Black Holes’ Magnetism Surprisingly Wimpy

Black holes are famous for their muscle: an intense gravitational pull known to gobble up entire stars and launch streams of matter into space at almost the speed of light.

It turns out the reality may not live up to the hype.

In a paper published today in the journal Science, University of Florida scientists have discovered these tears in the fabric of the universe have significantly weaker magnetic fields than previously thought.

A 40-mile-wide black hole 8,000 light years from Earth named V404 Cygni yielded the first precise measurements of the magnetic field that surrounds the deepest wells of gravity in the universe. Study authors found the magnetic energy around the black hole is about 400 times lower than previous crude estimates.

The measurements bring scientists closer to understanding how black holes’ magnetism works, deepening our knowledge of how matter behaves under the most extreme conditions — knowledge that could broaden the limits of nuclear fusion power and GPS systems.

The measurements also will help scientists solve the half-century-old mystery of how “jets” of particles traveling at nearly the speed of light shoot out of black holes’ magnetic fields, while everything else is sucked into their abysses, said study co-author Stephen Eikenberry, a professor of astronomy in UF’s College of Liberal Arts and Sciences.

“The question is, how do you do that?” Eikenberry said. “Our surprisingly low measurements will force new constraints on theoretical models that previously focused on strong magnetic fields accelerating and directing the jet flows. We weren’t expecting this, so it changes much of what we thought we knew.”

Study authors developed the measurements from data collected in 2015 during a black hole’s rare outburst of jets. The event was observed through the lens mirror of the 34-foot Gran Telescopio Canarias, the world’s largest telescope, co-owned by UF and located in Spain’s Canary Islands, with the help of its UF-built infrared camera named CIRCE (Canarias InfraRed Camera Experiment).

Smaller jet-producing black holes, like the one observed for the study, are the rock stars of galaxies. Their outbursts occur suddenly and are short-lived, said study lead author Yigit Dalilar and co-author Alan Garner, doctoral students in UF’s astronomy department. The 2015 outbursts of V404 Cygni lasted only a couple of weeks. The previous time the same black hole had a similar episode was in 1989.

“To observe it was something that happens once or twice in one’s career,” Dalilar said. “This discovery puts us one step closer to understanding how the universe works.”

New Stellar Stream Discovered By Astronomers

An international team of astronomers has detected a new thin stellar stream in the halo of the Milky Way galaxy. The newly discovered feature, named “jet stream,” could help researchers answer fundamental questions about the mass distribution of the Milky Way’s dark matter halo. The finding was presented November 24 in a paper published on the arXiv pre-print server.

Stellar streams are remnants of dwarf galaxies or globular clusters that once orbited a galaxy but have been disrupted and stretched out along their orbits by tidal forces of their hosts. So far, nearly 20 stellar streams have been identified in the Milky Way, just a few in the Andromeda galaxy, and about 10 outside the Local Group.

Astronomers are interested in finding new stellar streams in the Milky Way, as they hope that such features could answer some crucial questions about the the galaxy. For instance, stellar streams could help us understand the large-scale mass distribution of the galactic dark matter halo. Moreover, they could confirm whether or not our galaxy contains low-mass dark matter subhalos.

Now, a group of researchers led by Prashin Jethwa of the European Southern Observatory (ESO) has found a new stellar stream in the Milky Way using the Search for the Leading Arm of Magellanic Satellites (SLAMS) optical survey. SLAMS utilizes the 4-m Blanco telescope at Cerro Tololo InterAmerican Observatory in Chile and is used to look for satellites of the Magellanic Clouds. However, the observations conducted by Jethwa’s team in December 2016 and January 2017, have accidentally revealed the presence of a new stellar stream in the Milky Way’s halo.

“We recently carried out a mini survey which led to the fortuitous discovery of a thin stellar stream in the outer halo, which we name the jet stream,” the researchers wrote in the paper.

