Optical Photometry of Dwarf Nova QZ Serpentis in Quiescence

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1 Optical Photometry of Dwarf Nova QZ Serpentis in Quiescence Erica D. Jones Center for Astrophysics, Space Physics and Engineering Research at Baylor University Dr. Dwight Russell Department of Physics at Baylor University Richard Campbell Department of Mechanical Engineering at Baylor University Abstract We present quiescent photometry of dwarf nova QZ Serpentis. Given the limited amount of published information on this object, we present photometry at different wavelengths and comment on the spectral class. We collect data in four filters, BVRI, and calibrate data using bias subtraction, dark subtraction, and flat division. We also use multiple aperture photometry in the AstroImageJ software to obtain the brightness of our target object and our comparator star. We calculate the average magnitude for QZ Serpentis in each filter. We present light curves and folded light curves for each observing run. We use differential photometry to yield an average R magnitude of ± and B-V = ± We report a secondary spectral class ranging from an early G-type star to an early K-type star. Light curves exhibit sinusoidal features, which is typical of contact binaries. Our findings are consistent with previously published data. Keywords dwarf novae, accretion disk, cataclysmic variables, variable star photometry C I. INTRODUCTION ATACLYSMIC variables (CVs) are close binary star systems divided into subcategories based on frequencies and amplitudes of luminosity variations [1]. These categories are novalike variables (NL), novae (N), recurrent novae (RN), and dwarf novae. The CV of interest in this paper is the dwarf nova. Dwarf novae are close binaries with a late-type main sequence secondary star that fills its Roche lobe, transferring material through the Langrangian point L1, the inner Lagrangian point [2]. This transfer of material forms an accretion disk around the white dwarf primary star [2]. These objects exhibit interesting effects due to the presence of an accretion disk, and various theories exists to explain these effects observed in dwarf novae. Theories such as change in mass-transfer rate and disk instability, offer explanations for effects observed when studying dwarf novae [3]. These binary systems are known to have periods of quiescence, outbursts, and superoutbursts. In quiescence, SS Cygni has an average magnitude of 11.2; however, in outburst, there is a reported magnitude of 9 [4]. Dwarf novae also exhibit larger changes in magnitude during outburst. An example of this is dwarf nova V725 Aquilae. With an outburst magnitude of 13.7 and a quiescent magnitude of 19.32, the object increases over 100 times in brightness [5]. The dwarf nova QZ Serpentis was discovered in 1998 by Katsumi Haseda [6]. QZ Ser is located at right ascension 15 h 56 m s and declination QZ Ser has an orbital period, P orb, of days or minutes [6]. Thorstensen et. al (2002) report a the magnitude of the secondary star as V = 17.9 ± 0.4 [6]. In this paper, we present optical photometry of QZ Ser in the BVR and I filters. We discuss observing runs, data calibration techniques, data analysis, and light curve features. The majority of the data were collected using the R filter. We create folded light curves using the orbital period, P orb = days, and T 0 = HJD published in [6] in order to analyze the features of QZ Ser based on the position, or phase, of the binary. II. METHODOLOGY We collected data on 6 nights at the Paul and Jane Meyer Observatory using the 0.6 m Ritchey-Chretien telescope. Throughout the various observing runs, data were collected in four filters: BVRI. On June 10, 2013, we collected 10 exposures in the V filter with 60 s exposure time. Similar data were collected the next night in the I filter. Data in the R filter were collected on both June 13, 2013 and June 14, On the second night, we were able to

