A&A 407, 527-539 (2003)
DOI: 10.1051/0004-6361:20030831
G. Baume1,2 - S. Villanova1 - G. Carraro1
1 - Dipartimento di Astronomia, Università di Padova,
Vicolo Osservatorio 2, 35122 Padova, Italy
2 -
Facultad de Ciencias Astronómicas y Geofísicas de la
UNLP, IALP-CONICET, Paseo del Bosque s/n, La Plata, Argentina
Received 25 February 2003 / Accepted 16 May 2003
Abstract
We present CCD UBV(I)
observations obtained in the field of the previously unstudied northern open clusters NGC 1582 and NGC 1663. For the former, we also
provide high-resolution spectra of the brightest stars and complement our data
with Two-Micron All-Sky-Survey (2MASS) near-infrared photometry and with
astrometric data from the Tycho-2 catalog.
From the analysis of all these data, we argue that NGC 1582 is a very poor,
quite large and heavily contaminated open cluster. It turns out to have a
reddening
,
to be situated
pc from the
Sun and to have an age of
Myr.
On the other hand, we were not able to unambiguously clarify the nature of
NGC 1663. By assuming it is a real cluster and from the analysis of its
photometric diagrams, we found a color excess value
EB-V = 0.20,
an intermediate age value (2000 Myr) and a distance of about 700 pc.
The distribution of the stars in the region however suggests we are probably
facing an open cluster remnant. As an additional result, we obtained aperture
photometry of three previously unclassified galaxies placed in the field of
NGC 1663 and performed a preliminary morphological classification of them.
Key words: Galaxy: open clusters and associations: individual: NGC 1582 and NGC 1663 - open clusters and associations: general
This study is part of a long term project aimed at providing accurate CCD photometry for poorly known or unstudied northern open clusters (Carraro 2002a, and references therein). Here we focus our attention on the open clusters NGC 1582 and NGC 1663. Both objects have never been studied before, apart from the identification and a preliminary estimate of their angular size. Their basic data are summarized in Table 1.
By inspecting the finding charts of these two clusters (see Figs. 2 and 10), we notice mainly that:
The plan of this study is as follows: in Sect. 2 we briefly present the observations and data reduction. In Sects. 3 and 4 we illustrate our analysis and results for NGC 1582 and NGC 1663, respectively. Section 5 is dedicated to a preliminary analysis of three previously unclassified galaxies in the field of NGC 1663. Finally, in Sect. 6 we draw our conclusions.
Name |
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l | b |
NGC 1582 | 04:32:15.4 | +43:50:43 |
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NGC 1663 | 04:49:24.3 | +13:08:27 |
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CCD UBV(I)
observations were carried out with the AFOSC camera at the
1.82 m Copernico telescope of Cima Ekar (Asiago, Italy), in the photometric
night of November 8, 2002. AFOSC samples a
field
in a
nitrogen-cooled thinned CCD.
Field | Filter | Exposure time | Seeing | Airmass | ||
[s] | [
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NGC 1582 | U |
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180 | 20 | 2.5 | 1.196 |
B | 600 | 60 | 10 | 2.3 | 1.230 | |
V | 300 | 30 | 5 | 2.4 | 1.255 | |
I | 300 | 30 | 5 | 2.1 | 1.274 | |
NGC 1663 | U |
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- | - | 2.5 | 1.348 |
B | 600 | 60 | - | 2.4 | 1.215 | |
V | 300 | 30 | 3 | 2.2 | 1.188 | |
I | 300 | 30 | 3 | 2.0 | 1.190 | |
PG 0231+051 | U | 800 | 2.5 | 1.348 | ||
B | 300 | 2.4 | 1.324 | |||
V | 60 | 2.2 | 1.316 | |||
I | 90 | 2.2 | 1.315 | |||
PG 2213-006 | U | 600 | 2.5 | 1.447 | ||
B | 150 | 2.3 | 1.457 | |||
V | 30 | 2.3 | 1.465 | |||
I | 30 | 2.3 | 1.472 | |||
Calibration |
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coefficients |
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u3 = +0.58 | b3 = +0.29 | |||||
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v3 = +0.16 | i3 = +0.08 | |||||
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Figure 2:
Finding chart of the NGC 1582 region (
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Details of the observations are listed in Table 2, where the observed fields are
reported together with the exposure times, the typical seeing values and the
air masses. Figures 2 and 10 show the finding charts of the NGC 1582 and NGC 1663
regions respectively, indicating the covered areas and the object angular sizes.
