The symbiotic star CH Cygni – I. Non-thermal bipolar jets

Very Large Array surface brightness and spectral index maps of the evolving extended emission of the triple symbiotic star CH Cygni are presented. These are derived from observations at 4.8, 8.4 and 14 GHz between 1985 and 1999. The maps are dominated by thermal emission around the central bright peak of the nebula, but we also find unambiguous non-thermal emission associated with the extended regions. Our observations confirm that this is a jet. The central region has been associated with the stellar components through Hubble Space Telescope imaging. If the jets are the result of ejection events at outburst, expansion velocities are consistent with those from other measurement methods. We propose that the non-thermal emission is caused by material ejected in the bipolar jets interacting with the circumstellar wind envelope. The resulting shocks lead to local enhancements in the magnetic field from the compact component of the order of 3 mG.     

Eclipsing phenomena of the symbiotic star CH Cygni

Eclipsing phenomena of the inner binary with a period of 756 d in the triple symbiotic system CH Cyg may have been detected in detailed spectrophotometric observations. The eclipse of the hot component by the red giant started on 1994 October 11 and finished between 1995 January 8 and 18. The ingress duration was less than one day. The radius of the red giant is estimated to be 288 ± 15 R⊙ from the duration of the eclipse. Assuming the bolometric correction of the red giant (M7 III) as 4, the distance to this object is estimated to be 307 ± 32 pc, which agrees well with that obtained in the observations by Hipparcos . The interstellar extinction in the direction of this object may be much lower than that in the nearby areas. It has been suggested that the outer binary system with an orbital period of about 15 yr is an eclipsing one. It seems unlikely, however, that the variation of the activity of this object with a time-scale of more than 10 yr was a result of eclipses.     

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 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.     

The symbiotic system CH Cygni: An analysis of the shocked nebulae at different epochs

We analyse the line and continuum spectra of the symbiotic system CH Cygni. We adopt the colliding‐wind model to explain the symbiotic system at different phases. Peculiar observed features such as flickering, radio variation, X‐ray emission, as well as the distribution of the nebulae and shells throughout the system are investigated by modelling the spectra at different epochs. The models account consistently for shock and photoionization and are constrained by absolute fluxes. We find that the reverse shock between the stars leads to the broad lines observed during the active phases, as well as to radio and hard X‐ray emission, while the expanding shock is invoked to explain the data particularly during the transition phases (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A new interpretation of the remarkable X-ray spectrum of the symbiotic star CH Cyg

We have re-analysed the ASCA X-ray spectrum of the bright symbiotic star CH Cyg, which exhibits apparently distinct hard and soft X-ray components. Our analysis demonstrates that the soft X-ray emission can be interpreted as scattering of the hard X-ray component in a photoionized medium surrounding the white dwarf. This is in contrast to previous analyses in which the soft X-ray emission was fitted separately and assumed to arise independently of the hard X-ray component. We note the striking similarity between the X-ray spectra of CH Cyg and Seyfert 2 galaxies, which are also believed to exhibit scattering in a photoionized medium.     

The Manchester occulting mask imager (MOMI): first results on the environment of P Cygni

The design and first use of the Manchester occulting mask imager (MOMI) is described. This device, when combined with the Cassegrain or Ritchey–Chretien foci of large telescopes, is dedicated to the imagery of faint-line emission regions around bright central sources. Initial observations, with MOMI on the Nordic Optical Telescope (NOT), of the V =4.8 mag P Cygni environment have revealed a ≥5 arcmin long [N  II ] λ6584-Å emitting filament projecting from the outer nebular shell of this luminous blue variable (LBV) star. The presence of a monopolar lobe older than both the inner (22 arcsec diameter) and outer (1.6 arcmin diameter) shells is suggested.