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  • richardmitnick 2:28 pm on January 12, 2019 Permalink | Reply
    Tags: Astronomers find signatures of a ‘messy’ star that made its companion go supernova, , , , , , It takes many astronomers and a wide variety of types of telescopes working together to understand transient cosmic phenomena, , SN 2015cp, , , White dwarf stars   

    From University of Washington: “Astronomers find signatures of a ‘messy’ star that made its companion go supernova” 

    U Washington

    From University of Washington

    January 10, 2019
    James Urton

    An X-ray/infrared composite image of G299, a Type Ia supernova remnant in the Milky Way Galaxy approximately 16,000 light years away.NASA/Chandra X-ray Observatory/University of Texas/2MASS/University of Massachusetts/Caltech/NSF

    NASA/Chandra X-ray Telescope

    Caltech 2MASS Telescopes, a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center (IPAC) at Caltech, at the Whipple Observatory on Mt. Hopkins south of Tucson, AZ, Altitude 2,606 m (8,550 ft) and at the Cerro Tololo Inter-American Observatory at an altitude of 2200 meters near La Serena, Chile.

    Many stars explode as luminous supernovae when, swollen with age, they run out of fuel for nuclear fusion. But some stars can go supernova simply because they have a close and pesky companion star that, one day, perturbs its partner so much that it explodes.

    These latter events can happen in binary star systems, where two stars attempt to share dominion. While the exploding star gives off lots of evidence about its identity, astronomers must engage in detective work to learn about the errant companion that triggered the explosion.

    On Jan. 10 at the 2019 American Astronomical Society meeting in Seattle, an international team of astronomers announced that they have identified the type of companion star that made its partner in a binary system, a carbon-oxygen white dwarf star, explode. Through repeated observations of SN 2015cp, a supernova 545 million light years away, the team detected hydrogen-rich debris that the companion star had shed prior to the explosion.

    “The presence of debris means that the companion was either a red giant star or similar star that, prior to making its companion go supernova, had shed large amounts of material,” said University of Washington astronomer Melissa Graham, who presented the discovery and is lead author on the accompanying paper accepted for publication in The Astrophysical Journal.

    The supernova material smacked into this stellar litter at 10 percent the speed of light, causing it to glow with ultraviolet light that was detected by the Hubble Space Telescope and other observatories nearly two years after the initial explosion. By looking for evidence of debris impacts months or years after a supernova in a binary star system, the team believes that astronomers could determine whether the companion had been a messy red giant or a relatively neat and tidy star.

    The team made this discovery as part of a wider study of a particular type of supernova known as a Type Ia supernova. These occur when a carbon-oxygen white dwarf star explodes suddenly due to activity of a binary companion. Carbon-oxygen white dwarfs are small, dense and — for stars — quite stable. They form from the collapsed cores of larger stars and, if left undisturbed, can persist for billions of years.

    Type Ia supernovae have been used for cosmological studies because their consistent luminosity makes them ideal “cosmic lighthouses,” according to Graham. They’ve been used to estimate the expansion rate of the universe and served as indirect evidence for the existence of dark energy.

    An image of SN 1994D (lower left), a Type Ia supernova detected in 1994 at the edge of galaxy NGC 4526 (center).NASA/ESA/The Hubble Key Project Team/The High-Z Supernova Search Team.

    NASA/ESA Hubble Telescope

    Yet scientists are not certain what kinds of companion stars could trigger a Type Ia event. Plenty of evidence indicates that, for most Type Ia supernovae, the companion was likely another carbon-oxygen white dwarf, which would leave no hydrogen-rich debris in the aftermath. Yet theoretical models have shown that stars like red giants could also trigger a Type Ia supernova, which could leave hydrogen-rich debris that would be hit by the explosion. Out of the thousands of Type Ia supernovae studied to date, only a small fraction were later observed impacting hydrogen-rich material shed by a companion star. Prior observations of at least two Type Ia supernovae detected glowing debris months after the explosion. But scientists weren’t sure if those events were isolated occurrences, or signs that Type Ia supernovae could have many different kinds of companion stars.

