Interstellar water ice analogue properties as a function of temperature

Abstract

Context. Infrared observations of water ice in the interstellar medium are hindered by uncertainties in the band strength, density, and porosity, which introduce considerable errors in the estimated column density of the ice on a line of sight. Aims. We revise the infrared band strength values and band positions of water ice grown under simulated interstellar conditions at different deposition and warm-up temperatures. We also explore other physical ice parameters: density, refractive index, and porosity. Methods. We grew water ice in simulated interstellar conditions in ultra-high vacuum with temperatures between 10 and 150 K. We used infrared spectroscopy and laser interferometry to obtain updated values for the band strengths. With these updated values, we calculated and report the density, refractive index, porosity, and infrared band position for water ice. Results. Previous measurements of these properties were inaccurate because ice was considered non-porous. Our results show that the band strength for the O-H stretching vibration varies significantly with deposition temperature and should not be considered constant. There is also measurable variation of the band strength during the warm-up of the ice after deposition. Conclusions. Previous ice density values were overestimated due to inaccurate band strengths and the omission of porosity, which can only be considered negligible at very slow growth rates and deposition temperatures close to sublimation. The infrared band position shifts with varying porosity.