James Webb Space Telescope spies mysterious high-energy radiation in star nursery

Using the James Webb Space Telescope, astronomers have detected unexpected high-energy ultraviolet radiation around five infant stars, or protostars, in the Ophiuchus star birthing region. The discovery could prompt a change in our models of star formation.

Protostars represent the initial stage of stars’ lifetimes following their formation via the collapse of dense patches of gas and dust in molecular clouds. They are still surrounded by envelopes of the material from which they form, and from which they gather mass. This mass-gathering process continues until the protostar is massive enough to trigger the fusion of hydrogen to helium in its core, the process that defines an adult, “main sequence” star.

“We wanted to take a closer look at protostars, young stars that are still forming deep inside their parent molecular clouds,” team member Iason Skretas, researcher from Max Planck Institute for Radio Astronomy, said in a statement. “As protostars accrete mass, they launch part of it outward in the form of jets.”

This team discovered that in order to understand these powerful outflows from infant stars, scientists have to factor in ultraviolet radiation.

“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,” research team member Agata Karska, from the Center for Modern Inter-disciplinary Technologies at Nicolaus Copernicus University in Torun, Poland, said in the statement. “Where did it come from? What is its source: internal or external? We decided to investigate this.”

Tantrum-throwing young stars

To conduct this investigation, the team turned the James Webb Space Telescope (JWST) and its Mid-Infrared Instrument (MIRI) toward the Ophiuchus molecular cloud in the constellation of Ophiuchus, the Serpent-bearer. Located around 450 light-years away from Earth, this molecular cloud is home to many young, hot “B-type” stars that emit strong ultraviolet radiation.

This allowed the team to make detailed observations of five protostars located at different distances from these massive stars. In particular, the scientists were interested in emissions from molecular hydrogen.

Composed of two hydrogen atoms, molecular hydrogen is the most abundant molecule in the universe. Molecular hydrogen can’t be seen by ground-based telescopes through the atmosphere of Earth, but it is still tricky to detect in molecular clouds from space. That’s because the temperature of these vast complexes of dense star-forming gas is too low to excite these molecules.

However, when outflows from young stars strike surrounding molecular clouds, shockwaves are created that heat matter and cause molecular hydrogen to emit telltale emissions. These are emissions that can be spotted by the JWST and MIRI. making them the ideal scientific tag-team to study outflows from protostars.

The JWST observations clearly demonstrated to the researchers that ultraviolet radiation is present around protostars in Ophiuchus. But the big question is, where is this radiation coming from?

One possible source of this ultraviolet radiation is processes occurring immediately around the protostars. This could include shocks caused when material from the molecular cloud falls to the protostar. Alternatively, the radiation may be generated by shocks along the jet of material erupting from these tantrum-throwing infant stars.

Another possibility is that the ultraviolet radiation is coming from close massive stars, which are shining their light on their protostar neighbors. To eliminate this external source, the team considered the properties of the surrounding stars and their distances to the protostars. The researchers then factored in the capability of the dust around the protostars to absorb the ultraviolet radiation and re-emit it at longer wavelengths of light.

“Using these two methods, we showed that ultraviolet radiation, in terms of external conditions, varies significantly between our protostars, and therefore we should see differences in molecular emission,” Skretas said. “As it turns out, we don’t see them.”

This meant the team could reject external sources like neighboring stars as the sources of this radiation.

“We can say with certainty that UV radiation is present in the vicinity of the protostar, as it undoubtedly affects the observed molecular lines,” Karska said. “Therefore, its origin has to be internal.”

The researchers will continue to study JWST observations of not just the gas and protostars of the Ophiuchus molecular cloud, but also the dust and ice in this region. The goal is to get to the bottom of unexplained ultraviolet radiation around these objects.

The team’s research was published on Nov. 13 in the journal Astronomy & Astrophysics.