The data has been reduced with the
IRAF
packages CCDRED, DAOPHOT, and PHOTCAL using the point spread function (PSF)
method (Stetson 1987). The calibration equations obtained by observing Landolt
(1992) PG 0231+051 and PG 2213-006 fields during the night, are:
u = U + u1 + u2 (U-B) + u3 X | (1) | ||
b = B + b1 + b2 (B-V) + b3 X | (2) | ||
v = V + v1bv + v2bv (B-V) + v3 X | (3) | ||
v = V + v1vi + v2vi (V-I) + v3 X | (4) | ||
i = I + i1 + i2 (V-I) + i3 X | (5) |
High-resolution (
)
spectra of 10 stars (see Table 3 and Fig. 5) in the field of NGC 1582 were obtained during
the nights of January 14-15 and February 14, 2003, using the REOSC Echelle
Spectrograph attached to the 1.82 m telescope of Asiago Astronomical
Observatory. This instrument works with a Thomson 1024
1024 CCD and the
allowed wavelength coverage is approximately
4500-6650 Å. Details on this
instrument are given in Munari & Zwitter (1994) and on the Asiago
Obervatory Home page
.
The exposure times were 45 min for all the stars. In order to improve the signal-to-noise ratio, two exposures were taken for each star reaching at the end S/N values up to 70. The data have been reduced with the IRAF package ECHELLE using thorium lamp spectra for wavelength calibration purposes. By comparing final known sky line positions along the spectra we derived an error of about 0.01 Å.
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Figure 3: Stellar surface density profile in the region of NGC 1582 as a function of the radius. Data have been taken from 2MASS catalog. Dotted lines indicate the adopted limits for the cluster and for the comparison field related with Fig. 8b. |
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Figure 4:
a) Vector point plot for the stars
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Spectra were classified mainly on the basis of the intensity of the Fe, H and He lines - depending on the spectral type - and by comparing them with spectrophotometric standards. The results are shown in Fig. 5, where we plot the Echelle spectra of the 10 measured stars degraded to a resolution of about 90 Å/mm. A few main lines are also shown. These spectra have been then compared with stellar libraries from Montes et al. (1997, 1999) and Jaschek & Jaschek (1987) to obtain estimates of spectral types and luminosity classes.
Individual radial velocity values were obtained with the RVIDLINES task using up to 10 lines. Typical errors in the radial velocity amount to 3.0 km s-1. The resulting classification together with the radial velocity values are listed in Table 3.
NGC 1582 (=
+43) is commonly considered an open cluster with a
diameter in the range of
(Dias et al. 2002) to
(Lyngå 1987). Our CCD observations only cover the cluster central region
(see Fig. 2). Therefore, in order to better probe the cluster size and nature,
we complemented our data with near IR photometry from the 2MASS catalog.
To derive the radial stellar surface density we adopted the cluster center
given by Dias et al. (2002, see Table 1), and we use: a) the
corresponding DSS-2
red image of
,
and b) all the stars from
the 2MASS catalog inside a circle of radius 30
,
centered on the
adopted cluster center. The computation is done by performing star counts inside
increasing concentric rings
wide around the cluster center and then
dividing by their respective surfaces. The density profiles obtained in the two
cases are very similar. For the sake of illustration, only the one derived from
2MASS data is shown in Fig. 3. By inspecting this plot we notice that the
cluster appears as a weak over-density up to
from the center,
where the stellar density reaches the field value. As anticipated, the
cluster turns out to be much larger than the area we covered with our CCD observations and we adopt
as the radius. This way, the obtained
diameter has a value close to the previous qualitative suggestion by
Lyngå (1987).
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|
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Figure 5: Spectra of a sample of bright stars in the field of NGC 1582. A few interesting lines are indicated. See Table 3 for details. |
Important information on the kinematics of the brightest stars in and around NGC 1582 field can be derived from the study of proper motions and radial velocities. The former are available in the Tycho-2 catalog and in the HD Extension Charts (Nesterov et al. 1995) whereas the latter were measured for 10 stars (see Sect. 2.2).
