* Astronomy

Scientists have found the trigger for making massive stars.

Astronomers know that the recipe for creating a star in a stellar nurseries includes hydrogen gas, dust and some amount of heat and gravity, but until now they did not know quite how all the parts come together or what triggers the event.

Several theories had been proposed. One predicts that low-mass stars accrete surrounding material. Another calls for the forceful combination of two protostars. A third, called the "collect-and-collapse" model, says that a parent massive star influences the formation of second-generation stars.
Now, a collection of images presented by astronomers at the Laboratoire d'Astrophysique de Marseille in France provides the most complete and detailed evidence supporting the collect-and-collapse model, without ruling out the other models.
The research 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.

In the Collect-and-collapse model, when a star reaches a mass eight times greater than the sun, it begins to emit intense amounts of energy in the form of ultraviolet photons, triggering a series of events that lead to the formation of massive stars. These highly energised photons react with surrounding gas molecules, and the region near the star becomes filled with ionised hydrogen gas. It becomes an HII region.
This bubble of ionised gas is hot. Like all hot things, it expands, and it does so continuously because the area outside the region is much cooler. As the region expands, dust and gas begin to collect along the outer edges, like a broom sweeping across a dirty floor.
After a while, gravitational instabilities cause the dust and gas to fragment into clumps, which are large enough to collapse and form second-generation stars.


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In this picture of the star-forming HII region called RCW 79, blue represents ionised hydrogen, orange represents the region's dust shell, and yellow represents areas where cold dust is condensing. The inset shows details from a second-generation star-forming region.

The MSX Point Source is the IR source G308.7452+00.5482
MSX position(2000): RA 13h 40m 53. s1 Dec -61. 45' 51''

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While many scientists subscribed to the collect-and-collapse model, no strong evidence existed until the release of this set of images of galactic HII region RCW 79, about 17,200 light-years away from Earth in southern Milky Way and appears in the constellation Centaurus. Astronomers believe the bubble has a diameter of about 70 light-years and took about one million years to form from the radiation and wind of hot, youthful stars.
The collection of images gives the most complete look into a "triggered" star-forming region, clearly depicting each of its characteristics.

One light-year is the distance light travels in a year, about 9.7 trillion kilometres. RCW 79 has spawned at least two groups of new stars along the edge of the large bubble. Some are visible inside the small bubble in the lower left corner. Another group of baby stars appears near the opening at the top.

In the most recently released set of images, an orange version shows the dust shell that surrounds the HII region RCW 79. The blue image highlights ionised hydrogen, clearly showing that it fills the inside of the region. The yellow represents the condensation of cold dust in the outer shell of the region.


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Position(2000): RA: 13h37m19.56s Dec: -61d19m56.1s
Credit: NASA/JPL-Caltech/E.

Using the mid-infrared Spitzer observations from the GLIMPSE survey, the team has identified newly formed, second-generation massive stars associated with the main condensations.
One of these second-generation stars has evolved enough to begin emitting ultraviolet photons itself and could give rise to a compact second-generation HII region.

Based on the locations and morphological relations between these structures, the team concluded that these observations agreed very well with the collapse-and-collect process and that it is the main trigger for massive star formation along the borders of this region.

These images were produced by Annie Zavagno and Lise Deharveng using several telescopes: NASA's Spitzer Space Telescope, the SuperCOSMOS Sky Survey, the ESO Swedish Submillimetre Telescope and the ESO-New Technology Telescope.

This research is detailed in an upcoming issue of the journal Astronomy & Astrophysics.

Read more (PDF, French)