Presentation #120.04 in the session The Diffuse Gas in the Milky Way II: The Atomic Gas across the Milky Way.
The transition from HI to H2 is one of the major bottleneck processes toward star formation. Among the two flavors of HI, the cold and warm neutral medium (CNM and WNM), the cold one likely plays a critical role thanks to its higher density and lower temperature. Despite its expected importance for the HI-to-H2 transition and ensuing star formation, however, the CNM in and around molecular clouds remains largely unexplored. In this talk, we present ESPOIR (hope in French; Earliest stage of Star formation Probed by Observations of InterstellaR absorption), a ~100 hr VLA project to measure HI absorption toward 70 continuum sources behind the Perseus molecular cloud in the Milky Way. Leveraged by the high sensitivity of the VLA, this project almost triples the number of existing measurements with a factor of 20 higher source number density (~1 src/deg2). By analyzing the obtained HI absorption spectra along with GALFA HI emission data, we derive the physical properties of the CNM and WNM and examine how they change across the molecular cloud. In addition, we construct the 3D distribution of the CNM in and around Perseus based on the comparison with 3D dust data. With the unprecedentedly high source number density, ESPOIR enables us to examine the properties of the CNM in and around Perseus in detail and to evaluate the role of the CNM in the HI-to-H2 transition for the first time.