arXiv subjects

Maria Lugaro

Publications and source records attributed to Maria Lugaro.

At least 55 records · Page 3Linked to original sources

arXiv2019

Galactic Chemical Evolution of Radioactive Isotopes

The presence of short-lived ($\sim$\,Myr) radioactive isotopes in meteoritic inclusions at the time of their formation represents a unique opportunity to study the circumstances that led to the formation of the Solar System. To interpret these observations we need to calculate the evolution of radioactive-to-stable isotopic ratios in the Galaxy. We present an extension of the open-source galactic chemical evolution codes NuPyCEE and JINAPyCEE that enables to track the decay of radioactive isotopes in the interstellar medium. We show how the evolution of isotopic ratio depends on the star formation history and efficiency, star-to-gas mass ratio, and galactic outflows. Given the uncertainties in the observations used to calibrate our model, our predictions for isotopic ratios at the time of formation of the Sun are uncertain by a factor of 3.6. At that time, to recover the actual radioactive-to-stable isotopic ratios predicted by our model, one can multiply the steady-state solution (see Equation~1) by $2.3^{+3.4}_{-0.7}$. However, in the cases where the radioactive isotope has a half-life longer than $\sim$\,200\,Myr, or the target radioactive or stable isotopes have mass- and/or metallicity-depended production rates, or they originate from different sources with different delay-time distributions, or the reference isotope is radioactive, our codes should be used for more accurate solutions. Our preliminary calculations confirm the dichotomy between radioactive nuclei in the early Solar System with $r$- and $s$-process origin, and that $^{55}$Mn and $^{60}$Fe can be explained by galactic chemical evolution, while $^{26}$Al cannot.

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arXiv2019

Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities

There is now solid experimental evidence of at least one supernova explosion within 100 pc of Earth within the last few million years, from measurements of the short-lived isotope 60Fe in widespread deep-ocean samples, as well as in the lunar regolith and cosmic rays. This is the first established example of a specific dated astrophysical event outside the Solar System having a measurable impact on the Earth, offering new probes of stellar evolution, nuclear astrophysics, the astrophysics of the solar neighborhood, cosmic-ray sources and acceleration, multi-messenger astronomy, and astrobiology. Interdisciplinary connections reach broadly to include heliophysics, geology, and evolutionary biology. Objectives for the future include pinning down the nature and location of the established near-Earth supernova explosions, seeking evidence for others, and searching for other short-lived isotopes such as 26Al and 244Pu. The unique information provided by geological and lunar detections of radioactive 60Fe to assess nearby supernova explosions make now a compelling time for the astronomy community to advocate for supporting multi-disciplinary, cross-cutting research programs.

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arXiv2018

The consequences of a nearby supernova on the early Solar System

If the Sun was born in a relatively compact open cluster, it is quite likely that a massive (10MSun) star was nearby when it exploded in a supernova. The repercussions of a supernova can be rather profound, and the current Solar System may still bear the memory of this traumatic event. The truncation of the Kuiper belt and the tilt of the ecliptic plane with respect to the Sun's rotation axis could be such signatures. We simulated the effect of a nearby supernova on the young Solar System using the Astronomical Multipurpose Software Environment. Our calculations are realized in two subsequent steps in which we study the effect of the supernova irradiation on the circumstellar disk and the effect of the impact of the nuclear blast-wave which arrives a few decades later. We find that the blastwave of our adopted supernova exploding at a distance of $0.15$--$0.40$\,pc and at an angle of $35^\circ$--$65^\circ$ with respect to the angular-momentum axis of the circumsolar disk would induce a misalignment between the Sun's equator and its disk to $5^\circ.6\pm1^\circ.2$, consistent with the current value. The blast of a supernova truncates the disk at a radius between $42$ and $55$\,au, which is consistent with the current edge of the Kuiper belt. For the most favored parameters, the irradiation by the supernova as well as the blast wave heat the majority of the disk to $\sim 1200$\,K, which is sufficiently hot to melt chondrules in the circumstellar disk. The majority of planetary system may have been affected by a nearby supernova, some of its repercussions, such as truncation and tilting of the disk, may still be visible in their current planetary system's topology. The amount of material from the supernova blast wave that is accreted by the circumstellar disk is too small by several orders of magnitude to explain the current abundance of the short live radionuclide $^{26}$Al.

