Astron. Astrophys. 318, 841-869 (1997)

8. Gravity from Strömgren index

For cool stars, surface gravity may be derived from the Balmer discontinuity as measured by the Strömgren index. Fig. 21 shows that, for a given metallicity and a given , the color index may be used to fix the surface gravity of stars with included between 5500 K and 7500 K. The c - relation is plotted for different gravities for both COLK95 (full lines) and COLNOVER (dashed lines) grids. We used the c index of the Strömgren colors as computed by Kurucz. It is the c index at zero airmasses obtained from the c₁ and c₂ indices at 1 and 2 airmasses by using the relation c =2c₁ -c₂. The effect of the "overshooting option" is quite large. In order to estimate the differences related with a given c index, we derived at a given by interpolating in the COLNOVER grid for the c index of the COLK95 grid. The differences (COLNOVER) (COLK95) are displayed in Fig. 22. The gravities from the COLNOVER grid are larger than those from the COLK95 grid. For each there is a maximum in the - curve. This maximum decreases and shifts toward higher gravities with increasing . For instance, at =6500 K, the maximum occurs for =2.5 and it is about 0.5 dex.

Fig. 21. c - relations from COLK95 (full lines) and COLNOVER (dashed lines) synthetic grids for solar metallicity and different gravities

Fig. 22. Gravities differences as a function of for different when is derived from the c index by using both the COLK95 and the COLNOVER synthetic Strömgren indices

The differences in the c - relations, when the grids COLK95 and COLNOVER for [M/H]=0.0 are used, do not change very much when the metallicity [M/H]=-3.0 is considered. The differences become smaller for 6250 K.

Sect. 10 will show that, for Procyon, derived from the mass and radius of the star is larger than derived from the c index and K95 models, while it agrees well with from c index and NOVER models.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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