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  • richardmitnick 1:36 pm on July 30, 2020 Permalink | Reply
    Tags: , , , , , White dwarfs   

    From JHU HUB: “Johns Hopkins astrophysicists observe long-theorized quantum phenomena” 

    From JHU HUB

    7.30.20
    Saralyn Cruickshank

    1
    Planetary nebula NGC 2440’s central star, HD62166, is possibly the hottest known white dwarf star discovered yet. White dwarfs exhibit puzzling quantum phenomena: As they gain mass, they shrink in size.
    Image credit: Pixabay / WikiImages

    At the heart of every white dwarf star—the dense stellar object that remains after a star has burned away its fuel reserve of gases as it nears the end of its life cycle—lies a quantum conundrum: as white dwarfs add mass, they shrink in size, until they become so small and tightly compacted that they cannot sustain themselves, collapsing into a neutron star.

    This puzzling relationship between a white dwarf’s mass and size, called the mass-radius relation, was first theorized by Nobel Prize-winning astrophysicist Subrahmanyan Chandrasekhar in the 1930s. Now, a team of Johns Hopkins astrophysicists has developed a method to observe the phenomenon itself using astronomical data collected by the Sloan Digital Sky Survey and a recent dataset released by the Gaia Space Observatory. The combined datasets provided more than 3,000 white dwarfs for the team to study.

    SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude2,788 meters (9,147 ft)

    ESA/GAIA satellite

    A report of their findings, led by Hopkins senior Vedant Chandra, is now in press in The Astrophysical Journal.

    “The mass-radius relation is a spectacular combination of quantum mechanics and gravity, but it’s counterintuitive for us—we think that as an object gains mass, it should get bigger,” says Nadia Zakamska, an associate professor in the Department of Physics and Astronomy who supervised the student researchers. “The theory has existed for a long time, but what’s notable is that the dataset we used is of unprecedented size and unprecedented accuracy. These measurement methods, which in some cases were developed years ago, all of a sudden work so much better and these old theories can finally be probed.”

    The team obtained their results using a combination of measurements, including primarily the gravitational redshift effect, which is the change of wavelengths of light from blue to red as light moves away from an object. It is a direct result of Einstein’s theory of general relativity.

    “To me, the beauty of this work is that we all learn these theories about how light will be affected by gravity in school and in textbooks, but now we actually see that relationship in the stars themselves,” says fifth-year graduate student Hsiang-Chih Hwang, who proposed the study and first recognized the gravitational redshift effect in the data.

    The team also had to account for how a star’s movement through space might affect the perception of its gravitational redshift. Similar to how a fire engine siren changes pitch according to its movement in relation to the person listening, light frequencies also change depending on movement of the light-emitting object in relation to the observer. This is called the Doppler effect, and is essentially a distracting “noise” that complicates the measurement of the gravitational redshift effect, says study contributor Sihao Cheng, a fourth-year graduate student.

    To account for the variations caused by the Doppler effect, the team classified white dwarfs in their sample set by radius. They then averaged the redshifts of stars of a similar size, effectively determining that no matter where a star itself is located or where it’s moving in relation to Earth, it can be expected to have an intrinsic gravitational redshift of a certain value. Think of it as taking an average measurement of all the pitches of all fire engines moving around in a given area at a given time—you can expect that any fire engine, no matter which direction it’s moving, will have an intrinsic pitch of that average value.

    These intrinsic gravitational redshift values can be used to study stars that are observed in future datasets. The researchers say that upcoming datasets that are larger and more accurate will allow for further fine-tuning of their measurements, and that this data may contribute to the future analysis of white dwarf chemical composition.

    They also say their study represents an exciting advance from theory to observed phenomena.

    “Because the star gets smaller as it gets more massive, the gravitational redshift effect also grows with mass,” Zakamska says. “And this is a bit easier to comprehend—it’s easier to get out of a less dense, bigger object than it is to get out of a more massive, more compact object. And that’s exactly what we saw in the data.”

    The team is even finding captive audiences for their research at home—where they’ve conducted their work amid the coronavirus pandemic.

    “The way I extolled it to my granddad is, you’re basically seeing quantum mechanics and Einstein’s theory of general relativity coming together to produce this result,” Chandra says. “He was very excited when I put it that way.”

    See the full article here .


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    About the Hub
    We’ve been doing some thinking — quite a bit, actually — about all the things that go on at Johns Hopkins. Discovering the glue that holds the universe together, for example. Or unraveling the mysteries of Alzheimer’s disease. Or studying butterflies in flight to fine-tune the construction of aerial surveillance robots. Heady stuff, and a lot of it.

    In fact, Johns Hopkins does so much, in so many places, that it’s hard to wrap your brain around it all. It’s too big, too disparate, too far-flung.

    We created the Hub to be the news center for all this diverse, decentralized activity, a place where you can see what’s new, what’s important, what Johns Hopkins is up to that’s worth sharing. It’s where smart people (like you) can learn about all the smart stuff going on here.

    At the Hub, you might read about cutting-edge cancer research or deep-trench diving vehicles or bionic arms. About the psychology of hoarders or the delicate work of restoring ancient manuscripts or the mad motor-skills brilliance of a guy who can solve a Rubik’s Cube in under eight seconds.

    There’s no telling what you’ll find here because there’s no way of knowing what Johns Hopkins will do next. But when it happens, this is where you’ll find it.

