A&A 404, 621-629 (2003)
DOI: 10.1051/0004-6361:20030500
H. Jin1 - S.-L. Kim1 - S.-G. Kwon1 - J.-H. Youn1 - C.-U. Lee1 - D.-J. Lee1 - K.-S. Kim2
1 - Korea Astronomy Observatory, Daejeon 305-348, Korea
2 - Department of Astronomy and Space Science, Kyunghee University, Yongin 449-701, Korea
Received 31 January 2003 / Accepted 21 March 2003
Abstract
We present multi-passband CCD photometry of 20 ROTSE-I
Scuti type
pulsating stars and 1 RR Lyrae star to re-classify their variable
types using the comparison of amplitudes between V and Ipassbands. For the re-classification, we used a criterion that
pulsating stars have larger amplitude differences between
passbands than eclipsing binaries because brightness changes of
pulsating stars are mainly due to the temperature variations. As a
result, only six stars were re-confirmed as
Scuti
variables and thirteen stars turned out to be W UMa type eclipsing
binaries. The other two stars were identified as one cataclysmic
variable and one non-variable, respectively. Our results suggest
that a number of ROTSE-I
Scuti type stars, which do not
show typical pulsating light curves of high amplitude
Scuti stars, are W UMa type eclipsing binaries.
Key words: stars: variables:
Sct - stars: binaries: eclipsing - techniques:
photometric
The Robotic Optical Transient Search Experiment (ROTSE), a famous
gamma-ray burst observing group, discovered a number of new
variable stars using the ROTSE-I instruments (Akerlof et al.
2000). Among their 1781 periodic variable stars, they identified
a total number of 91
Scuti type pulsating stars. They classified
their variable types according to the periods and ratios of the
Fourier coefficients. However, W UMa type eclipsing binaries have
no restrictions for periods and show sinusoidal light curves,
which are similar to those of RRc and
Scuti type
pulsating stars. The ratios of the Fourier coefficients for W UMa
stars tend to overlap with RRc and
Scuti stars.
Therefore, Akerlof et al. (2000) applied another criterion, sign
of the greatest deviation. They proposed that the greatest
deviation is fainter than mean magnitudes for eclipsing binaries,
on the other hand, it tends to be brighter than the mean for
pulsating stars.
Table 1: Observation log of selected variable stars.
From the visual inspections of light curves for ROTSE-I
Scuti stars, we found that several stars appear to have very broad
maximum brightness. This tendency is not typical for high
amplitude
Scuti stars (hereafter, HADS) which are defined
as
Scuti stars with V amplitudes larger than 0
3. The HADS are known to be excited in one or two radial modes (Breger
2000). As shown in the OGLE database, the Fourier decomposition
parameters of the HADS are quite similar to those of RRab type
pulsating stars (Poretti 2001). Most of the HADS show asymmetric
light curves with fast increase and slow decrease. On the other
hand, small amplitude
Scuti stars show sinusoidal
symmetric light curves.
Recently, Branicki & Pigulski (2002) found that V357 Her, a
previously known HADS, seemed to be a W UMa type eclipsing binary.
In addition, TU UMi was discovered as a variable star by the Hipparcos satellite (ESA 1997) and then catalogued as a Scuti star by Kazarovets et al. (1999), but later, turned out to
be a W UMa-type eclipsing binary by Rolland et al. (2002).
In this paper we analyze new photometric data of 20 ROTSE-I
Scuti stars with two passbands in order to re-classify
their variable types. In contrast with eclipsing binaries,
pulsating stars have large amplitude differences between passbands
because their brightness changes are mainly due to the temperature
variations. We introduce the observation and data reduction of 20
Scuti stars and 1 RRa star classified by the ROTSE-I
group in Sect. 2. In Sect. 3, we present our V, I photometric
results and in Sect. 4, we discuss on the classification method of
variable stars discovered in photometric survey projects.
CCD photometric observations of 21 ROTSE-I variable stars were
carried out for 25 nights from March to November, 2002. There was
no special criterion to choose the 21 variable stars among the 91
ROTSE-I
Scuti stars, except for a few stars which showed
well defined light curves of the HADS. Instead we selected the
observing targets to have the best observing coordinates during
our available observation time. ROTSE-I
Scuti data were
collected from their database which has made the data publicly
available in their web
site
. One of the 21 targets, ROTSE J163117.94+115952.4
= DY Her, was classified as a RRa variable by the ROTSE-I group
but also as a HADS by others (Hoffmeister et al. 1985;
Rodríguez & Breger 2001).
We obtained time series CCD images with a SITe 2K CCD camera
attached to a 61 cm telescope at the Sobaeksan Optical Astronomy
Observatory (SOAO) in Korea. The field of view of a CCD image is
about
arcmin2 on the f/13.5 Cassegrain focus of the
telescope.
The CCD chip was cooled down to -110 using liquid nitrogen so
that there were negligible dark currents in our images. In order
to compare amplitudes between two passbands, the observations were
performed with V and I filters. The exposure times was
adjusted from 20 to 150 s depending on brightness of
variable stars and atmospheric seeing condition.
The CCD frames were automatically obtained using the observation
software SOAO (Kim et al. 2001), which was optimized for the
differential photometry of variable stars. Typical atmospheric
seeing was about 2.5 arcsec. Using the IRAF/CCDRED package, we
processed the CCD images to subtract bias frames and correct the
pixel-to-pixel inhomogeneity of quantum efficiency, flat fielding.
