New photoelectric photometry of 32 stars and photographic photometry of 415 stars in the field of the X-ray globular cluster NGC 1851 are reported. A new reddening estimate of E(B-V) = 0.02m with an uncertainty less than approximately 0.02 is obtained for the cluster from four-color and H-beta photometry of foreground stars. The cluster displays a color-magnitude diagram that is normal for its metal abundance, although the distribution of stars on the horizontal branch is slightly peculiar: there is a sharply defined clump of stars near the red end of the red horizontal branch, and a sparser distribution of stars spread out across the RR Lyrae gap, perhaps reaching another maximum near the blue end of the blue horizontal branch. With the new reddening estimate, the integrated colors of the cluster also seem normal for its metallicity, suggesting that the X-ray source, whatever it is, does not significantly affect the integrated photometry at optical wavelengths.
No takes yet. Share an insight, caveat, or question.
P. B. Stetson (1981) studied this question.