Astronomy

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Massive stars about eight times more massive than the Sun explode as supernovae at the end of their lives. The explosions, which leave behind a black hole or a neutron star, are so energetic they can outshine their host galaxies for months. However, astronomers appear to have spotted a massive star that skipped the explosion and turned directly into a black hole.

Stars are balancing acts between the outward force of fusion and the inward force of their own gravity. When a massive star enters its last evolutionary stages, it begins to run out of hydrogen, and its fusion weakens. The outward force from its fusion can no longer counteract the star’s powerful gravity, and the star collapses in on itself. The result is a supernova explosion, a calamitous event that destroys the star and leaves behind a black hole or a neutron star.

However, it appears that sometimes these stars fail to explode as supernovae and instead turn directly into black holes.

New research shows how one massive, hydrogen-depleted supergiant star in the Andromeda galaxy (M31) failed to detonate as a supernova. The research is “The disappearance of a massive star marking the birth of a black hole in M31.” The lead author is Kishalay De, a postdoctoral scholar at the Kavli Institute for Astrophysics and Space Research at MIT.

These types of supernovae are called core-collapse supernovae, also known as Type II. They’re relatively rare, with one occurring about every one hundred years in the Milky Way. Scientists are interested in supernovae because they are responsible for creating many of the heavy elements, and their shock waves can trigger star formation. They also create cosmic rays that can reach Earth.

This new research shows that we may not understand supernovae as well as we thought.

Artist’s impression of a Type II supernova explosion. These supernovae explode when a massive star nears the end of its life and leaves behind either a black hole or a neutron star. But sometimes, the supernova fails to explode and collapses directly into a black hole. Image Credit: ESO

The star in question is named M31-2014-DS1. Astronomers noticed it brightening in mid-infrared (MIR) in 2014. For one thousand days, its luminosity was constant. Then, for another thousand days between 2016 and 2019, it faded dramatically. It’s a variable star, but that can’t explain these fluctuations. In 2023, it was undetected in deep optical and near-IR (NIR) imaging observations.

The researchers say that the star was born with an initial mass of about 20 stellar masses and reached its terminal nuclear-burning phase with about 6.7 stellar masses. Their observations suggest that the star is surrounded by a recently ejected dust shell, in accordance with a supernova explosion, but there’s no evidence of an optical outburst.

“The dramatic and sustained fading of M31-2014-DS1 is exceptional in the landscape of variability in massive, evolved stars,” the authors write. “The sudden decline of luminosity in M31-2014-DS1 points to the cessation of nuclear burning together with a subsequent shock that fails to overcome the infalling material.” A supernova explosion is so powerful that it completely overcomes infalling material.

“Lacking any evidence for a luminous outburst at such proximity, the observations of M31-2014-DS1 bespeak signatures of a ‘failed’ SN that leads to the collapse of the stellar core,” the authors explain.

What could make a star fail to explode as a supernova, even if it’s the right mass to explode?

Supernovae are complex events. The density inside a collapsing core is so extreme that electrons are forced to combine with protons, creating both neutrons and neutrinos. This process is called neutronization, and it creates a powerful burst of neutrinos that carries about 10% of the star’s rest mass energy. The outburst is called a neutrino shock.

Neutrinos get their name from the fact that they’re electrically neutral and seldom interact with regular matter. Every second, about 400 billion neutrinos from our Sun pass right through every person on Earth. But in a dense stellar core, the neutrino density is so extreme that some of them deposit their energy into the surrounding stellar material. This heats the material, which generates a shock wave.

The neutrino shock always stalls, but sometimes it revives. When it revives, it drives an explosion and expels the outer layer of the supernova. If it’s not revived, the shock wave fails, and the star collapses and forms a black hole.

This image illustrates how the neutrino shock wave can stall, leading to a black hole without a supernova explosion. A shows the initial shock wave with cyan lines representing neutrinos being emitted and the red circle representing the shock wave propagating outward. B shows the neutrino shock stalling, with white arrows representing infalling matter. The outer layers fall inward, and the neutrino heating isn’t powerful enough to revive the shock. C shows the failed shock dissipating as a dotted red line and the stronger white arrows represent the collapse accelerating. The outer layers are falling in rapidly, and the core is becoming more compact. D shows the black hole forming, with the blue circle representing the event horizon and the remaining material forming an accretion disk. (Credit: Original illustration created for this article.)

