ST/ Origin of ultra-diffuse galaxies explained

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Paradigm

32 min readSep 8, 2021

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Space biweekly vol.34, 26th August — 8th September

TL;DR

Ultra-diffuse galaxies, or UDGs, are dwarf galaxies whose stars are spread out over a vast region, resulting in extremely low surface brightness, making them very difficult to detect. An international team of astronomers reports it has used sophisticated simulations to detect a few ‘quenched’ UDGs in low-density environments in the universe. A quenched galaxy is one that does not form stars.
New evidence suggests that white dwarfs could continue to burn hydrogen in the final stages of their lives, causing them to appear more youthful than they actually are. This discovery could have consequences for how astronomers measure the ages of star clusters.
Researchers have uncovered the physical and chemical effects of the impact of a protostellar jet in the interior of the Orion Nebula. The observations show evidence of compression and heating produced by the shock front, and the destruction of dust grains, which cause a dramatic increase in the gas phase abundance of the atoms of iron, nickel, and other heavy elements in the Orion Nebula.
The Very Large Array Sky Survey gave astronomers the first clue that ultimately revealed a dramatic story — the remnant of a star that exploded long ago had plunged into the core of its companion star causing it, too, to explode as a supernova.
A human space mission would be viable if it doesn’t exceed four years, an international research team concludes in new research.
The asteroid Vesta, like Earth, is composed of rock in its crust and mantle, and it has an iron core. Therefore, studying Vesta helps us understand the very early days of our planetary neighborhood and how our own planet formed.
Researchers combined observations and modeling to infer the distribution of cold planets in the Milky Way. The results suggest that this distribution is not strongly dependent on the distance from the galactic center. Cold planets seem to be present throughout our galaxy, even in the galactic bulge, where their existence was uncertain. The findings could improve our understanding of both planetary formation and its history in the Milky Way.
Using a supercomputer simulation, a research team has succeeded in following the development of a galaxy over a span of 13.8 billion years. The study shows how, due to interstellar frontal collisions, young and chaotic galaxies over time mature into spiral galaxies.
Brown dwarfs are astronomical objects with masses between those of planets and stars. The question of where exactly the limits of their mass lie remains a matter of debate, especially since their constitution is very similar to that of low-mass stars. Scientists has identified five objects that have masses near the border separating stars and brown dwarfs that could help scientists understand the nature of these mysterious objects.
Theoretical astrophysicist Kevin Heng has derived novel solutions to an old mathematical problem needed to calculate light reflections from planets and moons. Now, data can be interpreted in a simple way to understand planetary atmospheres. The new formulae will likely be incorporated into future textbooks.
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Analysts at Morgan Stanley and Goldman Sachs have predicted that economic activity in space will become a multi-trillion-dollar market in the coming decades. Morgan Stanley’s Space Team estimates that the roughly USD 350 billion global space industry could surge to over USD 1 trillion by 2040.

Source: Satellite Industry Association, Morgan Stanley Research, Thomson Reuters. *2040 estimates

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Quiescent ultra-diffuse galaxies in the field originating from backsplash orbits

by José A. Benavides, Laura V. Sales, Mario. G. Abadi, Annalisa Pillepich, Dylan Nelson, Federico Marinacci, Michael Cooper, Ruediger Pakmor, Paul Torrey, Mark Vogelsberger & Lars Hernquist in Nature Astronomy

As their name suggests, ultradiffuse galaxies, or UDGs, are dwarf galaxies whose stars are spread out over a vast region, resulting in extremely low surface brightness, making them very difficult to detect. Several questions about UDGs remain unanswered: How did these dwarfs end up so extended? Are their dark matter halos — the halos of invisible matter surrounding the galaxies — special?

Now an international team of astronomers, co-led by Laura Sales, an astronomer at the University of California, Riverside, reports that it has used sophisticated simulations to detect a few “quenched” UDGs in low-density environments in the universe. A quenched galaxy is one that does not form stars.

“What we have detected is at odds with theories of galaxy formation since quenched dwarfs are required to be in clusters or group environments in order to get their gas removed and stop forming stars,” said Sales, an associate professor of physics and astronomy. “But the quenched UDGs we detected are isolated. We were able to identify a few of these quenched UDGs in the field and trace their evolution backward in time to show they originated in backsplash orbits.”

On the left, one of the ultra-diffuse galaxies that was analyzed in the simulation. On the right, the image of the DF2 galaxy, which is almost transparent. (ESA/Hubble)

Here, “in the field” refers to galaxies isolated in quieter environments and not in a group or cluster environment. Sales explained that a backsplash galaxy is an object that looks like an isolated galaxy today but in the past was a satellite of a more massive system — similar to a comet, which visits our sun periodically, but spends the bulk of its journey in isolation, far from most of the solar system.

“Isolated galaxies and satellite galaxies have different properties because the physics of their evolution is quite different,” she said. “These backsplash galaxies are intriguing because they share properties with the population of satellites in the system to which they once belonged, but today they are observed to be isolated from the system.”

