Radiation pressure mystery 'solved' for massive stars

BY KULVINDER SINGH CHADHA

ASTRONOMY NOW

Posted: 16 January, 2009

Stars larger than 100 solar masses shouldn’t exist, as radiation pressure should prevent any more infalling material. And yet they do. A study published this week by Science explains why. It shows how the growth of massive stars can continue despite outward-flowing radiation pressure exceeding the net gravitational force pulling material inwards.

Lead author of the study, Professor Mark Krumholz of the University of California (Santa Cruz) says, “The new findings may also explain why massive stars tend to occur in binary or multiple star systems.” This aspect, the formation of companion stars, emerged as an unexpected by-product from the computer simulations that the researchers have been using to model the physics of massive star formation.

"We didn't set out to solve that question, so it was a nice side
benefit of the study," Krumholz says. "The main finding is that
radiation pressure does not limit the growth of massive stars."

Radiation pressure is the physical force exerted by photons
Striking physical objects. Under ordinary circumstances this radiation pressure effect is negligible, but it becomes significant inside the cores of stars due to the sheer intensity of the radiation. In the most massive stars, radiation pressure is the dominant force counteracting gravity, preventing the further collapse of the star under its own mass.

"When you apply the radiation pressure from a massive star to the dusty interstellar gas around it (which is much more opaque than the star's internal gas), it should explode the gas cloud," Krumholz says. So stars much greater than 20 solar masses (as earlier work has suggested) should blow away their own building material into space. Yet time and time again astronomers observe stars well-over 100 solar masses, such as Eta Carinae and the Pistol Star. So what is the secret to this seemingly paradoxical problem?

Still showing the computer simulation undertaken by Krumholz and colleagues that shows how massive stars can form before blowing away much of the material that makes them. Click on the image to see for yourself a video of the simulation occuring. Image: Krumholz et al.

Krumholz and his colleagues (at UC Berkeley and Lawrence Livermore National Laboratory) spent years perfecting their simulation of star formation processes and they have managed to create a detailed three-dimensional simulation of the collapse of an interstellar gas cloud to form a massive star.

It showed that as the dusty gas accretes onto the ever growing
core of a massive star (with radiation pressure inhibiting the gravitational force of the coalesced material), instabilities form into channels, where radiation blows through the dust in one channel whilst gas continues falling inwards through other channels. In effect, radiation pressure and gravitational attraction, instead of fighting one another, form ‘lanes’ like that for traffic going in different directions. Massive stars are central ‘stellar motorway hubs’.

Krumholz says, "You can see fingers of gas falling in and radiation leaking out between those fingers. This shows that you don't need any exotic mechanisms; massive stars can form through accretion processes just like low-mass stars."

For the formation of several stars, it appears that the rotation of the collapsing gas cloud is crucial. The disc is gravitationally unstable and breaks into clumps, forming a series of small secondary stars (but most of those coalesce with the central protostar). But in Krumholz’s simulation, one secondary star becomes massive enough to break free, complete with its own disc. This grows into a massive companion star.

When the researchers stopped the simulation, after allowing it to run for the equivalent of 57,000 years, the two stars had masses of 41.5 and 29.2 times the mass of the Sun, and both were circling each other in a fairly wide orbit.

Krumholz says that this is a typical situation for massive stars and boldly claims: "I think we can now consider the mystery of how massive stars are able to form to be solved. The age of supercomputers and the ability to simulate the process in three dimensions has made the solution possible."