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Keywords:

  • binaries: symbiotic;
  • stars: individual: AG Dra;
  • stars: magnetic field;
  • stars: rotation

ABSTRACT

We analyse the historical optical light curve of the symbiotic system AG Draconis, covering the last 120 years. During the first 31 years the brightness of the star has not been varying by more than 0.1 mag. A weak periodic signal with the binary period of the system of ∼550 d can be detected in this section of the light curve, as well as in all other later quiescence sections of it.

Around the year 1922, the quiescence brightness of the star increased by 0.29 mag. Since then, the star’s photometric history is marked by a series of brightness fluctuations with an amplitude of 1–2 mag and a typical duration of 100–200 d. The time intervals between outbursts are integral numbers of the period 373.5 d. The outbursts are grouped in six dense clusters, each one lasting some 1500 d, which are well separated from one another along the time axis with a quasi-periodicity of 5300 d.

We suggest that the outbursts of the system are triggered by episodes of intense mass outflow from the atmosphere of the cool star on to the environment of the hot component. The 373.5 d cycle is the length of a ‘day’ on the surface of the giant that rotates in retrograde direction with a sidereal period of 1160 d. A weak signal with this periodicity is also present in the light curve. The modulation of mass transfer in the system is a combined effect of a dipole magnetic field of the giant star and the tides induced in its atmosphere by its binary companion. The 5300 d quasi-period is that of a solar-like magnetic dynamo process that operates in the outer layers of the giant. The combined effect of the 5300  and 373.5 d cycles induces a second mode of pulsation of the giant star with the period of 350 d.

AG Dra is the fifth symbiotic system that shows in its historical light curve this type of intense magnetic and magnetically induced activity.