Dwarf galaxies are small galaxies that contain anywhere from 100 million to a few billion stars. In contrast, the Milky Way has 200 billion to 400 billion stars. While all UDGs are dwarf galaxies, all dwarf galaxies are not UDGs. For example, at similar luminosity, dwarfs show a very large range of sizes, from compact to diffuse. UDGs are the tail end of most extended objects at a given luminosity. A UDG has the stellar content of a dwarf galaxy, 10–100 times smaller than the Milky Way. But its size is comparable to the Milky Way, giving it the extremely low surface brightness that makes it special.

Sales explained that the dark matter halo of a dwarf galaxy has a mass at least 10 times smaller than the Milky Way, and the size scales similarly. UDGs, however, break this rule and show a radial extension comparable to that of much larger galaxies.

“One of the popular theories to explain this was that UDGs are ‘failed Milky Ways,’ meaning they were destined to be galaxies like our own Milky Way but somehow failed to form stars,” said José A. Benavides, a graduate student at the Institute of Theoretical and Experimental Astronomy in Argentina and the first author of the research paper. “We now know that this scenario cannot explain all UDGs. So theoretical models are arising where more than one formation mechanism may be able to form these ultradiffuse objects.”

*The image shows the fall of a blue ultra diffuse galaxy into a galaxy system and its subsequent ejection as a red ultra diffuse galaxy (having already lost its gas). (Vanina Rodriguez)*

According to Sales, the value of the new work is twofold. First, the simulation used by the researchers, called TNG50, successfully predicted UDGs with characteristics similar to observed UDGs. Second, the researchers found a few rare quenched UDGs for which they have no formation mechanism.

“Using TNG50 as a ‘time machine’ to see how the UDGs got to where they are, we found these objects were satellites several billion years before but got expelled into a very elliptical orbit and look isolated today,” she said.

The researchers also report that according to their simulations, quenched UDGs can commonly make up 25% of an ultradiffuse population of galaxies. In observations, however, this percentage is much smaller.

“This means a lot of dwarf galaxies lurking in the dark may have remained undetected to our telescopes,” Sales said. “We hope our results will inspire new strategies for surveying the low-luminosity universe, which would allow for a complete census of this population of dwarf galaxies.”

The study is the first to resolve the myriad of environments — from isolated dwarfs to dwarfs in groups and clusters — necessary to detect UDGs, and with high-enough resolution to study their morphology and structure. Next, the research team will continue its study of UDGs in TNG50 simulations to better understand why these galaxies are so extended compared to other dwarf galaxies with the same stellar content. The researchers will use the Keck Telescope in Hawaii, one of the most powerful telescopes in the world, to measure the dark matter content of UDGs in the Virgo cluster, the closest galaxy cluster to Earth.

“Future telescopes, such as the Large Synoptic Survey Telescope or the Roman Space Telescope, come online in the next five to 10 years with capabilities of detecting many more of these intriguing UDGs,” Sales said.

Slowly cooling white dwarfs in M13 from stable hydrogen burning

by Chen, J., Ferraro, F.R., Cadelano, M. et al. in Nature Astronomy

The prevalent view of white dwarfs as inert, slowly cooling stars has been challenged by observations from the NASA/ESA Hubble Space Telescope. An international group of astronomers have discovered the first evidence that white dwarfs can slow down their rate of ageing by burning hydrogen on their surface.
“We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,” explained Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research. “This was quite a surprise, as it is at odds with what is commonly believed.”

White dwarfs are the slowly cooling stars which have cast off their outer layers during the last stages of their lives. They are common objects in the cosmos; roughly 98% of all the stars in the Universe will ultimately end up as white dwarfs, including our own Sun. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well.

To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity but the populations of stars which will eventually give rise to white dwarfs are different. In particular, the overall colour of stars at an evolutionary stage known as the Horizontal Branch are bluer in M13, indicating a population of hotter stars. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool.

“The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters,” continued Chen. “This allowed us to really contrast how stars evolve in M3 and M13.”

Using Hubble’s Wide Field Camera 3 the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two clusters. They found that M3 contains standard white dwarfs which are simply cooling stellar cores. M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those which have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly.

Comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that roughly 70% of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling.

WD cooling time for models with and without hydrogen burning.

This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way. The evolution of white dwarfs has previously been modelled as a predictable cooling process. This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters. However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years.

“Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old,” added Francesco Ferraro of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who coordinated the study. “We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly”.

A transient radio source consistent with a merger-triggered core collapse supernova

by D. Z. Dong, G. Hallinan, E. Nakar, A. Y. Q. Ho, A. K. Hughes, K. Hotokezaka, S. T. Myers, K. De, K. P. Mooley, V. Ravi, A. Horesh, M. M. Kasliwal, S. R. Kulkarni in Science

Astronomers have found dramatic evidence that a black hole or neutron star spiraled its way into the core of a companion star and caused that companion to explode as a supernova. The astronomers were tipped off by data from the Very Large Array Sky Survey (VLASS), a multi-year project using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA).

“Theorists had predicted that this could happen, but this is the first time we’ve actually seen such an event,” said Dillon Dong, a graduate student at Caltech and lead author on a paper.

