Topic: Mid November Astronomy Bulletin

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THREE NEW NEAR EARTH ASTEROIDS FOUND

Association of Universities for Research in Astronomy (AURA)

An international team using the Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF's NOIRLab, has discovered three new near-Earth asteroids (NEAs) hiding in the inner Solar System, the region interior to the orbits of Earth and Venus. This is a notoriously challenging region for observations because asteroid hunters have to contend with the glare of the Sun. By taking advantage of the brief yet favourable observing conditions during twilight, however, the astronomers found an elusive trio of NEAs. One is a 1.5-kilometer-wide asteroid called 2022 AP7, which has an orbit that may someday place it in Earth's path. The other asteroids, called 2021 LJ4 and 2021 PH27, have orbits that safely remain completely interior to Earth's orbit. Also of special interest to astronomers and astrophysicists, 2021 PH27 is the closest known asteroid to the Sun. As such, it has the largest general-relativity effects [1] of any object in our Solar System and during its orbit its surface gets hot enough to melt lead. Finding asteroids in the inner Solar System is a daunting observational challenge. Astronomers have only two brief 10-minute windows each night to survey this area and have to contend with a bright background sky resulting from the Sun's glare. Additionally, such observations are very near to the horizon, meaning that astronomers have to observe through a thick layer of Earth's atmosphere, which can blur and distort their observations. Discovering these three new asteroids despite these challenges was possible thanks to the unique observing capabilities of DECam. The state-of-the-art instrument is one of the highest-performance, wide-field CCD imagers in the world, giving astronomers the ability to capture large areas of sky with great sensitivity. Astronomers refer to observations as 'deep' if they capture faint objects. When hunting for asteroids inside Earth's orbit, the capability to capture both deep and wide-field observations is indispensable.

PLANETARY SCAN COMFIRMS MARTIAN CORE

Australian National University

Seismologists have developed a new method to scan the deep interior of planets in our solar system to confirm whether they have a core at the heart of their existence.  The scanning method, which works in a similar way to an ultrasound scan using sound waves to generate images of a patient's body, requires only a single seismometer on a planet's surface in order to work. It can also be used to confirm the size of a planet's core. Using the ANU model to scan the entirety of Mars' interior, the researchers confirmed the Red Planet has a large core at its centre -- a theory first confirmed by a team of scientists in 2021. Confirming the existence of a planetary core, which the researchers refer to as the "engine room" of all planets, can help scientists learn more about a planet's past and evolution. It can also help scientists determine at what point in a planet's history a magnetic field formed and ceased to exist. The core plays an active role in sustaining a planet's magnetic field. In the case of Mars, it could help explain why, unlike Earth, the Red Planet no longer has a magnetic field -- something that is critical to sustaining all life forms.   Using a single seismometer on Mars' surface, the ANU team measured specific types of seismic waves. The seismic waves, which were triggered by marsquakes, give off a spectrum of signals, or "echoes," that change over time as they reverberate throughout the Martian interior.  These seismic waves pierce through and bounce off the Martian core. 

The researchers say their method of using a single seismometer to confirm the presence of a planetary core is also a "cost-effective solution. There is a single seismic station on Mars. There were four of them on the Moon in 1970s. The situation of having a limited number of instruments is unlikely to change in the coming decades or even this century due to high cost. The researchers hope this new ANU-developed technique involving a single seismometer could be used to help scientists learn more about our other planetary neighbours, including the Moon.  Although there are many studies on planetary cores, the images we have of planetary interiors are still very blurry. But with new instruments and methods scientists will be able to get sharper images which will help us answer questions such as how big the cores are and whether they take a solid or liquid form. 

