Grain Alignment in Starless Cores

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 $P_K\mathrm{/}{\tau }_K$ 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.