Topic: Early May Astronomy Bulletin

[Clive]

WHY VENUS ROTATES SLOWLY
University of California - Riverside

If not for the soupy, fast-moving atmosphere on Venus, Earth's sister planet would likely not rotate. Instead, Venus would be locked in place, always facing the Sun the way the same side of the Moon always faces Earth. The gravity of a large object in space can keep a smaller object from spinning, a phenomenon called tidal locking. Because it prevents this locking, it can be argued the atmosphere needs to be a more prominent factor in studies of Venus as well as other planets. Venus takes 243 Earth days to rotate one time, but its atmosphere circulates the planet every four days. Extremely fast winds cause the atmosphere to drag along the surface of the planet as it circulates, slowing its rotation while also loosening the grip of the Sun's gravity. Slow rotation in turn has dramatic consequences for the sweltering Venusian climate, with average temperatures of up to 900 degrees Fahrenheit -- hot enough to melt lead. One reason for the heat is that nearly all of the Sun's energy absorbed by the planet is soaked up by Venus' atmosphere, never reaching the surface. This means that a rover with solar panels like the one NASA sent to Mars wouldn't work. The Venusian atmosphere also blocks the Sun's energy from leaving the planet, preventing cooling or liquid water on its surface, a state known as a runaway greenhouse effect. It is unclear whether being partially tidally locked contributes to this runaway greenhouse state, a condition which ultimately renders a planet uninhabitable by life as we know it.

Not only is it important to gain clarity on this question to understand Venus, it is important for studying the exoplanets likely to be targeted for future NASA missions. Most of the planets likely to be observed with the recently launched James Webb Space Telescope are very close to their stars, even closer than Venus is to the Sun. Therefore, they're also likely to be tidally locked. Since humans may never be able to visit exoplanets in person, making sure computer models account for the effects of tidal locking is critical. Venus is our opportunity to get these models correct, so we can properly understand the surface environments of planets around other stars. Gaining clarity about the factors that contributed to a runaway greenhouse state on Venus, Earth's closest planetary neighbour, can also help improve models of what could one day happen to Earth's climate.


TITAN IS EARTH-LIKE ALIEN WORLD
Stanford University

Saturn's moon Titan looks very much like Earth from space, with rivers, lakes, and seas filled by rain tumbling through a thick atmosphere. While these landscapes may look familiar, they are composed of materials that are undoubtedly different -- liquid methane streams streak Titan's icy surface and nitrogen winds build hydrocarbon sand dunes. The presence of these materials -- whose mechanical properties are vastly different from those of silicate-based substances that make up other known sedimentary bodies in our solar system -- makes Titan's landscape formation enigmatic. By identifying a process that would allow for hydrocarbon-based substances to form sand grains or bedrock depending on how often winds blow and streams flow, scientists have shown how Titan's distinct dunes, plains, and labyrinth terrains could be formed. Titan, which is a target for space exploration because of its potential habitability, is the only other body in our solar system known to have an Earth-like, seasonal liquid transport cycle today. The new model shows how that seasonal cycle drives the movement of grains over the moon's surface. In order to build a model that could simulate the formation of Titan's distinct landscapes, scientists first had to solve one of the biggest mysteries about sediment on the planetary body: How can its basic organic compounds -- which are thought to be much more fragile than inorganic silicate grains on Earth -- transform into grains that form distinct structures rather than just wearing down and blowing away as dust? On Earth, silicate rocks and minerals on the surface erode into sediment grains over time, moving through winds and streams to be deposited in layers of sediments that eventually -- with the help of pressure, groundwater, and sometimes heat -- turn back into rocks. Those rocks then continue through the erosion process and the materials are recycled through Earth's layers over geologic time. On Titan, researchers think similar processes formed the dunes, plains, and labyrinth terrains seen from space. But unlike on Earth, Mars, and Venus, where silicate-derived rocks are the dominant geological material from which sediments are derived, Titan's sediments are thought to be composed of solid organic compounds. Scientists haven't been able to demonstrate how these organic compounds may grow into sediment grains that can be transported across the moon's landscapes and over geologic time.

