Astronomers discover a whole new type of nebula

Most readers of Universe Today are familiar with nebulae. They are gaseous structures lit by the radiation of nearby stars, and they are among the most beautiful forms in nature.

With the help of amateur astronomers who laid the groundwork, an international team of astronomers have discovered a new type of nebulae around binary stars that they call galactic emission nebulae.

The binary star YY Hya is at the center of this discovery. YY Hya is a periodic variable star composed of a dwarf star K and a warm white dwarf partner (WD). The stars are nestled in a common envelope of gas given off by the star which eventually became a white dwarf. Before that, it went through a red giant phase when it dispersed its outer layers of gas into space. Within this common envelope, the two stars continue to evolve as if they were alone, stellar radiation illuminating the dispersed gas.

The study presenting this discovery is entitled “YY Hya and his interstellar environmentAnd is published in Astrophysics and Astronomy. The main author is Stefan Kimeswenger from the Department of Astronomy and Particle Physics at the University of Innsbruck, Austria.

In a press release, Kimeswenger said, “Towards the end of their life, normal stars swell into red giant stars. Since a very large fraction of stars are in binary pairs, this affects evolution at the end of their life. In close binary systems, the swollen outer part of a star merges into a common envelope around the two stars. However, inside this gas envelope, the nuclei of the two stars are hardly disturbed and follow their evolution as independent individual stars.

Earlier discoveries paved the way for this one. Astronomers have found binary stars in this same arrangement but without a fully developed envelope. The reason they haven’t seen one might be the size of the envelope.

These are H-alpha images of the galactic nebula around YY Hya. To the left is the northeast lobe of the nebula and to the right is the southwest lobe. These two parts of the nebula are located at the periphery of the primary part of the nebula. Image credit: Kimeswenger et al 2021.

The envelope is vast, more than 15 light years in diameter. At this size, astronomers expect the envelope to be distorted and disturbed by other stars. But YY Hya is above the galactic plane and undisturbed by other gas clouds.

“The diameter of the main cloud is 15.6 light-years in diameter, nearly a million times greater than the distance of the earth from the sun and much greater than the distance from our sun to its closest neighboring star. close. Additionally, fragments as large as 39 light years apart have also been found. Since the object is slightly above the Milky Way, the nebula was able to develop largely undisturbed by other clouds in the surrounding gas, ”Kimeswenger said.

The size of these common envelopes could also inhibit their discovery. This is larger than the field of view of modern telescopes. “They are too big for the field of view of modern telescopes, and at the same time, they are very weak. Moreover, their lifespan is rather short, at least when viewed on cosmic time scales. It’s only been a few hundred thousand years, ”Kimeswenger said.

These are H-alpha images of the central part of the nebula with YY Hya dotted pink in the center.  The image on the right has other stars removed.  The tip of the nebula's southwest lobe is visible to the right of the images.  Image credit: Kimeswenger et al 2021.
These are H-alpha images of the central part of the nebula with YY Hya dotted pink in the center. The image on the right has other stars removed. The tip of the nebula’s southwest lobe is visible to the right of the images. Image credit: Kimeswenger et al 2021.

This discovery began with a group of French and German amateur astronomers. They were examining digitized historical astronomical images from the 1980s. They were looking for unknown objects when they found a fragment of a nebula.

They passed this discovery on to experts at the Department of Astronomy and Particle Physics at the University of Innsbruck. These professional astronomers have combined the discoveries of amateurs with observations from the past 20 years and with observations from various space telescopes and telescopes, including the Spitzer Space Telescope. With this data and this hand, astronomers at the University of Innsbruck ruled out a planetary nebula, their first consideration when they saw the data from amateurs.

Subsequent observations with the CHILESCOPE reveals the vast scale of the nebula. Astronomers in the United States have continued their spectroscopic observations, which provide essential data for discoveries like this. Existing UV images from GALEX and Catalina Real-time Transient Survey and TESS were also part of the mix, along with other observations from multiple sources.

The result?

The team built a model with all of this data that reveals a nearby binary star pair. They found that the temperature of the white dwarf is around 66,000 degrees Celsius (120,000 F) – which is hot for a white dwarf – and the K dwarf is around 4,400 degrees Celsius (8,000 F. ) The K dwarf is about a solar mass. The stars orbit rapidly at around 8 o’clock and are only 2.2 solar rays apart.

Because they are so close, the white dwarf heats the side of the K dwarf facing it. Heating leads to “extreme phenomena in the star’s spectrum and very regular variations in luminosity”.

The couple continues their evolution inside the shell of gas ejected by the white dwarf. The shell itself is gigantic, about a solar mass. This makes her more massive than the white dwarf and the K dwarf in the main sequence. The white dwarf ejected the gaseous shell about 500,000 years ago, according to the study.

And that leads to another interesting possibility.

The team of astronomers speculates, and it is not proven, that this binary star is related to the nova, or “guest star,” which ancient Chinese and Korean astronomers saw in 1086. “It is even possible that this system is linked to a nova observation made by Korean and Chinese astronomers in 1086. In any case, the positions of the historical observations correspond very well to those of our object described here, ”Kimeswenger said.

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