    Johns Hopkins niversity opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

    The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

    What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

     
  • richardmitnick 2:14 pm on July 23, 2020 Permalink | Reply
    Tags: "Spectacular ultraviolet flash may finally explain how white dwarfs explode", , , , , For just the second time ever astrophysicists have spotted a spectacular flash of ultraviolet (UV) light accompanying a white dwarf explosion., , Perlmutter Riess and Schmidt proved the accelerating expansion of the universe using Type 1A supernovae., Saul Perlmutter [The Supernova Cosmology Project] Adam Riess and Brian Schmidt [The High-z Supernova Search Team] shared the 2011 Nobel Prize in Physics., SN2019yvq, , White dwarfs,   

    From Northwestern University: “Spectacular ultraviolet flash may finally explain how white dwarfs explode” 

    Northwestern U bloc
    From Northwestern University

    July 23, 2020

    Amanda Morris
    (847) 467-6790
    amandamo@northwestern.edu

    White dwarf explosion caused extremely rare burst of ultraviolet (UV) light.
    This is just the second time a UV flash accompanied a type Ia supernova.
    White dwarfs are cool objects; UV light requires something incredibly hot.
    Researchers will have a better understanding of how white dwarfs explode as they continue to watch the event throughout the year.

    For just the second time ever, astrophysicists have spotted a spectacular flash of ultraviolet (UV) light accompanying a white dwarf explosion.

    An extremely rare type of supernova, the event is poised to offer insights into several long-standing mysteries, including what causes white dwarfs to explode, how dark energy accelerates the cosmos and how the universe creates heavy metals, such as iron.

    “The UV flash is telling us something very specific about how this white dwarf exploded,” said Northwestern University astrophysicist Adam Miller, who led the research. “As time passes, the exploded material moves farther away from the source. As that material thins, we can see deeper and deeper. After a year, the material will be so thin that we will see all the way into the center of the explosion.”

    At that point, Miller said, his team will know more about how this white dwarf — and all white dwarfs, which are dense remnants of dead stars — explode.

    1
    Zwicky Transient Facility composite image of SN2019yvq (blue dot in the center of the image) in the host galaxy NGC 4441 (large yellow galaxy in the center of the image), which is nearly 140 million light-years away from Earth. SN 2019yvq exhibited a rarely observed ultraviolet flash in the days after the star exploded.
    Credit: ZTF/A. A. Miller (Northwestern University) and D. Goldstein (Caltech)

    Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

    Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

    The paper was published today (July 23) in The Astrophysical Journal.

    Common event with a rare twist

    Using the Zwicky Transient Facility [above] in California, researchers first spotted the peculiar supernova in December 2019 — just a day after it exploded. The event, dubbed SN2019yvq, occurred in a relatively nearby galaxy located 140 million light-years from Earth, very close to tail of the dragon-shaped Draco constellation.

    Within hours, astrophysicists used NASA’s Neil Gehrels Swift Observatory to study the phenomenon in ultraviolet and X-ray wavelengths.

    NASA Neil Gehrels Swift Observatory

    They immediately classified SN2019yvq as a type Ia (pronounced “one-A”) supernova, a fairly frequent event that occurs when a white dwarf explodes.

    “These are some of the most common explosions in the universe,” Miller said. “But what’s special is this UV flash. Astronomers have searched for this for years and never found it. To our knowledge, this is actually only the second time a UV flash has been seen with a type Ia supernova.”

    Heated mystery

    The rare flash, which lasted for a couple days, indicates that something inside or nearby the white dwarf was incredibly hot. Because white dwarfs become cooler and cooler as they age, the influx of heat puzzled astronomers.

    “The simplest way to create UV light is to have something that’s very, very hot,” Miller said. “We need something that is much hotter than our sun — a factor of three or four times hotter. Most supernovae are not that hot, so you don’t get the very intense UV radiation. Something unusual happened with this supernova to create a very hot phenomenon.”

    Miller and his team believe this is an important clue to understanding why white dwarfs explode, which has been a long-standing mystery in the field. Currently, there are multiple competing hypotheses. Miller is particularly interested in exploring four different hypotheses, which match his team’s data analysis from SN2019yvq.

    Potential scenarios that could cause a white dwarf to explode with a UV flash are:

    A white dwarf consumes material from its companion star and becomes so massive and unstable that it explodes. The white dwarf’s expelled material and the companion star collide, causing a flash of UV emission;
    Extremely hot radioactive material in the white dwarf’s core mixes with its outer layers, causing the outer shell to reach higher temperatures than usual;
    An outer layer of helium ignites carbon within the white dwarf, causing an extremely hot double explosion and a UV flash;
    Two white dwarfs merge, triggering an explosion with colliding ejecta that emit UV radiation.

    “Within a year,” Miller said, “we’ll be able to figure out which one of these four is the most likely explanation.”

    Earth-shattering insights

    Once the researchers know what caused the explosion, they will apply those findings to learn more about planet formation and dark energy.

    Because most of the iron in the universe is created by type Ia supernovae, better understanding this phenomenon could tell us more about our own planet. Iron from exploded stars, for example, formed the core of all rocky planets, including Earth.

    “If you want to understand how the Earth formed, you need to understand where iron came from and how much iron was needed,” Miller said. “Understanding the ways in which a white dwarf explodes gives us a more precise understanding of how iron is created and distributed throughout the universe.”

    Illuminating dark energy

    White dwarfs already play an enormous role in physicists’ current understanding of dark energy as well. Physicists predict that white dwarfs all have the same brightness when they explode. So type Ia supernovae are considered “standard candles,” allowing astronomers to calculate exactly how far the explosions lie from Earth. Using supernovae to measure distances led to the discovery of dark energy, a finding recognized with the 2011 Nobel Prize in Physics.

    _____________________________________________
    4

    4 October 2011

    The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2011

    with one half to

    Saul Perlmutter
    The Supernova Cosmology Project
    Lawrence Berkeley National Laboratory and University of California,
    Berkeley, CA, USA

    and the other half jointly to

    Brian P. Schmidt
    The High-z Supernova Search Team
    Australian National University,
    Weston Creek, Australia

    and

    Adam G. Riess
    The High-z Supernova Search Team
    Johns Hopkins University and Space Telescope Science Institute,
    Baltimore, MD, USA

    “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae”

    Saul Perlmutter [The Supernova Cosmology Project] shared the 2006 Shaw Prize in Astronomy, the 2011 Nobel Prize in Physics, and the 2015 Breakthrough Prize in Fundamental Physics with Brian P. Schmidt and Adam Riess [The High-z Supernova Search Team] for providing evidence that the expansion of the universe is accelerating.

