Astronomy and space news summarized by Don Lynn from NASA and other sources

Milky Way Disk – Astronomers have known for more than 20 years that the disk of our Milky Way galaxy consists of two distinct populations of stars, differing in age and composition with the “Thin Disk” embedded within the “Thick Disk.” A new study of data from the star cataloging spacecraft Gaia and the ground-based Sloan Digital Sky Survey has found a third population, which the research team led by Daniela Carollo, of the Italian National Institute for Astrophysics, had dubbed the metal-weak thick disk (MWTD). The stars of the MWTD have about half the heavy element content, slower average orbital speeds and orbits that reach farther above and below the disk plane, as compared to thick disk stars. Unexpectedly, the abundance of some heavier elements, such as magnesium, is higher in the MWTD. The differing elemental content indicates that the MWTD formed at a different time or possibly different place than the thick disk as the abundance of heavier elements in a galaxy generally rises over billions of years as those elements are manufactured in stars and supernovas.

Thick Disk Age – In recent years, astronomers developed a technique using measurements of the oscillations in the surfaces of stars – asteroseismology – which can help calculate a star’s age more precisely than other methods. The Kepler planet-finding spacecraft produced a huge archive of star brightness oscillations, becoming a treasure trove of data for asteroseismologists. Previous calculations of star ages based on Kepler data from the Milky Way’s thick disk resulted in age distributions that did not agree with computer simulations of star ages. When this was initially released, there was skepticism whether the Kepler-derived ages for the thick disk should be believed. A new study by researchers at Australia’s ARC Centre of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO-3D), found that the chemical composition of thick disk stars used in the computer simulations did not agree with the latest spectroscopic observations of thick disk stars. They corrected the computer simulations and found agreement with the Kepler ages that the Milky Way thick disk is about 10 billion years old.

Dark Matter Missing – The discovery in the last two years of two dwarf galaxies that appeared to have no dark matter halo has been met with considerable skepticism. The total mass, including dark matter, was calculated from the speeds of orbiting globular clusters. The mass of stars was estimated from their brightness. In both cases, the total mass was about the same as star mass, whereas essentially all previously weighed galaxies had total mass far in excess of the star mass, indicating the presence of dark matter. To address the skepticism, a consortium of Chinese astronomers analyzed 324 more dwarf galaxies and found 19 more with little or no dark matter. They were found in data from the Arecibo radiotelescope that measured orbital motions of gas clouds around galaxies. It’s been hypothesized that dark-matterless galaxies could result if a neighboring galaxy gravitationally stole the dark matter, but many of the 19 have no neighbors. More work is needed to explain these.

Most Massive Black Hole – Spectral observations of the central galaxy in the galaxy cluster Abell 85 show that its black hole has a mass of 40 billion Sun masses, the largest confirmed mass for a black hole. At a distance of 700 million light-years, it is also the farthest away that the mass of a black hole has been measured directly from speeds of matter orbiting it. Surprisingly, the center of the galaxy is unusually faint which astronomers theorize might be because many of the stars near the center of the galaxy have been thrown out by interactions during galaxy collisions. Dim centers have been seen in some other massive galaxies for this reason. The astronomers at the Max Planck Institute for Extraterrestrial Physics (MPE) and the University Observatory Munich (USM) who discovered the black hole think it may be possible to use the dimness of galaxy centers to estimate the mass of a central black hole in cases where direct mass measurements have not been or cannot be made.

Tiger Stripes Explained – In 2005, the Cassini spacecraft took photos of Saturn’s icy moon Enceladus that showed four, 80-mile-long parallel cracks in the surface near the moon’s south pole, known as the “tiger stripes.” Scientists have been trying to explain how they formed, why they are parallel, why they formed at that location and why there is nothing like them elsewhere. A team of scientists from the Carnegie Institution for Science, the University of California, Davis and UC Berkeley conducted simulations of the structure of Enceladus and appear to have found answers. The first crack formed from internal heating stresses due to tidal forces. The tidal forces are a result of Enceladus having a slightly elliptical orbit, varying its distance from Saturn as it orbits, which varies the gravitational force from the planet. This internal heating melted a significant amount of internal ice, resulting in a subsurface ocean. After the crack formed, subsurface water erupted from the crack. The simulation showed that the layer of ice covering the ocean was thinnest at both of the poles, so the stresses would crack the layer first at one of the poles. It was likely chance that it was the south pole. The eruptions rained ice and snow onto the surface all along the first crack. Eventually the weight of this rained material flexed the surface until a parallel crack formed. This continued until there were four cracks. The whole process depends on a balance of moon size, distance from its planet, how non-circular its orbit is as well as the presence of a lot of water ice. This combination didn’t happen anywhere else in our Solar System, so the stripes are unique.

Bennu Particle Ejection – Soon after the OSIRIS-Rex spacecraft settled into orbit around its target asteroid Bennu, it captured images showing particles being ejected from the surface. Analysis of images of the three largest particle ejection events has narrowed the cause of these to three possibilities: Meteoroid impacts, thermal stress or the evaporation of a pocket of ice. The three events all happened in the asteroid afternoon and occurred at differing locations. The first two causes should commonly occur on any asteroid, and the third only on asteroids with substantial water ice content.

Martian Ice – Scientists at NASA’s Jet Propulsion Laboratory have figured out a new way to detect near-surface water ice on Mars using its inherent thermal properties. Because ice is good at storing heat, by looking at how quickly the Martian surface warms in summer and cools in winter, scientists can infer whether there is a large mass of ice present. The closer the ice is to the surface, the stronger the effect. This is more precise in determining small depths than previous methods, such as ground-penetrating radar. A team applied this method to archived temperature data and found that ice is within inches of the surface over huge portions of Mars, including equatorial areas. The Phoenix lander scraped up ice within inches of the surface where it landed in the Martian Arctic, but it was not known until now if that would occur over widespread areas. This is good news for future crewed missions to Mars, which will require easy access to water.

