From The University of California-San Diego
6.10.24
Michelle Franklin
m1franklin@ucsd.edu
A newly discovered L subdwarf is on an unusual journey through our galaxy.
A simulation of a hypothetical J1249+36-white dwarf binary pair ends with the white dwarf exploding into a supernova. (cr: Adam Makarenko / W.M. Keck Observatory)
It may seem like the Sun is stationary while the planets in its orbit are moving, but the Sun is actually orbiting around the Milky Way galaxy at an impressive rate of about 220 kilometers per second — almost half a million miles per hour. As fast as that may seem, when a faint red star was discovered crossing the sky at a noticeably quick pace, scientists took notice.
Thanks to the efforts of a citizen science project called Backyard Worlds: Planet 9 and a team of astronomers from around the country, a rare hypervelocity L subdwarf star has been found racing through the Milky Way. More remarkably, this star may be on a trajectory that causes it to leave the Milky Way altogether. The research, led by University of California San Diego Professor of Astronomy and Astrophysics Adam Burgasser, was presented today at a press conference during the 244th national meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.
The star, charmingly named CWISE J124909+362116.0 (“J1249+36”), was first noticed by some of the over 80,000 citizen science volunteers participating in the Backyard Worlds: Planet 9 project, who comb through enormous reams of data collected over the past 14 years by NASA’s Wide-field Infrared Survey Explorer (WISE) mission.
This project capitalizes on the keen ability of humans, who are evolutionarily programmed to look for patterns and spot anomalies in a way that is unmatched by computer technology. Volunteers tag moving objects in data files and when enough volunteers tag the same object, astronomers investigate.
J1249+36 immediately stood out because of the speed at which it is moving across the sky, initially estimated at about 600 kilometers per second (1.3 million miles per hour). At this speed, the star is fast enough to escape the gravity of the Milky Way, making it a potential “hypervelocity” star.
To better understand the nature of this object, Burgasser turned to the W.M. Keck Observatory in Maunakea, Hawai’i to measure its infrared spectrum.
These data revealed that the object was a rare L subdwarf — a class of stars with very low mass and temperature. Subdwarfs represent the oldest stars in the Milky Way.
The insight into J1249+36’s composition was made possible by a new set of atmosphere models created by UC San Diego alumnus Roman Gerasimov, who worked with UC LEADS scholar Efrain Alvarado III to generate models specifically tuned to study L subdwarfs. “It was exciting to see that our models were able to accurately match the observed spectrum,” said Alvarado, who is presenting his modeling work at the AAS meeting.
The spectral data, along with imaging data from several ground-based telescopes, allowed the team to accurately measure J1249+36’s position and velocity in space, and thereby predict its orbit through the Milky Way. “This is where the source became very interesting, as its speed and trajectory showed that it was moving fast enough to potentially escape the Milky Way,” stated Burgasser.
What gave this star a kick?
Researchers focused on two possible scenarios to explain J1249+36’s unusual trajectory. In the first scenario, J1249+36 was originally the low-mass companion of a white dwarf. White dwarfs are the remnant cores of stars that have depleted their nuclear fuel and died out. When a stellar companion is in a very close orbit with a white dwarf, it can transfer mass, resulting in periodic outbursts called novae. If the white dwarf collects too much mass, it can collapse and explode as a supernova.
Binary Star System Supernova Explosion.
A simulation of a possible explanation for an L subdwarf named CWISE J124909+362116.0’s speed shows it as a part of a white dwarf binary pair that ended with the white dwarf exploding into a supernova. (cr: Adam Makarenko / W.M. Keck Observatory)
“In this kind of supernova, the white dwarf is completely destroyed, so its companion is released and flies off at whatever orbital speed it was originally moving, plus a little bit of a kick from the supernova explosion as well,” said Burgasser. “Our calculations show this scenario works. However, the white dwarf isn’t there anymore and the remnants of the explosion, which likely happened several million years ago, have already dissipated, so we don’t have definitive proof that this is its origin.”