    Written in the stars

    “Some say the world will end in fire, some say in ice…” *
    What will be the final destiny of the Universe? Probably it will end in ice, if we are to believe this year’s Nobel Laureates in Physics. They have studied several dozen exploding stars, called supernovae, and discovered that the Universe is expanding at an ever-accelerating rate. The discovery came as a complete surprise even to the Laureates themselves.

    In 1998, cosmology was shaken at its foundations as two research teams presented their findings. Headed by Saul Perlmutter [The Supernova Cosmology Project], one of the teams had set to work in 1988. Brian Schmidt headed another team [The High-z Supernova Search Team], launched at the end of 1994, where Adam Riess was to play a crucial role.

    The research teams raced to map the Universe by locating the most distant supernovae. More sophisticated telescopes on the ground and in space, as well as more powerful computers and new digital imaging sensors (CCD, Nobel Prize in Physics in 2009), opened the possibility in the 1990s to add more pieces to the cosmological puzzle.

    The teams used a particular kind of supernova, called type Ia supernova. It is an explosion of an old compact star that is as heavy as the Sun but as small as the Earth. A single such supernova can emit as much light as a whole galaxy. All in all, the two research teams found over 50 distant supernovae whose light was weaker than expected – this was a sign that the expansion of the Universe was accelerating. The potential pitfalls had been numerous, and the scientists found reassurance in the fact that both groups had reached the same astonishing conclusion.

    For almost a century, the Universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the Universe will end in ice.

    The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma – perhaps the greatest in physics today. What is known is that dark energy constitutes about three quarters of the Universe. Therefore the findings of the 2011 Nobel Laureates in Physics have helped to unveil a Universe that to a large extent is unknown to science. And everything is possible again.
    _____________________________________________

    Standard Candles to measure age and distance of the universe from supernovae. NASA

    5
    Evolution of the ultraviolet and visible light emitted from SN2019yvq. Most Type Ia SNe emit far more light in the visible region of the electromagnetic spectrum than in the ultraviolet. As shown here, SN 2019yvq exhibited a spectacular ultraviolet flash just after it exploded. Credit: A. A. Miller/Northwestern University

    “We don’t have a direct way to measure the distance to other galaxies,” Miller explained. “Most galaxies are actually moving away from us. If there is a type Ia supernova in a distant galaxy, we can use it to measure a combination of distance and velocity that allows us to determine the acceleration of the universe. Dark energy remains a mystery. But these supernovae are the best way to probe dark energy and understand what it is.”

    And by better understanding white dwarfs, Miller believes we potentially could better understand dark energy and how fast it causes the universe to accelerate.

    “At the moment, when measuring distances, we treat all of these explosions as the same, yet we have good reason to believe that there are multiple explosion mechanisms,” he said. “If we can determine the exact explosion mechanism, we think we can better separate the supernovae and make more precise distance measurements.”

    The paper, “The spectacular ultraviolet flash from the peculiar type Ia supernova 2019yvq,” was partially supported by the Large Synoptic Survey Telescope Corporation, the Brinson Foundation and the Moore Foundation.

    See the full article here .

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    Stem Education Coalition

    Northwestern South Campus
    South Campus

    Northwestern is recognized nationally and internationally for its educational programs.

    On May 31, 1850, nine men gathered to begin planning a university that would serve the Northwest Territory.

    Given that they had little money, no land and limited higher education experience, their vision was ambitious. But through a combination of creative financing, shrewd politicking, religious inspiration and an abundance of hard work, the founders of Northwestern University were able to make that dream a reality.

    In 1853, the founders purchased a 379-acre tract of land on the shore of Lake Michigan 12 miles north of Chicago. They established a campus and developed the land near it, naming the surrounding town Evanston in honor of one of the University’s founders, John Evans. After completing its first building in 1855, Northwestern began classes that fall with two faculty members and 10 students.
    Twenty-one presidents have presided over Northwestern in the years since. The University has grown to include 12 schools and colleges, with additional campuses in Chicago and Doha, Qatar.

     
  • richardmitnick 12:42 pm on August 6, 2019 Permalink | Reply
    Tags: "Ghosts of Ancient Explosions Live on in Stars Today", , , , , , , White dwarfs,   

    From Caltech: “Ghosts of Ancient Explosions Live on in Stars Today” 

    Caltech Logo

    From Caltech

    August 05, 2019

    Contact
    Lori Dajose
    (626) 395‑1217
    ldajose@caltech.edu

    The chemical composition of certain stars gives clues about their predecessors, stars that have long since exploded and faded.

    1
    Image of a Type Ia supernova. Credit: Zwicky Transient Facility

    Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

    Edwin Hubble at Caltech Palomar Samuel Oschin 48 inch Telescope, (credit: Emilio Segre Visual Archives/AIP/SPL)

    Caltech Palomar Intermediate Palomar Transient Factory telescope at the Samuel Oschin Telescope at Palomar Observatory,located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

    Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

    When small, dense stars called white dwarfs explode, they produce bright, short-lived flares called Type Ia supernovae. These supernovae are informative cosmological markers for astronomers—for example, they were used to prove that the universe is accelerating in its expansion.

    White dwarfs are not all the same, ranging from half of the mass of our sun to almost 50 percent more massive than our sun. Some explode in Type Ia supernovae; others simply die quietly. Now, by studying the “fossils” of long-exploded white dwarfs, Caltech astronomers have found that early on in the universe, white dwarfs often exploded at lower masses than they do today. This discovery indicates that a white dwarf could explode from a variety of causes, and does not necessarily have to reach a critical mass before exploding.

    A paper about the research, led by Evan Kirby, assistant professor of astronomy, appears in The Astrophysical Journal.

