Topic: Early August Astronomy Bulletin

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COSMIC RAY UPDATE -- NEW RESULTS FROM THE MOON
Spaceweather.com

2019 is not a good year to fly into deep space. In fact, it's shaping up to be one of the worst of the Space Age. One of the deepest Solar Minima of the past century is under way now. As the Sun's magnetic field weakens, cosmic rays from deep space are flooding into the Solar system, posing potential health risks to space travellers. NASA is monitoring the situation with a radiation sensor in lunar orbit. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been circling the Moon on NASA's Lunar Reconnaissance Orbiter spacecraft since 2009. The overall decrease in solar activity in this period has led to an increased flux of energetic particles, to levels that are approaching those observed during the previous solar minimum in 2009/2010, which was the deepest minimum of the Space Age.  This always happens during Solar Minimum. As solar activity goes down, cosmic rays go up. The last two Solar Minima have been unusually deep, leading to high cosmic-ray fluxes in 2008-2010 and again in 2018-2019.  These are the worst years since humans first left Earth in the 1960s. Solar Minimum may actually be more dangerous than Solar Maximum.

In 2011, NASA launched the Curiosity rover to Mars. Inside its spacecraft, the rover was protected by about as much shielding (20 gm/cm^2) as a human astronaut would have. A radiation sensor tucked inside kept track of Curiosity's exposure. The results were surprising. During the 9-month journey to Mars, radiation from solar flares (including the strongest flare of the previous solar cycle) accounted for only about 5% of Curiosity's total dose. The remaining 95% came from cosmic rays. As 2019 unfolds, Solar Minimum is still deepening. Cosmic rays haven't quite broken the Space Age record set in 2009-2010, but they're getting close, only percentage points from the highest values CRaTER has ever recorded.


THREE NEW 'NEARBY' PLANETS
Massachusetts Institute of Technology

NASA's Transiting Exoplanet Survey Satellite, or TESS, has discovered three new worlds that are among the smallest, nearest exoplanets known to date.  The planets orbit a star just 73 light-years away and include a small, rocky super-Earth and two sub-Neptunes. The sub-Neptune furthest out from the star appears to be within a 'temperate' zone, meaning that the very top of the planet's atmosphere is within a temperature range that could support some forms of life. However, scientists say that the planet's atmosphere is probably a thick, ultradense heat trap that renders the planet's surface too hot to host water or life. Nevertheless, this new planetary system, which astronomers have dubbed TOI-270, is proving to have other curious qualities.  For instance, all three planets appear to be relatively close in size. In contrast, our own Solar System is populated with planetary extremes, from the small, rocky worlds of Mercury, Venus, Earth, and Mars, to the much more massive Jupiter and Saturn, and the more remote ice giants of Neptune and Uranus. There's nothing in our Solar System that resembles an intermediate planet, with a size and composition somewhere in the middle of Earth and Neptune. But TOI-270 appears to host two such planets: both sub-Neptunes are smaller than our own Neptune and not much larger than the rocky planet in the system. Astronomers believe TOI-270's sub-Neptunes may be a 'missing link' in planetary formation, as they are of an intermediate size and could help researchers determine whether small, rocky planets like Earth and more massive, icy worlds like Neptune follow the same formation path or evolve separately.

TOI-270 is an ideal system for answering such questions, because the star itself is relatively nearby and therefore bright, and also unusually quiet.  The star is an M-dwarf, a type of star that is normally extremely active, with frequent flares and solar storms. TOI-270 appears to be an older M-dwarf that has quietened down, giving out a steady brightness, against which scientists can measure many properties of the orbiting planets, such as their mass and atmospheric composition. Astronomers detected the three new planets after looking through measurements of stellar brightness taken by TESS. That MIT-developed satellite stares at patches of the sky for 27 days at a time, monitoring thousands of stars for possible transits -- characteristic dips in brightness that could signal a planet temporarily blocking the star's light as it passes in front of it. The team isolated several such signals from a nearby star, located 73 light years away in the southern sky. They named the star TOI-270, for the 270th "TESS Object of Interest" identified to date. The researchers used ground-based instruments to follow up on the star's activity, and confirmed that the signals are the result of three orbiting exoplanets: planet b, a rocky super-Earth with a roughly three-day orbit; planet c, a sub-Neptune with a five-day orbit; and planet d, another sub-Neptune slightly further out, with an 11-day orbit.  Astronomers note that the planets seem to line up in what astronomers refer to as a "resonant chain", meaning that the ratios of their orbits are close to whole integers -- in this case, 3:5 for the inner pair, and 2:1 for the outer pair -- and that the planets are therefore in 'resonance' with each other. Astronomers have discovered other small stars with similarly resonant planetary formations. And in our own Solar System, the moons of Jupiter also line up in resonance with each other.

 
FASTEST KNOWN ECLIPSING BINARY
Association of Universities for Research in Astronomy (AURA)

Observations made with a new instrument developed for use at the 2.1-metre telescope at the Kitt Peak National Observatory have led to the discovery of the fastest eclipsing white-dwarf binary yet known. Clocking in with an orbital period of only 6.91 minutes, the rapidly orbiting stars are expected to be one of the strongest sources of gravitational waves detectable with LISA, the future space-based gravitational-wave detector. After expanding into a red giant at the end of its life, a star like the Sun will eventually evolve into a dense white dwarf, an object with a mass like that of the Sun squashed down to a size comparable to the Earth. Similarly, as binary stars evolve, they can engulf their companion in the red-giant phase and spiral close together, eventually leaving behind a close white-dwarf binary.  White-dwarf binaries with very tight orbits are expected to be strong sources of gravitational-wave radiation. Although anticipated to be relatively common, such systems have proven elusive, with only a few identified to date. A new survey of the night sky, currently under way at Palomar Observatory and Kitt Peak National Observatory, is changing this situation. Each night, Caltech's Zwicky Transient Facility (ZTF), a survey that uses the 48-inch telescope at Palomar Observatory, scans the sky for objects that move, blink, or otherwise vary in brightness. Promising candidates are followed up with a new instrument, the Kitt Peak Electron Multiplying Demonstrator (KPED), at the Kitt Peak 2.1-m telescope, to identify short-period eclipsing binaries. KPED is designed to measure with speed and sensitivity the changing brightness of celestial sources.

This approach has led to the discovery of ZTF J1539+5027 (or J1539 for short), a white-dwarf eclipsing binary with the shortest period known to date, a mere 6.91 minutes. The stars orbit so close together that the entire system could fit within the diameter of the planet Saturn. As the dimmer star passes in front of the brighter one, it blocks most of the light, resulting in the seven-minute blinking pattern seen in the ZTF data.  Closely orbiting white dwarfs are predicted to spiral together closer and faster, as the system loses energy by emitting gravitational waves. J1539's orbit is so tight that its orbital period is predicted to become measurably shorter after only a few years. The team was able to confirm the prediction from general relativity of a shrinking orbit, by comparing their new results with archival data acquired over the past ten years. J1539 is a rare gem.  It is one of only a few known sources of gravitational waves -- ripples in space and time -- that will be detected by the future European space mission LISA (Laser Interferometer Space Antenna), which is expected to be launched in 2034. LISA, in which NASA plays a role, will be similar to the National Science Foundation's ground-based LIGO (Laser Interferometer Gravitational- wave Observatory), which made history in 2015 by making the first direct detection of gravitational waves from a pair of colliding black holes. LISA will detect gravitational waves from space at lower frequencies. J1539 is well matched to LISA; the 4.8-Hz gravitational wave frequency of J1539 is close to the peak of LISA's sensitivity.


