Infrared Spectroscopy of Symbiotic Stars. VII. Binary Orbit and Long Secondary Period Variability of CH Cygni

TL;DR

This study refines the orbital parameters of the symbiotic binary CH Cyg, confirming its long 15.6-year period caused by a white dwarf and M giant, and investigates the origin of its 2.1-year long secondary period, suggesting g-mode pulsation as a likely cause.

Contribution

It provides an extended 29-year infrared velocity dataset for CH Cyg, refines its orbital elements, and proposes a plausible mechanism for its long secondary period.

Findings

Confirmed the 15.6-year binary orbit with a white dwarf and M giant.

Identified the 2.1-year period as a long secondary period on Wood's sequence D.

Proposed g-mode non-radial pulsation as the likely cause of the long secondary period.

Abstract

High-dispersion spectroscopic observations are used to refine orbital elements for the symbiotic binary CH Cyg. The current radial velocities, added to a previously published 13 year time series of infrared velocities for the M giant in the CH Cyg symbiotic system, more than double the length of the time series to 29 years. The two previously identified velocity periods are confirmed. The long period, revised to 15.6 +/- 0.1 yr, is shown to result from a binary orbit with a 0.7 solar mass white dwarf and 2 solar mass M giant. Mass transfer to the white dwarf is responsible for the symbiotic classification. CH Cyg is the longest period S-type symbiotic known. Similarities with the longer period D-type systems are noted. The 2.1 year period is shown to be on Wood's sequence D, which contains stars identified as having long secondary periods (LSP). The cause of the LSP variation in CH Cyg and other stars is unknown. From our review of possible causes, we identify g-mode non-radial pulsation as the leading mechanism for LSP variation in CH Cyg. If g-mode pulsation is the cause of the LSPs a radiative region is required near the photosphere of pulsating AGB stars.