    “All of the science to date that has been done using Type Ia supernovae, including research on dark energy and the expansion of the universe, rests on the assumption that we know reasonably well what these ‘cosmic lighthouses’ are and how they work,” said Graham. “It is very important to understand how these events are triggered, and whether only a subset of Type Ia events should be used for certain cosmology studies.”

    The team used Hubble Space Telescope observations to look for ultraviolet emissions from 70 Type Ia supernovae approximately one to three years following the initial explosion.

    “By looking years after the initial event, we were searching for signs of shocked material that contained hydrogen, which would indicate that the companion was something other than another carbon-oxygen white dwarf,” said Graham.

    In the case of SN 2015cp, a supernova first detected in 2015, the scientists found what they were searching for. In 2017, 686 days after the supernova exploded, Hubble picked up an ultraviolet glow of debris. This debris was far from the supernova source — at least 100 billion kilometers, or 62 billion miles, away. For reference, Pluto’s orbit takes it a maximum of 7.4 billion kilometers from our sun.

    In 2017, 686 days after the initial explosion, the Hubble Space Telescope recorded an ultraviolet emission (blue circle) from SN 2015cp, which was caused by supernova material impacting hydrogen-rich material previously shed by a companion star. Yellow circles indicate cosmic ray strikes, which are unrelated to the supernova. NASA/Hubble Space Telescope/Graham et al. 2019.

    By comparing SN 2015cp to the other Type Ia supernovae in their survey, the researchers estimate that no more than 6 percent of Type Ia supernovae have such a litterbug companion. Repeated, detailed observations of other Type Ia events would help cement these estimates, Graham said.

    The Hubble Space Telescope was essential for detecting the ultraviolet signature of the companion star’s debris for SN 2015cp. In the fall of 2017, the researchers arranged for additional observations of SN 2015cp by the W.M. Keck Observatory in Hawaii, the Karl G. Jansky Very Large Array in New Mexico, the European Southern Observatory’s Very Large Telescope and NASA’s Neil Gehrels Swift Observatory, among others. These data proved crucial in confirming the presence of hydrogen and are presented in a companion paper lead by Chelsea Harris, a research associate at Michigan State University.

    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level,

    NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

    ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
    •KUEYEN (UT2; The Moon ),
    •MELIPAL (UT3; The Southern Cross ), and
    •YEPUN (UT4; Venus – as evening star).
    elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo, with an elevation of 2,635 metres (8,645 ft) above sea level,

    NASA Neil Gehrels Swift Observatory

    “The discovery and follow-up of SN 2015cp’s emission really demonstrates how it takes many astronomers, and a wide variety of types of telescopes, working together to understand transient cosmic phenomena,” said Graham. “It is also a perfect example of the role of serendipity in astronomical studies: If Hubble had looked at SN 2015cp just a month or two later, we wouldn’t have seen anything.”

    Graham is also a senior fellow with the UW’s DIRAC Institute and a science analyst with the Large Synoptic Survey Telescope, or LSST.

    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes, altitude 2,663 m (8,737 ft),

    “In the future, as a part of its regularly scheduled observations, the LSST will automatically detect optical emissions similar to SN 2015cp — from hydrogen impacted by material from Type Ia supernovae,” said Graham said. “It’s going to make my job so much easier!”

    Co-authors are Harris; Peter Nugent at the University of California, Berkeley and the Lawrence Berkeley National Laboratory; Kate Maguire at Queen’s University Belfast; Mark Sullivan and Mathew Smith at the University of Southampton; Stefano Valenti at the University of California, Davis; Ariel Goobar at Stockholm University; Ori Fox at the Space Telescope Science Institute; Ken Shen, Tom Brink and Alex Filippenko at the University of California, Berkeley; Patrick Kelly at the University of Minnesota; and Curtis McCully at the University of California, Santa Barbara and the Las Cumbres Observatory. The research was funded by the National Science Foundation, NASA, the European Research Council and the U.K.’s Science and Technology Facilities Council.

    See the full article here .


