Observations of 6.7 GHz Methanol Masers with EAVN I: VLBI Images of the
  first Epoch of Observations
  Very long baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol
maser allows us to measure the internal proper motions of the maser spots and
therefore study the gas motion around high-mass young stellar objects. To this
end, we have begun monitoring observations with the East-Asian VLBI Network. In
this paper we present the results of the first epoch observation for 36
sources, including 35 VLBI images of the methanol maser. Since two independent
sources were found in three images, respectively, images of 38 sources were
obtained. In 34 sources, more than or equal to 10 spots were detected. The
observed spatial scale of the maser distribution was from 9 to 4900
astronomical units, and the following morphological categories were observed:
elliptical, arched, linear, paired, and complex. The position of the maser spot
was determined to an accuracy of approximately 0.1 mas, sufficiently high to
measure the internal proper motion from two years of monitoring observations.
The VLBI observation, however, detected only approximately 20% of all maser
emission, suggesting that the remaining 80% of the total flux was spread into
an undetectable extended distribution. Therefore, in addition to
high-resolution observations, it is important to observe the whole structure of
the maser emission including extended low-brightness structures, to reveal the
associated site of the maser and gas motion.
