Understanding of the star-formation process in the ISM is vital to the study of evolution of galaxies. Yet a number of issues remain to be answered, among which the accurate estimate of the molecular gas reservoir and the onset trigger are crucial for our understanding of subsequent star-formation process.
It has become evident that the traditional tracer of molecular gas, CO, can miss a large fraction of molecular gas (CO-dark gas) because of more effective photodissociation of CO relative to H2. HD 112 and 56µm and/or [CII] 158µm lines provide alternative, reliable means to probe the molecular gas component in our Galaxy and galaxies. SPICA will offer the first opportunity to study the molecular gas reservoir reliably, which cannot be probed by CO observations, and provide us with the accurate estimate how much molecular mass is waiting for the star-formation in our Galaxy and galaxies.
Large scale motions and molecular cloud formation in galaxies generate interstellar turbulence. The corresponding turbulent kinetic energy is either dissipated by velocity shears (non compressive) or lead to the formation of dense structures at the origin of star formation through shocks, which is therefore central to the regulation of star-formation. The current picture is that turbulence dissipation may occur in low-velocity shocks, which are closely related to the formation of filamentary structures. The dissipation process of low-velocity shocks is best studied by [OI] 63µm line and SPICA offers a unique opportunity to study the very first stage of star-formation process.
Herschel and Planck findings lead to a “filamentary paradigm” for star-formation, in which both magnetic field and turbulence dissipation play critical roles in the onset of star-formation, as materials accrete to form filaments along magnetic force lines and dissipate their energy via radiative shocks to form first hydrostatic cores, followed by the class 0 stage of the proto-stellar phase. SPICA will obtain the necessary high spatial resolution polarimetry observations and refresh our view of star-formation, raising our understanding to a new level, by virtue of its efficient polarimetry in the FIR and its highly sensitive spectroscopy from MIR to FIR.