We examine long-term low-dispersion International Ultraviolet Explorer SWP and LWP spectroscopy of the R Coronae Borealis (RCB) star V854 Cen, obtained across the deep (*V [ 6 mag) 1991, 1992È 1993, and 1994 declines. We also report the optical light curve for the star in the interval 1987È1998, including multicolor photometry obtained during 1989È1998. The light curve includes at least eight major declines where the amplitude exceeds 5 mag, many of which appear to be multiple decline events. Analysis of the UV emission-line spectra indicates most lines decay during the deep declines on characteristic timescales comparable to that reported for optical features. Fe, Mg, and neutral C lines decay on timescales of typically 50È100 days. Other lines, notably ionized C lines, decay on longer timescales ([200 days) or appear to be una †ected by the declines. The general nature of the UV emission lines and other UV features during the declines is consistent with the E1/E2/BL line region model developed from the behavior of optical spectral features during declines. However, the detailed line behavior indicates large intrinsic variability between decline events inconsistent with the simple E1/E2/BL model. Limited temporal coverage prevents detailed examination of the geometry of the emission-line region or the obscuring dust. We also report the Ðrst detection of the transition-region line C IV] j1550 in the spectrum of an RCB star. We Ðt the onset times of all declines from maximum light within the 1987È1998 interval, irrespective of decline amplitude, with a 43.23 day linear solution, thus improving the decline ephemeris. The linear term is probably the pulsation period of V854 Cen, further supporting the suspected link between radial pulsations and mass loss in these types of stars.

We present new absorbance spectra of the 3‐, 6‐ and 12‐μm features of amorphous and crystalline H2O ice obtained between 10 and 140 K. Three sets of measurements have been made. In series I, the ice film was initially deposited on to a CsI substrate at 10 K and successive spectra were then obtained at intermediate temperatures as the ice was warmed up to 140 K. The second set, series II, comprises spectra for ice films deposited and measured at temperatures between 10 and 140 K. In the third set of measurements, series III, spectra were obtained for an ice film deposited at 140 K and then at intermediate temperatures as the film was cooled down to 10 K. The series I and II results show that the ice undergoes an amorphous‐to‐crystalline phase transition in the 110–120 K range. The 3‐ and 12‐μm bands have similar trends in full width at half‐maximum (FWHM) and opposite peak wavelength shifts. The temperature behaviour of the 6‐μm band is different, as no clear phase transition temperature can be discerned from its FWHM and peak wavelength position data. In the series III spectra the peak wavelength positions and FWHM of the three bands remain relatively constant, thus demonstrating the stability of the crystalline phase against thermal cycling. A comparison between the laboratory results and astronomical spectra suggests that the identification of the librational band of H2O ice in OH 231.8 + 4.2 may be incorrect.

Radiative transfer modelling of the Infrared Space Observatory (ISO) spectrum of IRAS 22036+5306 has shown that its unusual 11‐μm band can be suitably modelled with an alumina‐olivine mixture substantially dominated by the former. The results of this work add further credence to recent findings that significant amounts of Al2O3 dust grains are present in the dust shells of stars near or beyond the tip of the asymptotic giant branch. Indeed, in the case of IRAS 22036+5306, Al2O3 dominates the dust composition to the extent that it shifts the 9.8‐μm band due to amorphous silicates to 11 μm. IRAS 22036+5306 may be an unusual case in that the inner dust torus is maintained at a sufficiently high temperature for Al2O3 condensation, but not silicate.

Radiative transfer models have been produced for stars surrounded by circumstellar dust shells in order to investigate the detectability of the 11.5-µm librational band due to water-ice (H 2 Oice). The dust grains were assumed to be composed of a core material of either amorphous silicate of olivine composition (MgFeSiO 4 ) or alumina (Al 2 O 3 ), both grain species being coated with a water-ice mantle. The models may be divided into three classes: those with small, intermediate and large optical depths. It is found that in all three cases, even with only olivine present, the water-ice librational band feature is masked by radiative transfer effects and is therefore difficult to detect.For the librational band to display its characteristic shape requires far larger amounts of water-ice than are present in any known circumstellar dust shell, as indicated by the depth of the much stronger 3.1-µm water-ice band. The best prospect for finding some evidence of the librational band is likely to be via using a small beam centred on the cooler outer regions of the dust shell; in this case the line of sight may encounter only water-ice coated grains, thereby reducing the contaminating effect of bare grains. If a grain species such as alumina is present, with a broad absorption band in the vicinity of 11.5 µm, the identification of the librational band may be possible, perhaps surprisingly, as result of the sharpening of the 11.5-µm feature. We have fitted the spectrum of IRAS 22036+5306 and find that, although water-ice is clearly present in this object, as evidenced by the presence of the strong 3.1-µm band, the librational band has no obvious effect on its 10-µm spectrum.