Magnetic loops carry gas and dust above disks of planet-forming material
circling stars, as shown in this artist's conception. These loops give
off extra heat, which NASA's Spitzer Space Telescope detects as infrared
light. The colors in this illustration show what an alien observer with
eyes sensitive to both visible light and infrared wavelengths might
see. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).
Astronomers say that magnetic storms in the gas orbiting young stars may explain a mystery that has persisted since before 2006.
Researchers
using NASA's Spitzer Space Telescope to study developing stars have had
a hard time figuring out why the stars give off more infrared light
than expected. The planet-forming disks that circle the young stars are
heated by starlight and glow with infrared light, but Spitzer detected
additional infrared light coming from an unknown source.
A new
theory, based on three-dimensional models of planet-forming disks,
suggests the answer: Gas and dust suspended above the disks on gigantic
magnetic loops like those seen on the sun absorb the starlight and glow
with infrared light.
"If you could somehow stand on one of these
planet-forming disks and look at the star in the center through the disk
atmosphere, you would see what looks like a sunset," said Neal Turner
of NASA's Jet Propulsion Laboratory, Pasadena, Calif.
The new
models better describe how planet-forming material around stars is
stirred up, making its way into future planets, asteroids and comets.
While
the idea of magnetic atmospheres on planet-forming disks is not new,
this is the first time they have been linked to the mystery of the
observed excess infrared light. According to Turner and colleagues, the
magnetic atmospheres are similar to what takes place on the surface of
our sun, where moving magnetic field lines spur tremendous solar
prominences to flare up in big loops.
Stars are born out of
collapsing pockets in enormous clouds of gas and dust, rotating as they
shrink down under the pull of gravity. As a star grows in size, more
material rains down toward it from the cloud, and the rotation flattens
this material out into a turbulent disk. Ultimately, planets clump
together out of the disk material.
In the 1980s, the Infrared
Astronomical Satellite mission, a joint project that included NASA,
began finding more infrared light than expected around young stars.
Using data from other telescopes, astronomers pieced together the
presence of dusty disks of planet-forming material. But eventually it
became clear the disks alone weren't enough to account for the extra
infrared light -- especially in the case of stars a few times the mass
of the sun.
One theory introduced the idea that instead of a disk,
the stars were surrounded by a giant dusty halo, which intercepted the
star's visible light and re-radiated it at infrared wavelengths. Then,
recent observations from ground-based telescopes suggested that both a
disk and a halo were needed. Finally, three-dimensional computer
modeling of the turbulence in the disks showed the disks ought to have
fuzzy surfaces, with layers of low-density gas supported by magnetic
fields, similar to the way solar prominences are supported by the sun's
magnetic field.
The new work brings these pieces together by
calculating how the starlight falls across the disk and its fuzzy
atmosphere. The result is that the atmosphere absorbs and re-radiates
enough to account for all the extra infrared light.
"The
starlight-intercepting material lies not in a halo, and not in a
traditional disk either, but in a disk atmosphere supported by magnetic
fields," said Turner. "Such magnetized atmospheres were predicted to
form as the disk drives gas inward to crash onto the growing star."
Over
the next few years, astronomers will further test these ideas about the
structure of the disk atmospheres by using giant ground-based
telescopes linked together as interferometers. An interferometer
combines and processes data from multiple telescopes to show details
finer than each telescope can see alone. Spectra of the turbulent gas in
the disks will also come from NASA's SOFIA telescope, the Atacama Large
Millimeter/submillimeter Array (ALMA) telescope in Chile, and from
NASA's James Webb Space Telescope after its launch in 2018.
JPL
manages the Spitzer Space Telescope mission for NASA's Science Mission
Directorate, Washington. Science operations are conducted at the Spitzer
Science Center at the California Institute of Technology in Pasadena.
Spacecraft operations are based at Lockheed Martin Space Systems
Company, Littleton, Colo. Data are archived at the Infrared Science
Archive housed at the Infrared Processing and Analysis Center at
Caltech. Caltech manages JPL for NASA.
Source: JPL/Spitzer Space Telescope