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  • richardmitnick 12:16 pm on October 20, 2017 Permalink | Reply
    Tags: , , , , , , White dwarf stars   

    From astrobites: “Energy transport in white dwarfs: what about magnetic fields?” 

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    Oct 20, 2017
    Ingrid Pelisoli

    Title: Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105-820
    Authors: N. P. Gentile Fusillo, P.-E. Tremblay, S. Jordan, B. T. Gänsicke, J. S. Kalirai, J. Cummings
    First Author’s Institution: University of Warwick, UK

    Status: Submitted to MNRAS, open access

    Did you know that the bright yellow ball that shines in the sky, which we call the Sun, is also a huge magnet? However, it is huge only in terms of spatial dimensions – the strength of the magnetic field is only about 1 Gauss (G), or 10-4 Tesla (T). This is 10,000 weaker than the strongest magnet you can buy. The strongest magnet ever built on Earth produces a magnetic field of at least 45 T. Meanwhile, there are some other tiny dots in the sky with fields as strong as 108G, or 104T!

    Tiny giant magnets

    These tiny dots are white dwarf stars, which are about the size of the Earth, but with a mass comparable to the Sun. They maintain their hydrostatic equilibrium thanks to the Pauli exclusion principle: gravity can not further compress the object without pushing electrons into the same energy states, so the electrons push back, causing what is known as degeneracy pressure. The high field observed in some white dwarf stars is probably related to the fact that they are tiny: their progenitors had much smaller fields, but when they are compressed into a planetary size, the field is strengthened due to the magnetic flux being conserved. However, the process of evolution involves lots of mass being lost, and we don’t know exactly what happens to the magnetic field during these stages. As a result, we cannot fully understand the origin of such high magnetic fields.

    Figure 1: The author’s spectral fit to the hydrogen Balmer lines, from H8 to Hß. The top panel shows the best fit using a convective model, and the bottom panel shows the best radiative model. The obtained values of effective temperature and logarithm of the surface gravity are indicated. Figure 1 in the paper

    With the data release 2 of Gaia, which has made many astronomers draw a big circle around April 2018 on their calendars, we should identify hundreds of thousands of new white dwarfs. Something between 5 and 30% of them should be magnetic, based on the fraction of known magnetic white dwarfs. So it’s about time we start learning more about these objects! One particular problem we currently have is that it is very hard to estimate the mass of magnetic white dwarfs. We usually cannot apply spectroscopic analysis, our main method of estimating masses, because the spectral lines of magnetic white dwarfs are affected by the Zeeman effect. This effect causes an extra broadening which we have not (yet) been able to model together with the other important effects. In summary, no complete model exists! Gaia will give us a hand with that by allowing us to estimate the radius of white dwarfs – which is related to their mass (more about it in this bite). But we still have to know the temperature of the white dwarf to be able to do further cool science, such as estimating the age of stellar populations (like here and here).

    See the full article here .

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  • richardmitnick 2:46 pm on August 17, 2017 Permalink | Reply
    Tags: , , , , , , White dwarf stars   

    From Many Worlds: “Of White Dwarfs, “Zombie” Stars and Supernovae Explosions” 

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    Many Words icon

    Many Worlds

    Marc Kaufman

    Artistic view of the aftermath of a supernova explosion, with an unexpected white dwarf remnant. These super-dense but no longer active stars are thought to play a key role in many supernovae explosion. (Copyright Russell Kightley)

    White dwarf stars, the remnant cores of low-mass stars that have exhausted all their nuclear fuel, are among the most dense objects in the sky.

    Their mass is comparable to that of the sun, while their volume is comparable to that of Earth. Very roughly, this means the average density of matter in a white dwarf would be on the order of 1,000,000 times greater than the average density of the sun.

    Thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star — a category that includes the sun and over 97% of the other stars in the Milky Way — they are dim objects first identified a century ago but only in the last decade the subject of broad study.

    In recent years the white dwarfs have become more and more closely associated with supernovae explosions, though the processes involved remained hotly debated. A team using the Hubble Space Telescope even captured before and after images of what is hypothesized to be an incomplete white dwarf supernova. What was left behind has been described by some as a “zombie star.”

