95-DAY VARIABILITY IN THE LIGHT CURVE OF THE SYMBIOTIC STAR CH CYG

95-DAY VARIABILITY IN THE LIGHT CURVE OF THE SYMBIOTIC STAR CH CYG

Ruslan Tavakkul Mammadov

doctoral student, Batabat Astrophysical Observatory of Nakhchivan branch of ANAS,

Azerbaijan, Nakhchivan

Khidir Mustafa Mikailov

phD of Physics, Associate Professor, Baku State University,

Azerbaijan, Baku

ABSTRACT

Using the AAVSO database of photometric observations, the light curve of the symbiotic star CH Cyg was constructed in the V filter for 2000-2020. A period has been searched for in this curve. To investigate the periodicity, we applied statistical spectral Fourier analysis using the windows version of Scargle. As a result, 95-day periodic changes in the brightness of the star were found. The obtained period coincides with the results obtained by previous researchers.

Keywords: symbiotic stars, CH Cyg, photometry, period.

1. INTRODUCTION

Symbiotic stars are interacting binary systems consisting of red giant and white dwarf cover surrounding it. The material source of the cloud is red giant which loses its substance by star wind and pulsation, the energy source is the hot white dwarf.

CH Cyg (HD 182917= HIP 95413) is the brightest symbiotic binary at visual wavelengths and the second brightest symbiotic in the 2 µm infrared. The CH Cyg system has been observed both photometrically and spectroscopically from radio through X-ray wavelengths. Both the light curves and the radial velocity curves show multiple periodicites (e.g. a ∼ 100d photometric period best visible in the VRI light curves, attributed to radial pulsations of the giant [1], and a secondary period of ∼ 756d also present in the radial velocity curve [2]. The symbiotic star CH Cyg is a very complex and mysterious variable that has been studied in detail over a wide spectral range. The symbiotic star CH Cyg was discovered in the 80s of the 19th century. His photometric observations are available for a period of more than 130 years [3]. CH Cyg is the brightest symbiotic stars and is situated in 270.66 parsecs distance [4]. For many years it was considered to be a simple red giant pulsating in 100 days of periodicity. Until 1963, no sign of a symbiotic system - ultraviolet continuum and hydrogen emission lines – were observed in its spectrum.

Its visual stellar magnitude is V=6.0^m at maximum, V=10.5^m at minimum, and brighter in the infrared. The first useful spectra of the symbiotic star CH Cyg were obtained by Joy between 1924 and 1927 [5]. Based on 5 well-distributed spectra over a 100-day period, the star's spectral type is M6, and its radial velocity is -52.5 km/s. The presence of emission lines was not reported. In 1952, Gaposchkin analyzed the observational materials of the Harvard patrol and classified CH Cyg as M6-M7 spectral class with a 90-100-day period of 1m amplitude [6]. Since CH Cyg is located in a favorable location (+50°) in the northern hemisphere, regular spectral, photometric, infrared and radio observations have been made of it. Observations have shown that repeated active processes occur in the star at different times. The composition of the CH Cyg system has not yet been determined: models of binary and triple star systems have been proposed [2].

Photographic and photometric observations of the symbiotic star CH Cyg have been made for almost the last century. Long periods were not detected during these observations. Only short-term periodic changes of 90-100 days were detected. Later, many researchers collected and analyzed photometric data and discovered several periodic changes in the brightness curve. Music, etc. They constructed the light curve for the period 1929-1977. Analyzing this curve, they found two short periods of 100 and 157 days in the system and one long period of 780 days from the light curve after 40 years.

Our goal in this work is to detect short-period variations of the star CH Cyg based on photometric observations of the symbiotic star CH Cyg by AAVSO (American Association of Variable Star Observers).

2. RESEARCH METHOD

In fig. 1, based on the results of photometric observations conducted by AAVSO [7], we have constructed the light curve for the period 2000-2020 in the V filter of the star. As can be seen from the figure, the CH Cyg symbiotic star showed changes of different nature at different times.

Figure 1. Light curve of the symbiotic star CH Cyg for the period in the V filter for 2000-2020, according to AAVSO

Then, we looked for periods from the graph of the constructed light curve. Periodic changes of 90-100 days are clearly visible in the photometric observations conducted by AAVSO. As can be seen from the picture, these changes are noticeable when the star becomes relatively quiet. Perhaps short small-amplitude fluctuations are lost against the background of strong activity processes. Another possibility is that short pulsating changes are continuous from time to time, disappear and start again after a while.

To investigate periodicity, we applied statistical spectral Fourier analysis to all observation results using the Scargle method. We have found a short-period change of ~95 days of the star. Periodic changes are clearly visible from the picture. The stellar magnitudes in the V filter of the star are taken from the AAVSO observational database, and the phase is conventionally calculated from the beginning of the observing season of year 2000 (JD date 2451700). Figure 2 shows the dependence curves of the brightness in the V-filter on the phase in the images on the left, and the Julian history of the brightness in the V-filter in the images on the right. As can be seen from the images, the photometric variations fit the ~95-day period curve very well.

Figure 2. 2000-2020 light curve of the symbiotic star CH Cyg demonstration of short-term (P=95d) changes

During the observation period, the brightness of the star changed by 0.7^m in the range of 2451755-2451870 and 2452800-2452950, 0.3^m in the range of 2453510-2453640, and 1.1^m in the range of 2458300-2458410.

3. CONCLUSION

The light curve of CH Cyg symbiotic star for a period of 20 years was constructed and the problem of period estimation was considered. Using the Scargle method, the values ​​found by us for the ∼95-day period in the luminosity variation of the symbiotic star CH Cyg are in good agreement with the values ​​found by different researchers at different times. The ~95-day periodic changes we found can be explained by the radial pulsation of the Red Giant, the main star of the symbiotic system. These short periods are not seen when the star is burning and when the star is passing through periaster. Perhaps short small-amplitude fluctuations are lost against the background of strong activity processes.

References:

1.  Mikołajewski M., Mikołajewska J., Khudyakova T.N., 1992, A&A, 254, 127
2.  Hinkle K.H., Fekel F.C., Johnson D.S., Scharlach W.W.G., The triple symbıotıc system CH Cygni // AJ,1993, p. 1074-1086.
3. Mikolajewski, M., Mikolajewska, J., Khudiakova, T. N., A long-period symbiotic binary CH Cygni. I - A hundred years' history of variability // Astronomy and Astrophysics , 1990,  vol. 235, no. 1-2, p. 219-223.
4. Munari U., Renzini A., Bernacca B. L. Hipparcos–Venice ’97 (ESA SP-402), 1997, p. 413.
5. Joy Alfred H., Survey of the Spectra and Radial Velocities of the Less Regular M-Type Variable Stars // ApJ, 1942, 96, p. 344.
6. Gaposchkin S. Variable stars in Milton field 8 // The Observatory, 1952, vol.118, p.155-163.
7. https://www.aavso.org/LCGv2/