  A quantitative analysis of grain alignment in the filamentary dark cloud
L1755 in Ophiuchus is presented. We show, that the observed decrease of the
polarization to extinction ratio for the inner parts of this quiescent dark
cloud can be explained as a result of the decrease of the efficiency of grain
alignment.We make quantitative estimates of grain alignment efficiency for six
mechanisms, involving grains with either thermal or suprathermal rotation,
interacting with either magnetic field or gaseous flow. We also make
semiquantitative estimates of grain alignment by radiative torques.We show that
in conditions typical of dark cloud interiors, all known major mechanisms of
grain alignment fail. All the studied mechanisms predict polarization at least
an order of magnitude below the currently detectable levels of order of 1%. On
the contrary, in the dark cloud environments where A_v<1 the grain alignment
can be much more efficient. There the alignment of suprathermally rotating
grains with superparamagentic inclusions, and possibly also radiative torques
account for observed polarization. These results apply to L1755, which we model
in detail and probably also to B216 and other similar dark clouds. Our study
suggests an explanation for the difference in results obtained through
polarimetry of background starlight and polarized thermal emission from the
dust itself. We conjecture that the emission polarimetry selectively reveals
aligned grains in the environment far from thermodynamic equilibrium as opposed
to starlight polarization studie that probe the alignment of grains all the way
along the line of sight, including the interiors of dark quiescent clouds,
where no alignment is possible.