The Tycho-2 proper motions are based on the comparison between contemporary mean
positions derived from the recent Tycho observations on-board Hipparcos and
early-epoch positions observed many decades ago (see Høg et al. 2000 and
references therein). Due to the long time-baseline they have rather high
precision and therefore directly indicate the long-term mean tangential motions
of the stars. We have collected proper motion components for 53 stars in a field
around the center of NGC 1582. They are shown in the vector point
diagram in Fig. 4a. The points distribution is characterized by a global spread
10 mas/yr with a noticeable concentration, which seems to indicate a
possible physical relation among these stars.
Spectra obtained with the Echelle spectrograph allow us to get radial velocity, as described in Sect. 2.2. The obtained values are shown in Fig. 4b, and range from -40 to +40 km s-1.
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Figure 6: Color-color diagrams (CCDs) of stars in the region of NGC 1582. a) U-B vs. B-V diagram. The symbols have the following meaning: big circles are adopted member stars (m), triangles are probable member stars (pm), empty squares are non-member stars (nm), and small open circles are stars without any membership assignment. Black symbols indicate stars with CCD measurements whereas grey ones are for stars with data from the Tycho-2 catalog. The solid line is Schmidt-Kaler's (1982) ZAMS, whereas the dashed and dotted lines are the same ZAMS, but shifted by EB-V = 0.35 and 1.2, respectively. The dashed arrow indicates the normal reddening path. b) B-V vs. V-I diagram. Symbols as in Fig. 5a. The solid lines represent the intrinsic positions for stars of luminosity classes V and III (Cousins 1978a,b). The dashed line gives the normal reddening path (R = 3.1). |
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Figure 7: CMDs for all the stars covered in the field of NGC 1582. Symbols as in Fig. 5a. The solid line is the Schmidt-Kaler (1982) empirical MS fitted to the apparent distance modulus V-MV = 11.4 ( V-MV = V0-MV + 3.1 EB-V, see Sect. 3.4). |
The color-color diagrams (CCDs) and the color-magnitude diagrams (CMDs) are
shown in Figs. 6-8. The first two figures include all the stars measured
in the direction of NGC 1582 and also several bright stars with available
spectral classification and Hipparcos (ESA 1997) or Tycho-2 magnitudes within
from the cluster center and not covered by our photometry. Tycho-2
magnitudes are converted to the Johnson system using the relations given by
Bessell (2000) and approximated (U-B) and (V-I) colors are obtained
according to the spectral types (when they are available) together with
Schmidt-Kaler (1982) and Cousins (1978a,b) calibrations. Figure 8 presents the CMDs
from 2MASS catalog for stars placed inside the cluster radius (
,
see Sect. 3.1) and for star placed in a ring around the cluster (
,
see Fig. 3) that is adopted as a comparison field. Radii are
selected in such a way that both diagrams in Fig. 8 cover equal sky areas.
If we inspect Fig. 6a and compare the star positions with Schmidt-Kaler's (1982)
Zero Age Main Sequence (ZAMS), using different reddening values, there seem to
be two populations: one having a lower excess (dashed curve) mainly defined
by the brightest stars, and another one with a much larger reddening. We claim
that the former define the open cluster NGC 1582, whereas the latter population
is identified as the Galactic disk component, made of stars placed at different
distances and with a different amounts of absorption. To guide the eye we have
placed another ZAMS reddened by
EB-V = 1.2 (dotted curve). A similar
conclusion can be deduced from Figs. 7 and 8, where we see that most of the
stars observed are just Galactic disk field stars and NGC 1582 looks like a
small group of stars brighter than
above the mean stellar
background.
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Figure 8: CMDs from 2MASS catalog. Symbols are as in Fig. 6a. a) Stars placed inside the cluster area. b) Stars placed in a ring around the cluster. The solid line in panel a) and the dotted one in panel b) are the intrinsic position for MS stars from the Schmidt-Kaler (1982) and Koornneef (1983) calibrations fitted to the apparent distance modulus J-MJ = 10.6 ( J-MJ = V0-MV + (3.1 - 2.3) EB-V, see Sect. 3.4). |
We derive cluster membership by comparing the distribution of the stars in the different photometric diagrams (e.g. Baume et al. 1999; Ortolani et al. 2002; Carraro 2002b). But we also take into account their location in the finding chart (Fig. 2), in the vector point diagram and the measured radial velocities (Fig. 4). At first, we use only Figs. 6 and 7 and we notice that there are five bright blue stars (#2, 3, 4, 5 and 7) that fit pretty well the empirical ZAMS shifted by EB-V = 0.35 (dashed line). These stars also are well placed in the vector point diagram, although three of them, stars #4, 5 and 7, slightly depart from the central concentration in this diagram. Additionally, star #4 has a radial velocity value very different from that of stars #2 and 3. Therefore, stars #2 and 3 are adopted as cluster members (m), stars #5 and 7 as probable members (pm), and star #4 is considered to not belong to the cluster (nm).