According to the study, the jet stream is located about 95,000 light years away from the Earth and crosses the constellations of Hydra and Pyxis. The researchers estimate that the stream has a width of approximately 293 light years, which morphologically places it in the category of thin stellar streams along with Pal 5, GD-1 and ATLAS. The authors suggest that such thinness could indicate that a globular cluster was a progenitor of this stream.

The research also revealed that the jet stream has a mass of about 25,000 solar masses, which makes it one of the least massive stellar streams known to date. Moreover, they found that the jet stream consists of mainly metal-poor stars, and its age was calculated to be about 12.5 billion years.

Although fundamental parameters of the jet stream were determined by Jethwa’s team, still more studies are necessary to further characterize it and confirm its origin.

“Additional imaging is planned to attempt to trace the stream beyond the current survey footprint, followed by a spectroscopic campaign to determine radial velocities, metallicities, and detailed abundances, shedding light on the nature and orbital history of the progenitor. Finally, deeper, uniform imaging along the stream track will be required to robustly detect density perturbations caused by possible subhalo encounters,” the astronomers concluded.

Trickle-Down Is The Solution (To The Planetary Core Formation Problem)

Scientists have long pondered how rocky bodies in the solar system — including our own Earth — got their metal cores. According to research conducted by The University of Texas at Austin, evidence points to the downwards percolation of molten metal toward the center of the planet through tiny channels between grains of rock.

The finding calls into question the interpretation of prior experiments and simulations that sought to understand how metals behave under intense heat and pressure when planets are forming. Past results suggested that large portions of molten metals stayed trapped in isolated pores between the grains. In contrast, the new research suggests that once those isolated pores grow large enough to connect, the molten metal starts to flow, and most of it is able to percolate along grain boundaries. This process would let metal trickle down through the mantle, accumulate in the center, and form a metal core, like the iron core at the heart of our home planet.

“What we’re saying is that once the melt network becomes connected, it stays connected until almost all of the metal is in the core,” said co-author Marc Hesse, an associate professor in the UT Jackson School of Geosciences Department of Geological Sciences, and a member of UT’s Institute for Computational Engineering and Sciences.

The research was published on Dec. 4 in the Proceedings of the National Academy of Sciences. The work was the doctoral thesis of Soheil Ghanbarzadeh, who earned his Ph.D. while a student in the UT Department of Petroleum and Geosystems Engineering (now the Hildebrand Department of Petroleum and Geosystems Engineering). He currently works as a reservoir engineer with BP America. Soheil was jointly advised by Hesse and Maša Prodanovic, an associate professor in the Hildebrand Department and a co-author.

Planets and planetesimals (small planets and large asteroids) are formed primarily from silicate rocks and metal. Part of the planet formation process involves the initial mass of material separating into a metallic core and a silicate shell made up of the mantle and the crust. For the percolation theory of core formation to work, the vast majority of metal in the planetary body must make its way to the center.

In this study, Ghanbarzadeh developed a computer model to simulate the distribution of molten iron between rock grains as porosity, or melt fraction, increased or decreased. The simulations were perfomed at the Texas Advanced Computing Center. Researchers found that once the metal starts to flow, it can continue flowing even as the melt fraction decreases significantly. This is in contrast to previous simulations that found that once the metal starts flowing, it only takes a small dip in the volume of melt for percolation to stop.

“People have assumed that you disconnect at the same melt fraction at which you initially connected…and it would leave significant amounts of the metal behind,” Hesse said. “What we found is that when the metallic melt connects and when it disconnects is not necessarily the same.”

According to the computer model, only 1 to 2 percent of the initial metal would be trapped in the silicate mantle when percolation stops, which is consistent with the amount of metal in the Earth’s mantle.

The researchers point to the arrangement of the rock grains to explain the differences in how well-connected the spaces between the grains are. Previous work used a geometric pattern of regular, identical grains, while this work relied on simulations using an irregular grain geometry, which is thought to more closely mirror real-life conditions. The geometry was generated using data from a polycrystalline titanium sample that was scanned using X-ray microtomography.

