Ian U. Roederer - Research

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. My work is based on the analysis and interpretation of high-resolution spectroscopy of stars. I am best known for my studies into the nature of the astrophysical r-process, which is one of the fundamental ways that stars and stellar remnants produce the heaviest elements found in nature. This process, which is almost certainly associated with the extreme conditions present at the births and deaths of neutron stars, uses a rapid (the “r” in “r-process”) burst of neutrons to overwhelm light nuclei.

Career research highlights:

Discovered fission products of elements heavier than uranium in stars, demonstrating that fission occurs in the r-process, and that nuclei with atomic masses > 260 are regularly produced in nature
Discovered 40% of all r-process enhanced stars known
Co-discovered the first-known r-process enhanced galaxy, Reticulum II
Published the most complete chemical inventory for any object beyond the Solar System
Made the first detections of Ga, As, Se, Cd, In, Sb, Te, Lu, W, and Re—more than 15% of the elements detectable—in the spectra of cool stars useful for studying nucleosynthesis
Released the largest set of hand-crafted abundance derivations (48 elements in each of 313 metal-poor stars), which is widely used as a calibration and comparison standard
Produced the first study of the orbital kinematics of r-process enhanced stars, revealing probable dwarf galaxy or extragalactic origins
Published the first study of the detailed chemistry of stars in any stellar stream representing one of the major building blocks of the Milky Way
Found the first evidence that the Milky Way globular/star cluster metallicity floor may extend to [Fe/H] ~ −3, a factor of 2.5 times lower than previously thought

Please follow this link for a complete list of my publications from NASA's Astrophysics Data System. A few high-impact studies are summarized below.

Element abundance patterns in stars indicate fission of nuclei heavier than uranium

Roederer, I. U., Vassh, N., Holmbeck, E.M., Mumpower, M.R., Surman, R., Cowan, J.J., Beers, T.C., Ezzeddine, R., Frebel, A., Hansen, T.T., Placco, V.M., Sakari, C.M.

Science, 382, 1177 (2023)

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.

The R-Process Alliance: A Nearly Complete R-Process Abundance Template Derived from Ultraviolet Spectroscopy of the R-Process-Enhanced Metal-Poor Star HD 222925

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

BONUS! (lower left): This star was featured during commissioning time for the new GHOST spectrograph at Gemini-South. (credit: International Gemini Observatory/NOIRLab/NSF/AURA/GHOST Consortium)

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. If you have a favorite kilonova model, for example, compare with these results to constrain your predicted r-process yields. Table 3 lists the the main results.

Kinematics of Highly r-process-enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites

Roederer, I.U., Hattori, K., & Valluri, M.

The Astronomical Journal, 156, 179 (2018)

This study presented the first large-scale study of the kinematics of r-process-enhanced stars in the Milky Way, based on newly released astrometry from the Gaia satellite. We discovered that r-process-enhanced stars are clustered in energy-action space (think: angular momentum is conserved), which revealed that these stars are the remnants of low-mass ultra-faint dwarf galaxies that were tidally disrupted by the Milky Way long ago.

Detailed Chemical Abundances in the r-process-rich Ultra-faint Dwarf Galaxy Reticulum 2

Roederer, I.U., Mateo, M., Bailey, J.I. III, Song, Y., Bell, E.F., Crane, J.D., Loebman, S., Nidever, D.L., Olszewski, E.O., Shectman, S.A., Thompson, I.B., Valluri, M., Walker, M.G.

The Astronomical Journal, 151, 82 (2016)

This paper presented the co-discovery of a galaxy (!) where most of the stars are rich in r-process material. This unusual property was not predicted by theory. Along with complementary work by Ji et al. (2016), we concluded that the existence of a very small fraction of low-mass dwarf galaxies with high levels of r-process enrichment implies that the r-process must occur in rare, prolific events, such as mergers of neutron stars or rare kinds of supernovae.

A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-poor Stars

Roederer, I.U., Preston, G.W., Thompson, I.B., Shectman, S.A., Sneden, C., Burley, G.S., Kelson, D.D.

The Astronomical Journal, 147, 136 (2014)

This paper presents the largest hand-crafted set of detailed abundances available: up to 48 elements in each of 313 metal-poor stars. It represents the last stage of the spectroscopic followup from the well-known Beers-Preston-Shectman objective-prism survey for metal-poor stars (also known as the "HK Survey"). It is widely used as a comparison sample of abundance standards for studies of the Milky Way, Local Group, and other stellar populations.

Are There Any Stars Lacking Neutron-capture Elements? Evidence from Strontium and Barium

Roederer, I.U.

The Astronomical Journal, 145, 26 (2013)

This study demonstrated that there are no stars known at present that are lacking in the heaviest elements listed on the periodic table. Why? Presumably some of the first stars (Population III stars) were capable of producing small amounts of these heavy elements in every environment where stars formed - the Milky Way, all dwarf galaxies, and globular clusters.

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

Roederer, I.U., Sneden, C., Thompson, I.B., Preston G.W., Shectman, S.A.

The Astrophysical Journal, 711, 573 (2010)

This was the first study of the detailed chemistry of stars in a stellar stream that formed from the disruption of one of the confirmed building blocks of the Milky Way halo, the Helmi Stream. We showed that the stellar chemistry strongly favored a dwarf galaxy progenitor system. This chemical approach is commonly used now as a way to determine the nature of the stellar progenitor system.

The first detections of lots of lines of elements previously undetected in stars useful for studying nucleosynthesis

This is not a single study, but a collection of studies that I have led over the years. We made the first clear detections of gallium (Ga, Z = 31), arsenic (As, Z = 33), selenium (Se, Z = 34), cadmium (Cd, Z = 48), indium (In, Z = 49), antimony (Sb, Z = 51), tellurium (Te, Z = 52), lutetium (Lu, Z = 71), tungsten (W, Z = 74), and rhenium (Re, Z = 75) in FGK-type stars. These detections allow us to derive abundances of each of these elements, and a lot of these elements are particularly useful for constraining the properties of r-process nucleosynthesis. We also did a lot of work to improve the detectability of aluminum (Al, Z = 13), phosphorus (P, Z = 15), zinc (Zn, Z = 30), osmium (Os, Z = 76), gold (Au, Z = 79), and lead (Pb, Z = 82). The key for all of these detections is high-resolution UV spectra collected using the Hubble Space Telescope.

I also am involved in developing the scientific motivations that guide the designs of high-resolution spectrographs on the next generation of telescopes.

ELT ANDES: I chair one of the science working groups for the ANDES instrument, the optical and near-infrared high-resolution spectrograph for the Extremely Large Telescope, which will be the largest optical telescope when it is opened in 2028 in Chile.
UV spectroscopy from space: I have long been an advocate for a high-resolution UV spectrograph on the next large UV-optical-infrared space telescope [1, 2, 3, 4, 5, 6], which the National Academies' Astro2020 Decadal Survey ranked as its highest priority for development, with a target launch date in the 2040s.
The US-ELT program: I led a group that gamed out what large projects on a US Extremely Large Telescope (US-ELT) Program might look like. Here were some of our ideas [1, 2, 3, 4] that helped the National Academies' Astro2020 Decadal Survey rank this program as its highest recommendation for ground-based astronomy in the next decade.
And more: I am also on the science or commissioning teams for the GHOST instrument at Gemini and the IFU-M instrument at Magellan. Years ago I also served on the science team for the Large Earth Finder (née G-CLEF) instrument at the Giant Magellan Telescope.