SMASHING YOUNG STARS LEAVE DWARFS IN THEIR WAKE
Young star (proto-stellar disk) collisions and fragmentation to form Brown Dwarfs :
Visualizations of simulations completed by Sijing Shen for her Masters Thesis (May 2006) under the supervision of James Wadsley (Asst. Professor, Dept Physics and Astronomy, McMaster University).
Click for MPEG Movie (5.9 MB)
START: Two proto-stellar disks
Disk Collision Movie description
The movie depicts a collision between two rotating disks around young stars. Each star will ultimately be similar in size to the Sun. Currently about half the material that will belong to each young star is currently in its disk. The disks are large: over 1000 times the distance from the Earth to the Sun in radius. It is expected that all regular stars will go through this phase early in their formation. In the absense of a collision, over a long timescale of millions of years, this material will fall onto the star. The collision shown below takes about 15000 years from the initial approach to the final state shown in the image.
The disks are depicted in blue with brighter colours indicating denser gas. The stars at the centre of each disk are not shown.
The stars approach each other off-centre so there is some rotation of the two disks about each other that is clockwise. The rotation of the disk that finishes on the left is counter-clockwise. The disk that finishes on the right rotates clockwise. The counter-clockwise disk's rotation tends to be cancelled out during the collision, driving gas inward which fragments to form brown dwarf stars. The clockwise disk's gas is drawn out as its rotation matches the rotation of the orbit to form tidal tails in an S-like shape that do not fragment.
The movie zooms in to look at the left, counter-clockwise disk in detail as the shrunken, heavy disk forms brown dwarfs from dense spirals of gas. One of the spirals fragments into a close pair of brown dwarfs.
The movie zooms in again to look at the close pair. It stops and rotates the pair around to show that each member of the pair consists of a flattenned disk of gas.
At the end the leftmost star has four brown dwarfs in orbit around it as shown in the end image above. (Each image is shown half size. Click for full size. Note that stills from the movie and other images are also available.)
The images below are of the same disk collision shown in the movie.
| Gas density just after collision |
[ Click Image for Animation ]
| Simulated Observed Image (dust added) just after collision |
[ Click Image for Animation ]
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 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.
For more information contact:
James Wadsley (905 525 9140 x 27106, firstname.lastname@example.org)
Sijing Shen (905 525 9140 x 23183, email@example.com)