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The Stellar Zoo

The Stellar Zoo. Weird stars across the HR diagram What causes stellar abundance anomalies?. The Upper Upper Main Sequence. 100 (or so) solar masses, T~20,000 – 50,000 K Luminosities of 10 6 L Sun Generally cluster in groups (Trapezium, galactic center, eta Carinae, LMC’s R136 cluster).

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The Stellar Zoo

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  1. The Stellar Zoo Weird stars across the HR diagram What causes stellar abundance anomalies?

  2. The Upper Upper Main Sequence • 100 (or so) solar masses, T~20,000 – 50,000 K • Luminosities of 106 LSun • Generally cluster in groups (Trapezium, galactic center, eta Carinae, LMC’s R136 cluster)

  3. Wolf-Rayet Stars • Luminous, hot supergiants • Spectra with emission lines • Little or no hydrogen • 105-106 Lsun • Maybe 1000 in the Milky Way • Losing mass at high rates, 10-4 to 10-5 Msun per year • T from 50,000 to 100,000 K WC stars (carbon rich) NO hydrogen C/He = 100 x solar or more Also high oxygen • WN stars (nitrogen rich) • Some hydrogen (1/3 to 1/10 HE) • No carbon or oxygen • Outer hydrogen envelopes stripped by mass loss • WN stars show results of the CNO cycle • WC stars show results of helium burning • Do WN stars turn into WC stars?

  4. Types of Massive Stars • Luminous Blue Variables (LBVs) • Large variations in brightness (9-10 magnitudes) • Mass loss rates ~10-3 Msun per year, transient rates of 10-1 Msun per year • Episodes of extreme mass loss with century-length periods of “quiescence” • Stars’ brightness relatively constant but circumstellar material absorbs and blocks starlight • UV absorbed and reradiated in the optical may make the star look brighter • Or dimmer if light reradiated in the IR • Hubble-Sandage variables are also LBVs, more frequent events • Possibly double stars? • Radiation pressure driven mass loss? • Near Eddington Limit?

  5. Venn 1995: Fig 11 [C/N] ratios in A supergiants vs. effective temperature. The dashed line is the mean [N/C] ratio for B stars; the dotted line is the first dredge-up prediction for a 10 solar mass star.

  6. Chemically Peculiar Stars of the Upper Main Sequence • Ap stars • SrCrEu stars • Silicon Stars • Magnetic fields • Oblique rotators • Slow rotators • Am-Fm stars • Ca, Sc deficient • Fe group, heavies enhanced • diffusion • HgMn stars • The l Boo stars • Binaries?

  7. Solar Type Stars • Pulsators • The delta Scuti stars • SX Phe stars • Binaries • FK Comae Stars • RS CVn stars • W UMa stars • Blue Stragglers

  8. Boesgaard & Tripicco 1986: Fig 2 The famous lithium dip!

  9. The Lower Main Sequence • UV Ceti Stars • M dwarf flare stars • About half of M dwarfs are flare stars (and a few K dwarfs, too) • A flare star brightens by a few tenths up to a magnitude in V (more in the UV) in a few seconds, returning to its normal luminosity within a few hours • Flare temperatures may be a million degrees or more • Some are spotted (BY Dra variables) • Emission line spectra, chromospheres and coronae; x-ray sources • Younger=more active • Activity related to magnetic fields (dynamos) • But, even stars later than M3 (fully convective) are active – where does the magnetic field come from in a fully convective star? • These fully convective stars have higher rotation rates (no magnetic braking?)