    Now a team of astronomers led by Stephane Vennes of the Czech Academy of Sciences has detected another zombie white dwarf, LP-40-365 , that they put forward as a far-flung remnant of a long-ago supernova explosion. This is considered important and unusual because it would represent a first detection of such a remnant long after the supernova conflagration.

    This dynamic is well captured in an animation accompanying the Science paper that describes the possible remnant.

    A supernova — among the most powerful forces in the universe — occurs when there is a change in the core of a star. A change can occur in two different ways, with both resulting in a thermonuclear explosion.

    Type Ia supernova occurs at the end of a single star’s lifetime. As the star runs out of nuclear fuel, some of its mass flows into its core. Eventually, the core is so heavy that it cannot withstand its own gravitational force. The core collapses, which results in the giant explosion of a supernova. The sun is a single star, but it does not have enough mass to become a supernova.

    The second type takes place only in binary star systems. Binary stars are two stars that orbit the same point. One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova.

    Type Ia supernovae, which are the result of the complete destruction of the star in a thermonuclear explosion, have a fairly uniform brightness that makes them useful for cosmology. The light emitted by the supernova explosion can be, for a short while at least, as bright as the whole of the Milky Way.

    Recently, astronomers have discovered a related form of supernova, called Type Iax, which look like Type Ia, but are much fainter. Type Iax supernovae may be caused by the partial destruction of a white dwarf star in such an explosion. If that interpretation is correct, part of the white dwarf should survive as a leftover object.

    And that leftover object is precisely what Vennes et al claim to have found.

    They have identified LP 40-365 as an unusual white dwarf with a low mass, high velocity and strange composition of oxygen, sodium and magnesium – exactly as might be expected for the leftover star from a Type Iax event. Vennes describes the white dwarf remnant his team has detected as a “compact star,” and perhaps the first of its kind in terms of the elements it contains.

    The team calculate that the explosion must have occurred between five and 50 million years ago.

    The two inset images show before-and-after images captured by NASA’s Hubble Space Telescope of Supernova 2012Z in the spiral galaxy NGC 1309, what some call a “zombie star.”. The white X at the top of the main image marks the location of the supernova in the galaxy. A supernova typically obliterates the exploding white dwarf, or dying star. In 2014, scientists found that this faint supernova may have left behind a surviving portion of the white dwarf star.(NASA,ESA)

    In an email exchange, Vennes told me that he has been studying the local white dwarf population for thirty years.

    “These compact, dead stars tell us a lot about the “old” Milky Way, how stars were born and how they died,” he wrote.

    “Tens of thousands of these white dwarfs have been catalogued over this past century, most of them in the last decade, but we keep an eye on outliers, objects that are out of the norm. We look for exceedingly large velocity, peculiar chemical composition or abnormal mass or radii.

    “The strange case of LP40-365 came unexpectedly, but this was a classic case of serendipity in astronomy. Out of hundreds of targets we observed at the telescope, this one was uniquely peculiar. Fortunately, theorists are very imaginative and the model we adopted to interpret the observed properties of this object were only recently published. Our research on this object was certainly inspired and directed by their theory.”

    Vennes says the team was surprised to learn that the white dwarf LP40-365 is relatively bright among its peers and that similar objects did not show up in large-scale surveys such as the Sloan Digital Sky Survey.

    “This fact has convinced us that many more similarly peculiar white dwarfs await discovery. We should search among fainter, more distant samples of white dwarfs,” he wrote.

    And that search can be done by the European Space Agency’s Gaia astrometric space telescope, with follow-up observations at large telescopes such as the European Southern Observatory’s Very Large Telescope and the Gemini observatory in Chile.

    ESA/GAIA satellite

    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet

    “It is also likely that our adopted model involving a subluminous {faint} Type Ia supernova will be modified or even superseded by teams of theorists coming up with new ideas. But we remain confident that these new ideas would still involve a cataclysmic event on the scale of a supernova.”

    A supernova burns for only a short period of time, but it can tell scientists a lot about the universe.