MARS’S CRUST MORE COMPLEX

University of Iowa

Early crust on Mars may be more complex than previously thought -- and it may even be similar to our own planet's original crust. The Martian surface is uniformly basaltic, a product of billions of years of volcanism and flowing lava on the surface that eventually cooled. Because Mars did not undergo full-scale surface remodelling like the shifting of continents on Earth, scientists had thought Mars' crustal history was a relatively simple tale. But in a new study, researchers found locations in the Red Planet's southern hemisphere with greater concentrations of silicon, a chemical element, than what would be expected in a purely basaltic setting. The silica concentration had been exposed by space rocks that slammed into Mars, excavating material that was embedded miles below the surface, and revealing a hidden past. Scientists believe Mars formed about 4.5 billion years ago. Exactly how the Red Planet came into being is a mystery, but there are theories. One idea is that Mars formed via a titanic collision of rocks in space that, with its intense heat, spawned an entirely liquefied state, also known as a magma ocean. The magma ocean gradually cooled, the theory goes, yielding a crust, like a layer of skin, that would be singularly basaltic. Another theory is that the magma ocean was not all-encompassing, and that parts of the first crust on Mars had a different origin, one that would show silica concentrations different from basaltic.

Researchers analysed data gathered by the Mars Reconnaissance Orbiter for the planet's southern hemisphere, which previous research had indicated was the oldest region. The researchers found nine locations -- such as craters and fractures in the terrain -- that were rich in feldspar, a mineral associated with lava flows that are more silicic than basaltic. Feldspar had been found previously in other regions on Mars, but further analysis showed the chemical composition in those areas was more basaltic. That did not deter the researchers, who turned to another instrument, called THEMIS, which can detect silica concentrations through infrared wavelength reflections from the Martian surface. With data from THEMIS, the team determined the terrain at their chosen locations was more silicic than basaltic. Adding further credence to their observations, meteorites such as Erg Chech 002, discovered in the Sahara and dating roughly to the birth of the solar system, show similar silicic and other mineral compositions that the team observed in the nine locations on Mars. The researchers also dated the crust to about 4.2 billion years, which would make it the oldest crust found on Mars to date. While Mars' crustal origin remains shrouded, Earth's crustal history is even less clear, as any vestiges of our planet's original crust have been long erased due to the shifting of continental plates for billions of years. Still, the finding may offer insights into Earth's origins.

OLDEST PLANETARY DEBRIS IN OUR GALAXY

University of Warwick

Astronomers led by the University of Warwick have identified the oldest star in our galaxy that is accreting debris from orbiting planetesimals, making it one of the oldest rocky and icy planetary systems discovered in the Milky Way. The fate of most stars, including those like our Sun, is to become a white dwarf. A white dwarf is a star that has burnt up all of its fuel and shed its outer layers and is now undergoing a process of shrinking and cooling. During this process, any orbiting planets will be disrupted and in some cases destroyed, with their debris left to accrete onto the surface of the white dwarf. For this study the team of astronomers modelled two unusual white dwarfs that were detected by the space observatory GAIA of the European Space Agency. Both stars are polluted by planetary debris, with one of them being found to be unusually blue, while the other is the faintest and reddest found to date in the local galactic neighbourhood -- the team subjected both to further analysis. Using spectroscopic and photometric data from GAIA, the Dark Energy Survey and the X-Shooter instrument at the European Southern Observatory to work out how long it has been cooling for, the astronomers found that the 'red' star WDJ2147-4035 is around 10.7 billion years old, of which 10.2 billion years has been spent cooling as a white dwarf. Spectroscopy involves analysing the light from the star at different wavelengths, which can detect when elements in the star's atmosphere are absorbing light at different colours and helps determine what elements those are and how much is present. By analysing the spectrum from WDJ2147-4035, the team found the presence of the metals sodium, lithium, potassium and tentatively detected carbon accreting onto the star -- making this the oldest metal-polluted white dwarf discovered so far.