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arXiv2018

Heavy-element yields and abundances of Asymptotic Giant Branch models with a Small Magellanic Cloud metallicity

We present new theoretical stellar yields and surface abundances for asymptotic giant branch (AGB) models with a metallicity appropriate for stars in the Small Magellanic Cloud (SMC, $Z= 0.0028$, [Fe/H] $\approx -0.7$). New evolutionary sequences and post-processing nucleosynthesis results are presented for initial masses between 1$M_{\odot}$ and 7$M_{\odot}$, where the 7$M_{\odot}$ is a super-AGB star with an O-Ne core. Models above 1.15$M_{\odot}$ become carbon rich during the AGB, and hot bottom burning begins in models $M \ge 3.75 M_{\odot}$. We present stellar surface abundances as a function of thermal pulse number for elements between C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g., $^{12}$C/$^{13}$C), which can be compared to observations. The integrated stellar yields are presented for each model in the grid for hydrogen, helium and all stable elements from C to Bi. We present evolutionary sequences of intermediate-mass models between 4--7$M_{\odot}$ and nucleosynthesis results for three masses ($M=3.75, 5, 7M_{\odot}$) including $s$-process elements for two widely used AGB mass-loss prescriptions. We discuss our new models in the context of evolved AGB stars and post-AGB stars in the Small Magellanic Clouds, barium stars in our Galaxy, the composition of Galactic globular clusters including Mg isotopes with a similar metallicity to our models, and to pre-solar grains which may have an origin in metal-poor AGB stars.

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arXiv2017

Stellar yields and abundances: new directions from planetary nebulae

Planetary nebulae retain the signature of the nucleosynthesis and mixing events that occurred during the previous AGB phase. Observational signatures complement observations of AGB and post-AGB stars and their binary companions. The abundances of the elements heavier than iron such as Kr and Xe in planetary nebulae can be used to complement abundances of Sr/Y/Zr and Ba/La/Ce in AGB stars, respectively, to determine the operation of the slow neutron-capture process (the s process) in AGB stars. Additionally, observations of the Rb abundance in Type I planetary nebulae may allow us to infer the initial mass of the central star. Several noble gas components present in meteoritic stardust silicon carbide (SiC) grains are associated with implantation into the dust grains in the high-energy environment connected to the fast winds from the central stars during the planetary nebulae phase.

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arXiv2016

Intermediate-mass asymptotic giant branch stars and sources of $^{26}$Al, $^{60}$Fe, $^{107}$Pd, and $^{182}$Hf in the solar system

We explore the possibility that the short-lived radionuclides $^{26}$Al, $^{60}$Fe, $^{107}$Pd, and $^{182}$Hf inferred to be present in the proto-solar cloud originated from $3-8 M_{\odot}$ Asymptotic Giant Branch (AGB) stars. Models of AGB stars with initial mass above 5$M_{\odot}$ are prolific producers of $^{26}$Al owing to hot bottom burning (HBB). In contrast, $^{60}$Fe, $^{107}$Pd, and $^{182}$Hf are produced by neutron captures: $^{107}$Pd and $^{182}$Hf in models $\lesssim 5 M_{\odot}$; and $^{60}$Fe in models with higher mass. We mix stellar yields from solar-metallicity AGB models into a cloud of solar mass and composition to investigate if it is possible to explain the abundances of the four radioactive nuclides at the Sun's birth using one single value of the mixing ratio between the AGB yields and the initial cloud material. We find that AGB stars that experience efficient HBB ($\geq 6 M_{\odot}$) cannot provide a solution because they produce too little $^{182}$Hf and $^{107}$Pd relative to $^{26}$Al and $^{60}$Fe. Lower-mass AGB cannot provide a solution because they produce too little $^{26}$Al relative to $^{107}$Pd and $^{182}$Hf. A self-consistent solution may be found for AGB stars with masses in-between ($4-5.5 M_{\odot}$), provided HBB is stronger than in our models and the $^{13}$C($α$, n)$^{16}$O neutron source is mildly activated. If stars of M $< 5.5 M_{\odot}$ are the source of the radioactive nuclides, then some basis for their existence in proto-solar clouds needs to be explored, given that the stellar lifetimes are longer than the molecular cloud lifetimes.

