Research, teaching and outreach in Physics at UW–Madison
Magnetic fields implicated in the mysterious midlife crisis of stars
This post was originally published by the Royal Astronomical Society. UW–Madison physics graduate student Bindesh Tripathi is the lead author of the scientific publication.
Middle-aged stars can experience their own kind of midlife crisis, experiencing dramatic breaks in their activity and rotation rates at about the same age as our Sun, according to new research published today in Monthly Notices of the Royal Astronomical Society: Letters. The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring ages of stars past their middle age, and the transition of solar-like stars to a magnetically inactive future.
Astronomers have long known that stars experience a process known as ‘magnetic braking’: a steady stream of charged particles, known as the solar wind, escapes from the star over time, carrying away small amounts of the star’s angular momentum. This slow drain causes stars like our Sun to gradually slow down their rotation over billions of years.
In turn, the slower rotation leads to altered magnetic fields and less stellar activity – the numbers of sunspots, flares, outbursts, and similar phenomena in the atmospheres of stars, which are intrinsically linked to the strengths of their magnetic fields.
This decrease in activity and rotation rate over time is expected to be smooth and predictable because of the gradual loss of angular momentum. The idea gave birth to the tool known as ‘stellar gyrochronology’, which has been widely used over the past two decades to estimate the age of a star from its rotation period.
However recent observations indicate that this intimate relationship breaks down around middle age. The new work, carried out by Bindesh Tripathi at UW–Madison and the Indian Institute of Science Education and Research (IISER) Kolkata, India, provides a novel explanation for this mysterious ailment. Prof. Dibyendu Nandy, and Prof. Soumitro Banerjee of IISER are co-authors.
Using dynamo models of magnetic field generation in stars, the team show that at about the age of the Sun the magnetic field generation mechanism of stars suddenly becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. A middle aged star like the Sun can often switch to the low activity mode resulting in drastically reduced angular momentum losses by magnetized stellar winds.
Prof. Nandy comments: “This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena, such as why stars beyond their midlife do not spin down as fast as in their youth, the breakdown of stellar gyrochronology relations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.”
The new work provides key insights into the existence of low activity episodes in the recent history of the Sun known as grand minima – when hardly any sunspots are seen. The best known of these is perhaps the Maunder Minimum around 1645 to 1715, when very few sunspots were observed.
The team hope that it will also shed light on recent observations indicating that the Sun is comparatively inactive, with crucial implications for the potential long-term future of our own stellar neighbor.
Celebrating IceCube’s first decade of discovery
It was the beginning of a grand experiment unlike anything the world had ever seen. Ten years ago today, the IceCube Neutrino Observatory fully opened its eyes for the first time.
Over the course of the previous seven years, dozens of intrepid technicians, engineers, and scientists had traveled to the South Pole—one of the coldest, driest, and most isolated places on Earth—to build the biggest, strangest telescope in the world. Crews drilled 86 holes nearly two-and-a-half kilometers deep and lowered a cable strung with 60 basketball-sized light detectors into each hole. The result was a hexagonal grid of sensors embedded in a cubic kilometer of ice about a mile below the surface of the Antarctic ice sheet. On December 18, 2010, the 5,160th light sensor was deployed in the ice, completing the construction of the IceCube Neutrino Observatory.
The purpose of the unconventional telescope was to detect signals from passing astrophysical neutrinos: mysterious, tiny, extremely lightweight particles created by some of the most energetic and distant phenomena in the cosmos. IceCube’s founders believed that studying these astrophysical neutrinos would reveal hidden parts of the universe. Over the course of the next decade, they would be proven right.
IceCube began full operations on May 13, 2011 — ten years ago today — when the detector took its first set of data as a completed instrument. Since then, IceCube has been watching the cosmos and collecting data continuously for a decade.
During its first few years of operation, IceCube accumulated vast amounts of data, but it wasn’t until 2013 that the observatory yielded its first major results.
Ellen Zweibel elected to the National Academy of Sciences
Astronomy and physics Professor Ellen Zweibel has been honored with membership in the National Academy of Sciences.
Zweibel is among 120 new members — and one of 59 women, the largest group ever — elected to the academy, one of the highest honors that can be conferred on an American scientist. Members are chosen “in recognition of their distinguished and continuing achievements in original research.”
Zweibel, the W.L. Kraushaar Professor of Astronomy and Physics, came to UW–Madison in 2003. She studies the way magnetic fields shape the universe, including the physics of plasmas in stars and galaxies and the cosmic rays they throw out into the universe.
A founding member of the Center for Magnetic Self-Organization, a Physics Frontier Center funded by the National Science Foundation and the U.S. Department of Energy, Zweibel won the American Physical Society’s Maxwell Prize for Plasma Physics in 2016.
The National Academy of Sciences — with the National Academy of Engineering and the National Academy of Medicine — provides science, engineering, and health policy advice to the federal government and other organizations. It is a private, nonprofit institution established in 1863 under a congressional charter signed by President Abraham Lincoln.