2 observe QZ Ser over one complete orbital period. We collected data in three filters (BVR) on the night of June 19, One July 9, 2013, we observed two full orbital periods of QZ Ser in the R band filter using 60 second exposures. More details of the observing runs are included in Table I. UT Date TABLE I. OBSERVATION LOG FOR QZ SER UT Start Time (first exposure) UT End Time (last exposure) # of Exposures /Filter (exptime) 2013 June 10 07:09:33 07:35:14 10/V(60s) 2013 June 11 04:21:11 04:52:40 10/I(60 s) 2013 June 13 03:57:45 04:08:22 11/R(60 s) 2013 June 14 04:39:44 06:57:43 130/R(60 s) 2013 June 19 03:24:41 03:51: June 19 03:38:32 05:58:06 6/V(90 s) 6/R(90 s) 16/B(180 s) 16/V(120 s) 16/R(120 s) 2013 July 9 03:48:08 08:01:59 240/R(60 s) The data were calibrated using the AstroImageJ software. Bias frames, darks frames, and flat fields were median combined to create masters using the software. The master dark with the same exposure time as the flats was subtracted from all flats before the master flat using the appropriate filter was created. Images were bias subtracted, dark subtracted, and flat divided. The CCD was kept at -35 C for all exposures. Multi-aperture photometry was completed using the AstroImageJ software. Photometry was measured for the target, QZ Ser, and various comparator stars in the field of view. Comparator star 4, or C4 as shown in Figure 1, was used for differential photometry. To plot the data, we used an IDL program. The IDL program uses the HJD to calculate the phase of an object. As mentioned in the introduction, we use an orbital period, P orb, of days and a T 0 of HJD. These values were reported in [6]. The phase was calculated based on this initial time correlating to a phase of 0.0 and this orbital period. The HJD of each data point was user to calculate the phase of QZ Ser at that time, with phase ranging from 0.0 to 1.0. We used an IDL program to convert the Julian date (JD) to the heliocentric Julian date (HJD). By converting to HJD, we take into account the motion of the Earth around the sun and its effect on the movement towards or away from the object, depending on the right ascension and declination of the object. In order to determine the HJD, astronomers must consider the time it would take light to travel from a celestial object to the center of the Sun rather than to the Earth [7]. This calculation provides a reference for the amount of time it takes light to reach one point, regardless of the motion of the Earth. We also use phased average binning to calculate the mean magnitude at a phase step of 0.05, and we calculated the error using the standard deviation of the mean. AstroImageJ software outputs a flux error for the flux measurement of both the target and the comparator star. The flux measurements are converted to magnitude and magnitude error. The average magnitude of QZ Ser in each filter is calculated using the average of the magnitude measurements and error of this average is calculated using traditional error analysis on the sum of values in the numerator of the average and on the constant in the denominator. III. LIGHT CURVES In this section we present light curves from each observing run. Figures 2 through 11 contain a light curve with magnitude versus time in the upper panel and magnitude versus phase in the lower panel. The lower panel of Figures 2-11 shows the changes in the brightness of QZ Ser over two periods, with data from one period repeated for Figure 1. Finding chart of QZ Ser based on data taken in the R band Filter (60 s exposure) including the target and various comparator stars. The filter of view is with north at the bottom and east to the left. The light curve in Figure 10 contains data collected using the B filter. The brightness of QZ Ser in the B magnitude ranges from 19.2 to 18.0 within error bars. Figures 2, 6, and 8, represent the brightness of QZ Ser in the V filter. The V magnitude ranges from 18.2 to 17.4, including error bars. The R magnitude ranges from 17.6 to 17.1 in Figures 4, 7, 9, and 1l. Figure 5 shows the R magnitude ranging from 18.0 to In the I filter, the magnitude of QZ Ser ranges from 17.2 to 16.9 (Figure 3). The brightness of QZ Ser appears to increase at redder wavelengths.

3 Figure 2. Upper panel light curve of data set collect 2013 June 10. Data were collected using the V filter. Lower panel over two periods. Figure 4. Upper panel light curve of data set collect 2013 June 13. Data were collected using the R filter. Lower panel over two periods. Figure 3. Upper panel light curve of data set collect 2013 June 11. Data were collected using the I filter. Lower panel over two periods. Figure 5. Upper panel light curve of data set collect 2013 June 14. Data were collected using the R filter. Lower panel over two periods for

4 Figure 6. Upper panel light curve of data set collect 2013 June 19. Data were collected using the V filter with an exposure time of 90 s. Lower panel folded light curve of the same data plotted twice to view over two periods. Figure 8. Upper panel light curve of data set collect 2013 June 19. Data were collected using the V filter with an exposure time of 120 s. Lower panel folded light curve of the Figure 7. Upper panel light curve of data set collect 2013 June 19. Data were collected using the R filter with an exposure time of 90 s. Lower panel folded light curve of the same data plotted twice to view over two periods. Figure 9. Upper panel light curve of data set collect 2013 June 19. Data were collected using the R filter with an exposure time of 120 s. Lower panel folded light curve of the

5 Observing run #5 was separated in Figures 6 to 11 based on filter selection and exposure time. In the 5a observing run, the V and R data was taken using 90s exposures. In the 5b observing run, the data in V and R were taken using 120s exposures. The 5a data in the B filter was excluded due to the exposure time being insufficient to detect QZ Ser. Figure 12 contains all the data collected in the R filter during the summer of Data were collected in R during multiple observing runs. By calculating the HJD and using it to determine the phase of QZ Ser, we were able to plot the brightness of the object versus the phase. Because we used the phase, we are able to plot all data points from different observing runs and view them on one plot, as opposed to using the light curves in the upper panels of Figures 2 through 11. Figure 10. Upper panel light curve of data set collect 2013 June 19. Data were collected using the B filter with an exposure time of 180 s. Lower panel folded light curve of the The plot of hundreds of data points and corresponding errors in Figure 12 presents a challenge when trying to extract basic features in the light curve of an eclipsing binary. We used phase binning to help address this problem. Based on the phase, data points were binned, and the magnitude was averaged. The error on each point was calculated using the standard deviation of the mean. In the lower panel of Figure 12, we see a much cleaner light curve with features that were washed out in the plot with all data points and error bars. Figure 11. Upper panel light curve of data set collect 2013 July 9. Data were collected using the R filter. Lower panel folded light curve of the same data plotted twice to view over two periods for Figure 12. Upper panel folded light curve of all data set collected throughout the observing runs during the summer of 2013 in the R filter. Lower panel phased binned light curve of the

