EG Andromedae

EG Andromedae (often abbreviated to EG And) is a symbiotic binary in the constellation Andromeda. Its apparent visual magnitude varies between 6.97 and 7.80.[2]

EG Andromedae
Observation data
Epoch J2000      Equinox J2000
Constellation Andromeda
Right ascension 00h 44m 37.1876s[1]
Declination +40° 40′ 45.70303″[1]
Apparent magnitude (V) 6.97 – 7.80 variable [2]
Characteristics
Spectral type M2IIIep[2]
Apparent magnitude (U) 10.54[3]
Apparent magnitude (B) 8.93[3]
Apparent magnitude (V) 7.22[3]
Apparent magnitude (G) 6.3090[1]
Apparent magnitude (J) 3.65[3]
Apparent magnitude (H) 2.79[3]
Apparent magnitude (K) 2.56[3]
U−B color index 3.32[3]
B−V color index 1.71[3]
Variable type Symbiotic[2]
Astrometry
Radial velocity (Rv)−94.80±0.30[4] km/s
Proper motion (μ) RA: 8.452±0.072 [1] mas/yr
Dec.: −15.354±0.053[1] mas/yr
Parallax (π)1.4860 ± 0.0389[1] mas
Distance2,190 ± 60 ly
(670 ± 20 pc)
Orbit
Period (P)482.5±1.3 days[5]
Eccentricity (e)0[5]
Inclination (i)60[6]°
Semi-amplitude (K1)
(primary)		7.30±0.13[5] km/s
Details
White dwarf
Mass0.4[7] M☉
Radius1.9–2.3×10−2[6] R☉
Luminosity12.9-38.4[6] L☉
Surface gravity (log g)7.5[6] cgs
Temperature80–95×103[6] K
Donor star
Mass1.1 – 2.4[5] M☉
Temperature3730±130[5] K
Other designations
2MASS J00443718+4040456, BD+39 167, HD 4174, HIP 3494, SAO 36618, TYC 2801-1704-1
Database references
SIMBADdata

System

The EG Andromedae system hosts a white dwarf and an evolved giant star, with an orbital period of 482 days and a half. The giant star is losing mass through its stellar wind at a rate higher than 10−6 M☉/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 1-10×10−7 M☉/yr.[7]

The giant star doesn't 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]

The white dwarf is best studied in the ultraviolet, where also highly ionized species sulfur, oxygen, nitrogen, carbon and phosphorus can be identified with their absorption or emission lines.[6]

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]

Variability

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.[8]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. 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.
  3. 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.
  4. de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
  5. Kenyon, S. J.; Garcia, M. R. (2016). "EG Andromedae: A New Orbit and Additional Evidence for a Photoionized Wind". The Astronomical Journal. 152 (1): 1. arXiv:1604.04635. Bibcode:2016AJ....152....1K. doi:10.3847/0004-6256/152/1/1. S2CID 119203162.
  6. Sion, E. M.; Godon, P.; Mikolajewska, J.; Sabra, B.; Kolobow, C. (2017). "FUSE Spectroscopy of the Accreting Hot Components in Symbiotic Variables". The Astronomical Journal. 153 (4): 160. Bibcode:2017AJ....153..160S. doi:10.3847/1538-3881/AA62A9. PMC 5810147. PMID 29456255.
  7. Nuñez, N. E.; Nelson, T.; Mukai, K.; Sokoloski, J. L.; Luna, G. J. M. (2016). "Symbiotic Stars in X-Rays. III. Suzaku Observations". The Astrophysical Journal. 824 (1): 23. arXiv:1604.05980. Bibcode:2016ApJ...824...23N. doi:10.3847/0004-637X/824/1/23. S2CID 119292446.
  8. Skopal, A.; Shugarov, S.; Vaňko, M.; Dubovský, P.; Peneva, S. P.; Semkov, E.; Wolf, M. (2012). "Recent photometry of symbiotic stars". Astronomische Nachrichten. 333 (3): 242. arXiv:1203.4932. Bibcode:2012AN....333..242S. doi:10.1002/asna.201111655.
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