A&A 371, 107-114 (2001)
G. Carraro1,2 - F. Patat2 - H. Baumgardt3
1 - Dipartimento di Astronomia, Universitá di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
2 - European Southern Observatory, Karl-Schwartzschild-Str 2, 85748 Garching b. München, Germany
3 - Department of Mathematics and Statistics, University of Edinburgh, Edinburgh EH9 3JZ, UK
Received 4 December 2000 / Accepted 27 February 2001
We report on UBVRI photometry of two fields in the region of the young open cluster NGC 3324. One of our fields covers the core region, while the other is closer to the tidal radius of the cluster. Our study provides the first CCD photometry of NGC 3324. We find that the cluster is very young and probably contains several pre Main Sequence (MS) stars. 25 members are identified on the basis of their position in the (U-B) vs. (B-V) diagram. We investigate the relation of the red super-giant HD 92207 with NGC 3324, suggesting that it probably does not belong to the cluster. Our second field is close to Loden 165, a possible cluster of stars that has never been studied so far. We show that this object is a probable open cluster, much older than NGC 3324 and much closer to the Sun.
Key words: photometry: optical - open clusters and associations: NGC 3324: individual - open clusters and associations: Loden 165: individual
We already reported on Bochum 9, 10 and 11 in Patat & Carraro (2001). In this work we concentrate on the results we have obtained for NGC 3324 and Loden 165, while the remaining clusters (see Feinstein 1995 for a review on the subject) will be discussed in Romaniello et al (2001).
NGC 3324 is particularly interesting because it is believed to be extremely young and to have a significant pre-MS population. Moreover the membership of the red super-giant star HD 92207 (HIP 52004) to NGC 3324 is still uncertain. Although the projected distance of HD 92207 from the cluster center seems much larger than the cluster radius, there are reasons to believe that this star is a member of NGC 3324, although we are far from having a firm answer to this problem (Baumgardt et al. 2000).
As for Loden 165 (C1035-584 according to IAU), no data are available in the literaure to our knowledge. Therefore it is not clear whether it is a star cluster or not, and whether it is related or not to the Carina complex.
The layout of the paper is as follows: Sect. 2 presents
the data acquisition and reduction. In Sect. 3 we discuss the open
cluster NGC 3324 and the relation of HD 92007 to it.
Section 4 is dedicated to Loden 165, a triplet
of stars that Lodèn (1973) suggested to be the signature
of a possible star cluster, demanding further investigation.
Our conclusions are summarized in Sect. 5.
|NGC 3324||Loden 165|
|Field||Filter||Exp. Time||Seeing||Filter||Exp. Time||seeing|
|(s)||( )||(s)||( )|
To allow for a proper photometric calibration and to asses the night quality, the standard fields RU149, PG1323, PG1657, SA 109 and SA 110 (Landolt 1992) were monitored each night. Finally, a series of flat-field frames on the twilight sky were taken. The scientific exposures have been flat-field and bias corrected by means of standard routines within IRAF. Further reductions were performed using the DAOPHOT package (Stetson 1991) under the IRAF environment.
The instrumental magnitudes have been transformed into standard Bessel UBVRImagnitudes using fitting coefficients derived from observations of the standard field stars from Landolt (1992), after including exposure time normalization and airmass correction. Aperture corrections have also been applied. Typical rms errors in the zero points amount at 0.02 mag. More details on the data reduction have been presented in Patat & Carraro (2001).
|Figure 1: DSS map of the region around NGC 3324. The region of the center is covered by by the upper left square, which is a mosaic of the observed fields, whereas the lower right square defines the mosaic of the fields covered by our photometry for the outer cluster parts. The circle confines the three stars observed by Lodén (1975). The bright star on the left of our second field is HD 92207, which is presumed to be part of NGC 3324. Note the strong nebulosity which encloses the cluster and HD 92207|
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Later this cluster was investigated in detail by Clariá (1977), who obtained UBV photo-electric photometry for 45 stars in a region of about , extending the photometry down to V=14. From this study, Clariá concluded that NGC 3324 is very young ( 2.2 106 yrs), has a mean color excess of , and is 3.1 kpc away from the Sun, basically confirming Moffat & Vogt (1975) results.
