The Interstellar $^7$Li/$^6$Li Isotope Ratio Toward $\zeta$ Oph
Session 35 -- ISM: Abundances and Dust
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

[35.01] The Interstellar $^7$Li/$^6$Li Isotope Ratio Toward $\zeta$ Oph

I. Hawkins, I. Crawford, M. Barlow, F. Diego, K. McDonald, and J. Sommers (Eureka Scientific, Inc.; Univ. College, London; Univ. College, London; Univ. College, London; Eureka Scientific, Inc.; Eureka Scientific, Inc.)

The interstellar $^7$Li/$^6$Li isotope ratio is an important parameter which is governed by primordial nucleosynthesis during the Big Bang and by the chemical evolution of our Galaxy. Reported measurements of this isotope ratio in the interstellar medium thus far have included the following results: $^7$Li/$^6$Li$\sim$38 toward $\zeta$ Oph (Ferlet and Dennefeld 1984); $^7$Li/$^6$Li$=12.5\pm 3.8$ toward $\rho$ Oph (Lemoine et.al. 1993); $^7$Li/$^6$Li$=6.8\pm 1.6$ toward $\zeta$ Oph (Meyer, Hawkins, and Wright 1993); $^7$Li/$^6$Li$=5.5\pm 1.2$ toward $\zeta$ Per (Meyer, Hawkins, and Wright 1993); $^7$Li/$^6$Li$=8.6\pm 0.8$ and $^7$Li/$^6$Li$=1.4\pm 0.6$ for two components derived through profile fitting toward $\zeta$ Oph (Lemoine, Ferlet, and Vidal-Madjar 1995). Except for the early measurement by Ferlet and Dennefeld, the values above are in general agreement within the error bars. These low values of this important isotope ratio are not predicted by models of Galactic chemical evolution which take into account various Li production mechanisms and also the differential destruction of both isotopes in stellar interiors (Meyer, Hawkins, and Wright 1994 and references therein). Such Galactic chemical evolution models predict that the $^7$Li/$^6$Li isotope ratio value should be greater than 12.5 (the solar value) by about a factor of two. All the above measurements have been carried out with resolutions of at best 1 km/s, or $\lambda/\delta\lambda \sim 3 \times 10^5$. Despite the very high S/N ratio of the data which produced the above quoted measurements (4,000 -- 7,500 per pixel), the small separation of the lithium isotopic doublets makes it difficult to resolve individual components unless better resolution is achieved. We present ultra high resolution observations ($\lambda/\delta\lambda \sim 3 \times 10^6$) of the $^7$Li/$^6$Li isotope ratio toward $\zeta$ Oph, carried out with the Ultra High Resolution Facility (UHRF) and the Anglo-Australian Telescope.