The Henize sample of S stars I . The technetium dichotomy

This paper is the first one in a series investigating the properties of the S stars belonging to the Henize sample (205 S stars with δ < −25◦ and R < 10.5) in order to derive the respective properties (like galactic distribution and relative frequencies) of intrinsic (i.e. genuine asymptotic giant branch) S stars and extrinsic (i.e. post mass-transfer binary) S stars. High-resolution (R=30 000 to 60 000) spectra covering the range λλ4230−4270Å have been obtained for 76 S stars, 8 M stars and 2 symbiotic stars. The λ4262Å and λ4238Å blends involving a Tc i line were analysed separately and yield consistent conclusions regarding the presence or absence of technetium. Only one ‘transition’ case (Hen 140 = HD 120179, a star where only weak lines of technetium are detectable) is found in our sample. A resolution greater than R = 30 000 is clearly required in order to derive unambiguous conclusions concerning the presence or absence of technetium. The Tc/no Tc dichotomy will be correlated with radial velocity and photometric data in a forthcoming paper.