Instrumental magnitudes were obtained via the simple aperture
photometric routine in the IRAF/APPHOT package. The aperture
radius was chosen to be 7.2 arcsec. Then, we calculated
differential magnitudes of each variable star using a nearby
comparison star with similar brightness and color to the variable.
Detailed observation log is listed in Table 1.
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Figure 1:
Light curves of six ![]() |
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Figure 2: Light curves re-classified as W UMa type eclipsing binary stars. The data symbols are the same as Fig. 1. |
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Figure 2: continued. |
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Figure 2: continued. |
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Figure 1 shows the phase diagrams of 6 HADS (including the known variables, YZ Boo and DY Her) which have been re-confirmed in the present work. These diagrams are made with the use of the ROTSE-I data, our V and I magnitudes, and (V-I) colors. The mean magnitudes of each data set are expressed in arbitrary scales. The observed light curves are quite similar to typical ones of HADS (e.g. Hintz & Joner 1997), implying that our identification is reasonable. Phase diagrams of 13 W UMa type stars are shown in Fig. 2. The periods of eclipsing binaries are about two times larger than the values estimated by the ROTSE-I group. Their (V-I) color light curves does not show any remarkable changes with the maximum variations of (V-I) color less than 20% of their magnitude variations. The light curves are also different from those of typical HADS but quite similar to those of W UMa type eclipsing variables (Hoffmeister et al. 1985).
Figure 3 represents the phase diagrams of two other variables, ROTSE J181613.28+495205.0 and ROTSE J191516.94+475914.5. The former object is a AM Her, strongly magnetic cataclysmic variable. It does not have any systematic variation phases (de Martino et al. 2002). It is the reason that ROTSE phase diagram shows a quite different light curve in comparison with our result. The latter object did not show any noticeable light variations in our observations even though the ROTSE-I reported systematic variations. In addition, we could not find any variable stars around this object within our observation field of view. The complete data sets of Figs. 1-3 are listed in Table 3.
Figure 4 displays the amplitude differences of eclipsing binaries
as well as pulsating stars between two passbands, V and I. The
slopes of
against
are about 0.92 for thirteen
W UMa stars but 0.59 for six HADS. This large difference indicates
that the light variation mechanism of pulsating stars is quite
different from that of eclipsing variables. That is, pulsating
stars have much larger amplitude differences between passbands
than eclipsing binaries because their brightness changes are
mainly due to the temperature variations. The amplitudes of
pulsating stars decrease rapidly in the regime of the infrared
passband. Figure 5 shows the relationship between V-amplitudes
and periods for
Scuti stars. As seen in the figure, the
relationships of our re-confirmed six HADS are consistent with
those of previously known
Scuti stars (Rodríguez et al. 2000).
Figure 6 displays the color-magnitude diagram, (B-R) versus R,
to show difference of color distributions between pulsating and
eclipsing stars. The left panel was made from the observation
results of Everett et al. (2002) with 222 variable stars. The
right panel shows the diagram for 91 ROTSE-I
Scuti stars
whose B and R magnitudes were obtained from the USNO
catalogue. Although it does not show the difference clearly due to
the reddening effect, it should be noted that the pulsating stars
concentrate in a small region of bluer (B-R) color indexes. We
suggest a possibility that most of the objects with (B-R) color
larger than 1.1 are eclipsing variables.
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Figure 3: Peculiar light curves of two variable stars. (Left) ROTSE J181613.28+495205.0 is a AM Her, strongly magnetic cataclysmic variable. (Right) ROTSE J191516.94+475914.5 does not show any noticeable light variations in our observations. |
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Figure 4:
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Figure 5:
V-amplitudes versus periods diagram of ![]() ![]() ![]() |
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From the comparison of amplitudes between two passbands, we have
re-confirmed only six
Scuti stars out of our 21 target
stars. They are qualified to include in the catalogue of
Scuti stars (e.g., Rodriguez et al. 2000). Our results show that
other 13 stars are not
Scuti variables but W UMa type
eclipsing binaries.
We have found that a number of ROTSE-I
Scuti type stars,
which do not show typical pulsating light curves of the HADS such
as fast increasing, slow decreasing and sharp maximum brightness,
seem to be W UMa type eclipsing binaries. This fact implies that
previous ROTSE-I classifications of variable types to the HADS are
partially wrong. We suggest that the classifying methods of survey
data should be more precise by adding more criteria, specifically
to discriminate between the HADS and W UMa stars.
Blake et al. (2003) studied the period changes of 18 ROTSE-I
Scuti stars by comparing their result with ROTSE-I data.
Four of their objects are the same with this study. According to
our results, three objects are W UMa eclipsing variables and the
other one is a
Scuti star, YZ Boo. Even though they
reported the period change and identified as a new pre-main
sequence
Scuti star for ROTSE 4861 (J173937.28+501202.0),
our results show this star to be an eclipsing variable star.
Table 2:
Observation summary of 20
Scuti type
variable stars and 1 RRa in ROTSE-I field. Last two columns
represent the identifications of ROTSE-I group and ours.
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Figure 6:
Difference of color distributions between pulsating and eclipsing stars.
(Left) (B-R) versus R diagram using the observational data of Everett et al. (2002).
Filled circles are EA/EW stars, open circles are SX Phe/![]() ![]() |
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Acknowledgements
We appreciate the referee Dr. T. Arentoft's valuable comments. We thank Dr. Yong-Jae Moon for his careful readings. This work has been supported by the basic research funds of the Korea Astronomy Observatory. This research made use of the SIMBAD database, operated at CDS, Strasbourg, France.