Using stars #2, 3, 5 and 7, we compute colour excess values for each of them
applying the relations
EU-B/EB-V = 0.72 + 0.05 EB-V and
(U-B)0 =
3.69 (B-V)0 according to Vázquez & Feinstein (1991); this yields a
mean value
(s.d.), here adopted as the cluster color
excess. Beside, following the reddened ZAMS path onto CCDs and CMDs, we select
four additional stars and adopt them as probable cluster members (pm).
Star #2 is of spectral type A0 V, as derived from its spectrum and from its position in the CCD. However by comparing its position in the CMDs with respect to other members, we notice that it appears over-luminous by 0.75 mag. This can be explained by assuming it is a binary system. Of course, this is only a suggestion, which can be confirmed for instance by looking for radial velocity variations in other-epoch spectra. Therefore we keep it on the adopted members list. On the other hand, star #6 is well placed on the CCDs (see Fig. 6) as a F-type star, but its position in the CMDs (see Fig. 7) contradicts this hypothesis and it is therefore considered a non-member (nm).
At this point, we are ready to use Fig. 8 as well. In this figure we superimpose
on the data an empirical Main Sequence (MS) obtained by the combination of the
Schmidt-Kaler (1982) and Koornneef (1983) calibrations shifted according to the
relation among colors obtained from the van de Hulst extinction curve #15
(Johnson 1968). By closely inspecting this figure, we notice that there are some
stars not covered by our survey, but within the cluster radius, which are
properly located both in the CMD (Fig. 8a) and in the proper motion diagram of
Fig. 4a (grey symbols in those figures), and one of them (TYC 2892-1756-1) also
has a radial velocity value compatible with those of adopted member stars #2
and 3 (see Fig. 4b). This group of stars has a counterpart in the corresponding
comparison field (Fig. 8b). We compute then the J distributions for stars with J-K < 0.2 (see Table 4) in each diagram and we compare them by using a test. We find that they are different with a probability higher than 95%.
Therefore, although we are aware that we are dealing with small numbers of
stars and that a statistical analysis is therefore only indicative, the difference
between both distributions turns out to be noticeable and therefore we are
inclined to consider the group of stars mentioned above as cluster members (m). An
exception are stars TYC 2892-1306-1, TYC 2892-1503-1 and TYC 2892-1801-1. The
first two have high errors in their B-V measurements and are situated leftward
of the MS in Fig. 7b, and the third has a radial velocity value with a huge
error and is far from the adopted values for cluster members (see Fig. 4b and
Table 3). We cannot exclude that this might be due to a binary effect. Thus we
consider these three particular stars as only probable members (pm). On the
other hand, the photometric diagrams also reveal that some stars with available
spectral classification and Tycho-2 data do belong to the field stellar
population; they are therefore taken as non-members (nm).
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8-9 | 9-10 | 10-11 | 11-12 | 12-13 |
Cluster | 2 | 3 | 16 | 20 | 22 |
Comparison field | 1 | 1 | 4 | 7 | 6 |
Star #1 (GSC 02892-01209) deserves special attention. It is the brightest and one of the reddest stars in our sample. Unfortunately, we were not able to find out either its distance or its proper motion components. This star might in principle be a giant cluster member. However, from its spectrum we obtained a radial velocity value that disagrees with the adopted one for adopted cluster members (see Fig. 4b). Also, it is classified as a M-type star and its parameters (reddening and distance) differ from those of the cluster, ruling out the possibility that it is a member star.
In conclusion, after the above detailed analysis of the brightest stars in the NGC 1582 region, we conclude that i) the brightest ones are merely field stars, and ii) we only consider the stars classified as m and pm as the main members of the open cluster NGC 1582. They are indicated in Fig. 2, and demonstrate that we are dealing with a sparse, poor and severely field-star-contaminated open cluster.
