Grain Alignment in Starless Cores

TL;DR

This study investigates dust grain alignment in starless cores using near-infrared and submillimeter polarimetry, revealing that grain alignment diminishes significantly at high optical depths without internal radiation sources.

Contribution

It provides new observational evidence that dust grains in dense starless cores cease to align with magnetic fields beyond a certain optical depth, supported by a simple predictive model.

Findings

Grain alignment declines with increasing optical depth in starless cores.

Alignment effectively ceases at $A_V \,\gtrsim\, 20$ in the absence of internal sources.

A simple model accurately reproduces the observed decline in grain alignment.

Abstract

We present near infrared polarimetry data of background stars shining through a selection of starless cores taken in the $K$ band, probing visual extinctions up to $A_{V} \sim 48$. We find that $P_K/{\tau _K}$ continues to decline with increasing $A_{V}$ with a power law slope of roughly -0.5. Examination of published submillimeter (submm) polarimetry of starless cores suggests that by $A_{V} \gtrsim 20$ the slope for $P$ vs. $\tau$ becomes $\sim -1$, indicating no grain alignment at greater optical depths. Combining these two data sets, we find good evidence that, in the absence of a central illuminating source, the dust grains in dense molecular cloud cores with no internal radiation source cease to become aligned with the local magnetic field at optical depths greater than $A_V \sim 20$. A simple model relating the alignment efficiency to the optical depth into the cloud reproduces the observations well.