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The densest O-rich circumstellar shells with the 9.7 micron silicate line in strong absorption are classified from IRAS low resolution spectra as type 38 or 39. Some have white dwarf companions entraining an accretion disk from their red giant's wind, that provides an extra source of heat to surrounding dust, and gives them bluer integrated IR colors when it is on the near side of the shell (Ap.J., 396, 251). 26 % (11/42) of the class have distinctive blue IR colors.
A similar number must occur on the far side of the shell, leaving their colors unaffected, so >50% of the whole class have degenerate companions. Since just 50% of systems from an unbiased sample can have a red giant as a secondary, the further consequence is that almost all stars capable of achieving a superwind mass-loss rate that puts them into the 38-39 class are members of binary systems which have had components of comparable mass.
32 of 38 exhibit 1612 MHz masers. It is therefore evident that UV generated by an accretion disk is usually insufficient to eliminate most molecules from dense shells. Nevertheless six lack masers. These 6 O-rich stars are presumptively "fossil" symbiotic novae, that in the recent past collected enough mass at the surface of their companion white dwarfs for ignition. While a nova outburst quickly degrades the entire molecular content of a shell, it takes O(1000 yrs) to replace molecules from the wind once the outburst has died away. Recently active symbiotic novae, not surprisingly, exhibit no masers. We can recognize fossil symbiotic novae by an absence of 1612 MHz masers from dense O-rich shells.
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