“The numerical model Soheil developed in his Ph.D. thesis allowed for finding three-dimensional melt networks of any geometrical complexity for the first time,” said Prodanovic. “Having a three-dimensional model is key in understanding and quantifying how melt trapping works.”

The effort paid off because researchers found that the geometry has a strong effect on melt connectivity. In the irregular grains, the melt channels vary in width, and the larges ones remain connected even as most of the metal drains away.

“What we did differently in here was to add the element of curiosity to see what happens when you drain the melt from the porous, ductile rock,” said Ghanbarzadeh.

The researchers also compared their results to a metallic melt network preserved in an anchondrite meteorite, a type of meteorite that came from a planetary body that differentiated into discernable layers. X-ray images of the meteorite taken in the Jackson School’s High-Resolution X-Ray CT Facility revealed a metal distribution that is comparable to the computed melt networks. Prodanovic said that this comparison shows that their simulation capture the features observed in the meteorite.

Gravitational Waves Could Shed Light On The Origin Of Black Holes

A new study published in Physical Review Letters outlines how scientists could use gravitational wave experiments to test the existence of primordial black holes, gravity wells formed just moments after the Big Bang that some scientists have posited could be an explanation for dark matter.

“We know very well that black holes can be formed by the collapse of large stars, or as we have seen recently, the merger of two neutron stars,” said Savvas Koushiappas, an associate professor of physics at Brown University and coauthor of the study with Avi Loeb from Harvard University. “But it’s been hypothesized that there could be black holes that formed in the very early universe before stars existed at all. That’s what we’re addressing with this work.”

The idea is that shortly after the Big Bang, quantum mechanical fluctuations led to the density distribution of matter that we observe today in the expanding universe. It’s been suggested that some of those density fluctuations might have been large enough to result in black holes peppered throughout the universe. These so-called primordial black holes were first proposed in the early 1970s by Stephen Hawking and collaborators but have never been detected — it’s still not clear if they exist at all.

The ability to detect gravitational waves, as demonstrated recently by the Laser Interferometer Gravitational-Wave Observatory (LIGO), has the potential to shed new light on the issue. Such experiments detect ripples in the fabric of spacetime associated with giant astronomical events like the collision of two black holes. LIGO has already detected several black hole mergers, and future experiments will be able to detect events that happened much further back in time.

“The idea is very simple,” Koushiappas said. “With future gravitational wave experiments, we’ll be able to look back to a time before the formation of the first stars. So if we see black hole merger events before stars existed, then we’ll know that those black holes are not of stellar origin.”

Cosmologists measure how far back in time an event occurred using redshift — the stretching of the wavelength of light associated with the expansion of the universe. Events further back in time are associated with larger redshifts. For this study, Koushiappas and Loeb calculated the redshift at which black hole mergers should no longer be detected assuming only stellar origin.

They show that at a redshift of 40, which equates to about 65 million years after the Big Bang, merger events should be detected at a rate of no more than one per year, assuming stellar origin. At redshifts greater than 40, events should disappear altogether.

“That’s really the drop-dead point,” Koushiappas said. “In reality, we expect merger events to stop well before that point, but a redshift of 40 or so is the absolute hardest bound or cutoff point.”

A redshift of 40 should be within reach of several proposed gravitational wave experiments. And if they detect merger events beyond that, it means one of two things, Koushiappas and Loeb say: Either primordial black holes exist, or the early universe evolved in a way that’s very different from the standard cosmological model. Either would be very important discoveries, the researchers say.

For example, primordial black holes fall into a category of entities known as MACHOs, or Massive Compact Halo Objects. Some scientists have proposed that dark matter — the unseen stuff that is thought to comprise most of the mass of the universe — may be made of MACHOs in the form of primordial black holes. A detection of primordial black holes would bolster that idea, while a non-detection would cast doubt upon it.

The only other possible explanation for black hole mergers at redshifts greater than 40 is that the universe is “non-Gaussian.” In the standard cosmological model, matter fluctuations in the early universe are described by a Gaussian probability distribution. A merger detection could mean matter fluctuations deviate from a Gaussian distribution.

