4 Radial extension

To study the radial extension and richness of Tr 5 we used the data obtained with the 0.9-m telescope. That set of photometry becomes significantly incomplete for V>20.5. On the other hand, the upper main-sequence of the cluster terminates at $V\approx 16.5$. We limited therefore our attention to stars with 16.5<V<21.0 while studying the radial extend of Tr 5. The average stellar density was calculated in successive, 68 arcsec (100 pixels) wide annuli around the cluster center. Figure 2 shows the stellar surface density as a function of distance from the cluster center. The density profile flattens at radius $r\approx 700$ arcsec. Adopting r=700 arcsec for the cluster radius we derived $0.25\ 10^{-2}$ for the surface density of the field stars. Adopting the above listed parameters we may estimate from our data, that about 3030 stars with 16.5<V<20.5 populate the upper main-sequence of the cluster. This estimate is in fact just a conservative lower limit. First of all we made no correction for the incompleteness of the photometry. Moreover, open clusters often possess extended coronas harboring a significant fraction of their member stars. Considering the number of upper-main sequence stars possessed by Tr 5 we may conclude that it is one of the richest objects in the sample of known old open clusters. Its mass contained only in the upper-main sequence stars exceeds 3000 $m_{\odot}$. This can be compared with M 67 for which the estimated total mass does not exceed 1000 $m_{\odot}$ (Montgomery et al. 1993). The linear diameter of the cluster can be estimated at 20 pc for the adopted heliocentric distance of 3.0 kpc. This places Tr 5 among the most extended objects in the sample of known old open clusters.

  

Figure 2: Star density as a function of radial distance from the center of Tr 5 for objects with 16.5<V<20.5