dust growth scenario, dust-to-gas ratios, star formation histories, and initial mass functions of the stars. Results. For all the cases considered, we find that both photo-evaporation from young stars and from PNe are negligible with respect to other dust removal processes such as destruction from supernovae shocks, astration and possibly outflow. Grains are stable against photo-evaporation if they are exposed to a radiation field which is up to 10^7 times the interstellar radiation field. Conclusions. Dust grains of size >= 0.01 microns are not efficiently destroyed by photo-evaporation also in the presence of a strong radiation field.

astro-ph.GA↗

arXiv2023

On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios

Using a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age vs metallicity or abundance ratios vs age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] vs metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary vs secondary or odd elements) and on the lifetimes of their sources (short-lived vs long-lived ones). The latter property is critical in determining the single- or double- branch behavior of an elementary abundance ratio in the Solar neighborhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different timescales and help determining the site of e.g. the r-process(es). We define the "abundance distance" between the thin and thick disc sequences as an important element for such a separation. We also show how the inside-out evolution of the Milky Way disc leads rather to a single-branch behavior in other disc regions.

astro-ph.GA↗

arXiv2022

Presolar Silicon Carbide Grains of Types Y and Z: Their Strontium and Barium Isotopic Compositions and Stellar Origins

We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of types Y (11) and Z (7), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that the Y and Z grains show higher 88Sr/87Sr and more variable 138Ba/136Ba ratios than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the grains came from low mass AGB stars with 0.15 Zsun <= Z < 1.00 Zsun, in which the 13C neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z8 AGB star. This scenario is in line with the previous finding based on Ti isotopes, but it fails to explain the indistinguishable Mo isotopic compositions of MS, Y, and Z grains.

astro-ph.SR↗

arXiv2022

Chandrasekhar-mass white dwarfs are the progenitors of a small fraction of Type Ia supernovae according to nucleosythesis constraints

The precise progenitor system of type Ia supernovae (SNe Ia), whether it is a white dwarf (WD) close to the Chandrasekhar limit or substantially less massive, has been a matter of debate for decades. Recent research by our group on the accretion and simmering phases preceding the explosion of a massive WD has shown that the central density at thermal runaway lies in the range $(3.6-6.3)\times10^9$ g cm$^{-3}$ for reasonable choices of accretion rate onto the WD and progenitor metallicity. In this work, we have computed one-dimensional simulations of the explosion of such WDs, with special emphasis on the chemical composition of the ejecta, which in all cases is extremely rich in neutronized isotopes of chromium ($^{54}$Cr) and titanium ($^{50}$Ti). We show that, in order to reconcile such a nucleosynthesis with the isotopic abundances of the Solar System, Chandrasekhar-mass white dwarfs can account for at most 26 per cent of normal-luminosity SNe Ia, or at most 20 per cent of all SNe Ia.

astro-ph.SR↗

arXiv2022

Slow Neutron-Capture Process: Low-mass AGB stars and presolar silicon carbide grains

Presolar grains are microscopic dust grains that formed in the stellar winds or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar silicon carbide (SiC) grains, including types mainstream (MS), Y, and Z, came from low-mass C-rich asymptotic giant branch (AGB) stars, which is supported by the ubiquitous presence of SiC dust observed in the circumstellar envelope of AGB stars and the signatures of slow neutron-capture process preserved in these grains. Here, we review the status of isotope studies of presolar AGB SiC grains with an emphasis on heavy-element isotopes and highlight the importance of presolar grain studies for nuclear astrophysics. We discuss the sensitives of different types of nuclei to varying AGB stellar parameters and how their abundances in presolar AGB SiC grains can be used to provide independent, detailed constraints on stellar parameters, including 13C formation, stellar temperature, and nuclear reaction rates.

astro-ph.SR↗

arXiv2022

Pre-explosive accretion and simmering phases of Type Ia Supernovae

In accreting WDs approaching the Chandrasekhar limit, hydrostatic carbon burning precedes the dynamical breakout. During this \textit{simmering} phase, $e-$captures are energetically favored in the central region of the star, while $β-$decays are favored more outside, and the two zones are connected by a growing convective instability. We analyze the interplay between weak interactions and convection, the so-called convective URCA process, during the simmering phase of SNe Ia progenitors and its effects on the physical and chemical properties at the explosion epoch. At variance with previous studies, we find that the convective core powered by the carbon burning remains confined within the ${^{21}(Ne,F)}$ URCA shell. As a result, a much larger amount of carbon has to be consumed before the explosion which eventually occurs at larger density than previously estimated. In addition, we find that the extension of the convective core and its average neutronization depend on the the WD progenitor initial metallicity. For the average neutronization in the convective core at the explosion epoch we obtain ${\overlineη_{exp}} = (1.094\pm 0.143)\times 10^{-3} + (9.168\pm 0.677)\times 10^{-2}\times Z$. Outside the convective core, the neutronization is instead determined by the initial amount of C+N+O in the progenitor star. Since S, Ca, Cr and Mn, the elements usually exploited to evaluate the pre-explosive neutronization, are mainly produced outside the heavily neutronized core, the problem of too high metallicity estimated for the progenitors of the historical Tycho and Kepler SNe Ia remains unsolved.

