Topic: Mid January Astronomy Bulletin

[Clive]

WEBB TELESCOPE MIRROR UNFOLDS
NASA

NASA's James Webb Space Telescope team fully deployed its 21-foot, gold-coated primary mirror, successfully completing the final stage of all major spacecraft deployments to prepare for science operations. A joint effort with the European Space Agency (ESA) and Canadian Space Agency, the Webb mission will explore every phase of cosmic history -- from within our solar system to the most distant observable galaxies in the early Universe. The two wings of Webb's primary mirror had been folded to fit inside the nose cone of an Arianespace Ariane 5 rocket prior to launch. After more than a week of other critical spacecraft deployments, the Webb team began remotely unfolding the hexagonal segments of the primary mirror, the largest ever launched into space. This was a multi-day process, with the first side deployed Jan. 7 and the second Jan. 8. Mission Operations Center ground control at the Space Telescope Science Institute in Baltimore began deploying the second side panel of the mirror at 8:53 a.m. EST. Once it extended and latched into position at 1:17 p.m. EST, the team declared all major deployments successfully completed. The world's largest and most complex space science telescope will now begin moving its 18 primary mirror segments to align the telescope optics. The ground team will command 126 actuators on the backsides of the segments to flex each mirror -- an alignment that will take months to complete. Then the team will calibrate the science instruments prior to delivering Webb's first images this summer. Soon, Webb will also undergo a third mid-course correction burn -- one of three planned to place the telescope precisely in orbit around the second Lagrange point, commonly known as L2, nearly 1 million miles from Earth. This is Webb's final orbital position, where its sunshield will protect it from light from the Sun, Earth, and Moon that could interfere with observations of infrared light. Webb is designed to peer back over 13.5 billion years to capture infrared light from celestial objects, with much higher resolution than ever before, and to study our own solar system as well as distant worlds.  JWST is carrying enough fuel to operate for up to 20 years. 


THE SUN HAD RINGS BEFORE PLANETS
Rice University

Before the solar system had planets, the Sun had rings -- bands of dust and gas similar to Saturn's rings -- that likely played a role in Earth's formation, according to a new study. In the solar system, something happened to prevent the Earth from growing to become a much larger type of terrestrial planet called a super-Earth. The study by astronomers, astrophysicists and planetary scientists draws on the latest astronomical research on infant star systems. Their model assumes three bands of high pressure arose within the young Sun's disk of gas and dust. Such "pressure bumps" have been observed in ringed stellar disks around distant stars, and the study explains how pressure bumps and rings could account for the solar system's architecture. For decades, scientists believed gas and dust in protoplanetary disks gradually became less dense, dropping smoothly as a function of distance from the star. But computer simulations show planets are unlikely to form in smooth-disk scenarios. When particles move faster than the gas around them, they "feel a headwind and drift very quickly toward the star. At pressure bumps, gas pressure increases, gas molecules move faster and solid particles stop feeling the headwind. That's what allows dust particles to accumulate at pressure bumps. Astronomers have observed pressure bumps and protoplanetary disk rings with the Atacama Large Millimeter/submillimeter Array, or ALMA, an enormous 66-dish radio telescope that came online in Chile in 2013. In the simulations, pressure bumps at the sublimation lines of silicate, water and carbon monoxide produced three distinct rings. At the silicate line, the basic ingredient of sand and glass, silicon dioxide, became vapor. This produced the Sun's nearest ring, where Mercury, Venus, Earth and Mars would later form. The middle ring appeared at the snow line and the farthest ring at the carbon monoxide line. Protoplanetary disks cool with age, so sublimation lines would have migrated toward the Sun. The study showed this process could allow dust to accumulate into asteroid-sized objects called planetesimals, which could then come together to form planets. Previous studies assumed planetesimals could form if dust were sufficiently concentrated, but no model offered a convincing theoretical explanation of how dust might accumulate. Many previous solar system simulations produced versions of Mars as much as 10 times more massive than Earth. The model correctly predicts Mars having about 10% of Earth's mass because "Mars was born in a low-mass region of the disk.


