Context. Hydrogenated amorphous carbon (HAC) materials have been considered as a laboratory analog of cosmic carbonaceous nanoparticles in the interstellar medium (ISM). In the diffuse ISM, UV radiation can modify the electronic and atomic structure of HAC materials. Aims. Studying structural and optical properties of HAC materials in correlation with UV processing is very important to understand more clearly the effect of the UV radiation on carbonaceous dust grains in the diffuse ISM. This scenario can explain some astronomical spectral features such as the interstellar UV bump at 4.6 μm −1 . Methods. Laser ablation has been used to produce nano-sized HAC materials which are subsequently irradiated by strong UV doses in a high vacuum. Transmission electron microscope images and spectroscopic analyses show the evolution of the internal structure of the material with the UV irradiation. Results. It is found that hydrogen content and the sp 3 /sp 2 hybridization ratio decrease with the UV irradiation. The graphene layers become longer in processed materials. Also, graphitic fibers are observed in modified materials. The variation in the internal structure leads to dramatic changes in the spectral properties in the FUV-VIS range. The UV irradiation of HAC materials, coresponding to 21-33% of the average dose of the UV radiation in diffuse ISM, has produced a new band centered at 4.6 μm −1 (217.5 nm). Conclusions. Consequently, these results confirm for the first time the suggestion by Mennella et al. (1996) that irradiated HAC materials might be considered the carrier of the interstellar UV bump at 4.6 μm −1 . However, so far the amount of carbon needed to produce the interstellar 4.6 μm −1 band is higher than that available for interstellar carbon dust grains. So the ideal structure of irradiated HAC materials that would produce a band of sufficient strength is not yet clear for the interstellar dust.

Context. Mid-infrared (MIR) bands are characteristic for the short-range and medium-range structure of hydrogenated amorphous carbon (HAC) materials that exist in the interstellar medium (ISM) and are sensitive to processing in the harsh interstellar radiation field. Aims. We study the variability of the MIR features from the spectrum of non-processed to that of UV-processed HAC materials and compare them to spectra of interstellar carbonaceous materials. Methods. Nano-sized HAC materials produced by laser ablation were irradiated by vacuum-UV photons with doses comparable to those relevant for interstellar processing. They were subsequently analyzed by IR spectroscopy. Results. In the MIR range, the spectra of HAC materials show many absorption bands such as the sp 3 aliphatic ≡C-H stretching vibration at 3.03 μm, the sp 3 aliphatic -C-H stretching vibration at 3.4 μm and also the sp 3 aliphatic C-H bending vibration at both 6.85 μm and 7.25 μm. All these are recognizable bands of HAC materials. Other absorption bands such as the sp 2 aromatic =C-H stretching vibration at about 3.3 μm and the sp 2 -C=C stretching vibration close to 6.25 μm are observed. The HAC materials also possess bands which represent the aromatic out-of-plane bending at 11.65, 12.46 and 12.9 μm in addition to the aromatic -C-C-C in-plane bending at 15.87 μm. With UV irradiation, the mass absorption coefficient of the 3.03 μm band completely disappears and that of the aliphatic C-H bands (3.4, 6.85 and 7.25 μm) decreases. This reduction shows that the UV radiation destroys most of the aliphatic C-H bonds inside the HAC structure. On the other hand, the strength of the aromatic 6.2 μm band increases, which is evidence of the partial graphitization within UV-irradiated HAC materials. Because UV irradiation is not uniform, this band agrees well with the C-class PAH toward HD 100764. The C-H out-of-plane vibration bands are strongly affected by UV irradiation. Bands at 11.35, 12.14 and 12.64 μm (solo, duo and trio, respectively), which are found for PAHs of many interstellar spectra were observed partially and were compared to those of non-processed materials, in particular, those at 11.35 μm, which represent the aromatic structures. Conclusions. UV irradiation has a variable effect on both aliphatic and aromatic bands in the MIR region. The aliphatic C-H structure decreases while the aromatic C=C structure, which might lead to the graphitic colonies for PAHs, increases. UV irradiation has revealed solo, duo and trio bands that are relatively consistent with those of the A-and B-class PAHs.

Aims. The X-shooter archive of several thousand telluric standard star spectra was skimmed for Be and Be shell stars to derive the stellar fundamental parameters and statistical properties, in particular for the less investigated late-type Be stars and the extension of the Be phenomenon into early A stars.Methods. An adapted version of the BCD method is used, using the Balmer discontinuity parameters to determine effective temperature and surface gravity. This method is optimally suited for late B stars. The projected rotational velocity was obtained by profile fitting to the Mg ii lines of the targets, and the spectra were inspected visually for the presence of peculiar features such as the infrared Ca ii triplet or the presence of a double Balmer discontinuity. The Balmer line equivalent widths were measured, but they are only useful for determining the pure emission contribution in a subsample of Be stars owing to uncertainties in determining the photospheric contribution. Results. A total of 78, mostly late-type, Be stars, were identified in the X-shooter telluric standard star archive, out of which 48 had not been reported before. We confirm the general trend that late-type Be stars have more tenuous disks and are less variable than earlytype Be stars. The relatively large number (48) of relatively bright (V > 8.5) additional Be stars casts some doubt on the statistics of late-type Be stars; they are more common than currently thought. The Be/B star fraction may not strongly depend on spectral subtype.