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Figure 9:
MV vs. (B-V)0 diagram of the members and probable members
in the region of NGC 1582. Symbols as in Fig. 6a. Dashed curves are the
isochrones from Girardi et al. (2000). The reported numbers give the
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In Fig. 9 we plot the reddening-corrected MV vs. (B-V)0 diagram
for the cluster members and probable cluster members adopting a distance modulus
of
(error from inspection). Last value fits very
nicely the empirical Schmidt-Kaler (1982) ZAMS. We also apply the
spectroscopic parallax method to three classified stars, obtaining
.
These values are in quite good agreement and
imply that the few stars identified as NGC 1582 are located
pc
away from the Sun in the outer edge of the Orion arm.
From the obtained spectral classification (see Table 3) and from the location of adopted cluster members stars in Fig. 5a along the shifted ZAMS (dashed curve), we infer that the spectral types range from B7 to F2. If the stars having B7 spectral type are still on the main sequence, we derive an age of about 300 Myr for NGC 1582 (Girardi et al. 2000). A similar result is obtained by the isochrone fitting method shown in Fig. 9.
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Figure 10:
Finding chart of the NGC 1663 region (
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Figure 11: Stellar densities in the region of NGC 1663 as a function of the radius from DSS-2 image. The dotted lines indicate the adopted limits for the cluster and for the comparison field related with Fig. 14b. |
NGC 1663 (=
+130) is located quite high above the galactic plane
for an open cluster (see Table 1) and does not emerge much from the general
Galactic field toward its direction. According to Lyngå (1987) it has a
diameter of
and therefore our observations cover most of the
cluster region (see Fig. 10). Unfortunately, proper motions are available only
for four stars in the region
around NGC 1663 (see Table 5),
so we must rely mostly on photometric data to derive cluster members and cluster
fundamental parameters.
![]() - Letters T indicate data obtained from Tycho-2 catalog. - Proper motion and parallax values are expressed in mas/yr and mas respectively. |
NGC 1663 appears as a loose aggregate of a few relatively bright stars. As for NGC 1582, we perform stellar counts in concentric rings around the cluster
center, but this time, since the cluster does not appear much extended, we only
use the DSS-2 red data. The stellar density profile is shown in Fig. 11. One
can readily see the lack of any clear trend, and therefore it is a difficult
task to define the cluster radius. Anyway, if we inspect the DSS-2 image, the
region where is located the over-density of stars is about
radius
(see Fig. 10). In conclusion, it is hard to decide upon the real nature of this
cluster, although, broadly speaking, the over-density we found seems to suggest
that we are looking at an aggregate of the kind suggested by Bica et al. (2001),
namely a POCR. The lack of an unambiguous cluster center, and the loose
distribution of the brightest stars across the field support this suggestion.
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Figure 12: CCDs of the stars in the region of NGC 1663. a) U-B vs. B-V diagram. As for NGC 1582, symbols have the following meaning: circles are adopted member stars (m), triangles are probable member stars (pm), empty squares are non-members stars (nm) and small open circles are stars without any membership assignment. Field galaxies are indicated as white circles. The solid line is Schmidt-Kaler's (1982) ZAMS, whereas the dashed one indicates the same ZAMS, but shifted by EB-V = 0.2. The dashed arrow indicates the normal reddening path. b) B-V vs. V-I diagram. Symbols as in Fig. 12a. The solid lines are the intrinsic positions for stars of luminosity classes V and III (Cousins 1978a,b), while the dashed one provides the normal reddening path (R = 3.1). |
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Figure 13:
Color-magnitude diagrams (CMDs) for all the stars covered in the
field of NGC 1663. Symbols as in Fig. 12a. The solid line and dashed curves are
the Schmidt-Kaler (1982) empirical ZAMS and isochrones from Girardi et al.
(2002) respectively. They were fitted to the apparent distance modulus
V-MV
= 9.8 (
V-MV = V0-MV + 3.1 EB-V, see Sect. 4.2). The numbers
indicate
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Figure 14: CMDs from the 2MASS catalog. Symbols as in Fig. 12a. a) Stars placed inside the cluster area b) Stars placed in a ring around the cluster (see Fig. 10). |
We follow the same method applied above for NGC 1582 to derive preliminary
individual reddening and membership of the cluster. Photometric diagrams are
shown in Figs. 12-14, where all the observed stars are presented in the
first two, whereas the last one is obtained from 2MASS data. Let us fix our
attention in Fig. 12a: here all the stars seem to crowd along an empirical ZAMS
shifted by
EB-V = 0.20. It is however clear that in this case a reddening
solution cannot be found, since all the stars lie well beyond the location of A5 spectral type stars (Ortolani et al. 2002). The low color excess and the
small dispersion in reddening are not unexpected, due to the position of the
cluster high above the Galactic plane and toward the anti-center direction. The
CMDs are not easy to interpret, since most of the stars are just Galactic disk
field stars, as is readily seen by inspecting both panels in Fig. 14. From this
figure it is evident that NGC 1663 emerges as an over-density of a dozen stars
brighter than
above the mean stellar background.
