Color - magnitude diagrams

Color - magnitude diagram in B, V system

The V, B-V color-magnitude diagram for Palomar 13 derived in this study is shown in Fig. 3 (click here). The stars within 24 arcsec of the cluster center (1 core radii, Djorgovski 1993) are presented by crosses. The core of the cluster was well resolved and all stars down to (n=47) were measured.

  
Figure 3: The observed V vs. B-V color-magnitude diagram of Palomar 13 based on all stars listed in Table 6. The stars within 1 core radii are represented by crosses

  
Figure 4: The observed V vs. B-V color-magnitude diagram of Palomar 13 based on stars listed in Table 6 (crosses) and stars added from study of Ortolani et al. (1985) (open squares)

In Fig. 4 (click here). is shown the composed CMD based on our CCD data (crosses) and the photographic data of Ortolani et al. (1985) transformed to our photometric system. It contains 154 stars, covering arcmin area around the cluster.

The basic characteristics of the CMD are:

1. An extreme poorness of the horizontal branch (HB) which is populated by only one star () in addition to the three RR Lyr variables in our field.
2. A sparsely populated, moderately steep red giant branch (RGB) and a full lack of the red giants in the cluster's core (see Fig. 3 (click here)). All bright giants are located at a distance arcmin from the center of the of the cluster.
3. A well defined subgiant branch and upper main sequence turnoff region. As can be seen in the Fig. 3 (click here) the subgiants are the brightest stars in the core of the cluster.

     
   Table 4: BSS candidates
   

4. A number of stars form an extension of the main sequence up to about 1.5 mag brighter than TO point. These are possible blue straggler stars (BSS). Seven BSS candidates are identified in our field. Careful checks was made to ensure that the internal errors of the BSS candidates are the same as those of the subgiants at the same level. According to the galaxy model of Ratnatunga & Bahcall (1985) the predicted number of field stars (0.2 stars at 19 < V < 21 and (B-V) < 0.8) is not sufficient to explain this population. Ortolani et al. (1985) don't mention the presence of BSS candidates but two of them can be seen on their color-magnitude diagram. The BSS candidates lie at arcsec from the cluster center and four of them are in the core of the cluster. It is obvious that the BSS candidates present a marked tendency to populate the central part of the cluster. Comparison with other clusters of Palomar group (Pal4, Pal5, Pal14 and Pal15 - Fusi Pecci et al. 1992) shows that Palomar 13 can be included in class BS1 (Fusi Pecci et al. 1992).

   Data for BSS candidates in Palomar 13 - magnitudes, colors and distances from the cluster center are separately listed in Table 4 (click here).

5. The main sequence turnoff is found to be at and . The magnitude level of the HB determined as the magnitude of the nonvariable star is (the error is the internal error of the photometry). The magnitude level of the HB determined as mean magnitude of the RR Lyr variable stars is . Lee et al. (1990) found that the mean V magnitude of the red HB would be . brighter then that of the RR Lyrae variables. The agreement between the apparent magnitude of the horizontal branch determined as a magnitude of the nonvariable star and mean magnitude of the RR Lyr stars is good. This implies a magnitude difference between the HB and the TO .
6. There are no Palomar 13 stars in the asymptotic giant branch (AGB) phase of evolution.

Color - magnitude diagram in g ,r system

The g, g - r color-magnitude diagram for 175 stars measurable on the Focal Reducer of the Max Plank Institute of Aeronomy CCD images is presented in Fig. 5 (click here). Because of the smaller scale of the Reducer only 25 stars have been measured in the core of the cluster. The CMD in g, g - r verified the main features of the V, B-V CMD and especially:

1. Ortolani et al. (1985) noted that the existence of some ``evaporated" HB stars can not be excluded. In our field we have not found such stars. There is only one star (No. 46 in Table 7) above the HB in our g, g - r diagram which has an uncertain status.

     
   Figure 5: The observed g vs. g - r color-magnitude diagram of Palomar 13 based on all stars listed in Table 7
   

2. No more BSS candidates have been identified in this region. There are two stars on the diagram which fall in the region of the BSS candidates. One of them (No. 83 in Table 7) is a contact star and its photometry is poor, another (No. 164 in Table 7) is located too far from the center of the cluster () so we mean that it is more probable a field star. There is some discrepancy of the status of two of the BSS candidates listed in Table 7 (No. 168 and Ort 24). According g, r photometry they are subgiant stars. More precise photometry is needed for these stars.
3. The main sequence turnoff is found to be at and . The magnitude level of the HB determined as the magnitude of the nonvariable star is (the error is the internal error of the photometry). The magnitude level of the HB determined as the mean magnitude of the RR Lyr variable stars is .

The models of Ratnatunga & Bahcall (1985) have been used to estimate how many field stars are predicted in the region of Pal 13. The observed CMD has been statistically decontaminated by applying the procedure described in Ferraro et al. (1995). The analysis of the CMDs for Palomar 13 in the two photometric system allows us to separate 158 cluster members brighter than , including 4 variable stars and 51 field stars.

The fiducial lines of the main branches of Pal 13 have been determined from the statistically corrected CMD by dividing the various sequences into bins and computing in each bin the mode of the distribution color.

4.3. Metallicity and distance modulus

The cluster metallicity can be estimated by means of an appropriate calibration of RGB observations. , the intrinsic RGB color at the level of the HB in the Zinn & West (1984) calibration gives . The observed color is where the error has been estimated by combining the error made in merely locating the point with the photometric error. We have adopted 0.05 at E(B-V) (Harris & Racine 1979). The derived metal abundance by Ortolani et al. (1985) is between -1 and -1.5, a result consistent with our estimate.

We estimate the distance modulus of Palomar 13 by assuming the calibration of Lee (1990): . For this gives . The apparent magnitude of the HB is and thus the apparent distance modulus is (internal uncertainty only). As can be seen below the distances derived from the isochrone fitting are quite consistent with this estimate.