Moreover Clariá discusses the membership of HD 92207 to
NGC 3342. He suggests that the star is a foreground red supergiant
since it is far away from the cluster and appears to have a somewhat
lower distance modulus.
On the other hand Forte (1976) comes to a different conclusion, emphasizing
the possible membership since the star seems to be part of the same
complex of NGC 3324, at least looking at the strong gaseous structure
surrounding both the cluster and the star.
The same conclusion is drawn by Baumgardt et al. (2000).
In fact the cluster has an angular diameter of
which means a core radius of about
and a tidal radius
Since HD 92207 (HIP 52004) lies about
southwards of NGC 3324, it stays inside the cluster tidal
|Figure 2: Map of all stars brighter than V=14.5 in NGC 3324. Note that all the star groups usually blended in previous photo-electric photometries, are now resolved. The size of the circles is proportional to the magnitude of the stars. The upper right dashed circle is the size of the diaphragm ( ) in the photo-electric photometry of Moffat & Vogt (1975)|
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In both cases the differences are not negligible. In fact Moffat & Vogt (1975) adopted a diaphragm (Moffat, private communication), while Clariá (1977) a diaphragm (Clariá, private communication). Since the cluster is rather compact, and several stars have mean separation less than , we expect that stars blending is effective and is probably the cause of the reported differences. We cannot however exclude possible deviations due to differences in the adopted photometric systems.
CCD photometry allowed us to resolve blended stars and to obtain fairly precise measurements. In fact we find that the photometric errors amount at 0.03, 0.04, 0.04, 0.0.4 and 0.05, at V= 12, 14, 16, 18 and 20, respectively.
|Figure 3: Two colors diagram of NGC 3324. The arrow indicates the reddening vector, while the continuous and dashed lines are the empirical ZAMS from Schmidt-Kaler (1982). Cluster probable members are indicated with filled symbols, while non members with open symbols. The position of HD 92207 is indicated with a starred symbol. See the text for details|
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|Figure 4: The reddening corrected CMDs of NGC 3324. Only probable members are plotted, together with the Schmidt-Kaler sequence. See text for details|
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Our estimate for the reddening is consistent with that obtained by Clariá (1977). HD 92207 (starred symbol) lies somewhat outside the cluster mean line, and this raises some doubts on its membership to NGC 3324.
|Figure 5: The CMDs of NGC 3324 for all the detected stars|
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The Color Magnitude Diagrams (CMDs) of NGC 3324 in different colors are shown in Figs. 4 and 5. Figure 4 shows the reddening corrected CMD in the planes V0 -(B-V)0 (left panel) and V0-(U-B)0 (right panel), where only the probable cluster members have been plotted. super-imposed is the empirical ZAMS from Schmidt-Kaler (1982) shifted by E(B-V)=0.48and E(U-B)=0.34, respectively. The vertical shift provides an estimate of the apparent distance modulus , where the uncertainty has been computed allowing the reddening to lie in the range derived in the previous section. Therefore the corrected distance modulus (m-M)0 turns out to be , in agreement with Clariá's (1977) estimate. Accordingly, the distance of NGC 3324 to the Sun is kpc.
On the other hand, Fig. 5 shows the CMD in the planes V-(V-I) and V-(V-R). Note the extension of the MS down to V=20, and the widening of the MS at increasing magnitudes. Since the width of the MS below V = 14 is much larger than the mean photometric errors, it is quite probable that many pre-MS stars are actually present in this cluster (see below).
Estimating the precise age of this cluster is a challenging task. Looking again at the CMDs, apparently there are no bright stars which are significantly redder than the MS. This means that all the stars we see are still in the H-burning phase. The exact age of the cluster strongly depends on the membership of HD 92207. If this star belongs to the cluster, it must be an evolved star due to its red color. In this case the cluster would be as old as 2-3 Myr, as Clariá (1977) pointed out. On the other hand, if HD 92207 does not belong to the cluster, the 2-3 Myr estimate has to be considered only as an upper limit for the age of NGC 3324.
|Figure 6: Two colors diagrams for all the stars in the region of Loden 165, grouped according to the targeted fields (see Table 1). In the upper panels two different sequences are clearly visible. Open symbols indicate the stars studied by Lodén (1973)|
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Much firmer conclusions can be obtained by measuring the H emission from these candidates or by obtaining infrared photometry (Hillebrandt et al 1998).
