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Using near-infrared United Kingdom Infrared Deep Sky Survey (UKIDSS) Galactic Plane Survey data, we make extinction measurements to individual stars along the same line of sight as molecular clouds. Using an existing 3D extinction map of the inner Galaxy, that provides line of sight specific extinction–distance relationships, we convert the measured extinction of molecular clouds to a corresponding distance. These distances are derived independently from kinematic methods, typically used to derive distances to molecular clouds, and as such they have no near/far ambiguity. The near/far distance ambiguity has been resolved for 27 clouds, and distances have been derived to 20 clouds. The results are found to be in good agreement with kinematic measurements to molecular clouds where the ambiguity has already been resolved, using H i self-absorption techniques. [ABSTRACT FROM AUTHOR]

The origin of grain size distribution in the interstellar medium is one of the most fundamental problems in the interstellar physics. In the Milky Way, smaller grains are more abundant in number, but their origins are not necessarily specified and quantified. One of the most efficient drivers of small grain production is interstellar turbulence, in which dust grains can acquire relative velocities large enough to be shattered. Applying the framework of shattering developed in previous papers, we show that small grains reach the abundance level observed in the Milky Way in (i.e. within the grain lifetime) by shattering in warm neutral medium. We also show that if part of grains experiences additional shattering in warm ionized medium, carbonaceous grains with are redistributed into smaller sizes. This could explain the relative enhancement of very small carbonaceous grains with Å. Our theory also explains the ubiquitous association between large grains and very small grains naturally. Some tests for our theory are proposed in terms of the metallicity dependence. [ABSTRACT FROM AUTHOR]

GGD30 has been suggested to be either a small reflection nebulosity or a Herbig–Haro (HH) object formed in the outflow from a nearby obscured star. Observations to date have not been able to distinguish between these two scenarios. In addition, there are conflicting proposals for the location of the exciting source for GGD30. To resolve these questions, we have carried out optical spectroscopy and near-infrared ( J, K and 3.6-μm) imaging of GGD30. Taken together, these observations reveal that the bright optical knot in GGD30 must be a HH object, excited by the outflow from an optically obscured pre-main-sequence (PMS) star located ∼3 arcsec to the southwest. Based on mid-infrared fluxes from the Mid-course Space Experiment ( MSX) satellite, we estimate the luminosity of this PMS star to be which suggests it is an intermediate-mass object rather than low-mass as previously proposed. The optical spectroscopy indicates projected velocities of associated with the HH object. The fact that these velocities are blueshifted and relatively high compared to the velocities typical of HH flows suggests that the outflow from the PMS star must be almost aligned with the line of sight. There is an additional low-velocity component but its origin is not clear. [ABSTRACT FROM AUTHOR]