In a study in the July 22 issue of Nature, a team of astronomers led by Joel Kastner, professor of imaging science at the Rochester Institute of Technology, found that the interaction between the young star's magnetic field and an orbiting disk of gas can cause dramatic, episodic increases in light from the star and disk, illuminating the surrounding gas.
“The story of McNeil's Nebula is a wonderful example of the importance of serendipity in science,” Kastner says. “Visible-light images were made of this region several months before Jay McNeil made his discovery, so it could be determined approximately when and by how much the star flared up to produce McNeil's nebula.”
The small nebula, which lies in the constellation Orion, about 1,300 light years from earth, was discovered with a 3-inch telescope by McNeil, an amateur astronomer from Paducah, Ky., in January 2004.
After the announcement of McNeil's discovery, optical, infrared and X-ray astronomers rushed to observe the region again. They found that a young star buried in the nebula had flared up, and was illuminating the nebula.
The team of astronomers headed by Kastner included David Weintraub of Vanderbilt University and Michael Richmond of RIT, among others.
Chandra observations obtained by the astronomers just after the optical outburst showed that the source had brightened 50-fold in X-rays when compared to earlier observations. The star's flare up points to the cause of the X-ray outburst: the sudden fall of matter onto the surface of the star from an orbiting disk of gas.
In the Nature paper Kastner and colleagues, referring to the work of theoretical astrophysicists, describe how magnetic field interactions between such a young star and its circumstellar or “protoplanetary” disk regulate the inflow of dust and gas from the disk onto the star. This slow, steady inflow can accelerate if a large amount of gas accumulates in the disk, and the disk and the star are rotating at different rates.
The differing rotation rates would twist and shear the magnetic fields near the star, storing up energy. A violent rearranging of the magnetic field back to a more stable state would release the energy-dumping a large amount of dust and gas onto the star-and produce intense X-rays. The discovery by Kastner's team of just such an outburst of X-rays from the star in McNeil's Nebula offers strong, new support for this theoretical scenario.
New buildups of gas in the disk could lead to new outbursts in the future.
“Such episodic outbursts may lead to periodic ejections of material from McNeil's star, explaining the appearance of its surrounding nebulosity,” Kastner says.
Kastner, Richmond and Weintraub have just been awarded additional observing time with Chandra to measure the new star's X-ray activity after it becomes visible again in October.
To talk to Joel Kastner, please contact Susan Gawlowicz at 585-475-5061.