RECLASSIFICATION OF ROTSE-I SCUTI STARS WITH MULTIBAND PHOTOMETRY AND FOURIER DECOMPOSITION

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1 The Astronomical Journal, 128: , 2004 October # The American Astronomical Society. All rights reserved. Printed in U.S.A. RECLASSIFICATION OF ROTSE-I SCUTI STARS WITH MULTIBAND PHOTOMETRY AND FOURIER DECOMPOSITION H. Jin, S.-L. Kim, C.-U. Lee, and D.-J. Lee Korea Astronomy Observatory, Taejon , Korea; jinho@kao.re.kr, slkim@kao.re.kr and K.-S. Kim Department of Astronomy and Space Science, Kyunghee University, Yongin , Korea; kskim@khu.ac.kr Receivved 2004 April 21; accepted 2004 June 29 ABSTRACT We present new multiband photometric results for 29 ROTSE-I Scuti stars and a Fourier decomposition light-curve analysis in order to reclassify their variability types. For the classification between eclipsing and pulsating stars, we use the criterion that pulsating stars have larger amplitude differences between passbands than eclipsing binaries, because the brightness changes in pulsating stars are mainly due to variations in temperature. From this investigation, we find that 24 of our 29 observation targets are W Ursae Majoris type eclipsing binaries. Fourier analysis of the data for all 91 ROTSE-I Scuti stars shows that the phase parameters 21 of the high-amplitude Scuti variables identified in this study are concentrated around 4.0. This high-probability area characterized by 21 is in good agreement with the finding from OGLE data. Our results suggest that most of the ROTSE-I Scuti stars located outside the high-probability area are in fact W UMa type eclipsing binaries. Key words: binaries: eclipsing methods: data analysis Scuti techniques: photometric 1. INTRODUCTION The Robotic Optical Transient Search Experiment (ROTSE), a famous project to observe gamma-ray bursts, has discovered a number of new variable stars within ROTSE-I observing fields. Akerlof et al. (2000) identified 91 Scuti type pulsating stars among a total of 1781 periodic variable stars. In a previous study (Jin et al. 2003), we showed that a number of ROTSE-I Scuti stars that do not show pulsating light curves typical of high-amplitude Scuti (HADS) stars are actually W Ursae Majoris type eclipsing binaries. For the classification between eclipsing and pulsating stars, we adopted the criterion that pulsating stars have larger amplitude differences between passbands than eclipsing binaries do, because the brightness changes of pulsating stars are mainly due to temperature variations. In this study, we present new multiband photometric results for 29 ROTSE-I Scuti stars in order to reclassify their variability types, as well as the results of a Fourier decomposition analysis of the data for all 91 ROTSE-I Scuti stars. Fourier decomposition of light curves has proved to be a powerful method for the classification of pulsating stars (Morgan 2003; Poretti 2001). Another useful application of Fourier decomposition is in the comparison of theoretical and observed light and radial velocity curves, which can provide clues to the discrepancies between theoretical approximations and observations (Simon & Aikawa 1986; Feuchtinger & Dorfi 1996). We describe our new follow-up observations and the data reduction in x 2. Our V and I photometric results are presented in x 3, and the Fourier decomposition analysis is in x 4. In x 5, we discuss methods of classifying variable stars in photometric survey projects. 2. OBSERVATIONS AND DATA REDUCTION CCD photometric observations were carried out on 31 nights between 2003 March and 2004 May. We obtained time series CCD images through V and I filters with a 1K Apogee CCD 1847 camera attached to the Korea Astronomy Observatory s 1 m automatic telescope on Mount Lemmon, Arizona (Han et al. 2000). The observations in 2004 were made with a 2K CCD camera. The telescope was operated by remote control from Korea via a network connection. The field of view of the 1K CCD image is about 11:2 ; 11:2 arcmin 2 on the f=7.5 Cassegrain focus of the telescope. Since the CCD chip was cooled to 30 C with a thermoelectric cooler, we took several dark frames during our observations for dark noise correction. In order to compare amplitudes between two passbands, the observations were performed with V and I filters. The exposure time ranged from 20 to 150 s depending on the brightness of the variable and atmospheric seeing conditions. Typical seeing, including atmospheric and instrumental effects, was estimated to be about 2B3. Time series CCD frames were automatically obtained using the telescope control system software, which includes a routine to optimize for monitoring variable stars. Using the IRAF CCDRED package, we processed the CCD images to subtract bias and dark frames and corrected pixel-topixel inhomogeneities in quantum efficiency by flat-fielding. Instrumental magnitudes were obtained using the simple aperture photometry routine in the IRAF APPHOT package. The aperture radius was chosen to be We then calculated differential magnitudes for each variable star using a nearby comparison star with similar brightness and color to the variable. A detailed observation log is given in Table PHOTOMETRIC RESULTS We examined the V- andi-band light curves of 29 variable stars. From the V I color variations and a comparison of amplitudes between the V and I bands (see Fig. 3 below), we reclassified 24 stars as W UMa type eclipsing binaries. Our photometric results are summarized in Table 2. Figure 1 shows phase diagrams for the 24 W UMa type eclipsing variables that have been reclassified in the present work. We adopted the

