Subaru Stares into a Cradle of Stars
February 13, 2001
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Object Name: Star-formation Region S106 IRS4
Telescope: Subaru Telescope / Cassegrain Focus
Filter: J (1.25 micron) , H (1.65 micron) , K' (2.15 micron)
Color: Blue (J), Green (H) , Red (K')
Date: UT1999 May 25, June 5, 6
Exposure: 6 min (J) , 4 min (H, K')
Field of View: 5 arcmins
Orientation: North up, east left
Position: RA(J2000.0)=20h27m, Dec(J2000.0)=+37d (Cygnus)
Subaru Telescope has successfully taken a sharp and deep infrared image of the star-forming region, S106. In addition, many objects with masses less than that of an ordinary star have been discovered in this region.
S106 is at a distance of approximately 2000 light-years from the Earth. There is a large massive star called IRS4 (Infrared Source 4) at the center of S106. The star is approximately one hundred thousand years old, and its mass is approximately 20 times that of the Sun. The hourglass appearance of S106 is thought to be the result of the way material is flowing outwards from the central star. A huge disk of gas and dust surrounding IRS4 produces the constriction at the center.
Ultraviolet rays emitted from IRS4 ionize the surrounding hydrogen gas, creating what astronomers call an HII region. As the excited hydrogen gas relaxes, it emits the blue glow we see in the inner part of the nebula. We call this an emission nebula. The red region towards the edge of the nebula is a reflection nebula, made as surrounding dust particles directly reflect the light emitted from IRS4. Since this infrared image is extremely sharp, we can see subtle details like ripples inside the emission nebula. Furthermore, the differences in color and structure between the emission and reflection nebulae are beautifully displayed. In comparison, a visible-light image of the same region taken with the Hubble Space Telescope barely shows any detail in the upper part of the nebula because visible light is strongly absorbed by the region's dust.
A study of this deep S106 image has revealed hundreds of faint young objects around IRS4 and throughout the surrounding nebula. The mass of these objects is less than 0.08 times that of our Sun, too small to sustain the nuclear burning of hydrogen gas that causes a normal star to shine. They are considered to be young brown dwarfs.
The lightest and faintest objects discovered have an estimated mass of only a few times that of Jupiter. A joint group of astronomers from the National Astronomical Observatory of Japan and the University of Tokyo have observed similar light objects elsewhere in the sky, in the nearby star-forming regions towards the constellation of Taurus and Chameleon. Other researchers have seen such objects in the constellation of Orion. While such objects would be called "planets" if they orbited a star, this is not appropriate for these independent objects. For this reason, we would refer to them as "floating small objects."
From the observation with Subaru Telescope, it's clear that many light objects are born out in space along with the ordinary stars we see, and that the relative number of such objects differs from place to place. But the birth mechanism for these objects is still unclear.