Science & Technology - Posted by David Salisbury-VU on Wednesday, July 11, 2012 16:28 - 1 Comment
Baby star’s flares light up dust cloud
VANDERBILT (US) — Astronomers have observed a newly born star caught in the act of flaring.
The star has a pair of spots on its surface that are heated to more than one million degrees. The presence of these spots, which are hot enough to produce X-ray radiation, confirms a theory for how stellar infants grow.
The stellar infant in question, labeled V1647 Ori, burst into visibility in 2004 when it flared up a hundredfold in brightness. It pumped out enough visible light to illuminate the dark cloud of dust and gas surrounding it, allowing Jay McNeil, an amateur astronomer in western Kentucky, to spot its appearance in the vicinity of the Orion Nebula.
Discovery of an embryonic star like this is an exceedingly rare event, having occurred only a few times in the last century, and it is the first that has been caught in the act of flaring in modern times when its behavior can be monitored in radio, infrared, and X-ray wavelengths, as well as in visible light.
David Weintraub, professor of astronomy at Vanderbilt University, and his colleague Joel Kastner from the Rochester Institute of Technology advanced the stellar growth theory a few years ago. When they heard of the discovery, they got special permission for viewing time on NASA’s Chandra X-ray Observatory because the flaring episodes of newborn stars are normally short in duration.
This allowed them to determine that V1647 Ori was a strong X-ray source while it was flaring but that its X-ray emissions weakened and disappeared as the flare died away. These measurements confirmed that the physical process that produces the flare in visible light also produces the X-rays.
Many stars, including the sun, generate X-rays by a mechanism that depends on the star’s rotation rate and convection depth. However, the established mechanism could not explain the pattern of changes in intensity or the temperature of the protostar’s X-ray emissions.
That led Weintraub and Kastner to propose that both the visible and X-ray flare-ups were caused when large clumps of dust and gas fall onto the surface of the protostar from a dense disk of dust and gas called a protoplanetary disk that surrounds the protostar.
The young protostar’s rotation should generate intense magnetic fields that act like invisible funnels that direct large slugs of electrically charged dust and gas down to the surface. The protostar’s rotation rate should also wind up the magnetic fields, stretching them until they break. Each time the field lines break, they should deposit enough energy into the dust and gas to heat it to the million degree plus temperatures required to generate the X-ray signals that they observed.
When V1647 Ori flared up again in 2008, a team of astronomers, including Weintraub and Kastner, were ready. They pointed a trio of orbiting X-ray telescopes—Chandra, the Japan-led Suzaku satellite, and the European XMM satellite—at the still-forming star.
A new study based on these observations, scheduled for publication in the July 20 issue of the Astrophysical Journal, has confirmed that intense magnetic fields funnel torrents of dust and gas down to two spots on the protostar’s surface and, in the process, heat them to the high temperatures required to produce X-ray radiation.
Super fast rotation
The new observations also allowed the scientists to determine the rotation rate of the youngster.
“One of the most interesting findings is the fact that the star is rotating once every 24 hours,” says Weintraub. “Just a little bit faster and it would break up!”
While the process of absorbing material from the protoplanetary disk increases a forming star’s rotation rate, for the same reason that spinning skaters spin faster when they pull their arms in toward their bodies, young stars are also good at getting rid of their spin energy.
By the time a young protostar is a million years old, they typically rotate in five to ten days. Over their billions-of-years lifetimes, they lose more and more angular momentum. Mature stars like the Sun rotate very slowly, with periods of 20 to 30 days.
The extremely fast rotation rate of V1647 Ori, combined with observations that show the stellar infant is still partly swaddled in its birth cloud and is still growing by gulping more mass, have led the scientists to estimate that it is very young, only 100,000 years old.
With a rotation rate of only one day, this is one of the fastest-rotating Sun-like stars known and therefore perhaps one of the youngest.
“Now we just have to figure out what slows them down,” says Weintraub. ” While we know they must slow down, the mechanism as to how that happens remains a real mystery.”
More news from Vanderbilt University: http://news.vanderbilt.edu/research/