LOCAL VOID MAPPED
University of Hawaii at Manoa

The Universe is a tapestry of galaxy congregations and vast voids. A team of astronomers has mapped the size and shape of an extensive empty region called the Local Void that borders the Milky Way galaxy. Using the observations of galaxy motions, they infer the distribution of mass responsible for that motion, and construct three-dimensional maps of our local Universe.  Galaxies not only move with the overall expansion of the Universe, they also respond to the gravitational tugs of their neighbours and regions with a lot of mass. As a consequence, relative to the overall expansion they are moving towards the densest areas and away from regions with little mass -- the voids. Although we live in a cosmic metropolis, back in 1987 astronomers noted that our Milky Way galaxy is also at the edge of an extensive empty region that they called the Local Void. The existence of the Local Void has been widely accepted, but it remained poorly studied because it lies behind the centre of our Galaxy and is therefore heavily obscured from our view.

Now, the team has measured the motions of 18,000 galaxies in the Cosmicflows-3 compendium of galaxy distances, constructing a cosmographic map that highlights the boundary between the collection of matter and the absence of matter that defines the edge of the Local Void. The same technique was used in 2014 to identify the full extent of our home supercluster of over one hundred thousand galaxies, giving it the name Laniakea, meaning "immense heaven" in Hawaiian. For 30 years, astronomers have been trying to identify why the motions of the Milky Way, our nearest large galaxy neighbour Andromeda, and their smaller neighbours deviate from the overall expansion of the Universe by over 600 km/s. The new study shows that roughly half of this motion is generated 'locally' from the combination of a pull from the massive nearby Virgo Cluster and our participation in the expansion of the Local Void as it becomes ever emptier.

 
EARLY DAYS OF MILKY WAY REVEALED
Instituto de Astrofisica de Canarias (IAC)

The Universe 13,000 million years ago was very different from the Universe we know today. It is understood that stars were forming at a very rapid rate, forming the first dwarf galaxies, whose mergers gave rise to the more massive present-day galaxies, including our own. However, the exact chain of the events which produced the Milky Way was not known until now. Exact measurements of position, brightness and distance for around a million stars of our galaxy within 6,500 light years of the Sun, obtained with the Gaia space telescope, have allowed a team from the IAC to reveal some of its early stages. They have analyzed, and compared with theoretical models, the distribution of colours and magnitudes of the stars in the Milky Way, splitting them into several components; the so-called stellar halo (a spherical structure which surrounds spiral galaxies) and the thick disc (stars forming the disc of our Galaxy, but occupying a certain height range). Previous studies had discovered that the Galactic halo showed clear signs of being made up of two distinct stellar components, one dominated by bluer stars than the other. The movement of the stars in the blue component quickly allowed us to identify it as the remains of a dwarf galaxy (Gaia-Enceladus) which impacted onto the early Milky Way. However, the nature of the red population, and the epoch of the merger between Gaia-Enceladus and our Galaxy were unknown until now. Analyzing the data from Gaia has allowed astronomers to obtain the distribution of the ages of the stars in both components and has shown that the two are formed by equally old stars, which are older than those of the thick disc. But if both components were formed at the same time, what differentiates one from the other? The final piece of the puzzle was given by the quantity of 'metals' (elements other than hydrogen or helium) in the stars of one component or the other. The stars in the blue component have a smaller quantity of metals than those of the red component. Those findings, with the addition of the predictions of simulations, have allowed the researchers to complete the history of the formation of the Milky Way.

Thirteen thousand million years ago stars began to form in two different stellar systems which then merged: one was a dwarf galaxy which we call Gaia-Enceladus, and the other was the main progenitor of our Galaxy, some four times more massive and with a larger proportion of metals. Some ten thousand million years ago there was a violent collision between the more massive system and Gaia-Enceladus. As a result, some of its stars, and those of Gaia-Enceladus, were set into chaotic motion, and eventually formed the halo of the present Milky Way. After that, there were violent bursts of star formation until 6,000 million years ago, when the gas settled into the disc of the Galaxy, and produced what we know as the thin disc. Until now all the cosmological predictions and observations of distant spiral galaxies similar to the Milky Way indicate that a violent phase of merging between smaller structures was very frequent. Now we have been able to identify the specificity of the process in our own Galaxy, revealing the first stages of our cosmic history with unprecedented detail.