Lecturer: Mr. Shmuel Bialy Affiliation: Department of Astronomy and Astrophysics, Tel-Aviv University Abstract: I will discuss recent results from our study of the atomic-to-molecular (HI- to-H2) transition in a UV-irradiated turbulent medium (Bialy, Burkhart & Sternberg 2017). Our approach is twofold, a numerical study based on magnetohydrodynamic (MHD) numerical simulations complemented by an analytic theory. We derive the HI-to-H2density profiles and the probability density function (PDF) of the HI column density. Importantly, the HI PDF depends on three basic parameters, (i) αG - the dissociation parameter, (ii) Ms - the sonic Mach number, and (iii) Ldec/LHI - the ratio of the turbulence decorrelation scale to the HI scale lengths. I will demonstrate how our theory may be applied to 21 cm observations to set constraints on the Mach number and the turbulence driving scale of the cold neutral medium (CNM). Additional details of the upcoming Astrophysics' seminars can be found on the following link. ‏האירוע הזה כולל שיחת וידאו ב-Google Hangouts. הצטרף: https://plus.google.com/hangouts/_/mail.huji.ac.il/astrophysics?hceid=bWFpbC5odWppLmFjLmlsX2c0czhydDlpcmhwZzRvdGNybWIzZGFqcjdvQGdyb3VwLmNhbGVuZGFyLmdvb2dsZS5jb20.hgraamg47ohp379d2rgrbvr4o4&hs=121