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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 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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    THE PROGENITORS OF TYPE IIP SUPERNOVAE Emma R. Beasor A thesis submitted in partial fulfilment of the requirements of Liverpool John Moores University for the degree of Doctor of Philosophy. April 2019 Declaration The work presented in this thesis was carried out at the Astrophysics Research Insti- tute, Liverpool John Moores University. Unless otherwise stated, it is the original work of the author. While registered as a candidate for the degree of Doctor of Philosophy, for which sub- mission is now made, the author has not been registered as a candidate for any other award. This thesis has not been submitted in whole, or in part, for any other degree. Emma R. Beasor Astrophysics Research Institute Liverpool John Moores University IC2, Liverpool Science Park 146 Brownlow Hill Liverpool L3 5RF UK ii Abstract Mass-loss prior to core collapse is arguably the most important factor affecting the evolution of a massive star across the Hertzsprung-Russel (HR) diagram, making it the key to understanding what mass-range of stars produce supernova (SN), and how these explosions will appear. It is thought that most of the mass-loss occurs during the red supergiant (RSG) phase, when strong winds dictate the onward evolutionary path of the star and potentially remove the entire H-rich envelope. Uncertainty in the driving mechanism for RSG winds means the mass-loss rate (M_ ) cannot be determined from first principles, and instead, stellar evolution models rely on empirical recipes to inform their calculations. At present, the most commonly used M_ -prescription comes from a literature study, whereby many measurements of mass- loss were compiled.