Astronomers observe unprecedented detail in pulsar 6,500 light-years from Earth

A team of astronomers has performed one of the highest resolution observations in astronomical history by observing two intense regions of radiation, 20 kilometres apart, around a star 6500 light-years away. The observation is equivalent to using a telescope on Earth to see a flea on the surface of Pluto.

The pulsar PSR B1957+20 is seen in the background through the cloud of gas enveloping its brown dwarf star companion
[Credit: Dr. Mark A. Garlick; Dunlap Institute for Astronomy & Astrophysics, University of Toronto]

The extraordinary observation was made possible by the rare geometry and characteristics of a pair of stars orbiting each other. One is a cool, lightweight star called a brown dwarf, which features a "wake" or comet-like tail of gas. The other is an exotic, rapidly spinning star called a pulsar.

"The gas is acting as like a magnifying glass right in front of the pulsar," says Robert Main, lead author of the paper describing the observation being published in the journal Nature. "We are essentially looking at the pulsar through a naturally occurring magnifier which periodically allows us to see the two regions separately."

Main is a PhD astronomy student in the Department of Astronomy & Astrophysics at the University of Toronto, working with colleagues at the University of Toronto's Dunlap Institute for Astronomy & Astrophysics and Canadian Institute for Theoretical Astrophysics, and the Perimeter Institute.

The pulsar is a neutron star that rotates rapidly--over 600 times a second. As the pulsar spins, it emits beams of radiation from the two hotspots on its surface. The intense regions of radiation being observed are associated with the beams.

The brown dwarf star is about a third the diameter of the Sun. It is roughly two million kilometres from the pulsar--or five times the distance between the Earth and the moon--and orbits around it in just over 9 hours. The dwarf companion star is tidally locked to the pulsar so that one side always faces its pulsating companion, the way the moon is tidally locked to the Earth.

Because it is so close to the pulsar, the brown dwarf star is blasted by the strong radiation coming from its smaller companion. The intense radiation from the pulsar heats one side of the relatively cool dwarf star to the temperature of our Sun, or some 6000°C.

The blast from the pulsar could ultimately spell its companion's demise. Pulsars in these types of binary systems are called "black widow" pulsars. Just as a black widow spider eats its mate, it is thought that the pulsar, given the right conditions, could gradually erode gas from the dwarf star until the latter is consumed.

In addition to being an observation of incredibly high resolution, the result could be a clue to the nature of mysterious phenomena known as Fast Radio Bursts, or FRBs.

"Many observed properties of FRBs could be explained if they are being amplified by plasma lenses," say Main. "The properties of the amplified pulses we detected in our study show a remarkable similarity to the bursts from the repeating FRB, suggesting that the repeating FRB may be lensed by plasma in its host galaxy."

Source: Dunlap Institute for Astronomy & Astrophysics [May 23, 2018]

Lightening up dark galaxies

Despite substantial progress over the past half a century in understanding of how galaxies form, important open questions remain regarding how precisely the diffuse gas known as the 'intergalactic medium' is converted into stars. One possibility, suggested in recent theoretical models, is that the early phase of galaxy formation involves an epoch when galaxies contain a great amount of gas but are still inefficient at forming stars. Direct proof of such a 'Dark Phase' has been so far elusive, however --- after all, dark galaxies do not emit much visible light. The observational discovery of such galaxies would therefore fill an important gap in our understanding of galaxy evolution.

One of the new dark-galaxy candidates, identified through a combination of spectral information (left) and images reflecting
the emission of gas (middle) and stars (right). The position of the dark-galaxy candidate is marked by the red circle
[Credit: R. A. Marino/MUSE]

There are ways to bring dark galaxies to lighten up though. An international team led by Dr. Raffaella Anna Marino and Prof. Sebastiano Cantalupo from the Department of Physics at ETH Zurich has now done just that and thus was able to search the sky for potential dark galaxies with unprecedented efficiency. And successfully so, as they report in a paper published in The Astrophysical Journal, they have identified at least six strong candidates for dark galaxies.

