New measurements imply dramatically higher abundance of helium hydride ions in the early universe

**New measurements imply dramatically higher abundance of helium hydride ions in the early universe
Figure 1: Scheme of the CSR ring structure with stored HeH+ ion beam (red), merged electron beam (blue), reaction products (green) and particle detector (detailed reaction scheme below). Credit: MPIK

Physicists report the first laboratory measurements of electron reactions with helium hydride ions in the cryogenic storage ring CSR at the Max Planck Institute for Nuclear Physics in Heidelberg. At temperatures down to 6 K, the reaction rates destroying the molecule were found to be significantly lower compared to previous measurements at room temperature. This translates into a strongly enhanced abundance of this primordial molecule acting as a coolant for first star and galaxy formation in the early universe.

Just three minutes after the Big Bang, the chemical composition of the universe was settled: 75 percent hydrogen, 25 percent helium, and trace amounts of lithium, all created by primordial nucleosynthesis. However, in this early state, all matter was fully ionized, consisting of free bare nuclei and a hot electron gas, a "foggy" plasma for the cosmological background radiation.

About 400,000 years later, the expanding universe cooled down to a level where electrons and nuclei started to combine into neutral atoms. Space became transparent, but no stars were yet born; thus, this era is called the "dark ages." As the temperature dropped further, collisions of neutral helium with still abundant free protons formed the first molecule—the helium hydride ion (HeH⁺), which marks the dawn of chemistry. HeH+ and other early molecular species played an essential role in cooling primordial gas clouds via infrared emission, a necessary step for star formation.

The understanding and modeling of the latter processes require a detailed knowledge of abundances and of the relevant molecules. However, information up to now has been rather limited, particularly in the low-temperature regime (< 100 K) of the late dark ages, about 300 Million years after the Big Bang, when the first stars formed. Very recently, HeH⁺ was discovered in our galaxy by detecting its far-infrared emission.

**New measurements imply dramatically higher abundance of helium hydride ions in the early universe
Figure 2: Plasma temperature dependence of the recombination rate coefficients, measured here for individual rotational states (J = 0, 1, 2, ...), compared to previous data tables. Credit: MPIK

The abundance of HeH⁺ is critically determined by destructive reactions. At low temperatures, this is dominated by so-called dissociative recombination (DR) with free electrons: once neutralized by an electron capture, helium hydride dissociates into helium and hydrogen atoms. Previous results available in data tables for the reaction rates were based on laboratory experiments at . Under these conditions, the molecules are in highly excited rotational states that were suspected to influence the electron capture processes.

In order to gain insight into the low-temperature behavior, physicists from the division of Klaus Blaum at the Heidelberg Max Planck Institute for Nuclear Physics (MPIK) investigated collisions of HeH⁺ with electrons at the institute's cryogenic storage ring CSR. This unique facility was designed and built for laboratory astrophysics under space-like conditions regarding temperature and density. The CSR provides an environment of temperatures below 10 K and an excellent vacuum (observed down to < 10⁻¹⁴ mbar). The researchers studied the recombination using an electron target in which the stored ion beam is immersed in a co-propagating electron beam over a distance of about one meter (Figure 1). The relative velocities can be tuned down to zero, which provides access to very low-collision energies. The reaction products from the electron-ion interaction zone are detected downstream, thus providing absolute reaction rates (Figure 1).

At a temperature of 6 K inside the CSR, the scientists observed the stored HeH⁺ ions to cool down to the rotational ground state within a few tens of seconds. During this radiative cooling process, the researchers followed the population of the individual rotational states and extracted the state-selective DR probability (Figure 2).

"We find the electron recombination rates for the lowest rotational levels of HeH⁺ to be up to a factor of 80 below the values given in the data tables so far," says Oldřich Novotný, principal investigator of the experiment. "This dramatic decrease is largely due to the lower temperatures used in our laboratory measurements. It translates in a strongly enhanced abundance of this primordial molecule in the era of first star and galaxy formation."

The new result, now provided with unprecedented details, is of great relevance for both the understanding of the reaction itself as well as for the modeling of the early universe. For collision theory, HeH⁺ is still a challenging system. Here, the measurements help to benchmark the theory codes. The experimental DR reaction rates, now available for various electron energies and rotational states, can be translated into the environmental properties used in model calculations for the chemistry of the primordial gas. This and prospective future studies using the CSR provide broadly applicable data. Considering the imminent launch of the James Webb Space Telescope, the new capabilities of laboratory astrophysics are particularly timely, since its search for the first luminous objects and galaxies after the Big Bang will benefit greatly from reliable predictions on chemistry.


Explore further

Physicists reveal the role of diffusion in the early universe

More information: Oldřich Novotný et al. Quantum-state–selective electron recombination studies suggest enhanced abundance of primordial HeH+, Science (2019). DOI: 10.1126/science.aax5921
Journal information: Science

Provided by Informationsdienst Wissenschaft
Citation: New measurements imply dramatically higher abundance of helium hydride ions in the early universe (2019, July 19) retrieved 22 August 2019 from https://phys.org/news/2019-07-imply-higher-abundance-helium-hydride.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
253 shares

Feedback to editors

User comments

Jul 20, 2019
Helium hydride ion

First produced in a laboratory in 1925
Is stable in isolation
But extremely reactive
Cannot be prepared in bulk
Because it reacts with any other molecule it comes into contact with
The strongest known acid
A cation with the chemical formula HeH+
Consists of a helium atom bonded to a hydrogen atom
With one electron removed
It can be viewed as protonated helium
It is the lightest heteronuclear ion
Is one of the first compounds formed in the Universe after the Bigbang

Fascinating
This Helium hydride ion in this vacuum of space detected this April 2019
Reacting with any other molecule it comes into contact in the vacuum
For it appears
The rules of molecule formation appear in this vacuum
A million miles before this plasma condenses to stellar starlet nurseries
https://en.wikipe...ride_ion

Jul 20, 2019
A factor 80 in less recombination rate would mean that much, or two orders of magnitude, more early "coolant". This can be huge, it will be interesting to see if it affects modeling of early universe and how quickly clouds, galaxies and stars formed.

Jul 22, 2019
To my mind, after the Big Bang, it took five thousand years to re-assimilate the scattered mass gave rise to dark matter. The dark matter was bombarded with anti-matter waves to form wave matter. Most probably quarks which combined to form a neutron. To my mind neutron released electron forming a proton. This was Hydrogen which within a minute transformed into Helium. The dark matter was probably matter of noble element as Helium, Neon, Argon, Xenon, Krypton, and Radon. Neon decomposed as fluorine. Thus electronegativity took place.

Jul 23, 2019
To my mind


I.e. free fantasies without support. FWIW, I stopped reading right there.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more