ETA CARINAE HAS ALMOST FINISHED ITS CHANGE OF STATE -- AND WHY THAT MATTERS
KRIS DAVIDSON -- UNIV MINNESOTA JOHN C. MARTIN -- UNIV ILLINOIS SPRINGFIELD KAZUNORI ISHIBASHI -- NAGOYA UNIV. ROBERTA M. HUMPHREYS -- UNIV MINNESOTA 1965 – 2005: ASTRONOMERS RECOGNIZED THAT SUPERNOVA IMPOSTORS OCCUR
-- A.K.A. GIANT ERUPTIONS
-- THEY RESEMBLE SN EXPLOSIONS ... -- BUT THE STARS SURVIVE ! -- FAR MORE MYSTERIOUS THAN TRUE SUPERNOVAE -- • -- INSTEAD OF BLAST WAVES, GIANT ERUPTIONS ARE DRIVEN BY RADIATION PRESSURE.
-- INSTEAD OF NUCLEAR REACTIONS, THEY’RE CAUSED BY HORRIBLY COMPLEX DYNAMICAL INSTABILITIES WHEN THE RADIATION / GAS RATIO BECOMES TOO HIGH.
-- NO THEORIST PREDICTED THEM. -- GIANT-ERUPTION MASS LOSS IS CRUCIAL FOR THE EVOLUTION OF THE MOST MASSIVE STARS.
-- ALMOST ALL KNOWN EXAMPLES OCCURRED IN GALAXIES MILLIONS OF LIGHTYEARS AWAY... EXCEPT ETA CARINAE, ONLY 7500 LIGHTYEARS AWAY.
THIS STAR IS THE ONLY GIANT-ERUPTION SURVIVOR CLOSE ENOUGH FOR US TO STUDY ANY DETAILS. 1835 – 1855, ONE OF THE BRIGHTEST STARS IN THE SKY STAR’S INTERIOR “SHOULD” HAVE RECOVERED BY ABOUT 1900. BUT INSTEAD -- USING HST, IN 1999 WE DISCOVERED THE ONSET OF RAPID BRIGHTENING. FIRST TWIST TO THE STORY: ETA HAS A COMPANION STAR. HOT SECONDARY STAR IS UNOBSERVABLE -- BUT IT SUPPLIES NEARLY ALL OF THE IONIZING EUV ! 5.5 –YEAR CYCLE: AT CLOSEST APPROACH, BRIGHTNESS DIPPED AND EUV DISAPPEARED “NORMAL” STATE: COLLIDING WINDS à SHOCKS Normal state: HIGH-EXCITATION SPECTRAL LINES He I, [ Fe III ], [ Ne III ]. etc. A FEW WEEKS BEFORE CLOSEST APPROACH, GAS-DYNAMIC INSTABILITIES DESTROYED THE SHOCK FRONTS. AFTER THE SHOCKED ZONE BREAKS UP, DENSE EJECTA FROM THE PRIMARY STAR CAN FALL ONTO ITS HOT COMPANION. THIS TEMPORARILY STOPPED THE EUV SUPPLY (NOAM SOKER’S IDEA, c. 2009) HST OBSERVED THREE PERIASTRON PASSAGES: 2003.5, 2014.6, 2020.2. BUT IN 2014 AND 2020, THE EUV DID NOT TURN OFF!
2003 HST SPECTRA 2014 EVIDENTLY -- SOMETIME BETWEEN 2003 AND 2014, ETA CARINAE’S MASS OUTFLOW BECAME TOO LOW TO DEPOSIT MUCH GAS ONTO THE HOT SECONDARY STAR.
INSTEAD OF A RECORD-SETTING MASS LOSS, IT HAS BECOME (ALMOST) A NORMAL HOT-STAR WIND. OTHER HST DATA INDICATE THE SAME.
THIS IS THE ONLY DATA ON SUPERNOVA-IMPOSTOR RECOVERY, AND IT HAS TWO SURPRISES FOR FUTURE THEORY:
-- THE RECOVERY TOOK ALMOST 200 YEARS INSTEAD OF 40, -- AND IT WAS UNSTEADY! NEAR-FUTURE VIEWS OF ETA CARINAE: 30 YEARS AGO, THE HOMUNCULUS EJECTA-NEBULA WAS 10X BRIGHTER THAN THE CENTRAL STAR.
TODAY THE STAR IS BRIGHTER THAN THE NEBULA. IT WILL GET ABOUT ONE MAGNITUDE BRIGHTER, AND IT WILL IONIZE THE HOMUNCULUS NEBULA. THE HOMUNCULUS WILL LOOK LIKE A PLANETARY NEBULA ON STEROIDS.
FOR MORE INFORMATION: google “etacar.umn.edu”.