1,000-LIGHT-YEAR BUBBLE SURROUNDS EARTH
Harvard-Smithsonian Center for Astrophysics

The Earth sits in a 1,000-light-year-wide void surrounded by thousands of young stars -- but how did those stars form? Astronomers at the Center for Astrophysics and the Space Telescope Science Institute (STScI) have reconstructed the evolutionary history of our galactic neighbourhood, showing how a chain of events beginning 14 million years ago led to the creation of a vast bubble that's responsible for the formation of all nearby, young stars. The paper's central figure, a 3D spacetime animation, reveals that all young stars and star-forming regions -- within 500 light years of Earth -- sit on the surface of a giant bubble known as the Local Bubble. While astronomers have known of its existence for decades, scientists can now see and understand the Local Bubble's beginnings and its impact on the gas around it. Using a trove of new data and data science techniques, the spacetime animation shows how a series of supernovae that first went off 14 million years ago, pushed interstellar gas outwards, creating a bubble-like structure with a surface that's ripe for star formation. Today, seven well-known star-forming regions or molecular clouds -- dense regions in space where stars can form -- sit on the surface of the bubble. The team has calculated that about 15 supernovae have gone off over millions of years to form the Local Bubble that we see today. The oddly-shaped bubble is not dormant and continues to slowly grow, the astronomers note. The expansion speed of the bubble, as well as the past and present trajectories of the young stars forming on its surface, were derived using data obtained by Gaia, a space-based observatory launched by the European Space Agency.


ECCENTRIC EXOPLANET DISCOVERED
University of Bern

Astronomers have discovered a sub-Neptune exoplanet orbiting a red dwarf star. Red dwarfs" are small stars and thus much cooler than our Sun. Around stars like these, liquid water is possible on planets much closer to the star than in our solar system. The distance between an exoplanet and its star is a crucial factor in its detection: the closer a planet is to its host star, the higher the probability that it can be detected. Exoplanets that are very far from our solar system cannot be observed directly with a telescope -- they are too small and reflect too little light. However, one way to detect such planets is the transit method. This involves using telescopes to look for dips in the star's brightness that occur when planets pass in front of the star. Repeated observations of the dips in the star's brightness give precise measurements of the planet's orbital period around the star, and the depth of the transit allows researchers to determine the planet's diameter. When combined with planet mass estimates from other methods, such as using radial velocity measurements, the planet density can be calculated. Planet TOI-2257 b was initially identified by data from NASA's Transiting Exoplanet Survey Satellite TESS space telescope. The small star was observed for a total of four months, but the gaps between observations meant that it was not clear whether the decrease in brightness could be explained by the transit of a planet with an orbit of 176, 88, 59, 44 or 35 days. Observation of the star with the Las Cumbres Observatory Global Telescope subsequently ruled out the possibility that a planet with a 59-day orbital period was causing the drop in brightness. Next, astronomers wanted to find out if the 35-day orbital period could be possible. With its 35-day orbital period, TOI-2257 b orbits the host star at a distance where liquid water is possible on the planet, and therefore conditions favourable for the emergence of life could exist. Planets in this so-called "habitable zone" near a small red dwarf star are easier to study because they have shorter orbital periods and can therefore be observed more often. The radius of TOI-2257 b (2.2 times larger than Earth's) suggests that the planet is rather gaseous, with high atmospheric pressure not conducive to life. The team found that TOI-2257 b does not have a circular, concentric orbit. In fact, it is the most eccentric planet orbiting a cool star ever discovered. "In terms of potential habitability. While the planet's average temperature is comfortable, it varies from -80°C to about 100°C depending on where in its orbit the planet is, far from or close to the star." A possible explanation for this surprising orbit is that further out in the system a giant planet is lurking and disturbing the orbit of TOI 2257 b. Further observations measuring the radial velocity of the star will help confirm the eccentricity and search for possible additional planets that could not be observed in transit.


SOURCE OF POWERFUL COSMIC GAMMA-RAYS
Association of Universities for Research in Astronomy (AURA)