“Evidence for non-Gaussianity would require new physics to explain the origin of these fluctuations, which would be a big deal,” Loeb said.

The rate at which detections are made past a redshift of 40 — if indeed such detections are made — should indicate whether they’re a sign of primordial black holes or evidence for non-Gaussianity. But a non-detection would present a strong challenge to those ideas.

Blowing In The Stellar Wind: Scientists Reduce The Chances Of Life On Exoplanets In So-Called Habitable Zones

Is there life beyond Earth in the cosmos? Astronomers looking for signs have found that our Milky Way galaxy teems with exoplanets, some with conditions that could be right for extraterrestrial life. Such worlds orbit stars in so-called “habitable zones,” regions where planets could hold liquid water that is necessary for life as we know it.

However, the question of habitability is highly complex. Researchers led by space physicist Chuanfei Dong of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University have recently raised doubts about water on — and thus potential habitability of — frequently cited exoplanets that orbit red dwarfs, the most common stars in the Milky Way.

Impact of stellar wind

In two papers in The Astrophysical Journal Letters, the scientists develop models showing that the stellar wind — the constant outpouring of charged particles that sweep out into space — could severely deplete the atmosphere of such planets over hundreds of millions of years, rendering them unable to host surface-based life as we know it.

“Traditional definition and climate models of the habitable zone consider only the surface temperature,” Dong said. “But the stellar wind can significantly contribute to the long-term erosion and atmospheric loss of many exoplanets, so the climate models tell only part of the story.”

To broaden the picture, the first paper looks at the timescale of atmospheric retention on Proxima Centauri b (PCb), which orbits the nearest star to our solar system, some 4 light years away. The second paper questions how long oceans could survive on “water worlds” — planets thought to have seas that could be hundreds of miles deep.

Two-fold effect

The research simulates the photo-chemical impact of starlight and the electromagnetic erosion of stellar wind on the atmosphere of the exoplanets. These effects are two-fold: The photons in starlight ionize the atoms and molecules in the atmosphere into charged particles, allowing pressure and electromagnetic forces from the stellar wind to sweep them into space. This process could cause severe atmospheric losses that would prevent the water that evaporates from exoplanets from raining back onto them, leaving the surface of the planet to dry up.

On Proxima Centauri b, the model indicates that high stellar wind pressure would cause the atmosphere to escape and prevent atmosphere from lasting long enough to give rise to surface-based life as we know it. “The evolution of life takes billions of years,” Dong noted. “Our results indicate that PCb and similar exoplanets are generally not capable of supporting an atmosphere over sufficiently long timescales when the stellar wind pressure is high.”

“It is only if the pressure is sufficiently low,” he said, “and if the exoplanet has a reasonably strong magnetic shield like that of the Earth’s magnetosphere, that the exoplanet can retain an atmosphere and has the potential for habitability.”

Evolution of habitable zone

Complicating matters is the fact that the habitable zone circling red stars could evolve over time. So high stellar wind pressure early on could increase the rate of atmospheric escape. Thus, the atmosphere could have eroded too soon, even if the exoplanet was protected by a strong magnetic field like the magnetosphere surrounding Earth, Dong said. “In addition, such close-in planets could also be tidally locked like our moon, with one side always exposed to the star. The resultant weak global magnetic field and the constant bombardment of stellar wind would serve to intensify losses of atmosphere on the star-facing side.”

Turning to water worlds, the researchers explored three different conditions for the stellar wind. These ranged from:

Winds that strike the Earth’s magnetosphere today.
Ancient stellar winds flowing from young, Sun-like stars that were just a toddler-like 0.6 billion years old compared with the 4.6 billion year age of the Sun.
The impact on exoplanets of a massive stellar storm like the Carrington event, which knocked out telegraph service and produced auroras around the world in 1859.
The simulations illustrated that ancient stellar wind could cause the rate of atmospheric escape to be far greater than losses produced by the current solar wind that reaches the magnetosphere of Earth. Moreover, the rate of loss for Carrington-type events, which are thought to occur frequently in young Sun-like stars, was found to be greater still.

