BETA PICTORIS b SPIN-ORBIT ALIGNMENT MEASURED
University of Exeter
Astronomers have made the first measurement of spin-orbit alignment for a distant 'super-Jupiter' planet, demonstrating a technique that could enable breakthroughs in the quest to understand how exoplanetary systems form and evolved. The team has carried out the measurements for the exoplanet Beta Pictoris b -- located 63 light years from Earth. The planet, found in the Pictor constellation, has a mass of around 11 times that of Jupiter and orbits a young star on a similar orbit as Saturn in our solar system. The study marks the first time that scientists have measured the spin-orbit alignment for a directly-imaged planetary system. Crucially, the results give a fresh insight into enhancing our understanding of the formation history and evolution of the planetary system. The degree to that a star and a planetary orbit are aligned with each other tells us a lot about how a planet formed and whether multiple planets in the system interacted dynamically after their formation. Some of the earliest theories of the planet formation process were proposed by prominent 18th century astronomers Kant and Laplace. They noted that the orbits of the solar system planets are aligned with each other, and with the Sun's spin axis, and concluded that the solar system formed from a rotating and flattened protoplanetary disc. It was a major surprise when it was found that more than a third of all close-in
exoplanets orbit their host star on orbits that are misaligned with respect to the stellar equator. A few exoplanets were even found to orbit in the opposite direction than the rotation direction of the star. These observations challenge the perception of planet formation as a neat and well-ordered process taking place in a geometrically thin and co-planar disc.
For the study, the researchers devised an innovative method that measures the tiny spatial displacement of less than a billionth of a degree that is caused by Beta Pictoris' rotation. The team used the GRAVITY instrument at the VLTI, which combines the light from telescopes separated 140 metres apart, to carry out the measurements. They found that the stellar rotation axis is aligned with the orbital axes of the planet Beta Pictoris b and its extended debris disc. Gas absorption in the stellar atmosphere causes a tiny spatial displacement in spectral lines that can be used to determine the orientation of the stellar rotation axis. The challenge is that this spatial displacement is extremely small: about 1/100th of the apparent diameter of the star, or the equivalent to the size of a human footstep on the Moon as seen from Earth. The results show that the Beta Pictoris system is as well-aligned as our own solar system. This finding favours planet-planet scattering as the cause for the orbit obliquities that are observed in more exotic systems with Hot Jupiters. However, observations on a large sample of planetary systems will be required to answer this question conclusively. The team proposes a new interferometric instrument that will allow them to obtain these measurements on many more planetary systems that are about to be discovered.
WHITE DWARFS REVEAL ORIGIN OF CARBON
University of California - Santa Cruz
Approximately 90 percent of all stars end their lives as white dwarfs, very dense stellar remnants that gradually cool and dim over billions of years. With their final few breaths before they collapse, however, these stars leave an important legacy, spreading their ashes into the surrounding space through stellar winds enriched with chemical elements, including carbon, newly synthesized in the star's deep interior
during the last stages before its death. Every carbon atom in the Universe was created by stars, through the fusion of three helium nuclei. But astrophysicists still debate which types of stars are the primary source of the carbon in our own galaxy, the Milky Way. Some studies favour low-mass stars that blew off their envelopes in stellar winds and became white dwarfs, while others favour massive stars that eventually exploded as supernovae. In the new study, a team of astronomers discovered and analyzed white dwarfs in open star clusters in the Milky Way, and their findings help shed light on the origin of the carbon in our galaxy. Open star clusters are groups of up to a few thousand stars, formed from the same giant molecular cloud and roughly the same age, and held together by mutual gravitational attraction. The study was based on astronomical observations conducted in 2018 at the W. M. Keck Observatory in Hawaii. From the analysis of the observed Keck spectra, it was possible to measure the masses of the white dwarfs. Using the theory of stellar evolution, it was possible to trace back to the progenitor stars and derive their masses at birth. The relationship between the initial masses of stars and their final masses as white dwarfs is known as the initial-final mass relation, a
fundamental diagnostic in astrophysics that integrates information from the entire life cycles of stars, linking birth to death. In general, the more massive the star at birth, the more massive the white dwarf left at its death, and this trend has been supported on both observational and theoretical grounds.
