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In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on analyzing 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high-resolution optical spectroscopy, and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening, we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. This conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.

Despite its relevance in astrophysical scenarios, optical properties and density of ammonia have been scarcely studied. This work presents new data on the real part of the refractive index of ammonia at 632.8 nm and density at different temperatures of deposition from 13 K up to its desorption temperature around 110 K. The results show a significant variability for both parameters versus temperature, representing an increase of 50 % for density and a 10 % for refractive index as temperature increases in the range 13 -60 K, and at temperatures higher than 60 K a constant value is reached for both parameters. This initial variation and the further plateau reflects structural differences in its form at low and high temperatures that can play an important role in its interaction with other molecules of astrophysical interest.

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In this work, we focus on the study of the structural and elastic properties of

Contact. The increasing capabilities of space missions like the James Webb Space Telescope or ground-based observatories like the European Extremely Large Telescope demand high quality laboratory data of species in astrophysical conditions for the interpretation of their findings. Aims. We provide new physical and spectroscopic data of solid methanol that will help to identify this species in astronomical environments. Methods. Ices were grown by vapour deposition in high vacuum chambers. Densities were measured via a cryogenic quartz crystal microbalance and laser interferometry. Absorbance infrared spectra of methanol ices of different thickness were recorded to obtain optical constants using an iterative minimization procedure. Infrared band strengths were determined from infrared spectra and ice densities. Results. Solid methanol densities measured at eight temperatures vary between 0.64 g cm−3 at 20 K and 0.84 g cm−3 at 130 K. The visible refractive index at 633 nm grows from 1.26 to 1.35 in that temperature range. New infrared optical constants and band strengths are given from 650 to 5000 cm−1 (15.4–2.0 μm) at the same eight temperatures. The study was made on ices directly grown at the indicated temperatures, and amorphous and crystalline phases have been recognized. Our optical constants differ from those previously reported in the literature for an ice grown at 10 K and subsequently warmed. The disagreement is due to different ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.

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Context. The knowledge of the sublimation energy of ices allows us to better understand the dynamics between surfaces and atmospheres of different environments of astrophysical interest where ices are present. Aims. This work is intended to provide sublimation energy values for a set of pure ices (CO, CH 4 , CO 2 , N 2 , and NH 3 ) using a new experimental procedure. The results were compared to some values obtained by other authors under different conditions and/or methods, to check the reliability of this new method. Methods. We used the frequency variation obtained from a quartz crystal microbalance to calculate the sublimation energy from the Polany-Wigner equation for the first time.Results. The results obtained are relevant since there are few previous values of sublimation energy reported on these molecules in these conditions of pressure and temperature, which are representative of astrophysical regions. These values are needed in models used to interpret dynamics of icy surfaces. In general, our results compare well to other ones obtained by different methods and complement those previously available.