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Interstellar Iron Isn't Missing, It's Just Hiding in Plain Sight

Iron (Fe) pseudocarbynes are likely widespread in the interstellar medium, where extremely cold temperatures would lead carbon chains to condense on Fe clusters. Over eons, the formation of complex organic molecules would be facilitated from Fe pseudocarbynes. The model shows a hydrogen-capped carbon chain attached to an Fe13 cluster (iron atoms are shown as reddish brown, carbon is gray, hydrogen is light gray). Credit: Pilarisetty Tarakeshwar, Arizona State University



By Robert Burnham (Arizona State University Communications) and Kimberly Mann Bruch (SDSC Communications)

XSEDE resources used to discover new class of molecules

Iron, which is largely known for steel manufacturing, is most typically found in gaseous form in stars such as the sun and in a more condensed form in planets such as Earth. Astrophysicists know that iron is one of the most abundant elements in the universe, after lightweight elements such as hydrogen, carbon, and oxygen.

Iron in interstellar environments should also be common, but astrophysicists detect only low levels of the gaseous kind. This implies to researchers that the missing iron exists in some kind of solid form or molecular state, yet identifying its hiding place has remained elusive.

A team of cosmo-chemists at Arizona State University, with support from the W.M. Keck Foundation, now claims that the mystery may be simpler than it seems. The iron isn't really missing, they say. Instead, it's hiding in plain sight. The iron is believed to have combined with carbon molecules to form molecular chains called iron pseudocarbynes. The spectra of these chains are almost identical with the much more common chains of carbon atoms, long known to be abundant in interstellar space.

The team used the National Science Foundation-funded Extreme Science and Discovery Environment (XSEDE)'s Comet supercomputer at the San Diego Supercomputer Center (SDSC), an Organized Research unit at the University of California San Diego, to validate their findings, published earlier this year in the Astrophysical Journal.

"We are proposing a new class of molecules that are likely to be widespread in the interstellar medium," said Pilarisetty Tarakeshwar, a research associate professor in ASU's School of Molecular Sciences. His co-authors, Peter Buseck and Frank Timmes, are both in ASU's School of Earth and Space Exploration; Buseck, an ASU regents professor, is also in the School of Molecular Sciences with Tarakeshwar.

The team examined how clusters containing only a few atoms of metallic iron might join with chains of carbon atoms to produce molecules combining both elements. Recent evidence obtained from stardust and meteorites indicates the widespread occurrence of clusters of iron atoms in the cosmos. In the extremely cold temperatures of interstellar space, these iron clusters act as deep-freeze particles, enabling carbon chains of various lengths to stick to them, thus producing different molecules from those that can occur during the gaseous phase of iron.

Said Tarakeshwar, "We used Comet to calculate what the spectra of these molecules would look like, and we found that they have spectroscopic signatures nearly identical to carbon-chain molecules without any iron." He added that because of this, "previous astrophysical observations could have overlooked these carbon-plus-iron molecules."

The researchers say this means that the missing iron in the interstellar medium is actually out in plain view but masquerading as common carbon-chain molecules.

"The calculations involving iron pseudocarbynes were computationally challenging because we had to optimize the geometries and calculate the spectroscopic properties of several open-shell systems," said Tarakeshwar. "The computational resources on Comet (including the software installed on it) were instrumental in enabling us to complete most preliminary calculations in a couple of months. The support of SDSC staff, especially Mahidhar Tatineni, was extremely valuable because many problems we encountered were expeditiously resolved as soon as we encountered them."

The new work may also solve another longstanding puzzle. Carbon chains with more than nine atoms are unstable, according to the research team. Yet observations have detected more complex carbon molecules in interstellar space. How nature builds these complex carbon molecules from simpler carbon molecules has been a longstanding mystery.

"Longer carbon chains are stabilized by the addition of iron clusters," said Buseck, noting that this opens a new pathway for building more complex molecules in space such as polyaromatic hydrocarbons, of which naphthalene — the main ingredient in mothballs — is a familiar example.

Said Timmes, "Our work provides new insights into bridging the yawning gap between molecules containing nine or fewer carbon atoms and complex molecules such as C60 buckminsterfullerene, better known as 'buckyballs.'"

Since its inception, computational chemistry has been extremely interdisciplinary. "Resources such as XSEDE, which are operational because of the shared expertise of several talented people, reinforce the idea that one can advance fundamental understanding and solve cutting-edge problems by using such resources