Discovery of an expanding molecular bubble in Orion BN/KL
  During their infancy, stars are well known to expel matter violently in the
form of well-defined, collimated outflows. A fairly unique exception is found
in the Orion BN/KL star-forming region where a poorly collimated and somewhat
disordered outflow composed of numerous elongated ``finger-like'' structures
was discovered more than 30 years ago. In this letter, we report the discovery
in the same region of an even more atypical outflow phenomenon. Using
$^{13}$CO(2-1) line observations made with the Submillimeter Array (SMA), we
have identified there a 500 to 1,000 years old, expanding, roughly spherically
symmetric bubble whose characteristics are entirely different from those of
known outflows associated with young stellar objects. The center of the bubble
coincides with the initial position of a now defunct massive multiple stellar
system suspected to have disintegrated 500 years ago, and with the center of
symmetry of the system of molecular fingers surrounding the Kleinmann-Low
nebula. We hypothesize that the bubble is made up of gas and dust that used to
be part of the circumstellar material associated with the decayed multiple
system. The Orion hot core, recently proposed to be the result of the impact of
a shock wave onto a massive dense core, is located toward the south-east
quadrant of the bubble. The supersonic expansion of the bubble, and/or the
impact of some low-velocity filaments provide a natural explanation for its
origin.