astro-ph.SR↗

arXiv2021

Oxygen and Aluminum-Magnesium Isotopic Systematics of Presolar Nanospinel Grains from CI Chondrite Orgueil

Presolar oxide grains have been previously divided into several groups (Group 1 to 4) based on their isotopic compositions, which can be tied to several stellar sources. Much of available data was acquired on large grains, which may not be fully representative of the presolar grain population present in meteorites. We present here new O isotopic data for 74 small presolar oxide grains (~200 nm in diameter on average) from Orgueil and Al-Mg isotopic systematics for 25 of the grains. Based on data-model comparisons, we show that (i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent (red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red giant stars and (ii) Group 1 grains with (26Al/27Al)0 >= 5x10^-3 and Group 2 grains with (26Al/27Al)0 <= 1x10^-2 all likely experienced extra circulation processes in their parent low-mass stars but under different conditions, resulting in proton-capture reactions occurring at enhanced temperatures. We do not find any large 25Mg excess in Group 1 oxide grains with large 17O enrichments, which provides evidence that 25Mg is not abundantly produced in low-mass stars. We also find that our samples contain a larger proportion of Group 4 grains than so far suggested in the literature for larger presolar oxide grains (~400 nm). We also discuss our observations in the light of stellar dust production mechanisms.

astro-ph.SR↗

arXiv2021

Bottom-up dust nucleation theory in oxygen-rich evolved stars I. Aluminium oxide clusters

Aluminum oxide (alumina, Al$_{2}$O$_{3}$) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina \textit{seed} particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood.} {To shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments by a quantum-chemical bottom-up approach.} {Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network describing the formation and destruction of aluminium-bearing molecules and dust-forming (Al$_{2}$O$_{3}$)$_n$ clusters up to the size of dimers ($n$=2) coagulating to tetramers ($n=$4). Intermediary species include the prevalent gas-phase molecules AlO and AlOH, and Al$_x$O$_y$ clusters with $x=$1$-$5, $y=$1$-$6. The resulting extensive network is applied to two model stars, representing a semi-regular variable and a Mira-type, and to different circumstellar gas trajectories including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al$_{2}$O$_{3}$)$_n$ clusters with $n=$4$-$10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated by density functional theory. We provide energies, bond characteristics, electrostatic properties and vibrational spectra of the clusters as a function of size $n$ and compare these to corundum corresponding to the crystalline bulk limit ($n\rightarrow \infty$). {The circumstellar aluminium gas-phase chemistry in oxygen-rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H$_2$, AlO+H$_2$O, and their reverse. Models of ...}

astro-ph.SR↗

arXiv2021

New Multielement Isotopic Compositions of Presolar SiC Grains: Implications for Their Stellar Origins

We report NanoSIMS Si and Mg-Al isotopic data (and C, N, and Ti isotopic data when available) for 85 submicron- to micron-sized presolar SiC grains from the CM2 Murchison meteorite, including 60 MS, 8 AB1, 8 X, 7 AB2, and 2 Y grains. The MS and Y grain data demonstrate that (1) C and N contamination mainly appears as surface contamination, and sufficient presputtering is needed to expose a clean grain surface for obtaining intrinsic C and N signals, and (2) Mg and Al contamination appears as adjacent grains and rims, and high-resolution imaging and the choice of small regions of interest during data reduction together are effective in suppressing the contamination. Our results strongly indicate that previous studies on presolar SiC grains could have sampled differing degrees of contamination for C, N, Mg, and Al. Compared to the literature data, our new MS and Y grains are in better agreement with carbon star observations for both the C and N isotopic ratios. By comparing our new, tighter distributions of 12C/13C, 14N/15N, and initial 26Al/27Al ratios for MS and Y grains with FRUITY AGB stellar models, we provide more stringent constraints on the occurrence of cool bottom processing and the production of 26Al in N-type carbon stars, classical asymptotic giant branch stars.

astro-ph.SR↗