To overcome the obstacle that their target objects are dark, the team used a flashlight of sorts, which came in the form of quasars. These emit intense ultraviolet light, which in turn induces fluorescent emission in hydrogen atoms known as the Lyman-alpha line. As a result, the signal from any dark galaxies in the vicinity of the quasar gets a boost, making them visible. Such 'fluorescent illumination' has been used before in searches for dark galaxies, but Marino et al. now looked at the neighbourhood of quasars at greater distances than has been possible in earlier observations.

Also, they acquired the full spectral information for each of the dark-galaxy candidates. Deep observations --- 10 hours for each of the six quasar fields they studied --- enabled Marino and her colleagues to efficiently tell dark-galaxy candidates apart from other sources. From initially 200 Lyman-alpha emitters, half a dozen regions remained that are unlikely to be normal star-forming stellar populations, making them robust candidates for dark galaxies.

The advances in observational capability have become possible thanks to the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. In essence, previous studies were limited to imaging a relative narrow band of frequencies, for which specific filters had to be designed. The MUSE instrument instead allowed hunting 'blindly' -- without filters -- for dark galaxies around quasars at larger distances from Earth than had been possible so far.

Source: ETH Zurich [May 23, 2018]

How small inner moons of Saturn were formed

The small inner moons of Saturn look like giant ravioli and spaetzle. Their spectacular shape has been revealed by the Cassini spacecraft. For the first time, researchers of the University of Bern show how these moons were formed. The peculiar shapes are a natural outcome of merging collisions among similar-sized little moons as computer simulations demonstrate.

Formation of Atlas, one of the small inner moons of Saturn. Its flat, ravioli-like shape is the result of a merging collision 
of two similar-sized bodies. The picture is a snapshot in mid-collision, before the moon’s reorientation 
due to tides is completed [Credit: A. Verdier]

When Martin Rubin, astrophysicist at the University of Bern, saw the images of Saturn’s moons Pan and Atlas on the internet, he was puzzled. The close-ups taken by the Cassini spacecraft in April 2017 showed objects that NASA described in its news release as flying-saucers with diameters of about 30 km. With their large ridges and bulbous centres, Pan and Atlas also resembled giant ravioli. Martin Rubin wondered how these peculiar objects had formed and asked his colleague Martin Jutzi whether they could be the outcome of collisions, similar to the one that formed comet Chury as Jutzi had demonstrated earlier with computer simulations.

The top row shows 3 small moons of Saturn imaged by the Cassini spacecraft. Shown at the bottom are the model outcomes
. The simulations not only reproduce the shapes, but may also explain why the ridges on Pan and Atlas look different from 
the rest of their bodies: They are made of smooth material that was squeezed out during the merging process. Cracks 
on the main body could be the result of tensile stresses caused by the deformation of the merging objects. The modelled 
Prometheus-like moon displays the same tips at both ends as seen on the Cassini images 
[Credit: NASA/JPL-Caltech/Space Science Institute/University of Bern]

Martin Jutzi and Adrien Leleu, both members of the NCCR PlanetS, took the challenge of calculating the formation process of the small inner moons of Saturn. The first, simple tests worked well. “But then, we took the tidal forces into consideration and the problems piled up,” remembers Adrien Leleu. “The conditions close to Saturn are very special,” confirms Martin Jutzi. Since Saturn has 95 times more mass than Earth and the inner moons orbit the planet at a distance of less than half the distance between Earth and Moon, the tides are enormous and pull almost everything apart. Therefore, Saturn’s inner moons couldn’t have formed with these peculiar shapes by gradual accretion of material around a single core. An alternative model called pyramidal regime suggests that these moons were formed by a series of mergers of similar sized little moonlets.

The top image shows Saturn’s large moon Iapetus as observed by Cassini. It has an 
oblate spheroid shape and an equatorial ridge. Bottom: The result of the simulation 
of a head-on merger oft two equal-sized bodies with half of the mass of Iapetus 
[Credit: NASA/JPL/Space Science Institute/University of Bern]

Having solved their initial problems, the researchers could verify the pyramidal regime, but even more: They showed that the collisions of the moonlets resulted in exactly the shapes imaged by Cassini. Close to head-on mergers lead to flattened objects with large equatorial ridges, as observed on Atlas and Pan. With slightly more oblique impact angles, collisions resulted in elongated spaetzle-like shapes that closely resembled the 90-km long moon Prometheus as it was photographed by Cassini.

