In general, what I did in order to determine the A_V (cluster) was to
use the OB stars listed by Humphreys (1978). I eliminated all the RSGs
in determining the average, and also excluded all stars for which she
did not list a distance modulus. I then threw out stars whose individual
distance modulus (i.e., their spectroscopic parallaxes) disagreed with
the cluster averages significantly---these were USUALLY (but not always)
ones that she flagged as foreground or non-members.
Since she used R_V=3.0 in determining the Av's of these early-type
stars, I then divided by 3.0 to get the values listed in Table 2 of
Paper I. For the purposes of comparing to our Av values for the RSGs,
I then multiplied by 3.1.
Here are two specific examples.
Cep OB2:
discarded all RSGs plus the following based upon DM:
HD205196
HD207260
HD239725
HD239725
HD204827
HD206773
We are left with 14 OB stars. The average of their
Avs is 1.92. The standard deviation for this sample is
0.49 mag.
This Av value was divided by 3.0 to get the
E(B-V)_Cluster=0.64 value listed in Table 2 in Paper I.
We multiply this by 3.1 (for consistency) and obtain
1.98, with a 1 sigma of 0.50 mag.
For the 3 RSGs in our sample in this cluster:
star Av(Paper I) Delta Av
HD202380 2.63 +0.65
HD206936 2.01 +0.03
BD+56-2793 2.32 +0.34
Cep OB1: Removed:
HD212312 (foreground)
HD216927
HD217476
plus all of the stars for which there are no DM given plus RSGs.
We then find an average A_V=1.76 (R_V=3.0) based upon 27 stars; the
standard deviation is again 0.47,
--->E(B-V)=0.59---A_V(R_V=3.1) of 1.83 with sigma=0.48 mag. There are
two RSGs:
Case 75: 6.05 4.18
Case 78: 4.65 2.79
So, I think it is clear that these stars are significantly
reddened (8 sigma and 5 sigma!)
Here are the details for the complete sample of clusters.
Ave sigma N
Cas OB4 2.25 0.66 19 H78
Cas OB7 2.60 0.57 11 H78
NGC457 1.59 0.13 3 H78
NGC581 See Becker & Fenkart 71
NGC663 See Becker & Fenart 71
Cas OB8 2.17 0.61 7 H78
PerOB1 2.07 0.62 57 H78
N884 2.03 0.30 3 H78
Cam OB1 2.16 0.47 13 H78
Aug OB1 1.64 0.45 5 H78
Gem OB1 2.27 0.67 13 H78
Coll 121 0.06 0.05 3 H78
N2439 1.72 0.49 13 H78
N3293 0.93 0.09 2 H78
CarOB1(al)1.68 0.75 25 H78 0.81 Av in Paper I a mistake