The first clue came when the scientists examined images from VLASS, which began observations in 2017, and found an object brightly emitting radio waves but which had not appeared in an earlier VLA sky survey, called Faint Images of the Radio Sky at Twenty centimeters (FIRST). They made subsequent observations of the object, designated VT 1210+4956, using the VLA and the Keck telescope in Hawaii. They determined that the bright radio emission was coming from the outskirts of a dwarf, star-forming galaxy some 480 million light-years from Earth. They later found that an instrument aboard the International Space Station had detected a burst of X-rays coming from the object in 2014.

The data from all these observations allowed the astronomers to piece together the fascinating history of a centuries-long death dance between two massive stars. Like most stars that are much more massive than our Sun, these two were born as a binary pair, closely orbiting each other. One of them was more massive than the other and evolved through its normal, nuclear fusion-powered lifetime more quickly and exploded as a supernova, leaving behind either a black hole or a superdense neutron star.

Probable electron-capture supernova SN 2018zd within the galaxy NGC 2146.

The black hole or neutron star’s orbit grew steadily closer to its companion, and about 300 years ago it entered the companion’s atmosphere, starting the death dance. At this point, the interaction began spraying gas away from the companion into space. The ejected gas, spiraling outward, formed an expanding, donut-shaped ring, called a torus, around the pair.

Eventually, the black hole or neutron star made its way inward to the companion star’s core, disrupting the nuclear fusion producing the energy that kept the core from collapsing of its own gravity. As the core collapsed, it briefly formed a disk of material closely orbiting the intruder and propelled a jet of material outward from the disk at speeds approaching that of light, drilling its way through the star.

“That jet is what produced the X-rays seen by the MAXI instrument aboard the International Space Station, and this confirms the date of this event in 2014,” Dong said.

The collapse of the star’s core caused it to explode as a supernova, following its sibling’s earlier explosion.

“The companion star was going to explode eventually, but this merger accelerated the process,” Dong said.

The material ejected by the 2014 supernova explosion moved much faster than the material thrown off earlier from the companion star, and by the time VLASS observed the object, the supernova blast was colliding with that material, causing powerful shocks that produced the bright radio emission seen by the VLA.

“All the pieces of this puzzle fit together to tell this amazing story,” said Gregg Hallinan of Caltech. “The remnant of a star that exploded a long time ago plunged into its companion, causing it, too, to explode,” he added.

The key to the discovery, Hallinan said, was VLASS, which is imaging the entire sky visible at the VLA’s latitude — about 80 percent of the sky — three times over seven years. One of the objectives of doing VLASS that way is to discover transient objects, such as supernova explosions, that emit brightly at radio wavelengths. This supernova, caused by a stellar merger, however, was a surprise.

“Of all the things we thought we would discover with VLASS, this was not one of them,” Hallinan said.

Age relationships of large-scale troughs and impact basins on Vesta

by Hiu Ching Jupiter Cheng, Christian Klimczak, Caleb I. Fassett in Icarus

The asteroid Vesta is the second largest asteroid in our solar system. With a diameter of about 330 miles, it orbits the sun between the planets Mars and Jupiter.
“When we think of asteroid belts, we probably picture Han Solo maneuvering the millennium falcon through a dense set of irregularly shaped gray rocks in space,” Christian Klimczak, associate professor in the Franklin College of Arts and Sciences department of geology. “While most rocks are indeed irregularly shaped and gray, they are far apart and NASA’s Dawnspacecraft did not have to maneuver around any other asteroids to reach and explore Vesta.”

*Image of* *Asteroid* *4 Vesta, showing the locations of the Divalia Fossae and Saturnalia Fossae, and the Veneneia and Rheasilvia impact basins. The image was captured by the NASA Dawn mission on 24 July 2011.*

Dawnwas the space probe launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt, Vesta and Ceres.

Vesta, like Earth, is composed of rock in its crust and mantle, and it has an iron core. Because of its large size (for an asteroid) and because Vesta has a crust, mantle and core, it is considered a planetesimal. Planetesimals are building blocks out of which planets form. Earth formed by accretion of several such planetesimals. “Vesta was on the way to becoming an Earth-like planet, too, but planet formation stopped along the way there early in the history of our solar system,” Klimczak said. “Therefore, studying Vesta helps us understand the very early days of our planetary neighborhood and how our own planet formed.” Klimczak is co-author on a new study that examines the large-scale troughs and impact basins on Vesta.

*Schematic diagram illustrating the* *crater* *counting procedure for Divalia and Saturnalia Fossae on Vesta. (a) Step 1 is to map the count areas, which are defined by the two trough-bounding* *scarps*. (b) Step 2 involves counting only those craters that directly superpose the troughs. Craters are not included if they are not superposed on the trough, that is, if they are crosscut by the scarp or if they are outside the count areas.

Vesta was hit by two other large asteroids which left large impact craters so big they cover most of the southern hemisphere of Vesta. These impacts are thought to have ejected rocky material into space. Some of these rocks reached Earth as meteorites so scientists now have actual rock samples from Vesta to study its geochemistry.

