Search for clues in the nearby star cradle
To clarify this, the astronomers set their sights on a particularly close and prominent star cradle: Rho Ophiuchi. This star-forming molecular cloud is only around 450 light-years away from us and contains several hot, bright young stars as well as some protostars that are still in their dust cocoons. Skretas and his colleagues examined five of these stars and their outflows using the MIRI spectrometer of the James Webb Telescope, which operates in the mid-infrared.
The MIRI images confirmed: Four of the five protostars and their jets show an excess of radiation from excited molecular hydrogen and ionized gases. This radiation is too strong to come from collisions of fast particles alone, according to a review using two different models. “The models show that an influence of strong UV radiation is necessary to explain the observed values,” write the astronomers.
Neighboring young stars are eliminated
But where does this excess UV radiation come from? “One possibility would be that the UV radiation comes from the massive young stars nearby,” says Skretas’ colleague Friedrich Wyrowski. Two such young stars lie close to the five protostars examined. If their UV radiation is the source, emissions from protostar jets should decrease as the distance to these UV sources increases. Because the five protostars are at different distances from the two young stars.
But that is not the case: “Although the strength of the UV field varies significantly depending on the protostar position, there are only small differences in the excitation states,” report the astronomers. “This suggests that neither these UV sources nor the environmental conditions play a major role in the excitation of molecular hydrogen.” In other words: The enigmatically strong glow of the hydrogen clouds and protostar cones does not seem to be due to an external influence.
What remains then?
But what is it then? So far, astronomers can only speculate about this. However, they consider it likely that the energy required to excite the protostar gases comes from its outflow itself. “Shock waves from the accretion of material on the protostars or shocks along the protostellar jets are the most likely sources of the UV radiation that excites the hydrogen gases,” write Skretas and his colleagues.
But this means that more energy is released during these star formation processes than previously assumed. If this is confirmed, the models for star formation will have to be rewritten accordingly. “In any case, more research is needed, both in the form of observations that limit the UV field strength more closely and in descriptive models,” explain the astronomers. (Astronomy & Astrophysics, 2025; doi: 10.1051/0004-6361/202554977)
Source: Astronomy & Astrophysics, Max Planck Institute for Radio Astronomy
December 1, 2025 – Nadja Podbregar
