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These are recent RPA papers, led by Core members and/or RPA students and postdocs. All are available in this ADS library.
The R-Process Alliance: Enrichment of r-process Elements in a Simulated Milky Way-Like Galaxy
Hirai, Y., Beers, T. C., Lee, Y. S., Wanajo, S., Roederer, I. U., Tanaka, M., Chiba, M., Saitoh, T. R., Placco, V. M., Hansen, T. T., Ezzeddine, R., Frebel, A., Holmbeck, E. M., Sakari, C. M.
The Astrophysical Journal, in press (arXiv:2410.11943)
Here we investigate the origin of r-process enriched stars with a detailed cosmological zoom-in simulation of a Milky Way-like galaxy. The simulations show that most stars with no overabundance in r-process elements, as well as the strongly r-process enhanced r-II stars ([Eu/Fe] > +0.7), are formed in dwarf galaxies accreted by the Milky Way within 6 Gyr after the Big Bang. In contrast, over half of the moderately enhanced r-I stars (+0.3 < [Eu/Fe] ≤ +0.7) are formed in the main in-situ disk after 6 Gyr. The results are compared with high-resolution spectroscopic observations of r-process-enhanced metal-poor stars in the halo and dwarf galaxies, including those collected by the R-Process Alliance.
The R-Process Alliance: Hunting for gold in the near-UV spectrum of 2MASS J05383296-5904280
Hansen, T. T., Roederer, I. U., Shah, S. P., Ezzeddine, R., Beers, T. C., Frebel, A., Holmbeck, E. M., Placco, V. M., Sakari, C. M., Ji, A. P., Marshall, J. L., Mardini, M. K., Chiti, A.
Astronomy & Astrophysics, 697, A127 (2025)
This paper presents the third r-process-enhanced star the RPA has observed with the Hubble Space Telescope. From the optical and near-UV spectra, we derive abundances for 43 elements, including 26 neutron-capture elements. In particular, abundances of the rarely studied elements Nb, Mo, Cd, Lu, Os, Pt, and Au are derived from the HST spectrum. These results, combined with RPA near-UV observations of two additional r-II stars, increase the number of Cd abundances derived for r-process-enriched stars from seven to ten and Au abundances from four to seven. A large star-to-star scatter is detected for both of these elements, highlighting the need for more detections enabling further investigations, specifically into possible non-LTE (local thermodynamic equilibrium) effects.
Roederer, I. U., Beers, T.C., Hattori, K., Placco, V.M., Hansen, T.T., Ezzeddine, R., Frebel, A., Holmbeck, E.M., Sakari, C.M.
The Astrophysical Journal, 971, 158 (2024)
This paper describes our analysis and interpretation of the most extreme r-process-enhanced star known yet. We detected uranium for only the 8th time ever in this star, which was useful in determining the age. We also calculated that this star is likely so extreme because it formed in a cloud of gas where the r-process material was not very diluted.
Bandyopadhyay, A., Ezzeddine, R., Allende Prieto, C., Aria, N., Shah, S. P., Beers, T. C., Frebel, A., Hansen, T. T, Holmbeck, E. M., Placco, V. M., Roederer, I. U., & Sakari, C. M.
The Astrophysical Journal Supplement Series, 274, 39, (2024)
This is the fifth RPA data release, presenting the analysis of 41 metal-poor stars observed with the HORuS spectrograph on the Gran Telescopio Canarias. The focus for this sample was on the fainter stars, so the sample extends down to V = 15.8. Four new r-II stars, three new r-I stars, and one new limited-r star were identified in the sample.
The R-Process Alliance: Analysis of limited-r stars
Xylakis-Dornbusch, T., Hansen, T. T., Beers, T. C., Christlieb, N., Ezzeddine, R., Frebel, A., Holmbeck, E. M., Placco, V. M., Roederer, I. U., Sakari, C. M., & Sneden, C.
