Topic: Late July Astronomy Bulletin

[Clive]

A DOZEN NEW MOONS OF JUPITER DISCOVERED
Carnegie Institution for Science

Twelve new moons orbiting Jupiter have been found -- 11 "normal" outer moons, and one that they're calling an "oddball." This brings Jupiter's total number of known moons to a whopping 79 -- the most of any planet in our Solar System.  Astronomers first observed the moons in the spring of 2017 while they were looking for very distant Solar System objects as part of the hunt for a possible massive planet far beyond Pluto.  In 2014, this same team found the object with the most-distant known orbit in our Solar System and was the first to realize that an unknown massive planet at the fringes of our Solar System, far beyond Pluto, could explain the similarity of the orbits of several small extremely distant objects. This putative planet is now sometimes popularly called Planet X or Planet Nine.  Jupiter just happened to be in the sky near the search fields where the team was looking for extremely distant Solar System objects, so they were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our Solar System.  The International Astronomical Union's Minor Planet Center used the team's observations to calculate orbits for the newly found moons.  It takes several observations to confirm an object actually orbits around Jupiter so the whole process took a year. 
Nine of the new moons are part of a distant outer swarm of moons that orbit it in the retrograde, or opposite direction of Jupiter's spin rotation. These distant retrograde moons are grouped into at least three distinct orbital groupings and are thought to be the remnants of three once-larger parent bodies that broke apart during collisions with asteroids, comets, or other moons. The newly discovered retrograde moons take about two years to orbit Jupiter.  Two of the new discoveries are part of a closer, inner group of moons that orbit in the prograde, or same direction as the planet's rotation. These inner prograde moons all have similar orbital distances and angles of inclinations around Jupiter and so are thought to also be fragments of a larger moon that was broken apart. These two newly discovered moons take a little less than a year to travel around Jupiter. 

The other discovery is a real oddball and has an orbit like no other known Jovian moon.  It's also likely Jupiter's smallest known moon, being less than one kilometre in diameter.  This new "oddball" moon is more distant and more inclined than the prograde group of moons and takes about one and a half years to orbit Jupiter. So, unlike the closer-in prograde group of moons, this new oddball prograde moon has an orbit that crosses the outer retrograde moons.  As a result, head-on collisions are much more likely to occur between the "oddball" prograde and the retrograde moons, which are moving in opposite directions.  This is an unstable situation as head-on collisions would quickly break apart and grind the objects down to dust.  It's possible the various orbital moon groupings we see today were formed in the distant past through this exact mechanism.  The team think this small "oddball" prograde moon could be the last-remaining remnant of a once-larger prograde-orbiting moon that formed some of the retrograde moon groupings during past head-on collisions. The name Valetudo has been proposed for it, after the Roman god Jupiter's great-granddaughter, the goddess of health and hygiene.  Elucidating the complex influences that shaped a moon's orbital history can teach scientists about our Solar System's early years.  For example, the discovery that the smallest moons in Jupiter's various orbital groups are still abundant suggests the collisions that created them occurred after the era of planet formation, when the Sun was still surrounded by a rotating disk of gas and dust from which the planets were born.  because of their sizes -- one to three kilometres -- these moons are more influenced by surrounding gas and dust. If these raw materials had still been present when Jupiter's first generation of moons collided to form its current clustered groupings of moons, the drag exerted by any remaining gas and dust on the smaller moons would have been sufficient to cause them to spiral inwards toward Jupiter. Their existence shows that they were likely formed after this gas and dust dissipated.  The initial discovery of most of the new moons were made on the Blanco 4-meter telescope at Cerro Tololo Inter-American in Chile.   


