Kinematic Analysis of a Protostellar Multiple System: Measuring the
  Protostar Masses and Assessing Gravitational Instability in the Disks of
  L1448 IRS3B and L1448 IRS3A
  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 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, H13CO$+$, and
H13CN, 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
$1.19^{+0.13}_{-0.07}$ for IRS3B-ab, $1.51^{+0.06}_{-0.07}$~Msun for IRS3A, and
place an upper limit on the central protostar mass for the tertiary IRS3B-c of
0.2~Msun. We measure the mass of the fragmenting disk of IRS3B to be 0.29~Msun
from the dust continuum emission of the circum-multiple disk and estimate the
mass of the clump surrounding IRS3B-c to be 0.07~Msun. We also find that the
disk 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$>$5), while
the IRS3B disk is consistent with a gravitationally unstable disk (Q$<$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.
