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

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8. Gravity [FORMULA] from [FORMULA] Strömgren index

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

[FIGURE] Fig. 21. c [FORMULA] - [FORMULA] relations from COLK95 (full lines) and COLNOVER (dashed lines) synthetic grids for solar metallicity and different gravities
[FIGURE] Fig. 22. Gravities differences as a function of [FORMULA] for different [FORMULA] when [FORMULA] is derived from the c [FORMULA] index by using both the COLK95 and the COLNOVER synthetic Strömgren indices

The differences in the c [FORMULA] - [FORMULA] 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 [FORMULA] differences become smaller for [FORMULA] [FORMULA] 6250 K.

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

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998