White dwarfs are remnants of most important sequence stars like our Solar which have depleted their hydrogen. Their lives of fusion have come to an finish, and so they’ll simmer with residual warmth for trillions of years. Their lifespans are longer than the present age of the Universe.
White dwarfs are fascinating objects for a number of causes. About 97% of stars will turn into white dwarfs in order that they have rather a lot to show us about stellar evolution. Since they cool at predictable charges, they function timekeepers, serving to astronomers decide the ages of the stellar clusters they reside in. Additionally they typically explode as Type 1a supernovae, which function commonplace candles within the cosmic distance ladder. Final however not least, they’re excessive objects that permit astrophysicists to check issues like theories of quantum mechanics.
White dwarfs aren’t all the identical. The Chandrasekhar limit units the higher mass restrict for white dwarfs at about 1.44 photo voltaic plenty. Above that, they cannot assist their very own mass and both explode or collapse into neutron stars. Most white dwarfs are nicely under that, and solely uncommon ones are above one photo voltaic mass. These are known as high-mass white dwarfs.
One recognized high-mass white dwarf is known as WD 0525+526. It is about 130 light-years away and is 20% extra huge than the Solar. New analysis primarily based on UV information from the Hubble Area Telescope means that the white dwarf is the results of a merger with one other star.
The brand new analysis is titled “A hot white dwarf merger remnant revealed by an ultraviolet detection of carbon.” It is revealed in Nature Astronomy and the lead writer is Dr. Snehalata Snahu, a Analysis Fellow on the College of Warwick within the UK.
WD 0525+526 is totally different from different white dwarfs not solely due to its larger mass. It is chemistry can be totally different.
White dwarfs are sometimes composed of carbon and oxygen cores, with atmospheres dominated by both hydrogen or helium. When astronomers detect metals in a white dwarf’s environment, that means any ingredient heavier than hydrogen or helium, they take discover. It may possibly imply that the white dwarf has engulfed a planetesimal.
However astronomers know of six high-mass white dwarfs which have some puzzling options. They’ve hydrogen-dominated atmospheres with indicators of solely tiny quantities of carbon that is probably dredged up from the core by convection. These stars might be the outcomes of stellar mergers and are known as DAQ white dwarfs.
“Discovering clear proof of mergers in particular person white dwarfs is uncommon.” – Professor Boris Gänsicke, Division of Physics, College of Warwick.
“These uncommon white dwarfs probably originate from stellar mergers, making them ‘smoking weapons’ for one of many binary evolution channels resulting in thermonuclear supernovae,” the authors write. (A thermonuclear supernova is a Kind 1a supernova.) “Nevertheless, optical spectroscopy can uncover solely essentially the most carbon-enriched objects, suggesting that many extra merger remnants could masquerade as regular pure-hydrogen-atmosphere white dwarfs.”
Whereas optical spectroscopy struggles to detect fainter carbon indicators, UV doesn’t. On this analysis, the astronomers used the Hubble’s UV-sensing functionality to look at WD 0525+526.
“In optical mild (the type of mild we see with our eyes), WD 0525+526 seems like a heavy however in any other case abnormal white dwarf,” mentioned first writer Dr. Snehalata Sahu in a press release. “Nevertheless, by means of ultraviolet observations obtained with Hubble, we had been in a position to detect faint carbon signatures that weren’t seen to optical telescopes.”
This determine locations WD 0525+526 in context with the six different recognized hydrogen-rich ultra-massive white dwarfs. WD 0525+526 is indicated by a purple star, whereas the six revealed white dwarfs with spectral sort DAQ (hydrogen and carbon strains current) are proven as blue circles. Panel b exhibits the carbon abundances of the celebs, measured by log(C/H) in spectroscopy. It represents the logarithmic ratio of carbon to hydrogen. WD 0525+526 is considerably decrease in carbon. Picture Credit score: Snahu et al. 2025. NatAst.
“Discovering small quantities of carbon within the environment is a tell-tale signal that this huge white dwarf is prone to be the remnant of a merger between two stars colliding,” mentioned Dr. Snahu. “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 be capable to reveal them to us.”
A white dwarf’s thick hydrogen and helium atmospheres hold the core tightly-wrapped. There’s little or no convection in white dwarfs, since they’re supported not by thermal strain however by electron degeneracy pressure. What convection they do expertise is weak and restricted to the skinny atmospheric ranges close to their surfaces. Because of this it’s extremely troublesome for a white dwarf to dredge carbon up from its core.
However when a white dwarf merges with one other star, the hydrogen and helium layers are virtually fully burned off. The ensuing star has solely a really skinny layer of hydrogen and helium, and carbon can now be dredged up from the star’s inside. This describes WD 0525+526.
This determine exhibits the spectrum of WD 0525+526. It corroborates the white dwarf’s hydrogen environment, and likewise exhibits the presence of carbon. Blue strains signify sturdy carbon transitions and inexperienced dashed strains signify weaker ones or ones with much less convincing information. The insets present close-ups of the C III (left) and C II (proper) strains that had been used to find out the photospheric carbon abundance. Picture Credit score: Snahu et al. 2025. NatAst.
Co-first writer Antoine Bédard, additionally from Warwick College, mentioned “We measured the hydrogen and helium layers to be ten-billion occasions thinner than in typical white dwarfs. We predict these layers had been stripped away within the merger, and that is what now permits carbon to look on the floor.”
“However this remnant can be uncommon: it has about 100,000 occasions much less carbon on its floor in comparison with different merger remnants. The low carbon degree, along with the star’s excessive temperature (practically 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 understanding of the destiny of binary star programs, which is important for associated phenomena like supernova explosions.”
Astrophysicists do not count on carbon to achieve the floor of a star so sizzling. That may occur later within the post-merger, when the star has cooled and convection can act extra effectively to dredge carbon to the floor the place it is seen. As a substitute, the researchers recognized a kind of convective mixing known as semi-convection going down within the star. Semi-convection can permit some carbon to penetrate the outer layers and attain the floor, and that is the primary time it has been noticed in a white dwarf. It creates a lot slower mixing than common convection, and is just partial; it does not create homogenization. Semi-convection takes place in stars aside from white dwarfs, too, however may be very troublesome to mannequin.
This determine exhibits semi-convection delivers much less carbon to the white dwarf’s higher layers than common convection. Crimson represents hydrogen, blue represents helium, and inexperienced represents carbon. Semi-convection permits a skinny hydrogen-helium shell to drift on WD 0525+526’s floor, which explains why this purple dwarf has much less floor carbon than cooler DAQ stars noticed lengthy after their mergers. Picture Credit score: Snahu et al. 2025. NatAst.
“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 information for this examine. “However ultraviolet spectroscopy offers us the power to detect these indicators early, when the carbon continues to be invisible at optical wavelengths. As a result of the Earth’s environment blocks ultraviolet mild, these observations should be carried out from house, and presently solely Hubble can do that job.”
The authors observe that there are different potential explanations for the carbon noticed on WD 0525+526′ floor. “One other potential supply of photospheric carbon is accretion of fabric from the ISM, a disrupted planetary system or an in depth (sub)stellar companion,” they write. “Nevertheless, in all instances, different heavy components can be detected alongside carbon within the UV spectrum of the white dwarf.” Astronomers would anticipate finding issues like silicon, nitrogen, phosphorous, and sulfur can be current, relying on the accretion supply.
After testing their information towards all of those eventualities, they rejected all of them.
“We didn’t detect photospheric silicon or any heavy ingredient aside from carbon within the COS spectrum of WD 0525+526,” the authors concluded. “Subsequently, all variants of the accretion state of affairs appear extremely unlikely.”