But analysis of the newly discovered white dwarfs in old open clusters gave a surprising result: the masses of these white dwarfs were notably larger than expected, putting a "kink" in the initial-final mass relation for stars with initial masses in a certain range. In the last phases of their lives, stars twice as massive as our Sun produced new carbon atoms in their hot interiors, transported them to the surface, and finally spread them into the interstellar medium through gentle stellar winds. The team's detailed stellar models indicate that the stripping of the carbon-rich outer mantle occurred slowly enough to allow the central cores of these stars, the future white dwarfs, to grow appreciably in mass. Analyzing the initial-final mass relation around the kink, the researchers concluded that stars bigger than 2 solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not. In other words, 1.5 solar masses represents the minimum mass for a star to spread carbon-enriched ashes upon its death. These findings place stringent constraints on how and when carbon, the element essential to life on Earth, was produced by the stars of our galaxy, eventually ending up trapped in the raw material from which the Sun and its planetary system were formed 4.6 billion years ago. Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses. One of most exciting aspects of this research is that it impacts the age of known white dwarfs, which are essential cosmic probes to understand the formation history of the Milky Way. The initial-to-
final mass relation is also what sets the lower mass limit for supernovae, the gigantic explosions seen at large distances and that are really important to understand the nature of the Universe. By combining the theories of cosmology and stellar evolution, the researchers concluded that bright carbon-rich stars close to their death, quite similar to the progenitors of the white dwarfs analyzed in this study, are presently contributing to a vast amount of the light emitted by very distant galaxies. This light, carrying the signature of newly produced carbon, is routinely collected by large telescopes to probe the evolution of cosmic structures. A reliable interpretation of this light depends on understanding the synthesis of carbon in stars.
DISAPPEARANCE OF MASSIVE STAR
ESO
Astronomers have discovered the absence of an unstable massive star in a dwarf galaxy. Scientists think this could indicate that the star became less bright and partially obscured by dust. An alternative explanation is that the star collapsed into a black hole without producing a supernova. If true, this would be the first direct detection of such a monster star ending its life in this manner. Between 2001 and 2011, various teams of astronomers studied the mysterious massive star, located in the Kinman Dwarf galaxy, and their observations indicated it was in a late stage of its evolution. Astronomers wanted to find out more about how very massive stars end their lives, and the object in the Kinman Dwarf seemed like the perfect target. But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the telltale signatures of the star. Instead, they were surprised to find out that the star had disappeared. Located some 75 million light-years away in the constellation of Aquarius, the Kinman Dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them. From 2001 to 2011, the light from the galaxy consistently showed evidence that it hosted a ‘luminous blue variable’ star some 2.5 million times brighter than the Sun. Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness. Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star. The group first turned the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-metre telescopes simultaneously. But they were unable to find the signs that previously pointed to the presence of the luminous star. A few months later, the group tried the X-shooter instrument, also on ESO’s VLT, and again found no traces of the star.
The team then turned to older data collected using X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere. The old data indicated that the star in the Kinman Dwarf could have been undergoing a strong outburst period that likely ended sometime after 2011. Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically. Based on their observations and models, the astronomers have suggested two explanations for the star’s disappearance and lack of a supernova, related to this possible outburst. The outburst may have resulted in the luminous blue variable being transformed into a less luminous star, which could also be partly hidden by dust. Alternatively, the team says the star may have collapsed into a black hole, without producing a supernova explosion. This would be a rare event: our current understanding of how massive stars die points to most of them ending their lives in a supernova.
