Density and refractive index of interstellar water ice analogs at different deposition temperatures

Context. Water is the dominant component in interstellar ice mantles covering dust grains. Infrared observations of icy grains show evidence of structural variations in water ice. Aims. This work presents the experimental study of the morphology and optical properties of interstellar water ice analogs formed under various conditions of deposition. Methods. The real part of the refractive index (n) is obtained by double laser interferometry, and the average density (ρav) is measured with a quartz crystal microbalance. The experiments consisted of background-deposited water ice in a high-vacuum system, from 33 to 155 K, with a 532 nm laser. Results. The experiments show two regimes for n and ρav below and above 110 K. Below 110 K, both n and ρav increase with the deposition temperature. This increase is not linear with temperature, but exhibits a few steps at distinct temperatures. At 115 K, n and ρav decrease, marking the start of crystallite formation. At higher temperatures, both values increase again up to 150 K. The final values at 155 K are lower than at 150 K, indicating that the crystalline structure begins to change from cubic to hexagonal. Conclusions. We discuss the validity of the Lorenz-Lorentz equation for a polar molecule (water), depending on the ice structure, with hydrogen bonding influencing the polarizability. We find that a faster deposition produces lower values of n and ρav. This effect is less notable at lower temperatures. Our lowest deposition rate lets us compare results obtained in ultrahigh- and high-vacuum systems and extrapolate results to astrophysical conditions. The attenuation of the reflected light at different temperatures unveils a homogeneous structure for amorphous ices and non-compact crystalline ices. The high ice porosity derived from this work, 55% porosity at the lowest temperatures near 30 K, enhances grain growth and enables a more efficient trapping of chemical species, and consequently their reactions within the pores.