A. Gáspár1,5 - L. L. Kiss2,5, -
T. R. Bedding2 - A. Derekas2,5 - S. Kaspi6
- Cs. Kiss4 - K. Sárneczky3,5 - Gy. M. Szabó1 -
M. Váradi1,5
1 - Department of Experimental Physics and Astronomical Observatory,
University of Szeged,
Szeged, Dóm Tér 9, 6720, Hungary
2 - School of Physics, University of Sydney 2006, Australia
3 - Astronomical Observatory, Szeged, Hungary
4 - Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67,
1525 Budapest, Hungary
5 - Guest Observer at Konkoly Observatory
6 - School of Physics and Astronomy and the Wise Observatory,
Tel-Aviv University, Tel-Aviv 69978, Israel
Received 1 April 2003 / Accepted 18 August 2003
Abstract
We present the first CCD photometric observations of the northern
open cluster NGC 2126. Data were taken on eight nights in February and December
2002 with a total time span of 57 hours. Almost 1000 individual V-band
frames were examined to find short-period variable stars. We discovered six new
variable stars, of which one is a promising candidate for an
eclipsing binary with a pulsating component. Two stars were classified
as
Scuti stars and one as Algol-type eclipsing binary. Two stars are
slow variables with ambiguous classification. From absolute
photometry we
have estimated the main characteristics of the cluster:
and
kpc. Cluster membership
is suggested for three variable stars from their positions on the
colour-magnitude diagram.
Key words: open cluster and associations: general - open clusters
and associations: individual: NGC 2126 - stars: variables: general -
Sct
Variable stars in clusters are crucial tools of stellar astrophysics:
the application of stellar evolutionary theories via isochrone fitting
of the colour-magnitude diagrams yields temperature, luminosity and
age values for the member stars, which in turn draw strong constraints
on pulsational properties (e.g. Frandsen & Arentoft 1998) or
binary evolution (Rucinski et al. 1996). Early research of
Scuti
pulsation in open clusters (Breger 1972) and its revival with the wide
application of the CCD technique (for a review of the current state see
Rodríguez & Breger 2001) have recently been extended toward
the new class of
Dor variables (Handler 1999), which have been
found in a number of young and intermediate-age open clusters (Arentoft et al.
2001; Kim et al. 2001; Choo et al. 2003 and references therein). Clusters
of different parameters can help map out the dependence of pulsational
properties on age and metallicity, thus allowing better understanding of
physical mechanisms driving the pulsation. On the other hand, detached
eclipsing double-lined spectroscopic binaries can serve as very accurate
distance indicators (e.g. Thompson et al. 2001), with the potential of
feeding back to the isochrone fitting method itself.
This work aims to contribute to these issues with the
first CCD observations of the northern open cluster NGC 2126, with particular
emphasis on its variable star content.
NGC 2126 (=C0559+499,
02
55,
52
,
Trümpler class III 2 m:b) is a
moderately rich typical galactic cluster with several dozens of members
scattered in a region of 5-6
(Lyngå 1987) in the constellation
Auriga. The only previous photometric study was presented by Cuffey
(1943), who made blue and red (
4300-6200) photographic
observations. He estimated the distance to NGC 2126 as 950 pc, based on a
comparison to M 35. The WEBDA catalog
reflects this lack of data, as no other source is
listed for NGC 2126.
This neglect turned our attention toward NGC 2126 and
the present study presents an analysis of 8 nights of observations
obtained in 2002. The measurements and data reduction are described in
Sect. 2. Cluster parameters
and the six new variable stars are discussed in Sect. 3, and a brief
summary is given in Sect. 4.
CCD
observations were carried out on five consecutive nights in February 2002
and three nights in December 2002.
We used the 60/90/180 cm Schmidt telescope mounted at the Piszkésteto
Station of the Konkoly Observatory. The detector was a Photometrics AT-200 CCD
camera (
1024 pixels, KAF-1600 chip with UV-coating). The image scale
was 1
1/pixel, giving a 29
field of view.
The observations consisted of V-band time-series observations
(mostly 180 s exposures) and
calibrated
photometric
observations (from 60 s to 600 s exposures).
The full observing log is presented in Table 1. The observed field
and the new variable stars are shown in Fig. 1.
Table 1: The journal of observations.
