A&A 385, 471-478 (2002)
DOI: 10.1051/0004-6361:20020165
G. Carraro
Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2, 35122 Padova, Italy
Received 23 November 2001 / Accepted 17 January 2002
Abstract
We present CCD UBVI observations obtained in the field
of the two previously unstudied dissolving open cluster candidates NGC 7036
and NGC 7772. Our analysis suggests that both the objects are Open
Cluster Remnants (OCR). NGC 7036 is an open
cluster remnant with a core radius of about 3-4 arcmin.
We derive for the first time
estimates of its fundamental parameters. We identify 17 likely members
that define a group of stars at 1 kpc from the Sun,
with a low reddening
,
and with an age of about 3-4 Gyr. As for NGC 7772, we identify 14 likely members, that define
a group of stars with a very low reddening (
),
are 1.5 Gyr old
and are located about 1.5 kpc from the Sun.
Key words: open clusters and associations: individual: NGC 7036, NGC 7772 - open clusters and associations: general
Recently, Bica et al. (2001) drew attention to a sample of
high Galactic latitude (
)
star clusters presumably in an advanced stage of dynamic evolution, which
they baptized Probable Open Cluster Remants (POCR).
The prototype of this class of objects is NGC 6994 (M 73), recently studied
by Bassino et al. (2001) and Carraro (2001), who performed the first
multicolor photometric studies of this cluster, but
arrived at opposite conclusions on the nature of this object.
While Bassino et al. suggest that M 73 is the remnant of a star cluster, Carraro
(2000) proposes that it is just a chance alignment
of four bright stars.
Although it is a difficult task to unravel the nature of a star concentration based only on photometry, it however remains the first necessary step. Indeed sometimes the Color Magnitude Diagrams (CMDs) and Color-Color Diagrams (CCDs) are sufficient to disentangle a real bound system or a random enhancement of stars (Carraro & Patat 1995; Piatti & Clarià 2001). In many cases however, photometry cannot help to decide unambiguously about the nature of a star concentration: in this situation radial velocities and/or proper motion studies are necessary (Baumgardt 1998; Baumgardt et al. 2000; Odenkirchen & Soubiran 2002).
The census provided by Bica et al. (2001) lists 20 candidate dissolving open clusters, some of which are completely unstudied. This is the case for NGC 7036 and NGC 7772, two high latitude objects traditionally considered genuine open clusters, which are the subject of the present study. The basic idea of this paper is to present the first photometric study of these clusters and to provide a list of probable member stars to be further studied with high resolution spectroscopy.
We note the recent spectroscopic follow-up of NGC 6994 by Odenkirchen & Soubiran (2002), who confirmed the Carraro (2000) suggestions that this object is a chance alignment of four bright stars.
In Sect. 2 we briefly present the observations and data reduction. Sections 3 and 4 illustrate our results for NGC 7036 and NGC 7772, and, finally, Sect. 5 draws some conclusions and suggests further lines of research.
Name | ![]() |
![]() |
l | b |
hh:mm:ss | ![]() ![]() ![]() |
![]() |
![]() |
|
NGC 7036 | 21:10:02 | +15:31:06 | 64.55 | -21.44 |
NGC 7772 | 23:51:46 | +16:14:48 | 102.74 | -44.27 |
Observations were carried out with the AFOSC camera at the
1.82 m Copernico telescope of Cima Ekar (Asiago, Italy), on the photometric
night of October 21,
2001. AFOSC samples a
field in a
1K
1K thinned CCD. The typical seeing was between 2.0
and 2.5 arcsec.
The basic data of the studied clusters are summarized in Table 1, and
the details of the observations are listed in Table 2.
The covered regions are shown in Figs. 1 and 6, where two DSS
maps
are presented for NGC 7036 and NGC 7772, respectively.
The data have been reduced using the IRAF
packages CCDRED, DAOPHOT, and PHOTCAL.
The calibration equations obtained by observing the Landolt (1992)
PG 02331 field 3 times during the night, are:
Field | Filter | Time integration | Seeing |
(s) | (
![]() |
||
NGC 7036 | |||
U | 240, 600 | 2.2 | |
B | 30, 300 | 2.3 | |
V | 30, 300 | 2.3 | |
I | 30, 200 | 2.3 | |
NGC 7772 | |||
U | 300 | 2.2 | |
B | 30, 120 | 2.4 | |
V | 5, 15, 60 | 2.3 | |
I | 5, 15, 60 | 2.3 | |
PG 02331 | |||
U | 60, 120, 60 | 2.0 | |
B | 60, 60, 120 | 2.1 | |
V | 15, 15, 30, 30 | 2.0 | |
I | 30, 30, 15 | 2.1 |
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Figure 1: A DSS red map of the covered region in the field of NGC 7036. North is up, East on the left. |
Open with DEXTER |
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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). |
Open with DEXTER |
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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. |
Open with DEXTER |
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Figure 4: NGC 7036 stars brighter than V = 17 in the (B-V) vs. (B-I) plane. |
Open with DEXTER |
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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
![]() |
Open with DEXTER |
ID |
![]() |
![]() |
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. |
Open with DEXTER |
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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). |
Open with DEXTER |
In the field of the cluster
there is the star GSC 01722-01669 (TYC 1722 1669 1),
in our numbering system,
which presumably has no relation with NGC 7772, since it lies outside
the cluster core. It is probably a blue star located between us and NGC 7772.
