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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Dying stars' cocoons could be new source of gravitational waves    
      
     Date:   
         June 5, 2023   
     Source:   
         Northwestern University   
     Summary:   
         Although astrophysicists theoretically should be able to detect   
         gravitational waves from a single, non-binary source, they have yet   
         to uncover these elusive signals. Now researchers suggest looking   
         at a new, unexpected and entirely unexplored place: The turbulent,   
         energetic cocoons of debris that surround dying massive stars.   
      
      
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   FULL STORY   
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   Although astrophysicists theoretically should be able to detect   
   gravitational waves from a single, non-binary source, they have yet   
   to uncover these elusive signals. Now researchers suggest looking at a   
   new, unexpected and entirely unexplored place: The turbulent, energetic   
   cocoons of debris that surround dying massive stars.   
      
   Now Northwestern University researchers suggest looking at a new,   
   unexpected and entirely unexplored place: The turbulent, energetic   
   cocoons of debris that surround dying massive stars.   
      
   For the first time ever, the researchers have used state-of-the-art   
   simulations to show that these cocoons can emit gravitational waves. And,   
   unlike gamma-ray burst jets, cocoons' gravitational waves should be   
   within the frequency band that the Laser Interferometer Gravitational-Wave   
   Observatory (LIGO) can detect.   
      
   "As of today, LIGO has only detected gravitational waves from binary   
   systems, but one day it will detect the first non-binary source of   
   gravitational waves," said Northwestern's Ore Gottlieb, who led the   
   study. "Cocoons are one of the first places we should look to for this   
   type of source."  Gottlieb will present this research during a virtual   
   press briefing at the 242nd meeting of the American Astronomical   
   Society. "Jetted and turbulent stellar deaths: New LIGO-detectable   
   sources of gravitational waves" will take place on Monday, June 5, as a   
   part of a session on "Discoveries in Distant Galaxies."  Gottlieb is a   
   CIERA Fellow at Northwestern's Center for Interdisciplinary Exploration   
   and Research in Astrophysics (CIERA). Northwestern co-authors of the   
   study include professors Vicky Kalogera and Alexander Tchekovskoy,   
   postdoctoral associates Sharan Banagiri and Jonatan Jacquemin-Ide and   
   graduate student Nick Kaaz.   
      
   New source was 'impossible to ignore' To conduct the study, Gottlieb   
   and his collaborators used new state-of-the-art simulations to model the   
   collapse of a massive star. When massive stars collapse into black holes,   
   they may create powerful outflows (or jets) of particles traveling close   
   to the speed of light. Gottlieb's simulations modeled this process --   
   from the time the star collapses into a black hole until the jet escapes.   
      
   Initially, he wanted to see whether or not the accretion disk that   
   forms around a black hole could emit detectable gravitational waves. But   
   something unexpected kept emerging from his data.   
      
   "When I calculated the gravitational waves from the vicinity of the black   
   hole, I found another source disrupting my calculations -- the cocoon,"   
   Gottlieb said. "I tried to ignore it. But I found it was impossible   
   to ignore. Then I realized the cocoon was an interesting gravitational   
   wave source."  As jets collide into collapsing layers of the dying star, a   
   bubble, or a "cocoon," forms around the jet. Cocoons are turbulent places,   
   where hot gases and debris mix randomly and expand in all directions from   
   the jet. As the energetic bubble accelerates from the jet, it perturbs   
   space-time to create a ripple of gravitational waves, Gottlieb explained.   
      
   "A jet starts deep inside of a star and then drills its way out to   
   escape," Gottlieb said. "It's like when you drill a hole into a wall. The   
   spinning drill bit hits the wall and debris spills out of the wall. The   
   drill bit gives that material energy. Similarly, the jet punches through   
   the star, causing the star's material to heat up and spill out. This   
   debris forms the hot layers of a cocoon."  Call to action to look at   
   cocoons If cocoons do generate gravitational waves, then LIGO should be   
   able to detect them in its upcoming runs, Gottlieb said. Researchers   
   have typically searched for single-source gravitational waves from   
   gamma-ray bursts or supernovae, but astrophysicists doubt that LIGO   
   could detect those.   
      
   "Both jets and supernovae are very energetic explosions," Gottlieb   
   said. "But we can only detect gravitational waves from higher frequency,   
   asymmetrical explosions. Supernovae are rather spherical and symmetrical,   
   so spherical explosions do not change the balanced mass distribution   
   in the star to emit gravitational waves. Gamma-ray bursts last dozens   
   of seconds, so the frequency is very small -- lower than the frequency   
   band that LIGO is sensitive to."  Instead, Gottlieb asks astrophysicists   
   to redirect their attention to cocoons, which are both asymmetrical and   
   highly energetic.   
      
   "Our study is a call to action to the community to look at cocoons as a   
   source of gravitational waves," he said. "We also know cocoons to emit   
   electromagnetic radiation, so they could be multi-messenger events. By   
   studying them, we could learn more about what happens in the innermost   
   part of stars, the properties of jets and their prevalence in stellar   
   explosions."  The study, "Jetted and turbulent stellar deaths: New   
   LVK-detectable gravitational wave sources," was supported by the National   
   Science Foundation, NASA and the Fermi Cycle 14 Guest Investigator   
   program.These advanced simulations were made possible by the Department of   
   Energy's Oak Ridge National Laboratory supercomputer Summit and National   
   Energy Research Scientific Computing Center's supercomputer Perlmutter   
   through the ASCR Leadership Computing Challenge computational time award.   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Black_Holes # Stars # Astrophysics # Solar_Flare #   
                   Cosmic_Rays # Galaxies # Astronomy # Sun   
       * RELATED_TERMS   
             o Gravitational_wave o Jupiter o General_relativity o   
             Teleportation o Galaxy o Dark_matter o Red_supergiant_star   
             o Stellar_nucleosynthesis   
      
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   Story Source: Materials provided by Northwestern_University. Original   
   written by Amanda Morris. Note: Content may be edited for style and   
   length.   
      
      
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/06/230605181202.htm   
      
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