A star, eventually !

It is a story many parents are hoping for: Imagine there’s a very young kid that seems like a real low-performer, but then it turns out that secretly it has its very powerful moments — such that the odds are pretty good it will become a real star, eventually. In fact, this is very much what an international team of astronomers found when they examined the very young protostar IRAM 04191+1522 (IRAM 04191, in short). This protostar – located almost 500 light years away from us in the Taurus cloud — is still deeply embedded in its parental molecular cloud, accreting material from its surroundings in order to build up its final mass. It is so young that it is not yet generating energy from nuclear fusion. Instead it is heated by the infalling material. IRAM 04191, however, doesn’t seem to get much of that heat that makes it glow. It is what is called a “very low luminosity object” (a “VeLLO”). If a mass accretion rate is derived from its observed low luminosity, i.e. a measure of how much matter it receives in a certain amount of Ame, it looks like IRAM 04191 is not going to reach a final mass sufficient to become a proper low mass star. Instead it would become a brown dwarf, a substellar object heavier than gas giants but not heavy enough to start nuclear fusion of ordinary hydrogen. But is this really the whole story?

Astronomers from France, Germany, and Italy led by Sibylle Anderl and Sébastien Maret from the Université Grenoble Alpes and CNRS used observations performed with the NOEMA observatory to observe molecular radiation around IRAM 04191. NOEMA is an array of telescopes located in the French Alps that is run by the InsAtut de Radioastronomie Millimétrique. The observations were part of the CALYPSO-project (Continuum And Lines in Young ProtoStellar Objects) aiming at understanding the properties and the evolution of young protostellar objects in our galaxy. The basic question they asked: Has IRAM 04191 always been accreting material at the present rate, or could there have been Ames when much more material was raining down on that object? The tool they used to answer this question was chemistry.

Like a radiant heater on a snowy plane

The researchers observed something that is called “snow lines”: When a protostellar object heats its surrounding molecular cloud consisting of gas and ice-covered dust grains, there is an inner spherical region where the temperature is high enough to remove the ice from the grains and transfer its molecules into the gas phase – just like a radiant heater put on a snowy plane will create a circular hole in the snow. The boundary of this area is called snow line or ice line. Its radius depends on the ice composition: Each molecule requires a different temperature to be released and, accordingly, has its snow line at a different location. Once the molecule is released into the gas, it can be observed in astronomical spectroscopic observations. As the temperature in the molecular cloud depends on the luminosity of the protostellar source, the radius of a given snow line carries information on the young protostar: The further out it is, the more luminous the protostar is.

Anderl and her team were looking for the snow line of carbon monoxide by observing C18O, an isotopologue of this molecule, directly. But the CO-snow line is also traced by another molecule: diazenylium, N2H+, is chemically destroyed by CO and should therefore form a ring outside the region where CO is present in the gas. This pattern, a circular region of C18O emission and an adjacent ring of N2H+, was observed by Anderl and her colleagues around other protostellar sources in a previous study. To their surprise, it turned out that IRAM 04191 is different, however. The snow line traced by C18O is at located at a much smaller radius than the snowline traced by N2H+. “When we first saw the observations of IRAM 04191, it was clear that something very odd was going on there. So we decided not to analyze it together with the other sources, but to first put it aside and have a much closer look at it later”, says Anderl.

Chemistry as a tool to look back in time

Using numerical models of the protostellar cloud and its chemistry, they could show that this peculiar chemical pattern can be explained if the luminosity of IRAM 04191 had changed in the past. This conclusion uses the fact that chemistry can be used to look back in Ame. Because chemical reactions take a certain amount of time, sometimes chemical pattern still trace physical conditions that since have long changed. So when IRAM 04191 had a luminosity outburst in the past, the temperature in the surrounding cloud was higher and the snow line was further out, as it is still traced by N2H+. When the burst was over, the temperature went down again. But while CO was fast to be bound in ices again, the reformation of N2H+ in the gas phase took longer even though there was no CO anymore to destroy it: Thus the observed gap between C18O and N2H+. According to the researchers’ models, the luminosity burst should have occurred no longer than a thousand years ago from now and made IRAM 04191 shine more than 150-fold as luminous as it appears today. Assuming that bursts like this happen roughly every 13,000 years as indicated by models, this means that IRAM 04191 will indeed be able to reach a final mass of about 20 percent that of our sun – and become a proper low mass star.

The result demonstrates that the accretion history of very young protostars can be very dynamic. This insight has to be incorporated into respective models of protostellar evolution in order to connect the different “snapshots” of stellar populations at different ages into one coherent evolutionary picture.

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Référence

Probing episodic accretion with chemistry: CALYPSO observations of the very-low-luminosity Class 0 protostar IRAM 04191+1522, S. Anderl, S. Maret, S. Cabrit, A. J. Maury, A. Belloche, Ph. André, A. Bacmann, C. Codella, L. Podio, and F. Gueth, Astronomy & Astrophysics (2020). [DOI : https://doi.org/10.1051/0004-6361/201936926]

Further information INSU Brève (in French): Une étoile en devenir

Updated on 13 November 2020