“Rock properties are influenced by environmental conditions like surrounding stresses and the presence of water,” said Jupiter Cheng, doctoral candidate in the department of geography and co-author on the study. “Since Vesta is much smaller than Earth, or even the moon, it has a weaker gravity, and rock would deform differently near the surface than what we see on Earth.”

According to Cheng, one big question is what triggered the formation of these large troughs. The two troughs are concentric around the two massive impact basins, Rheasilvia and Veneneia, respectively, and widely considered to be simultaneously formed by the impact events, though this assumed age relationship has never been tested before.

*Maps of* *crater* *count areas and craters of the Divalia Fossae displayed in Dawn* FC images colour-coded by elevation. (a) Map showing count areas outlined by white solid lines with a total area of 22,016 km2 (topographically corrected). (b) Crater count map with the certain craters outlined in black (n = 2491) and marked (uncertain) craters outlined in yellow (n = 887).

“Our work used crater counting methods to explore the relative age of the basins and troughs,” Cheng said. Crater counting is a common method for estimating the age of a planet’s surface, a method based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known.

“Consequently, counting the number of craters of various sizes in a given area allows us to determine how long they have accumulated and, consequently, how long ago the surface formed,” she said. “Our result shows that the troughs and basins have a similar number of the crater of various sizes, indicating they share a similar age. However, the uncertainties associated with the crater counts allow for the troughs to have formed well after the impacts.

The origin of the troughs has long been a point of conjecture within the scientific community. Klimczak hopes their new geologic evidence can promote a more-durable theory about the troughs on Vesta.

*Crater* size-frequency distributions of Saturnalia Fossae with spatial randomness analyses (this study), the entire Rheasilvia basin floor, and the top of the Rheasilvia basin peak. (a) Differential crater size-frequencies of the Saturnalia Fossae with certain craters only (red squares) and all craters including those marked as uncertain (orange squares), the top of the Rheasilvia basin peak (blue circle), and the entire basin floor (green cross) are plotted against the mean crater diameter of each bin. (b) The relative crater size-frequency distribution for the Saturnalia Fossae with the same symbology as in (a). The lunar-derived production functions for 3.7 Ga are also plotted as grey lines in both of the plots. Error bars are of 90% confidence in Poisson counting statistics. (c)The Mean 2nd Closest Neighbor Distance (M2CND) spatial randomness analyses results are plotted for Saturnalia Fossae with the same symbology as in (a) shown in standard deviations above or below the Monte-Carlo-derived means.

“The leading hypothesis suggests that these troughs are fault-bounded valleys with a distinct scarp on each side that together mark the down-drop (sliding) of a block of rock. However, rock can also crack apart and form such troughs, an origin that has not been considered before,” said Cheng, who is investigating the origin of the troughs as part of her dissertation at UGA.
“Our calculations also show that Vesta’s gravity is not enough to induce surrounding stresses favorable for sliding to occur at shallow depths, instead, the physics shows that rocks there are favored to crack apart,” she said. “Therefore, the formation of these troughs must involve the opening of cracks, which is inconsistent with the leading hypothesis in the scientific community. Taken all together, the overall project provides alternatives to the previously proposed trough origin and geological history of Vesta, results that are also important for understanding similar landforms on other small planetary bodies elsewhere in the solar system.”

Photoionized Herbig–Haro Objects in the Orion Nebula through Deep High Spectral Resolution Spectroscopy. II. HH 204

by J. E. Méndez-Delgado, W. J. Henney, C. Esteban, J. García-Rojas, A. Mesa-Delgado, K. Z. Arellano-Córdova in The Astrophysical Journal

An international team led by researchers from the Instituto de Astrofísica de Canarias (IAC) has uncovered, with an new high degree of detail, the physical and chemical effects of the impact of a protostellar jet in the interior of the Orion Nebula. The study was made using observations with the Very Large Telescope (VLT) and 20 years of images with the Hubble Space Telescope (HST). The observations show evidence of compression and heating produced by the shock front, and the destruction of dust grains, which cause a dramatic increase in the gas phase abundance of the atoms of iron, nickel, and other heavy elements in the Orion Nebula.

The Orion Nebula, one of the known and brightest objects in the night sky, is the nearest region of massive star formation to Earth, and it has a complex and extensive gas structure. Some of the newborn stars within it emit jets of gas at high speed which, when they impact their surroudings, produce shock fronts which compress and heat the nebular gas. These impact zones are bow-shaped, and are called Herbig-Haro objects, after their discoverers, the US astonomer George Herbig, and the Mexican astronomer Guillermo Haro.

These objects have been observed previously in many dark nebulae, where the cold gas is neutral, and its main source of energy is the heat generated by the shock. However, the jets of gas in the Orion Nebula are immersed in a large radiation field produced by the most massive stars in the Trapezium of Orion, situated at the centre of the nebula. Due to this radiation the gas within the shock front and also the gas compressed after it has passed through, is warm and ionized, and this allows us to measure precisely the physical conditions and the chemical composition of the jet.

