Star formation in molecular clouds is fast but extremely inefficient

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The image on the left shows that the positions of molecular clouds (blue) and the radiation of young stars (pink) do not match on a small scale. The two branches of the 'tuning fork' on the right measure how strong this deviation is and show that the clouds and young stars only coincide when averaging takes place over a large part of the galaxy (1000 pc, around 3000 light years). - Image Credit: Diederik Kruijssen & Nature.

Star formation in molecular clouds occurs at a high pace but in an extremely inefficient manner. Most gas is quickly dispersed by the radiation from newborn stars. A team led by Dutch astronomer Diederik Kruijssen (University of Heidelberg, Germany) will publish this result tomorrow in the journal Nature.

Kruijssen and colleagues (including Leiden astronomer Ewine van Dishoeck) base their findings on observations of the spiral galaxy NGC300, which is located at a distance of six million light years from Earth. One of the outstanding questions in astrophysics is determining the mechanisms that drive star formation. The team has reconstructed the evolution over time of interstellar clouds of gas and dust and the star formation processes that take place in these molecular clouds for the first time .

The observations of NGC300 show that the star formation process is happening at lightning speed, but at the same time in an extremely inefficient way. Most interstellar gas is not converted into stars, but dispersed by the intense radiation from the new-born stars in the cloud. The analysis shows that the molecular clouds have very short life cycles due to this strong radiation. The galaxies in which they are located are therefore extremely dynamic systems, which constantly change shape.

To find out which model is appropriate for the life cycle of molecular clouds, the researchers made two maps based on data from the ALMA telescope (Atacama Large Millimeter / submillimeter Array) and the 2.2 meter telescope from the Max Planck Society and ESO, both in Chile. The first map (based on observations by co-author Andreas Schruba of the Max Planck Institute in Garching, Germany) shows the light emitted by carbon monoxide and shows where the molecular clouds are located. The second map shows hot, ionized hydrogen and shows the positions of heavy, newly formed stars. With a new statistical method the astronomers could very accurately determine the relative positions of the clouds and the newborn stars.

The results left no doubt about it: molecular clouds and young, heavy stars are rarely in the same place. This effect becomes stronger as the scale becomes smaller and means that stars are formed very quickly and then destroy their "mother cloud" at the same rate. Gas and young stars thus form successive phases in the life cycle of molecular clouds.

Kruijssen explains that molecular clouds in NGC300 have been around for 10 million years and only take 1.5 million years to be destroyed, well before most of the heavy stars have reached the age of dying as supernova. Mélanie Chevance, researcher in the Kruijssen group and co-author of the article, adds that due to the intense radiation from young stars, the molecular clouds are torn apart and hot bubbles of interstellar gas are created. As a result, only two to three percent of the mass in these clouds is converted into stars.

Co-author Ewine van Dishoeck emphasizes the importance of ALMA for this type of research stating that thanks to this observatory, they can now finally map individual clouds in galaxies other than our own Milky Way; before ALMA became operational, the sharpness they could achieve was lower by a factor of 50 and everything looked like one big blurry spot.

The astronomers now want to aim their telescopes at large numbers of near and far galaxies to investigate, using their new method, how star formation has taken place throughout the history of the universe.

Source: NOVA / press release

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