6. Metallicity effects
The colours of late-A and F stars can be significantly affected by the effects of metallicity, due to the vast amount of metal lines. Unfortunately, there are no such objects as fundamental metallicity stars; all abundance determinations are model dependent. The Strömgren uvby system has a metallicity index () which can be used to estimate the overall metal abundance ([M/H]) of late-A, F and early-G stars (Strömgren 1966). Several good empirical relationships are available (Smalley 1993a and references therein).
One of the major discrepancies found by Relyea & Kurucz (1978) was in the index, calculated from the Kurucz (1979a) model fluxes. The index for A and F stars disagreed with the observed values. Fig. 6 shows how well the various solar-composition grids compare with the main-sequence values listed in Philip & Egret (1980). It is clear that none of the models agrees completely with the main-sequence values, and all are discrepant for late-type stars. Large differences are caused by the different treatments of convection. Hence, the appears to be a sensitive indicator of convection in cool stars. Interestingly, for cool models, the Relyea & Kurucz (1978) values are coincident with the MLT OV line, even though the newer MLT OV models contain much improved line opacity. This may indicate that the problem is more fundamental, and not due to the effects of line opacity alone. In addition, is sensitive to the adopted value of microturbulence (Strömgren 1966; Kurucz 1991b). All the models discussed here were calculated with a microturbulence of 2 km s-1, but lowering its value reduces the discrepancy for the coolest models. Microturbulence is a free parameter in the models, but is probably closely related to the small-scale part of the photospheric convective flow pattern (Holweger & Stürenburg 1993; see also Cowley 1996). Clearly, further investigations into the cause of this discrepancy need to be performed.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998