Stars and planets form in cold, dense clouds of gas and dust. One such cloud is called Barnard 68, located about 500 light years from Earth in the constellation Ophiuchus. Its interior is 9 Kelvin (−264 °C) cold and so dense (and thus opaque) that light can hardly penetrate it. Cosmic rays play an important role here, as high-energy, charged particles from space ionise* the gas and regulate the chemistry and temperature, enabling the buildup of complex chemistry in these cold and dense regions.

The importance of cosmic rays is encoded in a key parameter, the cosmic-ray ionisation rate – the rate at which cosmic rays ionise molecular hydrogen per second. The cosmic-ray ionisation rate is one of the fundamental parameters in the chemistry of the molecular universe. Until now, estimations of the ionisation relied primarily on estimations through chemical line observations and models. ‘Previously, researchers had to take a roundabout approach by observing rare molecules such as protonated molecular hydrogen or molecular ions and then attempting to calculate the ionisation rate from their concentrations,’ says Gaches. However, such models depend on many assumptions – about density, temperature, electron abundance, and reaction pathways – and lead to highly variable results.

Researchers recently developed the idea of using the new, extremely sensitive James Webb Space Telescope to measure extremely faint infrared lines that arise when cosmic rays directly excite the gas. The theoretical concept of these directly excited lines dates back decades, but direct observations have evaded astronomers. Previous studies, including work by Gaches, used chemical models to show that these near-infrared lines are a reliable tracer of cosmic-ray ionization.

However, an international team led by the Technion Israel Institute of Technology has now clearly detected three of these lines – exactly as theoretical models had predicted for decades. Pointing the spectrometer of the James Webb Space Telescope at Barnard 68, it detected a faint glow of directly stimulated molecular hydrogen (H₂). This is the first time that cosmic rays have been observationally confirmed to directly stimulate measurable infrared lines. A follow up study, now in press, used these observations to directly measure rapidly cosmic rays lose energy in dense interstellar clouds.
These observations open up a new window of observational investigations into cosmic-ray physics and chemistry in star-forming regions. Future observations with the highly sought-after James Webb Space Telescope have also been approved to extend this analysis to another nearby cloud.


* During ionisation, electrons are stripped from atoms and molecules. The resulting ions are positively charged.