Silhouette Reveals Hidden Shape of Young Star's Envelope
April 20, 2005
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Object: Protostar M17-S01
Instrument: Infrared Camera and Spectrograph with Adaptive Optics
Observation Date: 2003/08/15 and 2003/05/23 (UT)
FOV: 14.8 arcsec x 7.4 arcsec
Orientation: North is 56 degrees clockwise from twelve-o-clock position
Position: R.A. 18h 20m 26.18s Dec. -16d 12m 10.2s
Top Image: A
near-infrared color composite image based on
2.1 μm (red), 1.6 μm (green), and
1.3 μm (blue) images. The dust in the gaseous
envelope surrounding the star is silhouetted
against the background light. Some of the scattered
illumination from the central light escaping
from the envelope can be seen in blue above
and below the envelope.
Detailed new images of the starbirth nursery in the Omega Nebula (M17) have revealed a multi component structure in the envelope of dust and gas surrounding a very young star. The stellar newborn, called M17-SO1, has a flaring torus of gas and dust, and thin conical shells of material above and below the torus. Shigeyuki Sako from University of Tokyo and a team of astronomers from the National Astronomical Observatory of Japan, the Japan Aeorospace Exploration Agency, Ibaraki University, the Purple Mountain Observatory of the Chinese Academy of Sciences, and Chiba University obtained these images and analyzed them in infrared wavelengths in order to understand the mechanics of protoplanetary disk formation around young stars. Their work is described in a detailed article in the April 21, 2005 edition of Nature.
The research team wanted to find a young star located in front of a bright background nebula and use near-infrared observations to image the surrounding envelope in silhouette, in a way comparable to how dentists use X-rays to take images of teeth. Using the Infrared Camera and Spectrograph with Adaptive Optics on the Subaru telescope, the astronomers looked for candidates in and around the Omega Nebula, which lies about 5,000 light-years away in the constellation Sagittarius. They found a large butterfly-shaped near-infrared silhouette of an envelope about 150 times the size of our solar system surrounding a very young star. They made follow-up observations of the region using the Cooled Mid-Infrared Camera and Spectrograph on the Subaru telescope and the Nobeyama Millimeter Array at the Nobeyama Radio Observatory. By combining the results from the near-infrared, mid-infrared, and millimeter wave radio observations, the researchers determined that the M17-SO1 is a protostar about 2.5 to 8 times the mass of the Sun, and that the butterfly-like silhouette reveals an edge-on view of the envelope.
The near-infrared observations reveal the structure of the surrounding envelope with unprecedented levels of detail. In particular, observations using the 2.166 emission line of hydrogen (called the Brackett gamma (Br γ) line) show that the envelope has multiple components instead of one simple structure. Around the equator of the protostar, the torus of dust and gas increases in thickness farther way from the star. Thin cone-shaped shells of material extend away from both poles of the star.
The discovery of the multi-component structure puts new constraints on how an envelope feeds material to a protostellar disk forming within its boundaries. "It's quite likely that our own solar system looked like M17-SO1 when it was beginning to form," said Sako. "We hope to confirm the relevance of our discovery for understanding the mechanism of protoplanetary disk formation by using the Subaru telescope to take infrared images with high resolution and high sensitivity of many more young stars.”
The Sun and the solar system formed from a dense cloud of gas and dust similar to M17-SO1 some 4.6 billion years ago. All the material that makes up the Earth and the creatures that live upon it originated in that primordial cloud. Once the Sun formed, its gravity pulled gas and dust inward. When the Sun's gravitational pull and the centrifugal force of the infalling material balanced, the remaining material settled into orbit around the Sun. The resulting disk of gas and dust was a protoplanetary disk. Repeated collisions of gas and dust within this disk led to the formation of the planets. To understand what the early solar system was like, and to understand how planetary systems form in general, astronomers are actively studying stars that could be similar to the Sun as it was 4.6 billion years ago.
Astronomers think that protoplanetary
disks surround young stars that are only a million
years old. Such stellar newborns are called T-Tauri
stars, named after the star T-Tauri in the constellation
Taurus. To understand how protoplanetary disks
form, astronomers must look further back in a
star’s evolution, at objects that are only
100,000 years old. Such protostars are surrounded
by an envelope of dust and gas. A disk forms as
material in the envelope settles into orbit around
the newly formed star. Although studying young
stars with envelopes is essential for understanding
the process of planet formation, it's also observationally
challenging since the envelope itself obscures
the process of how it feeds the proto-planetary
disk. A simple, direct solution is to look for
protoplanetary disks and clouds that are silhouetted
by radiation from nearby stars and study the characteristics
in near-infrared wavelengths.