Head-on collisions have high probability

Based on the current orbit of the moons and their orbital environment, the researchers were able to estimate that the impact velocities were of the order of a few 10 m/s. Simulating collisions in this range for various impact angles, they obtained various stable shapes similar to ravioli and spaetzle, but only for low impact angles. “If the impact angle is bigger than ten degrees, the resulting shapes are not stable anymore,” says Adrien Leleu. Any duck-shaped object like comet Chury would fall apart because of Saturn’s tides. “That is why Saturn’s small moons look very different to comets that often have bilobed shapes,” explains Martin Jutzi.

Collison of similar-sized moonlets orbiting around Saturn [Credit: Simulation by Adrien Leleu, 

Martin Jutzi and Martin Rubin / University of Bern]

Interestingly, the head-on collisions are not as rare as one might think. The small inner moons are believed to originate from Saturn’s rings, a thin disk located in the planet’s equatorial plane. Since Saturn isn’t a perfect sphere but rather oblate, it makes it hard for any object to leave this narrow plane. So, near head-on collisions are frequent and the impact angle tends to get even lower in subsequent encounters. “A significant fraction of such merging collisions take place either at the first encounter or after 1-2 hit-and-run events,” the authors summarize in their paper published in Nature Astronomy. “In this respect, Saturn is almost a toy system to study these processes,” says Martin Rubin.

Although the researchers mainly focussed on the small inner moons of Saturn, they also found a possible explanation for a long-standing mystery concerning Saturn’s third largest moon named Iapetus. Why does Iapetus have an oblate shape and a distinctive equatorial ridge? “Our modelling results suggest that these features may be a result of a merger of similar-sized moons taking place with a close to head-on impact angle, similar to the smaller moons,” the researchers summarize.

Source: University of Bern [May 22, 2018]

First interstellar immigrant discovered in the solar system

A new study has discovered the first known permanent immigrant to our Solar System. The asteroid, currently nestling in Jupiter's orbit, is the first known asteroid to have been captured from another star system. The work is published in Monthly Notices of the Royal Astronomical Society: Letters.

These are images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its
retrograde co-orbital nature. The bright stars and the asteroid (circled in yellow) appear black and the sky white
 in this negative image [Credit: C. Veillet/Large Binocular Telescope Observatory]

The object known as 'Oumuamua was the last interstellar interloper to hit the headlines in 2017. However it was just a tourist passing through, whereas this former exo-asteroid - given the catchy name (514107) 2015 BZ509 - is a long-term resident.

All of the planets in our Solar System, and the vast majority of other objects as well, travel around the Sun in the same direction. However 2015 BZ509 is different - it moves in the opposite direction in what is known as a 'retrograde' orbit.

"How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery," explains Dr Fathi Namouni, lead author of the study. "If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them."

Stellar nursery NGC 604 (NASA/HST), where star systems are closely packed and asteroid exchange is thought
to be possible. Asteroid (514107) 2015 BZ 509 emigrated from its parent star and settled around the Sun
 in a similar environment [Credit: NASA/Hubble Heritage Team (AURA/STScI)]

However the team ran simulations to trace the location of 2015 BZ509 right back to the birth of our Solar System, 4.5 billion years ago when the era of planet formation ended. These show that 2015 BZ509 has always moved in this way, and so could not have been there originally and must have been captured from another system.

"Asteroid immigration from other star systems occurs because the Sun initially formed in a tightly-packed star cluster, where every star had its own system of planets and asteroids," comments Dr Helena Morais, the other member of the team.

"The close proximity of the stars, aided by the gravitational forces of the planets, help these systems attract, remove and capture asteroids from one another."

