A&A 419, 149-159 (2004)
DOI: 10.1051/0004-6361:20035642
S. Villanova1 - G. Baume1,2 - G. Carraro1 - A. Geminale1
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 7 November 2003 / Accepted 30 January 2004
Abstract
We present and discuss broad band CCD UBV
photometry and low resolution spectroscopy for stars in the region of the open cluster NGC 6996, located in the North
America Nebula. The new data allow us to tightly constrain the
basic properties of this object. We revise the cluster
size, which in the past has been significantly underestimated.
The width of the Main Sequence is mainly
interpreted in terms of differential reddening, and indeed the stars'
color excess EB-V ranges from 0.43 to 0.65, implying the presence
of a significant and evenly distributed dust component.
We cross-correlate our optical photometry with near infrared photometry from 2MASS,
and by means of spectral classification we are able to build
extinction curves for an handful of bright members. We find that the
reddening slope and the total to selective absorption ratio RVtoward NGC 6996 are anomalous. Moreover
the reddening-corrected colors and magnitudes allow us to derive
estimates for the cluster distance and age, which turn out to be
pc (
)
and
350 Myr,
respectively. Based on our results, we suggest that NGC 6996 is located in
front of the North America Nebula, and does not seem to have any apparent
relationship with it.
Key words: Galaxy: open clusters and associations: individual: NGC 6996 - Galaxy: open clusters and associations: general
Galactic open clusters are fundamental tools to probe the global properties and evolution of the Galactic disk. The determination of their fundamental parameters (age, distance, reddening pattern, size) allows us not only to better understand the properties and evolution of the Galactic open cluster system as a whole, but in many cases to put constrains on large structures like super-clusters, complexes, HII regions and molecular clouds systems, which they might be part of or not.
This is the case of NGC 6996 = C2054+444
(
,
), which is
believed to lie
in front of the North America Nebula (NGC 7000) HII region,
near its western edge (see Fig. 1), where
an important dust cloud separates this nebula from the nearby
Pelican Nebula.
NGC 6996 is a sparse and moderately young open cluster (108 Myr,
Zdanavicius & Straizys 1990).
However, its basic parameters, in particular distance and reddening,
are not well known, and this was the main motivation driving
this study.
Previous investigations carried out in this area include the photographic studies by Muller (1936) and Barkhatova (1958). Later, a more detailed study was conducted by Zdanavicius & Straizys (1990), who obtained photoelectric measurements of several bright stars in the region of NGC 6996 in the Vilnius system. More recently, Subramaniam et al. (1995) catalogued this cluster as a probable binary together with the twin cluster Collinder 428. This latter object is located in the opposite (the eastern) border of the North America Nebula, and indeed both clusters might be related to the HII region. The entire area belongs to the Cygnus region, which has been studied rather intensively.
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Figure 1: A map of the North America Nebula region. The region of NGC 6996 covered by our study is marked by a dashed square (see also Fig. 3). North is up, East to the left. |
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This region (from
to
)
has the singular property that stars present an
anomalous interstellar reddening law, with a larger slope
(see Straizys et al. 1999 and references therein).
In particular, the visual absorption distribution
in this region was studied by Goudis & White (1979) with the H
surface brightness technique, by Bally & Scoville (1980)
by using 12CO observations and, finally, by
Cambrésy et al. (2002) from 2MASS data. These latter authors discovered some new possible star
clusters toward this area, which might be physical objects and as a
consequence deserve further investigation.
To derive better estimates of the NGC 6996 basic parameters
(reddening, distance and age), to study the reddening law in
this region and to understand whether some
relation exists between the cluster and the North America Nebula, we performed
CCD UBV
photometry covering most of the cluster region,
and complement it with
low resolution spectroscopic observation of bright stars in this region.
Our data were also
cross-correlated with near infrared (JHK
)
data from the 2MASS catalogue and with
astrometric information available from the Tycho-2 catalogue
(Høg et al. 2000) for the brightest stars.
The layout of the paper is as follows. In Sect. 2 we describe our observations and the reduction procedure. In Sect. 3 we present the data analysis. Section 4 deals with the cluster basic parameters determination. Section 5 is devoted to a brief discussion of the outcome of this study.
Table 1: Journal of observations of NGC 6996 and standard star fields together with calibration coefficients.