Related papers by other researchers:
Chini et al, 2004, Nature, 329 155-157
Padgett et al. 1999, The Astronomical Journal, 117, 1490-1504
Jiang et al. 2002, Vol 577, 245 - 259
Figure 1: Mechanism for Creating an Near-Infrared Silhouete
Around the Omega Nebula (M17), high-energy radiation from massive stars has created regions of ionized gas. Far from the massive stars, gas is in molecular form and mixed with dust in large clouds. Stars form when gas and dust begin to condense inside a molecular cloud. The research team observed the southwestern area of the Omega Nebula (Figure 3) where a molecular cloud lies in front of ionized gas along our line of sight. Visible light emitted by the ionized gas is blocked by the molecular cloud, but some of the infrared light can pass through, allowing astronomers to take images of what is inside the molecular cloud. In broadband 2.1 μm, 1.6 μm, and 1.3 μm images, protostars also contribute to the infrared light. However, only the ionized background gas emits light at the 2.166 μm emission line of hydrogen so the details of the envelope show up in silhouette in the 2.166 μm images.
Figure 2: Near-Infrared Images of M17-SO1
Infrared images of M17-SO1 from the Infrared Camera and Spectrograph on Subaru telescope with adaptive optics. The top half is a near-infrared color composite of K-band (2.1 μm; red), H-band (1.6 μm; green), and J-band (1.3 μm; blue) images. The dust in the gaseous envelope surrounding the star blocks the background light and can be seen in silhouette. Some of the scattered light from the central light escaping from the envelope can be seen in blue above and below the envelope. The bottom half is an image in the 2.166 μm near-infrared hydrogen emission line (Br γ). Only the background has emission at this wavelength, so the detailed structure of the envelope is visible in silhouette. both images have the same filed of view (4.8 arcsec x 7.4 arcsec).
Figure 3: Near-Infrared Image of a Wide Area Surrounding M17-SO1
(Image without white boxes, Image with different stretch around M17-SO1)
The white boxes labeled a, b, and c each correspond to the areas in Figure s 2, 5, and 6, respectively. A color composite of K-band (2.1 μm; red), H-band (1.6 μm; green), and J-band (1.3 μm; blue) images from Subaru telescope's Infrared Camera and Spectrograph using adaptive optics. The field of view is 1 arcminute x 1 arcminute. M17-SO1 is in the molecular cloud to the lower right. As the background light passes through the interstellar gas and dust, shorter wavelength light is preferentially removed, making the area surrounding the molecular cloud appear reddish. The bright star slightly left of the center was used as the adaptive optics reference star.
Figure 4: Wide Field Images of the Area Surrounding the Omega Nebula (Messier 17)
Upper Left: Visible light image (B, V, and R-bands) of the Omega Nebula from the 2kCCd Camera on the University of Tokyo Kiso Observatory 105cm Schmidt Telescope.
Upper right: Near-infrared image (J, H, and K-bands) of from the SIRIUS camera on the IRSF 1.4 m telescope in South Africa.
Lower Left: Wide-angle image of the Milky Way by astrophotographer Shino Kato.
Lower right: Same as Figure 3.
Figure 5: Mid-Infrared Image of M17-SO1
Image from the Cooled Mid-Infrared Spectrograph and Camera on the Subaru telescope at 12.8 μm. The field of view is 14.8 arcsecond x 14.8 arcsecond. The image shows the mid-infrared silhouette of the envelope similar in shape to the near-infrared silhouette. The lack of a mid-infrared source in the middle of the envelope indicates that the young star is cannot be very massive.
Figure 6: The Distribution of Carbon Monoxide (CO) around M17-SO1
Upper Panel: A contour map of the intensity distribution of carbon monoxide (13CO) from the Nobeyama Millimeter Array at the Nobeyama Radio Observatory superimposed upon a false color image of the near-infrared silhouette of M17-SO1. Molecular gas (CO) is clearly associated with M17-SO1 and surrounding material.
Lower Panel: A velocity map of the carbon monoxide. Red regions are moving away from Earth, blue regions are moving towards Earth. The fact that the blue and red regions are on opposite sides of M17-SO1 suggest that the CO gas orbits around M17-SO1.
Notable Features in the Envelope of M17-SO1
1. The butterfly-shaped flaring structure
Figure 8: Three Dimensional Model of the Envelope
By assuming that the data show the envelope surrounding M17-SO1 from a vantage point close to the equatorial plane of the central star and the surrounding structures, the researchers can create a three-dimensional model of the envelope. The diagram shows this model with a section removed to show the interior detail of the structures
The flare in the torus is likely
to originate from the gravitational pull of
the central star causing material to sink towards
its equatorial plane. Outflows of molecular
gas or a jet from the central star can create
the shell like structures by pushing away material
from the star's polar regions. (Diagram;
of a jet observed with the Subaru telescope.)