In M31-2014-DS1, the neutrino shock was not revived. The researchers were able to constrain the amount of material ejected by the star, and it was far below what a supernovae would eject. “These constraints imply that the majority of stellar material (?5 solar masses) collapsed into the core, exceeding the maximum mass of a neutron star (NS) and forming a BH,” they conclude. About 98% of the star’s mass collapsed and created a black hole with about 6.5 solar masses.

M31-2014-DS1 isn’t the only failed supernova, or candidate failed supernova, that astronomers have found. They’re difficult to spot because they’re characterized by what doesn’t happen rather than what does. A supernova is hard to miss because it’s so bright and appears in the sky suddenly. Ancient astronomers recorded several of them.

In 2009, astronomers discovered the only other confirmed failed supernova. It was a supergiant red star in NGC 6946, the “Fireworks Galaxy.” It’s named N6946-BH1 and has about 25 solar masses. After disappearing from view, it left only a faint infrared glow. In 2009, its luminosity increased to a million solar luminosities, but by 2015, it had disappeared in optical light.

A survey with the Large Binocular Telescope monitored 27 nearby galaxies, looking for disappearing massive stars. The results suggest that between 20% and 30% of massive stars can end their lives as failed supernovae. However, M31-2014-DS1 and N6946-BH1 are the only confirmed observations.

The post A Star Disappeared in Andromeda, Replaced by a Black Hole appeared first on Universe Today.

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About half a century ago, astronomers theorized that the Solar System is situated in a low-density hot gas environment. This hot gas emits soft X-rays that displace the dust in the local interstellar medium (ISM), creating what is known as the Local Hot Bubble (LHB). This theory arose to explain the ubiquitous soft X-ray background (below 0.2 keV) and the lack of dust in our cosmic neighborhood. This theory has faced some challenges over the years, including the discovery that solar wind and neutral atoms interact with the heliosphere, leading to similar emissions of soft X-rays.

Thanks to new research by an international team of scientists led by the Max Planck Institute for Extraterrestrial Physics (MPE), we now have a 3D model of the hot gas in the Solar System’s neighborhood. Using data obtained by the eROSITA All-Sky Survey (eRASS1), they detected large-scale temperature differences in the LHBT that indicate that the LHB must exist, and both it and solar wind interaction contribute to the soft X-ray background. They also revealed an interstellar tunnel that could possibly link the LHB to a larger “superbubble.”

The research was led by Michael C. H. Yeung, a PhD student at the MPE who specializes in the study of high-energy astrophysics. He was joined by colleagues from the MPE, the INAF-Osservatorio Astronomico di Brera, the University of Science and Technology of China, and the Dr. Karl Remeis Observatory. The paper that details their findings, “The SRG/eROSITA diffuse soft X-ray background,” was published on October 29th, 2024, by the journal Astronomy & Astrophysics.

This image shows half of the X-ray sky projected onto a circle with the center of the Milky Way on the left and the galactic plane running horizontally. Credit ©: MPE/J. Sanders/eROSITA consortium

The eROSITA telescope was launched in 2019 as part of the Russian–German Spektr-RG space observatory. It is the first X-ray observatory to observe the Universe beyond Earth’s geocorona, the outermost region of the Earth’s atmosphere (aka. the exosphere), to avoid contamination by the latter’s high-ultraviolet light. In addition, the eROSITA All-Sky Survey (eRASS1) was timed to coincide with the solar minimum, thus reducing contamination by solar wind charge exchanges.

For their study, the team combined data from the eRASS1 with data from eROSITA’s predecessor, the X-ray telescope ROSAT (short for Röntgensatellit). Also built by the MPE, this telescope complements the eROSITA spectra by detecting X-rays with energies lower than 0.2 keV. The team focused on the LHB located in the western Galactic hemisphere, dividing it into about 2000 regions and analyzing the spectra from each. Their analysis showed a clear temperature difference between the parts of the LHB oriented towards Galactic South (0.12 keV; 1.4 MK) and Galactic North (0.10 keV; 1.2 MK).

According to the authors, this difference could have been caused by supernova explosions that expanded and reheated the Galactic South portion of the LHB in the past few million years. Yeung explained in an MPE press release: “In other words, the eRASS1 data released to the public this year provides the cleanest view of the X-ray sky to date, making it the perfect instrument for studying the LHB.”

In addition to obtaining temperature data from the diffuse X-ray background spectra information, the combined data also provided a 3D structure of the hot gas. In a previous study, Yeung and his colleagues examined eRASS1 spectra data from almost all directions in the western Galactic hemisphere. They concluded that the density of the hot gas in the LHB is relatively uniform. Relying on this previous work, the team generated a new 3D model of the LHB from the measured intensity of X-ray emissions.

