The explosive growth of a young star

Using the telescope ALMA, a research team led by Danish Niels Bohr Institute researchers analyzed a protostar in the Milky Way and discovered that in the earlier stages of its development was about one hundred times brighter than it is now.



The graph shows the young protostar in the center surrounded by clouds of gas and dust. In the red emission of the organic molecule methanol are detected around the center. In blue, the distribution of the HCO + molecule clearly shows an extended ring structure. The yellow circle indicates where the innermost currently the temperature is greater than 100 degrees above absolute zero (-173 C˚), the outer circle indicates where there was the same temperature when the star was a hundred times more brilliant. 

That protostar can be considered the embryonic stage in the development of a star. A phase of rapid growth resulting from the gravitational collapse of giant clouds of gas and dust , a turbulent thickening that precedes the actual ignition of the star, or the initiation of thermonuclear fusion reactions of hydrogen in the stellar core .

A particularly lively and explosive growth has characterized IRAS 15398-3359, a low-mass protostar that has been forming over the last 100,000 years within the Milky Way. According to a Danish research team who has studied with the ALMA radio telescope at ESO in Chile, this young star was in the initial stages of its development about 100 times brighter than it is now. The study is published in the Astrophysical Journal Letters.

“We studied the chemistry of gas and dust surrounding the protostar,” explains Jes Jorgensen, of Niels Bohr Institute at the University of Copenhagen, leader of the research. “In this dense cloud chemical reactions take place that lead to the formation of various complex organic molecules, including methanol. We expect to find these molecules close to the star, but for one of them instead we observed a ring arrangement: something has removed a specific molecule, HCO+, from a wide area around the protostar. ”

Jorgensen and colleagues believe that the disappearance of the HCO+ molecule was due to the water vapor produced during the process of star formation by heating the ice present on the dust grains. Then follow the traces of the missing molecule may know the trauma that the star has encountered in its growth.

“From the size of the area where the HCO+ molecule was dissolved by the water vapor can calculate how brilliant it was the young star – says Jorgensen. And what jumps out is that this area is a lot bigger than you would expect compared to the actual brightness of the star: the protostar was up to 100 times brighter than it is now the star. In addition, the chemistry involved, we can also say that this change has taken place in the last 100-1000 years, a short time ago from the astronomical point of view. ”

The researchers believe that this was not necessarily a single burst of light and heat, but a phenomenon that can be repeated several times during the process of star formation. Phenomenon that is also interesting to understand because it can have a decisive influence on the abundance of complex organic molecules that, at a later stage of stellar evolution, will be incorporated into planetary systems. But at the moment we do not know if these “eruptions” are a common phenomenon among protostars, or if IRAS 15398-3359 constitutes a notable exception.

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