Highest-energy Cosmic Rays Detected in Star Clusters
For decades, researchers assumed the cosmic rays that regularly bombard Earth from the far reaches of the galaxy are born when stars go supernova — when they grow too massive to support the fusion occurring at their cores and explode.
Those gigantic explosions do indeed propel atomic particles at the speed of light great distances. However, new research suggests even supernovae — capable of devouring entire solar systems — are not strong enough to imbue particles with the sustained energies needed to reach petaelectronvolts (PeVs), the amount of kinetic energy attained by very high-energy cosmic rays.
And yet cosmic rays have been observed striking Earth’s atmosphere at exactly those velocities, their passage marked, for example, by the detection tanks at the High-Altitude Water Cherenkov (HAWC) observatory near Puebla, Mexico. Instead of supernovae, the researchers — including UW–Madison’s Ke Fang — posit that star clusters like the Cygnus Cocoon serve as PeVatrons — PeV accelerators — capable of moving particles across the galaxy at such high energy rates.
New UW–Madison assistant professor of physics Lu Lu’s research program combines the past with the future. Her research looks for sources of ultrahigh energy particles, which is done by analyzing data that has already been collected. As she says, “Maybe data is already talking to us, we just haven’t looked.” But she is also working toward improving future data collection, which will require more technologically-advanced detectors. “My teachers, my great masters, have taught me that the current young generation has the responsibility to look into new techniques to go to the future for younger generations to proceed forward,” she says about her work in sensor R&D.
On January 1, Professor Lu joined the Department of Physics and IceCube. Most recently, she was a postdoctoral fellow at the International Center for Hadron Astrophysics at Chiba University in Japan. To welcome her, we sat down for a (virtual) interview.
What are your research interests?
My prime interest is astroparticle physics, and my ultimate goal is to find the sources of the highest energy particles in the universe. These particles carry energy of about 1020 electronvolts. This is higher energy than what we have from the Large Hadron Collider and human technologies. The real attractiveness here is we don’t know how nature accelerates these particles. And once we identify the sources, we can test new theories beyond the Standard Model using sources crated by nature.
What are one or two main projects you focus your research on?
I’m involved in two experiments. One is IceCube, the other is Pierre Auger Observatory. I was doing cosmic ray analysis, but cosmic rays are usually charged particles and they are deflected in the magnetic field of the galaxy; they would not travel in a straight line. IceCube studies neutrinos which are neutral particles, they travel directly from the source. Pierre Auger detects ultrahigh energy photons, which are also neutral particles. One thing I want to do immediately after I join Madison is to combine these two experiments to do a joint analysis. We have photon candidates but we haven’t really tried to connect them in the multimessenger regime. By combining Pierre Auger photons with IceCube neutrinos, we could possibly find a transient source, a source that doesn’t constantly emit ultrahigh energy photons or neutrinos but all of a sudden there’s a flare. This type of analysis has never been done, but we have data on disks.
The second thing I’m interested in is using new sensor technologies. In IceCube, we have Gen2 being planned right now. Instead of using a single photon sensor, we’d use a more sensitive design and R&D. UW–Madison is taking the lead of designing this future detector. There’s also radio technology. So, to detect the highest energy neutrinos we need to build a large instrument volume. With optical array, it is really hard to scale up because one has to drill holes inside the South Pole, which is really expensive. But radio technology doesn’t have to go so deep, so they can bury their detectors on the surface areas, and the radiowaves can transmit further away than the optical photons in ice. For optical you have to make the detectors very dense, but for radio you can make the antennas further apart, so that means you can have a larger area and detect more events easily. I think radio is the way to go for the future.
You said you have a lot of data collected already and just need to analyze it. How do you analyze the data from these detectors?
We would have to search for photon candidates from the data from Auger, and identify where it comes from and what the time this event happened. Correspondingly, do we see neutrinos from IceCube coming from the same direction and at the same time? Because you can never be sure it’s a photon. It could be a proton. We then want to build a statistical framework to combine different multimessengers together in real time.
What does it mean if you find a photon in coincidence with a neutrino?
Cosmic rays were first detected more than 100 years ago, and there’s a rich history of studying where they come from. The mystery of origins still remains today because our poor knowledge on the galactic/extragalactic magnetic fields and mass composition of cosmic rays. In my opinion, the most probable way to solve this puzzle is to use neutral particles. If we can identify ultrahigh energy photons in coincidence with neutrinos, that is a smoking gun that we are actually looking at a source and we can finally pin down where in the universe is accelerating high energy particles. And therefore, we can study particle physics maybe beyond Standard Model. It’s just like a lab created by the universe to test particle physics.
What is your favorite element and/or elementary particle?
My favorite elementary particle is the electron anti-neutrino. I like muons, too. My favorite element is hydrogen.
What hobbies and interests do you have?
I’m afraid I’ll disappoint you because my hobby is related to my research: Augmented reality. When I heard about something called Microsoft Hololens, I thought, I could make IceCube a hologram. I bought these special glasses, and then made a program on it and used it for some outreach events. But the glasses are very expensive, so people said, “Okay we can’t buy hologram glasses.” So I moved it to mobile phones so that everyone could look at it for fun. It’s called IceCubeAR (note: download it for iPhones or Android phones). I made it with a group of friends in Tokyo.
