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Roederer I.U.,University of Michigan | Roederer I.U.,Joint Institute for Nuclear Astrophysics
Astrophysical Journal | Year: 2017

The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies in the Milky Way halo are generally underabundant by an order of magnitude or more when compared with stars in the halo field. Among the heavy elements produced by n-capture reactions, only Sr and Ba can be detected in red giant stars in most UFD galaxies. This limited chemical information is unable to identify the nucleosynthesis process(es) responsible for producing the heavy elements in UFD galaxies. Similar [Sr/Ba] and [Ba/Fe] ratios are found in three bright halo field stars, BD-18° 5550, CS 22185-007, and CS 22891-200. Previous studies of high-quality spectra of these stars report detections of additional n-capture elements, including Eu. The [Eu/Ba] ratios in these stars span +0.41 to +0.86. These ratios and others among elements in the rare Earth domain indicate an r-process origin. These stars have some of the lowest levels of r-process enhancement known, with [Eu/H] spanning -3.95 to -3.32, and they may be considered nearby proxies for faint stars in UFD galaxies. Direct confirmation, however, must await future observations of additional heavy elements in stars in the UFD galaxies themselves. © 2017. The American Astronomical Society. All rights reserved.

Seitenzahl I.R.,University of Würzburg | Seitenzahl I.R.,Max Planck Institute for Astrophysics | Timmes F.X.,Arizona State University | Timmes F.X.,Joint Institute for Nuclear Astrophysics | Magkotsios G.,Joint Institute for Nuclear Astrophysics
Astrophysical Journal | Year: 2014

We revisit the evidence for the contribution of the long-lived radioactive nuclides 44Ti, 55Fe, 56Co, 57Co, and 60Co to the UVOIR light curve of SN 1987A. We show that the V-band luminosity constitutes a roughly constant fraction of the bolometric luminosity between 900 and 1900 days, and we obtain an approximate bolometric light curve out to 4334 days by scaling the late time V-band data by a constant factor where no bolometric light curve data is available. Considering the five most relevant decay chains starting at 44Ti, 55Co, 56Ni, 57Ni, and 60Co, we perform a least squares fit to the constructed composite bolometric light curve. For the nickel isotopes, we obtain best fit values of M(56Ni) = (7.1 ± 0.3) × 10 -2 M ⊙ and M(57Ni) = (4.1 ± 1.8) × 10-3 M ⊙. Our best fit 44Ti mass is M(44Ti) = (0.55 ± 0.17) × 10-4 M ⊙, which is in disagreement with the much higher (3.1 ± 0.8) × 10-4 M ⊙ recently derived from INTEGRAL observations. The associated uncertainties far exceed the best fit values for 55Co and 60Co and, as a result, we only give upper limits on the production masses of M(55Co) < 7.2 × 10-3 M ⊙ and M(60Co) < 1.7 × 10-4 M ⊙. Furthermore, we find that the leptonic channels in the decay of 57Co (internal conversion and Auger electrons) are a significant contribution and constitute up to 15.5% of the total luminosity. Consideration of the kinetic energy of these electrons is essential in lowering our best fit nickel isotope production ratio to [57Ni/56Ni] = 2.5 ± 1.1, which is still somewhat high but is in agreement with gamma-ray observations and model predictions. © 2014. The American Astronomical Society. All rights reserved.

Denissenkov P.A.,University of Victoria | Denissenkov P.A.,Joint Institute for Nuclear Astrophysics | Herwig F.,University of Victoria | Herwig F.,Joint Institute for Nuclear Astrophysics | And 3 more authors.
Astrophysical Journal | Year: 2013

After off-center C ignition in the cores of super asymptotic giant branch (SAGB) stars, the C flame propagates all the way down to the center, trailing behind it the C-shell convective zone, and thus building a degenerate ONe core. This standard picture is obtained in stellar evolution simulations if the bottom C-shell convection boundary is assumed to be a discontinuity associated with a strict interpretation of the Schwarzschild condition for convective instability. However, this boundary is prone to additional mixing processes, such as thermohaline convection and convective boundary mixing. Using hydrodynamic simulations, we show that contrary to previous results, thermohaline mixing is too inefficient to interfere with the C-flame propagation. However, even a small amount of convective boundary mixing removes the physical conditions required for the C-flame propagation all the way to the center. This result holds even if we allow for some turbulent heat transport in the CBM region. As a result, SAGB stars build in their interiors hybrid C-O-Ne degenerate cores composed of a relatively large CO core (MCO 0.2 M⊙) surrounded by a thick ONe zone (ΔM⊙ONe ≳ 0.85 M⊙) with another thin CO layer above. If exposed by mass loss, these cores will become hybrid C-O-Ne white dwarfs. Otherwise, the ignition of C-rich material in the central core, surrounded by the thick ONe zone, may trigger a thermonuclear supernova (SN) explosion. The quenching of the C-flame may have implications for the ignition mechanism of SN Ia in the double-degenerate merger scenario. © 2013. The American Astronomical Society. All rights reserved.

