Di Li


Tracing the Transition Phase of Interstellar Medium through Spectroscopic Observations

Paul Goldsmith, William Langer, Gary Melnick


The transition from diffuse, largely atomic gas to dense, molecular gas is a key step in the life cycle of the interstellar medium. The results of this transition set up the conditions for star formation in dense molecular clouds.

 A comprehensive picture of ISM evolution and star formation requires the fundamental understanding of this transition. The conditions of diffuse, atomic ISM have been well studied by observing the emission from its dominant component, HI. The study of dense, molecular ISM has been carried out mostly through tracers, especially CO. The transition phase is not friendly to either probe.  In fact, this transition gas has been postulated to be a component of the so-called ''dark gas'' (e.g. Grenier et al. 2005), meaning hard-to-detect molecular gas without CO.

We studied this transition phase with three spectroscopic probes, ionized carbon emission (C+; Herschel),  atomic carbon emission (CI; SWAS), HI narrow self-absorption (HINSA; Arecibo and GBT).

These studies provide evidence for the existence of dark gas, the clumpy nature of star forming clouds, and a direct measurement of molecular clouds forming time scale.

Through an approved SOFIA basic science program, we plan to obtain  spatially and spectrally resolved C+ maps, thus further enhancing our understanding of the transition phase.