A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars

Aims. We have carried out a survey of 60 Herbig Ae/Be stars in the 3 micron wavelength region in search for the rare spectral features at 3.43 and 3.53 micron. These features have been attributed to the presence of large, hot, hydrogen-terminated nanodiamonds. Only two Herbig Ae/Be stars, HD 97048 and Elias 3−1 are known to display both these features. Methods. We have obtained medium-resolution spectra (R ∼ 2500) with the ESO near-IR instrument ISAAC in the 3.15−3.65 micron range. Results. In our sample, no new examples of sources with prominent nanodiamond features in their 3 micron spectra were discovered. Less than 4% of the Herbig targets show the prominent emission features at 3.43 and/or 3.53 µm. Both features are detected in our spectrum of HD 97048. We confirm the detection of the 3.53 µm feature and the non-detection of the 3.43 µm feature in MWC 297. Furthermore, we report tentative 3.53 µm detections in V921 Sco, HD 163296 and T CrA. The sources which display the nanodiamond features are not exceptional in the group of Herbig stars with respect to disk properties, stellar characteristics, or disk and stellar activity. Moreover, the nanodiamond sources are very different from each other in terms of these parameters. We do not find evidence for a recent supernova in the vicinity of any of the nanodiamond sources. We have analyzed the PAH 3.3 µm feature and the Pfund δ hydrogen emission line, two other spectral features which occur in the 3 micron wavelength range. We reinforce the conclusion of previous authors that flared-disk systems display significantly more PAH emission than self-shadowed-disk sources. The Pf δ line detection rate is higher in self-shadowed-disk sources than in the flareddisk systems. Conclusions. We discuss the possible origin and paucity of the (nano)diamond features in Herbig stars. Different creation mechanisms have been proposed in the literature, amongst others in-situ and supernova-induced formation. Our data set is inconclusive in proving or disproving either formation mechanism.