  10. On to the Giant Branch… • 1st dredge-up • LF Bump • Proton-capture reactions • Carbon Isotopes • Lithium Gilliland et al 1998 (47 Tuc)

  11. [a/Fe] Variations with Metallicity for Globular Clusters and Field Stars • Stellar data from Fulbright 2000

  12. CN-weak giants in M22 have lower metallicity than CN-strong giants (Brown and Wallerstein 1992) [a/Fe] Variations within Clusters [m/H]

  13. The [Ba/Eu] Ratio Stellar Data from Fulbright 2000 and Burris et al. 2000

  14. Abundance Changes on the Giant Branch • Bellman et al. show a steady decline in the carbon abundance as M92 giants evolve up the giant branch

  15. Oxygen and Sodium • Ivans et al. (1999) show a remarkable O vs. Na anti-correlation in several clusters

  16. Main Sequence Stars Giants Nature AND Nurture in M71 Cohen 1999 Penny et al. 1992

  17. 21Na 21Ne NeNa Cycle 28Si 20Ne 27Al 24Mg 26Mg 23Na 22Ne MgAl Cycle 26Al 25Al 25Mg Explaining Abundance Changes: • Proton-capture nucleosynthesis on the giant branch 22Na

  18. NGC 6752’s Main Sequence • VLT data show sodium and oxygen anti-correlation in unevolved stars

  19. Briley et al. 1996: Fig 1

  20. The 2nd Parameter Problem • Two clusters of similar age and metal content show different horizontal branch morphology

  21. Pilachowski et al. 1993: Fig 8

  22. Carbon Isotope Ratio

  23. Charbonnel 1995: Fig 1

  24. Real Red Giants • Miras (long period variables) • Periods of a few x 100 to 1000 days • Amplitudes of several magnitudes in V (less in K near flux maximum) • Periods variable • “diameter” depends greatly on wavelength • Optical max precedes IR max by up to 2 months • Fundamental or first overtone oscillators • Stars not round – image of Mira • Pulsations produce shock waves, heating photosphere, emission lines • Mass loss rates ~ 10-7 Msun per year, 10-20 km/sec • Dust, gas cocoons (IRC +10 216) some 10,000 AU in diameter • Semi-regular and irregular variables (SRa, SRb, SRc) • Smaller amplitudes • Less regular periods, or no periods

  25. Amplitude of Mira Light Curve

  26. More Red Giants • Normal red giants are oxygen rich – TiO dominates the spectrum • When carbon dominates, we get carbon stars (old R and N spectral types) • Instead of TiO: CN, CH, C2, CO, CO2 • Also s-process elements enhanced (technicium) • Double-shell AGB stars Peery 1971

  27. Weirder Red Giants • S, SC, CS stars • C/O near unity – drives molecular equilibrium to weird oxides • Ba II stars • G, K giants • Carbon rich • S-process elements enhanced • No technicium • All binaries! • R stars are warm carbon stars – origin still a mystery • Carbon rich K giants • No s-process enhancements • NOT binaries • Not luminous for AGB double-shell burning • RV Tauri Stars

  28. Smith & Lambert 1989: Fig 1 Red giants in the Magellanic Clouds

  29. Mass Transfer Binaries The more massive star in a binary evolves to the AGB, becomes a peculiar red giant, and dumps its envelope onto the lower mass companion • Ba II stars (strong, mild, dwarf) • CH stars (Pop II giant and subgiant) • Dwarf carbon stars • Nitrogen-rich halo dwarfs • Li-depleted Pop II turn-off stars

  30. McCarthy & Nemec 1997: Fig 11 Analysis of the anomalous Cepheid V19 in the globular cluster NGC 5466

  31. After the AGB • Superwind at the end of the AGB phase strips most of the remaining hydrogen envelope • Degenerate carbon-oxygen core, He- and H-burning shells, thin H layer, shrouded in dust from superwind (proto-planetary nebula) • Mass loss rate decreases but wind speed increases • Hydrogen layer thins further from mass loss and He burning shell • Star evolves at constant luminosity (~104LSun), shrinking and heating up, until nuclear burning ceases • Masses between 0.55 and 1+ solar masses (more massive are brighter) • Outflowing winds seen in “P Cygni” profiles • Hydrogen abundance low, carbon abundance high (WC stars) • If the stars reach T>25,000 before the gas/dust shell from the superwind dissipates, it will light up a planetary nebulae • Temperatures from 25,000 K on up (to 300,000 K or even higher) • Zanstra temperature - Measure brightness of star compared to brightness of nebula in optical hydrogen emission lines to estimate the uv/optical flux ratio to get temperature

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