Michael I. Andersen

We present Strömgren photometry of the two southern open clusters NGC 6134 and NGC 3680. The field of NGC 6134 was observed with different shifts of the telescope and rotations of the CCD camera in order to detect and remove any large scale flat field errors using the technique described by Andersen et al. (1995). By using calibrations of the Strömgren indices we find the metallicity and interstellar reddening of the clusters: For NGC 6134 the results are [Fe/H] = 0.28 ± 0.02 and E(by) = 0.263 ± 0.004 and for NGC 3680 we find [Fe/H] = 0.09 ± 0.02 and E(by) = 0.048 ± 0.011. When comparing the observed colour-magnitude diagrams theoretical isochrones we find that models which include overshooting in the convective cores agree with the extended hook after the turnoff. From the isochrone fit we find ages of 0.69 ± 0.10 Gyr and 1.45 ± 0.15 Gyr for NGC 6134 and NGC 3680 respectively. Strömgren photometry of six known δ Scuti stars in NGC 6134 is presented.

A "jump" in the BHB distribution in the V, u-y CMD was recently detected in the GC M13. It is morphologically best characterized as a discontinuity in u, u-y, with stars in the range 11,50011,500 K, dramatically enhancing their abundances in the atmospheres of BHB stars in the "critical" temperature region. Model atmospheres taking diffusion effects into account are badly needed, and will likely lead to better overall agreement between canonical evolutionary theory and observations for BHB stars.

A "jump" in the BHB distribution in the V, u-y CMD was recently detected in the GC M13. It is morphologically best characterized as a discontinuity in u, u-y, with stars in the range 11,500<Teff(K)11,500 K, dramatically enhancing their abundances in the atmospheres of BHB stars in the "critical" temperature region. Model atmospheres taking diffusion effects into account are badly needed, and will likely lead to better overall agreement between canonical evolutionary theory and observations for BHB stars.

We present deep CCD photometry of the globular cluster M13 (NGC 6205) in the Strömgren uvbyβ system and determine a foreground reddening of E(b-y)=0.015±0.01 mag. From a fit to the main sequence of metal-poor subdwarfs with Hipparcos parallaxes, we derive (m-M)₀=14.38±0.10, which implies an age near 12 Gyr assuming [Fe/H] = -1.61 and [α/Fe] = 0.3. The distance-independent [(b-y)₀, c₀] diagram indicates that M13 and metal-poor field subdwarfs of similar metallicity must be coeval to within ±1 Gyr. In addition, we find that, at any given (b-y)₀ color, there is a large spread in the c₀ index for M13 red giant branch (RGB) stars. We suspect that this scatter, which extends at least as faint as the base of the RGB, is most likely due to star-to-star variations in the atmospheric abundances of the CNO elements. We also note the existence of what appears to be two separate stellar populations on the horizontal branch of M13. Among other possibilities, it could arise as the result of differences in the extent to which deep mixing occurs in the precursor red giants.

We present deep CCD photometry of the globular cluster M13 (NGC 6205) in the Stromgren uvby-Beta system, and determine a foreground reddening of E(b-y)= 0.015mag. From a fit to the main-sequence of metal-poor subdwarfs with Hipparcos parallaxes, we derive (m-M)_0=14.38 +- 0.10 which implies an age near 12 Gyr assuming [Fe/H]=-1.61 and [Alpha/Fe]=0.3. The distance independent ((b-y)_0, c_0) diagram indicates that M13 and metal-poor field subdwarfs of similar metallicity must be coeval to within +-1 Gyr. In addition, we find that, at any given (b-y)_0 color, there is a large spread in the c_0 index for M13 red-giant branch (RGB) stars. We suspect that this scatter, which extends at least as faint as the base of the RGB, is most likely due to star-to-star variations in the atmospheric abundances of the CNO elements. We also note the existence of what appears to be two separate stellar populations on the HB of M13. Among other possibilities, it could arise as the result of differences in the extent to which deep mixing occurs in the precursor red giants.