A Z Andromedae star is a type of symbiotic star (which, in turn, is a type of cataclysmic variable) consisting of a close binary system in which a hot star ionizes part of an extended envelope of gas that has come from a cooler companion (of spectral class M, R, N, or S). The combined spectrum of the system shows the superposition of absorption and emission spectral features together with irregular variability (of up to 4 magnitudes in the visual region) that are characteristic of symbiotics.

 

The classification of symbiotics is confusing because of the great diversity of these stars and the many gaps in our knowledge about them. In the General Catalogue of Variable Stars (GCVS), Z Andromedae stars (ZANDs) are the only recognized subcategory of symbiotics, so that Z Andromedae itself is officially regarded as not only the prototype ZAND but also the prototype symbiotic. However, objects such as RR Telescopii stars, R Aquarii, and CH Cygni have unique characteristics that set them apart within the overall symbiotic family.

 

The strange nova-like spectral features and variability of Z Andromedae were discovered in 1901 by Williamina Fleming on Harvard College Observatory plates. When quiescent, which is most of the time, the only brightness changes shown are those of a small amplitude semi-regular red variable with an M-type spectrum, a period of about 700 days, and an average magnitude of about 11. Every 10 to 20 years, however, Z Andromedae becomes very active, brightening by about 3 magnitudes. The large amplitude outbursts are then followed by smaller outbursts of decreasing amplitude, after which the star fades again to quiescence. The brighter state occurs either abruptly or is preceded by a smaller outburst. The brightest recorded magnitude was during the 1939 outburst when the star reached a mean visual magnitude of 7.9. During outburst, the star gets bluer and the spectrum is dominated by the hot, compact star with a B-type shell spectrum. P Cygni profiles arise displaying blueshifted absorption lines, indicative of an expanding shell as seen in novae. As the shell dominated spectrum slowly fades, the star get redder, the P Cygni features disappear, the shell spectrum weakens and disappears, and the system returns to its slow and semi-regular variations. Its lines almost disappear at minimum, and the titanium oxide bands from the red star become prominent.

 

In the Z And system, the dwarf component appears to accrete mass from a stellar wind emanating from the red giant component. Since it seems to be a detached system, the stellar wind may be a key component during both quiescence and outburst. The accreted matter may then form a disk around the white dwarf component, however the existence of a disk has not been confirmed. The outbursts are thought to derive from the blue star, but the red star appears to be variable as well. Many of the basic stellar parameter such as stellar mass, and inclination of the system are unknown and prevent further knowledge and theoretical models of the system.