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Discovery of a Thorne-Żytkow Object Candidate in the Small Magellanic Cloud

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Discovery of a Thorne-Żytkow Object Candidate in the Small Magellanic Cloud Discovery of a Thorne-Żytkow Object Candidate in the Small Magellanic Cloud Emily Levesque University of Washington Philip Massey (Lowell Observatory), Nidia Morrell (LCO), Anna Żytkow (Cambridge) Trevor Dorn-Wallenstein, Kolby Weisenburger (UW), Je"rey Jennings (CU Boulder) Arizona State University Apr 27, 2016 Thorne-Żytkow Objects (TŻOs) are a theoretical class of star: a neutron star “core” surrounded by a large di"use envelope • Thorne & Żytkow (1977) predict supergiant TŻOs (Mc = 1.0M⦿, Mt ≥ 11.5M⦿) • Powered by a combination of thermonuclear reactions and gravitational accretion • Represent a completely new model for stable stellar interiors. There has never been a confirmed observation of a TŻO. Formation of TŻOs 1. Engulfing - an OB + NS HMXB; the OB companion leaves the main sequence, evolves into an RSG, expands, and engulfs the NS companion (Taam et al. 1978) 2. Collision - a massive binary system; one member collapses into a NS, and supernova kick velocity propels it into the massive companion (Leonard et al. 1994) TŻOs are... cool and luminous, lying at or beyond the Hayashi limit for massive stars (Thorne & Żtykow 1977) strongly mass-losing as a result (van Paradijs et al. 1995) potentially more common at low Z (Linden et al. 2010) metallicity TŻOs are... The cool periodic and luminous, table… lying at or beyond the Hayashi limit for massive stars (Thorne & Żtykow 1977) strongly mass-losing as a result (van Paradijs et al. 1995) X I - neutral atom potentially moreX IIcommon - singly-ionized atom at low Z (Linden et al. 2010) X III - double-ionized atom metallicity ^ TŻOs are... astronomers’ The cool periodic and luminous, table… lying at or beyond the Hayashi limit forH massive stars (Thorne & He Żtykow 1977) strongly mass-losing as a result (van Paradijs et al. 1995) X I - neutral atom “metals” (Z) potentially moreX IIcommon - singly-ionized atom at low Z (Linden et al. 2010) X III - double-ionized atom metallicity TŻOs are...almost indistinguishable from RSGs... cool and luminous, lying at or beyond the Hayashi limit for massive stars (Thorne & Żtykow 1977) strongly mass-losing as a result (van Paradijs et al. 1995) ? potentially more common at low Z (Linden et al. 2010) metallicity (anything beyond H and He) TŻOs are...almost indistinguishable from RSGs... cool and luminous, lying at or beyond the Hayashi limit for massive stars (Thorne & RSG surface Żtykow 1977) strongly mass-losing as a result (van Paradijs et al. 1995) potentially more common at low Z (Linden et al. 2010) He burning core TŻOs are...almost indistinguishable from RSGs... cool and luminous, lying at Mo Ru Zr Li Y Sr Rb or beyond the Hayashi limit for massive stars (Thorne & Żtykow 1977) TŻO surface irp-process Wallace & ! strongly mass-losing as a Woosley (1981) result (van Paradijs et al. Cannon (1993) 1995) Be-transport 7Be + e- ! Cameron (1955) potentially more common 7Li + v Podsiadlowski at low Z (Linden et al. 2010) (1995) ! unique chemical profile (Biehle 1994) p+ + 7Li ! p+ p+ + + p 4 p+ p + 2 He + + p NS core p p TŻOs are...almost indistinguishable from RSGs... cool and luminous,Red supergiant lying at Mospectrum Ru Zr Li Y Sr Rb or beyond the Hayashi limit for massive stars (Thorne & TŻO surface irp-process Żtykow 1977) Biehle (1994) Wallace & ! strongly mass-losing as a Woosley (1981) result (van Paradijs et al. Cannon (1993) 1995) Be-transport 7Be + e- ! Cameron (1955) potentially more common 7Li + v Podsiadlowski at low Z (Linden et al. 2010) (1995) ! unique chemical profile (Biehle 1994) p+ + 7Li ! p+ p+ + + p 4 p+ p + 2 He + + p NS core p p An e"ective large-scale search requires RSG samples with well-defined physical properties... …but red supergiant spectra are di$cult to model… Red supergiant spectrum ...and observations did not agree with theory... Hayashi limit from Ekström+ 2012 from Levesque+ 2005 ...and observations did not agree with theory... Step 1: Blame the theorists… - uncertain molecular opacities - high-velocity convective layers - highly extended atmosphere - treatment of mass-loss, rotation e"ects Step 2: …or could it be the data? - specifically, could it be the temperatures? ...and observations did not agree with theory... had some problems! •new observations (75 Milky Way RSGs) •new models (MARCS atmospheres) (K) ! e T Humphreys & McElroy (1984) Massey & Olsen (2003) • temperatures used rough estimate of a blackbody • temperatures used cold stars at lower masses Spectral Type (molecular band strength) Levesque+ 2005 New Observations of RSGs Fitting the spectra TiO band strengths New Observations of RSGs •new observations (75 Milky Way RSGs) •new models (MARCS atmospheres) (K) ! e T Massey & Olsen (2003) Humphreys & McElroy (1984) • temperatures used rough estimate of a blackbody • temperatures used cold stars at lower masses Spectral Type (molecular band strength) Levesque+ 2005 New Observations of RSGs Hayashi limit from Levesque+ 2005 Defining RSG samples Local Group ...and beyond! M31 Milky Way LMC Sextans A Sextans B Z~2Z⦿ Z~Z⦿ Z~0.47Z⦿ Z~0.07Z⦿ NGC 6822 SMC WLM Z~0.14Z⦿ Gemini/GMOS Z~0.4Z⦿ Z~0.27Z⦿ Z~0.1Z⦿ Weisenburger et al. (in prep) Metallicity E"ects Milky Way, Z=Z⦿ LMC, Z=0.47Z⦿ NGC 6822, Z=0.4Z⦿ Number SMC, Z=0.27Z⦿ WLM, Z=0.1Z⦿ Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 Massey 2012 Red supergiant subtypes Metallicity E"ects 3600 K Milky Way, Z=Z⦿ LMC, Z=0.47Z⦿ NGC 6822, Z=0.4Z⦿ Number SMC, Z=0.27Z⦿ WLM, Z=0.1Z⦿ Milky Way Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 from Levesque+ 2006 Massey 2012 Red supergiant subtypes Metallicity E"ects 3800 K Milky Way, Z=Z⦿ LMC, Z=0.47Z⦿ NGC 6822, Z=0.4Z⦿ Number SMC, Z=0.27Z⦿ WLM, Z=0.1Z⦿ LMC Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 from Levesque+ 2006 Massey 2012 Red supergiant subtypes Metallicity E"ects 4200 K Milky Way, Z=Z⦿ LMC, Z=0.47Z⦿ NGC 6822, Z=0.4Z⦿ Number SMC, Z=0.27Z⦿ WLM, Z=0.1Z⦿ SMC Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 from Levesque+ 2006 Massey 2012 Red supergiant subtypes Metallicity E"ects Milky Way, Z=Z⦿ LMC, Z=0.47Z⦿ NGC 6822, Z=0.4Z⦿ Number !? SMC, Z=0.27Z⦿ !? WLM, Z=0.1Z⦿ !? Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 Massey 2012 Red supergiant subtypes Metallicity E"ects Milky Way, Z=Z⦿ LMC, Z=0.47Z⦿ “...Years ago, Kip Thorne and myself ‘invented’ theoreticalNGC models 6822, Z=0.4Z of⦿ stars... Please let me know if there may be some interest in Number !? ! pursuingSMC, these Z=0.27Z lines⦿ of enquiry.” - Anna ytkow !? WLM, Z=0.1Z⦿ !? Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 Massey 2012 Red supergiant subtypes We selected the most likely candidates from our Galactic, LMC, and SMC samples… Reobserved with both high- and Milky Way, Z=Z⦿ 24 stars low-res spectrographs… LMC, Z=0.47Z⦿ 16 stars NGC 6822, Z=0.4Z⦿ APO 3.5m/ARCES Number SMC, Z=0.27Z⦿ 22 stars Magellan 6.5m/MIKE WLM, Z=0.1Z⦿ Levesque & K0-1 K2-3 K5-7 M0 M1 M2 M3 M4 M5 Massey 2012 Red supergiant subtypes Analyses - lines of interest Ni I, Rb I, Fe I Li I TŻO products (Ca I, K I) • “control” features Fe I, Mo I measure equivalent width ratios of TŻO/control Analyses - equivalent widths A A equivalent width Brightness Wavelength Analyses - equivalent widths • Kuchner et al. (2002) adopt method of “pseudo-equivalent widths” • definitions are based on the same features in each spectrum • all spectral features used depend on Te"... HV 2112; TŻO candidate! Mo I Li I Rb I Typical RSG Mo I Li I Rb I T!O candidate TŻO candidate (raw data) “I don’t know what it is, but I know that I like it!” ~Nidia Morrell T!O candidate Balmer series But couldn’t it be a... lower-mass giant star in the SMC/Milky Way? • quite unlikely that a giant in the Milky Way halo would just happen to match the exact motion of the background SMC • still would not explain element enhancements (giant stars cannot produce Ca... ...but TŻO formation can!) Tout et al. (2014) But couldn’t it be a... lower-mass giant star in the SMC/Milky Way? • quite unlikely that a giant in the Milky Way halo would just happen to match the exact motion of the background SMC • still would not explain element enhancements foreground dwarf? • radial velocity of 157 km s-1 agrees with SMC kinematics • not a flaring M dwarf; Balmer emission lasted over 2 nights some kind of strange binary? • not a binary within an ionized common envelope (lacks [NII], [OII], [OIII], etc.) • OB companion strong enough to produce the Balmer spectrum would produce a strong blue continuum Properties of our TŻO Candidate ✓ cool and luminous, lying at or beyond the Hayashi limit for massive stars (Thorne & Żtykow 1977) ✓ strongly mass-losing as a result (van Paradijs et al. 1995) ✓! ✓ potentially more common at low Z (Linden et al. 2010) ✓ unique chemical profile (Biehle 1994) This star represents the most encouraging detection of a TŻO to date. The existence of TŻOs would have profound implications for stellar astronomy. ‣ completely new model of stable stellar interiors ‣ a new fate of massive binaries ‣ new nucleosynthesis channels for Li and heavy elements Discovery of a Thorne-Żytkow Object Candidate in the Small Magellanic Cloud Emily Levesque University of Washington Philip Massey (Lowell Observatory), Nidia Morrell (LCO), Anna Żytkow (Cambridge) “ExtraordinaryTrevor Dorn-Wallenstein, claims require Kolby extraordinary Weisenburger evidence.” (UW), Je"rey Jennings (CU- theBoulder) “Sagan Standard” Arizona State University Apr 27, 2016 The existence of TŻOs would have profound implications for stellar astronomy.
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