The ongoing 2008���09 outburst of CI Cyg

Abstract

In this paper, we discuss the early phases of the ongoing outburst that CI Cyg, a prototype symbiotic star, is currently undergoing after 30-year quiescence. We have tightly monitored CI Cyg in B V R c I c bands, starting a whole year before the onset of the outburst, and in addition we obtained numerous Echelle high- and low-resolution absolutely flux-calibrated spectra. The outburst started while the accreting white dwarf (WD) was being eclipsed by the Roche lobe filling M giant companion, and it was discovered during the egress phase on the second half of 2008 August. The system reached peak V-band brightness in early 2008 October and has been characterized by amplitudes ΔB = 1.9, ΔV = 1.5, ΔR C = 0.9, ΔI C = 0.4 mag. At maximum V-band brightness, the outbursting WD had expanded to closely resemble an F3 II/Ib star, with M v = -3.5, T eff ∼ 6900 K and R = 28 R ☉ . The high-ionization emission lines ([Ne v], [Fe VII], He II) disappeared and only lower ionization lines were visible. Balmer and He I emission lines declined in equivalent width but increased in absolute flux. The output radiated by the hot component during the outburst corresponds to nuclear burning proceeding at a 2 x 10 -8 M ☉ yr -1 rate.