2014MNRAS.437.2980R

Of all the water-ice (H2O-ice) bands the librational band, occurring at a wavelength of about 12µm, has proved to be the most difficult to detect observationally and also to reproduce in radiative transfer models. In fact, the case for the positive identification of the feature is strong in only a few astronomical objects. A previously suggested explanation for this is that so-called radiative transfer effects may mask the feature. In this paper, radiative transfer models are produced which unambiguously reveal the presence of the librational band as a separate resolved feature provided that there is no dust present which radiates significantly in the 10-µm region, specifically silicate-type dust. This means that the maximum dust temperature must be ≲ 50K. In this case, the models indicate that the librational band may clearly be observed as an absorption feature against the stellar continuum. This suggests that the feature may be best observed by obtaining the 10-µm spectrum of stars either with very cool circumstellar dust shells, with Tmax ≲ 50K, or those without circumstellar dust shells at all but with interstellar extinction. The first option might, however, require unrealistically large amounts of dust in the circumstellar shell in order to produce measurable absorption. Thus, the best place to look for the water-ice librational band may not be protostars with the remnants of their dust cloud still present, or evolved objects with ejected dust shells, as one might first think, because of the warm dust (Tmax ≫ 50K) usually present in the shells of these objects. If objects associated with very cool dust exclusively do show the 3.1-µm water-ice band in deep absorption, but the librational band still does not appear, this may imply that it is not radiative transfer effects which suppress the librational band, and that some other mechanism for its suppression is in play. One possibility is that a low water-ice to silicate abundance may mask the librational band, even if the total amount of water-ice present is large enough to produce a deep 3.1-µm feature. Boogert et al. have recently presented the spectra of many heavily obscured stars behind isolated dense cores. Model results presented here indicate that one of these objects does show evidence for the presence of the librational band, seen as an absorption feature against the stellar continuum.