RCW 36 (also designated Gum 20) is an emission nebula containing an embedded cluster in the constellation Vela. This H II region is part of a larger-scale star-forming complex known as the Vela Molecular Ridge (VMR), a collection of molecular clouds in the Milky Way that contain multiple sites of ongoing star-formation activity. The VMR is made up of several distinct clouds, and RCW 36 is embedded in the VMR Cloud C. RCW 36 is one of the sites of massive-star formation closest to the Solar System, located at a distance of approximately 954 parsecs (3112 light-years). The most massive stars in the star cluster are two stars with late-O or early-B spectral types, but the cluster also contains hundreds of lower-mass stars and brown dwarfs. This region is also home to objects with Herbig–Haro jets, HH 1042 and HH 1043. It is about 1.1 million years old.
Star formation in RCW 36 Like most star-forming regions, the interstellar medium around RCW 36 contains both the gas from which stars form and some newly formed young stars. Here, young stellar clusters form in giant molecular clouds. Molecular clouds are the coldest, densest form of interstellar gas and are composed mostly of molecular hydrogen (H2), but also include more complex molecules, cosmic dust, and atomic helium. Stars form when the mass gas in part of a cloud becomes too great, causing it to collapse due to the Jeans instability. Most stars do not form alone, but in groups containing hundreds or thousands of other stars. RCW 36 is an example of this type of "clustered" star formation.
Molecular cloud and H II region
The Vela Molecular Ridge can be subdivided into several smaller clouds, each of which in turn can be subdivided into cloud "clumps". The molecular cloud clump from which the RCW 36 stars are forming is Clump 6 in the VMR C cloud. Early maps of the region were produced by radio telescopes that traced emission from several types of molecules found in the clouds, including CO, OH, and H2CO. More detailed CO maps were produced in the 1990s by a team of Japanese astronomers using the NANTEN millimeter-wavelength telescope. Using emission from C18O, they estimated the total mass of Cloud C to be 44,000 M☉. The cloud maps suggest that Cloud C is the youngest component of the VMR because of an ultra-compact H II region associated with RCW 36 and several sites of embedded protostars, while H II regions in other VMR clouds are more evolved. Observations from the Herschel Space Telescope show that the material within the cloud is organized into filaments and RCW 36 sits near the south end of a 10-parsec long filament. Star formation in RCW 36 is currently ongoing. In the dense gas at the western edge of RCW 36, where the far-infrared emission is greatest, are found protostellar cores, the Herbig Haro objects, and an ultra-compact H II region. However, more deeply embedded star-formation is obscured by dust, so radiation can only escape from the cloud surface and not from the embedded objects themselves. The H II region is an area around the cluster in which hydrogen atoms in the interstellar medium have been ionized by ultraviolet light from O- and B-type stars. The H II region in RCW 36 has an hourglass morphology, similar to the shape of H II regions around other young stellar clusters like W40 or Sh2-106. In addition, an ultra-compact H II region surrounds IRAS source 08576−4333.
Star cluster Due to its youth, most of the stars in RCW 36 are at an early stage of stellar evolution where they are known as young stellar objects or pre-main-sequence stars. These stars are still in the process of contraction before they reach the main sequence, and they may still have gas accreting onto them from either a circumstellar disk or envelope.
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