Astronomy & Astrophysics, 688, A123, (2024)
One focus of the RPA has been identifying stars with small Eu abundances and larger amounts of light r-process elements like Sr in their atmospheres compared to the heavy r-process elements like Eu, the so-called limited-r stars. In this paper, we analyse three of these stars and find that the lanthanide fraction of limited-r stars is lower than that of the 2017 kilonova (AT2017gfo). A dynamical analysis of the stars also showed that, unlike the r-I and r-II stars, the current sample of limited-r stars is largely born in the Galaxy and is not accreted.
Shah, S. P., Ezzeddine, R., Roederer, I. U., Hansen, T. T., Placco, V. M., Beers, T. C., Frebel, A., Ji, A. P., Holmbeck, E. M., Marshall, J., & Sakari, C. M.
Monthly Notices of the Royal Astronomical Society, 529, 1917 (2024)
This paper presents a detailed analysis of near-UV and optical observations of a highly r-process-enhanced star obtained with the Hubble Space Telescope and the Magellan Telescopes. Abundances are derived for 41 elements in total, including 23 r-process elements and rarely probed species such as Al II, Ge I, Mo II, Cd I, Os II, Pt I, and Au I. The star has an unusually high Ge enhancement of [Ge/Fe] = +0.10 and the highest Cd abundance observed for a metal-poor star to date.
Element abundance patterns in stars indicate fission of nuclei heavier than uranium
Press releases:
This meta analysis of the abundance patterns of 42 metal-poor stars found that some lighter r-process elements (Ru, Rh, Pd, and Ag) correlated with particular heavier ones (Eu, Gd, Dy, and more). The best explanation for this signature is that these two groups of elements share a common origin as the fission fragments of radioactive transuranic nuclei with A > 260. This finding indicates that r-process events can produce nuclei with A > 260, which is heavier than any others detected in space or naturally on Earth, and it is a nice confirmation of predictions that such heavy elements undergo fission.
Shank, D., Beers, T. C., Placco, V. M., Gudin, D., Catapano, T., Holmbeck, E. M., Ezzeddine, R., Roederer, I. U., Sakari, C. M., Frebel, A., & Hansen, T. T
The Astrophysical Journal, 943, 36 (2023)
The second paper presenting a dynamic analysis of r-process enhanced stars. This paper analyses 1700 stars, including stars from the RPA and GALAH surveys. The paper uses the HDBSCAN unsupervised learning algorithm to identify 36 chemodynamically tagged groups (CDTGs) containing between five and 22 members and 17 CDTGs with at least 10 member stars. In addition, previously known Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the splashed disk, the metal-weak thick disk, the Helmi stream, LMS-1 (Wukong), and Thamnos are unidentified, and associations with Milky Way globular clusters are determined for seven CDTGs.
Roederer, I. U., Cowan, J.J., Pignatari, M., Beers, T.C., Den Hartog, E.A., Ezzeddine, R., Frebel, A., Hansen, T.T., Holmbeck, E.M., Mumpower, M.R., Placco, V.M., Sakari, C.M., Surman, R., Vassh, N.
The Astrophysical Journal, 936, 84 (2022)
This paper shows that at least one relatively common source of r-process elements in the early Universe produced a consistent pattern among the *lighter* r-process elements Se, Sr, Y, Zr, Nb, Mo, and Te - including both the first and second r-process peaks. This surprising result has important consequences for interpreting the physics of the r-process and its host site(s).
Roederer, I. U., Lawler, J. E., Den Hartog, E. A., Placco, V. M., Surman, R., Beers, T. C., Ezzeddine, R., Frebel, A., Hansen, T. T., Hattori, K., Holmbeck, E. M., & Sakari, C. M.
The Astrophysical Journal Supplement Series, 260, 27 (2022)
Press release: Astronomers Find "Gold Standard" Star in Milky Way
This paper presents a new r-process abundance template - 42 r-process elements! - derived from a highly r-process-enhanced star that likely formed from the remnants of a single r-process event that occurred in the early Universe. We propose that this r-process pattern should serve as an alternative to the calculated Solar System r-process residual pattern when comparing with r-process model predictions. Table 3 lists the main results.
