Clues to Earth’s Origin Observed in Young Stars

RIT professor finds new evidence using Chandra X-ray Observatory

A Rochester Institute of Technology professor has used NASA’s Chandra X–ray Observatory in a new way to look at how planets develop.

To find the places planets and solar systems are born, Joel Kastner observes young stars surrounded by pancake-like disks of gas and dust. Kastner focused his recent study on two star systems—still young at 10 million years—in the loose cluster known as the TW Hydrae Association. Some of these stars could "grow up" to be suns surrounded by planets much like the 5 billion-year-old sun in our own solar system.

"Astronomers study young (T Tauri) stars to try to understand, among other things, the circumstances that led to the formation of the solar system and, eventually, life on planet Earth," says Kastner, associate professor at RIT’s Chester F. Carlson Center for Imaging Science and project leader.

Adding weight to existing theories of how stars and planets form, Kastner’s research shows that observations of X-rays provide a novel means to find stars "in the act" of forming. The study’s findings were released at a meeting of the American Astronomical Society in May.

Observations at infrared and other wavelengths have shown that planet-forming disks surround several stars in the TW Hydrae group. At a distance of about 180 light years away, these systems are among the nearest and most similar to the early solar nebula that produced Earth.

"X-rays give us an excellent new way to probe the disks around stars," Kastner says. "They can tell us whether a disk is very near to its parent star and dumping matter onto it, or if the disk has been assimilated into larger bodies—perhaps planets—or disrupted entirely."

The new X-ray technique used in Kastner’s study relies on the ability of Chandra’s spectrometers to measure precisely the energies of individual X-rays.

The technique involves determining temperature and density of particles and will help other astronomers identify the origin of intense X-ray emission from young stars.

Kastner’s study shows:

• New evidence that disks can dump matter onto the stars they encircle, as was shown by observing the TW Hydrae, namesake of the TW Hydrae Association. In this case, material spirals from the disk and amasses onto the surface of the star. The star’s magnetic field guides the matter onto one or more hot spots on its surface, producing X-rays in the process.
• Observations of a young multiple star system revealed that its brightest star produces X-rays like the sun does, from a hot upper atmosphere or corona. According to Kastner, this indicates that any disk around these stars has been greatly diminished or destroyed either by the formation of planets or by the gravitational influence of another star.

  "These first Chandra/ High Energy Transmission Gratings observations of T Tauri stars demonstrate the potential of the X–ray emission-line intensity method as another tool in the arsenal of astronomers attempting to determine the presence and longevity of planet-forming disks," Kastner says. "Additional HETGS spectroscopy of young Sun-like stars are now needed to establish the broader applicability of this method."

  Other members of the research team include David Huenemoerder, Norbert Shulz and Claude Canizares from the Chandra X-ray Center at Massachusetts Institute of Technology, and David Weintraub from Vanderbilt University.

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