History of Star Formation in the Milky Way and Its Neighbors

Extrapolation from experience with the LCO 2.5-m and CTIO 4-m telescopes indicates that it will be possible to obtain spectra with S/N=25 and R 12000 to V=19 in one night, and as faint as V=22 if four nights of integration on 40 similar stars (with 3 electron readout and on-chip extraction) are coadded. We enumerate three important initial projects that can be pursued with these capabilities of the spectrograph:

• SNI and SNII nucleosynthetic yields from first generation stars in Carina et al

  By the time Magellan comes into operation a CaII K-line interference filter survey conducted at LCO by McWilliam will have identified the numerous stars with [Fe/H] < -3 that must be present among the thousands of red giants in Carina and other dwarf spheroidals. Coadded spectra of stars binned by [Fe/H] can be used to investigate the ubiquity of abundance-dependent yields for Cr, Mn, and Co and the enormous dispersion in r-process yields found in Milky Way red giants (McWilliam et al 1995). These are basic constraints for theory of supernova nucleosynthesis.

• Calibration of SnI timescale from [/Fe] versus [Fe/H] relations derived from:

  • globular clusters in LMC with 10 Gyr age-spread.

  • star bursts in Carina dwarf spheroidal (Smecker-Hane et al 1994) with 10 Gyr age- spread. This calibration, never performed heretofore and a prerequisite for believable modeling of chemical evolution, is a natural for the spectrograph. The observations will also test the dogma that a sequence of starbursts is accompanied by a monotonic increase in metallicity.

• Use of thorium chronometers to derive age (dispersion?) in the Galactic Bulge

  A CaII K-line interference filter survey conducted at LCO by Preston, Shectman & Thompson will permit a follow-on search with intermediate resolution for r-process rich stars among those with [Fe/H] < -3. A search for thorium among such objects with the higher resolution of the Magellan Echelle Spectrograph will permit multiple estimates for the age and its dispersion in the central region of the Milky Way (Cowan et al 1996).