The left panel shows the Orion Nebula observed with the Hubble Space Telescope, picking out the area around HH204. In the right panel, we can see in detail the structure of HH204 and of its apparent companion, HH203. In this panel, the images by the Hubble Space Telescope taken during 20 years and artificially highlighted with different colours show the advance of the jets of gas through the Orion Nebula. Credit: Gabriel Pérez Díaz, SMM (IAC)

The research carried out by a team of astronomers in Spain, Mexico and the United States, led by José Eduardo Méndez Delgado, a doctoral student at the IAC and the University of La Laguna (ULL), has uncovered the complex relations between the ionic abundances of the gas and its physical conditions in HH204, one of the most prominent Herbig-Haro objects in the Orion Nebula.

“Our work shows that the in the shock front of HH204 the gas abundances of heavy elements such as iron and nickel are increased by up to 350% compared to the values usually found in the Orion Nebula, and this allows us to determine the proportion of other chemical elements more accurately, which contributes to an improved knowledge of the chemical evolution in the solar neighbourhood,” explains José Eduardo Méndez Delgado, the first author of the article.
“As well as the heavy element enrichment in the gas phase, we have observed a heated post-shock zone which comprises a very small fraction of the gas, and which lets us understand the different layers of the structure of the Herbig-Haro object generated by the impact of the shock front,” says César Esteban, and IAC researcher and a co-author of the article.

“The origin of HH204 appears to be associated with one of the most brilliant and star formation rich zones of the Orion Nebula, the regions called Orion South, although there are many interactions of gas which appear to feed it from several directions,” adds William Henney, a researcher at the Institute of Radioastronomy and Astrophysics at the National Autonomous University of Mexico, and a co-author of the article.

“Thanks to the images of the Hubble Space Telescope we have shown that HH204 is propagating at an angle of 32º with the plane of the sky, which lets us observe the compression of the gas transversely as we approach the shock front,” points out Karla Arellano Córdova, a researcher at the University of Texas at Austin, and a co-author of the article.

“We have seen that the impact of these objects can be important when determining the local physical conditions in ionized nebulae. In fact, if we don’t take these effects into account we can make incorrect determinations of the chemical composition of the ionized nebulae, which are fundamental techniques for understanding the chemical evolution of the Universe,” sums up Jorge García Rojas, an IAC researcher and a co-author of the article.

Beating 1 Sievert: Optimal Radiation Shielding of Astronauts on a Mission to Mars

by M.I. Dobynde, Y.Y. Shprits, A.Yu. Drozdov, J. Hoffman, J. Li in Space Weather

Sending human travelers to Mars would require scientists and engineers to overcome a range of technological and safety obstacles. One of them is the grave risk posed by particle radiation from the sun, distant stars and galaxies.

Answering two key questions would go a long way toward overcoming that hurdle: Would particle radiation pose too grave a threat to human life throughout a round trip to the red planet? And, could the very timing of a mission to Mars help shield astronauts and the spacecraft from the radiation?

In a new article, an international team of space scientists, including researchers from UCLA, answers those two questions with a “no” and a “yes.”

(a) Differential energy spectra of particle flux of various particle species in the interplanetary space near the Earth: galactic cosmic ray particles according to Matthiä, Berger, Mrigakshi, and Reitz (*2013): hydrogen and helium (red curve) and heavy ions (blue curve); the proton flux due to the solar particle event (Evaluated Solar Energetic Particle Data,* *2018*): September 1997 (cyan curve), October 2003 (green curve), and January 2005 (black curve). (b) Illustration of numeric simulations. The spherical aluminum shell (gray) and the spherical water phantom inside it (turquoise) are illuminated with unidirectional 3 GeV protons (blue), which are generated on a circular planar source on the left. Red dots show particle-matter interaction points. Secondary gamma-rays are shown in cyan, neutrons in yellow, protons on blue, leptons in red, and pions in magenta.

That is, humans should be able to safely travel to and from Mars, provided that the spacecraft has sufficient shielding and the round trip is shorter than approximately four years. And the timing of a human mission to Mars would indeed make a difference: The scientists determined that the best time for a flight to leave Earth would be when solar activity is at its peak, known as the solar maximum.

The scientists’ calculations demonstrate that it would be possible to shield a Mars-bound spacecraft from energetic particles from the sun because, during solar maximum, the most dangerous and energetic particles from distant galaxies are deflected by the enhanced solar activity.

A trip of that length would be conceivable. The average flight to Mars takes about nine months, so depending on the timing of launch and available fuel, it is plausible that a human mission could reach the planet and return to Earth in less than two years, according to Yuri Shprits, a UCLA research geophysicist and co-author of the paper.

“This study shows that while space radiation imposes strict limitations on how heavy the spacecraft can be and the time of launch, and it presents technological difficulties for human missions to Mars, such a mission is viable,” said Shprits, who also is head of space physics and space weather at GFZ Research Centre for Geosciences in Potsdam, Germany.

*Dependence of the net effective dose accumulated during an interplanetary flight on the mission duration (x axis) and launch date (y axis).*

The researchers recommend a mission not longer than four years because a longer journey would expose astronauts to a dangerously high amount of radiation during the round trip — even assuming they went when it was relatively safer than at other times. They also report that the main danger to such a flight would be particles from outside of our solar system.