Using the 4.1-meter SOAR Telescope in Chile, astronomers have discovered the first example of a binary system where a star in the process of becoming a white dwarf is orbiting a neutron star that has just finished turning into a rapidly spinning pulsar. The pair, originally detected by the Fermi Gamma-ray Space Telescope, is a "missing link" in the evolution of such binary systems. A bright, mysterious source of gamma rays has been found to be a rapidly spinning neutron star -- dubbed a millisecond pulsar -- that is orbiting a star in the process of evolving into an extremely-low-mass white dwarf. These types of binary systems are referred to by astronomers as "spiders" because the pulsar tends to "eat" the outer parts of the companion star as it turns into a white dwarf. NASA's Fermi Gamma-ray Space Telescope has been cataloguing objects in the Universe that produce copious gamma rays since its launch in 2008, but not all of the sources of gamma rays that it detects have been classified. One such source, called 4FGL J1120.0-2204 by astronomers, was the second brightest gamma-ray source in the entire sky that had gone unidentified, until now. Astronomers used the Goodman Spectrograph on the SOAR Telescope to determine the true identity of 4FGL J1120.0-2204. The gamma-ray source, which also emits X-rays, as observed by NASA's Swift and ESA's XMM-Newton space telescopes, has been shown to be a binary system consisting of a "millisecond pulsar" that spins hundreds of times per second, and the precursor to an extremely-low-mass white dwarf. The pair are located over 2600 light-years away. The optical spectrum of the binary system measured by the Goodman spectrograph showed that light from the proto-white dwarf companion is Doppler shifted -- alternately shifted to the red and the blue -- indicating that it orbits a compact, massive neutron star every 15 hours. The spectra also allowed astronomers to constrain the approximate temperature and surface gravity of the companion star. This allowed them to determine that the companion is the precursor to an extremely-low-mass white dwarf, with a surface temperature of 8200 °C (15,000 °F), and a mass of just 17% that of the Sun.

When a star with a mass similar to that of the Sun or less reaches the end of its life, it will run out of the hydrogen used to fuel the nuclear fusion processes in its core. For a time, helium takes over and powers the star, causing it to contract and heat up, and prompting its expansion and evolution into a red giant that is hundreds of millions of kilometres in size. Eventually, the outer layers of this swollen star can be accreted onto a binary companion and nuclear fusion halts, leaving behind a white dwarf about the size of Earth and sizzling at temperatures exceeding 100,000 °C. The proto-white dwarf in the 4FGL J1120.0-2204 system hasn't finished evolving yet. Currently it's bloated, and is about five times larger in radius than normal white dwarfs with similar masses. It will continue cooling and contracting and, in about two billion years, it will look identical to many of the extremely low mass white dwarfs that we already know about. Millisecond pulsars twirl hundreds of times every second. They are spun up by accreting matter from a companion, in this case from the star that became the white dwarf. Most millisecond pulsars emit gamma rays and X-rays, often when the pulsar wind, which is a stream of charged particles emanating from the rotating neutron star, collides with material emitted from a companion star. About 80 extremely low-mass white dwarfs are known, but this is the first precursor to an extremely low-mass white dwarf found that is likely orbiting a neutron star. . Consequently, 4FGL J1120.0-2204 is a unique look at the tail-end of this spin-up process. All the other white dwarf-pulsar binaries that have been discovered are well past the spinning-up stage.


MILKY WAY BLACK HOLE UNPREDICTABLE AND CHAOTIC
RAS

An international team of researchers has found that the black hole at the centre of our galaxy, Sagittarius A*, not only flares irregularly from day to day but also in the long term. The team analysed 15 years’ worth of data to come to this conclusion. Sagittarius A* is a strong source of radio, X-rays and gamma rays (visible light is blocked by intervening gas and dust). Astronomers have known for decades that Sagittarius A* flashes every day, emitting bursts of radiation that are ten to a hundred times brighter than normal signals observed from the black hole. To find out more about these mysterious flares, the team of astronomers searched for patterns in 15 years of data made available by NASA's Neil Gehrels Swift Observatory, an Earth-orbiting satellite dedicated to the detection of gamma-ray bursts. The Swift Observatory has been observing gamma rays from black hole since 2006. Analysis of the data showed high levels of activity from 2006 to 2008, with a sharp decline in activity for the next four years.  After 2012, the frequency of flares increased again - the researchers had a difficult time distinguishing a pattern. In the next few years, the team of astronomers expect to gather enough data to be able to rule out whether the variations in the flares from Sagittarius A* are due to passing gaseous clouds or stars, or whether something else can explain the irregular activity observed from our galaxy’s central black hole.


