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Session 79 - Special Objects in the Galaxy.
Oral session, Thursday, June 11
NGC 3201, located at \ell = 277^\circ, b = +9^\circ, with a radial velocity of +495 km/sec, is going around our Galaxy in the opposite direction of the general rotation of the Galactic disk.
Using CTIO 4-m echelle spectra of resolution 20-40,000 we have dervied elemental abundances in 18 red giants from a standard model atmosphere analysis. We find a mean metallicity given by [Fe/H] = -1.42 with an internal error of \pm0.03 and a likely external uncertainty of about \pm0.15 due to uncertain input parameters such as the effective temperature scale. We find a spread in the [O/Fe] ratio with 8 stars showing [O/Fe] = +0.4 \pm 0.1 and 5 stars showing [O/Fe] < +0.2. As in several other globular clusters Na and Al are enhanced in the stars with the least O. For Na the high O-stars show [Na/Fe] = +0.1 \pm 0.1 while the lower O-stars show [Na/Fe] = +0.4 \pm 0.1. For Al the high O-stars show [Al/Fe] = +0.25 \pm 0.1 and a spread in [Al/Fe] from +0.2 to +0.9 in the low O-stars. Our analysis of the relative abundances of the rare earths shows that [Ba/Eu] = -0.55 \pm 0.15. When the abundances of 8 species from Rb to Eu are considered the pattern shows that r-process production has dominated over s-process production in the material that was later incorporated in the stars in NGC 3201. A substantial contribution to the elements heavier than Ba by the r-process in both halo and globular cluster stars is evident from the review of the literature by McWilliam (ARAA, 35, 503, 1997).
In conclusion we note that the composition of the stars in NGC 3201 is similar to that of typical halo globulars despite its unusual Galactic orbit.
Program listing for Thursday