A540 Stellar Atmospheres Organizational Details

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A540 Stellar AtmospheresOrganizational Details

• Meeting times
• Textbook
• Syllabus
• Projects
• Homework

• Topical Presentations
• Exams
• Grading
• Notes

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Basic Outline

• Integrating Stars
• Stars in the astrophysical zoo
• Stellar activity
• Winds and mass loss
• White dwarf spectra and atmospheres
• M, L and T dwarfs
• Non LTE
• Metal poor stars
• Pulsating stars Asteroseismology
• Supergiants
• Wolf-Rayet stars
• AGB stars
• Post-AGB stars
• Chemically Peculiar Stars
• Pre-main sequence stars
• Binary star evolution
• Other ideas

• Textbook Topics
• Chapter 1 Review of relevant basic physics
• Chapter 3 Spectrographs
• Chapter 4 - Detectors
• Chapter 5 Radiation
• Chapter 6 Black bodies
• Chapter 7 Energy transport
• Chapter 8 Continuous Absorption
• Chapter 9 Model Photospheres
• Chapter 10 Stellar Continua
• Chapter 11 Line Absorption
• Chapter 12,13 Spectral Lines
• Chapter 14 Radii and Temperatures
• Chapter 15 - Pressure
• Chapter 16 - Chemical Analysis
• Chapter 17 Velocity Fields
• Chapter 18 - Rotation

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Goals

• Familiarity with basic terms and definitions
• Physical insight for conditions, parameters,
  phenomena in stellar atmospheres
• Appreciation of historical and current problems
  and future directions in stellar atmospheres

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History of Stellar Atmospheres

• Cecelia Payne Gaposchkin wrote the first PhD
  thesis in astronomy at Harvard
• She performed the first analysis of the
  composition of the Sun (she was mostly right,
  except for hydrogen).
• What method did she use?
• Note limited availability of atomic data in the
  1920s

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Useful References

• Astrophysical Quantities
• Holweger Mueller 1974, Solar Physics, 39, 19
  Standard Model
• MARCS model grid (Bell et al., AAS, 1976, 23,
  37)
• Kurucz (1979) models ApJ Suppl., 40, 1
• Solar composition "THE SOLAR CHEMICAL
  COMPOSITION " by Asplund, Grevesse Sauval in
  "Cosmic abundances as records of stellar
  evolution and nucleosynthesis", eds. F. N. Bash
  T. G. Barnes, ASP conf. series, in press see
  also Grevesse Sauval 1998, Space Science
  Reviews, 85, 161 or Anders Grevesse 1989,
  Geochem. Cosmochim. Acta, 53, 197
• Solar gf values Thevenin 1989 (AAS, 77, 137)
  and 1990 (AAS, 82, 179)

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What Is a Stellar Atmosphere?

• Basic Definition The transition between the
  inside and the outside of a star
• Characterized by two parameters
• Effective temperature NOT a real temperature,
  but rather the temperature needed in 4pR2sT4 to
  match the observed flux at a given radius
• Surface gravity log g (note that g is not a
  dimensionless number!)
• Log g for the Earth is 3.0 (103 cm/s2)
• Log g for the Sun is 4.4 (2.7 x 104 cm/s2)
• Log g for a white dwarf is 8
• Log g for a supergiant is 0
• Mostly CGS units

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Make it real

• During the course of its evolution, the Sun will
  pass from the main sequence to become a red
  giant, and then a white dwarf.
• Estimate the radius of the Sun (in units of the
  current solar radius) in both phases, assuming
  log g 1.0 when the Sun is a red giant, and log
  g8 when the Sun is a white dwarf.
• What assumptions are useful to simplify the
  problem?

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Basic Assumptions in Stellar Atmospheres

• Local Thermodynamic Equilibrium
• Ionization and excitation correctly described by
  the Saha and Boltzman equations, and photon
  distribution is black body
• Hydrostatic Equilibrium
• No dynamically significant mass loss
• The photosphere is not undergoing large scale
  accelerations comparable to surface gravity
• No pulsations or large scale flows
• Plane Parallel Atmosphere
• Only one spatial coordinate (depth)
• Departure from plane parallel much larger than
  photon mean free path
• Fine structure is negligible (but see the Sun!)

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Basic Physics Ideal Gas Law

• PVnRT or PNkT where Nr/m
• P pressure (dynes cm-2)
• V volume (cm3)
• N number of particles per unit volume
• r density (gm cm-3)
• n number of moles of gas (Avogadros
  6.02x1023)
• R Rydberg constant (8.314 x 107 erg/mole/K)
• T temperature in Kelvin
• k Boltzmans constant
• 1.38 x 1016 erg K-1 (8.6x10-5 eV K-1)
• mean molecular weight in AMU (1 AMU 1.66 x
  10-24 gm)
• Dont forget the electron pressure Pe NekT

Densities, pressures in stellar atmospheres are
low, so the ideal gas law generally applies.
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Make it real

• Using the ideal gas law, estimate the number
  density of atoms in the Suns photosphere and in
  the Earths atmosphere at sea level.
• For the Sun, assume P105 dyne cm-2.
• For the Earth, assume P106 dyne cm-2.
• How do the densities compare?

