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 1;2 , and as a result, almost half of all sun-like stars have at least one companion star 3 . 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 4;5 or smaller scale fragmentation of a massive protostellar disk due to gravitational instability 6;7 . Observational evidence for turbulent fragmentation on scales of >1000 AU has recently emerged 8;9 . Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems 10;11;12;13 . The triple protostar system L1448 IRS3B is an ideal candidate to search for evidence of disk fragmentation. L1448 IRS3B is in an early phase of the star formation process, likely less than 150,000 years in age 14 , and all protostars in the system are separated by <200 AU. Here we report observations of dust and molecular gas emission that reveal a disk with spiral structure surrounding the three protostars. Two protostars near the center of the disk are separated by 61 AU, and a tertiary protostar is coincident with a spiral arm in the outer disk at a 183 AU separation 13 . The inferred mass of the central pair of protostellar objects is ⇠1 M , while the disk surrounding the three protostars has a total mass of ⇠0.30 M . The tertiary protostar itself has a minimum mass of ⇠0.085 M . We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 AU and 320 AU, 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.L1448 IRS3B is located in the Perseus molecular cloud at a distance of ⇠230 pc 15 and contains three protostars out of the six that collectively make up L1448 IRS3 13;14 , spanning 0.05 pc. L1448 IRS3B is a Class 0 protostar system 16 , which signifies an early phase of the star formation process when the protostars are deeply enshrouded in an envelope of accreting material 17 . The three protostars in L1448 IRS3B (denoted -a, -b, and -c) have a hierarchical configuration; the central-most protostar, IRS3B-a, has projected separations from IRS3B-b and IRS3B-c of 61 AU and 183 AU, respectively 13 . The new observations of L1448 IRS3B conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) at a resolution of 0. 00 27⇥0. 00 16 (62 AU ⇥ 37 AU) provide images at 1.3 mm of the dust and gas emission surrounding the three protostars with 10⇥ higher sensitivity and 2⇥ higher resolution than previous studies.The ALMA 1.3 mm image of L1448 IRS3B is shown in Figure 1, revealing dust emission toward each of the three distinct protostars identified in previous Karl G. Jansky Very Large Array (VLA) observations 13 . The ALMA images also reveal a disk with substructure sur...

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