Jadhav M.,University of Hawaii at Manoa | Pignatari M.,University of Basel | Herwig F.,University of Victoria | Herwig F.,Joint Institute for Nuclear Astrophysics | And 3 more authors.
Astrophysical Journal Letters | Year: 2013

Graphite is one of the many presolar circumstellar condensate species found in primitive meteorites. While the isotopic compositions of low-density graphite grains indicate an origin in core-collapse supernovae, some high-density grains have extreme isotopic anomalies in C, Ca, and Ti, which cannot be explained by envelope predictions of asymptotic giant branch (AGB) stars or theoretical supernova models. The Ca and Ti isotopic anomalies, however, match the predictions of He-shell abundances in AGB stars. In this study, we show that the C, Ca, and Ti isotopic anomalies are consistent with nucleosynthesis predictions of the H-ingestion phase during a very late thermal pulse (VLTP) event in post-AGB stars. The low 12C/13C isotopic ratios in these grains are a result of abundant 12C efficiently capturing the protons that are being ingested during the VLTP. Very high neutron densities of ∼1015 cm-3, typical of the i-process, are achieved during this phase in post-AGB stars. The large 42, 43, 44Ca excesses in some graphite grains are indicative of neutron capture nucleosynthesis during VLTP. The comparison of VLTP nucleosynthesis calculations to the graphite data also indicate that apparent anomalies in the Ti isotopic ratios are due to large contributions from 46, 48Ca, which cannot be resolved from the isobars 46, 48Ti during the measurements. We conclude that presolar graphite grains with moderate to extreme Ca and Ti isotopic anomalies originate in post-AGB stars that suffer a VLTP. © 2013. The American Astronomical Society. All rights reserved.

News Article | November 28, 2016

A research team led by University of Minnesota School of Physics and Astronomy Professor Yong-Zhong Qian uses new models and evidence from meteorites to show that a low-mass supernova triggered the formation of our solar system. The findings are published in the most recent issue of Nature Communications, a leading scientific journal. About 4.6 billion years ago, a cloud of gas and dust that eventually formed our solar system was disturbed. The ensuing gravitational collapse formed the proto-Sun with a surrounding disc where the planets were born. A supernova--a star exploding at the end of its life-cycle--would have enough energy to compress such a gas cloud. Yet there was no conclusive evidence to support this theory. In addition, the nature of the triggering supernova remained elusive. Qian and his collaborators decided to focus on short-lived nuclei present in the early solar system. Due to their short lifetimes, these nuclei could only have come from the triggering supernova. Their abundances in the early solar system have been inferred from their decay products in meteorites. As the debris from the formation of the solar system, meteorites are comparable to the leftover bricks and mortar in a construction site. They tell us what the solar system is made of and in particular, what short-lived nuclei the triggering supernova provided. "This is the forensic evidence we need to help us explain how the solar system was formed," Qian said. "It points to a low-mass supernova as the trigger." Qian is an expert on the formation of nuclei in supernovae. His previous research has focused on the various mechanisms by which this occurs in supernovae of different masses. His team includes the lead author of the paper, Projjwal Banerjee, who is a former Ph.D. student and postdoctoral research associate, and longtime collaborators Alexander Heger of Monash University, Australia, and Wick Haxton of the University of California, Berkeley. Qian and Banerjee realized that previous efforts in studying the formation of the solar system were focused on a high-mass supernova trigger, which would have left behind a set of nuclear fingerprints that are not present in the meteoric record. Qian and his collaborators decided to test whether a low-mass supernova, about 12 times heavier than our sun, could explain the meteoritic record. They began their research by examining Beryllium-10, a short-lived nucleus that has 4 protons (hence the fourth element in the periodic table) and 6 neutrons, weighing 10 mass units. This nucleus is widely distributed in meteorites. In fact the ubiquity of Beryllium-10 was something of a mystery in and of itself. Many researchers had theorized that spallation--a process where high-energy particles strip away protons or neutrons from a nucleus to form new nuclei--by cosmic rays was responsible for the Beryllium-10 found in meteorites. Qian said that this hypothesis involves many uncertain inputs and presumes that Beryllium-10 cannot be made in supernovae. Using new models of supernovae, Qian and his collaborators have shown that Beryllium-10 can be produced by neutrino spallation in supernovae of both low and high masses. However, only a low-mass supernova triggering the formation of the solar system is consistent with the overall meteoritic record. "The findings in this paper have opened up a whole new direction in our research," Qian said. "In addition to explaining the abundance of Beryllium-10, this low-mass supernova model would also explain the short-lived nuclei Calcium-41, Palladium-107, and a few others found in meteorites. What it cannot explain must then be attributed to other sources that require detailed study." Qian said the group would like to examine the remaining mysteries surrounding short-lived nuclei found in meteorites. The first step, however is to further corroborate their theory by looking at Lithium-7 and Boron-11 that are produced along with Beryllium-10 by neutrino spallation in supernovae. Qian said they may examine this in a future paper and urged researchers studying meteorites look at the correlations among these three nuclei with precise measurements. The research is funded by the Department of Energy Office of Nuclear Physics. Qian, Banerjee, and Heger are also scientific participants of the Joint Institute for Nuclear Astrophysics-Center for the Evolution of the Elements, a National Science Foundation Physics Frontier Center. To read the full paper, entitled "Evidence from stable isotopes and Be-10 for solar system formation triggered by a low-mass supernova," visit the Nature Communications website.