6 TABLE II. AVERAGE MAGNITUDE OF QZ SER, IN FILTERS BVRI Filter(s) Average Magnitude Error B ± V ± R ± I ± B-V ± V. CONCLUSION Quiescent photometry of dwarf nova QZ Ser yield various information about the brightness, temperature, and physical nature of the system. Data analysis was conducted using differential photometry to yield an average R magnitude of ± and B-V = ± Based on this calculate B-V, QZ Ser has a secondary spectral class ranging from an early G-type star to an early K-type star. The light curve in Figure 12 exhibits sinusoidal features. The features in an eclipsing binary are consistent with a contact binary, in which the secondary star fills its Roche lobe. Our findings are consistent with previous findings and classifications of QZ Ser. IV. DISCUSSION For this system, there are limitations when attempting to determine the spectral class of the secondary star. Although QZ Ser is an eclipsing binary, it has an inclination of 67 [6]. Both objects are visible at every phase. The two eclipses present in the lower panel of Figure 12 represent the dip in magnitude when part of each component of the binary system is eclipsed. The accretion disk and white dwarf are never completely eclipsed by the secondary, so the spectral class is determined using the B-V based on the average magnitudes of QZ Ser which include all components. B-V is ± 0.030, implying a spectral class between an early G-type star and an early K-type star. The average magnitude of QZ Ser in quiescence is reported for various filters in Table II. These values near 17 and 18 magnitudes are consistent with previous report of QZ Ser in quiescence [6]. Based on the larger magnitudes at bluer wavelengths, represented by values in Table II and by our B-V measurement, we can conjecture that the system is composed of objects with temperatures closer to that of the Sun than new O-type stars. The folded light curve features in Figure 12 are consistent with the physical nature of a contact binary system with the secondary filling its Roche lobe. The sinusoidal nature implies a physical system in which one component of the binary is always transiting another component with a short time period where we see both object. This short period of time is represented by the peak in magnitude, consistent with observing all components of the binary system. ACKNOWLEDGMENTS We would like to thank Aubrey Brickhouse, president of the Central Texas Astronomical Society, for giving us access to the Paul and Jane Meyer observatory. Special thanks to our telescope operator Willie Strickland who taught us how to navigate AstroImageJ. Thanks to Dr. Hyde, Dr. Matthews, and everyone involved in the CASPER REU/RET program. This research was supported by the NSF. REFERENCES [1] E. L. Robinson, "The structure of cataclysmic variables," Annual Review of Astronomy and Astrophysics, vol 14, pp , [2] D. Nogami, S. Masuda, and T. Kato, "The 1994 super-outburst of the new su uma-type dwarf nova sx leonis minoris," Publications of the Astronomical Society of the Pacific, pp , [3] L. Kotko, J. P. Lasota, G. Dubus, and J. M. Hameury, Models of am canum venaticorum star outbursts, Astronomy and Astrophysics, vol. 544, p. 13, [4] W. B. Honey, G. T. Bath, P. A. Charles, R. Whitehurst, D. H. P. Jones, J. Echevarria, M. J. Arevalo, J. E. Solheim, G. Tovmassian, and K. Takagishi, Quiescent and outburst photometry of the dwarf nova SS Cygni, Monthly Notices of the Royal Astronomical Society,vol. 236, pp , [5] M. Uemura, T. Kato, E. Pavlenko, A. Baklanov, and J. Pietz, Photometric observation of a new in-the-gap su uma-type dwarf nova v 725 qquilae during the 1999 superoutburst, Publications of the Astronomical Society of Japan, vol. 53, pp , [6] J. R. Thorstensen, W. H. Fenton, J. Patterson, J. Kemp, J. Halpern, and I. Baraffe, Qz serpentis: a dwarf nova with a 2 hour orbital period and an anomalously hot, bright secondary star, Publications of the Astronomical Society of the Pacific, vol. 114, no. 800, pp , [7] B. W. Carroll and D. A. Ostlie, An Introduction to Modern Astrophysics, 2nd ed. Pearson Education, Inc., 2007, p. 15. Future research on the dwarf nova QZ Ser may include obtaining light curves over full orbital periods at multiple wavelengths. This paper presents a complete light curve in the R filter. A similar light curve in the B and V filters would add to the very limited amount of published data for this object.