NUCLEAR ASTROPHYSICS WITH THE R-PROCESS
The r-process is one of the fundamental ways that stars produce the heaviest elements. We cannot reproduce the r-process in any laboratory on Earth, so the state of modeling the r-process relies heavily on matching astronomical observations. I’ve led numerous projects using the Space Telescope Imaging Spectrograph (STIS) and Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) to detect and measure abundances of previously-undetected elements in metal-poor stars. For example, the “peaks” of the r-process distribution are especially sensitive constraints on the nucleosynthesis models, and my work has lead to the first detection of the first and second r-process peaks in halo stars. The impact of this work is that models of r-process nucleosynthesis have new single-event abundance patterns, which are more complete among key elements, to use in excluding models and tuning the detailed physics.
I am also a leader of a team called the R-Process Alliance. We are a collection of observers, modelers, theoreticians, and nuclear experimentalists with an interest in solving the big questions related to the r-process. The early phases of this project aim to quadruple the number of known highly r-process-enhanced stars, and the later phases of the project will lean on the multi-disciplinary nature of our team to provide new constraints and raise new questions about the long-sought nature of the astrophysical site(s) of the r-process.
Representative publications:
Roederer, I.U., Sakari, C.M., Placco, V.M., et al.
Astrophysical Journal, 865, 129 (2018)
Roederer, I.U., Lawler, J.E., Sobeck, J.S., et al.
New Hubble Space Telescope Observations of Heavy Elements in Four Metal-Poor Stars.
Astrophysical Journal Supplement Series, 203, 27 (2012)
Detection of Elements at All Three r-process Peaks in the Metal-poor Star HD 160617.
Astrophysical Journal, 750, 76 (2012)
Roederer, I.U., Lawler, J.E., Cowan, J.J., et al.
Detection of the Second r-process Peak Element Tellurium in Metal-poor Stars.
Astrophysical Journal Letters, 747, L8 (2012)
Press releases:
An Element that’s rare on Earth is found far, far away
Selected External Funding:
The Origins of Germanium and the Transition to Neutron-Capture Nucleosynthesis.
HST-AR-13884, 2015-2018, $74,476
The Most Complete Template for r-process Nucleosynthesis beyond the Solar System.
HST-GO-12976, 2012-2015, $1,504
Production of the Heavy Elements in the Universe.
HST-GO-12268, 2011-2014, $54,960
NEAR-FIELD COSMOLOGY and DWARF GALAXY ARCHAEOLOGY
The galaxies and stellar systems in our Local Group provide a detailed view of the outcome of structure and galaxy formation in the early Universe. The chemistry and dynamics of the dwarf galaxies and stellar streams surrounding the Milky Way and the Andromeda Galaxy offer clues about the nature of the first stars, the first metals, the epoch of reionization, and the nature of dark matter. My team recently confirmed one of the most startling discoveries in the search for the site(s) of the r-process: one of the ultra-faint dwarf galaxies newly-discovered by the Dark Energy Survey, Reticulum II, is absolutely teeming with stars highly-enriched in r-process material. Many more fascinating stellar systems will be discovered in the next decade by the Dark Energy Survey, the Large Synoptic Survey Telescope, and the Gaia satellite. My team and I are working to collect, analyze, and interpret spectroscopy of stars in these systems to further our understanding of these local analogs of the high-redshift Universe.
Representative publications:
The Origin of the Heaviest Metals in Most Ultra-faint Dwarf Galaxies.
Astrophysical Journal, 835, 23 (2017)
Roederer, I.U., Mateo, M., Bailey, J.I.
Detailed Chemical Abundances in the r-process-rich Ultra-faint Dwarf Galaxy Reticulum 2.
Astronomical Journal, 151, 82 (2016)
Detailed abundance analysis of the brightest star in Segue 2, the least massive galaxy.
