Stars of all ages and masses emit electromagnetic energy in different ways, and these emissions attract the attention of astronomers. Each of these emissions is a clue to how stars form, evolve, and even die. Young stars are known for their high luminosity and their high level of activity. They have strong stellar winds and powerful magnetic fields.
One of young stars’ most spectacular features are their jets. Their powerful magnetic fields play an important role in these jets, as the material travels along magnetic field lines. New research found magnetic fields around one massive young star with a distinctive circular polarization (CP) feature. This is the first time that CP has been detected around such a massive young star.
The research is “First Detection of Circular Polarization in Radio Continuum Toward a Massive Protostar,” and it’s published in The Astrophysical Journal Letters. The lead author is Amal G. Cheriyan, a PhD student at the Indian Institute of Space Science and Technology. The researchers used the National Radio Astronomy Observatory’s (NRAO) Karl G Jansky Very Large Array (VLA) in the USA to observe the CP.
“Polarization measurements provide strong constraints on magnetic fields in star-forming systems,” the authors write. “While magnetic field estimates of a few kG (kilogauss) have been obtained near the surfaces of low-mass protostars, there are no analogous measurements in the immediate vicinity of the surface of massive protostars.”
While all protostars are scientifically interesting, massive protostars attract attention because of the effects they have on their surroundings as they evolve. Their extreme luminosity and powerful stellar winds shape their surroundings in ways that lower-mass stars don’t. Their powerful UV radiation can ionize the interstellar medium, and their winds can carve out bubbles in it. They can heat the surrounding gas and make it turbulent, which inhibits star formation.
Astronomers have detected CP around low-mass protostars, and even around black holes, but never around such a high-mass protostar before. The young star is named IRAS 18162-2048, has about 10 solar masses, and is about 5,500 light-years away. IRAS 18162 “drives the largest known, highly collimated and most luminous jet in our Galaxy—the HH 80–81 jet,” the authors explain. HH 80-81 is a Herbig-Haro object, which are created when jets from young stars send ionized gas into nearby clouds of gas and dust in the ISM.
This Hubble Space Telescope image shows HH 80 (right) and HH 81 (left). Herbig-Haro objects are created when jets slam into the ISM. Since the ISM is clumpy, HH objects change over time. CC SA 4.0 Image Credit: https://commons.wikimedia.org/w/index.php?curid=132051483
Previous research from 2010 detected magnetized jets coming from HH 80-81, the first time they were ever detected, showing that protostars can have magnetized jets. This discovery builds on that, and is the first time that magnetic fields have been found coming directly form the protostar itself. While these fields have been detected around less massive protostars, finding them around massive protostars like IRAS 18162-2048 has been difficult.
Measuring the CP allows scientists to estimate the magnetic field close to the stellar surface. “However, no analogous measurements are available for massive protostars,” they write. “Massive protostars – stars that will evolve to have mass more than 8-10 times that of the Sun – are much harder to study. The circular polarization we’re looking for is very faint and sporadic, making such measurements very challenging,” explains lead author Cheriyan.
“This is the first inference of the magnetic field strength using circular polarization in radio waves from a massive protostar,” said Prof Sarita Vig of the Indian Institute of Space Science and Technology (IIST), who conceptualized the work.
CP has also been detected at active galactic nuclei. So detecting it at massive protostars draws a link between them, lower mass protostars, and black holes.
“The detection of circular polarization is an exceptionally rare and challenging feat – even in active galactic nuclei (AGNs), where conditions are extreme, but better investigated,” said Prof Nirupam Roy from the Indian Institute of Science (IISc).
“Observing it in the environment of a massive protostar, buried in dense gas and dust, is even more difficult, making this result very remarkable,” added Prof Samir Mandal of IIST.
Only a small number of OB stars, which IRAS 18162-2048 is on its way to becoming, have surface magnetic fields of several hundreds to thousands of gauss. It’s possible that these magnetic fields are fossils from earlier stages of the star’s life. This is called the fossil fields hypothesis.
“According to this theory, magnetic fields from the interstellar medium permeate molecular clouds, and as these clouds undergo gravitational collapse into protostars, the fields are both advected and amplified,” the researchers write in their paper. “This process can generate magnetic fields up to a few hundred gauss in massive stars, which is consistent with the observed results.”
This illustration shows the binary star HD 45166. Its primary star is extremely magnetic, and IRAS 18162-2048 could be on the path to becoming one of these magnetic stars. Image Credit: By P. Marenfeld, M. Zamani – https://noirlab.edu/public/images/noirlab2323a/, CC BY 4.0.
All stars have magnetic fields, but some stars have very strong and very stable magnetic fields that set them apart. IRAS 18162-2048 could end up becoming one of them. “Given the strength of the magnetic field estimated toward I18162, we speculate that I18162 could be a precursor to a massive magnetic star,” the researchers explain. As a result, the magnetic measurements of this star could help astrophysicists constrain and develop their models of star formation, even though the exact mechanism behind the CP is unclear.
Astrophysicists have thought for a long time that the same processes drive jets from stars and black holes. This research supports that, and is evidence of a universal jet-launching mechanism.