Light-element abundances in the weak G-band star HR 6766

Abstract

A high-resolution spectroscopic study of the weak G-band star HR 6766 has been carried out. Atomic line equivalent widths and broad-band photometric colors have been used to derive T/sub eff/=4750 +- 100 K, log g=2.0 +- 0.4, and v/sub t/=2.0 +- 1.0 km s/sup -1/. All iron-peak elements have approximately the same abundance, (M/H)=-0.15 +- 0.25. A spectrum synthesis analysis of the CH G band yields (C/H)=-1.49, an underabundance by a factor of 10 relative to a typical giant of similar luminosity. From individual lines of the CN red system and the CN violet band heads, an abundance (N/H)=+0.23 is obtained. Oxygen is nearly normal; the forbidden lines are used to derive (O/H)=-0.38. The CN red system lines have been used to obtain /sup 12/C//sup 13/C=4.1 +- 1.5 and /sup 14/N//sup 15//N>10. Finally, log Li=+0.75 is derived, and no solid evidence for the presence of /sup 6/Li is found.HR 6766 is compared with other weak G-band stars, and it is found that all may have the common characteristics of low /sup 12/C/sup 13/C ratios and strong lithium features. The abundances of the CNO group of elements in HR 6766 are consistent with a heavy expure to the CNO cycle inmore » its interior, followed by mixing of the burning products to its surface. Various scenarios for accomplishing this in a low-luminosity giant star (log L/L/sub sun/=1.9) are discussed.« less