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arXiv2016

The intermediate neutron-capture process and carbon-enhanced metal-poor stars

Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display enrichments in heavy elements associated with either the s (slow) or the r (rapid) neutron-capture process (e.g., barium and europium respectively), and in some cases they display evidence of both. The abundance patterns of these CEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling since the s and the r processes require neutron densities that are more than ten orders of magnitude apart, and hence are thought to occur in very different stellar sites with very different physical conditions. We investigate whether the abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of the intermediate neutron-capture process (the i process), which is characterised by neutron densities between those of the s and the r processes. Using nuclear network calculations, we study neutron capture nucleosynthesis at different constant neutron densities n ranging from $10^7$ to $10^{15}$ cm$^{-3}$. With respect to the classical s process resulting from neutron densities on the lowest side of this range, neutron densities on the highest side result in abundance patterns that show an increased production of heavy s-process and r-process elements but similar abundances of the light s-process elements. Such high values of n may occur in the thermal pulses of asymptotic giant branch (AGB) stars due to proton ingestion episodes. Comparison to the surface abundances of 20 CEMP-s/r stars show that our modelled i-process abundances successfully reproduce observed abundance patterns that could not be previously explained by s-process nucleosynthesis. Because the i-process models fit the abundances of CEMP-s/r stars so well, we propose that this class should be renamed as CEMP-i.

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arXiv2016

Stellar yields from metal-rich asymptotic giant branch models

We present new theoretical stellar yields and surface abundances for three grids of metal-rich asymptotic giant branch (AGB) models. Post-processing nucleosynthesis results are presented for stellar models with initial masses between 1$M_{\odot}$ and 7.5$M_{\odot}$ for $Z=0.007$, and 1$M_{\odot}$ and 8$M_{\odot}$ for $Z=0.014$ (solar) and $Z=0.03$. We include stellar surface abundances as a function of thermal pulse on the AGB for elements from C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g., $^{12}$C/$^{13}$C), which can be obtained from observations of molecules in stars and from the laboratory analysis of meteoritic stardust grains. Ratios of elemental abundances of He/H, C/O, and N/O are also included, which are useful for direct comparison to observations of AGB stars and their progeny including planetary nebulae. The integrated elemental stellar yields are presented for each model in the grid for hydrogen, helium and all stable elements from C to Bi. Yields of Li are also included for intermediate-mass models with hot bottom burning. We present the first $slow$ neutron-capture ($s$-process) yields for super-solar metallicity AGB stars with $Z=0.03$, and the first complete $s$-process yields for models more massive than 6$M_{\odot}$ at all three metallicities.

