A 540 Stellar Atmospheres Organizational Details Meeting times

A 540 – Stellar Atmospheres Organizational Details • • • Meeting times Textbook Syllabus Projects Homework • Topical Presentations • Exams • Grading • Notes

Basic Outline • 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 • 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…

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

History of Stellar Atmospheres • Cecelia Payne Gaposchkin wrote the first Ph. D 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 1920’s

Useful References • Astrophysical Quantities • Holweger & Mueller 1974, Solar Physics, 39, 19 – Standard Model • MARCS model grid (Bell et al. , A&AS, 1976, 23, 37) • Kurucz (1979) models – Ap. J 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 (A&AS, 77, 137) and 1990 (A&AS, 82, 179)

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 4 p. R 2 s. T 4 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/s 2) • Log g for the Sun is 4. 4 (2. 7 x 104 cm/s 2) • Log g for a white dwarf is 8 • Log g for a supergiant is ~0 • Mostly CGS units…

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 g=8 when the Sun is a white dwarf. • What assumptions are useful to simplify the problem?

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!)

Basic Physics – Ideal Gas Law PV=n. RT or P=Nk. T where N=r/m Densities, pressures in stellar atmospheres are low, so the ideal gas law generally applies. P= pressure (dynes cm-2) V = volume (cm 3) N = number of particles per unit volume r = density (gm cm-3) n = number of moles of gas (Avogadro’s # = 6. 02 x 1023) R = Rydberg constant (8. 314 x 107 erg/mole/K) T = temperature in Kelvin k = Boltzman’s constant 1. 38 x 10– 16 erg K-1 (8. 6 x 10 -5 e. V K-1) m = mean molecular weight in AMU (1 AMU = 1. 66 x 10 -24 gm) Don’t forget the electron pressure: Pe = Nek. T

Make it real… • Using the ideal gas law, estimate the number density of atoms in the Sun’s photosphere and in the Earth’s atmosphere at sea level. • For the Sun, assume P=105 dyne cm-2. • For the Earth, assume P=106 dyne cm-2. • How do the densities compare?

Thermal Velocity Distributions • RMS velocity = (3 k. T/m)1/2 • Most probable velocity = (2 k. T/m)1/2 • Average velocity = (8 k. T/pm)1/2 • What are the RMS velocities of 7 Li, 16 O, 56 Fe, and 137 Ba in the solar photosphere (assume T=5000 K). • How would you expect the width of the Li resonance line to compare to a Ba line?

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)/k. T with k in units of electron volts per degree (k= 8. 6 x 10 -5 e. V K-1) since c is normally given in electron volts.

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, u 0 and u 1 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

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 4000 K? In the Sun? In an A star at 8000 K?

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

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 R 136 cluster) • Always variable – unstable. (Some of) The Brightest Stars in the Galaxy Star m. V MV Mbol Sp. T. Dist. Pistol Star … … -11. 8 HD 93129 A 7. 0 -12 O 3 If Eta Carina 6. 2 -10 -11. 9 B 0 0 2. 5 kpc Cyg OB 2#12 11. 5 -10. 9 B 5 Ia+e 1. 7 kpc Zeta-1 Sco 4. 7 -8. 7 -10. 8 B 1. 5 Ia+ 1. 9 kpc 7 kpc 3. 4 kpc

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 • WN stars (nitrogen rich) • Some hydrogen (1/3 to 1/10 He) • No carbon or oxygen WC stars (carbon rich) NO hydrogen C/He = 100 x solar or more Also high 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?

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?

Chemically Peculiar Stars of the Upper Main Sequence • • • Ap stars (magnetic, slow rotators, not binaries, spots) – Sr. Cr. Eu stars – Silicon Stars – Magnetic fields – Oblique rotators Am-Fm stars (metalliclined, binaries, slow rotators) – Ca, Sc deficient – Fe group, heavies enhanced – diffusion? Hg. Mn stars The l Boo stars Binaries?

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

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

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 M 3 (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? )

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

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

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 – …

Amplitude of Mira Light Curve

More Red Giants • Normal red giants are oxygen rich – Ti. O dominates the spectrum • When carbon dominates, we get carbon stars (old R and N spectral types) • Instead of Ti. O: CN, CH, C 2, CO 2 • Also s-process elements enhanced (technicium) • Double-shell AGB stars Peery 1971

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 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 • • • 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 (~104 LSun), 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

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

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)

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