NEW TREASURE TROVE OF GLOBULAR CLUSTERS
University of Arizona

A survey completed using a combination of ground and space-based telescopes yielded a treasure trove of previously unknown globular clusters -- old, dense groups of thousands of stars that all formed at the same time -- in the outer regions of the elliptical galaxy Centaurus A. The work presents a significant advance in understanding the architecture and cosmological history of this galaxy and offers new insights into galaxy formation in general and the distribution of dark matter in the Universe. Centaurus A, also known as NGC 5128, is a visually stunning, elliptical galaxy featuring a relativistic jet spewing from a supermassive black hole at its centre and spectacular streams of scattered stars left behind by past collisions and mergers with smaller galaxies orbiting Centaurus A. Located in the constellation Centaurus, 13 million light-years from Earth, Centaurus A is too far away to allow astronomers to see individual stars, but star clusters can be identified as such and used as "fossil evidence" of the galaxy's tumultuous evolution. The new catalogue contains approximately 40,000 globular cluster candidates in Centaurus A, recommending follow-up observations focused on a set of 1,900 that are most likely to be true globular clusters. The researchers surveyed globular cluster candidates out to a projected radius of approximately 150 kiloparsecs, nearly half a million light-years from the galaxy's centre. The data combines observations from the following sources: the Panoramic Imaging Survey of Centaurus and Sculptor, or PISCeS; Gaia, a space observatory of the European Space Agency, and the NOAO Source Catalog, which combines publicly accessible images from telescopes in both hemispheres covering nearly the entire sky.

Centaurus A has been a leading target for extragalactic globular cluster studies due to its richness and proximity to Earth, but the majority of studies have focused on the inner 40 kiloparsecs (about 130,500 light-years) of the galaxy, leaving the outer reaches of the galaxy largely unexplored. Ranking the candidates based on the likelihood that they are true globular clusters, the team found that approximately 1,900 are highly likely to be confirmed as such and should be the highest priority for follow-up spectroscopic confirmation. Centaurus A's structure tells astronomers that it went through several major mergers with other galaxies, leading to its glob-like appearance with river-like regions that have many more stars than the surrounding areas, Hughes said. Providing the closest example of an elliptical galaxy, Centaurus A offers astronomers an opportunity to study up close a galaxy that is very unlike our own. The Milky Way, as well as its closest neighbour, the Andromeda Galaxy, are both spiral galaxies. With their familiar, pinwheel-like appearance, spiral galaxies may seem like the "typical" galaxy, but it turns out that their less orderly elliptical cousins outnumber them in the cosmos. Star clusters form from dense patches of gas in the interstellar medium. Almost every galaxy has globular clusters, including the Milky Way, which boasts around 150 of them, but most stars are not arranged in such clumps. By studying globular clusters, astronomers can gather clues about the galaxy hosting them, such as its mass, its history of interactions with nearby galaxies and even the distribution of dark matter within. Globular clusters are interesting because they can be used as tracers of structures and processes in other galaxies where we can't resolve individual stars. They hold on to chemical signatures, such as the elemental composition of their individual stars, so they tell us something about the environment in which they formed. The researchers specifically looked for globular clusters far from the centre of the galaxy because Centaurus A's substructure hints at a large, undiscovered population of such clusters. Previous observations had found just under 600 clusters in the more central regions, but the outer regions of the galaxy had remained largely uncharted. The researchers looked farther out and discovered more than 100 new clusters already, and most likely there are more. They can then use that data to reconstruct the architecture and movements in that galaxy, and also figure out its mass. From that they can eventually subtract all its stars and see what's left -- that invisible mass must be its dark matter.


RED SUPERGIANT’S DEATH THROES CAPTURED
Northwestern University

For the first time ever, astronomers have imaged in real time the dramatic end to a red supergiant's life -- watching the massive star's rapid self-destruction and final death throes before collapsing into a type II supernova. The team observed the red supergiant during its last 130 days leading up to its deadly detonation. The discovery defies previous ideas of how red supergiant stars evolve right before exploding. Earlier observations showed that red supergiants were relatively quiescent before their deaths -- with no evidence of violent eruptions or luminous emissions. The new observations, however, detected bright radiation from a red supergiant in the final year before exploding. This suggests at least some of these stars must undergo significant changes in their internal structure, which then result in the tumultuous ejection of gas moments before they collapse. The team quickly captured the powerful flash and obtained the very first spectrum of the energetic explosion, named supernova 2020tlf (SN 2020tlf) using the W.M. Keck Observatory's Low Resolution Imaging Spectrometer on Maunakea, Hawai. The data showed direct evidence of dense circumstellar material surrounding the star at the time of explosion, likely the same gas that Pan-STARRS had imaged the red supergiant star violently ejecting earlier in the summer. The team continued to monitor SN 2020tlf after the explosion. Based on data obtained from Keck Observatory's Deep Imaging and Multi-Object Spectrograph and Near Infrared Echellette Spectrograph, the researchers determined SN 2020tlf's progenitor red supergiant star -- located in the NGC 5731 galaxy about 120 million light-years away from Earth -- was 10 times more massive than the Sun.