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arXiv2016

Origin of the p-process radionuclides 92Nb and 146Sm in the early Solar System and inferences on the birth of the Sun

The abundances of 92Nb and 146Sm in the early Solar System are determined from meteoritic analysis and their stellar production is attributed to the p process. We investigate if their origin from thermonuclear supernovae deriving from the explosion of white dwarfs with mass above the Chandrasekhar limit is in agreement with the abundance of 53Mn, another radionuclide present in the early Solar System and produced in the same events. A consistent solution for 92Nb and 53Mn cannot be found within the current uncertainties and requires that the 92Nb/92Mo ratio in the early Solar System is at least 50% lower than the current nominal value, which is outside its present error bars. A different solution is to invoke another production site for 92Nb, which we find in the alpha-rich freezeout during core-collapse supernovae from massive stars. Whichever scenario we consider, we find that a relatively long time interval of at least ~10 Myr must have elapsed from when the star-forming region where the Sun was born was isolated from the interstellar medium and the birth of the Sun. This is in agreement with results obtained from radionuclides heavier than iron produced by neutron captures and lends further support to the idea that the Sun was born in a massive star-forming region together with many thousands of stellar siblings.

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arXiv2015

HIP 10725: The First Solar Twin/Analogue Field Blue Straggler

[Context]. Blue stragglers are easy to identify in globular clusters, but are much harder to identify in the field. Here we present the serendipitous discovery of one field blue straggler, HIP 10725, that closely matches the Sun in mass and age, but with a metallicity slightly lower than the Sun's. [Methods]. We employ high resolution (R $\sim 10^5$) high S/N (330) VLT/UVES spectra to perform a differential abundance analysis of the solar analogue HIP 10725. Radial velocities obtained by other instruments were also used to check for binarity. We also study its chromospheric activity, age and rotational velocity. [Results]. We find that HIP 10725 is severely depleted in beryllium ([Be/H] <= -1.2 dex) for its stellar parameters and age. The abundances relative to solar of the elements with Z <= 30 show a correlation with condensation temperature and the neutron capture elements produced by the s-process are greatly enhanced, while the r-process elements seem normal. We found its projected rotational velocity (v sin i = 3.3+/-0.1 km/s) to be significantly larger than solar, and incompatible with its isochrone-derived age. Radial velocity monitoring shows that the star has a binary companion. [Conclusions]. Based on the high s-process element enhancements and low beryllium abundance, we suggest that HIP 10725 has been polluted by mass-transfer from an AGB star, probably with initial mass of about 2 M_Sun. The radial velocity variations suggest the presence of an unseen binary companion, probably the remnant of a former AGB star. Isochrones predict a solar-age star, but this is in disagreement with the high projected rotational velocity and high chromospheric activity. We conclude that HIP 10725 is a field blue straggler, rejuvenated by the mass transfer process of its former AGB companion.

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arXiv2015

Evolution and nucleosynthesis of helium-rich asymptotic giant branch models

There is now strong evidence that some stars have been born with He mass fractions as high as $Y \approx 0.40$ (e.g., in $ω$ Centauri). However, the advanced evolution, chemical yields, and final fates of He-rich stars are largely unexplored. We investigate the consequences of He-enhancement on the evolution and nucleosynthesis of intermediate-mass asymptotic giant branch (AGB) models of 3, 4, 5, and 6 M$_\odot$ with a metallicity of $Z = 0.0006$ ([Fe/H] $\approx -1.4$). We compare models with He-enhanced compositions ($Y=0.30, 0.35, 0.40$) to those with primordial He ($Y=0.24$). We find that the minimum initial mass for C burning and super-AGB stars with CO(Ne) or ONe cores decreases from above our highest mass of 6 M$_\odot$ to $\sim$ 4-5 M$_\odot$ with $Y=0.40$. We also model the production of trans-Fe elements via the slow neutron-capture process (s-process). He-enhancement substantially reduces the third dredge-up efficiency and the stellar yields of s-process elements (e.g., 90% less Ba for 6 M$_\odot$, $Y=0.40$). An exception occurs for 3 M$_\odot$, where the near-doubling in the number of thermal pulses with $Y=0.40$ leads to $\sim$ 50% higher yields of Ba-peak elements and Pb if the $^{13}$C neutron source is included. However, the thinner intershell and increased temperatures at the base of the convective envelope with $Y=0.40$ probably inhibit the $^{13}$C neutron source at this mass. Future chemical evolution models with our yields might explain the evolution of s-process elements among He-rich stars in $ω$ Centauri.