Shprits and colleagues from UCLA, MIT, Moscow’s Skolkovo Institute of Science and Technology and GFZ Potsdam combined geophysical models of particle radiation for a solar cycle with models for how radiation would affect both human passengers — including its varying effects on different bodily organs — and a spacecraft. The modeling determined that having a spacecraft’s shell built out of a relatively thick material could help protect astronauts from radiation, but that if the shielding is too thick, it could actually increase the amount of secondary radiation to which they are exposed.

The two main types of hazardous radiation in space are solar energetic particles and galactic cosmic rays; the intensity of each depends on solar activity. Galactic cosmic ray activity is lowest within the six to 12 months after the peak of solar activity, while solar energetic particles’ intensity is greatest during solar maximum, Shprits said.

No Large Dependence of Planet Frequency on Galactocentric Distance

by Naoki Koshimoto, David P. Bennett, Daisuke Suzuki, Ian A. Bond in The Astrophysical Journal Letters

Although thousands of planets have been discovered in the Milky Way, most reside less than a few thousand light years from Earth. Yet our Galaxy is more than 100,000 light years across, making it difficult to investigate the Galactic distribution of planets. But now, a research team has found a way to overcome this hurdle.

In a study, researchers led by Osaka University and NASA have used a combination of observations and modeling to determine how the planet-hosting probability varies with the distance from the Galactic center.

The observations were based on a phenomenon called gravitational microlensing, whereby objects such as planets act as lenses, bending and magnifying the light from distant stars. This effect can be used to detect cold planets similar to Jupiter and Neptune throughout the Milky Way, from the Galactic disk to the Galactic bulge — the central region of our Galaxy.

“Gravitational microlensing currently provides the only way to investigate the distribution of planets in the Milky Way,” says Daisuke Suzuki, co-author of the study. “But until now, little is known mainly because of the difficulty in measuring the distance to planets that are more than 10,000 light years from the Sun.”

Spiral galaxy illustration (stock image). *Credit: Alexandr Mitiuc / stock.adobe.com*

To solve this problem, the researchers instead considered the distribution of a quantity that describes the relative motion of the lens and distant light source in planetary microlensing. By comparing the distribution observed in microlensing events with that predicted by a Galactic model, the research team could infer the Galactic distribution of planets.

The results show that the planetary distribution is not strongly dependent on the distance from the Galactic center. Instead, cold planets orbiting far from their stars seem to exist universally in the Milky Way. This includes the Galactic bulge, which has a very different environment to the solar neighborhood, and where the presence of planets has long been uncertain.

“Stars in the bulge region are older and are located much closer to each other than stars in the solar neighborhood,” explains lead author of the study Naoki Koshimoto. “Our finding that planets reside in both these stellar environments could lead to an improved understanding of how planets form and the history of planet formation in the Milky Way.”

According to the researchers, the next step should be to combine these results with measurements of microlens parallax or lens brightness — two other important quantities associated with planetary microlensing.

Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

by Nolan Grieves, François Bouchy, Monika Lendl, Theron Carmichael, Ismael Mireles, Avi Shporer, Kim K. McLeod, Karen A. Collins, Rafael Brahm, et al. in Astronomy & Astrophysics

Brown dwarfs are astronomical objects with masses between those of planets and stars. The question of where exactly the limits of their mass lie remains a matter of debate, especially since their constitution is very similar to that of low-mass stars. So how do we know whether we are dealing with a brown dwarf or a very low mass star? An international team, led by scientists from the University of Geneva (UNIGE) and the Swiss National Centre of Competence in Research (NCCR) PlanetS, in collaboration with the University of Bern, has identified five objects that have masses near the border separating stars and brown dwarfs that could help scientists understand the nature of these mysterious objects.

Like Jupiter and other giant gas planets, stars are mainly made of hydrogen and helium. But unlike gas planets, stars are so massive and their gravitational force so powerful that hydrogen atoms fuse to produce helium, releasing huge amounts of energy and light.

Brown dwarfs, on the other hand, are not massive enough to fuse hydrogen and therefore cannot produce the enormous amount of light and heat of stars. Instead, they fuse relatively small stores of a heavier atomic version of hydrogen: deuterium. This process is less efficient and the light from brown dwarfs is much weaker than that from stars. This is why scientists often refer to them as ‘failed stars’.

SOAR/HRCam speckle interferometry imaging with I-band autocorrelation functions for TOI-148 (TIC 393 940 766; top left), TOI-587 (TIC 294 090 620; top right), TOI-681 (TIC 410 450 228; middle left), TOI 746 (TIC 167 418 903; middle right), and TOI 1213 (TIC 399 144 800; bottom). The 5σ contrast curves with a linear fit are shown with black solid lines. The auto-correlation functions obtained in I-band are shown within the contrast curve plots.