The discovery of the first permanent asteroid immigrant in the Solar System has important implications for the open problems of planet formation, solar system evolution, and possibly the origin of life itself.

Understanding exactly when and how 2015 BZ509 settled in the Solar System provides clues about the Sun's original star nursery, and about the potential enrichment of our early environment with components necessary for the appearance of life on Earth.

Source: Royal Astronomical Society [May 21, 2018]

Could recent supernovae be responsible for mass extinctions?

Two nearby supernovae that exploded about 2.5 and eight million years ago could have resulted in a staggered depletion of Earth’s ozone layer, leading to a variety of repercussions for life on Earth.

The ultraviolet radiation from a nearby supernova may have resulted in changes in life on Earth
[Credit: David Aguilar (CfA)]

In particular, two-and-a-half million years ago the Earth was changing dramatically. The Pliocene, which was a hot and balmy epoch, was ending and the Pleistocene, an era of repeated glaciation known as the Ice Age, was beginning. Natural variations in Earth’s orbit and wobble likely accounted for the change in climate, but the simultaneous event of a supernova could provide insight on the diversification of life during this epoch.

This supernova is thought to have occurred between 163 and 326 light years away (50–100 parsecs) from Earth. For perspective, our closest stellar neighbor, Proxima Centauri, is 4.2 light years away.

Consequences for Earth

Supernovae can sterilize any nearby inhabited planets that happen to be in the path of their harmful ionizing radiation. Could nearby supernovae wreak havoc on the existing biology of our planet? One researcher wanted to find out. Dr Brian Thomas, an astrophysicist at Washburn University in Kansas, USA, modeled the biological impacts at the Earth’s surface, based on geologic evidence of nearby supernovae 2.5 million and 8 million years ago. In his latest paper, Thomas investigated cosmic rays from the supernovae as they propagated through our atmosphere to the surface, to understand their effect on living organisms.

The globally averaged change in ozone density, as a percent difference at 100 years, 300 years,
and 1000 years after a nearby supernova explosion [Credit: Brian Thomas]

Looking at the fossil record during the Pliocene–Pleistocene boundary (2.5 million years ago), we see a dramatic change in the fossil record and in land cover globally. Thomas tells Astrobiology Magazine that “there were changes, especially in Africa, which went from being more forested to more grassland.” During this time the geologic record shows an elevated global concentration of iron-60 (60Fe), which is a radioactive isotope produced during a supernova.

“We are interested in how exploding stars affect life on Earth, and it turns out a few million years ago there were changesin the things that were living at the time,” says Thomas. “It might have been connected to this supernova.”

For example, there was a change in the abundance of species during the Pliocene–Pleistocene boundary. Although no major mass extinctions happened, there was a higher rate of extinction in general, more speciation and a change in vegetation.

Not quite so deadly

How would a nearby supernova affect life on Earth? Thomas laments that supernovae often are exemplified as “supernova goes off and everything dies”, but that is not quite the case. The answer lies in the atmosphere. Beyond sunscreen, the ozone layer protects all biology from harmful, genetically altering ultraviolet (UV) radiation. Thomas used global climate models, recent atmospheric chemistry models and radiative transfer (the propagation of radiation through the layers of the atmosphere) to better understand how the flux of cosmic rays from supernovae would alter Earth’s atmosphere, specifically the ozone layer.

One of the the last supernovae known to have exploded in our Milky Way Galaxy was the star that left behind the
Cassiopeia A supernova remnant over 300 years ago, which is 11,000 light years away – much too far
 to have affected Earth [Credit: NASA/JPL-Caltech/O. Krause (Steward Observatory)]

One thing to note is that cosmic rays from supernovae would not blast everything in their paths all at once. The intergalactic medium acts as a kind of sieve, slowing down the arrival of cosmic rays and “radioactive iron rain” (60Fe) over hundreds of thousands of years, Thomastells Astrobiology Magazine. Higher energetic particles will reach Earth first and interact with our atmosphere differently than lower energy particles arriving later. Thomas’s study modeled the depletion in ozone 100, 300, and 1,000 years after the initial particles from a supernovabegan penetrating our atmosphere. Interestingly, depletion peaked (at roughly 26 percent) for the 300-year case, beating out the 100-year case.

