The Intrinsic Colours of Galactic Cepheid Variables

Abstract

The intrinsic colours of type I galactic cepheids are determined from the photoelectric colours of Eggen, using a method in which the observed colours are corrected for reddening by means of a model of the reddening medium which gives the colour excesses of B stars as a function of distance and the galactic coordinates. Estimates of the distances and reddening of the cepheids are found from the model by converting colour excess into photographic absorption and solving the distance modulus equation using Baade's zero-point of the period–luminosity curve. Solutions by least squares are then made for the coefficients in linear relations for the intrinsic colours at maximum and minimum light for cepheids belonging to Eggen's types A, B and C. The following solution for the intrinsic colours on the P, V System is adopted: Types A, B and C: |$\text{(P-V)}_\text{max}=+{0}^\text{m.}17+{0}^\text{m.}18\,\text{log}\,P$| Types A and B : |$\text{(P-V)}_\text{min}=+{0}^\text{m.}08+{0}^\text{m.}78\,\text{log}\,P$| Type C: |$\text{(P-V)}_\text{min}=+{0}^\text{m.}09+{0}^\text{m.}77\,\text{log}\,P$|⁠. It is shown that these intrinsic colours give colour excesses and distance moduli which support the hypothesis that cepheid distances and periods are not correlated. The intrinsic colours are much bluer than hitherto supposed, and at maximum light there is some similarity with the colours of cepheid variables in the Small Magellanic Cloud. The intrinsic colours are applied to an examination of the connection between differential galactic rotation and the zero-point of the period–luminosity curve. It is found that Baade's zero-point gives an Oort constant A = 20 km/sec/kpc for a group of nearby cepheids for which the average absorption is about 2 magnitudes per kpc. The recovery of Joy's original value of A for the cepheids is shown to be due to the fact that the greater absorption required by the intrinsic colours compensates for the increase in Joy's distance moduli made necessary by Baade's revision of Shapley's zero-point of the period–luminosity curve.