 | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Hercules |
| Right ascension | 18h 46m 31.471s[1] |
| Declination | +12° 14′ 02.10″[1] |
| Apparent magnitude (V) | 5.3Max. 19.1Min.[2] |
| Characteristics | |
| Variable type | Nova,[2]eclipsing binary[3] |
| Astrometry | |
| Proper motion (μ) | RA: −2.691±1.666[1]mas/yr Dec.: −6.600±1.930[1]mas/yr |
| Parallax (π) | 0.4642±0.7106 mas[1] |
| Distance | 2530+3434 −636[2] pc |
| Other designations | |
| Nova Her 1991,AAVSO 1841+12,2MASS J18463156+1214007,Gaia DR2 4504548029183559552[4] | |
| Database references | |
| SIMBAD | data |
V838 Herculis, also known asNova Herculis 1991, was anova which occurred in the constellationHercules in 1991. It was discovered byGeorge Alcock ofYaxley, Cambridgeshire, England at 4:35 UT on the morning of 25 March 1991. He found it with 10×50 binoculars, and on that morning itsapparent visual magnitude was 5 (making it visible to the naked eye).Palomar Sky Survey plates showed that before the outburst, the star was atphotographic magnitude 20.6 (blue light) and 18.25 (red light).[5]
V838 Herculis declined from its peak brightness very quickly, fading by 2 magnitudes in less than three days, making it one of the fastest classical novae ever recorded.[7]
All novae are binary stars, with a "donor" star orbiting awhite dwarf. The two stars are so close to each other that material is transferred from the donor to the white dwarf. Because the distance between the two stars is comparable to the radius of the donor star, novae are ofteneclipsing binaries, and V838 Herculis does show such eclipses. The eclipses were first detected a few weeks after the nova outburst, and they show the system's orbital period to be 7 hours, 8 minutes and 36 seconds as of 1991. The shape of the eclipse light curve suggests that the white dwarf itself is not being eclipsed by the donor, but rather that theaccretion disk surrounding the white dwarf is being partially eclipsed. The depth of the eclipses was initially only 0.1 magnitudes, but grew over the year following the nova event to 0.7 magnitudes, indicating that the accretion disk re-established itself after the nova outburst during that time.[3][7]
The white dwarf in the V838 Herculis system is an oxygen-neon-magnesium white dwarf,[8] with a mass of about 1.35M☉, which is near theChandrasekhar limit for white dwarf masses.[9][10] The donor star is believed to be a main sequence star.[11]
