Expanding bubbles in Orion A: [CII] observations of M42, M43, and NGC
  1977
  The Orion Molecular Cloud is the nearest massive-star forming region. Massive
stars have profound effects on their environment due to their strong radiation
fields and stellar winds. Velocity-resolved observations of the [CII]
$158\,\mu\mathrm{m}$ fine-structure line allow us to study the kinematics of
UV-illuminated gas. Here, we present a square-degree-sized map of [CII]
emission from the Orion Nebula complex obtained by the upGREAT instrument
onboard SOFIA, covering the entire Orion Nebula (M42) plus M43 and the nebulae
NGC 1973, 1975, and 1977. We compare the stellar characteristics of these three
regions with the kinematics of the expanding bubbles surrounding them. The
bubble blown by the O7V star $\theta^1$ Ori C in the Orion Nebula expands
rapidly, at $13\,\mathrm{km\,s^{-1}}$. Simple analytical models reproduce the
characteristics of the hot interior gas and the neutral shell of this
wind-blown bubble and give us an estimate of the expansion time of
$0.2\,\mathrm{Myr}$. M43 with the B0.5V star NU Ori also exhibits an expanding
bubble structure, with an expansion velocity of $6\,\mathrm{km\,s^{-1}}$.
Comparison with analytical models for the pressure-driven expansion of H\,{\sc
ii} regions gives an age estimate of $0.02\,\mathrm{Myr}$. The bubble
surrounding NGC 1973, 1975, and 1977 with the central B1V star 42 Orionis
expands at $1.5\,\mathrm{km\,s^{-1}}$, likely due to the over-pressurized
ionized gas as in the case of M43. We derive an age of $0.4\,\mathrm{Myr}$ for
this structure. We conclude that the bubble of the Orion Nebula is driven by
the mechanical energy input by the strong stellar wind from $\theta^1$ Ori C,
while the bubbles associated with M43 and NGC 1977 are caused by the thermal
expansion of the gas ionized by their central later-type massive stars.