The image reduction was done with standard tasks in IRAF. For flat field
corrections, we used sky flat images taken during the evening and morning
twilights. We performed psf-photometry with the daophot package in IRAF
using the Moffatt point-spread function, which was found to be well-suited for
fitting the slightly distorted stellar profiles of the used instrument
(Kiss et al. 2001).
For photometric calibrations, we observed standard stars in the open
cluster M 67 from the list of Chevalier & Ilovaisky (1991). The instrumental
magnitudes were transformed with the photcal package, according to
the following standard transformation equations:
![]() |
(1) | ||
![]() |
(2) | ||
![]() |
(3) |
The transformation of the cluster was done in two steps. First, we
calibrated non-saturated stars on three images (one for each filter) confined
by consecutive M 67 observations on February 2, 2002. Then we co-added a number of
V band images and all
frames and measured fainter stars
relative to previously calibrated local standards. We estimate the final
photometric error as
0
05, which is likely to be
an overestimate for the brightest stars.
![]() |
Figure 1:
The observed CCD field (29
![]() ![]() |
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The light curves were further analysed to search for possible periodicities. For this, we calculated Fourier spectra with Period98 (Sperl 1998). For the eclipsing binaries, the phase dispersion minimisation (PDM, Stellingwerf 1978) provided a better period searching method. Our original data are available electronically at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/879
![]() |
Figure 2:
The colour-magnitude diagrams of NGC 2126. Top row: full field of view;
bottom row: the central field of 8![]() ![]() |
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The physical parameters of the cluster were estimated by fitting isochrones to the colour-magnitude diagram (CMD). In order to separate the cluster's stars from other field stars, we have examined proper motions taken from the USNO B1.0 catalog (Monet et al. 2003). We found that of the 800 stars, almost 200 have proper motions detected in the USNO plate material. However, those stars are evenly distributed over the field with no concentration around the cluster, so that NGC 2126 turned out to be in the background. Consequently, we removed all stars from the CMD with non-zero proper motions and the resulting CMDs are shown in the top row of Fig. 2. This procedure has been checked by two nearby galactic fields, offset by about 1 degree north and south. A comparison of proper motion histograms showed that we have indeed selected the galactic foreground.
In order to decrease background contamination, we kept only the inner 8of the cluster for the isochrone fitting (bottom row in Fig. 2). This
region contains 103 stars between
,
of which the majority seems
to be easily distinguishable from the background and some weak foreground.
We assumed solar composition
and included reddening determination in the fitting procedure. The isochrones
were taken from Bertelli et al. (1994) and shifted individually to match the
main sequence, turn-off point and red giant positions. We found that the overall
shape of the CMD is well reproduced with
isochrones of
ranging from 9.0 to 9.3, E(V-I) between 0
53 and
0
33, E(V-R) between 0
14 and 0
03 and distance moduli between 11
5 and
10
4. We plot the
isochrone in Fig. 2, showing the
"best'' fit, but the difference for the other isochrones is almost negligible.
Changing the composition makes the parameter ranges even wider, thus the
estimated parameter errors are quite large and a further spectroscopic study
addressing spectral types and chemical composition is desirable.
In this paper we adopt the following physical parameters:
,
,
.
The reddenings were converted to
E(B-V) using the coefficients listed in Rieke & Lebofsky (1985):
E(V-R)/E(B-V)=0.78,
E(V-I)/E(B-V)=1.60. The result is
,
and the distance to the cluster
(assuming
)
is
kpc.
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Figure 3: Individual light curves of the six new variable stars. The solid lines denote Fourier fits for V3 and V5. |
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Nightly light curves of the new variable stars are shown in Fig. 3 (numbered in order of increasing right ascension). Differential light curve data were shifted so that the mean values match the apparent magnitudes of the stars in the CMD. Therefore, the absolute V magnitudes in Fig. 3 are somewhat uncertain, depending on the magnitude range of the variables.
The light curve
shapes suggest the following classifications: V1 and V2 are slow variables of
ambiguous nature; V3 and V5 are short-period variables, most
likely of
Scuti type; V4 is an Algol-type eclipsing binary with only
one observed minimum, while the light curve
of V6 is a mixture of
Scuti-like oscillations and Algol-like eclipses.
