Pleiades distance modulus determination through the zams fitting method

Except for nearest clusters, the dimensions of these objects are small enough to be neglected, compared with their distance from earth. Is essentially for this reason that it is assumed that all the stars inside a cluster, are at the same distance from us. Thus we can consider the apparent magnitudes m of these stars, as different from their absolute magnitudes M, by the same amount. Difference that can be expressed like: m - M = 5 log R - 5. In the previous equation R is the distance of custer itself expressed in parsec and the quantity m - M is called distance modulus. When these apparent magnitudes are plotted with respect to their color index, we have the so called color-magnitude diagram. Comparing in such diagram, for instance [V vs. (B-V)], an open cluster distibution shape with one standard Zams [ Zero age main sequence Mv = f(B-V)o], we see that the open cluster distance modulus can be obtained simply matching comparable parts of the two arrays. In this manner the difference between apparent and absolute magnitudes, is simply the distance modulus m - M. Naturally any light dimming operated by cosmic dust and disipersion in cluster sequence, due to stellar evolution outside standard main sequence, must be taken into account. In other word for the light dimming due to cosmic dust, we proceed using a color-color or two color diagram to obtain the color excess E(B-V) and than we correct measured colors and magnitudes for reddening. After all previous correction we can finally calculate the corrected magnitude difference to obtain the true cluster distance modulus. Practically all this can be obtained also through a graphical procedure. We can see this using the following command: Pleiades true distance modulus determination - A graphical procedure". More info on empirical Zams in Hr Trace Lab Tool

© 2006 - Valter Arnò.