Does near-infrared polarimetry reveal the magnetic field in cold dark clouds?

We present near-infrared (JHK) observations of the polarization of background starlight seen through the filamentary dark cloud L1755. The mean position angle and dispersion of the polarization vectors measured in the near-infrared, for stars along lines of sight passing through the densest portions (1 < A_v < 10 mag) of L1755, are virtually identical to those in an optical polarization map of stars around the periphery (A_v ∼ 1 mag) of L1755. Furthermore, the percentage of polarization is not seen to increase, at all, with extinction in the near-infrared observations. We surmise that much of the dust in the dark cloud is extinguishing background starlight significantly, but not polarizing it efficiently, and thus that the polarization map of background starlight cannot reliably trace the magnetic field associated with the dense interior of the dark cloud. Our results in L1755 are remarkably similar to what we found in the dark cloud B216-217 (Goodman et al. 1992), which also shows no change in the polarization map associated with the cloud, and no rise in percentage polarization with extinction. Using our multiwavelength polarimetric observations of L1755, we have estimated the wavelength of maximum polarization, λ_max, for most of the 53 stars in our sample. We find an unusually broad distribution of λ_max, with a mean at 0.88 ± 0.34 μm. The large range of λ_max leads us to the hypothesis that there is a wide range of grain sizes and/or shapes along the lines of sight through L1755. We conclude that only a small subset of grains is responsible for producing the polarization of background starlight, and that these grains may be critically underrepresented in the dense interiors of cold dark clouds.