The second 'blue' star WDJ1922+0233 is only slightly younger than WDJ2147-4035 and was polluted by planetary debris of a similar composition to the Earth's continental crust. The science team concluded that the blue colour of WDJ1922+0233, despite its cool surface temperature, is caused by its unusual mixed helium-hydrogen atmosphere. The debris found in the otherwise nearly pure-helium and high-gravity atmosphere of the red star WDJ2147-4035 are from an old planetary system that survived the evolution of the star into a white dwarf, leading the astronomers to conclude that this is the oldest planetary system around a white dwarf discovered in the Milky Way. These metal-polluted stars show that Earth isn't unique, there are other planetary systems out there with planetary bodies similar to the Earth. 97% of all stars will become a white dwarf and they're so ubiquitous around the Universe that they are very important to understand, especially these extremely cool ones. Formed from the oldest stars in our galaxy, cool white dwarfs provide information on the formation and evolution of planetary systems around the oldest stars in the Milky Way. Astronomers can also use the star's spectra to determine how quickly those metals are sinking into the star's core, which allows them to look back in time and determine how abundant each of those metals were in the original planetary body. By comparing those abundances to astronomical bodies and planetary material found in our own solar system, we can guess at what those planets would have been like before the star died and became a white dwarf -- but in the case of WDJ2147-4035, that has proven challenging.

The red star WDJ2147-4035 is a mystery as the accreted planetary debris are very lithium and potassium rich and unlike anything known in our own solar system. This is a very interesting white dwarf as its ultra-cool surface temperature, the metals polluting it, its old age, and the fact that it is magnetic, makes it extremely rare. When these old stars formed more than 10 billion years ago, the Universe was less metal-rich than it is now, since metals are formed in evolved stars and gigantic stellar explosions. The two observed white dwarfs provide an exciting window into planetary formation in a metal poor and gas-rich environment that was different to the conditions when the solar system was formed.

CLOSEST BLACK HOLE TO EARTH

Association of Universities for Research in Astronomy (AURA)

Astronomers using the Gemini Observatory have discovered the closest-known black hole to Earth. This is the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its close proximity to Earth, a mere 1600 light-years away, offers an intriguing target of study to advance our understanding of the evolution of binary systems. Black holes are the most extreme objects in the Universe. Supermassive versions of these unimaginably dense objects likely reside at the centres of all large galaxies. Stellar-mass black holes -- which weigh approximately five to 100 times the mass of the Sun -- are much more common, with an estimated 100 million in the Milky Way alone. Only a handful have been confirmed to date, however, and nearly all of these are 'active' -- meaning they shine brightly in X-rays as they consume material from a nearby stellar companion, unlike dormant black holes which do not. This dormant black hole is about 10 times more massive than the Sun and is located about 1600 light-years away in the constellation Ophiuchus, making it three times closer to Earth than the previous record holder, an X-ray binary in the constellation of Monoceros. The new discovery was made possible by making exquisite observations of the motion of the black hole's companion, a Sun-like star that orbits the black hole at about the same distance as the Earth orbits the Sun.

Though there are likely millions of stellar-mass black holes roaming the Milky Way Galaxy, those few that have been detected were uncovered by their energetic interactions with a companion star. As material from a nearby star spirals in toward the black hole, it becomes superheated and generates powerful X-rays and jets of material. If a black hole is not actively feeding (i.e., it is dormant) it simply blends in with its surroundings. The team originally identified the system as potentially hosting a black hole by analyzing data from the European Space Agency's Gaia spacecraft. Gaia captured the minute irregularities in the star's motion caused by the gravity of an unseen massive object. To explore the system in more detail the team turned to the Gemini Multi-Object Spectrograph instrument on Gemini North, which measured the velocity of the companion star as it orbited the black hole and provided precise measurement of its orbital period. The Gemini follow-up observations were crucial to constraining the orbital motion and hence masses of the two components in the binary system, allowing the team to identify the central body as a black hole roughly 10 times as massive as our Sun. Astronomers' current models of the evolution of binary systems are hard-pressed to explain how the peculiar configuration of Gaia BH1 system could have arisen. Specifically, the progenitor star that later turned into the newly detected black hole would have been at least 20 times as massive as our Sun. This means it would have lived only a few million years. If both stars formed at the same time, this massive star would have quickly turned into a supergiant, puffing up and engulfing the other star before it had time to become a proper, hydrogen-burning, main-sequence star like our Sun. It is not at all clear how the solar-mass star could have survived that episode, ending up as an apparently normal star, as the observations of the black hole binary indicate. Theoretical models that do allow for survival all predict that the solar-mass star should have ended up on a much tighter orbit than what is actually observed.

