The EG Andromedae system hosts a white dwarf and an evolved giant star, with an orbital period of 482.5 days. The giant star is losing mass through its stellar wind at a rate higher than 10−6M☉/yr, and the white dwarf is accreting a fraction of this mass without forming an accretion disk. The white dwarf itself could emit a hot wind that interacts with the cooler one of the giant star, in addition to inducing the photoionization of the latter.[6] X-ray observations, however, failed to detect emission coming from colliding winds, but established the non-magnetic nature of the white dwarf and estimated its accretion rate at 1–10×10−7M☉/yr.[7]
The giant star does not fill its Roche lobe but there are still large uncertainties on its mass and radius.[5] Even the parameters of the white dwarf are not strictly constrained, but available models can give lower and upper limits.[6]
Spectrum
The optical spectral classification of EG Andromedae is M2IIIep,[2] the one of a cool giant star with a peculiar spectrum and strong emission lines. The white dwarf contaminates the spectrum of the giant star photoionizes the stellar wind, giving rise to the spectral peculiarities. Emission lines H-alpha and H-beta, as well as TiO and CaI ones, change in phase with the orbit.[5]
X-ray observation of EG Andromedae detected a hot plasma (at a temperature of 3 keV) that is likely situated in the outer boundary layer of the white dwarf, without any contribution from an accretion disk.[7]
Discovery of the photometric variability of EG Andromedae was announced in 1964 by Polish astronomer Tadeusz Jarzębowski, based on observations made from 1961 through 1963 at Wroclaw Observatory.[9]
To date, no outburst has been observed in EG Andromedae. The observed variability is well described by the two components eclipsing each other during the orbit. However, there is some evidence that the giant star and the wind flow have an intrinsic variation.[10]
Bow shock of a ghost planetary nebula
Sketch showing the direction of movement with an arrow. EG And is in the center of the ellipse, at the base of the arrow. The blue arc on the lower left is the bow shock SDSO1.
A giant nebula near the Andromeda galaxy (M31) was detected by amateur astronomers in doubly ionized oxygen and was cataloged as the Strottner–Drechsler–Sainty Object (SDSO1). Initially it was suspected to belong to M31, but other scenarios, such as supernova remnant, planetary nebula or stellar bow shock nebula were considered.[12][13] A later study did however find that SDSO1 is located within the Milky Way and it was considered to be a interstellar gas filament.[14] Another study found that EG Andromedae expelled a planetary nebula 400,000 years ago. This planetary nebula faded and is today a ghost planetary nebula (GPN) with a diameter of 20 parsec. EG And has a hypersonic speed of 107 km/s relative to the interstellar medium. Therefore the giant GPN formed the bow shock SDSO1 around it.[11]
Gallery
Discovery images of the giant oxygen nebula around M31
Wide-field image using H-alpha (red), oxygen [OIII] (blue) and a RGB combination.
Detailed view of part of the nebula by the group "Deep Sky Collective"
Image taken by the amateur astronomer group "Association of Widefield Astrophotographers"
1 2 3 4 5 EG And, database entry, Combined General Catalog of Variable Stars (GCVS4.2, 2004 Ed.), N. N. Samus, O. V. Durlevich, et al., CDS ID II/250 Accessed on line 2018-10-17.
1 2 3 4 5 6 7 8 Database entry, Catalogue of Stellar Photometry in Johnson's 11-color system (2002 Ed.), J. R. Ducati, CDS ID II/237 Accessed on line 2018-10-25.
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