“However, we still do not know exactly where the mass limits of brown dwarfs lie, limits that allow them to be distinguished from low-mass stars that can burn hydrogen for many billions of years, whereas a brown dwarf will have a short burning stage and then a colder life,” points out Nolan Grieves, a researcher in the Department of Astronomy at the UNIGE’s Faculty of Science, a member of the NCCR PlanetS and the study’s first author. “These limits vary depending on the chemical composition of the brown dwarf, for example, or the way it formed, as well as its initial radius,” he explains. To get a better idea of what these mysterious objects are, we need to study examples in detail. But it turns out that they are rather rare. “So far, we have only accurately characterised about 30 brown dwarfs,” says the Geneva-based researcher. Compared to the hundreds of planets that astronomers know in detail, this is very few. All the more so if one considers that their larger size makes brown dwarfs easier to detect than planets.

Today, the international team characterized five companions that were originally identified with the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOI) — TOI-148, TOI-587, TOI-681, TOI-746 and TOI-1213. These are called ‘companions’ because they orbit their respective host stars. They do so with periods of 5 to 27 days, have radii between 0.81 and 1.66 times that of Jupiter and are between 77 and 98 times more massive. This places them on the borderline between brown dwarfs and stars.

Radius-mass diagram for the 54 brown dwarfs and low-mass stars. The five companions presented in this work are highlighted in red. The gray vertical dashed lines display the 13 and 80 MJup approximate boundaries of the brown dwarf regime. The colored lines display isochrone models from Baraffe et al. (2003, *2015) for low mass stars and substellar objects at solar metallicity with ages of 0.1, 0.5, 1, 5, and 10 Gyr. The histogram at the top displays relative occurrence of these transiting objects.*

These five new objects therefore contain valuable information. “Each new discovery reveals additional clues about the nature of brown dwarfs and gives us a better understanding of how they form and why they are so rare,” says Monika Lendl, a researcher in the Department of Astronomy at the UNIGE and a member of the NCCR PlanetS.

One of the clues the scientists found to show these objects are brown dwarfs is the relationship between their size and age, as explained by François Bouchy, professor at UNIGE and member of the NCCR PlanetS: “Brown dwarfs are supposed to shrink over time as they burn up their deuterium reserves and cool down. Here we found that the two oldest objects, TOI 148 and 746, have a smaller radius, while the two younger companions have larger radii.”

Yet these objects are so close to the limit that they could just as easily be very low-mass stars, and astronomers are still unsure whether they are brown dwarfs. “Even with these additional objects, we still lack the numbers to draw definitive conclusions about the differences between brown dwarfs and low-mass stars. Further studies are needed to find out more,” concludes Grieves.

VINTERGATAN — I. The origins of chemically, kinematically, and structurally distinct discs in a simulated Milky Way-mass galaxy

by Oscar Agertz, Florent Renaud, Sofia Feltzing, Justin I Read, Nils Ryde, Eric P Andersson, Martin P Rey, Thomas Bensby, Diane K Feuillet in Monthly Notices of the Royal Astronomical Society

Using a supercomputer simulation, a research team at Lund University in Sweden has succeeded in following the development of a galaxy over a span of 13.8 billion years. The study shows how, due to interstellar frontal collisions, young and chaotic galaxies over time mature into spiral galaxies such as the Milky Way.

Mock HST/ACS and XMM images of the main galaxy at z = 3:5; 1:3; 0:8 and 0.2.

Soon after the Big Bang 13.8 billion years ago, the Universe was an unruly place. Galaxies constantly collided. Stars formed at an enormous rate inside gigantic gas clouds. However, after a few billion years of intergalactic chaos, the unruly, embryonic galaxies became more stable and over time matured into well-ordered spiral galaxies. The exact course of these developments has long been a mystery to the world’s astronomers. However, in a new study, researchers have been able to provide some clarity on the matter.

“Using a supercomputer, we have created a high-resolution simulation that provides a detailed picture of a galaxy’s development since the Big Bang, and how young chaotic galaxies transition into well-ordered spirals” says Oscar Agertz, astronomy researcher at Lund University.

Large scale (140 kpc across) gaseous environment around the galaxy at, from left to right, z = 1:9; 1:5; 1:3 and 1:0. From top to bottom, the rows show density-weighted average gas densities, [=Fe] and [Fe=H] along the line of sight. The [=Fe]-maps have their (diverging) colourmaps centered around [=Fe]= 0:39, the approximate divide between the high-and low-[=Fe] stellar sequences.

In the study, the astronomers, led by Oscar Agertz and Florent Renaud, use the Milky Way’s stars as a starting point. The stars act as time capsules that divulge secrets about distant epochs and the environment in which they were formed. Their positions, speeds and amounts of various chemical elements can therefore, with the assistance of computer simulations, help us understand how our own galaxy was formed.

“We have discovered that when two large galaxies collide, a new disc can be created around the old one due to the enormous inflows of star-forming gas. Our simulation shows that the old and new discs slowly merged over a period of several billion years. This is something that not only resulted in a stable spiral galaxy, but also in populations of stars that are similar to those in the Milky Way,” says Florent Renaud, astronomy researcher at Lund University.

Mono abundance populations colour-coded by vertical velocity dispersion (top row), scaleheight (middle row) and average stellar age (bottom row) at R = 3 5 kpc (left), R = 7 9 kpc (middle) and R = 11 13 kpc (right). Broadly the galaxy features an old, kinematically hot, high-[ =Fe] , thick disc, as well as a young, kinematically cold, low-[=Fe] thin disc.

