College of Warwick astronomers have uncovered compelling proof {that a} close by white dwarf is the truth is the remnant of two stars merging — a uncommon stellar discovery revealed by way of Hubble House Telescope ultraviolet observations of carbon within the star’s sizzling environment.
White dwarfs are the dense cores left behind when stars exhaust their gasoline and collapse. They’re Earth-sized stellar embers weighing sometimes half as a lot because the Solar, made up of carbon-oxygen cores with floor layers of helium and hydrogen. Whereas white dwarfs are frequent within the universe, these with exceptionally excessive mass (weighing greater than the Solar) are uncommon and enigmatic.
In a paper printed on August 6 in Nature Astronomy, Warwick astronomers report on their investigations of a recognized high-mass white dwarf 130 light-years away, known as WD 0525+526. With a mass 20% bigger than our Solar, WD 0525+526 is taken into account “ultra-massive,” and the way this star got here to be shouldn’t be totally understood.
Such a white dwarf might kind from the collapse of a large star. Nevertheless, ultraviolet knowledge from the Hubble House Telescope revealed WD 0525+526 to have small quantities of carbon rising from its core into its hydrogen-rich environment — suggesting this white dwarf didn’t originate from a single huge star.
“In optical gentle (the type of gentle we see with our eyes), WD 0525+526 seems like a heavy however in any other case strange white dwarf,” stated first writer Dr Snehalata Sahu, Analysis Fellow on the College of Warwick. “Nevertheless, by way of ultraviolet observations obtained with Hubble, we have been in a position to detect faint carbon signatures that weren’t seen to optical telescopes.
“Discovering small quantities of carbon within the environment is a telltale signal that this huge white dwarf is more likely to be a be the remnant of a merger between two stars colliding. It additionally tells us there could also be many extra merger remnants like this masquerading as frequent pure-hydrogen environment white dwarfs. Solely ultraviolet observations would have the ability to reveal them to us.”
Usually, hydrogen and helium kind a thick barrier-like envelope round a white dwarf core, conserving components like carbon hidden. In a merger of two stars, the hydrogen and helium layers can burn off nearly fully as the celebrities mix. The ensuing single star has a really skinny envelope that not prevents carbon from reaching the floor — that is precisely what’s discovered on WD 0525+526.
Antoine Bédard, Warwick Prize Fellow within the Astronomy and Astrophysics group at Warwick and co-first writer stated, “We measured the hydrogen and helium layers to be ten-billion occasions thinner than in typical white dwarfs. We predict these layers have been stripped away within the merger, and that is what now permits carbon to seem on the floor.
“However this remnant can also be uncommon: it has about 100,000 occasions much less carbon on its floor in comparison with different merger remnants. The low carbon stage, along with the star’s excessive temperature (almost 4 occasions hotter than the Solar), tells us WD 0525+526 is way earlier in its post-merger evolution than these beforehand discovered. This discovery helps us construct a greater perceive the destiny of binary star programs, which is important for associated phenomena like supernova explosions.”
Including to the thriller is how carbon reaches the floor in any respect on this a lot hotter star. The opposite merger remnants are later of their evolution and funky sufficient for convection to deliver carbon to the floor. However WD 0525+526 is way too sizzling for that course of. As a substitute, the group recognized a subtler type of mixing known as semi-convection, seen right here for the primary time in a white dwarf. This course of permits small quantities of carbon to slowly rise into the star’s hydrogen-rich environment.
“Discovering clear proof of mergers in particular person white dwarfs is uncommon,” added Professor Boris Gänsicke, Division of Physics, College of Warwick, who obtained the Hubble knowledge for this research. “However ultraviolet spectroscopy provides us the power to detect these indicators early, when the carbon remains to be invisible at optical wavelengths. As a result of the Earth’s environment blocks ultraviolet gentle, these observations have to be carried out from area, and at present solely Hubble can do that job.
“Hubble simply turned 35 years outdated, and whereas nonetheless going sturdy, it is rather essential that we begin planning for a brand new area telescope that can finally change it.”
As WD 0525+526 continues to evolve and funky, it’s anticipated that extra carbon will emerge at its floor over time. For now, its ultraviolet glow affords a uncommon glimpse into the earliest stage of a stellar merger’s aftermath — and a brand new benchmark for the way binary stars finish their lives.