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Figure 2: DAOPHOT errors in the colour indexes and V magnitude as a function of V. |
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CCD UBV
data were obtained during three observational runs. Two of them
were carried out with the AFOSC camera at the 1.82 m Copernico telescope of Cima
Ekar (Asiago, Italy), on the photometric nights of November 8, 2002 and September
18, 2003. AFOSC samples a
field in a
nitrogen-cooled thinned CCD and this camera was used to take a
central and a south frame in the cluster area. The other run was conducted at
Teramo Observatory (Italy) using a
CCD onboard the 0.72 m
Teramo Normale Telescope (TNT). Teramo observations were used to complement Asiago ones
since two peripheral and less deep fields were taken during a non-photometric
night. Figures 1 and 3 show the finding charts of the covered area.
Details of the observations are listed in Table 1, where the observed fields are
reported together with the exposure times, the typical seeing values and the
air-masses. The data were reduced with the
IRAF
packages CCDRED, DAOPHOT, and PHOTCAL using the point spread function (PSF)
method (Stetson 1987) for the frame obtained on 8/11/02 (Asiago) and using only
aperture photometry for the others. The calibration equations obtained by
observing Landolt (1992) PG 0231+051 and PG 2213-006 fields at the Asiago
Observatory, 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) |
The photometric data for some of the brightest stars in the region of NGC 6996 are shown in Table 2. The full table is only available electronically at CDS.
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Figure 3:
Finding chart of the NGC 6996 region (V filter). The black solid circle
indicates the adopted angular size (radius =
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Figure 4: Spectra of some bright stars in the field of NGC 6996. A few interesting lines are indicated. See Table 2 for details. Numbers correspond to the Zdanavicius & Straizys (1990) identification. |
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Spectroscopic observations were carried out with the AFOSC camera at the 1.82 m
Copernico telescope of Cima Ekar (Asiago, Italy), on nights of
July 21, September 18 and October 1, 2003.
The instrument was used in low dispersion mode (R=600), the grism
was chosen to have a large spectral coverage (3500-7000 A) and the exposure
time ranged from 10 to 30 min, according to the brightness of the stars.
The data has been reduced with the IRAF package for
one-dimensional spectroscopy CTIOSLIT and by using the Hg-Cd lamp spectra
for wavelength calibration purposes.
To have flux-calibration, a few standard spectrophotometric
stars were also observed.
Spectra (see Fig. 4) were classified in two different ways:
To derive an estimate of the cluster angular size, we computed surface stellar
densities at increasing concentric
wide annuli around the
adopted cluster center (
:56:30;
:38:00)
over: a) the corresponding DSS-2
red image, and b) the 2MASS infrared data. The results are shown in Fig. 5,
where the dotted line is the field star density level as derived
from mean star counts in two 2MASS
fields, 20
southward and northward NGC 6996, respectively.
The patchy distribution of dust and gas clearly renders it difficult
to fix a cluster radius. However,
by inspecting both DSS maps and this plot we argue that
the cluster radius is
7
,
the point (dashed vertical line in Fig. 5) at which the
stellar density more clearly reaches the field
level.
Therefore, we adopt this value as angular radius, which turns out
to be almost twice the estimates reported by Lyngå (1987) and Dias et al. (2002)
(
and
in diameter, respectively).
Table 2: Brightest stars in the region of NGC 6996.
Important information on the kinematics and membership of the brightest stars in and
around a star cluster might in principle be derived from the proper motions as
available in the Tycho-2 catalogue (Høg et al. 2000). With this aim in mind, we
collected proper
motion components for 22 stars in a field of
radius centered on
NGC 6996. They are shown as a vector point diagram in Fig. 6. The point
distribution is characterized by a clump of several stars and few others
placed around it. This can be readily interpreted as
indicative of the presence of a star cluster.
However, on the basis of the analysis here below, we found only 3 member stars
with available proper motion from Tycho-2, and therefore we refrain from
any kinematic analysis of NGC 6996 stars.
The color-color diagrams (CCDs) and the color-magnitude diagrams (CMDs) from our
data are shown in Figs. 7 and 8, respectively. In Fig. 9 we present the
corresponding CMDs from 2MASS data for the cluster region stars
(
)
and for a region around it (see caption) that is used as the
comparison field. Some remarkable features in these diagrams are the following
ones:
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Figure 5: Stellar density in the region of NGC 6996 as a function of the radius from 2MASS and DSS-2 red data. The dashed line indicates the adopted limit for the cluster. |
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Figure 6:
Vector point diagram for the stars
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As a first step, we base our membership assignment procedure
on a synoptic analysis of the star
positions in the various photometric diagrams (e.g. Baume 1999, 2003a,b; Carraro 2002).
By inspecting Fig. 7a, we notice that up to
stars are placed on an apparent easily recognizable MS composed of B and later type stars according to a Schmidt-Kaler's (1982) ZAMS. They have also
a compatible position on the CMDs of Figs. 8 and 9 down to
.
