Astronomers are announcing today that encounters between proto-stars young enough to have extended disks of gas can result in the creation of smaller brown dwarf stars with disks of their own. The report is being presented by Sijing Shen and Dr. James Wadsley of McMaster University to the Canadian Astronomical Society meeting in Calgary, Alberta. The computer simulated results provide a natural explanation for the origin of observed brown dwarfs with disks, outflows and giant planetary companions as well as systems with several brown dwarfs in orbit around a star.
Brown dwarf stars are as common in number as large stars but no more than 7.5 percent of the mass of the Sun and unable to burn hydrogen in ongoing fusion reactions. Observational studies indicate that young brown dwarfs have disks and jets that are reminiscent of the T-Tauri phase of a regular star. This phase indicates the star is accumulating material from a large gaseous disk around it. A brown dwarf has been observed with a planetary companion (2M1207) which may have condensed from such a disk. The challenge to theorists was to explain the origin of not only brown dwarfs but brown dwarfs with disks.
Stars are thought to form from cold dense cores in giant molecular gas clouds. The natural mass of a core is expected to be large, closer to that of a star than a brown dwarf. As cores condense they first form proto-stars with massive disks of gas slowly feeding onto them. Stars form in clusters and are likely to encounter another star at least once during this stage. Using SHARCNET parallel computing facilities, Shen and Wadsley simulated several such encounters at unprecedented resolution, seeing gas pile-ups, drawn-out tidal arms and huge masses of gas driven closer to the stars. Amid this chaos several small objects were seen to form, from Jupiter-sized objects up to brown dwarfs. Reports from lower resolution simulations by other groups had shown no indication of disks. However, in every case, the new objects had disks with sizes ranging up to 18 astronomical units (the size of Saturn's orbit). As these rapidly spinning disks evolve they should produce outflows and even result in the formation of planets orbiting the brown dwarfs.
"We had no idea the simulated results would be so beautiful and complex and then we found out that observations were revealing brown dwarfs with disks which matched what we were seeing.", said Sijing Shen who is studying for her Ph.D. in Physics and Astronomy at McMaster.
The simulated objects would either leave the system (singly or in groups) or remain as brown dwarf companions to the stars. This mechanism shows particular promise for making systems of many brown dwarfs in orbit around a star. Though the relative numbers of simulated brown dwarfs and smaller objects fit expectations, it remains to be determined exactly how often such star encounters occur in nature and what fraction of those encounters are productive for making brown dwarfs. For this, Shen and Wadsley are planning a much larger set of encounter simulations using SHARCNET's new supercomputers.
"We can't say anything definitive to rule out other ways for making brown dwarfs just yet.", said Dr. Wadsley, Assistant Professor of Physics and Astronomy at McMaster, "It may be that encounters are most helpful for creating multiple systems such as GL 569 where a triple of brown dwarf stars is orbiting a regular star."
The simulations reported here were run on computers maintained at McMaster University by SHARCNET, the Shared Hierarchical Academic Research Computer Network. This work was supported by the Natural Science and Engineering Research Council of Canada.
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- These results are incorporated into a Letter to the Astrophysical Journal: Shen, S. and Wadsley, J., 2006, Astrophysical Journal Letters (submitted)
- Long wavelength studies of brown dwarfs with disks have been carried out by several groups. The authors would be happy provide more details of these studies and potential observer contacts.
- 2M1207 is a brown dwarf with a gas giant planet imaged in orbit about it.
Publication: Chauvin, G., Lagrange, A.-M., A.-M., Dumas, C., Zuckerman, B., Mouillet, D., Song, I., Beuzit, J.-L., & Lowrance, P. 2005, Astronomy and Astrophysics, 438, L25
- GL569 is a set of three brown dwarfs orbiting an M star.
Preprint: Simon, M., Bender, C., & Prato, L. 2006, ArXiv Astrophysics e-prints, arXiv:astro-ph/0603165