RARE DOUBLE ASTEROID FOUND
NASA

New observations by three of the world's largest radio telescopes have revealed that an asteroid discovered last year is actually two objects, each about 900 metres in size, orbiting each other.  Near-Earth asteroid 2017 YE5 was discovered with observations provided by the Morocco Oukaimeden Sky Survey on Dec. 21, 2017, but no details about the asteroid's physical properties were known until the end of June. This is only the fourth "equal mass" binary near-Earth asteroid ever detected, consisting of two objects nearly identical in size, orbiting each other. The new observations provide the most detailed images ever obtained of this type of binary asteroid.  On June 21, the asteroid 2017 YE5 made its closest approach to Earth for at least the next 170 years, coming to within 6 million kilometres of Earth, or about 16 times the distance between Earth and the Moon. On June 21 and 22, observations by NASA's Goldstone Solar System Radar (GSSR) in California showed the first signs that 2017 YE5 could be a binary system. The observations revealed two distinct lobes, but the asteroid's orientation was such that scientists could not see if the two bodies were separate or joined. Eventually, the two objects rotated to expose a distinct gap between them.  Scientists at the Arecibo Observatory in Puerto Rico had already planned to observe 2017 YE5, and they were alerted by their colleagues at Goldstone of the asteroid's unique properties. On June 24, the scientists teamed up with researchers at the Green Bank Observatory (GBO) in West Virginia and used the two observatories together in a bi-static radar configuration (in which Arecibo transmits the radar signal and Green Bank receives the return signal). Together, they were able to confirm that 2017 YE5 consists of two separated objects. By June 26, both Goldstone and Arecibo had independently confirmed the asteroid's binary nature.  The observations indicate that the two objects revolve around each other once every 20 to 24 hours.


MOLECULAR OXYGEN IN COMET’S ATMOSPHERE
Imperial College London
     
Scientists have found that molecular oxygen around comet 67P is not produced on its surface, as some suggested, but may be from its body.  The European Space Agency's Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko on its journey round the Sun from August 2014 -- September 2016, dropping a probe and eventually crashing onto its surface.  When the comet is close enough to the Sun the ice on its surface 'sublimes' -- transforms from solid to gas -- forming a gas atmosphere called a coma. Analysis of the coma by instruments on Rosetta revealed that it contained not only water, carbon monoxide and carbon dioxide, as anticipated, but also molecular oxygen.  Molecular oxygen is two oxygen atoms joined together, and on Earth it is essential for life, where it is produced by photosynthesis. It has been previously detected around some of the icy moons of Jupiter, but it was not expected to be found around a comet.  The Rosetta science team originally reported that the oxygen was most likely from the comet's main body, or nucleus. This meant it was 'primordial' -- that it was already present when the comet itself formed at the beginning of the Solar System 4.6 billion years ago.  One group of outside researchers however suggested there might be a different source for molecular oxygen at comets. They had discovered a new way to produce molecular oxygen in space triggered by energetic ions -- electrically charged molecules. They proposed that reactions with energetic ions on the surface of comet 67P could instead be the source of the detected molecular oxygen.

Now, members of the Rosetta team have analysed the data on 67P's oxygen in light of the new theory and report that the proposed mechanism for producing oxygen on the surface of the comet is not sufficient to explain the observed levels in the coma.  The team tested the new theory of surface molecular oxygen production using observations of energetic ions, particles which trigger the surface processes which could lead to the production of molecular oxygen. It found that the amount of energetic ions present could not produce enough molecular oxygen to account for the amount of molecular oxygen observed in the coma.  Surface generation of molecular oxygen may still happen on 67P, but the majority of the molecular oxygen in the coma is not produced through such a process.  The new analysis is consistent with team's original conclusion, that molecular oxygen is most likely primordial. Other theories have been proposed, and can't yet be ruled out, but the primordial theory currently fits the data best.  This is also supported by recent theories which revisited the formation of the molecular oxygen in dark clouds and the presence of molecular oxygen in the early Solar System. In this model, molecular oxygen created froze onto small dust grains. These grains collected more material, eventually building up the comet and locking the oxygen in the nucleus.