Gudin, D., Shank, D., Beers, T. C., Yuan, Z., Limberg, G., Roederer, I. U., Placco, V., Holmbeck, E. M., Dietz, S., Rasmussen, K. C., Hansen, T. T., Sakari, C. M., Ezzeddine, R., & Frebel, A.
The Astrophysical Journal, 908, 79 (2021)
This paper presents the dynamical analysis of 446 r-process-enhanced metal-poor stars, mainly originating from the RPA data releases. Using the HDBSCAN algorithm, the work uncovers 30 individual chemodynamically tagged groups (CDTGs), with 21 containing between 3 and 5 stars and 9 containing between 6 and 12 stars. The r-process-enhanced stars in these CDTGs have likely experienced common chemical-evolution histories, presumably in their parent satellite galaxies or globular clusters, prior to being disrupted into the Milky Way's halo.
Holmbeck, E. M., Hansen, T. T., Beers, T. C., Placco, V. M., Whitten, D. D., Rasmussen, K. C., Roederer, I. U., Ezzeddine, R., Sakari, C. M., Frebel, A., Drout, M. R., Simon, J. D., Thompson, I. B., Bland-Hawthorn, J., Gibson, B. K., Grebel, E. K., Kordopatis, G., Kunder, A., Meléndez, J., Navarro, J. F., Reid, W. A., Seabroke, G., Steinmetz, M., Watson, F., & Wyse, R. F. Ġ.
The Astrophysical Journal Supplement Series, 249, 30 (2020)
In DR4, we use our 595-star sample to redefine the separation between r-I and r-II stars from [Eu/Fe]=+1.0 to [Eu/Fe]=+0.7, identifying a total of 72 new r-II stars.
Ezzeddine, R., Rasmussen, K., Frebel, A., Chiti, A., Hinojisa, K., Placco, V. M., Ji, A. P., Beers, T. C., Hansen, T. T., Roederer, I. U., Sakari, C. M., & Melendez, J.
The Astrophysical Journal, 898, 150 (2020)
The third data release paper from the RPA presents the analysis of 141 metal-poor stars from spectra obtained with Magellan/MIKE. The sample includes ten new r-II stars, 62 new r-I stars, and 17 new limited-r stars.
Cain, M., Frebel, A., Ji, A. P., Placco, V. M., Ezzeddine, R., Roederer, I. U., Hattori, K., Beers, T. C., Meléndez, J., Hansen, T. T., & Sakari, C. M.
The Astrophysical Journal, 898, 40 (2020)
In this paper, we report the discovery of J1521−3538, a bright (V = 12.2), very metal-poor ([Fe/H] = −2.8), strongly r-process enhanced field horizontal branch star. J1521−3538 shows the largest r-process element overabundance in any known r-process-enhanced star, with [Eu/Fe] = +2.2. We propose to extend the existing classification of r-process enhanced stars to also include an r-III class, with [Eu/Fe] > 2.0, or 100 times the solar ratio of europium to iron.
An MIT UROP student project.
The R-process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5-455510
Placco, V. M., Santucci, R. M., Yuan, Z., Mardini, M. K., Holmbeck, E. M., Wang, X., Surman, R., Hansen, T. T., Roederer, I. U., Beers, T. C., Choplin, A., Ji, A. P., Ezzeddine, R., Frebel, A., Sakari, C. M., Whitten, D. D., & Zepeda, J.
The Astrophysical Journal, 897, 78 (2020)
RAVE J183−4555 is an extremely metal-poor star, highly enhanced in CNO, and with discernible contributions from the rapid neutron-capture process. The presence of Ba, La, and Ce abundances above the solar system r-process predictions suggests that there must have been a non-standard source of r-process elements operating at such low metallicities. One plausible explanation is that this enhancement originates from material ejected at unusually high velocities in a neutron star merger event. We also explore the possibility that the neutron-capture elements were produced during the evolution and explosion of a rotating massive star. In addition, based on comparisons with yields from zero-metallicity faint supernovae, we speculate that RAVE J1830−4555 was formed from a gas cloud pre-enriched by both progenitor types.