The research team found an answer by looking at sediments on Earth called ooids, which are small, spherical grains most often found in shallow tropical seas, such as around the Bahamas. Ooids form when calcium carbonate is pulled from the water column and attaches in layers around a grain, such as quartz. What makes ooids unique is their formation through chemical precipitation, which allows ooids to grow, while the simultaneous process of erosion slows the growth as the grains are smashed into each other by waves and storms. These two competing mechanisms balance each other out through time to form a constant grain size -- a process the researchers suggest could also be happening on Titan. Armed with a hypothesis for sediment formation, the teamors used existing data about Titan's climate and the direction of wind-driven sediment transport to explain its distinct parallel bands of geological formations: dunes near the equator, plains at the mid-latitudes, and labyrinth terrains near the poles. Atmospheric modelling and data from the Cassini mission reveal that winds are common near the equator, supporting the idea that less sintering and therefore fine sand grains could be created there -- a critical component of dunes. The study authors predict a lull in sediment transport at mid-latitudes on either side of the equator, where sintering could dominate and create coarser and coarser grains, eventually turning into bedrock that makes up Titan's plains. Sand grains are also necessary for the formation of the moon's labyrinth terrains near the poles. Researchers think these distinct crags could be like karsts in limestone on Earth -- but on Titan, they would be collapsed features made of dissolved organic sandstones. River flow and rainstorms occur much more frequently near the poles, making sediments more likely to be transported by rivers than winds. A similar process of sintering and abrasion during river transport could provide a local supply of coarse sand grains -- the source for the sandstones thought to make up labyrinth terrains. It shows that on Titan -- just like on Earth and what used to be the case on Mars -- we have an active sedimentary cycle that can explain the latitudinal distribution of landscapes through episodic abrasion and sintering driven by Titan's seasons.


NEW LIGHT ON STRANGE SIGNAL FROM GALACTIC CENTRE
Australian National University

Researchers have found an alternative explanation for a mysterious gamma-ray signal coming from the centre of the Galaxy, which was long claimed as a signature of dark matter. Gamma-rays are the form of electromagnetic radiation with the shortest wavelength and highest energy. This particular gamma-ray signal -- known as the Galactic Centre Excess -- may actually come from a specific type of rapidly-rotating neutron star, the super-dense stellar remnants of some stars much more massive than our Sun. The Galactic Centre Excess is an unexpected concentration of gamma-rays emerging from the centre of our galaxy that has long puzzled astronomers. The work does not throw any doubt on the existence of the signal, but offers another potential source based on millisecond pulsars -- neutron stars that spin really quickly -- around 100 times a second. Scientists have previously detected gamma-ray emissions from individual millisecond pulsars in the neighbourhood of the solar system, so we know these objects emit gamma-rays. The model demonstrates that the integrated emission from a whole population of such stars, around 100,000 in number, would produce a signal entirely compatible with the Galactic Centre Excess. The discovery may mean scientists have to re-think where they look for clues about dark matter.


LIKELY LOCATION OF MEDIUM-SIZED BLACK HOLES
Washington State University

Intermediate-mass black holes are notoriously hard to find but a new study indicates there may be some at the centre of dense star clusters located throughout the Universe. The study sheds new light on when and where black holes of about 100-100,000 solar masses could form and how they came into being. One of the biggest open questions in black hole astrophysics right now is how do black holes form that are between the size of a stellar mass black hole and a supermassive black hole. Most of the theories for their formation rely on conditions that are found only in the very early Universe. Scientists wanted to test another theory that says they can form throughout cosmic time in these really dense star clusters. For decades, astronomers have detected smaller black holes equal in mass either to a few suns or giant black holes with mass similar to millions of suns but the missing-link of black holes in between those sizes have eluded discovery. The existence of these intermediate-sized or massive black holes has long been theorized but finding them has proven difficult as the light emitted by objects falling into them is not easy to detect. To address this challenge, the research team used the Chandra X-Ray Observatory, the world's most powerful X-ray telescope, to look for X-ray signatures of black holes in nuclear star clusters in 108 different galaxies. Nuclear star clusters are found at the centre of most small or low-mass galaxies and are the densest known stellar environments. Previous research has identified the presence of black holes in nuclear star clusters but little is known about the specific properties that make these regions conducive for the formation of black holes.

