HI Narrow Line Absorption--Molecular or Not? by Dr. Di Li (CfA)   The galactic 21cm absorption is studied in two ways. One is against the continuum background radiation. The other is to study the so--called HI self--absorption (HISA) features in HI emission spectra. Based on both methods, a new consensus has emerged in recent studies--there exists cold (<20 K) neutral hydrogen in the Galaxy. The correlation between HISA and molecular clouds is inconclusive in previous studies. Our Arecibo observations, however, reveal a good correlation between HI narrow line absorption (HINLA) and OH emission. These nearby dark clouds have also been mapped in CO isotopes at Five College Radio Astronomy Observatory (FCRAO). Spectra of atomic carbon line at 492 GHz have also been obtained using Submillimeter Wave Astronomy Satellite (SWAS). All these tracers of molecular clouds show remarkable correlation with HINLA. Based on this observational evidence, we propose the following statements as a first step toward a comprehensive understanding of cold atomic hydrogen in the galaxy. There are two types of HI self-absorption. The first type reflects the temperature fluctuation in the cold neutral medium (CNM). The second type, studied by our Arecibo 21 cm survey of dark clouds, is cold atomic hydrogen residing in molecular clouds. They are better termed HI Narrow Line Absorption (HINLA). An updated H2 formation model or a complex cloud model is needed to explain the abundance of cold HI we observed.