The new stellar systems contain only young, blue stars, which are distributed in an irregular pattern and seem to exist in surprising isolation from any potential parent galaxy.

The stellar systems — which astronomers say appear through a telescope as “blue blobs” and are about the size of tiny dwarf galaxies — are located within the relatively nearby Virgo galaxy cluster. The five systems are separated from any potential parent galaxies by over 300,000 light years in some cases, making it challenging to identify their origins.

The astronomers found the new systems after another research group, led by the Netherlands Institute for Radio Astronomy’s Elizabeth Adams, compiled a catalog of nearby gas clouds, providing a list of potential sites of new galaxies. Once that catalog was published, several research groups, including one led by UArizona associate astronomy professor David Sand, started looking for stars that could be associated with those gas clouds.

The gas clouds were thought to be associated with our own galaxy, and most of them probably are, but when the first collection of stars, called SECCO1, was discovered, astronomers realized that it was not near the Milky Way at all, but rather in the Virgo cluster, which is much farther away but still very nearby in the scale of the universe.

SECCO1 was one of the very unusual “blue blobs,” said Michael Jones, a postdoctoral fellow in the UArizona Steward Observatory and lead author of a study that describes the new stellar systems. Jones presented the findings, which Sand co-authored, during the 240th American Astronomical Society meeting in Pasadena, California, Wednesday.

“It’s a lesson in the unexpected,” Jones said. “When you’re looking for things, you’re not necessarily going to find the thing you’re looking for, but you might find something else very interesting.”

The team obtained their observations from the Hubble Space Telescope, the Very Large Array telescope in New Mexico and the Very Large Telescope in Chile. Study co-author Michele Bellazzini, with the Istituto Nazionale di Astrofisica in Italy, led the analysis of the data from Very Large Telescope and has submitted a companion paper focusing on that data.

Together, the team learned that most of the stars in each system are very blue and very young and that they contain very little atomic hydrogen gas. This is significant because star formation begins with atomic hydrogen gas, which eventually evolves into dense clouds of molecular hydrogen gas before forming into stars.

“We observed that most of the systems lack atomic gas, but that doesn’t mean there isn’t molecular gas,” Jones said. “In fact, there must be some molecular gas because they are still forming stars. The existence of mostly young stars and little gas signals that these systems must have lost their gas recently.”

The combination of blue stars and lack of gas was unexpected, as was a lack of older stars in the systems. Most galaxies have older stars, which astronomers refer to as being “red and dead.”

“Stars that are born red are lower mass and therefore live longer than blue stars, which burn fast and die young, so old red stars are usually the last ones left living,” Jones said. “And they’re dead because they don’t have any more gas with which to form new stars. These blue stars are like an oasis in the desert, basically.”

The fact that the new stellar systems are abundant in metals hints at how they might have formed.

“To astronomers, metals are any element heavier than helium,” Jones said. “This tells us that these stellar systems formed from gas that was stripped from a big galaxy, because how metals are built up is by many repeated episodes of star formation, and you only really get that in a big galaxy.”

There are two main ways gas can be stripped from a galaxy. The first is tidal stripping, which occurs when two big galaxies pass by each other and gravitationally tear away gas and stars.

The other is what’s known as ram pressure stripping.

“This is like if you belly flop into a swimming pool,” Jones said. “When a galaxy belly flops into a cluster that is full of hot gas, then its gas gets forced out behind it. That’s the mechanism that we think we’re seeing here to create these objects.”

The team prefers the ram pressure stripping explanation because in order for the blue blobs to have become as isolated as they are, they must have been moving very quickly, and the speed of tidal stripping is low compared to ram pressure stripping.

Astronomers expect that one day these systems will eventually split off into individual clusters of stars and spread out across the larger galaxy cluster.

What researchers have learned feeds into the larger “story of recycling of gas and stars in the universe,” Sand said. “We think that this belly flopping process changes a lot of spiral galaxies into elliptical galaxies on some level, so learning more about the general process teaches us more about galaxy formation.”

“The decision making process for e-scooter companies is complex. One of the first questions is where to locate the scooters — In the transportation network, where do e-scooters need to be placed to meet demand? The second question is how to distribute them. It gets more complicated when you introduce different electric charging methods, so that some scooters are being collected by paid contractors and others are being charged by customers, through incentives,” Cheng said.

As the researchers see it, the main benefits of shared mobility are threefold:

1. It’s an affordable first/last mile option for people who cannot afford to own a vehicle;
2. It’s environmentally friendly, using fewer resources more efficiently;
3. It offers a novel solution to the problem of traffic congestion.

The model they created can provide decision makers with a robust solution that enables low cost and high service quality.

What Does the Model Do?

