REDDENING AT THE NORTH GALACTIC POLE - COSECANT VARIATION, A(B)=0.0 OR A(B)=0.2

Color excesses in the uvby beta-system for most A3-G0 stars brighter t han B=11.(m)5 and above b=+70 degrees have been averaged in one degree latitude zones for reference and to see if the cosecant postulate has any meaning for the polar cap itself. Two latitude intervals have tol erable cosecant relations: 70 less than or equal to b less than or equ al to 81 <(E(b-y))over bar>=+0.025 cosec(b)-0.012 all E(b-y) 83 less t han or equal to b less than or equal to 90 <(E(b-y))over bar>=+1.805 c osec(b)-1.800 all E(b-y) for the complete cap the correlation may be i mproved if an E(b-y) selection is done according to what could be a si mple distinction between intercloud and cloud lines of sight, E(b-y) a pproximate to 0.017: 70 less than or equal to b less than or equal to 90 <(E(b-y))over bar>=+0.021 cosec(b)-0.022 E(b-y) <0.017 70 less than or equal to b less than or equal to 90 <(E(b-y))over bar>=+0.056 cose c(b)-0.023 E(b-y) >0.018 70 less than or equal to b less than or equal to 90 <(E(b-y))over bar>=+0.105 cosec(b)-0.063 E(b-y) >0.030 between 81 degrees and 83 degrees the general average oscillates. Correlation coefficients for the five relations are 0.35, 0.98, 0.45, 0.50 and 0.2 1 respectively. E(b-y) = 0.030 is the typical reddening in a diffuse c loud. These results are somewhat surprising: averages for the complete zone do display a marginal cosecant variation in the range from 70 de grees to 82 degrees whereafter a marked drop to the polar reddening E( b-y)(89-90) = 0.(m)004 takes place. The physical relevance of the aver age latitude trend may however be questioned since each of the four qu adrants show distinctly different trends despite each containing simil ar to 1200 stars. In the quadrant 90 less than or equal to 1 < 180 the re is a sharp increase from E(b-y) = 0.008 to 0.028 when b is varied f rom 70 degrees to 76 degrees. Some 30 degrees longitude sectors, e.g. 1: 120 - 150, have significantly increasing average reddenings and som e display a stronger decrease than the average cosecant slope, e.g. 1: 300 - 330 degrees. The three 30 degrees sectors of the 4th quadrant a re all different. The plain average value of E(b-y) is 0.0137 mag and the standard deviation 0.0220, a value also valid for most of the lati tude zones. The mean corresponds to Ag = 5.59 x 0.0137 = 0.077(m). The cosecant fits to the one degree averages predict a polar reddening at b=90 degrees of E(b-y)=0.013 - 0.016 for all stars and those with a r eddening exceeding -0.015. These values correspond to a semi thickness es of the disk A(v) approximate to 0.056 - 0.068. These values are rem arkably close to the minimum projected value recently observed for a 2 kpc path through the disk at a latitude +4 degrees A(v)=0.073 at 1 = 262 degrees, Jonch-Sorensen & Knude (1994). In the cap beyond approxim ate to 84 degrees the average reddening is significantly smaller than the csc extrapolation and the grand average: E(b-y,b>87 degrees) = 0.0 07, but excesses larger than 0.030 are also E found at these high lati tudes. A combination of the data presented here to previously publishe d uvby beta excesses may indicate a shift in the reddening's latitude dependence between approximate to 40 degrees and approximate to 45 deg rees. A downwards jump of about 0.040 in E(b-y) seems possible. For ex tra Galactic investigations there has sometimes been a discussion of a clear window above b = 50 degrees or a cosecant variation all the way to the pole with A(B,pole) approximate to 0.2 AS the discussion shows the number A(B,pole) approximate to 0.2 is valid for several subsampl es of the polar stars related to the cloud reddening, but approximate to 2/3 of the lines of sight have E(b-y) < 0.018 producing an average A(B,pole) = 0.0. A simple general reddening law is thus not aplicable for the polar cap and if an accuracy of Ag better than 0.10 is require d the reddening must be measured. If the object is known to be reddene d the relation valid for E(b-y) > 0.018 should be used.