ANALYSIS OF ROSS 128 HELPS UNDERSTAND ITS EXOPLANET
Carnegie Institution for Science

Last autumn, the world was excited by the discovery of an exoplanet called Ross 128 b, which is just 11 light years away from Earth. Now researchers  have for the first time determined detailed chemical abundances of the planet's host star, Ross 128.  Understanding which elements are present in a star in what abundances can help researchers estimate the makeup of the exoplanets that orbit them, which can help predict how similar the planets are to the Earth.  Like the exoplanet's host star Ross 128, about 70 percent of all stars in the Milky Way are red dwarfs, which are much cooler and smaller than our Sun. Based on the results from large planet-search surveys, astronomers estimate that many of these red dwarf stars host at least one exoplanet. Several planetary systems around red dwarfs have been newsmakers in recent years, including Proxima b, a planet which orbits the nearest star to our own Sun, Proxima Centauri, and the seven planets of TRAPPIST-1, which itself is not much larger in size than our Solar System's Jupiter.  Using the Sloan Digital Sky Survey's APOGEE spectroscopic instrument, the team measured the star's near-infrared light to derive abundances of carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium, and iron.  When stars are young, they are surrounded by a disk of rotating gas and dust from which rocky planets accrete. The star's chemistry can influence the contents of the disk, as well as the resulting planet's mineralogy and interior structure. For example, the amount of magnesium, iron, and silicon in a planet will control the mass ratio of its internal core and mantle layers.

The team determined that Ross 128 has iron levels similar to our Sun. Although they were not able to measure its abundance of silicon, the ratio of iron to magnesium in the star indicates that the core of its planet, Ross 128 b, should be larger than Earth's.  Because they knew Ross 128 b's minimum mass, and stellar abundances, the team was also able to estimate a range for the planet's radius, which is not possible to measure directly due to the way the planet's orbit is oriented around the star.  Knowing a planet's mass and radius is important to understanding what it's made of, because these two measurements can be used to calculate its bulk density. What's more, when quantifying planets in this way, astronomers have realized that planets with radii greater than about 1.7 times Earth's are likely surrounded by a gassy envelope, like Neptune, and those with smaller radii are likely to be more-rocky, as is our own home planet.  The estimated radius of Ross 128 b indicates that it should be rocky.  Lastly, by measuring the temperature of Ross 128 and estimating the radius of the planet the team was able to determine how much of the host star's light should be reflecting off the surface of Ross 128 b, revealing that our second-closest rocky neighbour likely has a temperate climate.


VLT IMAGES NEWBORN PLANET
ESO

Astronomers at the Max Planck Institute for Astronomy in Heidelberg, Germany have captured a snapshot of planetary formation around the young dwarf star PDS 70. By using the SPHERE instrument on ESO’s Very Large Telescope (VLT) the international team has made the first robust detection of a young planet, named PDS 70b, cleaving a path through the planet-forming material surrounding the young star.  The SPHERE instrument also enabled the team to measure the brightness of the planet at different wavelengths, which allowed properties of its atmosphere to be deduced.  The planet stands out very clearly in the new observations, visible as a bright point to the right of the blackened centre of the image. It is located roughly three billion kilometres from the central star, roughly equivalent to the distance between Uranus and the Sun. The analysis shows that PDS 70b is a giant gas planet with a mass a few times that of Jupiter. The planet's surface has a temperature of around 1000°C, making it much hotter than any planet in our own Solar System.  The discovery of PDS 70’s young companion is an exciting scientific result that has already merited further investigation. A second team, involving many of the same astronomers as the discovery team, has in the past months followed up the initial observations to investigate PDS 70’s fledgling planetary companion in more detail. They not only made the spectacularly clear image of the, but were even able to obtain a spectrum of the planet. Analysis of this spectrum indicated that its atmosphere is cloudy.  PDS 70’s planetary companion has sculpted a transition disc — a protoplanetary disc with a giant “hole” in the centre. These inner gaps have been known about for decades and it has been speculated that they were produced by disc-planet interaction. Now we can see the planet for the first time.   


