Star and Planet FormationThe Orion Nebula is the nearest HII region of its kind, and the Orion Molecular Cloud is the nearest high-mass star-forming region. The Orion Nebula, ionized by the hot stars in the Trapezium, has therefore always been the prototypical example of interactions between young, massive stars and star-forming molecular clouds.
The discovery of a plasma bubble inside the Nebula was, on the one hand, predicted based on calculations made 3 decades ago. The winds of hot stars are supposed to carve shock-heated bubbles into the dense molecular environment until these bubbles expand and burst into the interstellar medium. But searches for hot diffuse gas clouds in star-forming regions have met with only partial success, very hot distributed gas being found in regions where supernovae have exploded, or in star-forming regions where many very early-type stars shed winds, some of which may collide and shock-heat gas.
The Orion Nebula neither hosts a supernova remnant, nor does in contain early O-type stars. Energetically, it is dominated by one O7 star, theta1 Ori C. We suggest that it is the wind of theta1 Ori C alone that shock-heats to 2 MK and fills the Orion cavity. We have also estimated that the gas now visible in X-rays is in approximate pressure equilibrium with the gas of the much cooler, optically visible Orion Nebula. Pressure thus does not build up although the cavity gas must be continuously replenished given the wind mass-loss rate of theta1 Ori C. The pressure argument rather suggests that the gas is flowing in a stead-state fashion, leaking out into interstellar space. Gamma-ray emission from the 26Al isotope, indeed tracing a a channel from the Orion region to the so-called Eridanus Superbubble in the nearby interstellar medium, suggest that the Orion plasma fills this bubble, carrying with it 26Al from the Trapezium region.
Because Orion-like star-forming regions with only few massive stars are common across the Galaxy, we suggest that star-forming regions contribute to a network of channels and bubbles in the hot interstellar medium.
XMM-Newton study of hot, diffuse plasma in the Orion Nebula region. Left (A): Part of a large mosaic of XMM-Newton exposures, covering the Orion Nebula with its Trapezium cluster (bright, saturated region to the upper left of the center). The red hue in the right half of the mosaic reveals the presence of soft X-ray emission from relatively cool, widely distributed plasma. — Right (B): After removal of X-ray point sources and adaptive smoothing, the diffuse X-ray source has been superimposed (in blue) on a Spitzer IRAC mosaic (in orange), the latter showing regions of thermal dust emission. The white contour shows the EPIC detector border of the central exposure.