In conclusion, only preliminary membership information can be derived from the CMDs of Figs. 13 and 14. We therefore try to fit the distribution of the stars with both the empirical Schmidt-Kaler (1982) ZAMS and with several isochrones from Girardi et al. (2000) by conservatively assuming that the metal content is solar. A possible relationship might exist between some MS stars and the brightest star (star #1). This is the star HD 287125 with spectral type G5 and it could be a giant cluster member, but its parallax value (see Table 5) definitely rules out this possibility, and we are left with a nearby foreground star. The shape of the upper part of the CMDs seems to suggest that the turn-off point (TO) is located at about V = 13.2, B-V = 0.6, and that the stars rightwards of the TO are sub-giant stars. On this basis we will consider two stars as cluster members (m), four more as probable ones (pm) and a few bright ones were identified as non-members (nm).
Figure 13 shows the CMDs corresponding to the cluster area, and to a ring around
it adopted as a comparison field (see Fig. 11). As in Fig. 8 for NGC 1582, radii
were selected in such a way that both diagrams in Fig. 14 represent equal sky
areas. By comparing both diagrams we can see a notable over-density of bright
stars in the cluster region, which would favor the idea that NGC 1663 is a
physical cluster.
Name |
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Morphology | V | B-V | J |
2MASS ID |
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U-B | H | |
V-I | K | ||||
Galaxy 1 | 04:49:25.7 | E3 | 16.45 | 1.50 | 14.52 |
J0449257+1308 | 13:08:25.0 |
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1.08 | 13.76 | |
1.75 | 13.49 | ||||
Galaxy 2 | 04:49:29.9 | S0 | 17.38 | 0.98 | 16.33 |
J0449299+1308 | 13:08:15.4 |
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0.92 | 15.85 | |
1.56 | 14.57 | ||||
Galaxy 3 | 04:49:09.5 | E0 | 16.67 | 1.55 | 14.82 |
J0449095+1313 | 13:13:03.0 |
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1.09 | 13.99 | |
1.81 | 13.54 |
As discussed in the previous section, Fig. 13 allows us to derive a rough
estimate of the distance and the age of NGC 1663. These diagrams yield a
cluster distance of about 700 pc (
)
and an age of
about 2000 Myr (
). We also tried other combinations of
distance moduli and color excesses over the CMDs, in order to check whether the
reddest stars could be cluster members, but the obtained solutions did not
agree at all with the corresponding CCDs of Fig. 12.
In conclusion, if NGC 1663 really is a star cluster, we may be facing a dissolving aggregate of the kind proposed by Bica et al. (2001).
Since NGC 1663 is located well above the Galactic plane, it is not very improbable that we should observe field galaxies toward its direction. In our case three of them are clearly detectable and we notice that they were not catalogued so far, except for entries in the 2MASS catalog as point sources. Therefore, we compute their integrated photometric parameters by performing aperture photometry. We use the PHOT task at increasing radius until the resulting magnitudes converged. Our results are presented in the photometric diagrams and in Table 6 together with an estimate of their angular sizes and a preliminary morphological classification that - by the way - agrees quite well with the observed colors.
We have presented the first CCD multicolor study in the regions of the two poorly known northern open clusters NGC 1582 and NGC 1663 for which no investigations had been carried out insofar. In the case of NGC 1582 we also obtained Echelle spectra of the brightest stars. In detail, we found that:
This article is partially based on the Second Generation Digitized Sky Survey that was produced at the Space Telescope Science Institute under US government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions. This study has also made use of: a) the SIMBAD database, operated at CDS, Strasbourg, France, and b) the data from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by NASA and NSF
Acknowledgements
The authors acknowledge the Asiago Observatory staff for the technical assistance. Fruitful discussion with Roberto Barbon, Ruggero Stagni, Corrado Boeche and Silvano Desidera are also warmly acknowledged. We thank the anonymous referee for the detailed report which helped to significantly improve the paper presentation. The work of GB is supported by Padova University through a postdoctoral grant.