BEAM OF LIGHT FROM NEUTRON STAR MERGER
University of Warwick
 
Astronomers have had to wait over 100 days for the sight of the first of confirmed neutron star merger to remerge from behind the glare of the Sun.  They were rewarded with the first confirmed visual sighting of a jet of material that was still streaming out from merged star exactly 110 days after that initial cataclysmic merger event was first observed. Their observations confirm a key prediction about the aftermath of neutron star mergers.  The binary neutron star merger GW170817 occurred 130 million light years away in a galaxy named NGC 4993. It was detected in August 2017 by the Advanced Laser Interferometer Gravitational-Wave Observatory (Adv-LIGO), and by Gamma Ray Burst (GRB) observations, and then became the first ever neutron star merger to be observed and confirmed by visual astronomy.  After a few weeks the merged star then passed behind the glare of our sun leaving it effectively hidden from astronomers until it remerged from that glare 100 days after the merger event. It was at that point that the research team were able to use the Hubble Space Telescope to see the star was still generating a powerful beam of light in a direction that, while off centre to the Earth, was starting to spread out in our direction.  Early on, the team saw visible light powered by radioactive decay of heavy elements, over a hundred days later and this has gone, but now a jet of material can be seen, ejected at an angle to us, but at almost of the speed of light. This is quite different than some people have suggested, that the material wouldn't come out in a jet, but in all directions.

If the team had looked straight down this beam they would have seen a really powerful burst of gamma-ray. This means that it is quite likely that every neutron star that mergers actually creates a gamma-ray burst, but we only see a small fraction of them because the jet doesn't line up all that often. Gravitational waves are a whole new way to find this kind of event, and they might be more common than we think.  These types of events will reveal the structure of these jets of material travelling close to the speed of light.  The behaviour of the light from these jets, how it brightens and fades, can be used to determine the velocity of the material throughout the jet. As the afterglow brightens we are seeing deeper into the jet structure and probing the fastest components. This will help us understand how these jets of material, travelling close to the speed of light, are formed and how they are accelerated to these phenomenal velocities.


MAJOR COLLISION THAT CHANGED THE MILKY WAY
Simons Foundation
     
A team of astronomers has discovered an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed the "Sausage" galaxy. The cosmic crash was a defining event in the early history of the Milky Way and reshaped the structure of our galaxy, fashioning both its inner bulge and its outer halo.   The astronomers propose that around 8 billion to 10 billion years ago, an unknown dwarf galaxy smashed into our own Milky Way. The dwarf did not survive the impact: It quickly fell apart, and the wreckage is now all around us.  The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits that are long and narrow like needles.  The stars' paths take them very close to the centre of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.  The team used data from the European Space Agency's Gaia satellite. This spacecraft has been mapping the stellar content of our galaxy, recording the journeys of stars as they travel through the Milky Way. Thanks to Gaia, astronomers now know the positions and trajectories of our celestial neighbours with unprecedented accuracy.  The paths of the stars from the galactic merger earned them the moniker "the Gaia Sausage."
 
The Milky Way continues to collide with other galaxies, such as the puny Sagittarius dwarf galaxy. However, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter was more than 10 billion times the mass of our sun. When the Sausage crashed into the young Milky Way, its piercing trajectory caused a lot of mayhem. The Milky Way's disk was probably puffed up or even fractured following the impact and would have needed to regrow. And Sausage debris was scattered all around the inner parts of the Milky Way, creating the 'bulge' at the galaxy's centre and the surrounding 'stellar halo.'  In simulations run by the team, stars from the Sausage galaxy enter stretched-out orbits. The orbits are further elongated by the growing Milky Way disk, which swells and becomes thicker following the collision.  Evidence of this galactic remodelling is seen in the paths of stars inherited from the dwarf galaxy.  The Sausage stars are all turning around at about the same distance from the centre of the galaxy. These U-turns cause the density in the Milky Way's stellar halo to decrease dramatically where the stars flip directions. The new research also identified at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.  While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all.   
 

