A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk (1610.08524v1)

Abstract: Binary and multiple star systems are a frequent outcome of the star formation process, and as a result, almost half of all sun-like stars 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 $>$1000~AU 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 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, 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. The inferred mass of the central pair of protostellar objects is $\sim$1 M${sun}$, while the disk surrounding the three protostars has a total mass of $\sim$0.30 M${\sun}$. The tertiary protostar itself has a minimum mass of $\sim$0.085 M$_{sun}$. 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.

A Triple Protostar System as Evidence of Disk Fragmentation

The paper "A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk" presents a detailed observational paper of the L1448 IRS3B triple protostar system located in the Perseus molecular cloud. The paper leverages high-resolution data from the Atacama Large Millimeter/submillimeter Array (ALMA) to explore the formation mechanisms of multiple star systems, challenging existing paradigms by providing robust evidence of disk fragmentation at smaller scales as a concurrent pathway to turbulence-based fragmentation.

Observational Insights and Structural Analysis

L1448 IRS3B is an archetypal Class 0 protostar system, indicative of an embryonic stage in star formation, marked by an enveloped structure of accreting material. The paper meticulously details the spatial configuration of the triple system, where IRS3B-a, IRS3B-b, and IRS3B-c demonstrate a hierarchical configuration with separations of 61 AU and 183 AU. The ALMA observations revealed a circumstellar disk exhibiting distinct spiral arms that encapsulate the protostars, extending to a radius of approximately 400 AU. This structural configuration, highlighted by spiral arm dynamics, naturally invites analysis under gravitational instability frameworks, particularly Toomre's Q parameter, which the authors utilize to assess the disk's susceptibility to fragmentation.

Mass and Dynamical Stability Considerations

Quantifying the mass components of the system reveals a central pair of protostars, IRS3B-a/b, with a combined mass of approximately 1 M_\odot, surrounded by a disk mass totaling 0.30 M_\odot, and a tertiary protostar IRS3B-c with an estimated minimum mass of 0.085 M_\odot. The velocity structure, captured via the Doppler shift of C$^{18}$O molecular line emissions, corroborates the rotational dynamics centered on IRS3B-a/b rather than the more distant IRS3B-c, aligning with disk fragmentation models that suggest such mass distribution and structural features.

Implications of Gravitational Instabilities

The analysis hinges on the gravitational instability that can ensue in protostellar disks when the self-gravity becomes comparable to central gravitational forces. The analytic and modeling evidence indicates that the L1448 IRS3B disk did fragment due to instability along its spiral arms between radii of 150–320 AU. The Toomre analysis provided convincing evidence of marginal instability with Q approaching unity, a critical threshold for the onset of disk fragmentation.

Theoretical and Practical Implications

The findings presented offer significant theoretical implications, bolstering the hypothesis that disk fragmentation is a viable mechanism for forming closely bound multiple star systems at young protostellar stages. The spiral arm morphology observed aligns with simulated predictions for disks undergoing instability, providing a framework for extrapolating disk behavior in similar regions. Practically, the paper suggests that systems with similar configurations can be uncovered with extensive surveys, potentially shifting the understanding of binary and multiple star formation mechanisms.

Future Directions

As the field progresses, further exploration with facilities like ALMA will likely yield larger samples of fragmenting disks similar to IRS3B. Such data will facilitate comparing the frequency of turbulent and disk fragmentation, thus refining the theoretical models presented. Enhanced sensitivity in future observations could also lead to direct measurement techniques for probing protostar masses more precisely, expanding the disk instability characterization framework.

The paper stands as a lucid example of harmonizing observational data with theoretical models, offering a comparative analysis with previous simulation frameworks and challenging prevailing theories surrounding protostellar disk behavior.