A 3D interactive view of the LHB and the solar neighborhood, Credit: MPE

This model shows that the LHB extends farther toward the Galactic poles than expected since the hot gas tends to follow the path of least resistance (away from the Galactic disc). Michael Freyberg, a core author of this work, was a part of the pioneering work in the ROSAT era three decades ago. As he explained:

“This is not surprising, as was already found by the ROSAT survey. What we didn’t know was the existence of an interstellar tunnel towards Centaurus, which carves a gap in the cooler interstellar medium (ISM). This region stands out in stark relief thanks to the much-improved sensitivity of eROSITA and a vastly different surveying strategy compared to ROSAT.”

These latest results suggest the Centaurus tunnel may be a local example of a wider hot ISM network sustained by supernovae and solar wind-ISM interaction across the Galaxy. While astronomers have theorized the existence of the Centaurus tunnel since the 1970s, it has remained difficult to prove until now. The team also compiled a list of known supernova remnants, superbubbles, and dust and used these to create a 3D model of the Solar System’s surroundings. The new model allows astronomers to better understand the key features in the representation.

These include the Canis Major tunnel, which may connect the LHB to the Gum Nebula (the red globe) or the long grey superbubble (GSH238+00+09). Dense molecular clouds, represented in orange, are shown near the surface of the LHB in the direction of the Galactic Center (GC). Recent work suggests these clouds are moving away from the Solar System and likely formed from the condensation of materials swept up during the early formation of the LHB. Said Gabriele Ponti, a co-author of this work:

“Another interesting fact is that the Sun must have entered the LHB a few million years ago, a short time compared to the age of the Sun. It is purely coincidental that the Sun seems to occupy a relatively central position in the LHB as we continuously move through the Milky Way.”

Further Reading: MPE, Astronomy & Astrophysics

The post eROSITA All-Sky Survey Takes the Local Hot Bubble’s Temperature appeared first on Universe Today.

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The dividing line between stars and planets is that stars have enough mass to fuse hydrogen into helium to produce their own light, while planets aren’t massive enough to produce core fusion. It’s generally a good way to divide them, except for brown dwarfs. These are bodies with a mass of about 15–80 Jupiters, so they are large enough to fuse deuterium but can’t generate helium. Another way to distinguish planets and stars is how they form. Stars form by the gravitational collapse of gas and dust within a molecular cloud, which allows them to gather mass on a short cosmic timescale. Planets, on the other hand, form by the gradual accumulation of gas and dust within the accretion disk of a young star. But again, that line becomes fuzzy for brown dwarfs.

The problem arises in that, if brown dwarfs form within a molecular cloud like stars, they aren’t massive enough to form quickly. If a cloud of gas and dust has enough mass to collapse under its own weight, it has enough mass to form a full star. But if brown dwarfs form like planets, they would have to accumulate mass incredibly quickly. Simulations of planet formation show it is difficult for a planet to form with a mass of more than a few Jupiters. So what gives? The answer may lie in what are known as Jupiter-mass binary objects, or JuMBOs.

The Orion nebula is a stellar nursery. Credit: NASA, ESA, M. Robberto

JuMBOs are binary objects where each component has a mass between 0.7 and 13 Jupiter masses. If they form like planets, they should be extremely rare, and if they form like binary stars, they should have more mass. Recent observations by the JWST of the Orion nebula cluster discovered 540 free-floating Jupiter mass objects, so-called rogue planets. This was surprising in and of itself, but more surprising was the fact that 42 of them were JuMBOs. Far from being rare, they make up nearly 8% of these rogue objects. So how do they form?

One clue lies in their orbital separation. The components of JuMBOs are most commonly separated by a distance of 28–384 AU. This is similar to that of binary stars with components around the mass of the Sun, which typically are in a range of 50–300 AU. Binary stars are extremely common. More common than single stars like the Sun. The environment of stellar nurseries, such as the Orion nebula, is also extremely intense. Massive stars that form first can blast nearby regions with ionizing radiation. Given how common JuMBOs are, it is likely they began as binary stars, only to have much of their masses blasted away by photo-erosion. Rather than being binary planets, they are the failed remnants of binary stars.

This could also explain why so many rogue planets have super-Jupiter masses. The same intense light that would cause photo-erosion would also tend to push them out of star systems.

Reference: Diamond, Jessica L., and Richard J. Parker. “Formation of Jupiter-Mass Binary Objects through photoerosion of fragmenting cores.” The Astrophysical Journal 975.2 (2024): 204.

The post An Explanation for Rogue Planets. They Were Eroded Down by Hot Stars appeared first on Universe Today.

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