    One kind of supernova has shown scientists that we live in an expanding universe, one that is growing at an ever increasing rate.

    Scientists also have determined that supernovas play a key role in distributing elements throughout the universe. When the star explodes, it shoots elements and debris into space. Many of the elements we find here on Earth are made in the core of stars.

    These elements travel on to form new stars, planets and everything else in the universe — making white dwarfs and supernovae essential to the process that ultimately led to life.

    See the full article here .

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    About Many Worlds

    There are many worlds out there waiting to fire your imagination.

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    President Dwight D. Eisenhower established the National Aeronautics and Space Administration (NASA) in 1958 with a distinctly civilian (rather than military) orientation encouraging peaceful applications in space science. The National Aeronautics and Space Act was passed on July 29, 1958, disestablishing NASA’s predecessor, the National Advisory Committee for Aeronautics (NACA). The new agency became operational on October 1, 1958.

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  • richardmitnick 3:28 pm on February 10, 2017 Permalink | Reply
    Tags: , , , Dwarf star 200 light-years away contains life’s building blocks, The constellation Boötes, , WD 1425+540, White dwarf stars   

    From UCLA: “Dwarf star 200 light-years away contains life’s building blocks” 

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    February 09, 2017
    Stuart Wolpert

    UCLA-led team discovers object in the constellation Boötes with carbon, nitrogen, oxygen and hydrogen.

    Rendering of a white dwarf star (bright white spot), with rocky debris from former asteroids or a minor planet that has been broken apart by gravity (red rings). University of Warwick

    Many scientists believe the Earth was dry when it first formed, and that the building blocks for life on our planet — carbon, nitrogen and water — appeared only later as a result of collisions with other objects in our solar system that had those elements.

    Today, a UCLA-led team of scientists reports that it has discovered the existence of a white dwarf star whose atmosphere is rich in carbon and nitrogen, as well as in oxygen and hydrogen, the components of water. The white dwarf is approximately 200 light-years from Earth and is located in the constellation Boötes.

    Benjamin Zuckerman, a co-author of the research and a UCLA professor of astronomy, said the study presents evidence that the planetary system associated with the white dwarf contains materials that are the basic building blocks for life. And although the study focused on this particular star — known as WD 1425+540 — the fact that its planetary system shares characteristics with our solar system strongly suggests that other planetary systems would also.

    “The findings indicate that some of life’s important preconditions are common in the universe,” Zuckerman said.

    The scientists report that a minor planet in the planetary system was orbiting around the white dwarf, and its trajectory was somehow altered, perhaps by the gravitational pull of a planet in the same system. That change caused the minor planet to travel very close to the white dwarf, where the star’s strong gravitational field ripped the minor planet apart into gas and dust. Those remnants went into orbit around the white dwarf — much like the rings around Saturn, Zuckerman said — before eventually spiraling onto the star itself, bringing with them the building blocks for life.

    The researchers think these events occurred relatively recently, perhaps in the past 100,000 years or so, said Edward Young, another co-author of the study and a UCLA professor of geochemistry and cosmochemistry. They estimate that approximately 30 percent of the minor planet’s mass was water and other ices, and approximately 70 percent was rocky material.

    The research suggests that the minor planet is the first of what are likely many such analogs to objects in our solar system’s Kuiper belt. The Kuiper belt is an enormous cluster of small bodies like comets and minor planets located in the outer reaches of our solar system, beyond Neptune.

    Kuiper Belt. Minor Planet Center
    Kuiper Belt. Minor Planet Center

    Astronomers have long wondered whether other planetary systems have bodies with properties similar to those in the Kuiper belt, and the new study appears to confirm for the first time that one such body exists.

    White dwarf stars are dense, burned-out remnants of normal stars. Their strong gravitational pull causes elements like carbon, oxygen and nitrogen to sink out of their atmospheres and into their interiors, where they cannot be detected by telescopes.

    The research, published in the Astrophysical Journal Letters, describes how WD 1425+540 came to obtain carbon, nitrogen, oxygen and hydrogen. This is the first time a white dwarf with nitrogen has been discovered, and one of only a few known examples of white dwarfs that have been impacted by a rocky body that was rich in water ice.

