The Lower Main Sequence

1. 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?)

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

3. 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?

4. The 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)

5. 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?

6. 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?

7. 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

8. Amplitude of Mira Light Curve

9. 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

10. 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

11. 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

12. 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