    Near the end of their lives, a majority of stars like our sun dwindle down into dim, dense white dwarfs, with all their mass packed into a space about the size of Earth. Sometimes, white dwarfs explode in what’s called a Type Ia (pronounced one-A) supernova.

    It is uncertain why some white dwarfs explode while others do not. In the early 1900s, an astrophysicist named Subrahmanyan Chandrasekhar calculated that if a white dwarf had more than 1.4 times the mass of our sun, it would explode in a Type Ia supernova. This mass was dubbed the Chandrasekhar mass. Though Chandrasekhar’s calculations gave one explanation for why some more massive white dwarfs explode, it did not explain why other white dwarfs less than 1.4 solar masses also explode.

    Studying Type Ia supernovae is a time-sensitive process; they flare into existence and fade back into darkness all within a few months. To study long-gone supernovae and the white dwarfs that produced them, Kirby and his team use a technique colloquially called galactic archaeology.

    Galactic archaeology is the process of looking for chemical signatures of long-past explosions in other stars. When a white dwarf explodes in a Type Ia supernova, it pollutes its galactic environment with elements forged in the explosion—heavy elements like nickel and iron. The more massive a star is when it explodes, the more heavy elements will be formed in the supernova. Then, those elements become incorporated into any newly forming stars in that region. Just as fossils today give clues about animals that have long ceased to exist, the amounts of nickel in stars illustrates how massive their long-exploded predecessors must have been.

    Using the Keck II telescope, Kirby and his team first looked at certain ancient galaxies, those that ran out of material to form stars in the first billion years of the universe’s life.

    Keck 2 telescope Maunakea Hawaii USA, 4,207 m (13,802 ft)

    Most of the stars in these galaxies, the team found, had relatively low nickel content. This meant that the exploded white dwarfs that gave them that nickel must have been relatively low mass—about as massive as the sun, lower than the Chandrasekhar mass.

    Yet, the researchers found that the nickel content was higher in more recently formed galaxies, meaning that as more time went by since the Big Bang, white dwarfs had begun to explode at higher masses.

    “We found that, in the early universe, white dwarfs were exploding at lower masses than later in the universe’s lifetime,” says Kirby.”It’s still unclear what has driven this change.”

    Understanding the processes that result in Type Ia supernovae is important because the explosions themselves are useful tools for making measurements of the universe. Regardless of how they exploded, most Type Ia supernovae follow a well-characterized relationship between their luminosity and the time it takes for them to fade.

    “We call Type Ia supernovae ‘standardizable candles.’

    Standard Candles to measure age and distance of the universe from supernovae NASA

    If you look at a candle at a distance, it will look dimmer than when it’s up close. If you know how bright it is supposed to be up close, and you measure how bright it is at a distance, you can calculate that distance,” says Kirby. “Type Ia supernovae have been very useful in calculating things like the rate of expansion of the universe. We use them all the time in cosmology. So, it’s important to understand where they come from and characterize the white dwarfs that generate these explosions.”

    The next steps are to study elements other than nickel, in particular, manganese. Manganese production is very sensitive to the mass of the supernova that produces it, and therefore gives a precise way to validate the conclusions drawn by the nickel content.

    The paper is titled Evidence for Sub-Chandrasekhar Type Ia Supernovae from Stellar Abundances in Dwarf Galaxies. In addition to Kirby, co-authors are Justin L. Xie and Rachel Guo of Harvard University, Caltech graduate student Mithi A. C. de los Reyes, Maria Bergemann and Mikhail Kovalev of the Max Planck Institute for Astronomy, Ken J. Shen of University of California Berkeley, and Anthony L. Piro and Andrew McWilliam of the Observatories of the Carnegie Institution for Science. Funding was provided by the National Science Foundation, a Cottrell Scholar award from the Research Corporation for Science Advancement, and Caltech.

    See the full article here .


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    Please help promote STEM in your local schools.


    Stem Education Coalition

    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

    Caltech campus

     
  • richardmitnick 12:43 pm on December 19, 2018 Permalink | Reply
    Tags: , , , , , White dwarfs   

    From astrobites: “Hunting for Variable White Dwarfs in the GALEX Archives” 

    Astrobites bloc

    From astrobites

    Title: Detections and Constraints on White Dwarf Variability from Time-Series GALEX Observations
    Authors: Dominick M. Rowan, Michael A. Tucker, Benjamin J. Shappee, and J.J. Hermes
    First Author’s Institution: Institute for Astronomy, University of Hawaii, Honolulu, USA

    U Hawaii Institute for Astronomy


    Status: Submitted to MNRAS, open access

    1
    Figure 1: A plot of brightness against colour for stars that were observed by the Gaia satellite. White dwarf candidates that were selected for today’s paper are shown by the coloured points. Source: Figure 1 in today’s paper.

    White dwarfs are stars in their silver years. 97% of all stars will end their lives as white dwarfs. These stars have stopped all fusion in their cores and are powered by left-over heat from their younger lives. It’s easy to think of white dwarfs as ‘dead stars’, doing nothing but hanging in space while they slowly cool. However, these stars have plenty of activity left in them. Some white dwarfs pulsate: instabilities in their atmospheres cause them to shrink and stretch in size. Studying these pulsations can tell us about the internal structure of a white dwarf. Other white dwarfs are in binary systems and undergo eclipses when their companion star blocks their light from reaching us. Studying these eclipses can help us accurately measure the masses and radii of the two stars. And some white dwarfs have their own planetary systems, which collide, rip themselves apart and shower the white dwarf with debris. These can help us study the composition of planets and give us an insight into planetary systems in the late stages of their lives.

    See the full article here .


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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 5:40 am on August 30, 2018 Permalink | Reply
    Tags: Astronomers use Hubble to 'weigh' Dog Star's companion, , , , , , , Red shift, White dwarfs   

    From Hubble via Manu: “Astronomers use Hubble to ‘weigh’ Dog Star’s companion” 


    From Manu Garcia, a friend from IAC.

    The universe around us.
    Astronomy, everything you wanted to know about our local universe and never dared to ask.

