5 mins read 27 May 2024

Black Holes that Eat their own stars

A massive binary system in the Large Magellanic Cloud is shining a light onto a potential evolutionary pathway in which massive stars end their lives not with a huge bang, but instead a whimper, before suddenly collapsing into their own black holes.

Artist impression of the VFTS 243 system that features the massive O-class star and the compact, yet large stellar-mass black hole companion. Credit: ESO / l. Calçada.

Over the past few decades, our knowledge of stellar evolution in massive stars has advanced significantly through studies of supernovae and observations of their remnants, like pulsars and black holes. While the exact mechanics of these explosive events remain a mystery, we know that the fate of a star—whether it becomes a white dwarf, neutron star, or black hole — depends on factors like its mass, binary configuration, and environment.

Compact remnants, like neutron stars and black holes typically form from the collapse of very massive stars after supernova explosions. These violent events eject enormous amounts of material into space, seeding the surrounding environment with heavy elements like helium, carbon, oxygen, and silicon. These elements go on to form new stars, planets, and, in a special part of the Universe (right here), life. Astronomers study the light signatures (spectra) of supernovae to track the dispersal of these elements into the interstellar medium.

However, there's a theory that some black holes form without the dramatic supernova explosion, in which the supernova explosion is more of a slight puff, rather than a big celestial firework. In this complete collapse hypothesis, the star collapses directly into a black hole failing to ever trigger a significant explosion. A new paper in Physical Review Letters suggests we may have found the first example of this type of event.

The paper describes the stellar system VFTS 243, a massive binary system featuring an O-class star (the most massive and brightest type) and a stellar-mass black hole companion, about 10 times the mass of our Sun. The pair orbit each other every 10.4 days and are located in the Large Magellanic Cloud (LMC), a (relatively nearby - astrophysically speaking) satellite galaxy of the Milky Way about 160,000 light-years from Earth. The LMC is a familiar sight to many southern hemisphere sky watchers where it, and the Small Magellanic Cloud, can often be seen with the naked eye from dark enough skies.

In this new paper, researchers simulated various scenarios to explain the mass and configuration of VFTS 243, aiming to determine which formation channel the system has followed. Their findings suggest that this black hole likely formed through the complete collapse channel, with no supernova explosion—essentially, the star was consumed by its own black hole. 

Challenging Supernovae Status Quo

Stellar evolution models. Credit: NASA JPL.

Existing models indicate that stars about the mass of our Sun (an average star), which live for roughly 10 billion years, end their lives by shedding their outer layers, leaving behind a white dwarf core that cools over time. Stars with masses between 8 and 25 times that of the Sun end in supernova explosions. These explosions leave behind neutron stars, pulsars, and magnetars. The rare, very massive stars—over 25 times the solar mass—burn through their fuel in 10 to 100 million years and explode in dramatic supernovae, with their cores collapsing into black holes.

In the complete collapse formation channel, the progenitor star's core accumulates matter until it collapses directly into a black hole without ejecting material into space. The new paper on VFTS 243 may now be providing supporting evidence for this scenario.

This has implications for our understanding of supernovae. If the most massive stars vanish without a supernova explosion, we won’t observe these events. Additionally, the lack of material ejection means fewer new elements are dispersed into the interstellar medium, potentially affecting the formation of new generations of stars and planets - which would need to be accounted for.

To reach their conclusions, the researchers examined two factors: how mass ejection from supernovae changes binary star orbits and how "kicks" to compact remnants affect these configurations.

When stars explode and shed their outer layers, the mass distribution in the system changes rapidly, potentially disrupting the binary configuration. The sudden loss of material can alter the orbits, increase eccentricity, shift inclination angles, or even separate the binary companions entirely.

Additionally, supernova explosions are often asymmetrical, giving the resulting neutron star or black hole a "natal kick." This kick can change the system's configuration, as seen in numerous studies of the varied velocities of neutron stars and pulsars, which range from tens to over 1000 km/s, with an average of around 250 km/s. These kicks can widen orbits, increase eccentricity, or disrupt the system.

Therefore, a system with a massive star and a high-mass black hole that has low orbital eccentricity (nearly circular) and a low natal kick velocity could suggest a complete collapse scenario. In other words, with no supernova event occurring, there's minimal mass ejection or natal kick to disrupt the system.

In this new paper, researchers' models showed that the mass ejection from the black hole formation in VFTS 243 was just 0.3 solar masses, and the velocity kick was a mere 4 km/s. These small values suggest that mass loss was primarily due to neutrino emission exclusively. Neutrinos are weakly interacting particles that carry away most of the gravitational energy during core collapse event, as well as being responsible for the initial cooling phase of neutron stars post supernova.

With these findings, astronomers hope to discover more systems like VFTS 243, shedding light on this poorly understood black hole formation channel and enhancing our understanding of how massive stars live, die, and disperse elements in our Universe.


Video Credit: European Southern Observatory

Read the paper in the journal, Physical Review Letters