The circumstellar environment around the embedded protostar EC 53
  EC53 is an embedded protostar with quasi-periodic emission in the near-IR and
sub-mm. We use ALMA high-resolution observations of continuum and molecular
line emission to describe the circumstellar environment of EC 53. The continuum
image reveals a disk with a flux that suggests a mass of 0.075 Msun, much less
than the estimated mass in the envelope, and an in-band spectral index that
indicates grain growth to centimeter sizes. Molecular lines trace the outflow
cavity walls, infalling and rotating envelope, and/or the Keplerian disk. The
rotation profile of the C17O 3-2 line emission cannot isolate the Keplerian
motion clearly although the lower limit of the protostellar mass can be
calculated as 0.3 +- 0.1 Msun if the Keplerian motion is adopted. The weak
CH3OH emission, which is anti-correlated with the HCO+ 4-3 line emission,
indicates that the water snow line is more extended than what expected from the
current luminosity, attesting to bygone outburst events. The extended snow line
may persist for longer at the disk surface because the lower density increases
the freeze-out timescale of methanol and water.
