• Dying stars' cocoons might explain fast

    From ScienceDaily@1:317/3 to All on Wed Apr 20 22:30:50 2022
    Dying stars' cocoons might explain fast blue optical transients
    First model that is fully consistent with all FBOT observations

    Date:
    April 20, 2022
    Source:
    Northwestern University
    Summary:
    Using a newly developed model, astrophysicists present a new theory
    to explain fast blue optical transients, a new class of transients
    that has boggled researchers since their discovery in 2018. In the
    new study, astrophysicists find that FBOTs could result from the
    actively cooling cocoons that surround jets launched by dying stars.



    FULL STORY ==========================================================================
    Ever since they were discovered in 2018, fast blue optical transients
    (FBOTs) have utterly surprised and completely confounded both
    observational and theoretical astrophysicists.


    ==========================================================================
    So hot that they glow blue, these mysterious objects are the brightest
    known optical phenomenon in the universe. But with only a few discovered
    so far, FBOTs' origins have remained elusive.

    Now a Northwestern University astrophysics team presents a bold new
    explanation for the origin of these curious anomalies. Using a new model,
    the astrophysicists believe FBOTs could result from the actively cooling cocoons that surround jets launched by dying stars. It marks the first astrophysics model that is fully consistent with all observations related
    to FBOTs.

    The research was published April 11 in the Monthly Notices of the Royal Astronomical Society.

    As a massive star collapses, it can launch outflows of debris at rates
    near the speed of light. These outflows, or jets, collide into collapsing layers of the dying star to form a "cocoon" around the jet. The new
    model shows that as the jet pushes the cocoon outward -- away from the
    core of the collapsing star - - it cools, releasing heat as an observed
    FBOT emission.

    "A jet starts deep inside of a star and then drills its way out to
    escape," said Northwestern's Ore Gottlieb, who led the study. "As the
    jet moves through the star, it forms an extended structure, known as
    the cocoon. The cocoon envelopes the jet, and it continues to do so even
    after the jet escapes the star, this cocoon escapes with the jet. When
    we calculated how much energy the cocoon has, it turned out to be as
    powerful as an FBOT." Gottlieb is a Rothschild Fellow in Northwestern's
    Center for Interdisciplinary Exploration and Research in Astrophysics
    (CIERA). He coauthored the paper with CIERA member Sasha Tchekovskoy, an assistant professor of physics and astronomy in Northwestern's Weinberg
    College of Arts and Sciences.



    ==========================================================================
    The hydrogen problem FBOTs (pronounced F-bot) are a type of cosmic
    explosion initially detected in the optical wavelength. As their name
    implies, transients fade almost as quickly as they appear. FBOTs reach
    peak brightness within a matter of days and then quickly fade -- much
    faster than standard supernovae rise and decay.

    After discovering FBOTs just eight years ago, astrophysicists wondered
    if the mysterious events were related to another transient class: gamma
    ray bursts (GRBs). The strongest and brightest explosions across all wavelengths, GRBs also are associated with dying stars. When a massive
    star exhausts its fuel and collapses into a black hole, it launches jets
    to produce a powerful gamma ray emission.

    "The reason why we think GRBs and FBOTs might be related is because both
    are very fast -- moving at close to the speed of light -- and both are asymmetrically shaped, breaking the spherical shape of the star," Gottlieb said. "But there was a problem. Stars that produce GRBs lack hydrogen. We
    don't see any signs of hydrogen in GRBs, whereas in FBOTs, we see hydrogen everywhere. So, it could not be the same phenomenon." Using their new
    model, Gottlieb and his coauthors think they might have found an answer
    to this problem. Hydrogen-rich stars tend to house hydrogen in their
    outermost layer -- a layer too thick for a jet to penetrate.



    ========================================================================== "Basically, the star would be too massive for the jet to pierce through," Gottlieb said. "So the jet will never make it out of the star, and that's
    why it fails to produce a GRB. However, in these stars, the dying jet
    transfers all its energy to the cocoon, which is the only component to
    escape the star. The cocoon will emit FBOT emissions, which will include hydrogen. This is another area where our model is fully consistent with
    all FBOT observations." Putting the picture together Although FBOTs
    glow bright in optical wavelengths, they also emit radio waves and
    X-rays. Gottlieb's model explains these too.

    When the cocoon interacts with the dense gas surrounding the star, this interaction heats up stellar material to release a radio emission. And
    when the cocoon expands far enough away from the black hole (formed from
    the collapsed star), X-rays can leak out from the black hole. The X-rays
    join radio and optical light to form a full picture of the FBOT event.

    While Gottlieb is encouraged by his team's findings, he says more
    observations and models are needed before we can definitively understand
    FBOTs' mysterious origins.

    "This is a new class of transients, and we know so little about them,"
    Gottlieb said. "We need to detect more of them earlier in their evolution before we can fully understand these explosions. But our model is able
    to draw a line among supernovae, GRBs and FBOTs, which I think is very elegant." "This study paves the way for more advanced simulations
    of FBOTs," Tchekovskoy said. "This next-generation model will allow
    us to directly connect the physics of the central black hole to the observables, enabling us to reveal otherwise hidden physics of the FBOT
    central engine." The study, "Shocked jets in CCSNe can power the zoo
    of fast blue optical transients," was supported by the National Science Foundation (award numbers AST-1815304 and AST-2107839). The authors
    developed the simulation using supercomputers at the Texas Advanced
    Computing Center at the University of Texas at Austin.

    Video: https://youtu.be/MquOKdZEaDw

    ========================================================================== Story Source: Materials provided by Northwestern_University. Original
    written by Amanda Morris. Note: Content may be edited for style and
    length.


    ========================================================================== Journal Reference:
    1. Ore Gottlieb, Alexander Tchekhovskoy, Raffaella Margutti. Shocked
    jets in
    CCSNe can power the zoo of fast blue optical transients. Monthly
    Notices of the Royal Astronomical Society, 2022; DOI:
    10.1093/mnras/stac910 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/04/220420133628.htm

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