• Mysterious death of carbon star plays ou

    From ScienceDaily@1:317/3 to All on Mon Mar 28 22:30:40 2022
    Mysterious death of carbon star plays out like six-ring circus

    Date:
    March 28, 2022
    Source:
    National Radio Astronomy Observatory
    Summary:
    Scientists studying V Hydrae (V Hya) have witnessed the star's
    mysterious death throes in unprecedented detail. The team discovered
    six slowly- expanding rings and two hourglass-shaped structures
    caused by the high- speed ejection of matter out into space.



    FULL STORY ========================================================================== Scientists studying V Hydrae (V Hya) have witnessed the star's mysterious
    death throes in unprecedented detail. Using the Atacama Large Millimeter/ submillimeter Array (ALMA) and data from the Hubble Space Telescope (HST),
    the team discovered six slowly-expanding rings and two hourglass-shaped structures caused by the high-speed ejection of matter out into space. The results of the study are published in The Astrophysical Journal.


    ==========================================================================
    V Hya is a carbon-rich asymptotic giant branch (AGB) star located
    approximately 1,300 light-years from Earth in the constellation
    Hydra. More than 90-percent of stars with a mass equal to or greater
    than the Sun evolve into AGB stars as the fuel required to power nuclear processes is stripped away. Among these millions of stars, V Hya has
    been of particular interest to scientists due to its so-far unique
    behaviors and features, including extreme-scale plasma eruptions that
    happen roughly every 8.5 years and the presence of a nearly invisible
    companion star that contributes to V Hya's explosive behavior.

    "Our study dramatically confirms that the traditional model of how AGB
    stars die -- through the mass ejection of fuel via a slow, relatively
    steady, spherical wind over 100,000 years or more -- is at best,
    incomplete, or at worst, incorrect," said Raghvendra Sahai, an astronomer
    at NASA's Jet Propulsion Laboratory, and the principal researcher on the
    study. "It is very likely that a close stellar or substellar companion
    plays a significant role in their deaths, and understanding the physics
    of binary interactions is both important across astrophysics and one
    of its greatest challenges. In the case of V Hya, the combination of a
    nearby and a hypothetical distant companion star is responsible, at least
    to some degree, for the presence of its six rings, and the high-speed
    outflows that are causing the star's miraculous death." Mark Morris,
    an astronomer at UCLA and a co-author on the research added, "V Hydra has
    been caught in the process of shedding its atmosphere -- ultimately most
    of its mass -- which is something that most late-stage red giant stars do.

    Much to our surprise, we have found that the matter, in this case, is
    being expelled as a series of outflowing rings. This is the first and only
    time that anybody has seen that the gas being ejected from an AGB star
    can be flowing out in the form of a series of expanding 'smoke rings.'"
    The six rings have expanded outward from V Hya over the course of roughly
    2,100 years, adding matter to and driving the growth of a high-density
    flared and warped disk-like structure around the star. The team has
    dubbed this structure the DUDE, or Disk Undergoing Dynamical Expansion.

    "The end state of stellar evolution -- when stars undergo the transition
    from being red giants to ending up as white dwarf stellar remnants --
    is a complex process that is not well understood," said Morris. "The
    discovery that this process can involve the ejections of rings of gas, simultaneous with the production of high-speed, intermittent jets of
    material, brings a new and fascinating wrinkle to our exploration of how
    stars die." Sahai added, "V Hya is in the brief but critical transition
    phase that does not last very long, and it is difficult to find stars
    in this phase, or rather 'catch them in the act. We got lucky and were
    able to image all of the different mass-loss phenomena in V Hya to
    better understand how dying stars lose mass at the end of their lives."
    In addition to a full set of expanding rings and a warped disk, V Hya's
    final act features two hourglass-shaped structures -- and an additional jet-like structure -- that are expanding at high speeds of more than
    half a million miles per hour (240 km/s). Large hourglass structures
    have been observed previously in planetary nebulae, including MyCn 18
    -- also known as the Engraved Hourglass Nebula -- a young emission
    nebula located roughly 8,000 light-years from Earth in the southern constellation of Musca, and the more well-known Southern Crab Nebula,
    an emission nebula located roughly 7,000 light-years from Earth in the
    southern constellation Centaurus.

    Sahai said, "We first observed the presence of very fast outflows in 1981.

    Then, in 2022, we found a jet-like flow consisting of compact plasma blobs ejected at high speeds from V Hya. And now, our discovery of wide-angle outflows in V Hya connects the dots, revealing how all these structures
    can be created during the evolutionary phase that this extra-luminous red
    giant star is now in." Due to both the distance and the density of the
    dust surrounding the star, studying V Hya required a unique instrument
    with the power to clearly see matter that is both very far away and also difficult or impossible to detect with most optical telescopes. The
    team enlisted ALMA's Band 6 (1.23mm) and Band 7 (.85mm) receivers,
    which revealed the star's multiple rings and outflows in stark clarity.

    "The processes taking place at the end stages of low mass stars, and
    during the AGB phase in particular, have long fascinated astronomers and
    have been challenging to understand," said Joe Pesce, an astronomer and
    NSF program officer for NRAO/ALMA. "The capabilities and resolution
    of ALMA are finally allowing us to witness these events with the
    extraordinary detail necessary to provide some answers and enhance
    our understanding of an event that happens to most of the stars in the Universe." Sahai added that the incorporation of infrared, optical,
    and ultraviolet data into the study created a complete multi-wavelength
    picture of what might be one of the greatest shows in the Milky Way, at
    least for astronomers. "Each time we observe V Hya with new observational capabilities, it becomes more and more like a circus, characterized by
    an even bigger variety of impressive feats. V Hydrae has impressed us
    with its multiple rings and acts, and because our own Sun may one day experience a similar fate, it has us at rapt attention."

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


    ========================================================================== Related Multimedia:
    * Mysterious_death_of_carbon_star ========================================================================== Journal Reference:
    1. R. Sahai, P-S. Huang, S. Scibelli, M. R. Morris, K. Hinkle,
    C-F. Lee. The
    rapidly evolving AGB star, V Hya: ALMA finds a multi-ring circus
    with high velocity outflow. The Astrophysical Journal (preprint
    on arXiv), 2022 [abstract] ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/03/220328092058.htm

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