The team showed that nuclear star clusters that were above a certain mass and density threshold emitted the X-ray signatures indicative of a black hole at twice the rate of those below the threshold. Their work provides the first observational evidence supporting the theory that intermediate-sized black holes can form in nuclear star clusters. Basically, it means that star clusters that are sufficiently massive and compact should be able to form a blackhole. It is exciting because researchers expect many of these black holes to be in the intermediate mass regime between supermassive black holes and stellar mass black holes where there is very little evidence for their existence. The research team's work not only suggests that intermediate-sized black holes can form in nuclear star clusters but also provides a mechanism by which they could potentially form throughout cosmic time rather than just during the first few billion years of the Universe. One of the prevailing theories is that massive black holes could only have formed during the early Universe when things were more dense. This research is more consistent with the picture where massive blackholes don't need to form in the very early Universe but could rather continue to form throughout cosmic time in these particular environments. Moving forward, the researchers plan to continue using Chandra to collect x-ray measurements of nuclear star clusters with the ultimate goal of learning more about the specific conditions where massive black holes can form.


SUPERNOVA REVEALS SECRETS
University of Texas at Austin

Astronomers have used observations from the Hobby-Eberly Telescope (HET) at the university's McDonald Observatory to unlock a puzzling mystery about a stellar explosion discovered several years ago and evolving even now. When an exploding star is first detected, astronomers around the world begin to follow it with telescopes as the light it gives off changes rapidly over time. They see the light from a supernova get brighter, eventually peak, and then start to dim. By noting the times of these peaks and valleys in the light's brightness, called a "light curve," as well as the characteristic wavelengths of light emitted at different times, they can deduce the physical characteristics of the system. In the case of supernova 2014C, the progenitor was a binary star, a system in which two stars were orbiting each other. The more massive star evolved more quickly, expanded, and lost its outer blanket of hydrogen to the companion star. The first star's inner core continued burning lighter chemical elements into heavier ones, until it ran out of fuel. When that happened, the outward pressure from the core that had held up the star's great weight dropped. The star's core collapsed, triggering a gigantic explosion. This makes it a type of supernova astronomers call a "Type Ib." In particular, Type Ib supernovae are characterized by not showing any hydrogen in their ejected material, at least at first. The team has been following SN 2014C from telescopes at McDonald Observatory since its discovery that year. Many other teams around the world also have studied it with telescopes on the ground and in space, and in different types of light, including radio waves from the ground-based Very Large Array, infrared light, and X-rays from the space-based Chandra Observatory. But the studies of SN 2014C from all of the various telescopes did not add up into a cohesive picture of how astronomers thought a Type Ib supernova should behave.

For one thing, the optical signature from the Hobby-Eberly Telescope (HET) showed SN 2014C contained hydrogen -- a surprising finding that also was discovered independently by another team using a different telescope. For a second thing, the optical brightness (light curve) of that hydrogen was behaving strangely. Most of the light curves from SN 2014C -- radio, infrared, and X-rays -- followed the expected pattern: they got brighter, peaked, and started to fall. But the optical light from the hydrogen stayed steady. The problem, the team realized, was that previous models of this system assumed that the supernova had exploded and sent out its shockwave in a spherical manner. The data from HET showed that this hypothesis was impossible -- something else must have happened. The team proposes a model where the hydrogen envelopes of the two stars in the progenitor binary system merged to form a "common-envelope configuration," where both were contained within a single envelope of gas. The pair then expelled that envelope in an expanding, disk-like structure surrounding the two stars. When one of the stars exploded, its fast-moving ejecta collided with the slow-moving disk, and also slid along the disk surface at a "boundary layer" of intermediate velocity. The team suggests that this boundary layer is the origin of the hydrogen they detected and then studied for seven years with HET. Thus the HET data turned out to be the key that unlocked the mystery of supernova SN 2014C.