Symbiotic Star UV Emission and Theoretical Models

Abstract

Observations of symbiotic stars in the far UV have provided important information on the nature of these objects. The canonical spectrum of a symbiotic star, e.g. RW Hya, Z And, AG Peg, is dominated by strong allowed and semi forbidden lines of a variety of at least twice ionized elements. Weaker emission from neutral and singly ionized species is also present. The Mg II doublet is usually very strong and may be associated with the M giant primary. A continuum may or may not be present in the 1200 – 2000 A range but is generally present in the range 2000 – 3200 A range, the latter arising from free-free and bound-free emission in the same nebula that is responsible for the UV line emission (CI Cyg, RW Hya, RX Pup). The suspected hot subdwarf continuum is seen in some cases in the range 1200 – 2000 A (RW Hya, AG Peg, SY Mus). High resolution observations of lines are important because they yield information on densities, temperatures and sizes of the line emitting region(s); in general, however, such observations are difficult and time-consuming to obtain with “IUE”. Densities are found to range from a low of ∼ 106 cm−3 in R Aqr and V1016 Cya through typical values of 108 – 109 cm−3 in RW Hya to a high of ∼ 1011 cm−3 in Z And. Sizes range from ∼ 1011 in the resonance line emitting region in Z And to ∼ 1014 – 1015 cm in the more extended regions of R Aqr and V1016 Cyg. Temperatures are generally 20, 000 K. High resolution profiles generally show single component nebular emission (RW Hya, SY Mus, AG Peg, V1016 Cyg). Complex profiles showing multiple velocity structure present in rings and/or streamers have been detected in RX Pup. Continua, which very often are flat, are harder to interpret but it seems that line blanketed models of B, A and F-type stars generally fail. A combination of different sources of continua seems ro be required: nebular emission (particularly for λ 2000 A); hot subdwarf continuum; and/or continuum arising in an accretion disk. The presence of an accretion disk is difficult to demonstrate and to this date the best candidate for accretion to a main sequence star remains CI Cyg. A number of equations have been derived by the author that can yield the accretion parameters from the observable quantities. Boundary layer temperatures ∼105 K and accretion rates >∼ 10−5 M⊙s/yr are required for accreting main sequence companions. To this date, though, most of the symbiotics may only require the presence of a ∼ 105 K hot subdwarf.KeywordsAccretion DiskThick DiskMain Sequence StarSingle StarBolometric LuminosityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.