Dark Energy Survey makes public catalog of nearly 700 million astronomical objects
The second data release from the Dark Energy Survey, or DES, is the culmination of over a half-decade of astronomical data collection and analysis with the ultimate goal of understanding the accelerating expansion of the universe and the phenomenon of dark energy, which is thought to be responsible for this accelerated expansion. It is one of the largest astronomical catalogs released to date. Keith Bechtol, assistant professor of physics at UW–Madison, has served as the DES Science Release co-coordinator since 2017, guiding the effort to assemble, scientifically validate, and document data releases for both cosmology analysis by the DES Collaboration and exploration by the broad astronomical community.
Including a catalog of nearly 700 million astronomical objects, DR2 builds on the 400 million objects cataloged with the survey’s prior data release, or DR1, and also improves on it by refining calibration techniques, which, with the deeper combined images of DR2, lead to improved estimates of the amount and distribution of matter in the universe.
Astronomical researchers around the world can access these unprecedented data and mine them to make new discoveries about the universe, complementary to the studies being carried out by the Dark Energy Survey collaboration. The full data release is online and available to the public to explore and gain their own insights as well.
“Most of the nearly 700 million objects visible in DES DR2 images had never been seen by humans before the past few years,” Bechtol says. “If you take a moment to look at even a small patch of sky in the DES images, you can see asteroids of our Solar System, stars out to the edge of the Milky Way, and distant galaxies as they were billions of years ago. We look forward to see how our colleagues use this enormous new dataset for research and education.”
DES was designed to map hundreds of millions of galaxies and to discover thousands of supernovae in order to measure the history of cosmic expansion and the growth of large-scale structure in the universe, both of which reflect the nature and amount of dark energy in the universe. DES has produced the largest and most accurate dark matter map from galaxy weak lensing to date, as well as a new map, three times larger, that will be released in the near future.
One early result relates to the construction of a catalog of a type of pulsating star known as “RR Lyrae,” which tells scientists about the region of outer space beyond the edge of our Milky Way. In this area nearly devoid of stars, the motion of the RR Lyrae hints at the presence of an enormous “halo” of invisible dark matter, which may provide clues on how our galaxy was assembled over the last 12 billion years. In another result, DES scientists used the extensive DR2 galaxy catalog, along with data from the LIGO experiment, to estimate the location of a black hole merger and, independent of other techniques, infer the value of the Hubble constant, a key cosmological parameter. Combining their data with other surveys, DES scientists have also been able to generate a complete map of the Milky Way’s dwarf satellites, giving researchers insight into how our own galaxy was assembled and how it compares with cosmologists’ predictions.
Covering 5,000 square degrees of the southern sky (one-eighth of the entire sky) and spanning billions of light-years, the survey data enables many other investigations in addition to those targeting dark energy, covering a vast range of cosmic distances — from discovering new nearby solar system objects to investigating the nature of the first star-forming galaxies in the early universe.
“This is a momentous milestone. For six years, the Dark Energy Survey collaboration took pictures of distant celestial objects in the night sky. Now, after carefully checking the quality and calibration of the images captured by the Dark Energy Camera, we are releasing this second batch of data to the public,” said DES Director Rich Kron of Fermilab and the University of Chicago. “We invite professional and amateur scientists alike to dig into what we consider a rich mine of gems waiting to be discovered.”
Once captured, these images (and the large amount of data surrounding them) are transferred to the National Center for Supercomputing Applications for processing via the DES Data Management project. Using the Blue Waters supercomputer at NCSA, the Illinois Campus Cluster and computing systems at Fermilab, NCSA prepares calibrated data products for public and research consumption. It takes approximately four months to process one year’s worth of data into a searchable, usable catalog.
The detailed precision cosmology constraints based on the full six-year DES data set will come out over the next two years.
NCSA, NOIRLab and the LIneA Science Server collectively provide the tools and interfaces that enable access to DR2.
“Because astronomical data sets today are so vast, the cost to handle them is prohibitive for individual researchers or most organizations. CSDC provides open access to big astronomical data sets like DES DR2 and the necessary tools to explore and exploit them — then all it takes is someone from the community with a clever idea to discover new and exciting science,” said Robert Nikutta, project scientist for Astro Data Lab at CSDC.
“With information on the positions, shapes, sizes, colors and brightnesses of over 690 million stars, galaxies and quasars, the release promises to be a valuable source for astronomers and scientists worldwide to continue their explorations of the universe, including studies of matter (light and dark) surrounding our home Milky Way galaxy, as well as pushing further to examine groups and clusters of distant galaxies, which hold precise evidence about how the size of the expanding universe changes over time,” said Dark Energy Survey Data Management Project Scientist Brian Yanny of Fermilab.
This work is supported in part by the U.S. Department of Energy Office of Science.
The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Studies and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science, Technology and Innovation, the German Research Foundation and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.