• Giant stars undergo dramatic weight loss

    From ScienceDaily@1:317/3 to All on Thu Apr 14 22:30:46 2022
    Giant stars undergo dramatic weight loss program

    Date:
    April 14, 2022
    Source:
    University of Sydney
    Summary:
    Astronomers have found a slimmer type of red giant star for the
    first time. These stars have undergone dramatic weight loss,
    possibly due to a greedy companion. The discovery is an important
    step forward to understanding the life of stars in the Milky Way --
    our closest stellar neighbors.



    FULL STORY ========================================================================== Astronomers at the University of Sydney have found a slimmer type of red
    giant star for the first time. These stars have undergone dramatic weight
    loss, possibly due to the presence a greedy neighbour. Published in Nature Astronomy, the discovery is an important step forward to understanding
    the life of stars in the Milky Way -- our closest stellar neighbours.


    ========================================================================== There are millions of 'red giant' stars found in our galaxy. These
    cool and luminous objects are what our Sun will become in four billion
    years. For some time, astronomers have predicted the existence of slimmer
    red giants. After finding a smattering of them, the University of Sydney
    team can finally confirm their existence.

    "It's like finding Waldo," said lead author, PhD candidate Mr Yaguang Li
    from the University of Sydney. "We were extremely lucky to find about
    40 slimmer red giants, hidden in a sea of normal ones. The slimmer
    red giants are either smaller in size or less massive than normal red
    giants." How and why did they slim down? Most stars in the sky are
    in binary systems - - two stars that are gravitationally bound to each
    other. When the stars in close binaries expand, as stars do as they age,
    some material can reach the gravitational sphere of their companion and
    be sucked away. "In the case of relatively tiny red giants, we think a companion could possibly be present," Mr Li said.

    An intragalactic treasure hunt The team analysed archival data from NASA's Kepler space telescope. From 2009 to 2013, the telescope continuously
    recorded brightness variations on tens of thousands of red giants. Using
    this incredibly accurate and large dataset, the team conducted a thorough census of this stellar population, providing the groundwork for spotting
    any outliers.



    ==========================================================================
    Two types of unusual stars were revealed: very low-mass red giants,
    and underluminous (dimmer) red giants.

    The very low-mass stars weigh only 0.5 to 0.7 solar mass -- around half
    the weight of our Sun. If the very low-mass stars had not suddenly lost
    weight, their masses would indicate they were older than the age of the Universe -- an impossibility.

    "So, when we first obtained the masses of these stars, we thought there
    was something wrong with the measurement," Mr Li said. "But it turns out
    there wasn't." The underluminous stars, on the other hand, have normal
    masses, ranging from 0.8 to 2.0 solar mass. "However, they are much less 'giant' than we expect," said study co-author, Dr Simon Murphy from
    the University of Southern Queensland. "They've slimmed down somewhat
    and because they're smaller, they're also fainter, hence 'underluminous' compared to normal red giants." Only seven such underluminous stars were found, and the authors suspect many more are hiding in the sample. "The
    problem is that most of them are very good at blending in. It was a real treasure hunt to find them," Dr Murphy said.



    ========================================================================== These unusual data points could not be explained by simple expectations
    from stellar evolution. This led the researchers to conclude that another mechanism must be at work, forcing these stars to undergo dramatic weight
    loss: theft of mass by nearby stars.

    Stellar population census The researchers relied on asteroseismology --
    the study of stellar vibrations - - to determine the properties of the
    red giants.

    Traditional methods to study a star are limited to their surface
    properties, for example, surface temperature and luminosity. By contrast, asteroseismology, which uses sound waves, probes beneath this. "The
    waves penetrate the stellar interior, giving us rich information on
    another dimension," Mr Li said.

    The researchers could precisely determine stars' evolutionary stages,
    masses, and sizes with this method. And when they looked at the
    distributions of these properties, something unusual was immediately
    noticed: some stars have tiny masses or sizes.

    "It is highly unusual for a PhD student to make such an important
    discovery," said Professor Tim Bedding, Mr Li's academic supervisor. "By sifting carefully through data from NASA's Kepler space telescope,
    Yaguang spotted something that everyone else had missed."

    ========================================================================== Story Source: Materials provided by University_of_Sydney. Note: Content
    may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Yaguang Li, Timothy R. Bedding, Simon J. Murphy, Dennis Stello,
    Yifan
    Chen, Daniel Huber, Meridith Joyce, Dion Marks, Xianfei
    Zhang, Shaolan Bi, Isabel L. Colman, Michael R. Hayden,
    Daniel R. Hey, Gang Li, Benjamin T. Montet, Sanjib Sharma,
    Yaqian Wu. Discovery of post-mass-transfer helium-burning red
    giants using asteroseismology. Nature Astronomy, 2022; DOI:
    10.1038/s41550-022-01648-5 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/04/220414125101.htm

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