Resumen
Our goal is the study of the magnetic field (MF) structure of a pre-collapse structure of the interstellar medium - the Musca Dark Cloud (MDC), a nearby (200-250 pc), large (0.25° × 3°) filamentary cloud. A description of the MF, together with knowledge on turbulence and gravitational forces, is key to understanding the evolution of interstellar clouds. We have obtained linear polarization measurements in the H band (1.65μm) with the Brazilian's 60 cm and 160 cm telescopes located at the OPD observatory. By combining these with our earlier optical observations (Pereyra & Magalhaes 2004), we were able to probe regions denser than what was possible in the optical. Our studies in the optical band showed that the cloud is surrounded by a MF which is well aligned with the projected small axis of the cloud. Our H-band data show in general the same tendency in the inner parts of the MDC. The comparison between the V and H bands allow us to conclude that the same type of grains are polarizing the light throughout the cloud at least up to Av ∼ 8-9, and that these are the same as those in the general ISM. Utilizing the dispersion of the polarization vectors, we estimated the MF intensity (0.02-0.16 mG) across the cloud. We compared the magnetic and gravitational energies and concluded that Musca is a subcritical cloud. From the structure function of the polarization at the H band, we obtain 0.21-0.29 pc for the range of correlation lengths of the MF in the cloud, comparable to the size of the optical condensations.
