V Sagittae
Variable star in the constellation Sagitta
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V Sagittae or V Sge is a cataclysmic variable in the constellation Sagitta. It is the only super soft X-ray source non-magnetic cataclysmic variable found so far. Lidiya Tseraskaya discovered the variability of this star, in 1902.[7] It appeared with its variable star designation in Annie Jump Cannon's 1907 work Second catalogue of variable stars.[8]
 | |
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| Constellation | Sagitta |
| Right ascension | 20h 20m 14.691s[1] |
| Declination | +21° 06′ 10.44″[1] |
| Apparent magnitude (V) | 8.6-13.9[2] |
| Characteristics | |
| Evolutionary stage | White dwarf (primary)[3] |
| Spectral type | Be[4] |
| Variable type | eclipsing and cataclysmic[2] |
| Astrometry | |
| Proper motion (μ) | RA: −2.133 mas/yr[1] Dec.: −6.489 mas/yr[1] |
| Parallax (π) | 0.3310±0.0206 mas[1] |
| Distance | 9,900 ± 600 ly (3,000 ± 200 pc) |
| Absolute magnitude (MV) | −2.2[5] |
| Orbit | |
| Period (P) | 12.34[3] hours |
| Semi-major axis (a) | 4.36 R☉[6] |
| Inclination (i) | 65–80[3]° |
| Semi-amplitude (K1) (primary) | 320[6] km/s |
| Semi-amplitude (K2) (secondary) | 85[6] km/s |
| Details | |
| White dwarf (primary) | |
| Mass | 1.0[3] M☉ |
| Donor (secondary) | |
| Mass | 0.8–1.0[3] M☉ |
| Other designations | |
| AAVSO 1015+20, V Sge, GSC 01643-01764 | |
| Database references | |
| SIMBAD | data |
Characteristics
There are two models that have been proposed to explain V Sagittae's properties. One says that the system is composed of a white dwarf accreting mass from a companion via an accretion disk, while the other model says that the system is composed of two hot stars (nearly) forming a contact binary. Both models have been disputed. Smak (2022) notes that the primary component shows similarities with Wolf-Rayet stars and the model with a white dwarf and its accretion disk does not explain many aspects of the system, including orbital period variations and mass loss from the primary, supporting a configuration with a Wolf-Rayet star and a main sequence star.[10] However, a 2025 study by Hakala, Charles and Rodríguez-Gil found that the 'hot binary' model fails to explain multiple properties of V Sagittae, such as the system's variability, strong Hα emission and presence of stationary, double-peaked narrow emission lines, while the white dwarf model explains nearly all of them.[3]
Material from the donor is accreting onto the primary at an exponentially increasing rate, generating a huge stellar wind. The doubling time for the accretion rate, and hence for the system luminosity, is about 89±11 years.[5] It is predicted that the system will erupt as a nova some time between 2067 and 2099, at which point it will become one of the brightest stars in the sky.[11][12]