Pulsar Properties – An instrument named NICER aboard the International Space Station has been studying the X-rays emitted by pulsars. It records precise timing down to one tenth of a microsecond of every X-ray photon it detects. This has allowed scientists to calculate the diameter and mass of a pulsar more precisely than previous methods. The results for a pulsar named J0030+0451 are that it is 16 miles across and 1.3-1.4 solar masses. Two teams, one from University of Amsterdam and the other from the University of Maryland, independently worked on the calculations which agreed well. They were also able to map out where on the pulsar’s surface lie the hot spots that emit X-rays. It is a difficult calculation because the extreme gravity near the surface of the pulsar bends the paths of the X-rays. The two teams agreed fairly well on the location of two hot spots, but one team found a third spot. Surprisingly, the locations of the spots did not agree with a simple model of a north and south magnetic pole, opposite each other, creating the hot spots. This implies that pulsar magnetic fields are more complicated than thought. The pulsar spins 205 times per second and lies 1,100 light-years away in the constellation Pisces. Similar observations and calculations will be made on other pulsars.

Exoplanet Atmospheres – A team of researchers led by the University of Cambridge released a study of the temperature and spectral data of the atmospheres of 19 exoplanets, the most extensive such survey yet. The planets ranged in temperature from room temperature to over 3,600 degrees Fahrenheit, and ranged in size and mass from mini-Neptunes to super-Jupiters. Like our local giant planets, the atmospheres were rich in hydrogen. Water vapor was common, but in amounts less than expected, compared to the abundance of other elements, such as sodium and potassium. This would suggest that existing theories of how giant planets form need to be modified to include less water in those planets.

Exoplanet Orbiting White Dwarf – A ring of gas has been found orbiting a white dwarf star known as WDJ0914+1914. The ring contains oxygen and sulfur, and an occasional spike in hydrogen content which is largely blowing away in a comet-like tail. The best theory to explain these observations is that the star is evaporating an orbiting ice giant planet, similar to Uranus or Neptune. The star is hot enough and the cloud close enough to the star to supply the heat necessary to evaporate such a planet. This is the first time evidence of a giant planet orbiting a white dwarf star has been found. Because white dwarfs are extremely dense, an ice giant planet would be about four times the diameter of its star, though far smaller in mass. The system is 1,500 light-years away in the constellation Cancer. There have been previous observations of material that appeared to be rocky planet debris falling into white dwarf stars, but not evidence of giant planets doing so. It had even been theorized that planets would be destroyed by their star going through the red giant phase, which occurs before the white dwarf phase, but these observations seem to show that planets can survive until the white dwarf phase of their star. The team, led by astronomers at the University of Warwick in the UK, believe that planet evaporation might be relatively common, and finding more examples could tell them more about the content of exoplanet atmospheres. However a search of spectral data on 7,000 white dwarfs did not find another system evaporating a gas planet. But the European Space Agency’s Gaia spacecraft data has 260,000 white dwarfs in it, so there is much more data to be searched for similar systems.

Solar Discoveries – The Parker Solar Probe has completed the third of 24 planned passes close to the Sun, each pass approaching closer to our star. Already new discoveries have been published: The solar wind spins with the speed of the Sun’s rotation when it is emitted, but loses its spin by the time it reaches Earth. This had been predicted theoretically, but has now been measured directly by Parker flying through the wind close to the Sun. The shape of the Sun’s electric and magnetic fields also has a few surprises. There are kinks in these, where the field reverses briefly while traveling outward from the Sun. There are jets of material traveling faster than the rest of the solar wind. There is known to be cosmic dust all over the Solar System, but theory says that closer to the Sun there would be a dust-free zone caused by the Sun’s heat. Parker has begun to see thinning of dust, and is expected to see the totally dust-free zone on closer approaches. It is known that bursts of particles from the Sun can reach speeds of a significant fraction of the speed of light. On closer passes to the Sun, Parker will seek to collect data to help scientists understand how these particles attain such speeds. Parker has observed rare bursts with large content of heavier elements.

 Geminids – The Parker Solar Probe spacecraft imaged the stream of debris left behind by the asteroid Phaethon. When these debris strike Earth’s atmosphere in December, they cause the Geminid meteor shower. This is the first time the Geminids have been viewed from space.

Space Radiotelescope – The Chinese spacecraft QueQiao has been relaying communications with the Chang’e 4 rover, which is traversing across the far side of the Moon. QueQiao is in orbit about the Earth-Moon L2 Lagrange point, and from that vantage, its radios have a direct line-of-sight to both the Earth and the lunar far side. The spacecraft carries a radiotelescope, built jointly with a Netherlands team, and known as NCLE, short for the Netherlands-China Low Frequency Explorer. The plan was to deploy it once communications with the rover had settled down and more time on the communications was radio available. Now that this has happened, the three NCLE antennas were extended and observations have begun. NCLE is designed to detect the signals emitted by hydrogen clouds shortly after the Big Bang, that have redshifted into the radio range. These frequencies do not penetrate Earth’s atmosphere, so ground-based radiotelescopes cannot make this kind of observations. The relatively radio-noise-free conditions near the Moon also put NCLE in unique position to make such observations.

TESS – NASA’s Transiting Exoplanet Survey Satellite (TESS), an exoplanet-hunting space telescope, observed for a lunar month an area of the sky that included the comet 46P/Wirtanen. It captured images of the comet’s development every 30 minutes, which included an unexpected explosive emission of dust coming from the comet, the most complete and detailed observation ever made of such an outburst. Other comets are expected to pass through TESS’s view in coming months.