We continue our series of papers on open cluster distances with a critical assessment of the accuracy of main-sequence fitting using isochrones that employ empirical corrections to the color-temperature relations .
We use four nearby open clusters with multicolor photometry and accurate metallicities and present a new metallicity for Praesepe ( { [ Fe / H ] } = +0.11 \pm 0.03 ) from high-resolution spectra .
The internal precision of distance estimates is about a factor of 5 better than the case without the color calibrations .
After taking into account all major systematic errors , we obtain distances accurate to about 2 % – 3 % when there exists a good metallicity estimate .
Metallicities accurate to better than 0.1 dex may be obtained from BVI _ { C } K _ { s } photometry alone .
We also derive a helium abundance for the Pleiades of Y = 0.279 \pm 0.015 , which is equal within the errors to the Sun ’ s initial helium abundance and that of the Hyades .
Our best estimates of distances are ( m - M ) _ { 0 } = 6.33 \pm 0.04 , 8.03 \pm 0.04 , and 9.61 \pm 0.03 to Praesepe , NGC 2516 , and M67 , respectively .
Our Pleiades distance at the spectroscopic metallicity , ( m - M ) _ { 0 } = 5.66 \pm 0.01 ( internal ) \pm 0.05 ( systematic ) , is in excellent agreement with several geometric distance measurements .
We have made calibrated isochrones for -0.3 \leq { [ Fe / H ] } \leq + 0.2 available online .