Monthly Notices of the Royal Astronomical Society, 440, 2665 (2014)
External Funding:
NSF award 1815403, 2018-2021, $329,793
STELLAR ARCHAEOLOGY
Chemistry can play a vital role in interpreting the hierarchical buildup of the Milky Way. One method is by chemically tagging stars. I have led work in this area by analyzing the detailed chemistry of a kinematically-cold stellar stream, sorting out the chemistry of the so-called inner and outer halo populations, performing the first analysis of the orbital properties of r-process-enhanced stars using high-quality astrometric data from the Gaia mission, and leading an abundance analysis of more than 40 elements in each of 313 metal-poor field stars selected mostly from the HK objective-prism survey. This hand-crafted analysis did not rely on the use of unsupervised automated measurement routines, and it is still the largest-scale survey of its kind to-date. We continue to find gems in it worth exploring further.
Representative publications:
Roederer, I.U., Hattori, K., Valluri, M.
Astronomical Journal, 156, 179 (2018)
Roederer, I.U., Preston, G.W., Thompson, I.B., et al.
A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-poor Stars.
Astronomical Journal, 147, 136 (2014)
Roederer, I.U., Sneden, C., Thompson, I.B., et al.
Astrophysical Journal, 711, 573 (2010)
Chemical Inhomogeneities in the Milky Way Stellar Halo.
Astronomical Journal, 137, 272 (2009)
Press Releases:
Massive hyper-runaway star ejected from the Milky Way Disk
External Funding:
HST-AR-15051, 2017-2020, $68,070
STIS Observations of Metal-Poor Stars: Direct Confrontation with Nucleosynthesis Predictions.
HST-GO-14232, 2016-2019, $57,609
The nucleosynthetic origins and chemical evolution of phosphorus in the early universe.
HST-AR-13246, 2013-2016, $27,549
METALS FROM THE FIRST STARS
Nearly every halo star that has been studied contains a detectable amount of elements heavier than the iron-group elements. We also see this in the class of carbon-enhanced metal-poor stars with low levels of heavy elements (“CEMP-no stars”). My team uses both ground-based optical spectra from the MIKE spectrograph at Magellan and space-based UV spectra from STIS and COS on HST. We are working to investigate the relationship between the light-element excesses and the heavy-element deficiencies to identify the nature of the first zero-metallicity Pop III stars that were the sites of the earliest metal production.
Representative publications:
Roederer, I.U., Placco, V.M., Beers, T.C.
Detection of Phosphorus, Sulphur, and Zinc in the Carbon-enhanced Metal-poor Star BD+44 493.
Astrophysical Journal Letters, 824, L19 (2016)
Roederer, I.U., Preston, G.W., Thompson, I.B., et al.
Neutron-capture Nucleosynthesis in the First Stars.
Astrophysical Journal, 784, 158 (2014)
Are There Any Stars Lacking Neutron-capture Elements? Evidence from Strontium and Barium.
Astronomical Journal, 145, 26 (2013)
Press releases:
Relics of the Milky Way’s first generation of stars
Explosions of universe’s first stars spewed powerful jets
External Funding:
A New Opportunity to Detect Iron in the Most Iron-Poor Star Known.
HST-GO-13827, 2016-2019, $46,472
HST-GO-14151, 2016-2019, $53,547
The First Detections of Phosphorus, Sulphur, and Zinc in a Bona-Fide Second-Generation Star.
HST-GO-14231, 2015-2018, $54,292
GLOBULAR CLUSTER FORMATION AND EVOLUTION
The heavy element abundances in stars in some globular clusters reveal yet another way that these stellar systems are complex stellar populations. For reasons that were neither predicted nor are understood, some massive Milky Way globular clusters do not have uniform distributions of the r- or s-process elements. Experience suggests this will teach us something about how (the progenitors of) globular clusters formed in the early universe.
Representative publications:
Roederer, I.U., Mateo, M., Bailey, J.I., et al.
Monthly Notices of the Royal Astronomical Society, 455, 2417 (2016)
Yong, D., Roederer, I.U., Grundahl, F., et al.
Iron and neutron-capture element abundance variations in the globular cluster M2 (NGC 7089).
Monthly Notices of the Royal Astronomical Society, 441, 3396 (2014)
Roederer, I.U., Marino, A.F., Sneden, C.
Astrophysical Journal, 742, 37 (2011)
External funding:
NSF award 1613536, 2016-2019, $144,914
