Stars are formed from cold and extremely thin interstellar molecular clouds as the gas is compressed through the force of gravity. In order to continue contracting, the clouds must get rid of the energy produced by the compression, otherwise the thermal pressure will halt the contraction and stop the process. The forming stars are believed to radiate the necessary energy from water vapour, molecular oxygen, carbon monoxide and carbon. The picture of the complex chemical processes leading to the formation of molecules in the cloud is still unclear. Today we know about more than a 100 molecular species in these clouds, but we still do not know the abundance of the key species water (H2O) and oxygen (O2). To solve this issue, observations from space are needed, since the Earth's atmosphere absorbs the emission line from H2O, O2, C and, to some extent, CO. Odin is specifically designed for detecting H2O and O2, in addition to other molecules, and will observe hundreds of individual molecular clouds in our galaxy as well as assemblies of giant molecular clouds in other galaxies.
Topics for astronomical study by Odin are:
- Giant molecular clouds and nearby dark clouds: the purpose here is to improve our understanding of the chemistry and cooling processes of the interstellar medium and thereby the conditions of star formation.
- Detection of protostars.
- Comets: the physics of outgassing of water, the size of active regions and density estimates both in short- and long-period comets.
- Planets: height distribution of trace elements in the atmospheres of Jupiter and Saturn. Detection of minor components brought up by convection would provide information not only on the dynamics but also on the physics and chemistry of the deep atmospheres.
- Circumstellar envelopes: the dynamics and chemical composition of outflows.
- Nearby galaxies: estimates of star formation activity from observations of CO and H2O.