Dissipation of the hot dust envelope of CH Cygni (1996–2003)

Abstract

Our analysis of JHKLM photometric data obtained in 1978–2003 for the CH Cyg system shows that the “local” dust envelope that formed in the system in 1986 and reached its highest column density in 1996 had largely dispersed by mid-2001, so that the observed brightness of the system’s red giant has returned to its pre-1985 level. The giant’s spectral type varied in the range M6.5–M7.5 in 2001–2003. The optical depth of the dust envelope at 1.25 µm was τ(J)≈0.83 near JD 2450090. The increase in the dust envelope’s optical depth was approximately a factor of 2.3 slower than the decrease. The envelope of CH Cyg can be pictured as a “stationary” dust cloud with an optical depth at λ=1.25 µm of ∼0.4 and a dust-grain temperature of ∼750–800 K. There was probably an injection of matter into this cloud toward the end of 1985, initiating the condensation of dust grains. The optical depth began to increase and, by 1991, the dust envelope was transformed from a cloud into a local, almost spherically-symmetric envelope with a grain temperature of 750–800 K. Its optical depth reached its maximum in 1996, after which the local envelope began to disperse, becoming a cloud again by 2001. The detected 4000-day variability of the JHK brightness of CH Cyg is in good agreement with a model with an eclipsing binary consisting of two cool giants with effective temperatures differing by approximately 100 K, a luminosity ratio of L(M7)/L(M6)∼6.8, and a radius ratio of R(M7)/R(M6)∼3.6. The orbital ephemeris is given by JD (Min I)=2444180+4000E.