DOES NEAR-INFRARED POLARIMETRY REVEAL THE MAGNETIC-FIELD IN COLD DARKCLOUDS

We present near-infrared (JHK) observations of the polarization of bac kground starlight seen through the filamentary dark cloud L1755. The m ean 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 ident ical to those in an optical polarization map of stars around the perip hery (A(V) similar to 1 mag) of L1755. Furthermore, the percentage of polarization is not seen to increase, at all, with extinction in the n ear-infrared observations. We surmise that much of the dust in the dar k cloud is extinguishing background starlight significantly, but not p olarizing it efficiently, and thus that the polarization map of backgr ound starlight cannot reliably trace the magnetic field associated wit h the dense interior of the dark cloud. Our results in L1755 are remar kably similar to what we found in the dark cloud B216-217 (Goodman et al. 1992), which also shows no change in the polarization map associat ed with the cloud, and no rise in percentage polarization with extinct ion. Using our multiwavelength polarimetric observations of L1755, we have estimated the wavelength of maximum polarization, lambda(max), fo r most of the 53 stars in our sample. We find an unusually broad distr ibution of lambda(max), with a mean at 0.88 +/- 0.34 mu m. The large r ange of lambda(max) leads us to the hypothesis that there is a wide ra nge of grain sizes and/or shapes along the lines of sight through L175 5. We conclude that only a small subset of grains is responsible for p roducing the polarization of background starlight, and that these grai ns may be critically underrepresented in the dense interiors of cold d ark. clouds.