In the second scenario, J1249+36 was originally a member of a globular cluster, a tightly bound cluster of stars, immediately recognizable by its distinct spherical shape. The centers of these clusters are predicted to contain black holes of a wide range of masses. These black holes can also form binaries, and such systems turn out to be great catapults for any stars that happen to wander too close to them.
“When a star encounters a black hole binary, the complex dynamics of this three-body interaction can toss that star right out of the globular cluster,” explained Kyle Kremer, an incoming Assistant Professor in UC San Diego’s Department of Astronomy and Astrophysics. Kremer ran a series of simulations and found that on rare occasions these kinds of interactions can kick a low-mass subdwarf out of a globular cluster and on a trajectory similar to that observed for J1249+36.
“It demonstrates a proof of concept,” said Kremer, “but we don’t actually know what globular cluster this star is from.” Tracing J1249+36 back in time puts it in a very crowded part of the sky that may hide undiscovered clusters.
To determine whether either of these scenarios, or some other mechanism, can explain J1249+36’s trajectory, Burgasser said the team hopes to look more closely at its elemental composition. For example, when a white dwarf explodes, it creates heavy elements that could have “polluted” the atmosphere of J1249+36 as it was escaping. The stars in globular clusters and satellite galaxies of the Milky Way also have distinct abundance patterns that may reveal the origin of J1249+36.
“We’re essentially looking for a chemical fingerprint that would pinpoint what system this star is from,” said Gerasimov, whose modeling work has enabled him to measure the element abundances of cool stars in several globular clusters, work he is also presenting at the AAS meeting.
Whether J1249+36’s speedy journey was because of a supernova, a chance encounter with a black hole binary, or some other scenario, its discovery provides a new opportunity for astronomers to learn more about the history and dynamics of the Milky Way.
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The University of California-San Diego is a public land-grant research university in San Diego, California. Established in 1960 near the pre-existing Scripps Institution of Oceanography, The University of California-San Diego is the southernmost of the ten campuses of the University of California, and offers over 200 undergraduate and graduate degree programs. The University of California-San Diego occupies 2,178 acres (881 ha) near the coast of the Pacific Ocean, with the main campus resting on approximately 1,152 acres (466 ha). The University of California-San Diego is ranked among the best universities in the world by major college and university rankings.
The University of California-San Diego consists of twelve undergraduate, graduate and professional schools as well as seven undergraduate residential colleges. It regularly receives over 140,000 applications for undergraduate admissions. The University of California-San Diego San Diego Health, the region’s only academic health system, provides patient care, conducts medical research and educates future health care professionals at The University of California-San Diego Medical Center, Hillcrest, Jacobs Medical Center, Moores Cancer Center, Sulpizio Cardiovascular Center, Shiley Eye Institute, Institute for Genomic Medicine, Koman Family Outpatient Pavilion and various express care and urgent care clinics throughout San Diego.
The University of California-San Diego operates 19 organized research units as well as eight School of Medicine research units, six research centers at Scripps Institution of Oceanography and two multi-campus initiatives. The University of California-San Diego is also closely affiliated with several regional research centers, such as The Salk Institute, the Sanford Burnham Prebys Medical Discovery Institute, the Sanford Consortium for Regenerative Medicine, and The Scripps Research Institute. It is classified among “R1: Doctoral Universities – Very high research activity”.
The University of California-San Diego is considered one of the country’s “Public Ivies”. The University of California-San Diego faculty, researchers, and alumni have won Nobel Prizes as well as Fields Medals, National Medals of Science, MacArthur Fellowships, and Pulitzer Prizes. Additionally, of the current faculty, a number have been elected to The National Academy of Engineering, The National Academy of Sciences, The National Academy of Medicine and to The American Academy of Arts and Sciences.
