Studying the effects and cause of the massive star formation in Messier
  8 East
  Messier 8 (M8), one of the brightest HII regions in our Galaxy, is associated
with two prominent massive star-forming regions: M8-Main, the particularly
bright part of the large scale HII region (mainly) ionised by the stellar
system Herschel 36 (Her 36) and M8 East (M8 E), which is mainly powered by a
deeply embedded young stellar object (YSO), a bright infrared (IR) source,
M8E-IR. We aim to study the interaction of the massive star-forming region M8 E
with its surroundings and to compare the star-forming environments of M8-Main
and M8 E. We used the IRAM 30 m telescope to perform an imaging spectroscopy
survey of the molecular environment of M8E-IR. We imaged and analysed data for
the $J$ = 1 $\to$ 0 rotational transitions of $^{12}$CO, $^{13}$CO, N$_2$H$^+$,
HCN, H$^{13}$CN, HCO$^+$, H$^{13}$CO$^+$, HNC and HN$^{13}$C observed for the
first time toward M8~E. We used LTE and non-LTE techniques to determine column
densities of the observed species and to constrain the physical conditions of
the gas responsible for their emission. Examining the YSO population in M8~E
allows us to explore the observed ionization front (IF) as seen in GLIMPSE
8~$\mu$m emission image. We find that $^{12}$CO probes the warm diffuse gas
also traced by the GLIMPSE 8~$\mu$m emission, while N$_2$H$^+$ and HN$^{13}$C
trace the cool and dense gas. We find that the star-formation in M8~E appears
to be triggered by the earlier formed stellar cluster NGC~6530, which powers an
HII region giving rise to an IF that is moving at a speed $\geq$
0.26~km~s$^{-1}$ across M8~E. We derive temperatures of 80 K and 30 K for the
warm and cool gas components, respectively, and constrain H$_2$ volume
densities to be in the range of 10$^4$--10$^6$~cm$^{-3}$. Comparison of the
observed abundances of various species reflects the fact that M8~E is at an
earlier stage of massive star formation than M8-Main.
