Space telescopes find trigger-happy star formation

While astronomers have long understood that stars and planets form from the collapse of a cloud of gas, the question of the main causes of this process has remained open.

One option is that the cloud cools, gravity gets the upper hand, and the cloud falls in on itself. The other possibility is that a "trigger" from some external source -- like radiation from a massive star or a shock from a supernova -- initiates the collapse. Some previous studies have noted a combination of triggering mechanisms in effect.

By combining observations of the star-forming cloud Cepheus B from the Chandra X-ray Observatory and the Spitzer Space Telescope, researchers have taken an important step in addressing this question. Cepheus B is a cloud of mainly cool molecular hydrogen located about 2,400 light years from Earth. There are hundreds of very young stars inside and around the cloud -- ranging from a few million years old outside the cloud to less than a million in the interior -- making it an important testing ground for star formation.

"Astronomers have generally believed that it's somewhat rare for stars and planets to be triggered into formation by radiation from massive stars," said Konstantin Getman of Penn State University, University Park, Pa., lead author of the study. "Our new result shows this belief is likely to be wrong."

This particular type of triggered star formation had previously been seen in small populations of a few dozen stars, but the latest result is the first time it has been clearly observed in a rich population of several hundred stars.

While slightly farther away than the famous Orion star-forming region, Cepheus B is at a better orientation for astronomers to observe the triggering process. The Chandra observations allowed the astronomers to pick out young stars within and around Cepheus B. Young stars have turbulent interiors that generate highly active magnetic fields, which, in turn, produce strong and identifiable X-ray signatures.

This composite image, combining data from NASA's Chandra X-ray Observatory and Spitzer Space Telescope shows the star-forming cloud Cepheus B, located in our Milky Way galaxy about 2,400 light years from Earth. A molecular cloud is a region containing cool interstellar gas and dust left over from the formation of the galaxy and mostly contains molecular hydrogen. The Spitzer data, in red, green and blue shows the molecular cloud (in the bottom part of the image) plus young stars in and around Cepheus B, and the Chandra data in violet shows the young stars in the field.

The Chandra observations allowed the astronomers to pick out young stars within and near Cepheus B, identified by their strong X-ray emission. The Spitzer data showed whether the young stars have a so-called "protoplanetary" disk around them. Such disks only exist in very young systems where planets are still forming, so their presence is an indication of the age of a star system.

These data provide an excellent opportunity to test a model for how stars form. The new study suggests that star formation in Cepheus B is mainly triggered by radiation from one bright, massive star (HD 217086) outside the molecular cloud. According to the particular model of triggered star formation that was tested -- called the radiation- driven implosion model -- radiation from this massive star drives a compression wave into the cloud triggering star formation in the interior, while evaporating the cloud's outer layers.

Different types of triggered star formation have been observed in other environments. For example, the formation of our solar system was thought to have been triggered by a supernova explosion. In the star-forming region W5, a "collect-and-collapse" mechanism is thought to apply, where shock fronts generated by massive stars sweep up material as they progress outwards. Eventually the accumulated gas becomes dense enough to collapse and form hundreds of stars. The radiation-driven implosion model mechanism is also thought to be responsible for the formation of dozens of stars in W5. The main cause of star formation that does not involve triggering is where a cloud of gas cools, gravity gets the upper hand, and the cloud falls in on itself.

Credit: NASA/CXC/JPL-Caltech/PSU/CfA

The Spitzer data revealed whether the young stars have a disk of material (known as "protoplanetary" disks) around them. Since they only exist in very young systems where planets are still forming, the presence of protoplanetary disks -- or lack thereof -- is an indication of the age of a star system.

The new study suggests that star formation in Cepheus B is mainly triggered by radiation from one bright, massive star outside the molecular cloud. According to theoretical models, radiation from this star would drive a compression wave into the cloud-triggering star formation in the interior, while evaporating the cloud's outer layers. The Chandra-Spitzer analysis revealed slightly older stars outside the cloud, and the youngest stars with the most protoplanetary disks in the cloud interior -- exactly what is predicted from the triggered star formation scenario.

"We essentially see a wave of star and planet formation that is rippling through this cloud," said co-author Eric Feigelson, also of Penn State. "It's clear that we can learn a lot about stellar nurseries by combining data from these two Great Observatories."

A paper describing these results was published in the July 10 issue of the Astrophysical Journal. The team of astronomers that worked with Getman and Feigelson also included Kevin Luhman and Gordon Garmire from Penn State; Aurora Sicilia-Aguilar from Max-Planck-Institut fur Astronomie in Germany; and Junfeng Wang from Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

Source: Jet Propulsion Laboratory
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