2 1848 JIN ET AL. Vol. 128 TABLE 1 Observation Log for Our Selected Variable Stars ROTSE Name ROTSE Period (days) Observation Date Start HJD (2,452,000+) Data Points Duration ( hr) V I J Mar J Jun J Mar J Mar J Jun J Apr J Apr J Apr J Apr J Apr J Jun J May J May J May May May J Jun J May J May J May J May J Jun J Jun J Jun J Jun J Jun J May J Jun J Jun J Jun J Jun periods from the ROTSE-I group and then changed them slightly in order to match the phases between the ROTSE-I photometric data and ours. The orbital periods of the eclipsing binaries are about 2 times larger than the periods derived by the ROTSE-I group. These plots include the ROTSE-I data and our V and I magnitudes and V I colors. The mean magnitude for each data set is expressed on an arbitrary scale. The light curves of the 24 reclassified W UMa binaries appear to match the corresponding light curves from the ROTSE-I data well, except for one object: the light curve of ROTSE1 J shows a quite different amplitude from that of our data. Since we did not find any other variables in our observing field, we believe that the ROTSE-I data for this object were contaminated by a faint star about away. (The ROTSE-I instrument has a typical FWHM of about ) Usually, data obtained at only one observatory suffer from the effects of aliasing. In the case of an all-sky survey such as ROTSE-I, in which there are only a few data points obtained at nearly the same time during each observing night, this effect could be more significant. We examined the aliases of the ROTSE-I periods using the phase diagram in combination with the ROTSE-I data (a few data points each night but long runs) and ours (continuous data for several hours). For most of the reclassified W UMa type binaries that have sufficiently long observing time to cover orbital phases, we found that the ROTSE-I periods yield reasonably symmetric eclipsing light curves. For a few stars with insufficient observing time (e.g., ROTSE1 J and J ), we could not determine whether the real period was the ROTSE-I value or one of its aliases. For ROTSE1 J , the 1.0 cycle day 1 alias period of about days [=2:0=(1:0=0: :0)] produces a more reliable eclipsing curve. Phase diagrams of the three pulsating stars and an unclassified star are shown in Figure 2. ROTSE1 J is a well-known Scuti star, YZ Boo. ROTSE1 J is an RRab-type star, given its lightcurve shape and period. According to the General Catalogue of Variable Stars (Kholopov et al. 1990), ROTSE1 J was known as an RRab-type star, V619 Her. As shown in the figure, its light curve is typical of an RRab pulsator. Our periods for the latter two stars are very different from the ROTSE-I results. The difference seems to be due to aliasing; the frequency differences 1=P ours 1=P ROTSE are cycles day 1 for ROTSE1 J and cycles day 1 for ROTSE1 J The last star in Figure 2, ROTSE1 J , shows different light variations than seen in the ROTSE-I data. It looks like a long-period variable star. We could not find any variable object around this star in our observing field of view, and this object remains unclassified in this study. We observed one more star, ROTSE1 J , which is located in an outer field of the globular cluster M3 (NGC 5272).

