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3 NGC 7036

This object has never been studied before, and it is not included in the Lyngå  (1987) open cluster catalog. NGC 7036 (see Fig. 1) appears as a weak star enhancement, confined within a 3 square arcmin area in a relatively rich stellar field.


 \begin{figure}
\par\includegraphics[clip, width=8.3cm]{2126f2.eps}
\end{figure} Figure 2: Star counts in the field of of NGC 7036 as a function of the radius. The dashed line is the field number density estimate provided by Bica et al. (2001).

3.1 Star counts

The first signature of the possible presence of a star cluster is recorded in star counts. Bica et al. (2001) compared star counts in the region of NGC 7036 with a Galactic model and DSS maps, showing that NGC 7036 significantly emerges from the surrounding field.
This is confirmed by star counts based on the present photometric data.
We derived the surface stellar density by performing star counts in concentric rings around star #17 (selected as the approximate cluster center) and then dividing by their respective surfaces. The final density profile and the corresponding poisson error bars are depicted in Fig. 2. In this figure we take into account all the measured stars.
The surface density decreases smoothly over the region we covered, suggesting that the cluster has a radius of about 3.5-4 arcmin, and actually emerges significantly from the background. We count 5 stars brighter than V =13.7 in a field of about 9 square arcmin, exactly the same number reported by Bica et al. (2001, Table 1). However we notice that these stars are not in the cluster central region, but populate a sort of ring between 2 and 3 arcmin. The cluster center is on the other hand populated by fainter stars presumably belonging to the field. This can be interpreted as an indication of the dissolution the cluster is undergoing.


 \begin{figure}
\par\includegraphics[clip, width=8.5cm]{2126f3.eps}
\end{figure} Figure 3: Color-color diagram for all the stars in the field of NGC 7036 having UBV photometry. Filled circles indicate stars brighter than V = 17, whereas open squares indicate the remaining stars. The solid line is a Schmidt-Kaler (1982) empirical ZAMS, whereas the dashed line is the same ZAMS, but shifted by E(B-V) = 0.1.


 \begin{figure}
\par\includegraphics[clip, width=8.8cm]{2126f4.eps}
\end{figure} Figure 4: NGC 7036 stars brighter than V = 17 in the (B-V) vs. (B-I) plane.

3.2 Color-Color and Color-Magnitude Diagrams

In order to better understand the nature of NGC 7036, we constructed CCDs and CMDs. The goal is to get some information about the cluster reddening, age and distance. A hint of the cluster reddening is derived from the Schlegel et al. (1998) dust emission reddening maps. In the direction of NGC 7036 the reddening is 0.08. This is basically confirmed by the analysis of the (B-V) vs. (U-B)diagram presented in Fig. 3. Here we consider the stars brighter than V = 17 (filled circles) which lie very close to to the empirical Schmidt-Kaler (1982) ZAMS (solid line), and therefore are weakly reddened. The remaining stars (open squares) are much more dispersed.


 \begin{figure}
\par\includegraphics[clip, width=16.6cm]{2126f5.eps}
\end{figure} Figure 5: CMDs of the stars in the region of NGC 7036. In the left panel we plot all the stars. In the middle and right panel we consider only the stars in the ring defined by $1.5 \leq r \leq 3.5$. The dotted line provide a magnitude limit for probable cluster members. Finally, the solid line is a ZAMS shifted by E(B-V) = 0.1 and (m-M) = 10.7. Filled circles identify likely members.


