Kinematic Analysis of a Protostellar Multiple System: Measuring the Protostar Masses and Assessing Gravitational Instability in the Disks of L1448 IRS3B and L1448 IRS3A

Abstract

We present new Atacama Large Millimeter/submillimeter Array observations toward a compact (∼230 au separation) triple protostar system, L1448 IRS3B, at 879 μm with 011 × 005 resolution. Spiral arm structure within the circum-multiple disk is well resolved in dust continuum toward IRS3B, and we detect the known wide (∼2300 au) companion, IRS3A, also resolving possible spiral substructure. Using dense gas tracers, C17O (J = 3 2), H13CO+ (J = 4 3), and H13CN (J = 4 3), we resolve the Keplerian rotation for both the circum-triple disk in IRS3B and the disk around IRS3A. Furthermore, we use the molecular line kinematic data and radiative transfer modeling of the molecular line emission to confirm that the disks are in Keplerian rotation with fitted masses of M ⊙ for IRS3B-ab and M ⊙ for IRS3A and place an upper limit on the central protostar mass for the tertiary IRS3B-c of 0.2 M ⊙. We measure the mass of the fragmenting disk of IRS3B to be ∼0.29 M ⊙ from the dust continuum emission of the circum-multiple disk and estimate the mass of the clump surrounding IRS3B-c to be 0.07 M ⊙. We also find that the disk around IRS3A has a mass of ∼0.04 M ⊙. By analyzing the Toomre Q parameter, we find the IRS3A circumstellar disk is gravitationally stable (Q > 5), while the IRS3B disk is consistent with a gravitationally unstable disk (Q < 1) between the radii ∼200–500 au. This coincides with the location of the spiral arms and the tertiary companion IRS3B-c, supporting the hypothesis that IRS3B-c was formed in situ via fragmentation of a gravitationally unstable disk.

Kinematic Structures in Planet-Forming Disks

“…Rather than being in a self-regulating state of self-gravity, as is likely the case with the Elias 2-27 system, L1448 IRS 3B has undergone disk fragmentation into three protostellar objects objects (Tobin et al 2016). Interestingly, recent observations using the molecular lines 12 CO, SiO, H 13 CO + , H 13 CN/SO 2 and C 17 O did not show any kinematic perturbations caused by the embedded companions (Reynolds et al 2021). This may be because the spatial and spectral resolution was below that required to detect kinematic features such as velocity kinks and the GI Wiggle, but it is also possible that the presence of the companions suppresses kinematic evidence of GI as in Rowther et al (2020a).…”

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars V. A Characterization of Protostellar Multiplicity

“…Therefore, the close multiples we observe could be the result of the disks that already fragmented, and the disks may not remain in a nearunstable state for long enough time to enable many of them to be discovered. For example, we did not detect obvious analogs to the fragmenting triple system L1448 IRS3B in Perseus (Tobin et al 2016b;Reynolds et al 2021) within Orion, and spatial resolution was too low to detect analogues to [BHB2007] 11 (Alves et al 2019). We do note however that the instability calculations presented in Paper I made many simplifying assumptions to arrive at a disk-averaged limit, so those values should not be considered absolute.…”

“…These flux density ratios have no relation to the underlying mass ratios of these systems because there are clear examples where the brightest dust continuum source in a system is not the most massive (e.g., L1448 IRS3B; Tobin et al 2016b;Reynolds et al 2021). The ratios could be interpreted as a dust mass ratio of the circumstellar disks around each component of a multiple system, with the caveat that the VLA data can have contributions from free-free emission.…”

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars VI. Insights from Radiative Transfer Modeling

“…We do find some disks that appear to have been caught in the act (e.g. Tobin et al 2016bTobin et al , 2018Reynolds et al 2021). So it may simply be the case that these disks remain in this state for short periods of time and are therefore difficult to directly observe, or have already formed multiple systems that would have been excluded from our modeling.…”