SUPER-EARTHS ORBITING NEARBY RED DWARF
University of Göttingen
The nearest exoplanets to us provide the best opportunities for detailed study, including searching for evidence of life outside the Solar System. The RedDots team of astronomers has detected a system of super-Earth planets orbiting the nearby star Gliese 887, the brightest red dwarf star in the sky. Super-Earths are planets which have a mass higher than the Earth's but substantially below those of our local ice giants, Uranus and Neptune. The newly discovered super-Earths lie close to the red dwarf's habitable zone, where water can exist in liquid form, and could be rocky worlds. The RedDots team of astronomers monitored the red dwarf, using the HARPS spectrograph at the European Southern Observatory in Chile. They used a technique known as radial velocity which enables them to measure the tiny back and forth wobbles of the star caused by the gravitational pull of the planets. The regular signals correspond to orbits of just 9.3 and 21.8 days, indicating two super-Earths -- Gliese 887b and Gliese 887c -- both larger than the Earth yet moving rapidly, much faster even than Mercury. Scientists estimate the temperature of Gliese 887c to be around 70oC.
Gliese 887 is one of the closest stars to the Sun at around 11 light years away. It is much dimmer and about half the size of our Sun, which means that the habitable zone is closer to Gliese 887 than Earth's distance from the Sun. RedDots discovered two more interesting facts about Gliese 887, which turn out to be good news not only for the newly discovered planets but also for astronomers. The first is that the red dwarf has very few starspots, unlike our Sun. If Gliese 887 was as active as our Sun, it is likely that a strong stellar wind -- outflowing material which can erode a planet's atmosphere -- would simply sweep away the planets' atmospheres. This means that the newly discovered planets may retain their
atmospheres, or have thicker atmospheres than the Earth, and potentially host life, even though GJ887 receives more light than the Earth. The other interesting feature the team discovered is that the brightness of Gliese 887 is almost constant. Therefore, it will be relatively easy to detect the atmospheres of the super-Earth system, making it a prime target for the James Webb Space Telescope, a successor to the Hubble Telescope.
HIGH ENERGY GAMMA RADIATION FROM ETA CARINAE
Deutsches Elektronen-Synchrotron DESY
Scientists have identified the binary star Eta Carinae as a new kind of source for very high-energy (VHE) cosmic gamma-radiation. Eta Carinae is located 7500 light years away in the constellation Carina in the Southern Sky and, based on the data collected, emits gamma rays with energies up to 400 gigaelectronvolts (GeV), some 100 billion times more than the energy of visible light. With a specialised telescope in Namibia a researchers have proven a certain type of binary star as a new kind of source for very high-energy cosmic gamma-radiation. Eta Carinae is a binary system of superlatives, consisting of two blue giants, one about 100 times, the other about 30 times the mass of our sun. The two stars orbit each other every 5.5 years in very eccentric elliptical orbits, their separation varying approximately between the distance from our Sun to Mars and from the Sun to Uranus. Both these gigantic stars fling dense, supersonic stellar winds of charged particles out into space. In the process, the larger of the two loses a mass equivalent to our entire Sun in just 5000 years or so. The smaller one produces a fast stellar wind travelling at speeds around eleven million kilometres per hour (about one percent of the speed of light). A huge shock front is formed in the region where these two stellar winds collide, heating up the material in the wind to extremely high temperatures. At around 50 million degrees Celsius, this matter radiates brightly in the X-ray range. The particles in the stellar wind are not hot enough to emit gamma radiation, though. When particles are accelerated this rapidly, they can also emit gamma radiation. In fact, the satellites "Fermi," operated by the US space agency NASA, and AGILE, belonging to the Italian space agency ASI, already detected energetic gamma rays of up to about 10 GeV coming from Eta Carinae in 2009.