However, the considered B-type stars are more dispersed in these diagrams.
As a second step, we combine the magnitudes and colors with our spectral
classification and that given by Zdanavicius & Straizys (1990), when available,
and the result is that most of the brighter stars have very low excess values
or/and low distance modulus, and only A-type stars have acceptable solutions.
Therefore, the stars from the first group were considered as cluster non members
(nm, probably interlopers), whereas the later ones as
likely cluster members (lm). Additionally,
following the ZAMS path along the CCDs and CMDs toward later-than-A spectral types,
we identify other stars with compatible positions on the photometric
diagrams. Here however the stellar contamination by field stars starts to
become important, and therefore we adopt them as probable cluster members (pm). This
membership assignment produces a number of stars in different bins of V
magnitude that is in agreement with the over-excess of stars present in Fig. 9a
when compared to Fig. 9b.
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Figure 7: Color-color diagrams (CCDs) of stars in the region of NGC 6996. a) U-B vs. B-V diagram. Symbols have the following meaning: circles are likely member stars (lm), triangles are probable member stars (pm), white squares are non-members (nm) and small hollow circles are stars without any membership assignment. The solid line is the Schmidt-Kaler's (1982) ZAMS, whereas the dashed lines are the same ZAMS, but shifted by EB-V = 0.43 and 0.65, respectively (see also Fig. 10a). The dashed arrow indicates the reddening path. b) B-V vs. V-I diagram. Symbols as in Fig. 7a. Solid lines are the intrinsic positions for stars of luminosity classes V and III (Cousins 1978a,b). The two dashed lines give the typical excess ratio for the Cygnus region ( EV-I / EB-V = 1.25) and that adopted for the cluster ( EV-I / EB-V = 1.69). |
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Figure 8:
Color-magnitude diagrams (CMDs) for all the stars covered in the
field of NGC 6996. Symbols as in Fig. 7a. The solid line is the Schmidt-Kaler
(1982) empirical ZAMS shifted by the apparent distance modulus
V-MV =
11.2 (
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Figure 9:
CMDs from the 2MASS catalog. Symbols as in Fig. 7a. a) Stars
placed inside the cluster area (
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Stars
6 and 14 (numbering from Zdanavicius & Straizys 1990) deserve
special attention. The former lies below the ZAMS in the CCD of Fig. 7a, and
by inspecting also its position in the CMDs we interpret its color as due
to binarity, the secondary being a cool red star.
The latter is a red star classified as
a K2 III. Looking at its location in the CMDs, also in relation to the superposed
isochrones (see below), we are inclined to consider it a cluster member. Unfortunately, it
has no proper motion measurement (from Tycho-2) and therefore we adopt it as
a probable member (pm).
To infer the intrinsic colors, when spectral classification is available, we
derive individual excesses by using Schmidt-Kaler (1982) relations. Figures 7a and 10b
show that the typical relation
EU-B/EB-V = 0.81 + 0.05 EB-V for the
Cygnus region (Johnson 1965) fits the point distribution much better than
the normal one (
EU-B/EB-V = 0.72). We use then the former relation to
obtain additional individual excesses from the CCD of Fig. 7a for stars
without spectral classification. Then by considering the adopted likely members and probable
members with V < 14, we found a mean excesses of
and
.
The mean values are then adopted as representative of the
cluster color excess, whilst the lowest value is interpreted as the foreground
color excess.
The reddening law in the direction of NGC 6996 appears to
be anomalous, and therefore we proceeded to compute the ratio
RV = AV/EB-V
by using the Excesses Relation Method (Fig. 10c), the Variable Extinction
Method (Fig. 10d) and the Color Difference Method (Fig. 11).
To build
Figs. 10c and 10d, we compute individual EB-V, EV-I and V-MV values by means of
the Schmidt-Kaler (1982) and Cousins (1978a,b) calibration relations, a method
also applied in Tr 14 (Vázquez et al. 1996) and in NGC 3293 (Baume et al. 2003).
In the case of the Color Difference Method we combined the information from
spectral classification, optical photometry and near infrared one
from the 2MASS. For other applications of this method, see Thé & Graafland (1995)
and Carraro et al. (2003).
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Figure 10:
a) EB-V distribution for stars inside the
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It is clear from Fig. 10c that star positions follow a path different from the
typical one for Cygnus region (
EV-I/EB-V = 1.25) resulting in a higher
RV value. This is not evident in Fig. 10d where there is a larger spread due
probably to the presence of binaries. However, by performing least squares fittings over Figs. 10c and 10d and an extrapolation in Fig. 11, we obtain similar
values (see Table 3). Therefore we adopt RV = 4.1 to compute corrected
magnitudes
for cluster likely and probable members. This
RV value yields a mean cluster absorption
.
