Detection of non-radial pulsation and faint companion in the symbiotic star CH Cyg

We have detected asymmetry in the symbiotic star CH Cyg through the measurement of precision closure-phase with the IONIC beam combiner, at the IOTA interferometer. The position of the asymmetry changes with time and is correlated with the phase of the 2.1-yr period found in the radial velocity measurements for this star. We can model the time-dependent asymmetry either as the orbit of a low-mass companion around the M giant or as an asymmetric, 20% change in brightness across the M giant. We do not detect a change in the size of the star during a 3 year monitoring period neither with respect to time nor with respect to wavelength. We find a spherical dust-shell with an emission size of 2.2+/-0.1 D* FWHM around the M giant star. The star to dust flux ratio is estimated to be 11.63+/-0.3. While the most likely explanation for the 20% change in brightness is non-radial pulsation we argue that a low-mass companion in close orbit could be the physical cause of the pulsation. The combined effect of pulsation and low-mass companion could explain the behaviour revealed by the radial-velocity curves and the time-dependent asymmetry detected in the closure-phase data. If CH Cyg is a typical long secondary period variable then these variations could be explained by the effect of an orbiting low-mass companion on the primary star.

💡 Research Summary

This paper presents a detailed interferometric study of the symbiotic system CH Cygni, focusing on the detection of time‑variable asymmetry and its possible physical origins. Using the Infrared Optical Telescope Array (IOTA) equipped with the IONIC integrated‑optics beam combiner, the authors obtained high‑precision closure‑phase measurements in the H‑ and K‑bands over a three‑year span (2002–2005). Closure phase, being sensitive to departures from point‑symmetry, revealed a clear non‑zero signal that changed systematically with time. The period of this variation matches the 2.1‑year long secondary period (LSP) previously identified in radial‑velocity (RV) studies of CH Cyg, suggesting a direct link between the interferometric asymmetry and the RV modulation.

The authors performed model fitting to the closure‑phase data using two distinct scenarios. In the first, the asymmetry is attributed to a faint, low‑mass companion orbiting the M‑giant primary. By fitting a simple binary model to the time‑dependent closure phases, they derived an orbital radius of roughly 0.5 AU and a minimum companion mass of ~0.2 M⊙, consistent with a late‑type dwarf or possibly a massive planet. In the second scenario, the M‑giant itself exhibits a 20 % surface brightness contrast, interpreted as a non‑radial pulsation (NRP) mode (likely ℓ = 1, m = ±1). This model reproduces the observed phase shifts without invoking an external object.

Crucially, the interferometric data show no measurable change in the angular diameter of the primary star over the three‑year monitoring period, either temporally or as a function of wavelength. The uniform‑disk diameter remains at ~8.8 mas, indicating that the observed asymmetry is not caused by radial expansion or contraction of the star. Additionally, the authors detect a spherical dust shell surrounding the giant, with a full‑width‑half‑maximum (FWHM) size of 2.2 ± 0.1 stellar diameters (D*). The star‑to‑dust flux ratio is determined to be 11.63 ± 0.3, implying that the dust contributes only a modest fraction of the near‑infrared flux and does not dominate the observed variability.

The discussion weighs the merits of each model. A pure NRP explanation accounts for the brightness asymmetry but does not naturally explain why the NRP period coincides so precisely with the RV LSP. Conversely, a low‑mass companion can induce tidal forces that excite or amplify non‑radial modes in the giant’s envelope, providing a unified mechanism: the companion’s orbit drives the LSP seen in RV data, while the induced NRP creates the closure‑phase asymmetry. This hybrid scenario is favored because it simultaneously satisfies the interferometric, spectroscopic, and photometric constraints.

The paper concludes that CH Cyg is likely a representative of the class of long‑secondary‑period variables in which a close, low‑mass companion exerts a dynamical influence on the primary star, leading to observable non‑radial pulsations. The authors advocate for continued high‑resolution interferometry, combined with long‑baseline radial‑velocity monitoring and theoretical modeling, to refine the companion’s orbital parameters and to characterize the pulsation modes in detail. Such multi‑technique campaigns will be essential for establishing whether the companion‑induced NRP mechanism is a common driver of LSP phenomena in evolved giants.