    “If there is water in Kuiper belt-like objects around other stars, as there now appears to be, then when rocky planets form they need not contain life’s ingredients,” said Siyi Xu, the study’s lead author, a postdoctoral scholar at the European Southern Observatory in Germany who earned her doctorate at UCLA.

    “Now we’re seeing in a planetary system outside our solar system that there are minor planets where water, nitrogen and carbon are present in abundance, as in our solar system’s Kuiper belt,” Xu said. “If Earth obtained its water, nitrogen and carbon from the impact of such objects, then rocky planets in other planetary systems could also obtain their water, nitrogen and carbon this way.”

    A rocky planet that forms relatively close to its star would likely be dry, Young said.

    “We would like to know whether in other planetary systems Kuiper belts exist with large quantities of water that could be added to otherwise dry planets,” he said. “Our research suggests this is likely.”

    According to Zuckerman, the study doesn’t settle the question of whether life in the universe is common.

    “First you need an Earth-like world in its size, mass and at the proper distance from a star like our sun,” he said, adding that astronomers still haven’t found a planet that matches those criteria.

    The researchers observed WD 1425+540 with the Keck Telescope in 2008 and 2014, and with the Hubble Space Telescope in 2014.

    Keck Observatory, Mauna Kea, Hawaii, USA
    Keck Observatory, Mauna Kea, Hawaii, USA

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    They analyzed the chemical composition of its atmosphere using an instrument called a spectrometer, which breaks light into wavelengths. Spectrometers can be tuned to the wavelengths at which scientists know a given element emits and absorbs light; scientists can then determine the element’s presence by whether it emits or absorbs light of certain characteristic wavelengths. In the new study, the researchers saw the elements in the white dwarf’s atmosphere because they absorbed some of the background light from the white dwarf.

    In addition to Xu, Young and Zuckerman, co-authors of the research are Michael Jura, a UCLA professor of astronomy who died in 2016; Beth Klein, a former graduate student of Jura’s; and Patrick Dufour, an assistant professor of physics at the University of Montreal.

    See the full article here .

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  • richardmitnick 10:22 am on June 29, 2016 Permalink | Reply
    Tags: , , , Dying main sequence stars, White dwarf stars   

    From astrobites: “Probing the Galaxy with Dead Stars” 

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    Jun 29, 2016
    Ingrid Pelisoli

    Title: The Field White Dwarf Mass Distribution

    Authors: P.-E. Tremblay, J. Cummings, J. S. Kalirai, B. T. Gaensicke, N. Gentile-Fusillo, R. Raddi

    First author’s institution: Department of Physics, University of Warwick, UK

    Status: accepted for publication in MNRAS

    Our Sun is a main sequence star. This is the evolutionary phase where stars spend most of their lives, burning hydrogen into helium in their cores. When this hydrogen is exhausted, they evolve to the next evolutionary phase, in which they fuse hydrogen in a shell outside the core. Meanwhile, the core contracts until helium burning temperature is reached in it. When helium runs out in the core, hydrogen and helium fusion continue in shells around a hot core of carbon and oxygen. Most stars will never reach temperature high enough to burn these two elements, ending their lives with this composition. The characteristics of these phases depend strongly on the initial mass and metallicity of the star. However, one thing is common to over 95% of them: they will eject their external layers on a planetary nebula and end their lives as white dwarf stars.

    The structure of a white dwarf is quite simple: a core usually composed of carbon and oxygen (which are the heaviest elements most stars can synthesize), a thin layer of helium, and a thin outer layer of hydrogen. Moreover, white dwarfs have a peculiar but well-defined mass-radius relation, due to the fact that matter is mostly degenerate in its core. All that makes them quite easy to model, allowing us to obtain physical parameters from observations, such as mass, temperature, and age, which are all correlated.

    Joining the facts that most stars become white dwarfs and that they are easy to model, one could get the idea to study our Galaxy solely by modeling the characteristics of the white dwarf population. That’s exactly what the authors of today’s paper did, as others have done before. The authors selected two well-determined white dwarf mass distributions, one limited by distance and the other by magnitude, and compared it to simulated distributions with ingredients reflecting our knowledge of our Galaxy’s formation, evolution, and current structure.

