AN INVESTIGATION OF GRAIN PROPERTIES IN THE RHO OPHIUCHI DARK CLOUD

We present a comprehensive study of dust grains in the rho Ophiuchi dark cloud. By combining new polarimetric, optical photometric, and spectral classification data for 134 stars, cloud material is probed to an absorption depth of A(V) less-than-or-equal-to 5-7 mag. For stars with R determined from the color-difference method, a mean value of R = 3.99 +/- 0.18 at a mean extinction of A(V) = 3.25 mag is found. R scales with the wavelength of maximum polarization at the rate R = 5.7lambda(max), consistent with results from other studies. Grain size, as deduced from lambda(max), increases with optical depth in a manner that is consistent with a simple model of turbulence-driven coagulative grain growth. Grain alignment is characterized by a mean P(V)(%)/A(V)(mag) = 0.91. The alignment decreases with both optical depth and grain size at rates which are consistent with classical B field alignment and the grain growth model. Comparison of the relative grain size distributions for rho Oph with other dark clouds, shows a clear progression from that of the largest mean grain size and dispersion for the youngest clouds to that of the normal distribution for the general interstellar medium. These results suggest that large grains are produced predominantly in very early stages of cloud evolution and that, after 10(6) yr, grain evolution is mainly a process of destroying larger grains, at least for the outer several magnitudes of A(V).