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arXiv2014

Stellar origin of the 182Hf cosmochronometer and the presolar history of solar system matter

Among the short-lived radioactive nuclei inferred to be present in the early solar system via meteoritic analyses, there are several heavier than iron whose stellar origin has been poorly understood. In particular, the abundances inferred for 182Hf (half-life = 8.9 million years) and 129I (half-life = 15.7 million years) are in disagreement with each other if both nuclei are produced by the rapid neutron-capture process. Here, we demonstrate that contrary to previous assumption, the slow neutron-capture process in asymptotic giant branch stars produces 182Hf. This has allowed us to date the last rapid and slow neutron-capture events that contaminated the solar system material at roughly 100 million years and 30 million years, respectively, before the formation of the Sun.

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arXiv2013

The impact of updated Zr neutron-capture cross sections and new asymptotic giant branch models on our understanding of the s process and the origin of stardust

We present model predictions for the Zr isotopic ratios produced by slow neutron captures in C-rich asymptotic giant branch (AGB) stars of masses 1.25 to 4 Msun and metallicities Z=0.01 to 0.03, and compare them to data from single meteoritic stardust silicon carbide (SiC) and high-density graphite grains that condensed in the outflows of these stars. We compare predictions produced using the Zr neutron-capture cross section from Bao et al. (2000) and from n_TOF experiments at CERN, and present a new evaluation for the neutron-capture cross section of the unstable isotope 95Zr, the branching point leading to the production of 96Zr. The new cross sections generally presents an improved match with the observational data, except for the 92Zr/94Zr ratios, which are on average still substantially higher than predicted. The 96Zr/94Zr ratios can be explained using our range of initial stellar masses, with the most 96Zr-depleted grains originating from AGB stars of masses 1.8 - 3 Msun, and the others from either lower or higher masses. The 90,91Zr/94Zr variations measured in the grains are well reproduced by the range of stellar metallicities considered here, which is the same needed to cover the Si composition of the grains produced by the chemical evolution of the Galaxy. The 92Zr/94Zr versus 29Si/28Si positive correlation observed in the available data suggests that stellar metallicity rather than rotation plays the major role in covering the 90,91,92Zr/94Zr spread.

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arXiv2013

Rubidium abundances in the globular clusters NGC 6752, NGC 1904 and NGC 104 (47 Tuc)

Large star-to-star variations of the abundances of proton-capture elements, such as Na and O, in globular clusters (GCs) are interpreted as the effect of internal pollution resulting from the presence of multiple stellar populations. To better constrain this scenario we investigate the abundance distribution of the heavy element rubidium (Rb) in NGC 6752, NGC 1904, and NGC 104 (47 Tuc). Combining the results from our sample with those in the literature, we found that Rb exhibits no star-to-star variations, regardless the cluster metallicity, with the possible intriguing, though very uncertain, exception of the metal-rich bulge cluster NGC 6388. If no star-to-star variations will be confirmed for all GCs, it implies that the stellar source of the proton-capture element variations must not have produced significant amounts of Rb. This element is observed to be enhanced at extremely high levels in intermediate-mass AGB (IM-AGB) stars in the Magellanic Clouds (i.e., at a metallicity similar to 47 Tuc and NGC 6388). This may present a challenge to this popular candidate polluter, unless the mass range of the observed IM-AGB stars does not participate in the formation of the second-generation stars in GCs. A number of possible solutions are available to resolve this conundrum, also given that the Magellanic Clouds observations are very uncertain and may need to be revised. The fast rotating massive stars scenario would not face this potential problem as the slow mechanical winds of these stars during their main-sequence phase do not carry any Rb enhancements; however, these candidates face even bigger issues such as the production of Li and the close over-imposition with core-collapse supernova timescales. Observations of Sr, Rb, and Zr in metal-rich clusters such as NGC 6388 and NGC 6441 are sorely needed to clarify the situation.

