EG Andromedae

Lore & Background

The EG Andromedae system hosts a white dwarf and an evolved giant star. The giant star loses mass through its stellar wind at a rate higher than 10⁻⁶ M☉/yr, and the white dwarf accretes a fraction of this mass without forming an accretion disk. The white dwarf may emit a hot wind that interacts with the cooler wind of the giant star, in addition to photoionizing the latter. X-ray observations failed to detect emission from colliding winds but established the non-magnetic nature of the white dwarf and estimated its accretion rate at 1–10×10⁻⁷ M☉/yr. The giant star does not fill its Roche lobe, and there are large uncertainties on its mass and radius; even the white dwarf parameters are not strictly constrained.

The optical spectral classification is M2IIIep, indicating a cool giant with a peculiar spectrum and strong emission lines. The white dwarf contaminates the spectrum and photoionizes the stellar wind, causing spectral peculiarities. Emission lines H-alpha and H-beta, as well as TiO and CaI lines, change in phase with the orbit. The white dwarf is best studied in the ultraviolet, where highly ionized species of sulfur, oxygen, nitrogen, carbon, and phosphorus are identified. X-ray observations detected a hot plasma at 3 keV, likely in the outer boundary layer of the white dwarf, with no contribution from an accretion disk.

Discovery of photometric variability was announced in 1964 by Polish astronomer Tadeusz Jarzębowski, based on observations from 1961 through 1963 at Wroclaw Observatory. No outburst has been observed. The variability is well described by the two components eclipsing each other during the orbit, though there is some evidence of intrinsic variation in the giant star and wind flow.

Reader's Guide

EG Andromedae is significant as a symbiotic binary that has not exhibited outbursts, contrasting with many such systems. Its variability is primarily due to mutual eclipses, with some intrinsic variation suspected. The system is also notable for its association with a ghost planetary nebula (GPN) expelled 400,000 years ago, now 20 parsecs in diameter. The star's hypersonic speed of 107 km/s relative to the interstellar medium causes this GPN to form the bow shock SDSO1, initially mistaken for an object in the Andromeda galaxy but later identified as a Milky Way interstellar gas filament. This connection provides a rare example of a ghost planetary nebula interacting with the interstellar medium. The white dwarf's non-magnetic nature and lack of an accretion disk, along with the hot plasma detected in its boundary layer, offer insights into accretion processes in symbiotic systems. The large uncertainties in the stellar parameters underscore the challenges in modeling such binaries.

Did You Know?

• EG Andromedae has an orbital period of 482.5 days.
• Its photometric variability was discovered by Tadeusz Jarzębowski based on observations from 1961–1963.
• The white dwarf accretes mass without forming an accretion disk.
• The system expelled a planetary nebula 400,000 years ago, now a ghost planetary nebula 20 parsecs across.