HD 92207 seems also to have a higher reddening ( E(B-V)=0.52) than the mean NGC 3324 value (0.48), and according to Clariá (1977) a lower distance (1.9 kpc).
Summarizing, we conclude that HD 92207 is unlikely to be a genuine member of NGC 3324. It might however be spatially close to this cluster, since the HII gas in this area surrounds the star and seems to connect it with the cluster center. But this may just be a projection effect.
Nothing exists in the literature about this cluster, designed as C1035-584 by the IAU. It was suggested to be a possible physical group by Lodén (1973) who measured three stars (HDE 303088, HDE 303089 and HDE 303090), and found that they are of A0-A2 spectral type. Lodén suggests that this triplet is 1 kpc distant from the Sun and suffers from a low extinction, which he found to be E(B-V) = 0.11.
Since this triplet appears rather compact, the stars might in principle constitute a poor open cluster or an association. The coordinates of the triplet are given in Table 1, from which it can be seen that Loden 165 lies about west of our second field. This can also be seen in Fig. 1, where Lodén's stars are enclosed in a small circle, whereas the field covered by us is identified by the lower right square.
The area covered by our observations does not include Loden 165 stars. However this field is very interesting due to its vicinity to NGC 3324. According to the estimate of its tidal radius ( ), the field covered by us could simply be a portion of the halo of NGC 3324.
Alternatively, we could be simply looking at a region of Loden 165, whose nature of cluster has never been clarified.
The two color diagram for the stars in the field of Loden is shown in Fig. 6, where we have divided the regions according to the observed fields which, combined together, cover the region shown in Fig. 1. It is clear that the region is dominated by differential reddening. In particular in the upper frames the stars are rather dispersed, and seem to show several parallel sequences, characterized by a different mean reddening. The larger value of the reddening in this region with respect to the other fields, is marginally compatible with the reddening of HD 92207 and NGC 3324, which lie very close.
On the opposite side the reddening is lower and close to the one reported by Lodén for his triplet. However, on the average, the mean reddening of this region is much lower than in the case of NGC 3324 (cf. Sect. 3.3), being between 0.20-0.35. The mean reddening in this region, averaging over all four fields, turns out to be .
The three stars of Loden 165 are shown in the right panels of Fig. 6 with open symbols. They seem to lie somewhat apart from the ZAMS in both panels, supporting the conclusion that they are a triplet of stars lying closer to the Sun and suffering from a smaller reddening.
|Figure 7: CMD in the region of Loden 165. Superimposed are three solar metallicity isochrones for the ages of 2 (dotted line), 3 (solid line) and 4 (dashed line) Gyrs (Girardi et al 2000). The three filled circles indicate Lodén's triplet|
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The global CMD of Loden 165 is shown in Fig. 7. The general appearance is very different from the CMD of NGC 3324 (cf. Figs. 4 and 5). There is a clear MS extending for about 5 magnitudes from V = 20 to V = 15. At brighter magnitudes it sharply drops, showing a clear Turn Off at V = 14.5, (B-V) = 0.25. The population of bright MS stars is very poor, while there is clear evidence of a sub-giant and red giant branch population. This leads to the suggestion that the cluster is rather old, at least much older than NGC 3324.
To have an idea of the cluster age and distance we superimposed three solar metallicity isochrones for the ages of 2 (dotted line), 3 (solid line) and 4 (dashed line) Gyrs from the Padova models (Girardi et al. 2000). It turns out that the best match is obtained with the 3 Gyrs isochrone by assuming the mean reddening derived above. As a consequence, the apparent distance modulus turns out to be (m-M)=12.1. The corrected distance modulus turns out to be (m-M)0=11.42, which puts this object 1.9 kpc from the Sun. In the same plot we have included also the Lodén triplet using filled circles. The most plausible conclusion is that they seem to be simply field stars.
Loden 165, an object never studied so far, is an old star cluster with an age of approximately 3 Gyrs. The estimates of the age, reddening and distance are very different from those of NGC 3324, suggesting that Loden 165 has presumably nothing to do with the Carina open cluster system.
GC acknowledges kind hospitality from ESO, and expresses his gratitude to M. Romaniello, L. Pasquini and A. Brown for stimulating discussions. This study made use of Simbad and WEBDA.