Observational Projects for SOAR 4m telescope

a) High-Resolution Spectroscopy of Solar-Type Stars
(G. Porto de Mello)

This project seeks to obtain a more complete description of the 
detailed abundance distributions of the solar neighborhood G-type 
stars, in order to a) evaluate the uniformity of the local abundance 
pattern, b) uncover details of the local history of nucleosynthetic 
enrichment, c) quantify the role played by the spatial and temporal 
heterogeneity of the enrichment processes in the chemical evolution 
of the solar circle and d) establish exactly how typical the Sun 
truly is of its galactic vicinity. We intend to produce a large, 
homogeneous and reliable set atmospheric parameters, evolutionary 
status and abundance data for 25 chemical elements, sampling a wide 
variety of nucleosynthetic processes. The method of analysis consists 
in rigorously differential model atmosphere analysis, with the Sun as 
standard star, of equivalent widths of moderately weak metal lines, 
measured off high S/N(=300) and high-resolution (R=50000) echelle 
spectra of a sample of 79 bright (V<6.5) nearby (pi>0.040) and 
solar-type (5500K < Teff < 6100K) stars. 

An echelle spectrograph would be necessary for this project. The 
desired spectral coverage is at least 2000 angstrons, and the desired 
nominal resolution should be at least R=50000, preferebly R=60000.
S/N ratios should be in excess of 300 per pixel. The lunar phase is 
not restrictive for this project. For a 4m class telescope such as 
the CTIO/Blanco, expected integration times should be 20-40 minutes 
for V=6-7 stars, roughly.

b) The Chemical Evolution of Cu and the Heavy Elements in the Solar 
Neighborhood.
(G. Porto de Mello)

Recent work on super-metal-rich G-type disk stars has shown that the 
metallicity interval +0.30 < [Fe/H] < +0.50 probably represents the 
extreme tail of the disk metallicity distribution (Castro 1997, PhD 
thesis). Their abundance pattern shows that the alpha-elements, including 
O,  become underabundant with respect to Fe as the metallicity increases. 
Also, the heavy main s-process elements Y, Zr and Ba seem to become 
underabundant as the metallicity increases. These results are in good 
agreement with the data on Y, Zr and Ba  abundances found by Porto de Mello 
(1996, PhD thesis) in - 0.30 < [Fe/H] < +0.30 stars. This author has 
also found that the [Cu/Fe] ratio increases at the high-metallicities. Recent 
results on Cu and Ba abundances in a barium dwarf (Porto de Mello, 
1996, PhD thesis) and a barium giant (Pereira & Porto de Mello 1997, 
AJ, accepted) revealed that a depletion of Cu is probably associated with the 
operation of the s-process. The presen project aims to analyze Cu and heavy 
element abundances in the super metal rich stars, extending the sample studied 
by Castro (1997),  in order to clarify the correlation of  the s-process 
nucleosynthesis with the mechanisms operating in the destruction & production 
cycle of Cu. We also intend to  extend the metallicity interval analyzed by Porto de 
Mello (1996) to [Fe/H] = -1.0. We propose to study the run of the abundance ratios 
of  these elements in the whole span of metallicity of the galactic 
disk , -1.0  < [Fe/H] < +0.50, and achieve a better understanding of the nucleosynthetic 
processes which contribute to their galactic chemical  evolution.

The same technical setup as described in project (a) would be needed for 
this project. The magnitude interval, however, is V = 7-11, with 
correspondingly longer integration times. Gray lunar phases would be 
preferable for this project.