Cheng, along with Dr. Xiaofeng Li, Dr. Yao-Jan Wu, and doctoral student Abolhassan Mohammadi Fathabad, created a two-stage stochastic programming model, for the planning and operation of large shared mobility networks in the presence of demand uncertainty. The model, using a random probability pattern, can answer several questions that arise at the company and policy-maker levels:

• In the face of uncertain demand, how many e-scooters should be placed in the network and in which locations?
• How many charging facilities are needed, where and at what capacities?
• How should the network be evaluated and adjusted daily, to continue to meet demand while minimizing costs?

The model, developed using data from the City of Tucson, allows for optimal placement, charging, and rebalancing of unused (idle) scooters to meet demand in the most efficient manner possible. Successful operation of a shared mobility system requires both careful planning and strategies that minimize operational costs for the company while also increasing the customer satisfaction rate (i.e., an e-scooter is available when it is wanted). From a longer-term planning perspective, it is necessary to consider e-scooter demand patterns on a seasonal basis and plan accordingly for long-term economic benefits. The final report offers a detailed walk-through of the computational model.

Possibilities for Future Research

By predicting scooter rentals, relocations, idle periods, and charging times, the model offers e-scooter management companies a ready-made decision-making system to to effectively design and operate shared e-scooter systems, and thus help to ensure system reliability and cost-effectiveness.

In the future, this research could be extended in several directions. First, to increase the robustness in uncertainty modeling, researchers could develop a distributionally robust optimization framework (a modeling framework for decision-making under uncertainty). Another possible direction is to develop new approaches to solve the e-scooter planning problem when more variables are introduced that would require the model to be capable of solving more complex problems.

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Using a surprisingly comparable process, an interdisciplinary team of Northwestern Engineering researchers pieced together a method to determine how different 2D materials respond to disorder — testing some materials that could possibly replace silicon in new transistors and sensors.

“The analysis method will lead to a better understanding of disorder potentials in 2D materials to help make faster transistors, as well as better gas sensors that can more easily discriminate different gases,” said Matthew Grayson, professor of electrical and computer engineering at the McCormick School of Engineering, and one of the study’s authors.

In the paper “Field-effect Conductivity Scaling for Two-dimensional Materials with Tunable Impurity Density” published June 16 in the journal 2D Materials, the investigators developed a method to determine the fingerprint of the neighboring disorder as seen by a 2D material.

In science, disorder refers to imperfections or nearby charges that might scatter the otherwise straight path of an electron. 2D materials like graphene are particularly susceptible to nearby disorder because they are literally several atoms thick, at most.

“Disorder characterization is paramount in understanding and improving the performance of 2D materials,” Grayson said. “This paper shows that there exists a universal curve that serves as a fingerprint of that disorder. Even though different doses of disorder appear to result in completely different behaviors, these behaviors all represent individual threads of an overall tapestry.”

This is where the similarity comes in between the science and games you play on your phone or printed newspaper.

Using 2D material samples developed by the Hersam and Dravid groups, Grayson and his team implemented a new method to measure electrical conductivity curves using a cryostat, a device that preserves samples at low temperatures for microscopic examination. At room temperature, the charges that constitute disorder are free to shift around until they reach equilibrium, but when frozen in the cryostat, the disorder is frozen in place.

Each individual conductivity curve resembles a puzzle piece. The researchers then utilized an empirical rule to piece together all the curves until they formed a complete a picture.

Sound familiar?

They then used physical arguments to understand why this rule works as well as it does. As a result, they solved the riddle of how each of the materials under study respond to a specific class of imperfections.

“The impressive continuity of this picture when all the puzzle pieces were in place inspired us to dig deeper into the physics to understand what the underlying reason must be for this behavior,” Grayson said. “The same mentality that the general public uses to solve their daily Wordle or crossword puzzle is applied here.”

These findings also have implications for 2D materials research moving forward.

“Instead of seeing individual devices made from the same 2D materials as a bunch of puzzle pieces that each have to be studied independently, you can now locate where a given sample fits into the previously solved puzzle,” Grayson said, “so that each individual piece is instantly recognized as part of a greater picture.

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This is the first case of cosmic cannibalism in which astronomers have observed a white dwarf consuming both rocky-metallic material, likely from a nearby asteroid, and icy material, presumed to be from a body similar to those found in the Kuiper belt at the fringe of our own solar system.

“We have never seen both of these kinds of objects accreting onto a white dwarf at the same time,” said lead researcher Ted Johnson, a physics and astronomy major at UCLA who graduated last week. “By studying these white dwarfs, we hope to gain a better understanding of planetary systems that are still intact.”

The findings are based on an analysis of materials captured by the atmosphere of G238-44, a white dwarf some 86 light-years from Earth, using archival data from the Hubble Space Telescope and additional NASA satellites and observatories. A white dwarf is the burned-out core that remains after a star like our sun sheds its outer layers and stops burning fuel through nuclear fusion.

