This uncommon white dwarf appears to be like regular, till Hubble reveals its explosive secret

“It’s a discovery that underlines things may be different from what they appear to us at first glance,” stated the principal investigator of the Hubble program, Boris Gaensicke, of the University of Warwick within the United Kingdom. “Until now, this appeared as a normal white dwarf, but Hubble’s ultraviolet vision revealed that it had a very different history from what we would have guessed.”

A white dwarf is a dense object with the identical diameter as Earth, and represents the tip state for stars that aren’t huge sufficient to blow up as core-collapse supernovae. Our Sun will turn into a white dwarf in about 5 billion years.

In principle, a white dwarf can have a mass of as much as 1.4 occasions that of the Sun, however white dwarfs heavier than the Sun are uncommon. These objects, which astronomers name ultra-massive white dwarfs, can type both by the evolution of a single huge star or by the merger of a white dwarf with one other star, resembling a binary companion.

This new discovery, printed within the journal Nature Astronomy, marks the primary time {that a} white dwarf born from colliding stars has been recognized by its ultraviolet spectrum. Prior to this examine, six white dwarf merger merchandise had been found by way of carbon strains of their visible-light spectra. All seven of those are half of a bigger group that had been discovered to be bluer than anticipated for his or her plenty and ages from a examine with ESA’s Gaia mission in 2019, with the proof of mergers offering new insights into their formation historical past.

Astronomers used Hubble’s Cosmic Origins Spectrograph to research a white dwarf known as WD 0525+526. Located 128 light-years away, it’s 20% extra huge than the Sun. In seen gentle, the spectrum of WD 0525+526’s environment resembled that of a typical white dwarf. However, Hubble’s ultraviolet spectrum revealed one thing uncommon: proof of carbon within the white dwarf’s environment.

White dwarfs that type by the evolution of a single star have atmospheres composed of hydrogen and helium. The core of the white dwarf is usually composed largely of carbon and oxygen or oxygen and neon, however a thick environment often prevents these parts from showing within the white dwarf’s spectrum.

When carbon seems within the spectrum of a white dwarf, it will probably sign a extra violent origin than the standard single-star state of affairs: the collision of two white dwarfs, or of a white dwarf and a subgiant star. Such a collision can burn away the hydrogen and helium atmospheres of the colliding stars, forsaking a scant layer of hydrogen and helium across the merger remnant that permits carbon from the white dwarf’s core to drift upward, the place it may be detected.

WD 0525+526 is exceptional even inside the small group of white dwarfs identified to be the product of merging stars. With a temperature of virtually 21,000 kelvins (37,000 levels Fahrenheit) and a mass of 1.2 photo voltaic plenty, WD 0525+526 is hotter and extra huge than the opposite white dwarfs on this group.

WD 0525+526’s excessive temperature posed one thing of a thriller for the crew. For cooler white dwarfs, such because the six beforehand found merger merchandise, a course of known as convection can combine carbon into the skinny hydrogen-helium environment. WD 0525+526 is simply too sizzling for convection to happen, nonetheless. Instead, the crew decided a extra refined course of known as semi-convection brings a small quantity of carbon up into WD 0525+526’s environment. WD 0525+526 has the smallest quantity of atmospheric carbon of any white dwarf identified to consequence from a merger, about 100,000 occasions lower than different merger remnants.

The excessive temperature and low carbon abundance imply that figuring out this white dwarf because the product of a merger would have been not possible with out Hubble’s sensitivity to ultraviolet gentle. Spectral strains from parts heavier than helium, like carbon, turn into fainter at seen wavelengths for warmer white dwarfs, however these spectral alerts stay vibrant within the ultraviolet, the place Hubble is uniquely positioned to identify them.

“Hubble’s Cosmic Origins Spectrograph is the only instrument that can obtain the superb quality ultraviolet spectroscopy that was required to detect the carbon in the atmosphere of this white dwarf,” stated examine lead Snehalata Sahu from the University of Warwick.

Because WD 0525+526’s origin was revealed solely as soon as astronomers glimpsed its ultraviolet spectrum, it is possible that different seemingly “normal” white dwarfs are literally the results of cosmic collisions — a chance the crew is worked up to discover sooner or later.

“We would like to extend our research on this topic by exploring how common carbon white dwarfs are among similar white dwarfs, and how many stellar mergers are hiding among the normal white dwarf family,” stated examine co-leader Antoine Bedrad from the University of Warwick. “That will be an important contribution to our understanding of white dwarf binaries, and the pathways to supernova explosions.”

The Hubble Space Telescope has been working for greater than three many years and continues to make ground-breaking discoveries that form our basic understanding of the universe. Hubble is a undertaking of worldwide cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based mostly in Denver, additionally helps mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.