title: A triple protostar system formed via fragmentation of a gravitationally unstable disk.
abstract: Binary and multiple star systems are a frequent outcome of the star formation process and as a result almost half of all stars with masses similar to that of the Sun have at least one companion star. Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large-scale fragmentation of turbulent gas cores and filaments or smaller-scale fragmentation of a massive protostellar disk due to gravitational instability. Observational evidence for turbulent fragmentation on scales of more than 1,000 astronomical units has recently emerged. Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems. The triple protostar system L1448 IRS3B is an ideal system with which to search for evidence of disk fragmentation as it is in an early phase of the star formation process, it is likely to be less than 150,000 years old and all of the protostars in the system are separated by less than 200 astronomical units. Here we report observations of dust and molecular gas emission that reveal a disk with a spiral structure surrounding the three protostars. Two protostars near the centre of the disk are separated by 61 astronomical units and a tertiary protostar is coincident with a spiral arm in the outer disk at a separation of 183 astronomical units. The inferred mass of the central pair of protostellar objects is approximately one solar mass, while the disk surrounding the three protostars has a total mass of around 0.30 solar masses. The tertiary protostar itself has a minimum mass of about 0.085 solar masses. We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 and 320 astronomical units, overlapping with the location of the tertiary protostar. This is consistent with models for a protostellar disk that has recently undergone gravitational instability, spawning one or two companion stars.
authors: Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, Oklahoma 73019, USA.|Leiden Observatory, Leiden University, PO Box 9513, 2300-RA Leiden, The Netherlands.
authors: John J
authors: JJ
authors: Tobin
authors: Department of Astronomy and Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, Arizona 85721, USA.
authors: Kaitlin M
authors: KM
authors: Kratter
authors: Leiden Observatory, Leiden University, PO Box 9513, 2300-RA Leiden, The Netherlands.|Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden.
authors: Magnus V
authors: MV
authors: Persson
authors: Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
authors: Leslie W
authors: LW
authors: Looney
authors: Department of Physics, State University of New York Fredonia, Fredonia, New York 14063, USA.
authors: Michael M
authors: MM
authors: Dunham
authors: Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
authors: Dominique
authors: D
authors: Segura-Cox
authors: Department of Astronomy, University of Virginia, Charlottesville, Virginia 22903, USA.
authors: Zhi-Yun
authors: ZY
authors: Li
authors: National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA.
authors: Claire J
authors: CJ
authors: Chandler
authors: Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany.
authors: Sarah I
authors: SI
authors: Sadavoy
authors: Department of Astronomy, University of Illinois, Urbana, Illinois 61801, USA.
authors: Robert J
authors: RJ
authors: Harris
authors: Center for Astrophysics and Space Sciences, University of California, San Diego, California 92093, USA.
authors: Carl
authors: C
authors: Melis
authors: Max-Planck-Institut für Radio Astronomie, Auf dem Hügel 69, 53121 Bonn, Germany.
authors: Laura M
authors: LM
authors: Pérez