Kaspar von Braun — Page 6

arXiv1998

Color-color Relations for Red Giants in Star Clusters

New Johnson-Cousins UBVRI photometry of giants in globular clusters is combined with JHK photometry on the CIT system to produce color sequences for giants from the globular clusters M3, M5, M13, and M92. UBVRI data are also presented for giants in the metal-rich open cluster NGC 6791. These data fill a gap in the literature, especially for the R & I bands. We provide the empirical relations between broad band colors for various [Fe/H] values for metal-poor giants. The color sequences for U-B and B-V show clear separations for different [Fe/H] values. We also find weak, though unexpected, metallicity dependences of V-R, V-I, and J-K colors. H-K is metal-insensitive. The above colors are plotted as a function of V-K, and a literature (V-K) - T_{eff} relation is given.

astro-ph↗

arXiv1997

Tests of the Tully-Fisher Relation II: Scatter Using Optical Rotation Curves

We investigate the amount of scatter in the Tully-Fisher relation (TFR) when using optical long-slit H-alpha rotation curves to determine the velocity widths of spiral galaxies. We study a sample of 25 galaxies in the Coma region of the sky which were shown in Bernstein et al. (1994) to exhibit an extraordinarily low scatter of 0.10 mag RMS in the I magnitude vs 21-cm width TFR. Using the same I magnitudes with new widths derived from high-quality H-alpha rotation curves, we measure an RMS scatter of 0.14 mag in the TFR. This suggests that measurement errors and ``astrophysical errors'' (such as non-circular gas motion) on the H-alpha velocity widths are below 6%, and optical widths are nearly as good for TFR studies as 21-cm widths. The scatter and form of the TFR are found to be robust under choice of velocity width-extraction algorithm, as long as the central portions of the optical rotation curve are ignored and low-S/N points are not weighted too heavily. In this small sample there is no evidence that rotation curve shapes vary systematically with rotation velocity, nor that rotation curve shape can be used to reduce the scatter in the TFR.

astro-ph↗