Different models have been proposed to explain how this gamma radiation is produced. It could be generated by accelerated electrons or by high-energy atomic nuclei. Determining which of these two scenarios is correct is crucial: very energetic atomic nuclei account for the bulk of the so-called Cosmic Rays, a subatomic cosmic hailstorm striking Earth constantly from all directions. Despite intense research for more than 100 years, the sources of the Cosmic Rays are still not exhaustively known. Since the electrically charged atomic nuclei are deflected by cosmic magnetic fields as they travel through the Universe, the direction from which they arrive at Earth no longer points back to their origin. Cosmic gamma rays, on the other hand, are not deflected. So, if the gamma rays emitted by a specific source can be shown to originate from high-energy atomic nuclei, one of the long-sought accelerators of cosmic particle radiation will have been identified. In the case of Eta Carinae, electrons have a particularly hard time getting accelerated to high energies, because they are constantly being deflected by magnetic fields during their acceleration, which makes them lose energy again. Very high-energy gamma radiation begins above the 100 GeV range, which is rather difficult to explain in Eta Carinae to stem from electron acceleration. The satellite data already indicated that Eta Carinae also emits gamma radiation beyond 100 GeV, and H.E.S.S. has now succeeded in detecting such radiation up to energies of 400 GeV around the time of the close encounter of the two blue giants in 2014 and 2015. This makes the binary star the first known example of a source in which very high-energy gamma radiation is generated by colliding stellar winds. The analysis of the gamma radiation measurements taken by H.E.S.S. and the satellites shows that the radiation can best be interpreted as the product of rapidly accelerated atomic nuclei. This would make the shock regions of colliding stellar winds a new type of natural particle accelerator for cosmic rays. With H.E.S.S., which is named after the discoverer of Cosmic Rays, Victor Franz Hess, and the upcoming Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory currently being built in the Chilean highlands, the scientists hope to investigate this phenomenon in greater detail and discover more sources of this kind.
YOUNG GIANT PLANET GIVES CLUES TO FORMATION OF EXOTIC WORLDS
NASA
For most of human history our understanding of how planets form and evolve was based on the eight planets in our solar system. But over the last 25 years, the discovery of more than 4,000 exoplanets, or planets outside our solar system, changed all that. Among the most intriguing of these distant worlds is a class of exoplanets called hot Jupiters. Similar in size to Jupiter, these gas-dominated planets orbit extremely close to their parent stars, circling them in as few as 18 hours. We have nothing like this in our own solar system, where the closest planets to the Sun are rocky and orbiting much farther away. The questions about hot Jupiters are as big as the planets themselves: Do they form close to their stars or farther away before migrating inward? And if these giants do migrate, what would that reveal about the history of the planets in our own solar system? To answer those questions, scientists will need to observe many of these hot giants very early in their formation. Now, a new study reports on the detection of the exoplanet HIP 67522 b, which appears to be the youngest hot Jupiter ever found. It orbits a well-studied star that is about 17 million years old, meaning the hot Jupiter is likely only a few million years younger, whereas most known hot Jupiters are more than a billion years old. The planet takes about seven days to orbit its star, which has a mass similar to the Sun's. Located only about 490 light-years from Earth, HIP 67522 b is about 10 times the diameter of Earth, or close to that of Jupiter. Its size strongly indicates that it is a gas-dominated planet. HIP 67522 b was identified as a planet candidate by the Transiting Exoplanet Survey Satellite (TESS), which detects planets via the transit method: Scientists look for small dips in the brightness of a star, indicating that an orbiting planet has passed between the observer and the star. But young stars tend to have a lot of dark splotches on their surfaces - starspots, also called sunspots when they appear on the Sun - that can look similar to transiting planets. So scientists used data from the recently retired infrared observatory, the Spitzer Space Telescope, to confirm that the transit signal was from a planet and not a starspot. (Other methods of exoplanet detection have yielded hints at the presence of even younger hot Jupiters, but none have been confirmed.)
The discovery offers hope for finding more young hot Jupiters and learning more about how planets form throughout the Universe - even right here at home. There are three main hypotheses for how hot Jupiters get so close to their parent stars. One is that they simply form there and stay put. But it's hard to imagine planets forming in such an intense environment. Not only would the scorching heat vaporize most materials, but young stars frequently erupt with massive explosions and stellar winds, potentially dispersing any newly emerging planets. It seems more likely that gas giants develop farther from their parent star, past a boundary called the snow line, where it's cool enough for ice and other solid materials to form. Jupiter-like planets are composed almost entirely of gas, but they contain solid cores. It would be easier for those cores to form past the snow line, where frozen materials could cling together like a growing snowball. The other two hypotheses assume this is the case, and that hot Jupiters then wander toward closer to their stars. But what would be the cause and timing of the migration? One idea posits that hot Jupiters begin their journey early in the planetary system's history while the star is still surrounded by the disk of gas and dust from which both it and the planet formed. In this scenario, the gravity of the disk interacting with the mass of the planet could interrupt the gas giant's orbit and cause it to migrate inward. The third hypothesis maintains that hot Jupiters get close to their star later, when the gravity of other planets around the star can drive the migration. The fact that HIP 67522 b is already so close to its star so early after its formation indicates that this third hypothesis probably doesn't apply in this case. But one young hot Jupiter isn't enough to settle the debate on how they all form.
