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Image Of Circumstellar Disk Illuminates Massive Star Formation
by Staff Writers
Tokyo, Japan (SPX) Nov 23, 2009

Mid-infrared image of HD200775 taken with COMICS (Cooled Mid Infrared Camera and Spectrometer) mounted on the Subaru Telescope. The detected disk is the yellow portion in the lower right. HD200775 is in the central position, although the infrared emission mostly arises from dust grains in the vicinity of the star, not from the star itself. (Note that the circular ring immediately surrounding the central star is a diffracted light pattern created by the telescope aperture; it is not a real structure.)

The elliptical shape stretching in the north-south direction (top to bottom) is a configuration that indicates we are seeing the stars' system from an angle inclined from a disk pole (perpendicular to the disk). The structure imaged in red and extending toward the northeast reaches out toward the reflection nebula NGC7023. It probably bridges the disk and an envelope (cloud of dust and gas) that has a structure 30 times larger in scale (blue-white cloud).

A team of astronomers from Ibaraki University, Japan Aerospace Exploration Agency, Kanagawa University, University of Tokyo, Academica Sinica, and the National Astronomical Observatory of Japan (NAOJ) have used the Subaru Telescope's Cooled Mid-Infrared Camera and Spectrometer (COMICS) to capture the first direct, well-resolved infrared images of a circumstellar disk around a young massive star: HD 200775.

Their findings contribute to understanding the role of circumstellar disks in massive star formation in particular and to the birth of stars in general.

Massive stars are those with masses eight times greater than that of the Sun, and they are the most prominent objects in distant galaxies. They illuminate a large area around them and form a nebula of ionized gas as they develop into mature stars. At the end of their lives, they explode as supernovae and scatter many kinds of heavy elements like iron into interstellar space.

Scientists understand relatively well how stars with masses similar to our Sun's are born: their cloudy cocoon of natal gas contracts and forms circumstellar disks, which provide the mass that feeds their central stars. These circumstellar disks are referred to as protoplanetary disks since they are likely to become the birthplaces of planets.

Images of disks around low mass stars are relatively common, and are helpful for detailed studies of the properties of disks and how they relate to planet formation.

However, the formation of massive stars is less well understood, and there have been no direct images of circumstellar disks around massive stars in either the infrared or visible wavelengths. Scientists have proposed two models to explain the creation of massive stars.

In the disk accretion scenario massive stars develop much as low mass stars do: their disks provide the mass to feed them as they grow. In the merging scenario, massive stars are formed when less massive stars merge with each other.

Several observations made in the 2000s indicated that some massive stars were formed by accretion; their circumstellar disks fed them the mass to increase in size. Observations with the Subaru Telescope, fitted with its Coronagraphic Imager with Adaptive Optics (CIAO), also yielded these results in 2005.

In August 2006, June 2007, and July 2008, the current team used the Subaru Telescope, fitted with COMICS, to make infrared observations of the young massive star HD 200775 in the constellation Cepheus. What would its circumstellar disk reveal about how this massive star was born?

HD 200775 is located 1400 light-years from Earth and is actually a close binary system that contains at least one massive star that is about 10 times the mass of the Sun. A large cavity of molecular gas surrounds the star and extends east to west from the central stars.

Molecular gas is dense within the cavity wall, and HD 200775 illuminates the gas closest to the star, forming the reflection nebula NGC 7023. The binary orbit extends in the north-south direction, indicating that the binary is almost seen in its orbital plane.

The infrared images captured by COMICS revealed an elliptical emission with the same direction as the binary orbit. The research team found that the emission arises from a circumbinary disk (a disk that surrounds both stars of the binary system) around HD 200775, the orbital plane of which is similar to that of the binary.

The direction is consistent with the fact that an outflow generally occurs in a direction perpendicular to a disk. The images taken this time are significant because they are the first clear and direct infrared images of a disk around a massive star.

The team examined the disk images in detail and found that the center of the disk emission shifted from the central stars. This indicates that the disk is not flat but flared; the disk height increases as the disk radius increases. Such a flared disk is very similar to that found for star HD97048, which is twice as massive as the Sun.

The team also found that strong ultraviolet irradiation from the central star seems to evaporate the gas of the HD 200775 disk, leading them to believe that the star itself quickly dissipates the disk that surrounds the binary stars. This is one reason that it is difficult to find disks around massive stars. Though massive stars may be created through an accretion process similar to that of lower mass stars, the fate of the circumstellar disk is different.

In summary, this research shows that a massive star up to 10 solar masses has a circumstellar disk similar to stars like Earth's Sun. The team concluded that the star HD 200775 probably formed from the accretion of mass from its disk. Nevertheless, the results also indicate that disk evolution depends on the masses of the central stars.

In the future, it will be interesting to tackle more challenging questions. Do the disks of other massive stars form through similar processes? Do planetary systems form around massive stars? Subaru Telescope will be a powerful ally in studying these exciting issues.


Related Links
Ibaraki University
Stellar Chemistry, The Universe And All Within It

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