The star is a rare relic from the Milky Way's formative years. As such, it offers astronomers a precious opportunity to explore the origin of the first stars that sprung to life within our galaxy and the universe. A Brazilian-American team including Vinicius Placco, a research assistant professor at the University of Notre Dame and a member of JINA-CEE (Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements), and led by Jorge Meléndez from the University of São Paulo used two of European Southern Observatory's telescopes in Chile to discover this star, named 2MASS J18082002–5104378. The star was spotted in 2014 using ESO's New Technology Telescope. Follow-up observations using ESO's Very Large Telescope discovered that, unlike younger stars such as the sun, this star shows an unusually low abundance of what astronomers call metals—elements heavier than hydrogen and helium. It is so devoid of these elements that it is known as an ultra metal-poor star. Although thought to be ubiquitous in the early universe, metal-poor stars are now a rare sight within both the Milky Way and other nearby galaxies. Metals are formed during nuclear fusion within stars, and are spread throughout the interstellar medium when some of these stars grow old and explode. Subsequent generations of stars therefore form from increasingly metal-rich material. Metal-poor stars, however, formed from the unpolluted environment that existed shortly after the Big Bang. Exploring stars such as 2MASS J18082002–5104378 may unlock secrets about their formation, and show what the universe was like at its very beginning. The results have been published in Astronomy & Astrophysics. Explore further: Galactic archaeologists uncover new insights into the formation of the earliest stars and galaxies

Hinkel N.R.,Arizona State University | Hinkel N.R.,San Francisco State University | Timmes F.X.,Arizona State University | Timmes F.X.,Joint Institute for Nuclear Astrophysics | And 3 more authors.
Astronomical Journal | Year: 2014

We compile spectroscopic abundance data from 84 literature sources for 50 elements across 3058 stars in the solar neighborhood, within 150 pc of the Sun, to produce the Hypatia Catalog. We evaluate the variability of the spread in abundance measurements reported for the same star by different surveys. We also explore the likely association of the star within the Galactic disk, the corresponding observation and abundance determination methods for all catalogs in Hypatia, the influence of specific catalogs on the overall abundance trends, and the effect of normalizing all abundances to the same solar scale. The resulting stellar abundance determinations in the Hypatia Catalog are analyzed only for thin-disk stars with observations that are consistent between literature sources. As a result of our large data set, we find that the stars in the solar neighborhood may reveal an asymmetric abundance distribution, such that a [Fe/H]-rich group near the midplane is deficient in Mg, Si, S, Ca, Sc II, Cr II, and Ni as compared to stars farther from the plane. The Hypatia Catalog has a wide number of applications, including exoplanet hosts, thick- and thin-disk stars, and stars with different kinematic properties. © 2014. The American Astronomical Society. All rights reserved.

