U Gemini

The star was first discovered by D. R. Hind on December 15, 1855 ^[1] , which initially classified it as new , since its brightness very quickly fell below the limit value that could be viewed with his telescope . Its true nature was revealed three months later, when a flashed star was rediscovered by R. N. Pogson ^[11] on March 24, 1856 ^[1] . In Russia, L. P. Ceraskaya is considered its discoverer ^[12] . The star is still actively observed by professionals and amateur astronomers .

The period of revolution of a satellite star in the binary system of U Gemini is very short - 4 hours 11 minutes. This proximity of the orbit makes the system slightly variable, since the components overshadow each other at each revolution. Usually, the joint apparent magnitude varies between 14 ^m , 0 and 15 ^m , 1; but during a flash, a star can increase its brightness a hundred times: up to the 9th magnitude. The interval of flashes, in contrast to eclipses, is not periodic, but cyclical. Although the average interval is estimated at 100 days, in fact it is very irregular, ranging from 62 days to 257. Like all dwarf novae , outbreaks are the result of periodic bursts of matter from the accretion disk around the white dwarf , caused by instability in the disk itself. Estimation of the distance to Gemini U is very inaccurate and varies from 52 parsecs (170 light-years ) to 112 parsecs (370 light-years ), with the best estimate of 82 parsecs (270 light-years ) ^[2] .

Spectroscopic studies show that the Gemini U system consists of a white dwarf with a mass of 1.20 ± 0.05 M _☉ and a radius of 4000 km. ^[9] , as well as a red dwarf mass of 0.42 ± 0.04 M _☉ ^[8] . The stars rotate around each other with an inclination of 69.7 ° ± 0.7 ° as seen from Earth ^[2] . The semimajor axis of their orbit is 1.55 ± 0.02 R _☉ . The red dwarf is a dim star of spectral class M (M6V), its contribution to the total luminosity of the system is 5%. The white dwarf is surrounded by an accretion disk whose outer radius is 0.61 R _☉ , and its size is almost equal to the distance to the inner Lagrange point L1, which for this system is 0.63 R _☉ ^[8] . A comparison of the integrated spectrum and the synthesized spectral models indicates a temperature in the accretion disk of about 30,000 K. Also, these studies indicate an abundance of carbon in the system: about 10% of the solar value and nitrogen , which is almost 4 times more than in the Sun ^[2] .

There are two types of outbreaks in the U Gemini system: some are shorter and others are longer and slightly brighter. Outbreaks of the first type last about nine days, and the second 16 days ^[1] .

On the light curve of U Gemini, there are not only flashes, but also attenuations of various shapes associated with eclipses. In a calm state, which lasts about two hours, the brightness of the star, as a rule, has a value of 14 ^m , 6. About 1.7 hours before the eclipse, the total luminosity of the system constantly increases, reaching a maximum (V = 14 ^m , 0) 30 minutes before the middle of the eclipse. The eclipse begins about 15 minutes later (V = 14 ^m , 2). Then the system reaches the minimum luminosity level at V = 15 ^m , 1, after which the luminosity level is quickly restored to V = 14 ^m , 4. The total duration of the eclipse is approximately 20-25 minutes. In the next two hours, the system returns to its normal brightness level of V = 14 ^m .6 ^[1] .