    NASA Hubble Banner

    NASA/ESA Hubble Telescope

    From NASA/ESA Hubble Telescope

    13 December 2005
    Martin Barstow
    University of Leicester, United Kingdom
    Tel: +44-11-44-116-252-3492
    Cell: +44-776-62-333-62
    E-mail: mab@star.le.ac.uk or

    Lars Lindberg Christensen
    Hubble/ESA, Garching, Germany
    Tel: +49-(0)89-3200-6306
    Cellular: +49-(0)173-3872-621
    E-mail: lars@eso.org

    Julia Maddock
    PPARC Press Office
    Tel +44-17-93-44-20-94
    Email: Julia.maddock@pparc.ac.uk

    Howard Bond
    Space Telescope Science Institute, Baltimore, USA
    Tel: +1-410-338-4718
    E-mail: bond@stsci.edu

    Jay Holberg, Lunar Planetary Lab, Tucson, USA
    Tel: +1-520-621-4571
    E-mail: holberg@argus.lpl.arizona.edu

    1
    White dwarfs are important to theories of both stellar and cosmological evolution. New results published in the Monthly Notices of the Royal Astronomical Society provide for the first time an accurate measurement of the weight of the nearest white dwarf, Sirius B, companion of the brightest star in the sky. It turns out that Sirius’s companion, despite being smaller than the Earth, has a mass that is 98% that of our own Sun.

    2
    This picture is an artist’s impression showing how the binary star system of Sirius A and its diminutive blue companion, Sirius B, might appear to an interstellar visitor. The large, bluish-white star Sirius A dominates the scene, while Sirius B is the small but very hot and blue white-dwarf star on the right. The two stars revolve around each other every 50 years. White dwarfs are the leftover remnants of stars similar to our Sun. The Sirius system, only 8.6 light-years from Earth, is the fifth closest stellar system known. Sirius B is faint because of its tiny size. Its diameter is only 7,500 miles (about 12 thousand kilometres), slightly smaller than the size of our Earth. The Sirius system is so close to Earth that most of the familiar constellations would have nearly the same appearance as in our own sky. In this rendition, we see in the background the three bright stars that make up the Summer Triangle: Altair, Deneb, and Vega. Altair is the white dot above Sirius A; Deneb is the dot to the upper right; and Vega lies below Sirius B. But there is one unfamiliar addition to the constellations: our own Sun is the second-magnitude star, shown as a small dot just below and to the right of Sirius A. Credit: NASA, ESA and G. Bacon (STScI)

    3
    Based on the Hubble measurements made with the Space Telescope Imaging Spectrograph, an international team found that Sirius B has a mass that is 98 percent that of our own Sun. Despite this large mass Sirius B is only 12,000 kilometers in diameter, making it smaller than even the Earth and much denser. Sirius B’s powerful gravitational field is 350,000 times greater than Earth’s, meaning that a 68 kilogram person would weigh 25 million kilograms standing on its surface. Credit: NASA/ESA Hubble

    4
    This picture is an artist’s impression showing how the binary star system of Sirius A and its diminutive blue companion, Sirius B, might appear to an interstellar visitor. The large, bluish-white star Sirius A dominates the scene, while Sirius B is the small but very hot and blue white-dwarf star on the right. The two stars revolve around each other every 50 years. White dwarfs are the leftover remnants of stars similar to our Sun. The Sirius system, only 8.6 light-years from Earth, is the fifth closest stellar system known. Sirius B is faint because of its tiny size. Its diameter is only 7,500 miles (about 12 thousand kilometres), slightly smaller than the size of our Earth. The Sirius system is so close to Earth that most of the familiar constellations would have nearly the same appearance as in our own sky. In this rendition, we see in the background the three bright stars that make up the Summer Triangle: Altair, Deneb, and Vega. Altair is the white dot above Sirius A; Deneb is the dot to the upper right; and Vega lies below Sirius B. But there is one unfamiliar addition to the constellations: our own Sun is the second-magnitude star, shown as a small dot just below and to the right of Sirius A. Credit: NASA, ESA and G. Bacon (STScI)

    For astronomers, it’s always been a source of frustration that the nearest white-dwarf star is buried in the glow of the brightest star in the nighttime sky. This burned-out stellar remnant is a faint companion of the brilliant blue-white Dog Star, Sirius, located in the winter constellation Canis Major.

    Now, an international team of astronomers has used the keen eye of the NASA/ESA Hubble Space Telescope to isolate the light from the white dwarf, called Sirius B. The new results allow them to measure precisely the white dwarf’s mass based on how its intense gravitational field alters the wavelengths of light emitted by the star.

    “Studying Sirius B has challenged astronomers for more than 140 years,” said Martin Barstow of the University of Leicester, U.K., who is the leader of the observing team. “Only with Hubble have we at last been able to obtain the observations we need, uncontaminated by the light from Sirius, in order to measure its change in wavelengths.”

    “Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution. Our Sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions that are used to measure cosmological distances and the expansion rate of the universe.

    A white dwarf fed by a normal star reaches the critical mass and explodes as a type Ia supernova. Credit: NASA/CXC/M Weiss

    Measurements based on Type Ia supernovae are fundamental to understanding ‘dark energy,’ a dominant repulsive force stretching the universe apart.

    Dark energy depiction. Image: Volker Springle/Max Planck Institute for Astrophysics/SP)

    MPG Institute for Astrophysics

    Standard Candles to measure age and distance of the universe NASA

    Also, the method used to determine the white dwarf’s mass relies on one of the key predictions of Einstein’s theory of General Relativity; that light loses energy when it attempts to escape the gravity of a compact star.”