As surprising as the white dwarf’s wide-ranging diet is, the findings are also intriguing because astronomers believe icy objects crashed into and irrigated dry, rocky planets in our solar system — including Earth. Billions of years ago, comets and asteroids are thought to have delivered water to our planet, sparking the conditions necessary for life. The makeup of the material detected raining onto G238-44 implies that icy reservoirs might be common among planetary systems, said research co-author Benjamin Zuckerman, a UCLA professor of physics and astronomy.

“Life as we know it requires a rocky planet covered with a variety of volatile elements like carbon, nitrogen and oxygen,” Zuckerman said. “The abundances of the elements we see on this white dwarf appear to have come from both a rocky parent body and a volatile-rich parent body — the first example we’ve found among studies of hundreds of white dwarfs.”

Chaos and destruction: From living star to red giant to white dwarf

Theories of planetary-system evolution describe the demise of a star as a turbulent, chaotic event, one that begins when it first balloons exponentially into what is known as a red giant and then quickly loses its outer layers, collapsing into a white dwarf — a super-dense star about the size of Earth, with a mass of our sun. The process dramatically disrupts the remaining planets’ orbits, and smaller objects — asteroids, comets, moons — that venture too close to them can be scattered like pinballs and sent hurtling toward the white dwarf.

This study confirms the true scale of the chaos, showing that within 100 million years after the beginning of its white dwarf phase, the star is able to simultaneously capture and consume material from its nearby asteroid belt and its far-flung Kuiper belt-like regions.

Though astronomers have cataloged more than 5,000 planets outside our solar system, the only planet whose interior makeup we have some direct knowledge of is Earth. Because the materials accreting onto G238-44 are representative of the building blocks of major planets, this white dwarf cannibalism provides a unique opportunity to take planets apart and see what they were made of when they first formed around the star, said UCLA astronomy researcher Beth Klein, a member of the team.

The team measured the presence of nitrogen, oxygen, magnesium, silicon and iron, among other elements, in the white dwarf’s atmosphere. Their detection of iron in very high abundance is evidence for metallic cores of terrestrial planets, like Earth, Venus, Mars and Mercury, Johnson said. Unexpectedly high nitrogen abundances led them to conclude that icy bodies were also present.

“The best fit for our data was a nearly two-to-one mix of Mercury-like material and comet-like material, which is made up of ice and dust,” Johnson said. “Iron metal and nitrogen ice each suggest wildly different conditions of planetary formation. There is no known solar system object with so much of both.”

The researchers say the ultimate scenario for our own sun some 5 billion years from now will likely be quite similar to what has been seen with G238-44. During the sun’s red giant phase, the Earth might be completely vaporized along with the inner planets, they predict.

The orbits of many of the asteroids in our solar system’s main asteroid belt will be gravitationally perturbed by Jupiter and will also fall onto the white dwarf remnant that the sun will become, he said.

For more than two years, the research group at UCLA, along with colleagues at UC San Diego and the University of Kiel in Germany, has worked to unravel the mystery of G238-44 by analyzing the elements detected on the white dwarf star.

Their analysis included data from NASA’s retired Far Ultraviolet Spectroscopic Explorer, the Keck Observatory’s High Resolution Echelle Spectrometer in Hawaii and the Hubble Space Telescope’s Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

The team’s results were presented at an American Astronomical Society press conference on June 15.

The discovery forms part of a treasure trove of data made available in Gaia’s Data Release 3, which provides the most detailed survey of our galactic neighbourhood to date.

Observations made by the Gaia observatory, which orbits a point in space about 1.5 million km from Earth, will allow astronomers to create the most accurate and complete multi-dimensional map of the Milky Way and better understand our place in the Universe.

Martin Barstow, Professor of Astrophysics and Space Science at the University of Leicester and Director of Strategic Partnerships for Space Park Leicester, is part of the Gaia collaboration to have co-authored multiple papers using the new data.

Other highlights of Data Release 3 include description of ‘starquakes’, stellar DNA and a new binary star catalogue of more than 800,000 binary systems. Before Gaia launched in 2013, only around 30,000 binaries were known in our galaxy.

Professor Barstow said:

“Gaia data has been moving through astronomy like a tidal wave. It’s the most productive observatory we have available to use right now, and it’s transforming both astronomy and our understanding of the Universe.

“This Data Release 3 is a complete step change. All the data we have catalogued using spectra — stellar distances, ages, composition, and more — adds an extra dimension to what we know about the stars in our galaxy, and represents a huge leap forward.”

Data reveals a ‘super Jupiter’ companion

Observations of exoplanets orbiting white dwarfs is notoriously difficult. White dwarfs are the core remnant of stars not massive enough to become a black hole or neutron star.

