EI2GYB > ASTRO 26.11.25 11:45l 64 Lines 7003 Bytes #200 (0) @ WW
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Subj: Puzzling ultraviolet radiation in the birthplaces of stars
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Puzzling ultraviolet radiation in the birthplaces of stars
Researchers used the MIRI instrument onboard the James Webb Space Telescope (JWST) to identify the presence of ultraviolet radiation in five young stars in the Ophiuchus region, and to understand its role in the formation of stars. The discovery of UV radiation around these protostars and its significant impact on the surrounding material is a challenge to models describing the formation of stars.
The paper is published in Astronomy & Astrophysics, and the research team included Iason Skretas, a doctoral student at MPIfR, and Dr. Agata Karska (Center for Modern Inter-disciplinary Technologies at Nicolaus Copernicus University in Torun, Poland, and Max Planck Institute of Radio Astronomy (MPIfR), Bonn, Germany).
Investigating protostars and ultraviolet radiation
"We wanted to take a closer look at protostars, i.e., young stars that are still forming deep inside their parent molecular clouds. As protostars accrete mass, they launch part of it outward in the form of jets," says Skretas. These are called outflows, and are the most striking sign of star formation. The scientists were able to show that in order to understand the chemistry and physics of these molecular outflows from young stars, they must take into account the existence of 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. Where did it come from, what is its source: internal or external? We decided to investigate this," adds Karska.
Observing Ophiuchus with JWST's MIRI instrument
The JWST targeted young stars towards the constellation Ophiuchus (the Serpent-bearer) with MIRI, its Mid-Infrared Instrument. Located 450 light-years away from us, the Ophiuchus molecular cloud contains several B-type stars, which are very young, hot, and emit strongly in the ultraviolet (UV). Five objects located at different distances from these massive stars were selected for detailed observations.
The MIRI instrument allows us to observe astronomical objects in the wavelength range from 2 to 28 micrometers, covering multiple lines of molecular hydrogen (H2) which cannot be observed from the ground due to Earth's atmosphere. The JWST is indispensable for this type of observation, as it allows us to observe these lines even from very faint objects with high resolution.
The significance of molecular hydrogen
For astronomers, H2 is the most important molecule in the universe. First of all, it is the most abundant molecule as, on average, there is 10,000 times more H2 than carbon monoxide, the second most abundant molecule in space.
At the same time, the structure of H2 makes it very difficult to observe it in molecular clouds because the temperature is too low to excite the molecule. However, ejections from young stars produce shock waves that compress and heat up the matter, creating bright H2 emission. Therefore, JWST/MIRI is the perfect combination to study outflows from protostars.
Searching for the source of ultraviolet radiation
The analysis of the observations from JWST in Ophiuchus clearly demonstrates the presence of UV radiation in the vicinity of protostars and their outflows through the impact of the UV radiation on the molecular hydrogen. That leads to the question of, where is this radiation coming from? Is it related to processes occurring in the immediate vicinity of the protostar? For example, shocks formed during the infall of matter onto the protostar (accretion shocks), or shocks produced along the protostellar jet?
"One possibility is that the UV radiation originates from nearby massive stars that illuminate the birthplaces of the next generation of stars, so we started with this hypothesis," says Friedrich Wyrowski, also from the MPIfR. The astronomers used two methods to estimate the external UV radiation. The first relied on the properties of the surrounding stars and their distances from the observed sources. The second was based on the dust, which has the ability to absorb UV radiation and re-emit it at longer wavelengths.
"Using these two methods, we showed that UV radiation-in terms of external conditions-varies significantly between our protostars, and therefore we should see differences in molecular emission. As it turns out, we don't see them," adds Skretas.
"So we had to reject the hypothesis of an external source of radiation. However, we can say with certainty that UV radiation is present in the vicinity of the protostar, as it undoubtedly affects the observed molecular lines. Therefore, its origin has to be internal," adds Karska.
Implications for future star formation research
The results of this research indicate the necessity to include the production of UV radiation in the models describing the formation of stars. Future analysis of JWST observations will focus not only on the gas, but also on the composition of dust and ices, offering alternate ways to constrain the origin of UV radiation around protostars.
The increase of the number of observed sources, including observations covering the full extent of outflows, will be a crucial step in placing stricter constraints on the production sites of UV radiation.
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