V605 Eagle

V605 Eagle ( V605 Aquilae , V605 Aql ) is the variable central star of the planetary nebula Abell 58 in the constellation Eagle . It is an unusual carbon-rich and hydrogen-poor star.

Eagle V605 was originally noted as a new star in 1919, but then an unusual variability was noted. At the time of maximum brightness, the apparent magnitude was 10.4. ^[3] A study of the original photographs showed that until 1918 the apparent magnitude was not brighter than the 15th, after 1918 the star increased its brightness to 12th magnitude. The star had an 11th magnitude for about a year, then there was a fading. At the end of 1291, the star again increased its brightness to the 12th magnitude, as in 1923, then the brightness diminished significantly. ^[6] At the time of the flares, the spectral class was R0, which corresponded to a cold carbon star with a hydrogen deficiency similar to variable stars of the R type of the Northern Crown . ^{[2] [7]}

Eagle V605 was subsequently detected several times, while it had a visible magnitude of 18–20, however, it is possible that these were the discoveries of several parts of the nebula near the star. The images obtained by the Hubble telescope showed that the star itself is weaker than the 23rd magnitude, although the nebula is a bright infrared object of irregular shape with a diameter of 2.5 ". It is assumed that the star retained its luminosity , but the radiation is strongly absorbed by the material of the nebula. ^[4] Although the star cannot be observed directly, the scattered light showed that the spectral class of the star is [WC4], which differs from the class of the star at the peak of brightness. In 2013, the central star had a visible magnitude of 20.2, estimating the absorption etsya 4 star magnitude. Now spectral class is assumed to be [WC4] , which corresponds to a poor hydrogen, but carbon-rich and helium stars with strong emission lines. ^[2]

According to estimates in 1921, the surface layers of the star consisted of 98% helium and 1% carbon, which is typical for variable stars of the R type of the Northern Crown. In 2006, the content of chemical elements was estimated at 55% helium, 45% carbon, 5% oxygen, which is typical for WC type stars. Both classes of stars are unusual compared to most stars, consisting mainly of hydrogen. ^[four]

Starting around 1970, the temperature began to rise, now it is around 90,000 K. It is widely believed that a star is a reborn star that has passed the stage of the asymptotic branch of giants and is experiencing a late heat pulse. ^[4] Another explanation is that the outburst could be new, which was formed by an oxygen-neon white dwarf. To explain the difficulties with the new theory, a scenario is proposed for the merger of a white dwarf and an ordinary companion star. ^[2]

V605 Orla is located in the center of a planetary nebula and, possibly, is the ancestor of the nebula. The observed planetary nebula is almost spherically symmetrical and is older than the 1919 outbreak. The smaller nebula resulting from the flare is non-spherical. The shape as a whole can be the sum of a disk and a bipolar nebula or torus containing a dust band. A dust band or disc completely covers the center star. A comparison of the changes in the angular dimensions of the nebula and its radial velocity gives an estimate of the distance to the star of 4600 parsecs . ^[2]