MASSIVE QUANTUM MYSTERIES OF BLACK HOLES

University of Queensland

Bizarre quantum properties of black holes -- including their mind-bending ability to have different masses simultaneously -- have been confirmed. A team of theoretical physicists ran calculations that reveal surprising black hole quantum phenomena. Black holes are an incredibly unique and fascinating feature of our Universe. They're created when gravity squeezes a vast amount of matter incredibly densely into a tiny space, creating so much gravitational pull that even light cannot escape. It's a phenomenon that can be triggered by a dying star. But, until now, we haven't deeply investigated whether black holes display some of the weird and wonderful behaviours of quantum physics. One such behaviour is superposition, where particles on a quantum scale can exist in multiple states at the same time. This is most commonly illustrated by Schrödinger's cat, which can be both dead and alive simultaneously. But, for black holes, scientists wanted to see whether they could have wildly different masses at the same time, and it turns out they do. Imagine you're both broad and tall, as well as short and skinny at the same time -- it's a situation which is intuitively confusing since we're anchored in the world of traditional physics. But this is reality for quantum black holes."

To reveal this, the team developed a mathematical framework allowing us to "place" a particle outside a theoretical mass-superposed black hole. Mass was looked at specifically, as it is a defining feature of a black hole, and as it is plausible that quantum black holes would naturally have mass superposition. The research in fact reinforces conjectures raised by pioneers of quantum physics. It shows that the very early theories of Jacob Bekenstein -- an American and Israeli theoretical physicist who made fundamental contributions to the foundation of black hole thermodynamics -- were accurate. He postulated that black holes can only have masses that are of certain values, that is, they must fall within certain bands or ratios -- this is how energy levels of an atom works, for example. The new modelling showed that these superposed masses were, in fact, in certain determined bands or ratios -- as predicted by Bekenstein. The Universe is revealing to us that it's always more strange, mysterious and fascinating than most of us could have ever imagined.

RED GIANT SUPERNOVA REVEALS SECRETS OF EARLY UNIVERSE

University of Minnesota

A research team has measured the size of a star dating back 2 billion years after the Big Bang, or more than 11 billion years ago. Detailed images show the exploding star cooling and could help scientists learn more about the stars and galaxies present in the early Universe. The red supergiant in question was about 500 times larger than the Sun, and it's located at redshift three, which is about 60 times farther away than any other supernova observed in this detail. Using data from the Hubble Space Telescope with follow-up spectroscopy using the University of Minnesota's access to the Large Binocular Telescope, the researchers were able to identify multiple detailed images of the red supergiant because of a phenomenon called gravitational lensing, where mass, such as that in a galaxy, bends light. This magnifies the light emitted from the star. The gravitational lens acts as a natural magnifying glass and multiplies Hubble's power by a factor of eight. Three images can be seen but even though they can be seen at the same time, they show the supernova as it was at different ages separated by several days. We see the supernova rapidly cooling, which allows astronomers to basically reconstruct what happened and study how the supernova cooled in its first few days with just one set of images. The researchers combined this discovery with another supernova discovery from 2014 to estimate how many stars were exploding when the Universe was a small fraction of its current age. They found that there were likely many more supernovae than previously thought. Core-collapse supernovae mark the deaths of massive, short-lived stars. The number of core-collapse supernovae detected can be used to understand how many massive stars were formed in galaxies when the Universe was much younger.