A Duo Of Dancing Stars

Small stars like our Sun die with great beauty, encircled by beautiful shrouds of multicolored gases that were once their outer layers–leaving only their relic cores behind as silent testimony to the Universe that they once existed. Our Sun, like other small stars, will first become a bloated red giant that will swell in size to the ghastly point that its flames will engulf the inner planets Mercury, Venus, and possibly Earth. It will then wither into a tiny, dense white dwarf–its former core. In March 2020, an international team, led by University of Warwick (U.K.) astronomers, reported that they had discovered a strange phenomena involving a closely dancing duo of these dead stars. The scientists detected a massive white dwarf star with a weird carbon-rich atmosphere that could really be two white dwarfs that merged together as they performed their bizarre danse macabre in the space between stars–narrowly escaping an explosive destruction.

The astronomers spotted an unusual ultra-massive white dwarf located about 450 light-years from Earth with an atmospheric composition that had never been seen before. This important observation marked the first time that a merged dancing duo of white dwarfs had been discovered by astronomers using its atmospheric composition as a clue to solving the mystery of its true identity.

The discovery, published in the March 2, 2020, issue of the journal Nature Astronomy, could shed new light on the enduring question of how massive white dwarf stars evolve, as well as on the number of supernovae dwelling in our barred-spiral Milky Way Galaxy.

The ultra-massive white dwarf, named WD J0551+4135, was detected in a survey of data derived from the European Space Agency’s (ESA’s) Gaia telescope. The astronomers followed up their discovery with a spectroscopy obtained using the William Herschel Telescope. The scientists focused on those white dwarfs identified as especially massive–an accomplishment that was made possible by the Gaia mission. By breaking apart the light emitted by the strange star, the astronomers were able to determine the chemical composition of its atmosphere, and found that it contained an unusually high level of carbon.

Lead author Dr. Mark Hollands, from the University of Warwick’s Department of Physics, explained in a March 2, 2020 University of Warwick Press Release that “This star stood out as something we had never seen before. You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon. You don’t expect to see this combination of hydrogen and carbon at the same time as there should be a thick layer of helium in between that prohibits that. When we looked at it, it didn’t make sense.”

Most white dwarfs are relatively light, weighing-in at about 0.6 times the mass of our Sun. However, WD J0551+4135 weighs in at an impressive 1.14 times solar masses, which makes it almost double the average mass of other white dwarfs. Despite being more massive than our Sun, it is squeezed into a tiny dense ball that is only two-thirds the diameter of Earth.

In order to solve the intriguing mystery, the astronomer-detectives decided to uncover the star’s true origins. The age of WD J0551+4135 also provided them with an important clue. Older stars orbit our Milky Way Galaxy much more swiftly than younger ones, and this strange white dwarf zipped around our Galaxy faster than 99% of the other nearby white dwarfs with the same cooling age. This means that this dead star is much older than it looks.

Dr. Hollands continued to explain in the March 2, 2020 University of Warwick Press Release that “We have a composition that we can’t explain through normal stellar evolution, a mass twice the average for a white dwarf, and a kinematic age older than that inferred from cooling. We’re pretty sure of how one star forms one white dwarf and it shouldn’t do this. The only way you can explain it is if it was formed through a merger of two white dwarfs.”

The Death Of A Small Sun-Like Star

White dwarfs are all that is left of stars, like our own Sun, after they have finished burning their entire necessary supply of nuclear-fusing fuel. At this fatal point, the dying small star has shed its outer gaseous layers into space. A small star’s grand finale contrasts with the noisy and explosive demise of more massive stars, that die in violent and catastrophic supernova blasts. Small stars like our Sun “go gentle into that good night”, and perish with great beauty and relative peace. Indeed, their lovely multicolored gaseous shrouds have inspired astronomers to refer to them as the “butterflies of the Universe”, as homage to their celestial loveliness.

Solitary small stars like our Sun perish gently. However, if there is another stellar actor in the drama, ghastly complications develop. If a small star resides in a binary system with another star, a wild party will inevitably occur. When the first of the duo “dies”, leaving its dense white dwarf core behind, the stellar corpse will gravitationally sip up material from its still-living companion star–and victim. As the vampire-like dwarf continues to steal more and more material from its unlucky companion, it will at last sip up enough material to attain sufficient mass to “go critical.” At this point, the white dwarf pays for its crime and explodes–just like the big guys. This explosion is termed a Type Ia supernova, and it differs from the core-collapse Type II supernovae experienced by more massive stars.

The proposal that WD J0551+4135 is really an object that formed as the result of the merger of a duo of white dwarfs is based on a related, but not identical, theory of its formation. In this case, as one of the two stars expanded at the end of its life into a swollen red giant, it engulfed its companion star, drawing its orbit in ever closer and closer as the first star withered into its white dwarf stage. An encore performance then occurred when the other star became a bloated red giant. Over the passage of billions of years, gravitational wave emission shrunk the orbit further, to the point that the waltzing stellar duo merged together to form a single object.

The Dancers And Their Dance

Even though white dwarf mergers have been predicted to occur, the one involving the unusual WD J0551+4135 is stranger than expected. This is beause most of the mergers in our Milky Way occur between stars sporting different masses, whereas this odd merger likely occurred between a duo of similaly-sized stars. There is also a limit in respect to how big the resulting single white dwarf cam be. This is because, if the resulting stellar corpse weighs-in at over 1.4 times solar masses, it will “go critical” and blow itself to smithereens in a Type Ia supernova explosion. However, it is possible that such fatal stellar explosions can be triggered at slightly lower masses, and so this odd white dwarf is especially useful because it demonstrates how massive a white dwarf can get and still “live” to tell the story.

Because the merger restarts the process of the star’s cooling, astronomers find it difficult to calculate the star’s true age. The stellar corpse probably merged about 1.3 billion years ago–but the duo of original dead stars may have existed for many billions of years before that event.

WD J0551+4135 is important because it is one of only a handful of merged white dwarfs to be identified–and it is the only one to be identified so far on the basis of its composition.

Dr. Hollands explained in the March 2, 2020 University of Warwick Press Release that “There aren’t that many white dwarfs this massive, although there are more than you would expect to see which implies that some of them were probably formed by mergers.”

“In the future we may be able to use a technique called astroseismology to learn about the white dwarf’s core composition from its stellar pulsations which would be an independent method confirming this star formed from a merger. Maybe the most exciting aspect of this star is that it must have just about failed to explode as a suprnova–these gargantuan explosions are really important in mapping the structure of the Universe as they can be detected out to very large distances. However, there remains much uncertainty about what kind of stellar systems make it to the supernova stage,” he added.

“Strange as it may sound measuring the properties of this ‘failed’ supernova and future look alikes is telling us a lot about the pathways to thermonuclear self-annihilation,” Dr. Hollands continued to comment.

This research is published in the March 2, 2020 issue of the journal Nature Astronomy under the title An Ultra-massive white dwarf with a mixed hydrogen-carbon atmosphere as a likely merger remnant.