EVIDENCE OF A GRAVITATIONAL WAVE BACKGROUND
University of Birmingham

The results of a comprehensive search for a background of ultra-low frequency gravitational waves have been announced by an international team of astronomers. These light-year-scale ripples, a consequence of Einstein's theory of general relativity, permeate all of spacetime and could originate from mergers of the most massive black holes in the Universe or from events occurring soon after the formation of the Universe in the Big Bang. Scientists have been searching for definitive evidence of these signals for several decades. The International Pulsar Timing Array (IPTA), joining the work of several astrophysics collaborations from around the world, recently completed its search for gravitational waves in their most recent official data release, known as Data Release 2 (DR2). This data set consists of precision timing data from 65 millisecond pulsars -- stellar remnants which spin hundreds of times per second, sweeping narrow beams of radio waves that appear as pulses due to the spinning -- obtained by combining the independent data sets from the IPTA's three founding members: The European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA). These combined data reveal strong evidence for an ultra-low frequency signal detected by many of the pulsars in the combined data. The characteristics of this common-among-pulsars signal are in broad agreement with those expected from a gravitational wave "background." The gravitational wave background is formed by many different overlapping gravitational-wave signals emitted from the cosmic population of supermassive binary black holes (i.e. two supermassive black holes orbiting each other and eventually merging) -- similar to background noise from the many overlapping voices in a crowded hall. This result further strengthens the gradual emergence of similar signals that have been found in the individual data sets of the participating pulsar timing collaborations over the past few years. The detection of gravitational waves from a population of massive black hole binaries or from another cosmic source will give us unprecedented insights into how galaxy form and grow, or cosmological processes taking place in the infant Universe. A major international effort of the scale of IPTA is needed to reach this goal, and the next few years could bring us a golden age for these explorations of the Universe.
To identify the gravitational-wave background as the origin of this ultra-low frequency signal, the IPTA must also detect spatial correlations between pulsars. This means that each pair of pulsars must respond in a very particular way to gravitational waves, depending on their separation on the sky. These signature correlations between pulsar pairs are the "smoking gun" for a gravitational-wave background detection. Without them, it is difficult to prove that some other process is not responsible for the signal. Intriguingly, the first indication of a gravitational wave background would be a common signal like that seen in the IPTA DR2. Whether or not this spectrally similar ultra-low frequency signal is correlated between pulsars in accordance with the theoretical predictions will be resolved with further data collection, expanded arrays of monitored pulsars, and continued searches of the resulting longer and larger data sets. Consistent signals like the one recovered with the IPTA analysis have also been published in individual data sets more recent than those used in the IPTA DR2, from each of the three founding collaborations. The IPTA DR2 analysis demonstrates the power of the international combination giving strong evidence for a gravitational wave background compared to the marginal or absent evidences from the constituent data sets. Additionally, new data from the MeerKAT telescope and from the Indian Pulsar Timing Array (InPTA), the newest member of the IPTA, will further expand future data sets. The first hint of a gravitational wave background would be a signal like that seen in the IPTA DR2. Then, with more data, the signal will become more significant and will show spatial correlations, at which point we will know it is a gravitational wave background. We are very much looking forward to contributing several years of new data to the IPTA for the first time, to help achieve a gravitational wave background detection. Given the latest published results from the individual groups who now all can clearly recover the common signal, the IPTA is optimistic for what can be achieved once these are combined into the IPTA Data Release 3. Work is already ongoing on this new data release, which at a minimum will include updated data sets from the four constituent PTAs of the IPTA. The analysis of the DR3 data set is expected to finish within the next few years. If the signal the team is currently seeing is the first hint of a gravitational wave background, then based on our simulations, it is possible we will have more definite measurements of the spatial correlations necessary to conclusively identify the origin of the common signal in the near future."