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. 

The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-Poor Star with an Extreme R-Process Enhancement of [Eu/H] = -0.14

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)


Roederer, I.U., Lawler, J.E. 

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:


Old star, new trick


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:


Roederer, I.U. 

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)


Roederer, I.U., Kirby, E.N. 

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:


Collaborative Research: Dark Matter and Substructure in the Galactic Halo with Gaia and Multi-Object Spectroscopy

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. 

Kinematics of Highly R-Process-Enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites

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. 

Characterizing the Chemistry of the Milky Way Stellar Halo: Detailed Chemical Analysis of a Metal-poor Stellar Stream

Astrophysical Journal, 711, 573 (2010)


Roederer, I.U. 

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:


A New Test of Copper and Zinc Abundances in Late-Type Stars Using Cu II and Zn II Lines in the Near-Ultraviolet

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)


Roederer, I.U. 

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


Constraining Pop III supernova energies and the formation of the first low-mass stars with the iron-poor star HE1327-2326 (with [Fe/H] = -5.4).

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. 

Detailed chemical abundances in NGC 5824: another metal-poor globular cluster with internal heavy element abundance variations

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. 

Characterizing the Heavy Elements in Globular Cluster M22 and an Empirical s-process Abundance Distribution Derived from the Two Stellar Groups

Astrophysical Journal, 742, 37 (2011)



External funding:


A Focused Test of Whether Neutron-Capture Elements Are Another Expression of the Multiple Populations Phenomenon in Globular Clusters

NSF award 1613536, 2016-2019, $144,914



 

My research addresses fundamental problems in nuclear astrophysics and near-field cosmology, using stellar chemistry to understand the formation and evolution of the Milky Way and Local Group and the origin of the heaviest elements.