    Sirius B has a diameter of 12,000 kilometres, less than the size of Earth, but is much denser. Its powerful gravitational field is 350,000 times greater than Earth’s, meaning that a 68 kilogram person would weigh 25 million kilograms standing on its surface. Light from the surface of the hot white dwarf has to climb out of this gravitational field and is stretched to longer, redder wavelengths of light in the process. This effect, predicted by Einstein’s theory of General Relativity in 1916, is called gravitational redshift, and is most easily seen in dense, massive, and hence compact objects whose intense gravitational fields warp space near their surfaces.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Red shift and evidence for an expanding universe spiff.rit.edu

    Astronomical Red shift Imaging the Universe University of Iowa

    Red shift and wave length shift-The Earliest Stars And Galaxies In The Universe Science at ESA

    Based on the Hubble measurements of the redshift, made with the Space Telescope Imaging Spectrograph, the team found that Sirius B has a mass that is 98 percent that of our own Sun. Sirius itself has a mass of two times that of the Sun and a diameter of 2.4 million kilometres.

    NASA/ESA Hubble Space Telescope Imaging Spectrograph

    White dwarfs are the leftover remnants of stars similar to our Sun. They have exhausted their nuclear fuel sources and have collapsed down to a very small size. Despite being the brightest white dwarf known, Sirius B is about 10,000 times fainter than Sirius itself, making it difficult to study with telescopes on the Earth’s surface because its light is swamped in the glare of its brighter companion. Astronomers have long relied on a fundamental theoretical relationship between the mass of a white dwarf and its diameter. The theory predicts that the more massive a white dwarf, the smaller its diameter. The precise measurement of Sirius B’s gravitational redshift allows an important observational test of this key relationship.

    The Hubble observations have also refined the measurement of Sirius B’s surface temperature to be 25,000 degrees C. Sirius itself has a surface temperature of 10,000 degrees C.

    At 8.6 light-years away, Sirius is one of the nearest known stars to Earth. Stargazers have watched Sirius since antiquity. Its diminutive companion, however, was not discovered until 1862, when it was first glimpsed by astronomers examining Sirius through one of the most powerful telescopes of that time.

    See the full article here .


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    The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

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  • richardmitnick 1:28 pm on January 9, 2018 Permalink | Reply
    Tags: , , , , , White dwarf’s inner makeup is mapped for the first time, White dwarfs   

    From ScienceNews: “White dwarf’s inner makeup is mapped for the first time” 

    ScienceNews

    January 8, 2018
    Lisa Grossman

    The stellar corpse is richer in oxygen than expected, challenging long-standing theories about stellar evolution.

    1
    WHAT LIES WITHIN The inner structure of a white dwarf star (shown in this artist’s impression) has been mapped for the first time — and it’s more oxygen-rich than expected. Stéphane Charpinet.

    Astronomers have probed the inner life of a dead star. Tiny changes in a white dwarf’s brightness reveal that the stellar corpse has more oxygen in its core than expected, researchers report online January 8 in Nature. The finding could challenge theories of how stars live and die, and may have implications for measuring the expansion of the universe.

    As a star ages, it sheds most of its gas into space until all that remains is a dense core of carbon and oxygen, the ashes of a lifetime of burning helium (SN: 4/30/16, p. 12). That core, plus a thin shellacking of helium, is called a white dwarf.

    But the proportion of those elements relative to one another was uncertain. “From theory, we have a rough idea of how it’s supposed to be, but we have no way to measure it directly,” says astrophysicist Noemi Giammichele, now at the Institute of Research in Astrophysics and Planetology in Toulouse, France.

    Luckily, some white dwarfs encode their inner nature on their surface. These stars change their brightness in response to internal vibrations. Astrophysicists can infer a star’s internal structure from the vibrations, similar to how geologists learn about Earth’s interior by measuring seismic waves during an earthquake.

    Giammichele and her colleagues used data from NASA’s Kepler space telescope, which watched stars unblinkingly to track periodic changes in their brightness. Kepler’s chief aim was to find exoplanets, the worlds orbiting distant stars (SN Online: 10/31/17). But it also monitored white dwarf KIC 08626021, located 1,375 light-years away in the constellation Cygnus, for 23 months. The observations provided the highest-precision data ever on tiny changes in a white dwarf’s brightness and, indirectly, its vibrations.

    Next, Giammichele borrowed a computer simulation technique from her former life as an aeronautical engineer to figure out how the changes in vibrations related to the makeup of the core. The team ran millions of simulations, looking for one that reproduced the exact light changes that Kepler observed. One simulation fit the data perfectly, showing that the white dwarf had the expected carbon and oxygen core with a thin shell of helium.

    But the details were surprising. The core was about 86 percent oxygen, 15 percent greater than physicists had previously calculated. That suggests that something about the processes that convert helium to carbon and oxygen or mix elements in the star’s core during its active lifetime must boost the amount of oxygen.

    Four other white dwarfs show a similar trend, says study coauthor Gilles Fontaine, an astrophysicist at the University of Montreal. “We certainly will go ahead and analyze many more.” If other white dwarfs turn out to be similar, the results will send theorists who study stellar evolution back to the drawing board, he says.

    White dwarfs are also thought to be the precursors of type 1a supernovas. These catastrophic stellar explosions were once thought to have the same intrinsic brightness, meaning they appeared brighter or dimmer depending only on their distance from Earth. Measuring their actual distances led to the discovery that the universe is expanding at an accelerating rate (SN: 8/6/16, p. 10), which physicists explain by invoking a mysterious substance called dark energy.

    More recent observations suggest that these so-called standard candles may not be so standard after all. If the white dwarfs that help create supernovas have varying oxygen contents, that may help explain some of the differences, Fontaine says.

    Accounting for that difference may someday help reveal details of what dark energy is made of, says astrophysicist Alexei Filippenko of the University of California, Berkeley. But those implications are a long way off. “Just how much bearing it will have on cosmology remains to be seen,” he says.

    See the full article here .

    Science News is edited for an educated readership of professionals, scientists and other science enthusiasts. Written by a staff of experienced science journalists, it treats science as news, reporting accurately and placing findings in perspective. Science News and its writers have won many awards for their work; here’s a list of many of them.