3 No. 4, 2004 RECLASSIFICATION OF ROTSE-I STARS 1849 TABLE 2 Observation Summary for 29 Scuti Variables in the ROTSE-I Field ROTSE Name m (ROTSE) V I Classification Period (days) Epoch ROTSE This Work J a ,451, Scuti HADS J ,451, Scuti RRab J b ,451, Scuti RRab J Scuti Unclassifi Bed J Scuti Nonvariable J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa J ,451, Scuti W UMa a Aknown Scuti star, YZ Boo. b A known RR Lyrae star, V619 Her ( Kholopov et al. 1990). Its light curve is not presented in this paper, because we could not find any brightness variations. Even though there have been many photometric observations of the M3 region, this object has never been reported as a variable star (e.g., Bakos et al. 2000). We think there is a possibility that the ROTSE-I data for this object are contaminated by a faint RRab star about distant. Figure 3 displays the amplitude differences for the eclipsing binaries and for the pulsating stars between the two passbands, V and I. The slope of I against V is about 0.92 for the 24 W UMa stars but 0.63 for the three pulsating variables. Note that the pulsating stars have much larger amplitude differences between passbands than the eclipsing binaries do. The amplitudes of the pulsating stars rapidly decrease in the region of the infrared passband because their brightness changes are mainly due to variations in temperature. The behavior of the photometric variations between different passbands for the pulsating stars shown in Figure 3 is only valid for radially pulsating stars. It is known that most HADS stars are monoperiodic or double-mode radial pulsators (Breger 2000; Rodríguez et al. 1996). For nonradially pulsating stars, the respective weights of the geometric and temperature variation terms are different (Watson 1988; Dupret et al. 2003). 4. FOURIER DIAGRAM Fourier decomposition has proved to be a powerful method for the description and interpretation of the light-curve shapes of variable stars. The light curves are fitted with a function of the general Fourier form I(t) ¼ A 0 þ Xn k¼1 A k cos (k!t þ k ) (Simon & Lee 1981), where I(t) is the magnitude observed at time t, A 0 is the mean magnitude, A k is the amplitude of the kth component,! ¼ 2=period, n is the order of the fit, and k is the phase value of the kth component. The Fourier decomposition parameters are normally used in two forms, the amplitude ratios R ij ¼ A i =A j and the phase differences ji ¼ i j j i. We applied the Fourier decomposition technique to the data sets of the 91 ROTSE-I Scuti stars in order to select targets for follow-up observation. For this study we selected 29 follow-up objects including most of the stars that have Fourier parameters 21 located in the range between 3.0 and 5.0 radians, that is, the high-probability area for HADS stars (Morgan, Simet, & Bargenquast 1998; Poretti 2001). Figure 4 displays a diagram of 21 versus period for the 91 ROTSE-I Scuti stars. This diagram was made with the ROTSE-I periods, and the parameter 21 was calculated from the ROTSE-I photometric data using a least-squares fitting technique. Our photometric reclassification results, together with those from our previous study (Jin et al. 2003), are represented by different symbols for each type of variable star. The dashed ð1þ

4 Fig. 1. Phase diagrams for the stars reclassified as W UMa type eclipsing binaries. In each panel, the data are labeled ROTSE, 1mkat_V (our V magnitude), 1mkat_I (our I magnitude), and 1mkat_VI (our V I color). Mean magnitudes for each data set are expressed on an arbitrary scale. 1850

