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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). |
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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. |
ID |
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V | (B-V) | (U-B) | (V-I) |
hh:mm:ss | ![]() ![]() |
|||||
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
.
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
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
,
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.
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Figure 6: A red DSS map of the covered region in the field of NGC 7772. North is up, East on the left. |
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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). |
Copyright ESO 2002