Symbiotic star

(astronomy) A stellar object whose optical spectrum displays features indicative of two very different stellar regimes: a stellar spectrum whose flux distribution and absorption lines suggest the presence of a cool star, and emission lines which can be formed only in a much hotter medium.

A double star system in the late stage of stellar evolution. Since the symbiotic phase represents a brief span in the life of the binary, symbiotic stars are rare objects. The “near-official” list of symbiotic stars contains 188 safe entries; 15 of them extragalactic, and 30 suspected candidates.

Symbiotics are always associated with a nebular environment. The spectra indicate the presence of a cool M-type star with a surface temperature below 4000 K, and a hot nebula quite similar to a planetary nebula. The light output is variable on time scales of days, months, and years. See also Astronomical spectroscopy; Light curves; Nebula; Planetary nebula; Variable star.

Evidence has accumulated that all symbiotics are double stars with periods between one year and many dozens of years. Their orbits are sufficiently wide for the two stars not to be in direct contact, and both stars lie safely within their Roche lobe. Thus, for the two stars stellar evolution proceeds over the whole main-sequence lifetime practically uninfluenced by the partner. That changes dramatically in the later stages of their evolution. See also Doppler effect; Eclipsing variable stars.

The gas temperature in the symbiotic nebula, approximately 15,000 K, is much too low to collisionally ionize the atoms in the nebula to the observed degree. The nebular gas must be radiatively ionized by a small, very hot star. Observations and model calculations show the cool star to be a red giant. For the hot star, temperatures between 50,000 and 200,000 K are found. The star's size is that of a white dwarf or of a central star in a planetary nebula, between ∼0.01 and ∼0.1 solar radius.

The symbiotic nebula has, as a rule, the same chemical composition as expected from red giants. This is taken as evidence that the red giant suffers considerable mass loss. Symbiotics began as stars with masses of the order of 5 solar masses. The more massive star evolved faster through the main sequence, and when it arrived in the red giant region, shed most of its mass through a stellar wind. That material can occasionally still be detected in the wider environment. The star has become a hot white dwarf. The originally less massive star has retained its mass and has now entered the red giant phase, with large mass loss. See also Stellar evolution; White dwarf star.

A fraction of the mass lost by the red giant is captured by the white dwarf. The accretion liberates gravitational energy which can convert into radiative energy and be responsible for some of the irregular luminosity variations. When the white dwarf has accumulated a critical mass of hydrogen, thermonuclear reactions on its surface lead to an energy outburst lasting over 100 years. The energy production outburst can reach many thousand times that of the Sun. The mechanism resembles a nova explosion. However, a classical nova has higher peak energy output and shorter duration than a symbiotic nova. See also Cataclysmic variable; Nova.

Observations have shown traces of the mass formerly lost by the hot star. However, as roted above, the bulk of the matter now detected as an ionized nebula is due to the present mass loss of the red giant in its stellar wind. In its active phase the white dwarf may have a stellar wind of its own. In that case the nebular environment will be strongly structured by the collision of the two winds. This leads to shock zones with temperatures of several million kelvins. The origin of bipolar gaseous jets observed in several symbiotics is not yet known. Observational evidence indicates that the white dwarf possesses a strong magnetic field which could play a major role.