History
When the Regents of the University of California originally authorized The University of California-San Diego campus in 1956, it was planned to be a graduate and research institution, providing instruction in the sciences, mathematics, and engineering. Local citizens supported the idea, voting the same year to transfer to the university 59 acres (24 ha) of mesa land on the coast near the preexisting Scripps Institution of Oceanography. The Regents requested an additional gift of 550 acres (220 ha) of undeveloped mesa land northeast of Scripps, as well as 500 acres (200 ha) on the former site of Camp Matthews from the federal government, but Roger Revelle, then director of Scripps Institution and main advocate for establishing the new campus, jeopardized the site selection by exposing the La Jolla community’s exclusive real estate business practices, which were antagonistic to minority racial and religious groups. This outraged local conservatives, as well as Regent Edwin W. Pauley.
University of California President Clark Kerr satisfied San Diego city donors by changing the proposed name from University of California, La Jolla, to University of California-San Diego. The city voted in agreement to its part in 1958, and the University of California approved construction of the new campus in 1960. Because of the clash with Pauley, Revelle was not made chancellor. Herbert York, first director of The DOE’s Lawrence Livermore National Laboratory, was designated instead. York planned the main campus according to the “Oxbridge” model, relying on many of Revelle’s ideas.
According to Kerr, “San Diego always asked for the best,” though this created much friction throughout the University of California system, including with Kerr himself, because The University of California-San Diego often seemed to be “asking for too much and too fast.” Kerr attributed The University of California-San Diego’s “special personality” to Scripps, which for over five decades had been the most isolated University of California unit in every sense: geographically, financially, and institutionally. It was a great shock to the Scripps community to learn that Scripps was now expected to become the nucleus of a new University of California campus and would now be the object of far more attention from both the university administration in Berkeley and the state government in Sacramento.
The University of California-San Diego was the first general campus of the University of California to be designed “from the top down” in terms of research emphasis. Local leaders disagreed on whether the new school should be a technical research institute or a more broadly based school that included undergraduates as well. John Jay Hopkins of General Dynamics Corporation pledged one million dollars for the former while the City Council offered free land for the latter. The original authorization for The University of California-San Diego campus given by the University of California Regents in 1956 approved a “graduate program in science and technology” that included undergraduate programs, a compromise that won both the support of General Dynamics and the city voters’ approval.
Nobel laureate Harold Urey, a physicist from the University of Chicago, and Hans Suess, who had published the first paper on the greenhouse effect with Revelle in the previous year, were early recruits to the faculty in 1958. Maria Goeppert-Mayer, later the second female Nobel laureate in physics, was appointed professor of physics in 1960. The graduate division of the school opened in 1960 with 20 faculty in residence, with instruction offered in the fields of physics, biology, chemistry, and earth science. Before the main campus completed construction, classes were held in the Scripps Institution of Oceanography.
By 1963, new facilities on the mesa had been finished for the School of Science and Engineering, and new buildings were under construction for Social Sciences and Humanities. Ten additional faculty in those disciplines were hired, and the whole site was designated the First College, later renamed after Roger Revelle, of the new campus. York resigned as chancellor that year and was replaced by John Semple Galbraith. The undergraduate program accepted its first class of 181 freshman at Revelle College in 1964. Second College was founded in 1964, on the land deeded by the federal government, and named after environmentalist John Muir two years later. The University of California-San Diego School of Medicine also accepted its first students in 1966.
Political theorist Herbert Marcuse joined the faculty in 1965. A champion of the New Left, he reportedly was the first protester to occupy the administration building in a demonstration organized by his student, political activist Angela Davis. The American Legion offered to buy out the remainder of Marcuse’s contract for $20,000; the Regents censured Chancellor William J. McGill for defending Marcuse on the basis of academic freedom, but further action was averted after local leaders expressed support for Marcuse. Further student unrest was felt at the university, as the United States increased its involvement in the Vietnam War during the mid-1960s, when a student raised a Viet Minh flag over the campus. Protests escalated as the war continued and were only exacerbated after the National Guard fired on student protesters at Kent State University in 1970. Over 200 students occupied Urey Hall, with one student setting himself on fire in protest of the war.
Early research activity and faculty quality, notably in the sciences, was integral to shaping the focus and culture of the university. Even before The University of California-San Diego had its own campus, faculty recruits had already made significant research breakthroughs, such as the Keeling Curve, a graph that plots rapidly increasing carbon dioxide levels in the atmosphere and was the first significant evidence for global climate change; the Kohn–Sham equations, used to investigate particular atoms and molecules in quantum chemistry; and the Miller–Urey experiment, which gave birth to the field of prebiotic chemistry.