“Our analysis suggests that such space weather events may prove to be a key driver of atmospheric losses for exoplanets orbiting an active young Sun-like star,” the authors write.

High probability of dried-up oceans

Given the increased activity of red stars and the close-in location of planets in habitable zones, these results indicate the high probability of dried-up surfaces on planets that orbit red stars that might once have held oceans that could give birth to life. The findings could also modify the famed Drake equation, which estimates the number of civilizations in the Milky Way, by lowering the estimate for the average number of planets per star that can support life.

Authors of the PCb paper note that predicting the habitability of planets located light years from Earth is of course filled with uncertainties. Future missions like the James Webb Space Telescope, which NASA will launch in 2019 to peer into the early history of the universe, will therefore “be essential for getting more information on stellar winds and exoplanet atmospheres,” the authors say, “thereby paving the way for more accurate estimations of stellar-wind induced atmospheric losses.”

Scientists spot potentially habitable worlds with regularity. Recently, a newly discovered Earth-sized planet orbiting Ross 128, a red dwarf star that is smaller and cooler than the sun located some 11 light years from Earth, was cited as a water candidate. Scientists noted that the star appears to be quiescent and well-behaved, not throwing off flares and eruptions that could undo conditions favorable to life.

Collaborating with Dong on the PCb paper were physicists from Harvard University, the Harvard-Smithsonian Center for Astrophysics, the University of California, Los Angeles, and the University of Massachusetts. Support for the work came from a NASA Jack Eddy postdoctoral fellowship for Dong through the Princeton Center for Heliophysics, led by Prof. Amitava Bhattacharjee, head of the PPPL Theory Department who serves as Dong’s postdoctoral advisor, and the Max Planck-Princeton Research Center for Plasma Physics, jointly financed by the DOE Office of Science and the National Science Foundation. Collaborating on the water world research were scientists from the University of Michigan, the Harvard-Smithsonian Center for Astrophysics and Harvard University. The NASA Jack Eddy postdoctoral fellowship supported Dong.

Traces Of Life On Nearest Exoplanets May Be Hidden In Equatorial Trap

New simulations show that the search for life on other planets may well be more difficult than previously assumed, in research published today in the journal Monthly Notices of the Royal Astronomical Society. The study indicates that unusual air flow patterns could hide atmospheric components from telescopic observations, with direct consequences for formulating the optimal strategy for searching for (oxygen-producing) life such as bacteria or plants on exoplanets.

Current hopes of detecting life on planets outside of our own Solar System rest on examining the planet’s atmosphere to identify chemical compounds that may be produced by living beings. Ozone — a variety of oxygen — is one such molecule, and is seen as one of the possible tracers that may allow us to detect life on another planet from afar.

In Earth’s atmosphere, this compound forms the ozone layer that protects us from the Sun’s harmful UV radiation. On an alien planet, ozone could be one piece in the puzzle that indicates the presence of oxygen-producing bacteria or plants.

But now researchers, led by Ludmila Carone of the Max Planck Institute for Astronomy in Germany, have found that these tracers might be better hidden than we previously thought. Carone and her team considered some of the nearest exoplanets that have the potential to be Earth-like: Proxima b, which is orbiting the star nearest to the Sun (Proxima Centauri), and the most promising of the TRAPPIST-1 family of planets, TRAPPIST-1d.

These are examples of planets that orbit their host star in 25 days or fewer, and as a side effect have one side permanently facing their star, and the other side permanently facing away. Modelling the flow of air within the atmospheres of these planets, Carone and her colleagues found that this unusual day-night divide can have a marked effect on the distribution of ozone across the atmosphere: at least for these planets, the major air flow may lead from the poles to the equator, systematically trapping the ozone in the equatorial region.