    Published since 1922, the biweekly print publication reaches about 90,000 dedicated subscribers and is available via the Science News app on Android, Apple and Kindle Fire devices. Updated continuously online, the Science News website attracted over 12 million unique online viewers in 2016.

    Science News is published by the Society for Science & the Public, a nonprofit 501(c) (3) organization dedicated to the public engagement in scientific research and education.

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  • richardmitnick 3:26 pm on October 5, 2017 Permalink | Reply
    Tags: , , , , NAOJ Cray XC30 ATERUI, , , , White dwarfs   

    From NOAJ Subaru: “Surface Helium Detonation Spells End for White Dwarf” 

    NAOJ

    NAOJ

    October 4, 2017
    No writer credit

    An international team of researchers has found evidence that the brightest stellar explosions in our Universe could be triggered by helium nuclear detonation near the surface of a white dwarf star. Using Hyper Suprime-Cam mounted on the Subaru Telescope, the team detected a type Ia supernova within a day after the explosion, and explained its behavior through a model calculated using the supercomputer ATERUI.

    NAOJ Cray XC30 ATERUI, installed in the NAOJ Mizusawa campus

    1
    Figure 1: A type Ia supernova detected within a day after exploding. Taken with Hyper Suprime-Cam mounted on the Subaru Telescope. Figure without the labels is linked here. (Credit: University of Tokyo/NAOJ)

    NAOJ Subaru Hyper Suprime-Cam

    Some stars end their lives with a huge explosion called a supernova. The most famous supernovae are the result of a massive star exploding, but a white dwarf, the remnant of an intermediate mass star like our Sun, can also explode. This can occur if the white dwarf is part of a binary star system. The white dwarf accretes material from the companion star, then at some point, it might explode as a type Ia supernova.

    Because of the uniform and extremely high brightness (about 5 billion times brighter than the Sun) of type Ia supernovae, they are often used for distance measurements in astronomy. However, astronomers are still puzzled by how these explosions are ignited. Moreover, these explosions only occur about once every 100 years in any given galaxy, making them difficult to catch.

    An international team of researchers led by Ji-an Jiang, a graduate student of the University of Tokyo, and including researchers from the University of Tokyo, the Kavli Institute for the Physics and Mathematics of the Universe (IPMU), Kyoto University, and the National Astronomical Observatory of Japan (NAOJ), tried to solve this problem. To maximize the chances of finding a type Ia supernova in the very early stages, the team used Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope, a combination which can capture an ultra-wide area of the sky at once. Also they developed a system to detect supernovae automatically in the heavy flood of data during the survey, which enabled real-time discoveries and timely follow-up observations.

    They discovered over 100 supernova candidates in one night with Subaru/Hyper Suprime-Cam, including several supernovae that had only exploded a few days earlier. In particular, they captured a peculiar type Ia supernova within a day of it exploding. Its brightness and color variation over time are different from any previously-discovered type Ia supernova. They hypothesized this object could be the result of a white dwarf with a helium layer on its surface. Igniting the helium layer would lead to a violent chain reaction and cause the entire star to explode. This peculiar behavior can be totally explained with numerical simulations calculated using the supercomputer ATERUI. “This is the first evidence that robustly supports a theoretically predicted stellar explosion mechanism!” said Jiang.

    This result is a step towards understand the beginning of type Ia supernovae. The team will continue to test their theory against other supernovae, by detecting more and more supernovae just after the explosion. The details of their study are to be published in Nature on October 5, 2017 (Jiang et al. 2017, A hybrid type la supernova with an early flash triggered by helium-shell detonation, Nature).

    See the full article here .

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    The National Astronomical Observatory of Japan (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations – the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude Observatory of Mizusawa, and a part of Research Institute of Atmospherics of Nagoya University.

    In the 2004 reform of national research organizations, NAOJ became a division of the National Institutes of Natural Sciences.

    NAOJ Subaru Telescope

    NAOJ Subaru Telescope interior
    Subaru

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    sft
    Solar Flare Telescope

    Nobeyama Radio Telescope - Copy
    Nobeyama Radio Observatory

    Nobeyama Solar Radio Telescope Array
    Nobeyama Radio Observatory: Solar

    Misuzawa Station Japan
    Mizusawa VERA Observatory

    NAOJ Okayama Astrophysical Observatory Telescope
    Okayama Astrophysical Observatory

    The National Astronomical Observatory of Japan (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations – the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude Observatory of Mizusawa, and a part of Research Institute of Atmospherics of Nagoya University.

    In the 2004 reform of national research organizations, NAOJ became a division of the National Institutes of Natural Sciences.

     
  • richardmitnick 2:20 pm on July 17, 2017 Permalink | Reply
    Tags: , , , , , White dwarfs   

    From astrobotes: “How well do we measure the radii of white dwarfs?” 

    Astrobites bloc

    Astrobites

    Jul 17, 2017
    Ingrid Pelisoli

    Title: Testing the white dwarf mass-radius relationship with eclipsing binaries
    Authors: S. G. Parsons, B. T. Gänsicke, T. R. Marsh et al.
    First Author’s Institution: Department of Physics and Astronomy, University of Sheffield, UK

    Status: Accepted to MNRAS [open access]

    Look outside your window. Can you see the Sun? If it’s night-time, just pick a random star instead. Our Sun one day will become a white dwarf star, and the chance that the random star you’ve picked will follow the same path is over 95%. White dwarfs are by far the most common final evolutionary state for a star. The famous supernovas actually only occur when a star is massive enough to burn elements heavier than helium in its core, and that is usually not the case. What happens instead is that the star can only produce elements up to carbon and oxygen, and then nuclear reactions in the core cease to occur. With no release of energy to counteract the gravitational force, the carbon-oxygen core will contract more and more until it becomes degenerate. This degenerate core is essentially the white dwarf, which becomes visible when the outer layers of the star are ejected on its final breadth of hydrogen burning in outer shells.

    What does it mean to be degenerate?

    Degenerate matter occurs in stars when the density is so high that all the electrons are cramped in lower energy states. But there’s a catch: due to the Pauli exclusion principle, only two fermions, e.g. electrons, can occupy the same energy level. So other electrons end up in higher energy states and cannot move to already filled lower energy levels. This makes degenerate matter very resistible to compression, and is in fact what keeps white dwarfs from collapsing.

    Degenerate matter has a really cool property: the pressure does not depend on temperature. This is because the kinetic energies of these electrons that cannot move to lower states of energy are quite high, and the rate of collisions between electrons and other particles is quite low, so the electrons essentially travel at the speed of light. The pressure on the gas depends on this speed. As the speed of light is the fastest the electrons can travel, adding heat will not change the pressure at all. This has an effect somewhat counter-intuitive. As the only way to increase pressure is by adding mass, when you increase pressure you also increase gravity and make the particles become spaced closer together, so the object becomes smaller. In other words, the more massive the white dwarf, the smaller its radius. As a consequence of this weird property of degenerate matter, there’s a relationship between a white dwarf’s mass and its radius, so that if you know one you can estimate the other and the other way around. But how accurate is this mass-radius relationship? Today’s paper authors decided to test!

    Obtaining mass and radius independently

    Despite being widely used, the mass-radius relationship remained untested observationally until this paper. That is because there aren’t many ways to estimate the mass of a white dwarf without relying on this relationship at some point. What the authors realised is that we can use eclipsing binary stars for that. Eclipsing binaries orbit in a plane which intersects our line of sight, allowing us to detect dips in light when the stars transit in front of each other. For these objects, we can combine photometric and spectroscopic measurements to estimate the mass and the radius independently.

    The shape of the eclipses of the white dwarfs by their companions, which in this paper are all main-sequence stars, gives us two pieces of information: the width and the duration of the eclipses. However, there’s an issue: we have three unknown quantities, namely the orbital inclination and the radius of each star. So we need one more piece of information to be able to determine all the unknowns.

    The authors suggest different methods, but what works best in their sample is to use the gravitational redshift estimated from spectroscopy. As the gravity in white dwarfs is really high (about 350,000 times the gravity of Earth!), light gets shifted a measurable amount towards the red when it exits the white dwarf, as it was delayed. The amount of redshift depends on the mass and radius of the white dwarf. Combining that with Kepler’s third law, we obtain a relationship between the white dwarf’s radius and the binary inclination. All we need is to estimate the radial velocity semi-amplitudes for both stars, hence the necessity of spectroscopy, which was also used to constrain the effective temperature of the white dwarf. Going back to Kepler’s third law with the estimated inclination, we can also estimate the mass of the white dwarf.

    So the combined fit of the light curve and the radial velocities of both stars, together with Kepler’s law and the known relation for gravitational shift, give us the mass and the radius without having to recur to the mass-radius relationship. Just the independent measure we needed!

    The results

    Figure 1 shows the comparison between the values the authors obtained with the theoretical models for different temperatures. Black lines are for the more common C/O core white dwarfs, while green lines are for white dwarfs with a He core. He-core white dwarfs are less massive and are formed when the outer envelope is lost by the progenitor star before helium is even ignited. They would be formed by lower mass stars that do not achieve conditions to burn helium, but the Universe is not old enough for them to have evolved off the main sequence just yet, so this objects are explained by some form of enhanced mass loss (such as binary evolution). As the analysed white dwarfs have different temperatures and core compositions, is difficult to define how the results agree with the theory based on this figure.

    1
    Figure 1: Comparison between mass and radius obtained from observations, represented by the red data points, and theoretical mass-radius relationships for different effective temperatures. Black lines assume a C/O core and green lines assume a He core. [Adapted from figure 9 in the paper.]

    Instead, the authors analyse what we see in Figure 2, which is the ratio between the radii estimated observationally and using the mass-radius relationship as a function of mass. Below 0.5 solar masses, the authors assume both a relationship assuming a He core and a C/O core. The first result is that white dwarfs with masses below 0.5 solar masses are more consistent with a He core, which agrees with our theories of stellar evolution. However, this is the first time we have direct observational evidence for that.

    2
    Figure 2: Ratio of the estimated white dwarf radii to theoretical predictions as a function of mass. Below 0.5 solar masses, black points assume a C/O core white dwarf and red points assume a He core white dwarf. Almost all radius measurement at this range are more consistent with He core. [Figure 10 in the paper.]

    Another interesting test the authors made was to compare surface gravities obtained from their fit to the spectra and derived from the mass and radius obtained from the light curves. Fitting spectra is the most widely used method to obtain the physical parameters of a white dwarf, so testing this independently is important to check we are basing our science in correct estimates. As Figure 3 shows, in most cases there’s excellent agreement between the results, but there are a few outliers where the spectral fit overestimates the gravity. In most of these cases, this disagreement can easily be explained by contamination from the companion. There’s one exception where the white dwarf dominates the spectrum so contamination from the companion is not enough to cause the discrepancy, but for this one the discrepancy is only at a 2-sigma level. Additional broadening mechanisms, such as magnetic fields, might be the cause for that.

    3
    Figure 3: Comparison between surface gravities estimated from spectral fits, and those computed from the mass and radius values obtained from the light curves. [Figure 13 in the paper.]

    Why does it matter?

    The authors came to conclude that both our mass-radius relationship and spectral modelling are quite good at estimating the properties of white dwarfs. This result is important not only for those directly interested in white dwarfs, but has also implications for cosmology. The radius of a white dwarf is related to its cooling rate, which depends on its surface area. As you can read on this bite, the cooling times of white dwarfs can be used to estimate the ages of different stellar populations. The mass-radius relationship also sets an upper limit to the mass of a white dwarf, which is an important result to the study of type Ia supernovae, which in turn are used to measure the expansion of the Universe.

    Now that we know that our theoretical models are doing a good job, we can keep doing cool science with them!

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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