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 delta <-25(deg) 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 lambda lambda4230 -4270 Angstroms have been obtained for 76 S stars, 8 M stars and 2 symbiotic stars. The lambda4262 Angstroms and lambda4238 Angstroms 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. Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile)