Xavier Strottner and Marcel Drechsler


MASSIVE NEBULA AROUND SPECIAL PAIR OF STARS

Our small team of amateur astronomers around Marcel Drechsler from Germany and Xavier Strottner from France was able to make an important contribution to the study of the evolution of binary star systems: on old images of sky surveys we discovered in 2019 "Strottner-Drechsler Object 20", a previously unknown galactic nebula. In its center, a pair of stars surrounded by a common envelope was found. There could even be a connection to a historic nova.




MODEL OF A NEW CLASS OF GALACTIC NEBULAE

For the first time, amateurs and scientists have succeeded in providing evidence for a fully developed envelope of a so-called "common envelope system," by which is meant the phase of the common envelope of a binary star system.

The double star YY Hya is the focus of this discovery. YY Hya is a periodically variable star consisting of a K dwarf star and a hot white dwarf (WD) partner. The stars are in a common gas envelope emitted by the star that eventually became a white dwarf. Prior to that, it went through a red giant phase in which it dispersed its outer gas envelopes into space. Within this shared envelope, both stars continue to evolve as if they were alone, with stellar radiation stimulating the scattered gas to glow.

Already for a long time many star systems are known, which must be remnants of such evolutions due to various chemical and physical properties. Also star systems, which just develop a common envelope, have already been discovered by their special brightness. However, the fully developed envelope of a common envelope system and its ejection into interstellar space has so far been observed in this form for the first time.

The study presenting this discovery is titled "YY Hya and its interstellar environment" and was published in Astrophysics and Astronomy.

Toward the end of their lives, normal stars inflate into red giant stars. Since a very large fraction of stars are in binary stars, this affects the evolution at the end of their lives. In close binary systems, the inflating outer part of a star merges into a common envelope around both stars. Within this gas envelope, however, the cores of the two stars are virtually undisturbed and follow their evolution like independent individual stars.




DISCOVERY THANKS TO AMATEUR ASTRONOMERS

This is where amateur astronomy and our German-French team come into play: 
We painstakingly searched for unknown objects in the now digitized archives of historical sky images and finally found a fragment of the nebula on photographic plates from the 1980s that had not been listed in any catalog until now.

In the midst of this structure, which we previously assumed might be an unknown planetary nebula, was the tiny variable star YY Hya, which appeared absolutely unspectacular in searchable light. In ultraviolet light, however, the star shone brighter than any other star in the region, giving us valuable clues. A yellowish white star that appears so bright in the ultraviolet spectrum is almost always accompanied by a very hot star. Taking into account the variability of YY Hya, it became clear that the two stars orbit each other very closely and in only a few hours.

After studying the immediate vicinity of YY Hya in the ultraviolet spectrum, our team was also able to identify the two outbursts, which were axially symmetric to the northeast and southwest of YY Hya, well outside the central nebula.


(c) Marcel Drechsler

For further study of our discovery, we contacted Brazilian astrophotographer Maicon Germiniani, for whom the constellation Hydra was very accessible at the time.
Maicon provided 22 hours of H-alpha images that showed the full extent of the structure.

With our discovery and the collected data, we contacted scientific experts, including Stefan Kimeswenger from the Institute of Astro- and Particle Physics at the University of Innsbruck, who is very experienced in this field.

By collecting and combining observations from the last 20 years from public archives of various telescopes and with data from four different space satellites, the researchers were able to rule out the first assumption, namely the discovery of a planetary nebula caused by the remnants of dying stars. The enormous extent of the nebula finally became apparent with the help of our photo on the Chilescope, scientists in the USA finally completed these observations by means of spectrographs.

The diameter of the main cloud is 15.6 light-years, nearly 1 million times greater than the distance of Earth to the Sun and much greater than the distance of our Sun to its nearest neighboring star. Fragments have also been found that are up to 40 light-years apart. Because the object lies slightly above the Milky Way, the nebula was able to develop largely undisturbed by other clouds in the surrounding gas.


(c) Marcel Drechsler


INTERNATIONAL COLLABORATION SOLVES THE MYSTERY

The international team used all this data to create a model showing a close binary pair of stars. They found that the white dwarf has a temperature of about 66,000 degrees Celsius (120,000 F) - which is very hot for a white dwarf - and the K dwarf has a temperature of about 4400 degrees Celsius (8000 F). The K dwarf has about one solar mass. The stars orbit each other rapidly in about 8 hours and are only 2.2 solar radii apart.
Because they are so close, the white dwarf heats the side of the K dwarf facing it. The heating causes extreme phenomena in the star's spectrum and very regular variations in brightness.


(c) Stephan Kimeswenger

The entire shell itself is gigantic, about one solar mass. This makes it more massive than the white dwarf and the K dwarf of the main sequence. According to the study, the compact common envelope surrounding the two stars could have been ejected by the white dwarf about 500,000 years ago.

And that leads to another interesting possibility ...


STAR EXPLOSION ALMOST 1000 YEARS AGO

The astronomical team speculates that this double star could be responsible for the nova or "guest star" that Chinese and Korean astronomers observed as early as 1065. In any case, the positions of the historical observations match very well with those of our object described here.



The visibility of the region of YY Hya in the morning hours of September, 11th 1065 from Beijing, China using the medieval Chinese constellations. The circle marks the position of YY Hya. The plot was generated by Stellarium. (c) Stephan Kimeswenger, Stellarium


AN IMAGE OF SUPERLATIVES

As a final task, Marcel Drechsler and Xavier Strottner were faced with the great challenge of creating a new and spectacular photograph of this massive nebula.
To realize this very expensive, time consuming and laborious project, numerous donors and the Chilescope team generously supported us.
With this money and additional private funds we rented 3 large telescopes in the Chilean desert for one year and pointed them at the constellation Hydra every clear night. 

In over 100 nights more than 360 hours of exposure time could be collected.
The result shows a blood-red stellar explosion amid tens of thousands of stars, which the team has christened "the heart of Hydra." Already, the photo and the discovery have caused quite a stir internationally, and not just among scientists.

The result is a massive high resolution mosaic of 9 panels and 5 channels (red, green, blue, H-alpha and OIII).

By the way, the explosion of red glowing hydrogen gas is still expanding at a rapid speed of several hundred kilometers per second.
If we were to take another photo in 50 years, the nebula would already have grown by several percent.

The team's total working time amounts to several thousand hours, which is a small price to pay for a discovery like this.


Our special thanks to:
- Stephan Kimeswenger for his tremendous scientific work
- Professor Robert Fesen from Dartmore College for his support
- Maicon Germiniani for the first H-alpha data in 2020
- Sergey Pogrebisskiy and the Chilescope team
- Jean-Baptiste Auroux and Ola Skarpen for their help in combining the mosaic panels
- Lionel Mulato for the spectroscopic observation of the nebula
- and of course our many donors who made this project possible!


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A NOTE FROM THE AUTHORS
With the many fine details of the Nebula, the impression could arise that AI applications such as Topaz Denoise have been used excessively. However, the H-alpha raw data shows already very filigree structures, which did not need any significant sharpening.
The background was moderately denoised with Photoshop and Topaz Denoise. We were very careful that no structures were added by the AI.
Our team used techniques that are also applied in science to make fine details visible.
For the image of the YY Hya nebula, we also developed new techniques and adapted and refined existing methods.

Destructive techniques such as removing the stars using software were mostly avoided.
Instead, the stars were removed using a subtraction technique and the remaining tiny artefacts were logically filled.

Software used
- Stacking: PixInsight, Deep Sky Stacker, Photoshop
- Denoising: Topaz Denoise AI (moderate), Adobe Photoshop
- Image processing: PixInsight, Adobe Photoshop


H-alpha raw data, Linear stretch, 300% zoom