SUPERSTAR ETA CARINAE SHOOTS COSMIC RAYS
NASA/Goddard Space Flight Center

A new study using data from NASA's NuSTAR space telescope suggests that Eta Carinae, the most luminous and massive stellar system within 10,000 light-years, is accelerating particles to high energies -- some of which may reach Earth as cosmic rays.  Astronomers know that cosmic rays with energies greater than 1 billion electron volts (eV) come to us from beyond our solar system. But because these particles -- electrons, protons and atomic nuclei -- all carry an electrical charge, they veer off course whenever they encounter magnetic fields. This scrambles their paths and masks their origins.  Eta Carinae, located about 7,500 light-years away in the southern constellation of Carina, is famous for a 19th century outburst that briefly made it the second-brightest star in the sky. This event also ejected a massive hourglass-shaped nebula, but the cause of the eruption remains poorly understood.  The system contains a pair of massive stars whose eccentric orbits bring them unusually close every 5.5 years. The stars contain 90 and 30 times the mass of our Sun and pass 225 million kilometres apart at their closest approach -- about the average distance separating Mars and the Sun.  Both of Eta Carinae's stars drive powerful outflows called stellar winds.  Where these winds clash changes during the orbital cycle, which produces a periodic signal in low-energy X-rays that have been tracking for more than two decades.

NASA's Fermi Gamma-ray Space Telescope also observes a change in gamma rays -- light packing far more energy than X-rays -- from a source in the direction of Eta Carinae. But Fermi's vision isn't as sharp as X-ray telescopes, so astronomers couldn't confirm the connection.  To bridge the gap between low-energy X-ray monitoring and Fermi observations, the team turned to NuSTAR. Launched in 2012, NuSTAR can focus X-rays of much greater energy than any previous telescope. Using both newly taken and archival data, the team examined NuSTAR observations acquired between March 2014 and June 2016, along with lower-energy X-ray observations from the European Space Agency's XMM-Newton satellite over the same period.  Eta Carinae's low-energy, or soft, X-rays come from gas at the interface of the colliding stellar winds, where temperatures exceed 40 million degrees Celsius. But NuSTAR detects a source emitting X-rays above 30,000 eV, some three times higher than can be explained by shock waves in the colliding winds. For comparison, the energy of visible light ranges from about 2 to 3 eV.  The team's analysis shows that these "hard" X-rays vary with the binary orbital period and show a similar pattern of energy output as the gamma rays observed by Fermi.  The researchers say that the best explanation for both the hard X-ray and the gamma-ray emission is electrons accelerated in violent shock waves along the boundary of the colliding stellar winds. The X-rays detected by NuSTAR and the gamma rays detected by Fermi arise from starlight given a huge energy boost by interactions with these electrons.  Some of the superfast electrons, as well as other accelerated particles, must escape the system and perhaps some eventually wander to Earth, where they may be detected as cosmic rays.  Astronomers have known for some time that the region around Eta Carinae is the source of energetic emission in high-energy X-rays and gamma rays, but until NuSTAR was able to pinpoint the radiation, show it comes from the binary and study its properties in detail, the origin was mysterious.
 

MILKY WAY TYPE DUST PARTICLES FOUND IN DISTANT GALAXY
Faculty of Science - University of Copenhagen

