New photometric data for the red giants in the open cluster NGC 5822: membership, chemical composition and physical properties

New photoelectric UBV, DDO, and CMT₁T₂ data of 14 G and K stars in the field of the open cluster NGC 5822 are presented. By applying two independent photometric criteria 11 stars are found to have a high probability of being cluster giants, while three remaining stars are almost certainly red field objects. A reddening |$E(B-V)=0.15\pm0.01$|⁠, a distance modulus of |$V_0-M_V=8.77\pm0.18$|⁠, and an age of about 6−8×10⁸ yr have been derived. NGC 5822 has a mean ultraviolet excess of |$\delta(U-B)=0.00\pm0.01$|⁠, with respect to the field K giants, and a mean cyanogen anomaly of |$\delta \text {CN}=+0.031\pm0.008$|⁠, both implying a nearly solar iron abundance. Although the Fe+CNO-sensitive [Fe/H]_CM abundance index of the CMT₁T₂ system yielded a result close to those of the UBV and DDO systems, as expected on physical grounds, the abundance derived from the iron lines ([Fe/H]_MT = −0.4±0.1) suggests that the cluster giants are slightly enriched in elements of the CNO-group. Since there is no evidence for mixing or mass loss among any of the observed stars, we believe that the cluster stars may have formed from gas already contaminated by elements of the CNO-group. The effective temperatures, surface gravities and masses of the red giants have also been determined. The mean value of the masses of the red giants was found to be 1.7±0.2 (σ_m), which is consistent with the expected theoretical value.