Although these two stars showed clear variability, our dataset is too short to determine reliable periods. For V1, both PDM and Fourier analysis suggested P=1.64470 d or half that value, 0.82235 d (the uncertainties are a few in the last digit). We plot the resulting phase diagrams in Fig. 4 (the epoch of maximum is HJD 2 452 307.48).
With no colour light curve, the classification is uncertain.
For 0.822235 d, one can speculate on possible
Dor-like oscillations.
These stars are early F-type main-sequence or subgiant stars with periods
0.4-3.0 d and V-band amplitudes less than 0
1, pulsating in
non-radial gravity modes. According to the definition by Handler (1999),
the
Dor instability strip covers the 7200-7700 K range on the
zero-age main sequence (ZAMS) and the 6900-7500 K range one magnitude
above the ZAMS. Assuming cluster membership, the dereddened colours of V1 are
and
.
Standard tabulations
give an effective temperature of 6500-7000 K (Cox 2000), which
marginally supports
the classification as a
Dor star. Also, the cluster
membership would imply an absolute magnitude
;
the temperature and absolute magnitudes are consistent with
the coolest
Dor variables.
In this case, the apparent amplitude modulation is due to the presence
of other excited modes of pulsation and a proper asteroseismological analysis would
require a much longer dataset.
![]() |
Figure 4: Phase diagrams for V1 with two equally possible periods. |
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On the other hand, for
P = 1.6447 d, the double-peaked light curve
is similar to those observed for spotted active binaries of RS CVn
subtype. For instance, recent data collection of Cutispoto et al. (2003)
contains nice examples of very similar double-peaked curves of well-known
active stars (BI Cet, BC Phe). Based on the present dataset, it is impossible
to distinguish between the two classifications. The fact that V1 lies
quite far from the cluster centre (about 10)
weakens the possibility
of membership and consequently makes the temperature and absolute
magnitude estimates less reliable.
For V2, the data can be equally well folded with periods around 0.5 d and 1
d, but neither period gives a continuous phase diagram with no
gap. Therefore, we cannot decide the cause of the low-amplitude variability
of this star (the total range is about 0
08).
We observed only one minimum of the star (at HJD 2 452 308.387),
so that no firm conclusion can
be drawn on its period. However, if we plot only the February subset in one
light curve (Fig. 5), the shape suggests a clearly
visible reflection effect (about 0
1).
Assuming symmetry around the unobserved secondary
minimum, a tentative period of
3 days can be estimated (i.e. the next
primary minimum happened immediately before or after the last night of
observations in February).
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Figure 5: The 2002 February data for V4. |
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Both stars showed rapid oscillations with full amplitudes of a few tens
of millimags, characteristic of
Scuti variables. We have determined
the pulsational frequencies with Fourier analysis consisting of iterative
prewhitening steps (see Fig. 6).
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Figure 6: Fourier analysis of V3 and V5. |
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We could identify two frequencies
in both light curves (see Table 2) with S/N ratios (Breger et al. 1993)
larger than 4. For V5, the low-frequency component refers to
slight night-to-night variations of the mean brightness level, which we suspect
were caused by
a closely separated field star (see Fig. 1) lying
from V5. It is fainter by about 4 mag, so that
it may affect profile fitting to some small extent. Nightly
variations of the seeing may also introduce variable contribution
from this star.
In summary, V3 and V5 are probably multiply periodic pulsating
Scuti variables. The frequency
ratios (V3:
f1/f2=0.81, V5:
f1/f2=0.94) suggest
non-radial modes of pulsation for both stars. Small deviations from the
light curve
fits (Fig. 3) and residual structures of the
prewhitened spectra (especially for V3) suggest the probable existence of more
frequencies. Assuming their cluster membership implies
the following dereddened colours and absolute magnitudes:
V3 --
,
,
;
V5 --
,
,
.
The latter values are consistent with typical parameters
of
Sct stars (Rodríguez & Breger 2001),
we therefore conclude V5 is likely to be a member of the cluster.
Moreover, its
proximity to the cluster centre supports this conclusion.
On the other hand, V3 seems to be a background object, since its absolute
magnitude, assuming cluster membership, places it well below the main
sequence.
Table 2: The results of the period analyses.
The most intriguing star is V6, which shows complex light variations.
