Loose threads: Parsec-scale filamentation in the high-Galactic-latitude molecular clouds MBM 3 and MBM 16

Context. The existence of high-galactic-latitude molecular clouds has been known about for several decades, and studies of their dust and gas distributions reveal complicated morphological structures. Their dynamics involve turbulence even in the absence of internal energy sources such as stars. Aims. We study in detail two such clouds, MBM 3 and MBM 16, trying to recover the geometric structure and topology of the gas distribution. In particular, we address the evidence of superthermal asymmetric atomic and molecular line profiles as a result of filament superposition combined with turbulent motions. Methods. We used a variety of spectroscopic and imaging archival observations of the gas and dust components. The spectroscopic data set comprises H I 21 cm, 12 CO, 13 CO, and CH line profiles. We also used archival infrared images to study the dust distribution and temperature. To understand the topology of MBM 3 and MBM 16, we compared molecular and atomic spectra, along with a profile decomposition of the H I 21 cm line. Standard tools such as structure functions of velocity centroids were used to characterise the turbulence in MBM 3, and channel maps and position-velocity diagrams were employed to elucidate the filament topology of both clouds. Results. The unusually large linewidths previously reported for MBM 3 are due to the superposition of individual filaments whose superthermal linewidths are about 1 km s -1 . In MBM 16, the cloud appears to decompose into two adjacent structures with similar properties. The filaments have high aspect ratios, with lengths of about 1 pc and widths of about 0.1 pc. In general, the molecular gas is embedded within more extended neutral hydrogen structures. Velocity gradients found within these structures are not necessarily dynamical, convergent flows. Projection effects and topology of the driving flows produce signatures that mimic velocity shears even if they are simply distortions of ordered gas.