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

## TL;DR

This study combines optical polarization, Gaia parallaxes, and molecular line data to determine the distance, dust properties, and magnetic field structure of the dark globule LDN 1225, revealing the dominant role of magnetic fields in cloud support.

## Contribution

It provides the first detailed magnetic field morphology and strength analysis of LDN 1225 using combined polarization and molecular data, and characterizes dust grain growth and alignment.

## Key findings

- Distance to LDN 1225 is 830±83 pc.
- Magnetic fields are organized with a small dispersion of 12°.
- Magnetic pressure dominates over turbulence and gravity.

## Abstract

We present the results based on the optical $R$-band polarization observations of 280 stars distributed towards the dark globule LDN\,1225. {\it Gaia} data release 2 parallaxes along with the polarization data of $\sim$200 stars have been used to (a) constrain the distance of LDN\,1225 as 830$\pm$83~pc, (b) determine the contribution of interstellar polarization (ISP), and (c) characterize the dust properties and delineate the magnetic field (B-field) morphology of LDN\,1225. We find that B-fields are more organized and exhibit a small dispersion of 12$\degr$. Using the $^{12}$CO molecular line data from the Purple Mountain Observatory (PMO), along with the column density, dispersion in B-fields, we estimate B-field strength to be $\sim$56\,$\pm$\,10\,$\mu$G, magnetic to turbulence pressure to be $\sim$3\,$\pm$\,2, and the mass-to-magnetic flux ratio (in units of critical value) to be~$<$\,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 $^{12}$CO map) of the cloud, in contrast they are neither parallel nor perpendicular to the high-density core structures (traced by $^{13}$CO and C$^{18}$O maps). LDN\,1225 hosts two 70\,$\mu$m sources which seem to be of low-mass Class 0 sources. The total-to-selective extinction derived using optical and near-infrared photometric data is found to be anomalous ($R_{V}$~$=$~3.4), suggesting dust grain growth in LDN\,1225. Polarization efficiency of dust grains follows a power-law index of $-$0.7 inferring that optical polarimetry traces B-fields in the outer parts of the cloud.