Inside the molecular cloud

Rebuilt with an unprecedented level of detail, thanks to the array of radio telescopes in the Atacama, the structure and kinematics of an “OB association.” The study, published on ApJ, helps explain the mechanism of formation of clusters of massive stars.
If the stars were people, this offers us today ALMA would be a kind of detailed reconstruction of the internal structure of a department of obstetrics baby’s large size. Reconstruction obtained staked 66 antennas Large Millimeter / submillimeter Array of Atacama, in the Chilean Andes, to G33.92 + 12:11: a huge lump of molecular gas to 23 thousand light-years away in which he is, in fact, taking shape a particular star cluster , a so-called “OB association.” Thanks to the unrivaled resolution of ALMA, astronomers were able to see inside, in detail two arms of molecular gas incredibly long – we are talking about an extension that is around 3.2 light years – wrap them around two nuclei also of a molecular gas. A sign that it is these arms, the researchers write in the team led by Liu Hauyu Baobab of ASIAA Taiwan in the pages of The Astrophysical Journal, are the cradles of those nuclei in which form – or already are forming – the stars OB, the “Infants large size” to which we mentioned before: metaphorically, hot and massive stars.



Beyond the beauty of the images and the technological success inherent in having reached a resolution so push, what matters most astronomers is that this result will help to answer a crucial question, apparently simple but actually quite tricky: how can aggregate, are these clusters of stars held together by gravity? Insidious because, as soon as the gravitational pull brings the gas to condense up to create the first new stars, here is that the winds driven by the latter begin to dissipate the surrounding gas – a negative feedback mechanism that astronomers call stellar feedback – thus risk of extinguishing the bud the process of star formation.
Well, thanks to the high resolution afforded by ALMA, as well as its ability to discern between different emissions sources, the observation of the two nuclei and the arms of molecular gas of G33.92 + 12:11 have provided important clues. In particular, in addition to dust, there are three “tracers” used by ALMA to reconstruct the activities going on inside the molecular cloud. First, acetonitrile (CH 3 CN), abundant in the central parts, the more dense and bright is signalizing of the presence of massive stars. Second, the molecules of DCN (hydrocyanic acid with heavy hydrogen), whose distribution, overlapping to that of the powder, indicates that the molecular arms around the nuclei more peripheral, less massive ones, are relatively cold. The carbon monosulfide (13CS) present in the arms around the two cores, on the contrary, is a sign of gas at temperatures substantially higher.
Well, what all of this evidence suggests is that the gravitational collapse under way in the cloud is happening at great speed: a condition which, according to the models, could help to neutralize the mechanism of stellar feedback which we mentioned earlier. It is observed, moreover, as the angular momentum residue has led to the formation of the rotary structure of gas, dense and flat. As explained by one of the co-authors of the study, Roberto Galván-Madrid, “the structures of molecular gas similar to spiral arms should be common to many systems, and at different scales, provided that they are gravitationally unstable and present a significant rotation. And that’s exactly what we are starting to show us these spectacular images made with ALMA. “

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