2023A&A...676A.138M

Context. Given the structural and thermodynamical complexity of the interstellar medium (ISM), the variety of governing processes, such as stellar feedback, poses challenges to the investigation. High-latitude molecular clouds (HLMCs) with no evidence of internal star formation, as in the case of MBM 40, are excellent sites for studying the chemistry and dynamic evolution of the cold neutral ISM.
Aims. We used this high-latitude cloud as an exemplar for the dynamical and chemical processes in the diffuse interstellar medium.
Methods. We analyzed new and archival ¹²CO, ¹³CO, CH, HCO⁺, CS, H₂CO, and HCN data from Five College Radio Observatory (FCRAO), Onsala Space Observatory (OSO), Arizona Radio Observatory (ARO), and W. Gordon telescope (Arecibo) combined with the Galactic Arecibo L-band Feed Array H I (GALFA-HI) H I 21 cm data set, to study the chemistry, thermal state, and dynamics of MBM 40. A new dynamical analytical approach was adopted by considering each line profile as a line-of-sight probability distribution function (PDF) of the turbulence weighted by gas emissivity.
Results. The atomic and molecular gas are smoothly distributed in space and velocity. No steep transition is seen between circumcloud atomic and cloud molecular gas in either radial velocity or structure. We propose a topology of the cloud based on molecular tracers, as a contorted filamentary structure that is shaped by a broad embedding shear flow in the neutral atomic gas. A comparative examination of different molecular tracers shows that ¹³CO, H₂CO, and CS only arise from denser molecular cores, whereas ¹²CO, CH, and HCO⁺ trace diffuse gas with a broader range of dynamics.