Readers of Universe Today are familiar with the word nebulae. Some of nature's most beautiful forms are lit up with radiation from nearby stars.
An international team of astronomy have discovered a new type of emission around stars that they are calling galactic emission nebulae.
There is a star at the center of this discovery. YY Hya is a variable star with a K dwarf star and a hot white dwarf partner. The stars are surrounded by gas that was given off by the star that eventually turned into a white dwarf. It went through a red giant phase when it dispersed its outer layers of gas. Both stars evolve as if they were alone, with the stellar radiation lighting up the dispersed gas.
The study presenting this discovery is titled "YY Hya and its interstellar environment". The author is from the Department of Astro and Particle Physics at Innsbruck University.
Kimeswenger said that normal stars inflate into red giant stars at the end of their lives. The evolution of stars at the end of their lives is affected by the large fraction of stars in pairs. The inflating outer part of a star becomes a common envelope around both stars in close systems. The core of the two stars are completely undisturbed and follow their evolution like single stars.
The way for this one was paved by previous discoveries. Astronomers have found stars in the same arrangement but without an envelope. They have never seen one because of the size of the envelope.
The images are of the galactic nebula. The northeastern and southwestern lobes are on the left and right, respectively. The primary part of the nebula is on the outskirts of these two portions. Kimeswenger et al 2021.
Over 15 light-years, the envelope is vast. Astronomers think that the envelope would be disrupted by other stars at that size. YY Hya is undisturbed by other gas clouds.
The main cloud is more than a million times larger than the distance of the earth to the sun and more than a million times larger than the distance of our sun to its nearest neighbour. There are fragments as large as 39 light-years apart. Kimeswenger said that since the object is slightly above the Milky Way, the nebula was able to develop undisturbed.
The size of the common envelopes could make their discovery difficult. The field of view of modern telescopes is smaller than this one. They are too large for a field of view and are very faint. Their lifetime is shorter when considered in the time scales. Kimeswenger said it was only a few hundred thousand years.
There are images of the central portion of the nebula with YY Hya dotted pink. There are other stars removed from the image on the right. On the right of the images is a view of the southwestern part of the nebula. Kimeswenger et al 2021.
A group of French and German amateur astronomer discovered the discovery. They were looking at historical images from the 1980s. They found a fragment of a nebula while looking for unknown objects.
The University of Innsbruck has a Department for Astro and Particle Physics. The amateur findings were combined with observations from the last 20 years with the help of the professional astronomer. The astronomer from the University of Innsbruck looked at the amateur data and ruled out a planetary nebula.
The vast scale of the nebula was made apparent by subsequent observations. Astronomers in the USA continued to make observations that provide essential data. Existing UV images from GALEX and the Catalina Real-time Transient Survey were included in the mix.
The result?
The model the team constructed shows a close pair of stars. The white dwarf has a temperature of about 130,000 Celsius (120,000 F), while the K dwarf has a temperature of 4400 degrees Celsius (8000 F.) The K dwarf is small. The stars are only 2.2 solar radii apart and they are rapidly circling each other.
The K dwarf is in close proximity to the white dwarf. The heating causes extreme phenomena in the spectrum of the star.
The pair are inside the shell of gas ejected by the white dwarf. The shell is large and has a solar mass. It is more massive than the white dwarf and the main-sequence K dwarf. The white dwarf ejected the shell 500,000 years ago.
That leads to another possibility.
The team of astronomer theorize that the star is related to the nova, which was seen by Chinese and Korean astronomer in 1086. It is possible that this system is related to a nova observation made in 1086 by Korean and Chinese astronomer. Kimeswenger said that the positions of the historical observations match those of the object described here.
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