Evolution of a young star

After almost 20 years, astronomers have made two VLA images of a young star, eight times more massive than the Sun, which reveal valuable clues about the primordial stages of its evolution. They allow observing almost in “real time” dramatic changes of stellar structure and they offer a unique opportunity that will follow the next steps in the coming years. The results can be found in Science.
Two radio images of a protostar, shooting at a distance of 18 years, astronomers are proving a substantial difference of its structure, a result that provides a kind of unique window that allows you to see almost in “real time” the primordial stages of evolution of a massive star. The study, published in Science, attempts to provide new insight into this category of stellar objects whose knowledge is far less complete than the solar-type stars.



The observations of the object, named with the initials W75N (B) -VLA 2 and located about 4200 light years, have been made with the telescope “Karl G. Jansky” Very Large Array (VLA). The researchers compared a picture of 2014 with another obtained in 1996. “The comparison is substantial,” says Carlos Carrasco-Gonzalez of the Center of Radioastronomy and Astrophysics at the National Autonomous University of Mexico and author of the study.
The new radio image is the result of work dating back to 2009 when a group of researchers, led by Gabriele Surcis the Joint Institute for VLBI in Europe (JIVE) and co-author of the study, was able to measure for the first time with the European network of radio telescopes (European VLBI Network, EVN) the orientation of the magnetic field in the region where there is the star in question. “We noticed that the magnetic field surrounding W75N (B) -VLA 2 shows a preferred direction that is aligned with the global magnetic field present in that region,” says Surcis. “Now, we have found that the stellar wind follows the same direction of the magnetic field, a fact which indicates that they are very important magnetic forces in the process of formation of the star.”
“We are watching these dramatic changes ‘near real time’ as the object is developing a characteristic bipolar morphology,” adds Huib van Langevelde director of JIVE and co-author of the study. The image 1996 shows a compact region associated with a strong stellar wind, consisting of charged particles, which is emitted from the star. Instead, the image most recently observed as this region has assumed a much more elongated. “The observations of this object we are providing a unique opportunity, exciting, that will allow us to follow over the next few years as a protostar follows the initial stages of its evolution,” says Carrasco-Gonzalez.



Scientists believe that the object to be forming in a dense environment and gaseous and which is surrounded by a structure of dust donut-shaped, astronomers call bull. In addition, scientists have noticed that the star emits a strong stellar wind for some periods that can last several years. Initially, this flow of charged particles has expanded in all directions, forming a spherical envelope surrounding the star, and then ended by interacting with the bull which slows it down. The part of the stellar wind which moves outwards exiting from the two polar regions of the torus, where there is less resistance, propagates more quickly forming an elongated structure. “In the course of just 18 years, we have observed exactly what we had expected,” says Carrasco-Gonzalez.
“The evolution that we are looking after nearly 18 years following the models very carefully,” says Ciriaco Goddi of Radboud University Nijmegen, and co-author of the study. In fact, there are some models that have been developed specifically to explain why nearly spherical expansion of such stellar winds has been observed in a star more massive than the Sun, when it was expected to see a stream more content, in the form of beam, based on observations of solar-type stars, less massive, which are in a similar stage of development. Scientists estimate that W75N (B) -VLA 2 is at least eight times more massive than the sun. Usually, more uniform stellar winds are observed in the stars that have some thousands of years, which is the phase in which it is believed to be currently W75N (B) -VLA 2. “Our knowledge on the evolution of massive young stars and is much less complete than that relating to solar-type stars. At this point, it will be very interesting to be able to observe at least in a phase of transformation. Therefore, we expect to learn many things from this object, “concludes Carrasco-Gonzalez.

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