Star formation should be a relatively straightforward process. Dense clouds of molecular hydrogen collapse under gravity, fragmenting into cores that grow into protostars. These infant stars are cold, deeply embedded in their parent clouds, and shouldn’t produce ultraviolet radiation. They’re not hot enough. Yet when astronomers used the James Webb Space Telescope’s MIRI instrument to observe five young stars in the Ophiuchus molecular cloud, 450 light years away, they found clear evidence of UV radiation affecting molecular hydrogen in outflows around these protostars.
“This is the first surprise. Young stars are not capable of being a source of radiation; they cannot ‘produce’ radiation. So we should not expect it and yet we have shown that UV occurs near protostars” – Agata Karska from the University of Torun and the Max Planck Institute for Radio Astronomy.
The discovery emerged from analysing molecular hydrogen emissions, which JWST can observe in unprecedented detail. Molecular hydrogen is the universe’s most abundant molecule, outnumbering carbon monoxide, the second most common, by 10,000 to one. When UV radiation hits H2 molecules, it excites them in specific ways that create distinctive spectral signatures.
The team initially hypothesised that the UV came from external sources, specifically, hot B-type stars scattered throughout Ophiuchus that were churning out UV radiation. The researchers calculated expected UV levels based on the distances between these massive stars and the observed protostars, then cross checked using dust properties, since dust absorbs UV and re-emits it at longer wavelengths.
The UV signatures remained consistent across all five protostars despite dramatically different external UV environments. The hypothesis failed. It seemed that protostars launch jets and outflows as they accrete material, compressing and heating the surrounding gas through shock waves. Perhaps these shocks, either where material falls onto the protostar or along the jets themselves, generate enough energy to produce UV radiation locally.
Infrared images from instruments at Kitt Peak National Observatory (left) and NASA’s Spitzer Space Telescope document the outburst of HOPS 383, a young protostar in the Orion star-formation complex (Credit : NASA)
The findings indicate that current star formation models need updating. Astronomers have largely ignored UV radiation in protostellar environments because theory suggests it shouldn’t exist. Now it appears essential to understanding the chemistry and physics of molecular outflows from young stars.
The research demonstrates how JWST’s mid-infrared capabilities continue to challenge assumptions about stellar birth. Understanding the source and impact of this unexpected UV radiation could reshape predictions about which molecules survive in protostellar environments and how planetary systems eventually form from the remaining material around these young stars.
Source : Puzzling ultraviolet radiation in the birthplaces of stars