The new findings will help astronomers to interpret current and future mappings of the Milky Way. The study points to a new direction for research in which the main focus will be on the interaction between large galaxy collisions and how spiral galaxies’ discs are formed. The research team in Lund has already started new super computer simulations in cooperation with the research infrastructure PRACE (Partnership for Advanced Computing in Europe).

“With the current study and our new computer simulations we will generate a lot of information which means we can better understand the Milky Way’s fascinating life since the beginning of the Universe,” concludes Oscar Agertz.

Closed-form ab initio solutions of geometric albedos and reflected light phase curves of exoplanets

by Kevin Heng, Brett M. Morris, Daniel Kitzmann in Nature Astronomy

For millennia, humanity has observed the changing phases of the Moon. The rise and fall of sunlight reflected off the Moon, as it presents its different faces to us, is known as a “phase curve.” Measuring phase curves of the Moon and Solar System planets is an ancient branch of astronomy that goes back at least a century. The shapes of these phase curves encode information on the surfaces and atmospheres of these celestial bodies. In modern times, astronomers have measured the phase curves of exoplanets using space telescopes such as Hubble, Spitzer, TESS and CHEOPS. These observations are compared with theoretical predictions. In order to do so, one needs a way of calculating these phase curves. It involves seeking a solution to a difficult mathematical problem concerning the physics of radiation.

Approaches for the calculation of phase curves have existed since the 18th century. The oldest of these solutions goes back to the Swiss mathematician, physicist and astronomer, Johann Heinrich Lambert, who lived in the 18th century. “Lambert’s law of reflection” is attributed to him. The problem of calculating reflected light from Solar System planets was posed by the American astronomer Henry Norris Russell in an influential 1916 paper. Another well-known 1981 solution is attributed to the American lunar scientist Bruce Hapke, who built on the classic work of the Indian-American Nobel laureate Subrahmanyan Chandrasekhar in 1960. Hapke pioneered the study of the Moon using mathematical solutions of phase curves. The Soviet physicist Viktor Sobolev also made important contributions to the study of reflected light from celestial bodies in his influential 1975 textbook. Inspired by the work of these scientists, theoretical astrophysicist Kevin Heng of the Center for Space and Habitability CSH at the University of Bern has discovered an entire family of new mathematical solutions for calculating phase curves. The paper, authored by Kevin Heng in collaboration with Brett Morris from the National Center of Competence in Research NCCR PlanetS — which the University of Bern manages together with the University of Geneva — and Daniel Kitzmann from the CSH, has just been published.

“I was fortunate that this rich body of work had already been done by these great scientists. Hapke had discovered a simpler way to write down the classic solution of Chandrasekhar, who famously solved the radiative transfer equation for isotropic scattering. Sobolev had realised that one can study the problem in at least two mathematical coordinate systems.” Sara Seager brought the problem to Heng’s attention by her summary of it in her 2010 textbook.

By combining these insights, Heng was able to write down mathematical solutions for the strength of reflection (the albedo) and the shape of the phase curve, both completely on paper and without resorting to a computer. “The ground-breaking aspect of these solutions is that they are valid for any law of reflection, which means they can be used in very general ways. The defining moment came for me when I compared these pen-and-paper calculations to what other researchers had done using computer calculations. I was blown away by how well they matched,” said Heng.

“What excites me is not just the discovery of new theory, but also its major implications for interpreting data,” says Heng. For example, the Cassini spacecraft measured phase curves of Jupiter in the early 2000s, but an in-depth analysis of the data had not previously been done, probably because the calculations were too computationally expensive. With this new family of solutions, Heng was able to analyze the Cassini phase curves and infer that the atmosphere of Jupiter is filled with clouds made up of large, irregular particles of different sizes. This parallel study has just been published in collaboration with Cassini data expert and planetary scientist Liming Li of Houston University in Texas, U.S.A.

“The ability to write down mathematical solutions for phase curves of reflected light on paper means that one can use them to analyze data in seconds,” said Heng. It opens up new ways of interpreting data that were previously infeasible. Heng is collaborating with Pierre Auclair-Desrotour (formerly CSH, currently at Paris Observatory) to further generalize these mathematical solutions. “Pierre Auclair-Desrotour is a more talented applied mathematician than I am, and we promise exciting results in the near future,” said Heng.

In the paper, Heng and his co-authors demonstrated a novel way of analyzing the phase curve of the exoplanet Kepler-7b from the Kepler space telescope. Brett Morris led the data analysis part of the paper. “Brett Morris leads the data analysis for the CHEOPS mission in my research group, and his modern data science approach was critical for successfully applying the mathematical solutions to real data,” explained Heng. They are currently collaborating with scientists from the American-led TESS space telescope to analyze TESS phase curve data. Heng envisions that these new solutions will lead to novel ways of analyzing phase curve data from the upcoming, 10-billion-dollar James Webb Space Telescope, which is due to launch later in 2021.

“What excites me most of all is that these mathematical solutions will remain valid long after I am gone, and will probably make their way into standard textbooks,” said Heng.