West C.,University of Minnesota | West C.,Joint Institute for Nuclear Astrophysics | Heger A.,University of Minnesota | Heger A.,Joint Institute for Nuclear Astrophysics | Heger A.,Monash University
Astrophysical Journal | Year: 2013

All stellar evolution models for nucleosynthesis require an initial isotopic abundance set to use as a starting point. Generally, our knowledge of isotopic abundances of stars is fairly incomplete except for the Sun. We present a first model for a complete average isotopic decomposition as a function of metallicity. Our model is based on the underlying nuclear astrophysics processes, and is fitted to observational data, rather than traditional forward galactic chemical evolution modeling which integrates stellar yields beginning from big bang nucleosynthesis. We first decompose the isotopic solar abundance pattern into contributions from astrophysical sources. Each contribution is then assumed to scale as a function of metallicity. The resulting total isotopic abundances are summed into elemental abundances and fitted to available halo and disk stellar data to constrain the model's free parameter values. This procedure allows us to use available elemental observational data to reconstruct and constrain both the much needed complete isotopic evolution that is not accessible to current observations, and the underlying astrophysical processes. As an example, our model finds a best fit for Type Ia contributing ≃ 0.7 to the solar Fe abundance, and Type Ia onset occurring at [Fe/H] ≃ -1.1, in agreement with typical values. © 2013. The American Astronomical Society. All rights reserved.

Ibeling D.,Cambridge College | Ibeling D.,University of Minnesota | Heger A.,University of Minnesota | Heger A.,Monash University | Heger A.,Joint Institute for Nuclear Astrophysics
Astrophysical Journal Letters | Year: 2013

Understanding the progenitors of core-collapse supernovae (SNe) and their population statistics is a key ingredient for many current studies in astronomy, but as yet this remains elusive. Using the MESA stellar evolution code, we study the dependence of the lower mass limit for making core-collapse SNe a function of initial stellar metallicity. We find that this mass limit is smallest at [Z] -2 with a value of 8.3 M ⊙. At [Z] = 0 the limit is 9.5 M ⊙ and continues to rise with higher metallicity. As a consequence, for a fixed initial mass function the SN rate may be 20%-25% higher at [Z] = -2 than at [Z] = 0. This affects the association of observed SN rates as a probe for the cosmological star formation rate, rate predictions for SN surveys, and population synthesis studies. © 2013 The American Astronomical Society. All rights reserved.

Hawley W.P.,Arizona State University | Athanassiadou T.,Arizona State University | Athanassiadou T.,Joint Institute for Nuclear Astrophysics | Timmes F.X.,Arizona State University | Timmes F.X.,Joint Institute for Nuclear Astrophysics
Astrophysical Journal | Year: 2012

We systematically explore zero impact parameter collisions of white dwarfs (WDs) with the Eulerian adaptive grid code FLASH for 0.64 + 0.64 M ⊙ and 0.81 + 0.81 M ⊙ mass pairings. Our models span a range of effective linear spatial resolutions from 5.2 × 10 7 to 1.2 × 107cm. However, even the highest resolution models do not quite achieve strict numerical convergence, due to the challenge of properly resolving small-scale burning and energy transport. The lack of strict numerical convergence from these idealized configurations suggests that quantitative predictions of the ejected elemental abundances that are generated by binary WD collision and merger simulations should be viewed with caution. Nevertheless, the convergence trends do allow some patterns to be discerned. We find that the 0.64 + 0.64 M ⊙ head-on collision model produces 0.32 M ⊙ of 56Ni and 0.38 M ⊙ of 28Si, while the 0.81 + 0.81 M ⊙ head-on collision model produces 0.39 M ⊙ of 56Ni and 0.55 M ⊙ of 28Si at the highest spatial resolutions. Both mass pairings produce 0.2 M ⊙ of unburned 12C+16O. We also find the 0.64 + 0.64 M ⊙ head-on collision begins carbon burning in the central region of the stalled shock between the two WDs, while the more energetic 0.81 + 0.81 M ⊙ head-on collision raises the initial post-shock temperature enough to burn the entire stalled shock region to nuclear statistical equilibrium. © 2012. The American Astronomical Society. All rights reserved..

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