Grain growth in interstellar clouds

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Abstract
The phenomenon of grain growth in interstellar clouds, evidenced by increases in the wavelength of maximum polarization and ratio of total to selective extinction, is investigated by two distinct methods. A comparison of the empirical correlation between λmax and the colour excess ratio EV–K/EB–V with theoretical models published by McMillan indicates that growth occurs by the accretion of dielectric mantles on to dielectric grain cores of the same refractive index. A search for the 3.1 μm absorption band of water ice towards four dust-embedded stars in the ρ Ophiuchi molecular cloud was carried out using the 3.9-m AAT and IR photometer-spectrometer. Results indicate that the grain mantles in this cloud do not contain appreciable water ice: in particular, the highly obscured early-type star GS35 (AV = 12.5 mag) shows no 3.1 μm feature; an upper limit for the ratio of band absorption to visual extinction of 0.006 was deduced, which is comparable with that found for the diffuse interstellar medium. The lack of a spectral feature at 3.3–3.4 μm in our spectra similarly places chemical constraints on organic grain mantles. It is concluded that a grain model involving refractory compounds of oxygen (silicates and metal oxides) is capable of explaining the available data. The 3.1 μm absorption band is detected for the first time in the R Corona Australis dark cloud, towards an intense infrared source associated with the Herbig-Haro object H-H 100. The optical depth of the band is τ(3.1) ∼ 0.7, and the deduced τ(3.1)/AV ratio consistent with other reported detections in molecular clouds. Problems concerning the interpretation of the observed 3.1 μm feature are discussed in the final section.

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