The nebula flashing

Thanks to the observations of the Very Large Telescope , researchers led by Chris De Pree were able to observe changes in the intensity in the issue of the nebula Sgr B2 occurred over a period of just 23 years. This result helps to clarify the mechanisms that regulate the formation of massive stars and their interaction with the environment that surrounds them.

A false color image obtained by the Very Large Telescope shows the ionized gas in the star-forming region Sgr B2. The bright spots and filaments visible indicate areas where there is ionized gas around massive stars.



The emission nebulae, composed primarily of hydrogen and ionized hot (hence their technical name H II, where H stands for hydrogen, II is the term that indicates the state of ionization, due to the loss of its sole electron) shine light of its own thanks to the intense flux of radiation, especially ultraviolet, produced by massive young stars that lie within them. But why, despite the large flux of radiation received, these nebulae are not swept away? To give an answer to this fundamental question that racked the brains of astrophysicists for several decades, the research led by Chris De Pree, director of the Bradley Observatory at Agnes Scott College in the United States, recently published in an article in The Astrophysical Journal Letters was tested.

De Pree and his colleagues, observing the nebula known as Sgr B2 (Sagittarius B2 ) with the Jansky Very Large Array ( VLA ) radio telescope in New Mexico, have confirmed those theories that were previously only theoretical speculations . That is, that in the clouds where massive stars are forming filamentous structures also gather gas that absorb a major fraction of the ultraviolet radiation. Its result was that the nebula as a whole tends temporarily to cool and reduce its brightness.

“In the old theoretical model, it is formed when a massive star the H II region around it ‘ lights up ‘ and begins to expand. Everything was clear and orderly,” says De Pree. “But the group of theoretical astrophysicists has developed simulations that demonstrate how the continued growth phases of the star during its formation and that the matter would continue to fall toward the star even after the H II region had formed.”

This is because the interstellar gas surrounding massive stars does not fall evenly on the star but instead it forms filamentous concentrations modeled by the force of gravity. When the radiation produced by massive star invests filaments, these mainly absorb its ultraviolet component, shielding the surrounding gas. Considering this effect, it explains not only how gas can continue to fall toward the stars, but also the ionized nebulae observed with the VLA are so small so they tend to shrink when they are no longer ionized and over time their light that seems to flicker like a candle.

These transitions in which the gas passes from one state to one of rarefied high density and vice versa occur very quickly compared to other astronomical events, even within a few tens of years, as it is clear from a comparison of the VLA observations of the region in Sagittarius B2 1989 and 2012. “The long-term trend is always the same, namely that the H II regions expand with time,” says De Pree. “But to observe more accurately how they become brighter or weaker in a cyclic manner. Accurate measurements over time are able to show this process more complex. “

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