News & Publications
These are recent RPA papers, led by Core members and/or RPA students and postdocs. All are available in this ADS library.
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.
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 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)
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)
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 were 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 exhibit 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 bonafide 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)
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.