Engineering, particularly computer science, became an important part of the university’s academics as it matured. University researchers helped develop The University of California-San Diego Pascal, an early machine-independent programming language that later heavily influenced Java; the National Science Foundation Network, a precursor to the Internet; and the Network News Transfer Protocol during the late 1970s to 1980s. In economics, the methods for analyzing economic time series with time-varying volatility (ARCH), and with common trends (co-integration) were developed. The University of California-San Diego maintained its research intense character after its founding, racking up many Nobel Laureates affiliated within 50 years of history.
Under Richard C. Atkinson’s leadership as chancellor from 1980 to 1995, The University of California-San Diego strengthened its ties with the city of San Diego by encouraging technology transfer with developing companies, transforming San Diego into a world leader in technology-based industries. He oversaw a rapid expansion of the School of Engineering, later renamed after Qualcomm founder Irwin M. Jacobs, with the construction of the San Diego Supercomputer Center and establishment of the computer science, electrical engineering, and bioengineering departments. Private donations increased from $15 million to nearly $50 million annually, faculty expanded by nearly 50%, and enrollment grew during his administration. By the end of his chancellorship, the quality of The University of California-San Diego graduate programs was ranked highly in the nation by The National Research Council.
The University of California-San Diego continued to undergo further expansion during the first decade of the new millennium with the establishment and construction of two new professional schools — the Skaggs School of Pharmacy and Rady School of Management—and the California Institute for Telecommunications and Information Technology, a research institute run jointly with University of California-Irvine. The University of California-San Diego also reached two financial milestones during this time, becoming the first university in the western region to raise over $1 billion in its eight-year fundraising campaign in 2007 and also obtaining an additional $1 billion through research contracts and grants in a single fiscal year for the first time. Despite this, due to the California budget crisis, the university loaned $40 million against its own assets in 2009 to offset a significant reduction in state educational appropriations. The salary of Pradeep Khosla, who became chancellor in 2012, has been the subject of controversy amidst continued budget cuts and tuition increases.
On November 27, 2017, The University of California-San Diego announced it would leave its longtime athletic home of the California Collegiate Athletic Association, an NCAA Division II league, to begin a transition to Division I in 2020. At that time, it would join the Big West Conference, already home to four other UC campuses (Davis, Irvine, Riverside, and Santa Barbara). The transition period would run through the 2023–24 school year. The university prepared to transition to NCAA Division I competition on July 1, 2020.
Research
Applied Physics and Mathematics
The Nature Index lists The University of California-San Diego highly in the United States for research output by article count. The university operates several organized research units, including the Center for Astrophysics and Space Sciences (CASS), the Center for Drug Discovery Innovation, and the Institute for Neural Computation. The University of California-San Diego also maintains close ties to the nearby Scripps Research Institute and Salk Institute for Biological Studies. In 1977, The University of California-San Diego developed and released the University of California-San Diego Pascal programming language. The university was designated as one of the original national Alzheimer’s disease research centers in 1984 by the National Institute on Aging. In 2018, The University of California-San Diego received $10.5 million from The DOE’s National Nuclear Security Administration to establish the Center for Matters under Extreme Pressure (CMEC).
The University of California-San Diego founded The San Diego Supercomputer Center in 1985, which provides high performance computing for research in various scientific disciplines. In 2000, The University of California-San Diego partnered with The University of California-Irvine to create the Qualcomm Institute, which integrates research in photonics, nanotechnology, and wireless telecommunication to develop solutions to problems in energy, health, and the environment.
The University of California-San Diego also operates the Scripps Institution of Oceanography, one of the largest centers of research in earth science in the world, which predates the university itself. Together, SDSC and SIO, along with funding partner universities California Institute of Technology, San Diego State University, and The University of California-Santa Barbara, manage the High Performance Wireless Research and Education Network.
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