On the nature of absorption features toward nearby stars

Context. Diff use interstellar absorption bands (DIBs) of largely unknown chemical origin are regularly observed primarily in distant early-type stars. More recently, detections in nearby late-type stars have also been claimed. These stars' spectra are dominated by stellar absorption lines. Specifically, strong interstellar atomic and DIB absorption has been reported in tau Boo. Aims. We test these claims by studying the strength of interstellar absorption in high-resolution TIGRE spectra of the nearby stars tau Boo, HD 33608, and alpha CrB. Methods. We focus our analysis on a strong DIB located at 5780.61 angstrom and on the absorption of interstellar Na. First, we carry out a differential analysis by comparing the spectra of the highly similar F-stars, tau Boo and HD 33608, whose light, however, samples different lines of sight. To obtain absolute values for the DIB absorption, we compare the observed spectra of tau Boo, HD 33608, and alpha CrB to PHOENIX models and carry out basic spectral modeling based on Voigt line profiles. Results. The intercomparison between tau Boo and HD 33608 reveals that the difference in the line depth is 6.85 +/- 1.48 m angstrom at the DIB location which is, however, unlikely to be caused by DIB absorption. The comparison between PHOENIX models and observed spectra yields an upper limit of 34.0 +/- 0.3m angstrom for any additional interstellar absorption in tau Boo; similar results are obtained for HD 33608 and alpha CrB. For all objects we derive unrealistically large values for the radial velocity of any presumed interstellar clouds. In tau Boo we find Na D absorption with an equivalent width of 0.65 +/- 0.07m angstrom and 2.3 +/- 0.1m angstrom in the D-2 and D-1 lines. For the other Na, absorption of the same magnitude could only be detected in the D2 line. Our comparisons between model and data show that the interstellar absorption toward tau Boo is not abnormally high. Conclusions. We find no significant DIB absorption in any of our target stars. Any differences between modeled and observed spectra are instead attributable to inaccuracies in the stellar atmospheric modeling than to DIB absorption.