We present new formulations of binary colliding wind models appropriate to symbiotic star systems. The derived models differ from previous formulations in assuming mixing of the shocked material from both incoming streams, rather than postulating a self-sustaining contact discontinuity. The CWb model (colliding winds, binary) extends the work of Girard and Willson by the derivation of an adiabatic temperature, the consideration of radiative cooling, the inclusion of thermal pressures in the incoming winds, and the treatment of interaction shells of finite thickness and density. The finite thickness of the interaction shell allows for calculation of its radiative intensity distribution. The CWc model (colliding winds, concentric) is a similar extension of the model of Kwok, Purton, and Fitzgerald. It is derived in a manner parallel to that of the CWb model, thereby facilitating a unification of the two models. A unified model is desired since wind collisions in symbiotic systems should include aspects of both CWb and CWc interactions. Two examples of model applications are presented: a comparison of the flux densities arising from colliding winds (CWb model) with those arising from the ionization of the surrounding medium (STB model) in the galactic population of symbiotic stars, and model imaging of the symbiotic nova HM Sge.
AG Pegasi is one of about 30 symbiotic stars whose radio emission has been observed. It is also the only known symbiotic nova system to have taken longer than 100 years to return to quiescence after a major outburst (c.1850). The stellar components of the system include an M3111 giant and a WN6 compact object (Boyarchuk 1967a). In this study, the radio emission from AG Pegasi is observed Dedication What spectacle confronted them when they, first the host then the guest, emerged silently, doubly dark, from obscurity by a passage from the rere of the house into the penumbra of the garden?The heaventree of stars hung with humid nightblue fruit.
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.
HM Sge is a symbiotic binary star that underwent a nova‐like outburst in 1975. Its radio emission has been monitored using MERLIN and the VLA, and maps from 1992 to 1997 are presented. Thermal emission within 0.4 arcsec of the optical peak is elongated north‐‐south and hotspots appear to be moving around an anticlockwise ellipse. If this is due to motion in an inclined disc, the period is ∼ 90 yr. Eyres et al. predicted that if this is due to Mach shocks in the post‐nova wind, proper motions should be seen in expansion. This has not been detected since 1992, so an alternative model is adopted, based on the work of Eyres et al. and Kenny et al., of colliding winds in a binary system following the nova‐like outburst. The emission peaks appear to be corotating with the binary orbit as the ionization front and the hot wind from the white dwarf interact with the Mira wind. The positions of the stars are estimated, at a separation of ∼ 25 au. A distance of ∼ 1 kpc is most consistent with the observations reported here. On arcsecond scales the emission is extended east‐‐west, consistent with a biconical outflow arising from the collimation of the nova outburst through interaction with the pre‐existing cool wind. The presence of non‐thermal emission at a separation of ∼700 mas from the stars is confirmed. This is very unusual at such a distance from low‐mass stars, and could arise from synchrotron emission in a μT magnetic field. A model for this is developed, which shows that non‐thermal emission is expected to decline within decades as the nova wind decelerates. This could explain the non‐detection of non‐thermal emission in other symbiotic stars.
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