The historical light curve of the symbiotic star AG Draconis: intense, magnetically induced cyclic activity

TL;DR

This study presents a 120-year optical light curve analysis of AG Draconis, revealing cyclic outbursts linked to magnetic activity and stellar interactions, with implications for understanding symbiotic star behavior.

Contribution

It introduces evidence of magnetically induced cyclic activity in AG Draconis, connecting outbursts to magnetic dynamo cycles and tidal effects, a novel insight among symbiotic stars.

Findings

Outbursts occur in clusters with a ~5300-day cycle.

Outburst intervals are multiples of 373.5 days.

Multiple periodic signals correspond to stellar pulsations, rotation, and binary orbit.

Abstract

We analyze an optical light curve of the symbiotic system AG Draconis covering the last 120 years of its history. During the first 32 years the system was in a quiescence state. Around the year 1922 the star's quiescence luminosity brightened by 0.29 mag. The last 82 years of the light curve (LC) are characterized by a series of outbursts of 1-2 magnitude in brightness and about 100 days in duration. The outbursts are distributed along the time axis in 6 clusters with a quasi-periodic cycle of some 5300 days. The time intervals among the outbursts themselves are integral numbers of the period 373.5 days. During quiescence states the LC oscillates with the binary period of the system of 550 d. The LC contains also a weak periodic signal with a period of 350 d, attributed to pulsations of the giant star. Another period of 1160 d is also present in the light curve, being the sidereal rotation period of the giant star. We suggest that the outbursts are events of intense mass transfer from the giant onto the hot component. These are modulated by an interplay between a solar-like magnetic dynamo cycle operating in the outer layers of the giant, and a tidal deformation of these layers that circulates the surface of the giant with the synodic diurnal period of 373.5 Earth days. AG Dra is the 5th symbiotic system with a light curve that reflects such an intense magnetic and magnetically modulated activity. (Abridged)