An international research team, with participation from the Niels Bohr Institute at the University of Copenhagen, has found the same type of interstellar dust that we know from the Milky Way in a distant galaxy 11 billion light years from Earth. This type of dust has been found to be rare in other galaxies and the new discovery plays an important role in understanding what it takes for this particular type of interstellar dust to be formed.  Galaxies are complex structures comprised of many individual parts, such as stars, gas, dust and dark matter. Even though the dust only represents a small part of the total amount of matter in a galaxy, it plays a major role in how stars are formed and how the light from the stars escapes the galaxies. Dust grains can both absorb and scatter light. Dust particles also play a decisive role in the formation of planets and thus also for the understanding of our own existence on Earth.  The dust in galaxies consists of small grains of carbon, silicon, iron, aluminium and other heavier elements. The Milky Way has a very high content of carbonaceous dust, which has been shown to be very rare in other galaxies. But now a similar type of dust has been found in a few, very distant galaxies that researchers have been able to investigate using light from gamma-ray bursts. Gamma-ray bursts come from massive stars that explode when the when the fuel in its core is exhausted. The explosion causes the dying stars to emit powerful bursts of light that astronomers can use to analyse what the galaxies are comprised of. Specifically, they can measure the elemental content and analyse their way forward to the properties of the dust properties by examining the light that escapes from the galaxies.  The carbonaceous dust is registered in the measurements as a "dust bump," that is, a high value of dust with the said composition. This ultraviolet dust bump has now been detected in a gamma-ray burst, which has been named GRB180325A.   

GRB180325A was detected the Swift Observatory (NASA) on 28 March 2018. Swift is a satellite mission that detects gamma rays from the dying stars. When such a detection from the satellite hits the astronomers, a hectic period begins. The astronomers try to observe that part of the sky as quickly as possible in order to secure the crucial information that allows them to study the interior of the galaxy the explosion originated from. In this case a researcher activated the Nordic Optical Telescope (NOT) at La Palma and first observations of the light from the gamma-ray burst were secured only a few minutes after the discovery by Swift.  The observations from NOT showed that the star had exploded in a galaxy with a red shift of 2.25, which means that the light has travelled approximately 11 billion light years. The observations immediately showed that the dust bump, known from the Milky Way, was present in this galaxy. The team then observed the gamma-ray burst with the X-shooter spectrograph on ESO's Very Large Telescope (European Southern Observatory) on the Cerro Paranal in Chile. All in all, four spectra of the afterglow from the gamma-ray burst were secured -- all with a clear detection of the dust bump.  The spectra show that the presence of atomic carbon seems to be a requirement for the dust that causes the dust bump to be formed.  The dust bump has previously been seen in observations of four other gamma-ray bursts, the last of which was detected 10 years ago.


QUASAR SHINES LIGHT ON UNIVERSE’S YOUTH
Carnegie Institution for Science

A team of astronomers has found a quasar with the brightest radio emission ever observed in the early Universe, due to it spewing out a jet of extremely fast-moving material.  The discovery was followed up by the National Radio Astronomy Observatory, which allowed the team to see with unprecedented detail the jet shooting out of a quasar that formed within the universe's first billion years of existence.  The findings will allow astronomers to better probe the Universe's youth during an important period of transition to its current state.  Quasars are comprised of enormous black holes accreting matter at the centres of massive galaxies. This newly discovered quasar, called PSO J352.4034-15.3373, is one of a rare breed that doesn't just swallow matter into the black hole but also emits a jet of plasma travelling at speeds approaching that of light. This jet makes it extremely bright in the frequencies detected by radio telescopes. Although quasars were identified more than 50 years ago by their strong radio emissions, now we know that only about 10 percent of them are strong radio emitters.  What's more, this newly discovered quasar's light has been traveling nearly 13 billion of the Universe's 13.7 billion years to reach us here in Earth. P352-15 is the first quasar with clear evidence of radio jets seen within the first billion years of the Universe's history.  This is the most-detailed image yet of such a bright galaxy at this great distance. 

The Big Bang started the Universe as a hot soup of extremely energetic particles that were rapidly expanding. As it expanded, it cooled and coalesced into neutral hydrogen gas, which left the Universe dark, without any luminous sources, until gravity condensed matter into the first stars and galaxies. About 800 million years after the Big Bang, the energy released by these first galaxies caused the neutral hydrogen that was scattered throughout the Universe to get excited and lose an electron, or ionize, a state that the gas has remained in since that time.  It's highly unusual to find radio jet-emitting quasars such as this one from the period just after the Universe's lights came back on.  The jet from this quasar could serve as an important calibration tool to help future projects penetrate the dark ages and perhaps reveal how the earliest galaxies came into being.