The lower main sequence
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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

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The lower main sequence
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?)


Solar type stars
Solar Type Stars

  • Pulsators

    • The delta Scuti stars

    • SX Phe stars

  • Binaries

    • FK Comae Berenices Stars

    • RS CVn stars

    • W UMa stars

    • Blue Stragglers


Chemically peculiar stars of the upper main sequence
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?


The upper main sequence
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)


Types of massive stars
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?


Wolf rayet stars
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?


Red giants
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



More red giants
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


Weirder red giants
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


Mass transfer binaries
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


After the agb
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