The high-energy cosmic rays in the 100-year case would zip right through the stratosphere and deposit their energy below the ozone layer, depleting it less, while the less energetic cosmic rays arriving during the 300-year interval would deposit more energy in the stratosphere, depleting ozone significantly.

A decrease in ozone could be a concern for life on the surface. “This work is an important step towards understanding the impact of nearby supernovae on our biosphere,” says Dr Dimitra Atri, a computational physicist at the Blue Marble Space Institute of Science in Seattle, USA.

Mixed effects

Thomas examined several possible biologically-damaging effects (erythema, skin cancer, cataracts, marine phytoplankton photosynthesis inhibition and plant damage) at different latitudes as a result of increased UV radiation resulting from a depleted ozone layer. They showed heightened damage across the board, generally increasing with latitude, which makes sense given the changes we see in the fossil record. However, the effects aren’t equally detrimental to all organisms. Plankton, the primary producers of oxygen, seemed to be minimally affected. The results also suggested a small increase in the risk of sunburn and skin cancer among humans.

So, do nearby supernovae result in mass extinctions? It depends, says Thomas. “There is a subtler shift; instead of a ‘wipe-out everything’, some [organisms] are better off and some are worse off.” For example some plants showed increase yield, like soybean and wheat, while other plants showed reduced productivity.  “It fits,” Thomas states, referring to the change in species in the fossil record.

In the future, Thomas hopes to expand on this work and examine possible linkages between human evolution and supernovae.

Author: Julia Demarines | Source: Astrobiology Magazine [May 18, 2018]

A new map for a birthplace of stars

A Yale-led research group has created the most detailed maps yet of a vast seedbed of stars similar to Earth's Sun.

Gas in the Orion A cloud star-forming region. Each of the three colors (red, green and blue)
represents a different velocity range [Credit: NSF/S. Kong, J. Feddersen, H. Arce
& CARMA-NRO Orion Survey team]

The maps provide unprecedented detail of the structure of the Orion A molecular cloud, the closest star-forming region of high-mass stars. Orion A hosts a variety of star-forming environments, including dense star clusters similar to the one where Earth's Sun is believed to have formed.

"Our maps probe a wide range of physical scales needed to study how stars form in molecular clouds, and how young stars impact their parent cloud," said Yale postdoctoral associate Shuo Kong, first author of a study about the group's research that appears in the Astrophysical Journal Supplement.

The research team includes astronomers from institutions in the U.S., Chile, Japan, France, Germany, Spain, and the U.K. The team's principal investigators are Yale astronomy professor Héctor G. Arce, ALMA Observatory scientist John Carpenter, and Caltech astronomy professor Anneila Sargent.

Kong said the team constructed its maps of the Orion A cloud by combining data from a single-dish telescope and an interferometer. The Yale Center for Research Computing assisted in handling the large dataset and producing the images.

The dataset and maps are collectively known as the CARMA-NRO Orion Survey. The name refers to the Combined Array for Research in Millimeter Astronomy (CARMA), an interferometer that was located in California, and the Nobeyama Radio Observatory (NRO) telescope, in Japan.

"Our survey is a unique combination of data from two very different telescopes," said Yale graduate student Jesse Feddersen, a co-author of the study. "We have combined the zoom of CARMA with the wide-angle of NRO to simultaneously capture the details of individual forming stars and the overall shape and motions of the giant molecular cloud."

In addition, the maps will help researchers calibrate star formation models for extragalactic studies. "The data we provide here will benefit research on a broad range of evolutionary stages of the star formation process and on the environment stars form," Arce said.

Yale graduate student María José Maureira is also a co-author of the study.

"The combined observations are a great help for astronomers seeking to understand how fast and efficiently stars form. For example, their maps show the energy released by high-mass stars has a strong impact on the cloud environment," said Glen Langston, program director at the National Science Foundation. The research was supported by the National Science Foundation.

Source: Yale University [May 18, 2018]