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arXiv2013

On the internal pollution mechanisms in the globular cluster NGC 6121 (M4): heavy-element abundances and AGB models

Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass AGB stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for intermediate-mass AGB stellar models of 5 and 6 solar masses, including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing-length parameter, and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of intermediate-mass AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.

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arXiv2013

Decoding the message from meteoritic stardust silicon carbide grains

Micron-sized stardust grains that originated in ancient stars are recovered from meteorites and analysed using high-resolution mass spectrometry. The most widely studied type of stardust is silicon carbide (SiC). Thousands of these grains have been analysed with high precision for their Si isotopic composition. Here we show that the distribution of the Si isotopic composition of the vast majority of stardust SiC grains carry the imprints of a spread in the age-metallicity distribution of their parent stars and of a power-law increase of the relative formation efficiency of SiC dust with the metallicity. This result offers a solution for the long-standing problem of silicon in stardust SiC grains, confirms the necessity of coupling chemistry and dynamics in simulations of the chemical evolution of our Galaxy, and constrains the modelling of dust condensation in stellar winds as function of the metallicity.

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arXiv2013

Europium s-process signature at close-to-solar metallicity in stardust SiC grains from AGB stars

Individual mainstream stardust silicon carbide (SiC) grains and a SiC-enriched bulk sample from the Murchison carbonaceous meteorite have been analyzed by the Sensitive High Resolution Ion Microprobe - Reverse Geometry (SHRIMP-RG) for Eu isotopes. The mainstream grains are believed to have condensed in the outflows of 1.5 to 3 Msun carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived from our measurements are compared with previous astronomical observations of carbon-enhanced metal-poor (CEMP) stars enriched in elements made by slow neutron captures (the s-process). Despite the difference in metallicity between the parent stars of the grains and the metal-poor stars, the fr(151Eu) values derived from our measurements agree well with fr(151Eu) values derived from astronomical observations. We have also compared the SiC data with theoretical predictions of the evolution of Eu isotopic ratios in the envelope of AGB stars. Because of the low Eu abundances in the SiC grains, the fr(151Eu) values derived from our measurements show large uncertainties, being in most cases larger than the difference between solar and predicted fr(151Eu) values. The SiC aggregate yields a fr(151Eu) value within the range observed in the single grains and provides a more precise result (fr(151Eu) = 0.54 +/- 0.03, 95 percent conf.), but is approximately 12 percent higher than current s-process predictions. The AGB models can match the SiC data if we use an improved formalism to evaluate the contribution of excited nuclear states in the calculation of the 151Sm(n,g) stellar reaction rate.

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arXiv2012

Fluorine variations in the globular cluster NGC 6656 (M22): implications for internal enrichment timescales

Observed chemical (anti)correlations in proton-capture elements among globular cluster stars are presently recognized as the signature of self-enrichment from now extinct, previous generations of stars. This defines the multiple population scenario. Since fluorine is also affected by proton captures, determining its abundance in globular clusters provides new and complementary clues regarding the nature of these previous generations, and supplies strong observational constraints to the chemical enrichment timescales. In this paper we present our results on near-infrared CRIRES spectroscopic observations of six cool giant stars in NGC 6656 (M22): the main objective is to derive the F content and its internal variation in this peculiar cluster, which exhibits significant changes in both light and heavy element abundances. We detected F variations across our sample beyond the measurement uncertainties and found that the F abundances are positively correlated with O and anticorrelated with Na, as expected according to the multiple population framework. Furthermore, our observations reveal an increase in the F content between the two different sub-groups, s-process rich and s-process poor, hosted within M22. The comparison with theoretical models suggests that asymptotic giant stars with masses between 4 and 5 Msun are responsible for the observed chemical pattern, confirming evidence from previous works: the difference in age between the two sub-components in M22 must be not larger than a few hundreds Myr.

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