Sakari, C. M., Roederer, I. U., Placco, V. M., Beers, T. C., Ezzeddine, R., Frebel, A., Hansen, T., Sneden, C., Cowan, J. J., Wallerstein, G., Farrell, E. M., Venn, K. A., Matijevič, G., Wyse, R. F. G., Bland-Hawthorn, J., Chiappini, C., Freeman, K. C., Gibson, B. K., Grebel, E. K., Helmi, A., Kordopatis, G., Kunder, A., Navarro, J., Reid, W., Seabroke, G., Steinmetz, M., & Watson, F.
The Astrophysical Journal, 874, 148 (2019)
A high-resolution analysis of a low-alpha, r-II star. The low-alpha abundances indicate that the star was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. Without the contribution of this Type Ia material, the star would have been even more r-process enhanced, possibly similar to Reticulum II.
Placco, V. M., Santucci, R. M., Beers, T. C., Chanamé, J., Sepúlveda, M. P., Coronado, J., Rossi, S., Lee, Y. S., Starkenburg, E., Youakim, K., Barrientos, M., Ezzeddine, R., Frebel, A., Hansen, T. T., Holmbeck, E. M., Ji, A. P., Rasmussen, K. C., Roederer, I. U., Sakari, C. M., & Whitten, D. D.
The Astrophysical Journal, 870, 122 (2019)
Observing campaign to identify low-metallicity stars in the Best & Brightest Survey. From medium-resolution (R~1200–2000) spectroscopy of 857 candidates, we estimate the stellar atmospheric parameters, as well as carbon and α-element abundances. A subset of the confirmed low-metallicity stars was followed up with high-resolution spectroscopy, as part of the R-process Alliance, with the goal of identifying new highly and moderately r-process-enhanced stars. Comparison between the stellar atmospheric parameters estimated in this work and from high-resolution spectroscopy exhibits good agreement, confirming our expectation that medium-resolution observing campaigns are an effective way of selecting interesting stars for further, more targeted, efforts.
Sakari, C. M., Placco, V. M., Farrell, E. M., Roederer, I. U., Wallerstein, G., Beers, T. C., Ezzeddine, R., Frebel, A., Hansen, T., Holmbeck, E. M., Sneden, C., Cowan, J. J., Venn, K. A., Davis, C. E., Matijevič, G., Wyse, R. F. G., Bland-Hawthorn, J., Chiappini, C., Freeman, K. C., Gibson, B. K., Grebel, E. K., Helmi, A., Kordopatis, G., Kunder, A., Navarro, J., Reid, W., Seabroke, G., Steinmetz, M., & Watson, F.
The Astrophysical Journal, 868, 110 (2018)
The first northern hemisphere snapshot survey of 126 metal-poor targets, which identified new limited-r, r-I, and r-II stars.
Roederer, I. U., Sakari, C. M., Placco, V. M., Beers, T. C., Ezzeddine, R., Frebel, A., & Hansen, T. T.
The Astrophysical Journal, 865, 129 (2018)
This paper reports the identification of the brightest-known highly r-II star, HD 222925, and presents a chemical abundance analysis of its optical spectrum .
Cain, M., Frebel, A., Gull, M., Ji, A. P., Placco, V. M., Beers, T. C., Meléndez, J., Ezzeddine, R., Casey, A. R., Hansen, T. T., Roederer, I. U., & Sakari, C.
The Astrophysical Journal, 864, 43 (2018)
We present detailed chemical abundances of three new bright (V ∼ 11), extremely metal-poor ([Fe/H] ∼ −3.0), r-process-enhanced halo red giants. We measured abundances for 20–25 neutron-capture elements in each of our stars. J1432−4125 is among the most r-process-rich r-II stars, with [Eu/Fe] = +1.44. J2005−3057 is an r-I star with [Eu/Fe] = +0.94. J0858−0809 has [Eu/Fe] = +0.23, and exhibits [C/Fe]corr = +0.76, thus adding to the small number of known carbon-enhanced r-process stars.
