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We present new Atacama Large Millimeter/submillimeter Array (ALMA)observations towards a compact (230~au separation) triple protostar system,L1448 IRS3B, at 879~\micron with \contbeam~resolution. Spiral arm structurewithin the circum-multiple disk is well resolved in dust continuum towardIRS3B, and we detect the known wide (2300~au) companion, IRS3A, also resolvingpossible spiral substructure. Using dense gas tracers, C17O, H13CO$+$, andH13CN, we resolve the Keplerian rotation for both the circum-triple disk inIRS3B and the disk around IRS3A. Furthermore, we use the molecular linekinematic data and radiative transfer modeling of the molecular line emissionto confirm that the disks are in Keplerian rotation with fitted masses of$1.19^{+0.13}_{-0.07}$ for IRS3B-ab, $1.51^{+0.06}_{-0.07}$~Msun for IRS3A, andplace an upper limit on the central protostar mass for the tertiary IRS3B-c of0.2~Msun. We measure the mass of the fragmenting disk of IRS3B to be 0.29~Msunfrom the dust continuum emission of the circum-multiple disk and estimate themass of the clump surrounding IRS3B-c to be 0.07~Msun. We also find that thedisk around IRS3A has a mass of 0.04~Msun. By analyzing the Toomre~Q parameter,we find the IRS3A circumstellar disk is gravitationally stable (Q$>$5), whilethe IRS3B disk is consistent with a gravitationally unstable disk (Q$<$1)between the radii 200-500~au. This coincides with the location of the spiralarms and the tertiary companion IRS3B-c, supporting the hypothesis that IRS3B-cwas formed in situ via fragmentation of a gravitationally unstable disk.

the astrophysical journal. letters

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