Automated measurements of diffuse interstellar bands in early-type star spectra

Aims. Stellar spectroscopic surveys may bring useful statistical information on the links between diffuse interstellar bands (DIBs) and the interstellar environment. DIB databases can also be used as a complementary tool for locating interstellar (IS) clouds. Our goal is to develop fully automated methods of DIB measurements to be applied to extensive data from stellar surveys. Methods. We present a method that is appropriate for early-type nearby stars, along with its application to high-resolution spectra of similar to 130 targets recorded with the ESO FEROS spectrograph and comparisons with other determinations. Using a DIB average profile deduced from the most reddened stars, we performed an automated fitting of a combination of a smooth stellar continuum, the DIB profile, and, when necessary, a synthetic telluric transmission. Results. Measurements are presented for 16 DIBs in the optical domain that could be extracted automatically: 4726.8, 4762.6, 4963.9, 5780.4, 5797.1, 5849.8, 6089.8, 6196.0, 6203.0-6204.5, 6269.8, 6283.8, 6379.3, 6445.3, 6613.6, 6660.7, and 6699.3 angstrom. Two approaches to determining equivalent width (fitted equivalent width and continuum-integrated equivalent width) were tested and the two determinations were found to agree within the uncertainties, except for the 6203-6204 multiple band, and are compatible with previous non automated measurements. Errors arising from statistical noise and continuum placement were estimated conservatively by means of the " sliding window" method. We derived the mean relationship between the DIB equivalent width and the color excess. Parameters of the linear correlation relationships are positively compared with published values, and most correlation coefficients were found to be higher than those based on an earlier type, UV bright target stars, which confirms previous results on the influence of the radiation field. We provide the first correlation parameters for the 5849.8, 6089.8, 6269.8, 6445.3, 6660.7, and 6699.3 angstrom bands in the Galaxy. The coefficients are within the same range as for other optical bands. Conclusions. Automated DIB equivalent width extractions can be performed for nearby early-type stars without significant losses in accuracy. This opens the perspective of building extended DIB catalogs based on high-resolution spectroscopic surveys. We discuss the limitations of this method in the case of distant stars and potential improvements.