Emission-line CCD imaging of three southern symbiotic stars

Abstract

Symbiotic stars that are strong radio sources and have cool dust emitting in the IR are expected to have extended emission nebulae around them. In order to search for such emission nebulae, CCD imaging of three symbiotic stars (R Aqr, RR Tel and H1-36) with narrow-band filters centered at the emission lines of forbidden O III 5007, H-alpha 6563, N II 6584, S II 6717 + 6731 are carried out. RR Tel and H1-36 images do not show any extended nebulosities around them. The CCD image of the R Aqr nebulosity in the high excitation forbidden O III line is different from its image in H-alpha and the low excitation lines of forbidden N II and S II indicating ionization-stratification in the nebula. In H1-36 the optical nebulosity (if it exists) is smaller than about 2 arcsec while the radio image size is known to be large (about 5 arcsec). This behavior is opposite to that seen in R Aqr in which the radio emission comes from the core region of a much larger optical nebulosity. Interstellar and/or circumstellar extinctions are suggested to be responsible for this difference.