1. 3. Analysis of the photoelectric and CCD photometry.
The investigation of photometric variability of the novalike object V380 Oph using the archival and modern data.
S. Shugarov (1,2), K. Sokolovsky (3), P. Golysheva (2), D. Chochol (1)
1) Astronomical Institute of the Slovak Academy of Sciences,Tatranská Lomnica, Slovakia
2) Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, Moscow, Russia
3) Astro Space Center of Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia
Historical overview.
V380 Oph was discovered as a variable star by C. Hoffmeister (1929). Hope (1938) suggested that the object belongs to the class of Mira-type pulsating stars, variables with a period of around 100 – 1000 days. Meinunger (1965) examined the photographic plates of Sonneberg Observatory and indicated that the star may be either an eclipsing binary with the period close to 1 day, or a RR Lyrae-type pulsating variable. Bond (1979) first suspected that V380 Oph can be cataclysmic variable star. Shafter (1983,1985) showed that the spectra of V380 Oph exhibit strong Balmer emission lines and HeII 4686 line and by analysis of their radial velocities he found an orbital period of about 0.158d. The first detailed photometric study of V380 Oph was carried out by Shugarov et al. (2005, 2007). The authors analyzed about 220 photographic observations ( ) and about 1300 photoelectric and BVR CCD observations ( ) and showed that the object was at a low state (~17.5 pg) in 1979, and close to 14 mag at other times. They found two photometric periods of d and d in data, probably related to the orbital period and disc precession, respectively. The short orbital period of V380 Oph and detected large depression of luminosity include it in VY Scl-type nova-like cataclysmic variables (anti-dwarf novae). Rodriguez-Gil et al. (2007) used the Hɑ radial velocities of V380 Oph to find its orbital period d. Their Hɑ Doppler tomography of the object allowed classify it as the SW Sex type star.
2. Photometry of V380 Oph at the Crimea SAI archival photographic plates.
During 1976 – 1995 about 220 photo-plates of the 66 Oph area, where V380 Oph is located, were taken by the 40 cm astrograph of the Southern Station of the Sternberg Astronomical Institute in Crimea. In the past, the plates with V380 Oph were measured by the Iris micro-photometer. Recently, the same plates were digitized with a flatbed scanner. Thus, we have photographic magnitudes, obtained by different methods.Since the plate size is large (30 x 30 cm), the digitized plate was divided into 1x1 deg subfields that were processed independently of each other. The magnitude scale was calibrated using APASS B magnitudes of all UCAC4 stars in the subfield (Sokolovsky et al., 2014). V380 Oph, visible in two overlapping subfields, was measured twice on each plate using two different sets of comparison stars. Fig. 1 shows the measured magnitude of the variable at the first section of the plate versus its magnitude at the second section of the same plate. There is a very good correlation between the two independent measurements of the variable star. The standard deviation (SD) about 0.05 mag is the instrumental limit of the scanned measurements accuracy. The real accuracy of the measurements is smaller than this value. We calculated and used for the analysis the average star magnitude found from these two independent scans. Fig. 2 shows the magnitude BSCAN, obtained by scanning, with respect to the magnitude Bpg, measured by the micro-photometer. There is a small systematic discrepancy of magnitudes and the slope of the curve differs from 45 deg. We found the equation of transformation Bpg = 1.10BSCAN and constructed the Fig. 3, in which the transformation equation was applied. The SD of the measurement was 0.13 mag, close to the real measurements accuracy on photographic plates. Next, we calculated the average star magnitudes obtained by micro-photometer and scanning methods and used these values for further analysis. The Bpg magnitudes of V380 Oph ( ) are presented in
Fig. 4 together with our photoelectric and CCD photometry obtained by the telescopes in Russia and Slovakia ( ). The Rc brightness depression up to 4 mag was detected in 2015.
At the same time the optical companion located southwest from the variable at an angular distance close to 5" was seen (Fig. 5).
3. Analysis of the photoelectric and CCD photometry.
Photoelectric and CCD observations have obviously a better accuracy and time resolution than the photographic ones. The Fourier analysis of V and B photometry during the years was used to find the period P2 = d, which is 5% shorter than the value found by Shugarov et al. (2005). The V phase light curve is shown in Fig. 6. We removed the wave trend from the V and B data and found by Fourier analysis the best period P1 = d in residuals (Fig. 7) in full agreement with the value found by Shugarov et al. (2005) from observations taken during the years
Fig.2. The dependence of V380 Oph magnitude measured by micro-photometer versus its magnitude obtained by scanning of the photographic plate for 220 plates taken in
Fig.1 Measured magnitude of the variable at the first section
of the photographic plate versus its magnitude at the second section of the same plate for 220 plates taken in
Fig.3. The same as in Fig. 2 after application of the equation of transformation Bpg = 1.10BSCAN
4. The generalization of archival photographic and modern series of observations.
The Fourier period analysis of photographic data did not reveal the period of days, although there were some indications of periodicities close to this period with small probability. The amplitude of the periodic component is about 0.15 mag (see Fig. 7), close to the limit of precision of photographic measurements. Therefore, 220 photographic observations obtained in the interval of 20 years are insufficient for the purpose of periods search. In addition, there is a strong flickering at the light curve of V380 Oph (Fig. 8), which also makes difficult to find periodicities.
Fig.5. CCD frames of V380 Oph during its bright state (right) and low state (left). Variable is designated
by arrow. The optical companion is seen in the angular distance of 5" to the south-west from the variable.
Fig.4. The light curves in BVRC bands during the last 40 years. The depressions were detected in 1979 and 2015.
5. Conclusions.
Photographic observations are still relevant and important to investigate the long-term large amplitude brightness variations. Through these observations we found the low state of V380 Oph in 1979 and suggested the existence of short period, but our attempt to find its exact value was not successful. We found the accuracy of photographic measurement to be about 0.13 mag. It is plausible for the photographic plates obtained by 40/160 cm telescope for stars of 15 mag.
More precise photoelectric and CCD photometry allows us to find the period close to the orbital one, which can be interpreted as the period of negative superhumps (Rodriguez-Gil et al., 2007).
Fig.7. The phase diagram of the B,V data residuals after removing the d wave, folded with the period of superhumps d.
Fig.8. The photoelectric B light curve exhibits a strong flickering.
Fig.6. The phase diagram of V observations folded with the period of d.
Thus, the joint analysis of different methods of photometry is still actual and perspective for a comprehensive analysis of the light curves of peculiar stars.
Acknowledgments: The work was partially supported by the VEGA grant No. 2/0002/13, RFBR grants No and
References:
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Meinunger, L., 1965, MVS 2, h.7, Shugarov, S. Yu., Katysheva, N. A., Kroll, P. 2007, ASP Conf. Ser. 372, 515
Rodríguez-Gil, P., Schmidtobreick, L., Gänsicke, B. T. 2007, MNRAS, 374, Sokolovsky, K., Antipin, S., Kolesnikova D., et al., 2014, Proceedings “Astroplate 2014”(Prague), 79 .