Dust and Stellar Radiation Field

Interstellar space is permeated by radiation of all wavelengths originating from the stars. However, below 912 A, which is the ionization potential (Lyman 

6a und b. a) Typical observed interstellar extinction curve from the IR through the optical range to the UV. b) Schematic representation of the optical part of the extinction curve, defining the quantities A v, Ab and R 

It increases rapidly with decreasing wavelength. The extinction is measured in magnitudes. From optical observations of stars, it is easy to determine the relative extinction or color excess E = AB — A v, with AB and A v the total extinction at wavelengths B (= 4400 A) and V (= 5500 A). On the assumption that the extinction curve is similar in all directions in space, the total extinction in the visual is (Fig. 6b) 

The albedo of a dust grain is defined as the ratio of its scattering cross section to its cross section for total extinction. From the observations, values of the albedo as high as 0.9 at X 1500 A are obtained. A particle with no 

Most molecules observed to date in interstellar space can be dissociated by UV radiation of wave lengths longer than 912 A. In fact, their average lifetimes in interstellar space are 100 yr, unless they are protected by a dust layer (Section IV. E). This, and the fact that surface reactions on dust grains play an important role in the formation of interstellar molecules (see Section IV. 

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