It is worth noticing that this estimate is
comparable with the absorption map in this region obtained by Goudis & White
(1979).
Table 3: Computed RV values by using different methods.
The distance of NGC 6996 is derived by superposing the Schmidt-Kaler (1982) ZAMS
onto the reddening-free CMD (Fig. 12). The best ZAMS fitting was achieved for a
distance modulus
(error from inspection). We also apply
the spectroscopic parallax method to 10 likely and probable member stars of
luminosity class V (see Table 3) by using the relation of spectral types and
MV from Schmidt-Kaler (1982). This method yields a value
.
The large dispersion of the last value and its difference from the
ZAMS fit can be ascribed to the presence of binary and somewhat evolved stars included in
the computation. The adopted distance modulus is therefore
which in turn implies that NGC 6996 is located
pc away from the Sun.
For the age of NGC 6996, we superimposed on the CMD (see Fig. 12)
a set of isochrones derived from Girardi et al. (2000)
evolutionary models (computed with solar metallicity, mass loss and overshooting).
The fit wasperformed taking into account only MS stars
and is compatible with an age for the cluster of about
310 Myr (
). By assuming that stars having A0 spectral type are still along the
MS, we derive again an age near to 390 Myr (
)
for this
cluster. Both procedures yield then similar results and we
adopt
Myr as the cluster age.
We have presented the first multicolor CCD photometric study
in the region of the open cluster
NGC 6996 together with spectral classification of some bright stars.
NGC 6996 turns out to be a moderate-age open cluster (350 Myr)
located close to the west edge of NGC 7000, the North America Nebula.
Our analysis places the cluster at a distance of
pc.
Evidence emerges of a significant differential reddening affecting the stars'
positions in all the photometric diagrams.
We also point out that the reddening law has an anomalous
value in the direction of this cluster.
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Figure 11:
The color difference method. a) Solid lines are individual
curves computed for stars with spectral classification and luminosity class V
(see Table 2). b) Solid line is the obtained average curve together
with their extrapolation up to
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Figure 12:
V0 vs. (B-V)0 diagram of the likely and probable member
stars in the region of NGC 6996. Symbols as in Fig. 7a. Solid and dotted lines
are the Schmidt-Kaler (1982) ZAMS shifted by the adopted distance modulus
V0 - MV = 9.4 and the 0.75 envelope limit for binaries. Dashed curves are
the isochrones from Girardi et al. (2000). Numbers give the
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To address the issue of its possible connection with the Nebula,
we first summarize what it is known about the distance of the Nebula itself
(see for reference Fig. 1).
According to Straizys et al. (1999)
the dust cloud that separates the North America and Pelican nebulae is placed at
about 580 pc form the Sun. For the HII region (NGC 7000 itself), the distance is
far from being reasonably contrained.
Different works claim for very different distance
estimates, ranging from 420 pc (Beer 1964) to 1980 pc (Dieter 1967).
HD 199579, an O6 star considered responsible for at least part of the excitation
of the region, is placed at a distance of 1200 pc by Miller (1968) and 830 pc
by Garmany & Stencel (1992), who consider the star a member
of the Cyg OB7 association. According to many other authors
however, the most accepted value seems to be 1 kpc
(Downes & Rinehart 1966; Wendeker 1968; Goudis 1976; Bally
& Scoville 1980).
If we accept this value as the distance of NGC 7000, NGC 6996 turns out to have no apparent relationship to the HII region, the cluster being placed about 300 pc closer to the Sun. The cluster age and the absence of early spectral type stars further corroborate this hypotesis.
The computed RV value for the cluster region is higher than the common one holding for the Galaxy (3.1, Mathis 1990), and is at odds with previous studies (e.g. Cambrésy et al. 2002) which assign to NGC 7000 a normal RV ratio. High RV values are indicative of the presence of dust grain of large size, typically larger than 0.05 micron. Since NGC 6996 is dominated by A type stars, UV radiation is not very effective, and one expects that dust grains grow in size (Kim & Martin 1996). Another possibility would be that small size dust grains have been kicked off the cluster by a presumed population of massive stars that already have died as type II SNæ (McKee 1989), a scenario which is quite compatible with the age of the cluster.
Acknowledgements
The authors acknowledge the Asiago Observatory staff for the technical support and the director of Teramo Observatory for the generous time allocation. The work of G.B. is supported by the Università di Padova (Italy) through a postdoctoral grant.