5 Fig. 1. Continued 1851

6 Fig. 1. Continued 1852

7 Fig. 1. Continued 1853

8 1854 JIN ET AL. Vol. 128 Fig. 2. Light curves of three pulsating variable stars and an unclassified star. Top left,the Scuti star YZ Boo; top right, anrrab star; bottom left, anrrab star already known as V619 Her; bottom right, a star that looks to be a long-period variable but is unclassified in this paper. The labels are the same as in Fig. 1. Fig. 3. Diagram of V vs. I amplitude differences. Open circles represent the 24 W UMa type eclipsing binaries, and filled circles denote the three pulsating stars. The slopes of I against V are quite different between the two variable types. ellipse denotes the high-probability area for HADS stars. The inclination of the ellipse is due to the fact that the 21 of HADS stars has a slight trend to decrease toward shorter periods (Poretti 2001). All stars inside this ellipse except one have been observed. From our observations we confirmed six HADS stars, but we present five in this figure because one HADS star was not included among the 91 ROTSE-I Scuti type stars: ROTSE1 J was classified as an RRab type star by the ROTSE group (Jin et al. 2003). The phase parameters 21 of the five HADS stars are all concentrated around 4.0 radians, which is in good agreement with the result from OGLE data by Poretti (2001). Note that the concentration in the high-probability area is only valid for HADS stars and should not be generalized to all Scuti stars. Most Scuti stars that have low amplitude, less than 0.1 mag, show sinusoidal light variations, differing from the asymmetric light curves of the HADS stars. But all of the 91 ROTSE-I Scuti stars except one have amplitudes larger than 0.1 mag. Therefore, we assume that if most of the ROTSE-I Scuti stars are HADS stars, they should be concentrated around

9 No. 4, 2004 RECLASSIFICATION OF ROTSE-I STARS 1855 Fig. 4. Diagram of the Fourier parameter 21 vs. period for the 91 ROTSE-I Scuti stars. The diagram was constructed usting the ROTSE-I period and 21 calculated from the ROTSE-I photometric data. Each symbol denotes our reclassified variable type. The dashed ellipse represents the high-probability area for HADS stars. The error in 21 is 0.41 (median). 4.0 radians. But the ROTSE-I Scuti type stars, as shown in Figure 4, have a very wide spread in the Fourier phase diagram. Moreover, most of our targets for follow-up observation turned out to be W UMa eclipsing variables, which are located outside the high-probability area near 2 and 0 radians as a result of their symmetric light-curve shape. From our photometric results, we suggest that most of the ROTSE-I Scuti stars located outside the high-probability area are in fact W UMa type eclipsing binaries. Because the ROTSE-I data have large observational errors, as shown in the phase diagrams, the errors in 21, ranging from 0.09 to 2.88, are not so small. There are eight stars with errors above 1.0. All of them except one (ROTSE1 J ) were observed in our studies (Jin et al and this work) and turned out to be W UMa type binaries. For the 91 ROTSE-I Scuti stars in Figure 4, the median value of the error in 21 was estimated to be These errors are not so significant for determining whether the stars are located inside or outside the high-probability area. Thus, we believe that most of the unobserved ROTSE-I Scuti stars are located outside the high-probability area as shown in Figure 4, implying that they are W UMa type binaries. 5. CONCLUSION AND DISCUSSION By comparing amplitudes between V and I passbands, we have reclassified 24 W UMa type eclipsing variables out of our 29 target stars selected from the 91 ROTSE-I Scuti type stars. There is only one Scuti star among our observing targets, the well-known YZ Boo. Table 3 summarizes our main results for the 49 objects classified by our photometric results, including the previous study (Jin et al. 2003). There are only six HADS stars among 49 follow-up objects, and 37 targets have turned out to be W UMa type eclipsing binaries. This fact implies that previous ROTSE-I classifications of variable stars as Scuti types have some significant problems. Akerlof et al. (2000) applied the sign of the greatest deviation to classify variable stars. They suggested that the greatest deviation is fainter than the mean magnitudes for eclipsing binaries; on the other hand, it tends to be brighter than the mean for pulsating stars. This criterion is a reasonable method to apply to variable star classification for survey projects. However, our results show that a number of W UMa type binaries were misclassified as Scuti type pulsators in the ROTSE-I group. The misclassification seems to be due to the ROTSE-I method, which is susceptible to data quality. Except for a few targets, the photometric data for the ROTSE-I Scuti stars have large magnitude errors. The Fourier decomposition parameter 21 provides a useful method to classify variable types for photometric survey data such as that from ROTSE, although it is not an absolute criterion. As shown Figure 4, all the observing targets located TABLE 3 Reclassified Types for Our 49 Follow-up Targets Type No. HADS... 6 EW RRab... 2 Unclassified... 1 Nonvariable... 2 Cataclysmic... 1 Total Note. Includes results from Jin et al. (2003).

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