The collect and collapse process: a way of triggering the formation of massive stars.
In an upcoming issue, the journal Astronomy & Astrophysics will publish the most complete picture of a “triggered” star-forming region. Induced (or “triggered”) star formation is one of the processes that are supposed to lead to the formation of massive stars. Massive stars play a key role in the chemical and dynamical evolution of galaxies. The way massive stars form is still much debated among the astronomers’ community: it is currently one of the hottest astrophysical topics. Do they form by accretion as low-mass stars do or do they need the environment of a dense cluster to form through the merging of low mass protostars?
In this framework, the team led by Annie Zavagno and Lise Deharveng (from the Laboratoire d’Astrophysique de Marseille, France) selected regions where several generations of massive stars are likely to be formed. Stars more massive than 8 solar masses, once formed, emit intense UV photons that ionize the surrounding gas. The region filled with ionized hydrogen is called an HII region. Theory suggests that the expansion of the HII region can trigger massive star formation: after the HII region has formed, it expands continuously because the temperature inside the region is much higher than in the cold environment that surrounds it. During the expansion, a dense layer of gas and dust is collected around the HII region, after which gravitational instabilities in the layer cause it to fragment into dense clumps, which then go on to collapse into new stars. The fragments are massive and thus form massive objects (stars or clusters). The successive steps of this process, called collect and collapse process, are shown in Figure 1.
To characterize this process, the team selected the Galactic HII region RCW 79, located 14000 light-years from the Earth. They combined observational data obtained at different wavelengths, as well as from various origins (space, ground-based telescopes, and archived observations), to probe different parts of the region, as illustrated in Figure 2.
Jennifer Martin | alfa
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