Carone says: “Absence of traces of ozone in future observations does not have to mean there is no oxygen at all. It might be found in different places than on Earth, or it might be very well hidden.”

Such unexpected atmospheric structures may also have consequences for habitability, given that most of the planet would not be protected against ultraviolet (UV) radiation. “In principle, an exoplanet with an ozone layer that covers only the equatorial region may still be habitable,” Carone explains. “Proxima b and TRAPPIST-1d orbit red dwarfs, reddish stars that emit very little harmful UV light to begin with. On the other hand, these stars can be very temperamental, and prone to violent outbursts of harmful radiation including UV.”

The combination of advances in modelling and much better data from telescopes like the James Webb Space Telescope is likely to lead to significant progress in this exciting field. “We all knew from the beginning that the hunt for alien life will be a challenge,” says Carone. “As it turns out, we are only just scratching the surface of how difficult it really will be.”

Frictional Heat Powers Hydrothermal Activity On Enceladus

Heat from the friction of rocks caused by tidal forces could be the “engine” for the hydrothermal activity on Saturn’s moon Enceladus. This presupposes that the moon has a porous core that allows water from the overlying ocean to seep in, where the tidal friction exerted on the rocks heats it. This shows a computer simulation based on observations from the European-American Cassini-Huygens mission. It also offers among others an answer to the long-standing question of where the energy that can support water in liquid form on the small, cryovulcanic moon far from the sun comes from. The Heidelberg University research group led by planetary scientist Assistant Professor Dr Frank Postberg participated in the investigation.

In 2015, the researchers had already shown that there must be hydrothermal activity on Saturn’s moon. Icy volcanoes on Enceladus launch huge jets of gas and icy grains that contain fine particles of rock into space. A detector on the Cassini space probe was able to measure these particles. They originate on the seafloor more than 50,000 metres below the moon’s ice shell, which ranges in thickness from three to 35 kilometres. Using computer simulations and laboratory experiments, the scientists discovered signs that deep below the rock and the water interact — at temperatures of a least 90 degrees Celsius. But where does the energy for the hydrothermal systems that drive the transport of matter come from? And how exactly do the grains of rock get to the surface of the icy moon?

The current studies under the direction of the University of Nantes (France) offer an explanation. According to Dr Postberg, the rock core of Enceladus is probably porous, which is why the water from the overlying ocean is able to deeply permeate it. At the same time, strong tidal forces from Saturn affect the “loose” rock in the moon’s core. The new computer simulations show that the frictional heat is transferred very efficiently to the water circulating through the core, heating it to more than 90 degrees Celsius. This water dissolves some constituents of the rocky material. At certain hotspots, the hydrothermal fluids vent back into the ocean. Due to the cooling dissolved material now partially precipitates as fine particles, which are carried by the warm water to the ocean’s surface. The hotspots are located primarily at the poles of Enceladus.

The ascending hydrothermal fluids probably trigger local melting in the ice layer of the polar region. According to Dr Postberg, this explains why the ice layer at the poles is considerably thinner than at the equator — three to ten kilometres versus 35 kilometres. “At the south pole, the water can even rise through fissures almost to the moon’s surface. There, the microscopically small grains of rock from the core are catapulted along with ice particles into space, where they were measured by the instruments on the Cassini space probe,” explained the Heidelberg planetary scientist. The study also showed that only this heat source in the core can keep the overlying ocean water from freezing. Without it, the ocean would completely freeze in less than 30 million years. Dr Postberg conducts research at the Klaus Tschira Laboratory for Cosmochemistry. The laboratory ist part of the Institute of Earth Sciences at Heidelberg University. It is funded by the Klaus Tschira Foundation.

The aim of the Cassini-Huygens mission, a joint project of NASA, ESA, and Italy’s ASI space agency that began in 1997, was to gain new insights into the gas planet Saturn and its moons. The Cassini space probe began orbiting Saturn in 2004. The mission concluded in September of this year when the probe entered Saturn’s atmosphere. The latest research results were published in the journal “Nature Astronomy.”