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Thermal Velocity Distributions

• RMS velocity (3kT/m)1/2
• Most probable velocity (2kT/m)1/2
• Average velocity (8kT/pm)1/2
• What are the RMS velocities of 7Li, 16O, 56Fe,
  and 137Ba in the solar photosphere (assume
  T5000K).
• How would you expect the width of the Li
  resonance line to compare to a Ba line?

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Excitation the Boltzman Equation

• g is the statistical weight and Dc is the
  difference in excitation potential. For
  calculating the population of a level the
  equation is written as

u(T) is the partition function (see def in text).
Partition functions can be found in an appendix
in the text. Note here also the definition of q
5040/T (log e)/kT with k in units of electron
volts per degree (k 8.6x10-5 eV K-1) since c is
normally given in electron volts.
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Ionization The Saha Equation

• The Saha equation describes the ionization
  of atoms (see the text for the full equation).
• Pe is the electron pressure and I is the
  ionization potential in ev. Again, u0 and u1 are
  the partition functions for the ground and first
  excited states. Note that the amount of
  ionization depends inversely on the electron
  pressure the more loose electrons there are,
  the less ionization. For hand calculation
  purposes, a shortened form of the equation can be
  written as follows

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Make it real

• At (approximately) what Teff is Fe 50 ionized in
  a main sequence star? In a supergiant?
• What is the dominant ionization state of Li in a
  K giant at 4000K? In the Sun? In an A star at
  8000K?

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The Stellar Zoo

• Across the HR diagram
• What causes an ordinary star to become weird?

• basic stellar evolution
• mass loss winds
• diffusion radiative levitation
• pulsation (radial and non-radial)
• rotation
• mixing
• magnetic fields
• binary evolution mass transfer
• coalescence

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The Upper Upper Main Sequence

• 100 (or so) solar masses, T20,000 50,000 K
• Luminosities of 106 LSun
• Generally cluster in groups (Trapezium, Galactic
  Center, Eta Carinae, LMCs R136 cluster)
• Always variable unstable.

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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?

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More 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?

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Chemically Peculiar Stars of the Upper Main
Sequence

• Ap stars (magnetic, slow rotators, not binaries,
  spots)
• SrCrEu stars
• Silicon Stars
• Magnetic fields
• Oblique rotators
• Am-Fm stars (metallic-lined, binaries, slow
  rotators)
• Ca, Sc deficient
• Fe group, heavies enhanced
• diffusion?
• HgMn stars
• The l Boo stars
• Binaries?

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Solar Type Stars (F, G, K)

• Pulsators
• The delta Scuti stars, etc.
• SX Phe stars
• Binaries
• FK Comae Stars
• RS CVn stars
• W UMa stars
• Blue Stragglers

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Boesgaard Tripicco 1986 Fig 2
The famous lithium dip!
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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
• Flare temperatures may be a million degrees or
  more
• Some are spotted (BY Dra variables)
• Emission line spectra, chromospheres and coronae
  x-ray sources
• Youngermore 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?)

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On to the Giant Branch

• Convection
• 1st dredge-up
• LF Bump
• Proton-capture reactions
• CNO, Carbon Isotopes
• Lithium

Gilliland et al 1998 (47 Tuc)
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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

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Pulsators

• Found in many regions of the HR diagram
• Classical Cepheid Instability Strip
• Cepheids
• RR Lyrae Stars
• ZZ Ceti Stars
• Other pulsators
• Beta Cephei Stars
• RV Tauri
• LPVs
• Semi-Regulars
• PG 1159 Stars
• Ordinary red giants

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Amplitude of Mira Light Curve
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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
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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

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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

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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 Tgt25,000 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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R Corona Borealis Stars

• A-G type Supergiants
• Suddenly become much fainter (8 mag)
• He, Carbon rich, H poor
• Dust puff theory - Mass loss and dust
  obscuration?
• Origin - Double degenerate (He CO with mass
  transfer)?
• about 100 known

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White Dwarf Merger Scenario

• The camera aspect remains the same, but moves
  back to keep the star in shot as it expands.
  After the star reaches 0.1 solar radii, an octal
  is cut away to reveal the surviving disk and
  white dwarf core. The red caption (x) is a
  nominal time counter since merger. A rod of
  length initially 0.1 and later 1 solar radius is
  shown just in front of the star. (Saio Jeffrey
  - http//star.arm.ac.uk/csj/movies/merger.html)

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White Dwarf Soup

• Single Stars
• DO (continuous)
• DB (helium)
• DA (hydrogen)
• DZ (metals)
• DC (carbon)
• Evolutionary sequence still unclear

• Cataclysmic Variables
• WD low mass companion
• Neutron star companion
• Accretion disk