We observed well-defined minima - three deeper and two shallower -
characteristic of an eclipsing
binary with components of different temperatures. Moreover, the star showed
steady oscillations outside eclipses, with amplitude and cycle length
characteristic of
Scuti-like pulsation (about 0
05 and 0.13 d).
The symmetric stellar profile, even on the best CCD images, excluded the
possibility of two unresolved stars, so that we conclude V6 is an
eclipsing binary with at least one pulsating component. Such systems
are spectacular targets for asteroseismology, because independent
determination of the physical parameters (mass, radius, temperature), applying
binary star astrophysics, gives strong constraints on the possible mode
identification (e.g. Mkrtichian et al. 2002; Kim et al. 2002a).
In addition, eclipses of a pulsating star give spatial resolution across
the stellar surface and thereby provide the possibility for mode
identification (e.g. Reed et al. 2002).
The period analysis resulted in
d. The data were folded
with this period and the phase diagram is shown in Fig. 7. The
depth is 0
2 and 0
12 for the primary and secondary minima, respectively.
The oscillations are clearly visible outside minima and we note
that the scatter of the phase diagram after folding only the February data
was fairly small. This indicated that the oscillations were quite coherent
within 5 consecutive nights in February. Their amplitude was about 0
05
associated with a period
d.
![]() |
Figure 7: Phase diagram for V6. |
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Figure 8:
Outside-eclipse data folded with
![]() |
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We also performed a separate period analysis of data from which eclipse
minima were excluded. This showed the oscillations to be surprisingly
stable. They have a period
d and the phase diagram
shows remarkable small scatter (Fig. 8). The subsequent
prewhitening steps indicated a low-frequency component, approximately
2/
,
and
.
Another
interesting result is that
,
suggesting that there might be a 1:9 resonance between the
orbital motion and pulsation.
We have also observed V6 with the 40
telescope at
the Tel-Aviv University Wise Observatory on 2003 April 10. The spectrograph
10
long-slit was aligned at PA = 133.8
in order to
maximize the separation between the
V6 spectrum and the nearby objects residing south-east to it. In this
way, we minimized the possibility of light from the nearby objects
contaminating the V6 spectrum. The spectrum shows prominent hydrogen absorption
lines accompanied by the sodium D-doublet (Fig. 9). The spectral
lines and the continuum shape suggest an F-type star, typical for a
Scuti star, so that the oscillations may be attributed to
Scuti-like
pulsations.
Out of the six variables, only V6 has
non-zero proper motion (
mas/year,
mas/year), thus cluster membership can be excluded.
In the literature, only a few similar system are reported, making V6
a very interesting variable star. Rodríguez & Breger (2001) listed
nine
Scuti variables in
eclipsing binaries, of which RZ Cas (Ohshima et al. 2001) and AB Cas
(Rodríguez et al. 1998) are the best studied. Compared to the known
examples, V6 is different in two important respects: i) the relative
amplitude ratio (
)
is
much smaller than that of any other similar stars (the next is V577 Oph,
with a ratio of 13); ii) the period ratio (
)
is much smaller than usual - from Table 3 in Rodríguez &
Breger (2001), only WX Eri has a ratio smaller than 20 (5.005).
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Figure 9: A low-resolution spectrum obtained on April 10, 2003. |
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Figure 10: Variable stars on the colour-magnitude diagram excluding foreground stars (except V6). |
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Table 3: The basic data of new variable stars. Except V6, coordinates and identifications were taken from the Guide Star Catalog, Version 2.2 (STSci 2001).
This paper presents the first CCD photometric study of the northern open cluster
NGC 2126. The
observations revealed the main characteristics
of the cluster and led to the discovery of six new variable stars.
The locations of the variables on the CMD (Fig. 10) suggest
membership for V1, V4 and V5; V2 and V3 are most likely non-member stars,
while V6 is a foreground object. The most important results of this
research are the following:
We summarize the basic data of the variable stars in Table 3.
Acknowledgements
This work has been supported by the FKFP Grant 0010/2001, OTKA Grants #F043203, #T032258 and the Australian Research Council. Astronomy at the Wise Observatory is supported by grants from the Israeli Academy of Sciences. The NASA ADS Abstract Service was used to access data and references. This research has made use of the SIMBAD database, operated at CDS-Strasbourg, France.