Circumstellar Disk Cradles Young Massive Star (Forwarded)

Subaru Telescope
National Astronomical Observatory of Japan
Hilo, Hawaii

August 31, 2005

Circumstellar Disk Cradles Young Massive Star

An international group of astronomers has used the Coronagraphic Imager
for Adaptive Optics (CIAO) on the Subaru telescope in Hawai'i to obtain
very sharp near-infrared polarized-light images of the birthplace of a
massive proto-star known as the Becklin-Neugebauer (BN) object at a
distance of 1500 light years from the Sun (Note 1). The group's images led
to the discovery of a disk surrounding this newly forming star. This
finding, described in detail in the September 1 issue of Nature, deepens
our understanding of how massive stars form.

The research group, which includes astronomers from the Purple Mountain
Observatory, China, National Astronomical Observatories of Japan, and
University of Hertfordshire, UK, explored the region close to the
Becklin-Neugebauer object and analyzed how infrared light is affected by
dust. To do this, they took a polarized-light image of the object at a
wavelength of 1.6 micrometers (the H band of infrared light). Images of
the brightness of the object just show a circular distribution of light.
However, an image of the light's polarization shows a butterfly shape that
reveals details that are undetectable by looking at the brightness
distribution alone (Figure 1). To understand the environment around the
star and what the butterfly shape implies, the astronomers created a
computer model for comparison (Figure 2), along with a schematic of star
formation (Figure 3). These models show that the butterfly shape is the
signature of a disk and an outflow structure near the newborn star.

This discovery is the most concrete evidence for a disk around a massive
young star and shows that massive stars like the BN object (which is about
seven times the mass of the Sun) form the same way as lower-mass stars
like the Sun.

There are two main theories to explain the formation of massive stars. The
first states that massive stars are the results of the mergers of several
low-mass stars. The second says that they are formed through gravitational
collapse and mass accretion within circumstellar disks. Lower-mass stars
like the Sun are most likely to have formed through the second method. The
collapse-accretion theory assumes that a system has a star associated with
a bipolar outflow, a circumstellar disk and an envelope, while the merger
theory does not. The presence or absence of such structures can
distinguish between the two formation scenarios.

Until recently, there has been little direct observational evidence in
support of either theory of massive star formation. This is because,
unlike lower-mass stars, newly forming massive stars are so rare and so
far away from us that they have been difficult to observe. Large
telescopes and adaptive optics, which greatly improve image sharpness, now
make it possible to observe these objects with unprecedented clarity.
High-resolution infrared polarimetry is an especially powerful tool for
probing the environment hidden behind the bright glow of a massive star.

Polarization -- the direction that light waves oscillate in as they stream
away from an object -- is an important characteristic of radiation.
Sunlight doesn't have a preferred direction of oscillation, but can become
polarized when scattered by Earth's atmosphere, or after reflecting off
the surface of water. A similar action occurs in a circumstellar cloud
around a newborn star. The star lights up its surroundings -- the
circumstellar disk, the envelope and the cavity walls formed by the
outflow streams. The light can travel freely within the cavity and then
reflect off its walls. This reflected light becomes highly polarized. By
contrast, the disk and the envelope are relatively opaque to light. This
reduces the polarization of light coming from those regions. A schematic
view the process is shown in Figure 3. (Note 2)

The group's success in detecting evidence for a disk and outflow around
the BN object through high-resolution infrared polarimetry suggests that
the same technique can be applied to other forming stars. This would allow
astronomers to obtain a comprehensive observational description of the
formation of massive stars greater than ten times the mass of the Sun.

Note 1: The BN object was discovered in 1967 by Eric Becklin and Gerry
Neugebauer during their near-infrared survey of the Orion Nebula region,
and has been regarded as a prototype of massive protostars. It is one of
the brightest infrared objects in the sky, but invisible at optical
wavelengths except for the foreground nebula. The Orion constellation and
an optical image of the Orion nebula, an infrared image of the central
region of the nebula, and the position of the BN object is shown in Figure
5.

Note 2: Polarized light generally exhibits a so-called central-symmetric
pattern, as shown in Figure 2. However, in the BN system astronomers
observed a different polarization pattern, with the polarization vectors
parallel to each other. This is caused by an effect called "dichroic
extinction." If elongated dust grains exist in a foreground cloud, they
tend to align perpendicular to the magnetic field of the cloud. These
grains would block more light polarized in one direction than in the
other. Therefore, when light emerges from the cloud, it will be polarized
parallel to the magnetic field, as shown in Figure 4. This process happens
more frequently in hot dusty clouds than in cold clouds, since in hot
clouds dust particles would be more easily aligned.

[NOTE: Images supporting this release are available at
http://www.subaru.naoj.org/Pressrele.../31/index.html ]