1999AJ....118..421F

Photographic proper motions and apparent B and V magnitudes of 5x10⁴ stars in the range m_V=11 to m_V=14 over the sky area 18^h48^m≤α≤20^h04^m and 17°≤δ≤25° (equinox B1900) are used to investigate interstellar extinction as distant as 0.8 kpc from the Sun. The Vulpecula molecular cloud, linked to the Vul OB I stellar association, seems conspicuous at a distance of 0.3 kpc with a total extinction of about 1.5 mag, when analyzing 1.6x10⁴ stars with significant proper-motion components. Transverse velocities for a set of 10³ stars along the boundary layer of the molecular cloud suggest a value of 7x10² g.cm².s^–1 for the shear viscosity coefficient of the interstellar medium in this region. The characteristic timescale is on the order of 20 Myr, and the free path is on the order of 20 pc. A transfer of optical energy of 2x10^–5 M_☉.yr^–1 is indeed suggested from a sample of 400 nebular variable candidates at the interface between the suspected gas and dust components. Follow-up spectroscopic observations of three candidates with transverse velocity on the order of 23±14 km.s^–1 do not rule out the chances of detecting small-scale variations in the interstellar extinction.