The surface density decreases sharply up to a
radius of about 2 arcmin, afterwards the density profile
is basically flat. Therefore NGC 7772 appears to be very
compact with a core radius which at maximum amounts to 2 arcmin.
Also in this case star counts indicate that we are facing a significant
density contrast with respect to the field, whose
bright stars (B = 13.5, V = 13.0) density in this direction
is represented by the dashed line in Fig. 7. We count 5 stars
brighter than V =13.0 in a field of about 9 squared arcmin,
basically the same number reported by Bica et al. (2001, Table 1).
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Figure 8: Color-color diagram for all the stars in the field of NGC 7772 having UBV photometry. The solid line is a Schmidt-Kaler (1982) empirical ZAMS. |
Open with DEXTER |
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Figure 9: NGC 7772 stars in the (B-V) vs. (B-I) plane. |
Open with DEXTER |
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Figure 10:
CMDs of the stars in the region of NGC 7772. In the left
panel we plot all the stars in the (B-V) vs. V plane.
In the middle and right panel we consider
only the stars within
![]() |
Open with DEXTER |
ID |
![]() |
![]() |
V | (B-V) | (U-B) | (V-I) |
hh:mm:ss | ![]() ![]() |
|||||
2 | 23:51:42.816 | +16:14:10.40 | 11.183 | 1.310 | 1.459 | 1.399 |
3 | 23:51:46.129 | +16:14:23.22 | 12.529 | 0.719 | 0.262 | 0.855 |
4 | 23:51:47.355 | +16:15:53.19 | 12.718 | 0.578 | 0.047 | 0.741 |
5 | 23:51:52.413 | +16:11:17.04 | 12.662 | 0.817 | 0.554 | 0.953 |
6 | 23:51:39.154 | +16:18:11.99 | 13.171 | 0.553 | -0.071 | 0.786 |
7 | 23:51:48.158 | +16:15:05.47 | 13.340 | 0.929 | 0.837 | 1.031 |
8 | 23:51:46.383 | +16:14:58.90 | 13.461 | 0.628 | 0.102 | 0.772 |
9 | 23:51:35.455 | +16:12:49.35 | 13.557 | 0.880 | 0.587 | 1.018 |
10 | 23:51:48.000 | +16:14:00.87 | 13.635 | 0.705 | 0.286 | 0.839 |
11 | 23:51:57.916 | +16:17:28.75 | 14.065 | 0.715 | 0.444 | 0.831 |
12 | 23:51:44.723 | +16:14:45.51 | 14.242 | 0.681 | 0.199 | 0.831 |
13 | 23:51:48.862 | +16:10:58.80 | 14.278 | 0.743 | 0.372 | 0.864 |
14 | 23:51:52.312 | +16:11:71.07 | 15.209 | 0.835 | 0.543 | 0.964 |
15 | 23:51:37.259 | +16:18:02.02 | 15.346 | 0.532 | -0.077 | 0.715 |
16 | 23:51:47.501 | +16:12:48.21 | 15.618 | 0.997 | 1.108 | 1.072 |
The same result is obtained
by considering the distribution of the stars in the (B-V) vs. (B-I)diagram (see Fig. 9).
By applying the Munari & Carraro (1996) method we infer a reddening
E(B-V) =
.
As in the case of NGC 7036, we can infer the approximate spectral type by analysing
colours in the (B-V) vs. (U-B) plane. It turns out that the spectral type
of possible members ranges from about A8 to M3.
In Fig. 10 we present three CMDs for the stars in the field of NGC 7772.
In the left panel we plot all the stars, in the middle panel
we plot the stars lying within
where we have seen that the brightest stars are confined. Finally,
in the right panel we plot the same stars, but in the plane (V-I) vs. V.
Most of the stars in all the panels above V = 16 are probable MS stars.
There is only one exception, which is star ,
which lies within
the cluster core, and that we consider as a giant star probable
member of the cluster.
In the three panels of Fig. 10 we have overlaid a solar metallicity
isochrone taken from Girardi et al. (2001) for the age of 1.5 Gyr.
This isochrone provides a good fit of the data down to V=16.0 by
adopting E(B-V) = 0.03 and
.
Below V = 16.0, the MS starts to be ill defined. This is a clear
signature of low mass star depletion, in nice analogy with - for instance -
the cases of NGC 3680 (Anthony-Twarog et al. 1991) and NGC 7762
(Patat & Carraro 1995).
In conclusion, we confirm previous suggestions by Bica et al. (2001) that NGC 7772 is an OCR 1.5 Gyr old. We identify 14 likely members, which are plotted with filled symbols in Fig. 10, and whose properties are summarized in Table 4.
It is worth noting that the present results must be considered with some caution, and that the list of members must be better constrained by determining individual star radial velocities and proper motions. In this way these objects can become templates for N-body simulation aimed at investigating the dynamical evolution of open star clusters and the origin of the field star population
Acknowledgements
I am deeply indebted to Nando Patat and Sandro Villanova for numerous very useful conversations. I acknowledge Giorgio Martonara and Franco Bocchi for the kind night assistance. This study made use of the Simbad database.