ABSTRACT: 1I/`Oumuamua is the first of likely many small bodies of extrasolar origin to be found in the solar system. These interstellar objects (ISOs) are hypothesized to have formed in extrasolar planetary systems prior to being ejected into interstellar space and subsequently arriving at the solar system. This paper discusses necessary considerations for tracing ISOs back to their parent stars via trajectory analysis, and places approximate limits on doing so. Results indicate the capability to backtrace ISOs beyond the immediate solar neighborhood is presently constrained by the quality of stellar astrometry, a factor poised for significant improvement with upcoming Gaia data releases. Nonetheless, prospects for linking 1I or any other ISO to their respective parent star appear unfavorable on an individual basis due to gravitational scattering from random stellar encounters which limit traceability to the past few tens of millions of years. These results, however, do not preclude the possibility of occasional success, particularly after considering the potential for observational bias favoring the discovery of younger ISOs, together with the anticipated rise in the ISO discovery rate under forthcoming surveys.
ABSTRACT: The positions of multiple images in galaxy lenses are related to the galaxy mass distribution. Smooth elliptical mass profiles were previously shown to be inadequate in reproducing the quad population. In this paper, we explore the deviations from such smooth elliptical mass distributions. Unlike most other work, we use a model-free approach based on the relative polar image angles of quads, and their position in 3D space with respect to the Fundamental Surface of Quads. The FSQ is defined by quads produced by elliptical lenses. We have generated thousands of quads from synthetic populations of lenses with substructure consistent with ΛCDM simulations, and found that such perturbations are not sufficient to match the observed distribution of quads relative to the FSQ. The result is unchanged even when subhalo masses are increased by a factor of ten, and the most optimistic lensing selection bias is applied. We then produce quads from galaxies created using two components, representing baryons and dark matter. The transition from the mass being dominated by baryons in inner radii to being dominated by dark matter in outer radii can carry with it asymmetries, which would affect relative image angles. We run preliminary experiments using lenses with two elliptical mass components with nonidentical axis ratios and position angles, perturbations from ellipticity in the form of nonzero Fourier coefficients a4 and a6, and artificially offset ellipse centers as a proxy for asymmetry at image radii. We show that combination of these effects is a promising way of accounting for quad population properties. We conclude that the quad population provides a unique and sensitive tool for constraining detailed mass distribution in the centers of galaxies.
The gravitational wave event GW170817—detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and named after the date that it occurred—was swiftly identified as the merger of two neutron stars. Unlike previously detected black hole mergers, theoretical models predicted that merging neutron stars should emit electromagnetic radiation, potentially detectable by conventional astronomical telescopes. The race was on to find the electromagnetic counterpart before it faded away.
Combined data from the two LIGO detectors and the Virgo interferometer showed that the source was located somewhere in a 31-square-degree patch of sky. That’s 150 times the size of the full Moon—a large area to search blindly. A coincident gamma-ray burst was also detected, but that did not improve the direction constraint. However, the interferometers provided another crucial piece of information: the approximate distance. Cross-matching the direction and distance with catalogs of known galaxies narrowed the search to about 100 possible locations.
The search took less than an hour of nighttime observing, after waiting hours for the Sun to go down. A bright, but rapidly fading, new source in the galaxy NGC 4993 was quickly found using an optical telescope. Numerous other observatories then swung into action: Over the following weeks, dozens of teams used 70 separate telescopes to study the event. The papers in this section describe many of those observations, covering the electromagnetic spectrum from x-rays to radio waves, and interpret them as an explosion generated by the merger, known as a kilonova.
The scientific bonanza from GW170817 was enabled by the sensitivity of the gravitational wave interferometers, coupled with the rapid response from time-domain astronomers. The partnership of these two communities holds great promise for new discoveries.
On 17 August, scientists around the world witnessed something never seen before: One hundred and thirty million light-years away, two neutron stars spiraled into each other in a spectacular explosion that was studied by observatories ranging from gamma ray detectors to radio telescopes. The blast confirmed several key astrophysical models, revealed a birthplace of many heavy elements, and tested the general theory of relativity as never before. That first observation of a neutron-star merger, and the scientific bounty it revealed, is Science’s 2017 Breakthrough of the Year.
Especially remarkable was the way the event was spotted: by detecting the infinitesimal ripples in space itself, called gravitational waves, that the spiraling neutron stars radiated before they merged. Scientists first detected such waves just 27 months ago, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) sensed a space tremor from two massive black holes spiraling together in an invisible cataclysm. The discovery of gravitational waves was Science’s 2016 Breakthrough of the Year.
If that observation sounded the clarion of discovery, this year’s produced a scientific symphony. The difference comes down to matter. A black hole, the ghostly gravitational field that remains when a huge star collapses to a point, contains no matter to heat up and radiate. A neutron star, in contrast, is a ball of nearly pure neutrons, the densest stuff there is. Whereas the colliding black holes emitted nothing but gravitational energy, the neutron-star smashup put on a light show that was studied by more than 70 observatories. “The amount of information we have been able to extract with one event blows my mind,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta and deputy spokesperson for the LIGO team.
The gravitational waves from the twirling neutron stars tickled not only the enormous LIGO detectors in Hanford, Washington, and Livingston, Louisiana, but also the French-Italian Virgo detector near Pisa, Italy, which, after a 5-year upgrade, had started recording data just 17 days earlier. Researchers immediately knew they were witnessing the death spiral of two neutron stars. Unlike black-hole mergers, which produce secondslong pulses of low-frequency gravitational waves, the lighter neutron stars produced a telltale higher frequency hum that increased in frequency and strength over 100 seconds.
That crescendo cued the fireworks. Two seconds later, NASA’s orbiting Fermi Gamma-ray Space Telescope detected a pulse of gamma rays called a short gamma ray burst. Then, other telescopes took aim. Because the gravitational waves were spotted by three widely spaced detectors, researchers could triangulate the neutron star pair’s location in the sky. Within 11 hours, several teams of optical and infrared astronomers had found a new beacon on the edge of the galaxy NGC 4993. Over several days, the source faded from bright blue to dimmer red. Then, after 11 days, it began to glow in x-rays and radio waves. The explosion was easily the most studied event in the history of astronomy, with 3674 researchers from 953 institutions collaborating on a single paper summarizing the merger and its aftermath.
The observations bolstered the 25-year-old hypothesis that neutron-star mergers produce short gamma ray bursts. And the reddish afterglow fit the model of a so-called kilonova, in which neutron-rich matter flung into space by colliding neutron stars hosts a chain of nuclear interactions known as the r-process. The process is thought to produce half the elements heavier than iron, and the heaviest ones would soak up blue light, tinting the glowing radioactive cloud red. “It’s been superexciting to see something that was just an idea come to life,” says Daniel Kasen of the University of California, Berkeley, who has modeled kilonovas. “All this stuff was done basically with eyes-closed theory.” The observation even bolstered Albert Einstein’s general theory of relativity by confirming that gravitational waves travel at the same speed as light and not more slowly, as some alternative theories had predicted.
But the merger also poses puzzles that have whetted astrophysicists’ appetites for more data. For example, the gamma ray burst was surprisingly feeble, says Vicky Kalogera, an astrophysicist and LIGO team member at Northwestern University in Evanston, Illinois. Such bursts are thought to originate when narrow jets of material shoot out of a neutron-star merger at near–light-speed, like search beams. The simplest explanation is that the jet may not have pointed straight at Earth. However, it’s possible that astrophysicists’ model isn’t quite right and that neutron-star mergers produce only muted gamma ray bursts, Kalogera says. To resolve the issue, astrophysicists need to see more mergers.
They would also like to see the gravitational waves right up to the point at which the neutron stars spiral into each other. In this first observation, the LIGO and Virgo detectors tracked the stars whirling around each other at an accelerating pace, sending out higher and higher frequency gravitational waves. But at about 500 cycles per second, the waves’ frequency climbed out of LIGO’s sensitivity range, and the detectors couldn’t observe the final few revolutions leading up to the merger.
Those final revolutions could provide insights into the nature of neutron stars, orbs of pure nuclear matter slightly more massive than the sun but just 20 to 30 kilometers wide. Astrophysicists want to know how stiff or squishy neutron star matter is—a property encapsulated in the so-called equation of state. In principle, the gravitational waves can reveal that information: The stiffer the matter is, the larger the neutron stars will be, and the earlier they will tear each other apart as they spiral together, altering the signal. “If we want to determine the equation of state, we need to see the whole event,” says James Lattimer, a nuclear astrophysicist at the State University of New York in Stony Brook. Researchers plan to increase LIGO’s sensitivity at high frequencies—for instance, by manipulating the laser light circulating in the massive detectors—but doing so may take a few years.
Scientists also hope to see new types of events, such as mergers of a neutron star and a black hole, which theory suggests are rare. Supernova explosions of individual stars in our Milky Way galaxy should also produce detectable gravitational waves, which could help astrophysicists figure out exactly how the stars blow up. Spinning neutron stars called pulsars might broadcast a steady warble of gravitational waves. In coming decades, scientists hope to launch a space-based gravitational-wave detector that could spot lower frequency waves, such as those from the mergers of supermassive black holes in the centers of galaxies.
Most thrilling would be a signal that astrophysicists haven’t predicted at all, says Roger Blandford, a theorist at Stanford University in Palo Alto, California. “I’d love to see something that doesn’t fit the expectations.”
NASA has selected two missions to further explore past targets—Saturn’s largest moon, Titan, and the comet 67P/Churyumov-Gerasimenko—as the final candidates for its next billion-dollar robotic spacecraft, the agency announced today. The candidates for the next New Frontiers mission, chosen from a field of 12, will now have until January 2019 to refine their pitches to the agency, with a launch planned by 2025.
The first, Dragonfly, would send a semiautonomous quad-copter to fly between sites on the surface of Titan, which features an Earth-like landscape of rivers and lakes filled with liquid methane. The second candidate, Comet Astrobiology Exploration Sample Return (CAESAR), would capture and return to Earth a sample from the nucleus of 67P/Churyumov-Gerasimenko—a comet previously explored by the European Space Agency’s Rosetta spacecraft.
Rather than selecting three final candidates, as it has in the past, NASA opted for a head-to-head competition. “I selected these mission concepts based on their outstanding and visionary science,” Thomas Zurbuchen, associate administrator of NASA’s science mission directorate in Washington, D.C., said in a teleconference announcing the finalists. “I didn’t start with a number in mind.”
Dragonfly is led by Elizabeth “Zibi” Turtle, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. CAESAR is led by Steve Squyres, a planetary scientist at Cornell University who has long led the Spirit and Opportunity rovers on Mars. Dragonfly would be managed by APL, whereas CAESAR would be managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Both missions would feature a long wait: Dragonfly would arrive at Titan in 2034, and CAESAR’s samples would return to Earth in 2038.
With hydrocarbon seas that may contain amino acids and other interesting molecules, Titan is thought to be a place for testing ideas for how life arose on Earth. With a suite of spectrometers, drills, and cameras, Dragonfly would split its mission between science in the air and on the ground. The rotocraft could travel up to 100 kilometers between sampling sites, and recharge its batteries between flights with a nuclear power source. Although frigid, Titan is otherwise a relatively benign place, and the rotocraft could survive for several years. That could give the team time, Turtle said, to “evaluate how far prebiotic chemistry has progressed in an environment where we know we have the ingredients for life.”
CAESAR would also do something unprecedented, Squyres said, by not just sampling a comet, but capturing both dust and volatile ices from its interior. Comets are thought to contain the early building blocks of the solar system, and returning such samples to Earth would allow close analysis. By returning to 67P/Churyumov-Gerasimenko, a body mapped in detail by Rosetta, the mission can target the best sites for retrieving samples, which it will divide into volatile and nonvolatile components for the journey back to Earth. “We’re able to design our spacecraft specifically for the conditions we know,” Squyres said.
The picks deal another blow to the small pool of researchers who have longed for NASA to return to Venus. One-fourth of the proposed New Frontiers missions targeted Venus, but like other recent competitions, NASA could not convince itself to move a mission toward flight. One of the Venus proposals, led by Goddard’s Lori Glaze, was selected for further technology development, however, along with a mission targeting Enceladus, the saturnian moon with perhaps the best chance of supporting life, led by Chris McKay of NASA’s Ames Research Center in Mountain View, California.
The cost-capped New Frontiers program, with $850 million set out for the mission and about $150 million for the launch, is the largest planetary exploration line that NASA opens to outside competition and leadership. Each candidate must target research priorities from a list set by the scientific community, which this time included a return to Venus; probing of Saturn or its ocean moons; exploration of the moon’s south pole; or returning a sample from a comet, among other options.
Previous spacecraft launched under New Frontiers include New Horizons, which surveyed Pluto and is now due to visit MU69, an icy object in the farthest reaches of the solar system; Juno, now in orbit around Jupiter; and OSIRIS-REx, launched last year, which will collect samples from an asteroid and return them to Earth.
ABSTRACT: The XO project aims at detecting transiting exoplanets around bright stars from the ground using small telescopes. The original configuration of XO (McCullough et al. 2005) has been changed and extended as described here. The instrumental setup consists of three identical units located at different sites, each composed of two lenses equipped with CCD cameras mounted on the same mount. We observed two strips of the sky covering an area of 520 deg2 for twice nine months. We build lightcurves for ~20,000 stars up to magnitude R~12.5 using a custom-made photometric data reduction pipeline. The photometric precision is around 1-2% for most stars, and the large quantity of data allows us to reach a millimagnitude precision when folding the lightcurves on timescales that are relevant to exoplanetary transits. We search for periodic signals and identify several hundreds of variable stars and a few tens of transiting planet candidates. Follow-up observations are underway to confirm or reject these candidates. We found two close-in gas giant planets so far, in line with the expected yield.
ABSTRACT: We show that the rate of pollution of white dwarfs by asteroidal material implies a concomitant rate of material ejection that can contribute significantly to the population of interstellar minor bodies. We note also that the irradiation during post main sequence evolution implies that much of this ejected material may lose volatiles, providing a rationale for the curious properties of the recently discovered interstellar object Oumuamua.
The World Wide Web consortium (W3C) er et internationalt samfund, hvor medlemsorganisationer, fuldtidsansatte og private personer arbejder sammen om at udvikle Web-standarder. W3C ledes af Tim Berners-Lee, opfinderen af WWW, med det formål at føre “the Web” mod dets fulde potentiale.
The World Wide Web konsortiet arbejder mod dette fælles mål ved at bringe forskellige aktører sammen for at udvikle nye standarder af høj kvalitet i en klar og effektiv konsensus-process baseret på bidrag fra medlemsorganisationer, fuldtidsansatte og private personer.
CSS er et sprog, som beskriver gengivelsen af struturerede dokumenter (som HTML og XML) på skærm, på papir eller i tale.
CSS er en integreret del af det moderne Web, som afløser Tim Berners-Lee’s 25 år gamle første WWW.
ABSTRACT: The first confirmed interstellar interloper in our Solar System, 1I/`Oumuamua, is likely to be a minor body ejected from another star, but its brief flyby and faintness made it difficult to study. Two remarkable properties are its large (2-2.5 mag) rotational variability and its motion relative to the Sun before encounter. The former suggests an extremely elongated (>10:1) shape and the latter an origin from the protoplanetary disk of a young star in a nearby association. 1I/`Oumuamua’s variability can also be explained if it is a contact binary of near-equilibrium ellipsoidal components and heterogeneous surfaces, i.e. brighter, dust-mantled inner-facing hemispheres and darker, dust-free outer-facing poles. The probability that 1I/`Oumuamua has the same motion as a young stellar association is <1%. One explanation for the youth of 1I/`Oumuamua relative to the Solar neighborhood mean it that loss of dust mantles and darkening of lag surfaces by cosmic rays renders similar objects undetectable in a few 100’s of Myr. In this scenario, 1I/`Oumuamua is smaller and much less massive, but represents a more numerous population of ejected planetesimals. Studies of such objects are a potential means to probe early planet formation, complementing observations of protoplanetary disks and studies of meteorites.
ABSTRACT: Two recent short term dimmings of KIC 8462852 (Boyajian’s Star) exhibit clear reddening in the B, r’ and i’ photometric passbands. We show that the intensity ratios of the three pass bands agree well with cooling of an approximately 6800 K black body by about 30K. This agreement, together with other recent findings on the timing and longer term dimmings of this star, support our previous argument that the star’s photometric behavior is caused by internal storage of impeded convective flux, rather than by external sources of obscuration such the ISM or circumstellar material.