The JCMT Transient Survey: Stochastic and Secular Variability of
  Protostars and Disks In the Sub-Millimeter Observed Over Eighteen Months
  We analyze results from the first eighteen months of monthly sub-mm
monitoring of eight star-forming regions in the JCMT Transient Survey. In our
search for stochastic variability in 1643 bright peaks, only the previously
identified source, EC53, shows behavior well above the expected measurement
uncertainty. Another four sources, two disks and two protostars, show
moderately-enhanced standard deviations in brightness, as expected for
stochastic variables. For the two protostars, this apparent variability is the
result of single epochs that are much brighter than the mean. In our search for
secular brightness variations that are linear in time, we measure the
fractional brightness change per year for 150 bright peaks, fifty of which are
protostellar. The ensemble distribution of slopes is well fit by a normal
distribution with sigma ~ 0.023. Most sources are not rapidly brightening or
fading in the sub-mm. Comparison against time-randomized realizations shows
that the width of the distribution is dominated by the uncertainty in the
individual brightness measurements of the sources. A toy model for secular
variability reveals that an underlying Gaussian distribution of linear
fractional brightness change sigma = 0.005 would be unobservable in the present
sample, whereas an underlying distribution with sigma = 0.02 is ruled out. Five
protostellar sources, 10% of the protostellar sample, are found to have robust
secular measures deviating from a constant flux. The sensitivity to secular
brightness variations will improve significantly with a larger time sample,
with a factor of two improvement expected by the conclusion of our 36-month
survey.
