3 Age, reddening and distance

The presented data are not sufficient on their own for an accurate determination of basic properties of Tr 5. The main obstacle is a lack of an independent information on the cluster's reddening and metallicity. Our photometry confirms an earlier finding, based on the Kalinowski's (1975) photometry, that Tr 5 is an old open cluster (Janes & Adler 1982; Phelps et al. 1994). The cluster CMD shows a clearly marked red giant branch with a populous red giant clump. Also an observed color of the cluster turnoff can be determined with confidence from our data. We note that despite a low galactic latitude of Tr 5, its area is poorly populated by the background stars. The presented CMD's are contaminated mostly by the foreground objects. Apparently, the light of most of disk stars located behind the cluster is screened by a molecular cloud against which Tr 5 is projected.

Ages of old and intermediate-age open clusters can be estimated with a good accuracy using morphological parameters $\Delta V$, $\Delta (B-V)$ and/or $\Delta (V-I)$ (e.g. Castellani et al. 1992). The parameter $\Delta V$ is defined as a difference in magnitudes between a red giant clump and the brightest point of a given cluster's turnoff. The second age diagnostic is a difference in color, $\Delta (B-V)$ or $\Delta (V-I)$, between the cluster's turnoff and the red giant branch at the level of the clump. Unfortunately, the exact determination of the brightest V magnitude at the turnoff is impossible in case of Tr 5. Apparently, binary stars and/or field objects make that task difficult. We may only state that $\Delta V \gt 1.5$. Parameters $\Delta (B-V)$ and $\Delta (V-I)$ can be determined with confidence from the data presented in Fig. 1. We obtained $\Delta (B-V)=1.63 - 1.03 = 0.60$ and $\Delta (V-I) = 1.83 - 1.27 = 0.56$. These numbers can be compared with values of the respective parameters for M 67. From photometry obtained for that cluster by Montgomery et al. (1993) one gets $\Delta (B-V)=0.56$ and $\Delta (V-I)=0.50$.Hence, it turns that Tr 5 is slightly younger that M 67. Adopting the age 4.8 Gyr for M 67 (Carraro & Chiosi 1994) and using calibrations implied by the theoretical isochrones of Bertelli et al. (1994) we can estimate the age of Tr 5 at about 4.1 Gyr.

To estimate the reddening of Tr 5 let us assume for a moment that metallicity of the cluster is the same as metallicity of M 67. In that case the difference of cluster ages implies that the red giant branch of Tr 5 should be bluer by $\approx\!0.03$ mag than the red giant branch of M 67. The same differences are expected for both colors, B-V and V-I, based on isochrones published by Bertelli et al. (1994). The observed differences of colors measured at the level of the red giant clump are $\delta (B-V)=0.50$ and $\delta (V-I)=0.67$. Hence, the differential reddening of Tr 5 relatively to M 67 can be estimated at $\Delta E(B-V)=0.50+0.03=0.53$ and $\Delta E(V-I)=0.67+0.03=0.70$. Adopting E(B-V)=0.05 and E(V-I)=0.065 for M 67 (Montgomery et al. 1993) we obtain E(B-V)=0.58 and E(V-I)=0.765 for Tr 5. We note that the obtained estimates of the E(B-V) and E(V-I) are consistent with each other as they fulfill the standard relation $E(V-I)=1.28\times E(B-V)$ (e.g. Dean et al. 1978).

Tr 5 is located in the galactic anticenter and its galactocentric distance is significantly larger than that of M 67. Hence, it is likely that metallicity of Tr 5 is lower than metallicity of M 67. Lower metallicity would in turn imply a bluer color of the red giant branch for a fixed age of the cluster. Therefore E(B-V)=0.58 and E(V-I)=0.765 should be considered just lower limits on the reddening of Tr 5.

As it was shown by Paczynski & Stanek (1998) the average I-band luminosity of the red clump giants does not depend on their intrinsic color in the range 0.8<(V-I)₀<1.4. Based on the Hipparcos distances they derived $M_{\rm I}=-0.26$ for a sample of red clump giants from the solar vicinity. The clump of Tr 5 is observed at $I \approx 13.2$ which implies $(m-M)_{\rm I}\approx 13.46$. For E(V-I)=0.765 we have $A_{\rm I}=1.03$which leads to $(m-M)_{0}\approx 12.4$ for Tr 5. In that case the heliocentric distance of the cluster would be about 3.0 kpc. A higher value of the cluster reddening (see the previous paragraph) would lead to a distance lower than 3.0 kpc.