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Astrochemistry

Universe’s first molecules finally found in space

Observation of long-sought molecule could change astrochemical models

by Sam Lemonick
April 17, 2019 | APPEARED IN VOLUME 97, ISSUE 16

 

09716-leadcon-nebula.jpg
Credit: Hubble/NASA/ESA/judy Schmidt
After a long search for the molecule, researchers found HeH+ in this planetary nebula.

Around 380,000 years after the Big Bang, the plasma that was our universe cooled enough for nuclei and electrons to begin combining. Helium was the first atom, and helium atoms soon bonded with protons to make the universe’s first molecule, helium hydride (HeH+).

While chemists made HeH+ in the laboratory as early as 1925, it wasn’t until the 1970s that scientists suggested it might be found in the interstellar medium of space. Four decades later, astronomers report they have finally observed it (Nature 2019, DOI: 10.1038/s41586-019-1090-x). What they’re seeing may change models of chemical reactions in space.

The failure to spot HeH+ had been “nagging” scientists, University of Georgia physicist Phillip Stancil says. He was one of the authors of a 2002 paper that established HeH+ as the universe’s first molecule. Stancil says it bothered scientists that they couldn’t back up the 1970s models of the universe’s chemistry with experimental data. But now, he says, they can.

Rolf Güsten of the Max Planck Institute for Radio Astronomy and colleagues identified a spectral line characteristic of HeH+ in observations of the planetary nebula NGC 7027, one of the places HeH+ was thought likely to be found. The nebula’s HeH+ is not left over from the primordial universe. Planetary nebulae form after stars similar in size to our sun collapse, ejecting a shell of gas and leaving a white dwarf star at its center. The HeH+ Güsten’s group observed formed on the inside of that shell.

09716-leadcon-plane.jpg
Credit: NASA Photo/Jim Ross
Researchers detected HeH+ using NASA's plane-based telescope SOFIA.

A few factors complicated previous efforts to observe HeH+, the first being its relative scarcity. The researchers identified the molecule’s ground state rotational transition at 149.1 µm. A carbon-hydrogen bond transition at 149.09 µm often masks this signal. It’s also obscured by water and other molecules in Earth’s atmosphere, making ground observations unlikely.

No flying or planned space telescopes are designed to observe this part of the spectrum. So in 2016, the researchers flew a spectrometer on a converted airplane operated by NASA that flies a telescope above the thickest part of the atmosphere, at an altitude higher than 12,000 meters. Güsten’s team used a heterodyne spectrometer, which compares incoming light to a reference light, a technique that can increase sensitivity. The researchers were able to distinguish the HeH+ transition from C–H lines in their data.

“It has been a long search,” says Jérôme Loreau, a physicist who has developed models of how HeH+ and other molecules developed in the early universe. One reason this observation is interesting, he says, “is that there is a discrepancy between the predicted abundance of HeH+ and the observed abundance.” Based on their observations, Güsten’s team calculated an HeH+ abundance that’s a factor of 3 higher than predicted by models of chemical reactions in space.

That mismatch indicates room for improvement in astrophysical models, Loreau says. Güsten agrees. “I would expect that models of the early universe will change,” he says.

Stancil is less certain. He says he has already made plans with a colleague to recalculate those HeH+ reaction rates in light of this finding. But he says a lot of assumptions went into the researchers’ calculations that might be masking something scientists don’t yet fully understand. Nevertheless, he says this discovery is likely to spur new modeling and lab work that could improve our understanding of chemistry in space and the early universe.

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Comments
Bill (April 24, 2019 6:54 PM)
"But he says a lot of assumptions went into the researchers’ calculations that might be masking something scientists don’t yet fully understand. "

Well, that never stopped us before, why would it now? Scientists are great storytellers.
Dr.Paul C.Li (April 24, 2019 11:29 PM)
Wilson Cloud chamber with or without a magnetic field may give some insight according to calculated effective nuclear charge per unit surface where sizes and weights are counted in the experiment.
Dr. Paul C. Li (April 28, 2019 6:56 AM)
Dear Honorable C&EN Editors in Chief:
I have finished the estimation for the vibrational stretching frequency of HeH+ through the use of Richard M. Badger’s empirical equation and Hooke’s law.

k * (delta x) = [ (1.86 * 100,000)/ (delta x) square]

where delta x is the stretching length approximated by bond length * percent elongation

Hooke’s law in terms of vibrational frequency ( nu bar/ 1307) square = (k/mu)

Bond length of helium hydride cation is assumed to be just like He atom 128 pm, while the proton cation was about 6.6 x10 to the -4 in pm according to John Easley ref. 1988 Oxford.

The cubic nature of the denominator in RMB’s k value allows me to use mathematical approximation to ignore the hot (higher order term ) when the radius of proton is 10 thousands of that He.

Now it is straight forward to get what one expects in nu bar about 3325 wave numbers or 1/cm. The percent elongation is assumed 27.4 commonly known for many many bonds even in C60, the famous Buckminsterfullerene when treated as two hemispheres.

Submitted to you in memory of my previous honorable Advisors.

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