    The distance limited sample contains white dwarfs up to 20 pc, and it’s about 90 % complete, meaning we have detected about 9 out of every 10 white dwarfs in this region. That’s an advantage because there should be no strong selection bias. However, the sample has little over a hundred objects, so it’s statistically poor. To compare their simulations also with a larger sample, they also selected bright white dwarfs detected with the Sloan Digital Sky Survey (SDSS).

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    The sample is much larger, over a thousand objects, but in this case suffers from selection bias from the survey criteria and sky coverage.

    The main ingredients to their simulations are a star formation history (SFH), an initial mass function (IMF), an initial-to-final mass relation (IFMR), and a description to the vertical scale height of the Galactic disk. With only these four ingredients, one can cook up a nice mass distribution to compare with observations. The SFH describes how many stars were formed and when, the IMF defines the fraction of stars at each given mass interval when formation happens, and the IFMR determines the masses and ages of the resulting white dwarfs. The structure of the Galactic disk, determined by its scale height, will describe how the formed white dwarfs are distributed around us, allowing us to estimate how likely we are to detect them.

    The authors first simulated standard distributions with popular parameters from the literature: a constant SFH throughout 10 Gyr, which is the assumed age of the disk, a Salpeter IMF, a quadratic IFMR, and a variable scale height, increasing with the star’s age, i.e. allowing old stars to be further away from the plane of the disk than young stars. To compare with the 20 pc sample, they created stars in their simulation until they reached a significant number of detected stars within 20 pc. To compare with the SDSS sample, this was done until a fair number was reached in the SDSS covered region and within the upper and lower magnitude limits of the observational sample. They then built the mass distributions to their simulations and overplotted it, without any fit, over the observed distributions (Figs. 1 and 2).

    Figure 1: Comparison between the observed (black) and simulated (filled blue) mass distributions for the sample limited to 20 pc. Objects with masses below 0.45 solar masses (shown in red) are neglected for the computation of the mean mass and dispersion because they are the result of binary evolution, which is not taken into account on the simulations. The overall shape of the distributions, and the mean mass and dispersion labeled on the panel agree remarkably well, considering no fit was done.

    Figure 2: Observed (black) and simulated (filled blue) mass distributions for the white dwarfs in the SDSS sample. They are of types DA (hydrogen-dominated atmosphere) and DB (helium-dominated atmosphere). Binaries and magnetic white dwarfs were removed from the sample. Low-mass objects were neglected as mentioned on Fig. 1. A similar shape and agreeing mean mass and dispersion are obtained between simulation and observation, even without a fit.

    The result is quite impressive: the overall shape, mean mass, and dispersion of their simulations agree remarkably well with the observations. This reflects the fact that, given the uncertainties in our observations, our models are good enough to describe then. However, not all is perfect: they notice that their simulations predicted a higher fraction of higher mass white dwarfs than the observations by a factor of about 1.5, making their simulated mean mass higher than the observed in both cases.

    The authors then went one step further: they tweaked their ingredients to see how that would affect the obtained distribution, and whether it could result on a better agreement with the observations. Their main result is that a steeper IMF function could bring the fraction of higher mass stars closer to observed values. This would mean that the ever so popular Salpeter IMF may need a revision. The authors caution that, given the current uncertainties, we cannot rule out that the Salpeter function is correct and that what actually needs to improve is the IFMR. Changes to the SFH and to the description of the disk scale height cause less prominent effects.

    The roles of each of these ingredients and their influence on the white dwarf mass distribution should become much clearer in the near future when Gaia will have obtained parallax measurements to most of these white dwarfs, allowing us to much better constrain their physical parameters, as the authors point out. The authors give us an idea of where should we put our attention in the meantime with regards to modeling: mostly the IMF, closely followed by the IFMR. We have only a few more years to work on our models and improve our descriptions to try to explain what Gaia will reveal us. Better get to work!

    See the full article here .

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