Polarimetric and photometric investigation of the dark globule LDN 1225: distance, extinction law, and magnetic fields

Abstract

We present results based on the optical R-band observations of the polarization of 280 stars distributed toward the dark globule LDN 1225. Parallaxes from Gaia data release 2 along with the polarization data of ~200 stars have been used to (a) constrain the distance of LDN 1225 as 830 ± 83 pc, (b) determine the contribution of interstellar polarization, and (c) characterize the dust properties and delineate the magnetic field (B-field) morphology of LDN 1225. We find that B-fields are more organized and exhibit a small dispersion of 12°. Using the 12CO molecular line data from the Purple Mountain Observatory, along with the column density and dispersion in B-fields, we estimate the B-field strength to be ~56 ± 10 μG, the ratio of magnetic to turbulent pressure to be ~3 ± 2, and the ratio of mass to magnetic flux (in units of the critical value) to be <1. These results indicate the dominant role of B-fields in comparison to turbulence and gravity in rendering the cloud support. B-fields are aligned parallel to the low-density parts (traced by a 12CO map) of the cloud; in contrast, they are neither parallel nor perpendicular to the high-density core structures (traced by 13CO and C18O maps). LDN 1225 hosts two 70 μm sources, which seem to be low-mass Class 0 sources. The ratio of total to selective extinction derived using optical and near-infrared photometric data is found to be anomalous (R V = 3.4), suggesting the growth of dust grains in LDN 1225. The polarization efficiency of dust grains follows a power law with an index of −0.7, implying that optical polarimetry traces B-fields in the outer parts of the cloud.