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

Abstract

International audienceWe have detected asymmetry in the symbiotic star CH Cyg through the measurement of precision closure phase with the Integrated Optics Near-Infrared Camera (IONIC) beam combiner, at the infrared optical telescope array interferometer. The position of the asymmetry changes with time and is correlated with the phase of the 2.1-year 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 per cent 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* full width at half-maximum 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 per cent 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

First spatial resolution of the stellar components of the interacting binary CH Cygni

“…The 2‐mag difference between the visual optical brightness of the red giant and its companion is reasonable for CH Cyg's mild activity stage indicated by the AAVSO light curve and remarkable H i , Fe ii and [O iii ] line emission detected in 2004. Such a symbiotic blue companion is also consistent with the null detection in infrared reported by Pedretti et al (2009).…”

“…The mechanism causing the precession is not clear, although Crocker et al favoured warping of the collimating accretion disc by a magnetic white dwarf. We also note that the hypothetical close companion orbiting the red giant (Pedretti et al 2009) has little if any influence on the jet formation and geometry unless the jets originate from the red giant which in our opinion is very unlikely.…”

“…In 2004–2006, Pedretti et al (2009) performed interferometric observations of CH Cyg using the infrared optical telescope array and the Keck‐1 telescope with aperture masking. Combining their null detection of a companion at the better than 100:1 level at the H band (Δ H = H h – H g ≳5 mag) with our optical detection (Δ m opt ∼Δ V ∼2 mag), we estimate the near‐infrared colour of the companion relative to the red giant of ( V − H ) h − ( V − H ) g ≲−3.…”

“…Recently, Hinkle, Fekel & Joyce (2009, hereafter HFJ) refined the orbital elements of CH Cyg and showed that the long period indeed results from the symbiotic binary orbit, and identified the g‐mode non‐radial pulsation as the leading mechanism for the LSP modulation. Finally, Pedretti et al (2009) explained the LSP of CH Cyg by the combined effect of non‐radial pulsation and a low‐mass companion orbiting the giant.…”

CO observations of symbiotic stellar systems

“…The presence of a relatively high amount of mass in the inner circumstellar shells of CH Cyg is confirmed by its FIR dust emission and by the identification of a hot dust shell, a few times larger than the stellar photosphere, which significantly contributes to the total NIR flux (Pedretti et al 2009). The values derived here for the mass-loss rate and typical temperature in the inner shells are quite similar to those found by Taranova & Shenavrin (2007) from analysis of the dust FIR emission of recently ejected material.…”