The methanol emission in the $J_1-J_0$ A$^{-+}$ line series as a tracer
  of specific physical conditions in high-mass star-forming regions
  We present results of the investigations of the properties of the methanol
$J_1 - J_0$ A$^{-+}$ line series motivated by the recent serendipitous
detection of the maser emission in the $14_1 - 14_0$ A$^{-+}$ line at 349~GHz
in S255IR-SMA1 soon after the accretion burst. The study includes further
observations of several lines of this series in S255IR with the SMA, a
mini-survey of methanol lines in the 0.8 mm range toward a sample of bright
6.7~GHz methanol maser sources with the IRAM-30m telescope, and theoretical
modeling. We found that the maser component of the $14_1 - 14_0$ A$^{-+}$ line
in S255IR decayed by more than order of magnitude in comparison with that in
2016. No clear sign of maser emission is observed in other lines of this series
in the SMA observations except the $7_1 - 7_0$ A$^{-+}$ line where an
additional bright component is detected at the velocity of the maser emission
observed earlier in the $14_1 - 14_0$ A$^{-+}$ line. Our LVG model constrains
the ranges of the physical parameters that matches the observed emission
intensities. No obvious maser emission in the $J_1 - J_0$ A$^{-+}$ lines was
detected in the mini-survey of the 6.7~GHz methanol maser sources, though one
component in NGC7538 may represent a weak maser. In general, the maser effect
in the $J_1 - J_0$ A$^{-+}$ lines may serve as a tracer of rather hot
environments and in particular luminosity flaring events during high mass star
formation.