However, by analysing the motion of the metal-rich white dwarf WD 0141-675 and noting a ‘wobble’ in its orbit, researchers inferred the existence of a companion object with a mass around nine times that of Jupiter. Too small to be a star, this must be an exoplanet.

This ‘super Jupiter’ is only the third known exoplanet to orbit a white dwarf, and makes WD 0141-675 the closest white dwarf to Earth to host a planet.

Stars in their eyes: Gaia more than doubles number of known binaries

Gaia’s Data Release 3 also expands astronomers’ understanding of binary systems, where two stars are gravitationally bound to one another. Sirius, the brightest star visible from Earth (after our Sun), is a binary system comprising a main sequence star, Sirius A, and a faint white dwarf companion, Sirius B.

These new datasets both refine the stellar catalogue of known binaries and add many more new such systems, bringing the total of known binary systems from around 300,000 before Data Release 3 to more than 800,000. Researchers detect binaries using Gaia’s radial velocity spectrometer and a variety of techniques;

• astrometry: by detecting the motion of source objects which are not uniform and are observed to ‘wobble’ or otherwise change direction from what would otherwise be expected;
• photometry: when aligned with Gaia’s line of sight, where one star is observed to pass in front of another and periodically ‘eclipse’ its companion;
• and spectroscopy: these binaries have a radial velocity that varies periodically, depending on whether a star approaches or recedes from our viewpoint at Earth.

Professor Barstow continued:

“We have so much more data on binaries with this release and, crucially, that data is so much more precise than what has gone before.

“Once you have more precise data about a binary system you can work out all sorts of things such as ages and composition: all aspects we haven’t had information on before.

“And by answering those questions, we can start to understand more of the fundamentals about how our Universe works, including how stars live and die.”

HD 53143 — a roughly billion-year-old Sun-like star located 59.8 light-years from Earth in the Carina constellation — was first observed with the coronagraphic Advanced Camera for Surveys on the Hubble Space Telescope (HST) in 2006. It also is surrounded by a debris disk — a belt of comets orbiting a star that are constantly colliding and grinding down into smaller dust and debris — that scientists previously believed to be a face-on ring similar to the debris disk surrounding our Sun, more commonly known as the Kuiper Belt.

The new observations were made of HD 53143 using the highly-sensitive Band 6 receivers on ALMA, an observatory co-operated by the U.S. National Science Foundation’s National Radio Astronomy Observatory (NRAO), and have revealed that the star system’s debris disk is actually highly eccentric. In ring-shaped debris disks, the star is typically located at or near the center of the disk. But in elliptically-shaped eccentric disks, the star resides at one focus of the ellipse, far away from the disk’s center. Such is the case with HD 53143, which wasn’t seen in previous coronagraphic studies because coronagraphs purposely block the light of a star in order to more clearly see nearby objects. The star system may also be harboring a second disk and at least one planet.

“Until now, scientists had never seen a debris disk with such a complicated structure. In addition to being an ellipse with a star at one focus, it also likely has a second inner disk that is misaligned or tilted relative to the outer disk,” said Meredith MacGregor, an assistant professor at the Center for Astrophysics and Space Astronomy (CASA) and Department of Astrophysical and Planetary Sciences (APS) at CU Boulder, and the lead author on the study. “In order to produce this structure, there must be a planet or planets in the system that are gravitationally perturbing the material in the disk.”

This level of eccentricity, MacGregor said, makes HD 53143 the most eccentric debris disk observed to date, being twice as eccentric as the Fomalhaut debris disk, which MacGregor fully imaged at millimeter wavelengths using ALMA in 2017. “So far, we have not found many disks with a significant eccentricity. In general, we don’t expect disks to be very eccentric unless something, like a planet, is sculpting them and forcing them to be eccentric. Without that force, orbits tend to circularize, like what we see in our own Solar System.”

Importantly, MacGregor notes that debris disks aren’t just collections of dust and rocks in space. They are a historical record of planetary formation and how planetary systems evolve over time. and provide a peek into their futures. “We can’t study the formation of Earth and the Solar System directly, but we can study other systems that appear similar to but younger than our own. It’s a bit like looking back in time,” she said. “Debris disks are the fossil record of planet formation, and this new result is confirmation that there is much more to be learned from these systems and that knowledge may provide a glimpse into the complicated dynamics of young star systems similar to our own Solar System.”

Dr. Joe Pesce, NSF program officer for ALMA, added, “We are finding planets everywhere we look, and these fabulous results by ALMA are showing us how planets form — both those around other stars and in our own Solar System. This research demonstrates how astronomy works and how progress is made, informing not only what we know about the field but also about ourselves.”

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