An MIT UROP student project
Gull, M., Frebel, A., Cain, M. G., Placco, V. M., Ji, A. P., Abate, C., Ezzeddine, R., Karakas, A. I., Hansen, T. T., Sakari, C., Holmbeck, E. M., Santucci, R. M., Casey, A. R., & Beers, T. C.
The Astrophysical Journal, 862, 174 (2018)
We discovered RAVE J094921.8−161722, a bright (V = 11.3) metal-poor red giant star with [Fe/H] = −2.2. We find J0949−1617 to be a CEMP star with s-process enhancement that must have formed from gas enriched by a prior r-process event. Using nucleosynthesis model predictions, we conclude that J0949−1617 is the first bona fide CEMP-r + s star identified.
An MIT UROP student project.
The R-Process Alliance: 2MASS J09544277+5246414, the Most Actinide-enhanced R-II Star Known
Holmbeck, E. M., Beers, T. C., Roederer, I. U., Placco, V. M., Hansen, T. T., Sakari, C. M., Sneden, C., Liu, C., Lee, Y. S., Cowan, J. J., & Frebel, A.
The Astrophysical Journal, 859, L24 (2018)
Detailed analysis of the most actinide-enhanced star known based on a high-resolution, high signal-to-noise spectrum obtained with the Harlan J. Smith 2.7 m telescope. Using the Th/U chronometer, we estimate an age of 13.0 ± 4.7 Gyr for this star.
Spectroscopic Validation of Low-metallicity Stars from RAVE
Placco, V. M., Beers, T. C., Santucci, R. M., Chanamé, J., Sepúlveda, M. P., Coronado, J., Points, S. D., Kaleida, C. C., Rossi, S., Kordopatis, G., Lee, Y. S., Matijevič, G., Frebel, A., Hansen, T. T., Holmbeck, E. M., Rasmussen, K. C., Roederer, I. U., Sakari, C. M., & Whitten, D. D.
The Astronomical Journal, 155, 256 (2018)
Medium-resolution (R~2000) spectroscopic follow-up campaign of 1694 bright (V < 13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 and 5. Follow-up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allowed for the determination of carbon and α-element abundances. Selected stars from our validated list were followed-up with high-resolution spectroscopy by the RPA.
Hansen, T. T., Holmbeck, E. M., Beers, T. C., Placco, V. M., Roederer, I. U., Frebel, A., Sakari, C. M., Simon, J. D., & Thompson, I. B.
The Astrophysical Journal, 858, 92 (2018)
The first data release paper from the RPA presenting the scope and phases of the RPA along with the chemical analysis of 107 metal-poor targets from the southern hemisphere snapshot survey.
The r-process Pattern of a Bright, Highly r-process-enhanced Metal-poor Halo Star at [Fe/H] ~ -2
Sakari, C. M., Placco, V. M., Hansen, T., Holmbeck, E. M., Beers, T. C., Frebel, A., Roederer, I. U., Venn, K. A., Wallerstein, G., Davis, C. E., Farrell, E. M., & Yong, D.
The Astrophysical Journal, 854, L20 (2018)
A high-resolution analysis of an r-II star.
RAVE J203843.2-002333: The First Highly R-process-enhanced Star Identified in the RAVE Survey
Placco, V. M., Holmbeck, E. M., Frebel, A., Beers, T. C., Surman, R. A., Ji, A. P., Ezzeddine, R., Points, S. D., Kaleida, C. C., Hansen, T. T., Sakari, C. M., & Casey, A. R.
The Astrophysical Journal, 844, 18 (2017)
We report the discovery of RAVE J2038-0023, a bright (V = 12.73), very metal-poor, r-process-enhanced star selected from the RAVE survey. High-resolution spectroscopy allowed for the determination of chemical abundances for 24 neutron-capture elements, including thorium and uranium. RAVE J2038-0023 is among five metal-poor stars with a clearly measured U abundance. The derived chemical abundance pattern exhibits good agreement with those of other known highly r-process-enhanced stars. Age estimates were calculated using U/X abundance ratios, yielding a mean age of 13.0 ± 1.1 Gyr.
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