April 21, 2005

Hidden Shape of Young Star’s Envelope Seen

NAOJ -- 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 ÃŽ³) 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.


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