R Aqr—the 2022 Eclipse Has Started! (Abstract)

Volume 48 number 1 (2020)

Lee Anne Willson

5326 Springbrook Drive, Ames, IA 50014; lwillson@iastate.edu

Abstract

(Abstract only) The R Aqr symbiotic system consists of a Mira variable plus a companion that is presumably a white dwarf. Most of the time the light curve shows the pulsation of the Mira. However, every 44 years there is an event that distorts the light curve for about six cycles. Around 1978 this event reduced the brightness of the system while leaving the pulsation period of the Mira unchanged. In 1934 there was a dimming of maximum and a brightening of minimum. In 1981 Willson, Garnavich, and Mattei (IBVS 1961) proposed that the reason for the dimming is a partial eclipse of the Mira by an opaque source, and that the system has a 44-year orbital period, with a predicted next eclipse centered on 2022. In the interim, an orbit has been derived by Gromadzki and Mikołajewska (2009, A&A 495, p. 931); this has an eccentric orbit (e = 0.25) with periastron and eclipse occurring at nearly the same time, and with an orbital period of 43.6 years. A low minimum about three years ago prompted an alert to AAVSO members that has resulted in excellent coverage of the last three cycles, including BVRI photometry. The current cycle is so far depressed by about 2 magnitudes, again with normal light curve shape for the Mira. In addition, the R and I colors are also down by about the same amount, consistent with an opaque object eclipsing the Mira. For the eclipse to last six years and block more than 80% of the light means we are looking at a large structure. Is it an accretion disk around the companion, or an opaque flow between the two stars? Will this event include a brightening of minimum or just a depression of the light curve? The stars are too widely separated for Roche lobe overflow, but the Mira, with period 386 days, is expected to have a heavy wind, and this could lead to enhanced accretion near periastron. This poster will include our analysis of the light curve and discussion of possible models for the ongoing event in this unique system.