MYSTERY ASTRONOMICAL OBJECT IN 'MASS GAP'
California Institute of Technology
When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in supernovas and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our Sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: does anything lie in this so-called mass gap? Now, in a new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Europe, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole about 25 times the mass of the Sun (some of the merged mass was converted to a blast of energy in the form of gravitational waves). The newly formed black hole lies about 800 million light-years away from Earth. Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.
It's a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object. The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our Sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected. When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but none picked up any signals. So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. That event, discovered by the LIGO-Virgo network in August of 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Conversely, black hole mergers, in most circumstances, are thought not to produce light. According to the LIGO and Virgo scientists, the August 2019 event was not seen by light-based telescopes for a few possible reasons. First, this event was six times farther away than the merger observed in 2017, making it harder to pick up any light signals. Secondly, if the collision involved two black holes, it likely would have not shone with any light. Thirdly, if the object was in fact a neutron star, its 9-fold more massive black-hole partner might have swallowed it whole; a neutron star consumed whole by a black hole would not give off any light. How will researchers ever know if the mystery object was a neutron star or a black hole? Future observations with LIGO, Virgo, and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.
MONSTER BLACK HOLE FOUND IN EARLY UNIVERSE
W. M. Keck Observatory
Astronomers have discovered the second-most distant quasar ever found using three Maunakea Observatories in Hawai'i: W. M. Keck Observatory, the international Gemini Observatory, a Program of NSF's NOIRLab, and the University of Hawai'i-owned United Kingdom Infrared Telescope (UKIRT). It is the first quasar to receive an indigenous Hawaiian name, Poniua'ena, which means "unseen spinning source of creation, surrounded with brilliance" in the Hawaiian language. Poniua'ena is only the second quasar yet detected at a distance calculated at a cosmological redshift greater than 7.5 and it hosts a black hole twice as large as the other quasar known in the same era. The existence of these massive black holes at such early times challenges current theories of how supermassive black holes formed and grew in the young Universe. Quasars are the most energetic objects in the Universe powered by their supermassive black holes and since their discovery, astronomers have been keen to determine when they first appeared in our cosmic history. By systematically searching for these rare objects in wide-area sky surveys, astronomers discovered the most distant quasar (named J1342+0928) in 2018 and now the second-most distant, Poniua'ena (or J1007+2115, at redshift 7.515). The light seen from Poniua'ena travelled through space for over 13 billion years since leaving the quasar just 700 million years after the Big Bang. Spectroscopic observations from Keck Observatory and Gemini Observatory show the supermassive black hole powering Poniua'ena is 1.5 billion times more massive than our Sun. Poniua'ena is the most distant object known in the Universe hosting a black hole exceeding one billion solar masses. For a black hole of this size to form this early in the Universe, it would need to start as a 10,000 solar mass "seed" black hole about 100 million years after the Big Bang, rather than growing from a much smaller black hole formed by the collapse of a single star. Current theory holds the birth of stars and galaxies as we know them started during the Epoch of Reionization, beginning about 400 million years after the Big Bang. The growth of the first giant black holes is thought to have occurred during that same era in the Universe's history. The discovery of quasars like Poniua'ena, deep into the
reionization epoch, is a big step towards understanding this process of reionization and the formation of early supermassive black holes and massive galaxies. Poniua'ena has placed new and important constraints on the evolution of the matter between galaxies (intergalactic medium) in the reionization epoch.
Topic: Mid July Astronomy Bulletin (Read 864 times)