 
Table 3: Photometry of likely member stars in the field of NGC 7036 deserving further spectroscopic investigation.
ID $\alpha(J2000.0)$ $\delta(J2000.0)$ V (B-V) (U-B) (V-I)
  hh:mm:ss $^{\circ}$:$^{\prime}$:''        
2 21:10:06.898 +15:30:38.06 12.624 0.543 0.022 0.662
3 21:10:03.892 +15:32:24.83 12.965 0.657 0.112 0.765
5 21:09:47.118 +15:31:01.14 13.521 0.771 0.283 0.878
6 21:09:47.002 +15:30:59.42 14.866 -0.440 0.037 2.009
8 21:09:49.301 +15:30:08.71 13.854 0.560 0.023 0.690
13 21:09:49.755 +15:29:52.80 14.221 0.677 0.204 0.775
14 21:09:44.580 +15:29:52.80 14.318 0.660 0.162 0.733
18 21:09:47.613 +15:33:10.43 14.767 0.709 0.175 0.823
20 21:09:48.684 +15:30:59.42 14.791 0.848 0.542 0.904
23 21:09:50.022 +15:31:52.51 15.109 0.751 0.310 0.849
28 21:10:09.531 +15:32:22.10 15.571 0.950 0.556 1.071
29 21:10:01.266 +15:28:26.44 15.622 0.690 0.101 0.815
32 21:10:09.286 +15:32:37.38 15.747 0.979 0.858 1.064
35 21:10:00.299 +15:33:42.61 15.897 0.798 0.422 0.892
42 21:10:04.397 +15;31:31.97 16.061 0.903 0.665 1.020
44 21:09:54.668 +15:33:16.41 16.239 1.116 0.744 1.262
47 21:09:59.729 +15:33:15.34 16.410 1.322 1.641 1.535


To guide the eye, we plotted also an empirical ZAMS (dashed line) shifted by E(B-V) = 0.1.
It seems that when looking for possible members one has to consider the brightest stars. It is not possible on the other hand to select cluster members by using individual reddenings, since the stars in Fig. 3 are probably of spectral type later than G.

The same indication about the reddening derives from the analysis of the BVI photometry, following the method devised by Munari & Carraro (1996), which yields $E(B-V)=0.10\pm0.05$.

In Fig. 5 we present three CMDs for the stars in the field of NGC 7036. In the left panel we plot all the stars, in the middle panel we plot the stars lying in the ring $1.5 \leq r \leq 3.5$where we have seen that the brightest stars are confined. Finally, in the right panel we plot the same stars, but in the plane (U-B) vs. V. The dotted line indicates a magnitude limit for probable members. These latter are indicated with filled symbols.

It is very difficult to get an estimate of the cluster age and distance since apparently there are no evolved stars. This can be explained statistically, since the cluster is intrinsically poorly populated, and therefore the absence of evolved stars is not completely unexpected. The only star that stays in the evolved region of the CMD is also the brightest one, and is very probably a field star located between us and NGC 7036, since it appears projected apart from the cluster central region.

To have a rough estimate of cluster distance we have to rely only on MS stars, and we proceed as follows.

From the location of stars in the (B-V) vs. (U-B) plane, we infer that the stars' spectral types range from about G0 to M2 by deriving the absolute colors from the ZAMS at the same position of the stars. This implies that the distance modulus is $(m-M) \approx 10.7\pm0.5$, which corresponds to a distance of about 1 kpc. Moreover, if the stars having G0 spectral type are still along the Main Sequence (MS), we infer a probable age of about 3-4 Gyr.

In conclusion, we are tempted to suggest that NGC 7036 is indeed an OCR having 17 likely members, whose properties are summarized in Table 3. Star counts seem to support this suggestion. CCDs and CMDs are more difficult to interpret. Anyway, if NGC 7036 is a bound stellar aggregate, what remains is an old, weakly reddened star cluster 1 kpc away from the Sun.


 \begin{figure}
\par\includegraphics[clip, width=8.8cm]{2126f6.eps}
\end{figure} Figure 6: A red DSS map of the covered region in the field of NGC 7772. North is up, East on the left.


 \begin{figure}
\par\includegraphics[clip, width=8.1cm]{2126f7.eps}
\end{figure} Figure 7: Star counts in the field of of NGC 7772 as a function of the radius. The dashed line is the field number density estimate provided by Bica et al. (2001).


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