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Young Stars Swim in Pools of Molecular Hydrogen

June 27, 2007

Figure 1: The spectrum of ECHA J0843.3-7905 divided by that of RECX12 with the fit to the H2 1-0 S(1) line and residuals in the lower panel.

During a search for hydrogen emission in the disks of young stars, Suzanne K. Ramsay Howat (UK Astronomy Technology Centre) and Jane S. Greaves (University of St. Andrews) have discovered a massive layer of hot gas around a low-mass M3-type star in the 6 million-year-old Eta Chamaeleontis cluster. Both the strength and the kinematics of the emission imply that it arises from a disk illuminated by ultraviolet radiation produced by the central star.

Studying the gas content of protoplanetary disks around young stars is an important step in understanding the formation of planetary systems. Molecular hydrogen is one of the main constituents of the atmospheres of giant planets and so must be present in the disks for Jupiter-like planets to form. Moreover, the near-infrared emission from it is one of the best indicators of warm gas that may exist in gaps carved by massive new protoplanets. This emission may arise from excitation of gas molecules by a passing shock wave, or by the absorption of ultraviolet radiation (or x-rays) from the stars at the center of the disks.

Molecular hydrogen emission was detected from only one of the seven sources observed by Ramsay Howat and Greaves. The researchers used PHOENIX, the high-resolution infrared spectrograph on the Gemini South Telescope at Cerro Pachón, Chile to make the observations. This instrument was built by a National Optical Astronomy Observatory (NOAO) team led by Ken Hinkle and is on loan to Gemini from NOAO.

Detecting molecular hydrogen emission associated with disks has proven challenging, even with 8- to 10-meter class telescopes and advanced instruments. In fact, this measurement by Ramsay Howat and Greaves is one of very few published detections of this type of emission. Another definite detection was recently made around the star AB Aurigae by a team led by Marty Bitner using the very high-resolution mid-infrared spectrograph TEXES on Gemini North.

The mass of hot molecular gas inferred around this source, named ECHA J0843.4-7905, is about 0.03 solar mass. This is similar to the mass of the minimum solar nebula. From the shape and width of the line profile (Figure 1) the authors conclude that this circumstellar gas is orbiting at 2 AU (astronomical units) from the star. Since the system is ~6 million years old, these results indicate that a significant gas reservoir persists to the age when gas giant planets are presumed to form. Current theories show that Jupiter-like object should form in the first ~2 million years of a disk’s life with the thick atmosphere accreted onto the planet in around 5million years.

More details can be found in the article “Molecular Hydrogen Emission from Disks in the Eta Chamaeleontis Cluster”, by S. K. Ramsay Howat and J. S. Greaves, The Monthly Notices of the Royal Astronomical Society, 2007 in press.