Magnesium isotopes and s-process elements in the barium star HR 774.

We have analyzed low-noise high-resolution Reticon spectra of MgH and selected unblended s-process element lines in the Ba star HR 774. We find that the isotopic abundances of Mg are terrestrial (24Mg: 25Mg: 26Mg = 79:10:11). Large overabundances of the s-process elements (Y, +0.9dex; Zr, +0.8dex; Mo, +1.0dex; La, +1.1 dex; Nd, +1.Sdex) with respect to the normal K giant p Gem are determined. These results imply that the neutrons responsible for the s-process enhancements are provided by the `3C(a, n)'6O reaction and not the 22Ne(a, n)25Mg reaction. If the latter reaction were the primary neutron source, substantial overabundances of 25Mg and 26Mg would be predicted for HR 774, e.g., 24Mg 25Mg 26Mg. This is incompatible with our analysis of the MgH lines. Our conclusion accords with the theoretical expectations of Truran and Iben who suggest that in low-mass Ba stars, such as HR 774, `3C(a, n)'6O rather than 22Ne(a, n)25Mg is the major source of neutrons. Iron is found to be 0.2 dex deficient with respect to Gem. Carbon, nitrogen, and oxygen abundances are determined by analysis of Reticon spectra of Swan system C2 lines, red system CN lines, and the 6300 A forbidden 0 1 line. Nitrogen and oxygen have the same abundances as in a 0.2 dex metal-deficient, and otherwise normal, mixed K giant, while carbon is 0.4 dex overabundant. The 12C/13C ratio is 23 + 2. In addition, `2C/'3C ratios of 23 + 3 13 + 1, and 24 + 2 have been determined for the Ba stars HR 2392, HR 4474, and HR 5058, respectively. These results are used to interpret the evolutionary state of the Ba stars. Subject headings: nucleosynthesis - stars: abundances - stars: Ba ii - stars: individual - stars: late type