RADIATIVE TRANSFER AND STELLAR ATMOSPHERESdhuber/teaching/ASTR631/1_Introduction.pdfPolluted...
Transcript of RADIATIVE TRANSFER AND STELLAR ATMOSPHERESdhuber/teaching/ASTR631/1_Introduction.pdfPolluted...
RADIATIVE TRANSFER AND STELLAR ATMOSPHERES
Institute for AstronomySpring 2020
Dan Huber ([email protected])
http://www.ifa.hawaii.edu/~dhuber/teaching/ASTR631.html!1
LOGISTICS• Slides will be posted on course website
• Recommended to take notes; slides will not necessarily cover everything we discuss during class!
• Physics of stellar atmospheres are complex and we will cover lots of equations. Goal is to develop a qualitative understanding of basic principles and practical tools for analyzing spectra
• Questions and discussions are encouraged!!2
• Quiz to recap previous lecture at the beginning of each new lecture
• Gray, D.F., “The Observation and Analysis of Stellar Photospheres”, 3rd ed., Cambridge University Press, Cambridge, 2005 (obs-heavy)
• Hubeny, I. & Mihalas, D., “Theory of Stellar Atmospheres”, Princeton University Press, 2015 (theory-heavy)
• Rutten, R.J., “Radiative Transfer in Stellar Atmospheres”, 7th ed., 2000 (http://www.astro.uu.nl/~rutten/tmr/)
• Rybicki, G.B. & Lightman, A., “Radiative Processes in Astrophysics”, New York, Wiley, 1979
• Osterbrock, D.E., “Astrophysics of Gaseous Nebulae and Active Galactic Nuclei”, University Science Books, Mill Valley, 1989
ADDITIONAL RESOURCES
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HOMEWORK & GRADING• Course will be accompanied by 5-6 homework assignments
spread out through the semester
• Due at the start of class, typically 2 weeks after handout
• Most homework will require some amount of coding; if you do not have programming experience and/or access to a computer come to talk to me after class
• If there is time, homework solutions will be discussed as a group in class once coursework has been graded
• Final exam on May ~7, testing qualitative understanding of the material
• Final Grade: 60% homework, 40% final exam!4
CLASS SCHEDULE
• Mar 17+19 (week 10, spring break) • Mar 26 (week 11, Dan in Tucson)
No class:
Jan 14 - May 7, Tue & Thu 12:00-13:15
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• Jan 21+23: classes taught by Jen van Saders
Guest Lectures:
• Jan 28: Spectroscopy & Stellar Physics (Jamie Tayar)• Jan 30: Spectroscopy & Exoplanet Science (Lauren Weiss)
1. Introduction: The power of modern quantitative spectroscopy [1w]
COURSE OUTLINE
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history of spectroscopy, spectra of astronomical objects, tour of science highlights: ultra-metal poor stars, extragalactic astronomy, transients, galactic archeology, asteroseismology, stellar metallicity and planets
2. Basic assumptions for stellar atmospheres [1w]Geometry, stationarity, conservation of momentum, mass, energy, Local Thermodynamic Equilibrium (LTE)
3. Transport of Energy: Radiation [2w]Specific intensity, radiative flux, radiation pressure, absorption and emission coefficients, optical depth, equation of transfer, source function, integral operators, approximate solutions atmospheric temperature stratification, grey atmospheres, mean opacities, limb darkening
COURSE OUTLINE
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4. Transport of Energy: Convection [1.5w]Convection in stars, solar granulation, Schwarzschild-criterion, mixing-length theory, numerical simulation of convection, effects of 1D versus 3D model atmospheres
5. Atomic Radiation Processes [2w]Bound-bound transitions, Einstein coefficients, oscillator strengths, line broadening processes and profiles, continuous absorption and scattering
6. Stellar Spectra [1w]Excitation and ionization, Saha equation, stellar spectral classification, stellar opacities
7. Non-LTE [1w]LTE versus non-LTE, occupation numbers, rate equations, transition probabilities examples: hot stars, A supergiants, M supergiants
COURSE OUTLINE
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8. Spectral Line Formation [1w]Two level atom, Milne-Eddington model, Curve of growth
9. Stellar Winds & Expanding Atmospheres [1.5w]Stellar wind signatures across the HRD, effects on stellar evolution and stellar atmospheres; Effects of velocity fields on absorption coefficients, optical depth and radiative transfer, escape probabilities, interaction regions; Sobolev-approximation, P-Cygni profiles, mass-loss rates and IR/radio- excess, X-rays and stellar wind shocks
10. Physical Parameters from Stellar Spectra [1.5w]Temperature indicators, fundamental temperature calibrations, surface brightness relations, infrared flux method; gravity indicators; measurements of chemical abundances, radial velocities, projected rotational velocities, micro/macroturbulence.
1. Introduction & Science Examples
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"We understand the possibility of determining their shapes, their distances, their sizes and their movements; whereas we would never know how to study by any means their chemical composition, or their mineralogical structure, and, even more so, the nature of any organized beings that might live on their surface." Auguste Compte
(1798-1857)
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Joseph von Frauenhofer (1820)“It will reward enough for me if, by the publication of the present experiment, I have directed the attention of investigators to this subject, which still promises much for physical optics and appears to open a new field.”
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Modern Solar Spectrum
NOAO/AURA/NSF!13
A TOUR OF SPECTRA FOR
ASTRONOMICAL OBJECTS
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Examples of spectra: The Sun
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Examples of spectra: Blue Supergiants
Pauldrach, Puls, Kudritzki et al. 1994, SSRev, 66, 105 !16
Examples of spectra: Supernovae
Filippenko 1997, ARA&A, 35, 309!17
Examples of spectra: Planetary Nebulae
Zhang & Liu 2002, MNRAS, 337, 499
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Examples of spectra: Quasar Composite
Vanden Berk et al. 2001, AJ, 122, 549 (Sloan) !19
Examples of spectra: Quasar + Lyman α Forest
Bill Keel !20
Examples of spectra: Quasar + Lyman α Forest
Bill Keel !21
Examples of spectra: Seyfert Galaxies
Osterbrock 1978, Physica Scripta, 17, 137 !22
Examples of spectra: HII Regions of Galaxies
Bresolin & Kennicutt 2002, ApJ, 572, 838
HII regions in M83
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Examples of spectra: Stellar Populations in Elliptical Galaxies
van Dokkum & Conroy 2010, Nature
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Examples of spectra: ?
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Examples of spectra: ?
Woolf et al. 2002
Earthshine (reflected off the Moon!)
H20H20
O2O2
H20
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Examples of spectra: ?
Woolf et al. 2002
Earthshine (reflected off the Moon!)
H20H20
O2O2
H20
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Rayleigh scattering
Recap: Quiz
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A star (Vega)
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Spiral Galaxy (NGC2276)
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Quasar + Lyman alpha forest
Spiral Galaxy (NGC2276)
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Spiral Galaxy (NGC2276)
Elliptical Galaxy
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Emission Line Stars (Nova & Be Star)
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White Dwarf
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Polluted Atmosphere White Dwarf
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Planetary Nebula
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Exoplanet (51 Eri b)
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Mira Star (Red Giant Variable)
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The First Stars in the Universe
Science Examples:
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The First Stars…
today
Cosmic time (not to scale)
Big Bang
Larson & Bromm 2001
2nd and later generations of stars (<1 M◉)
First stars (100 M◉)
first galaxies today’s galaxies
Anna Frebel, 2009 !50
Taking a spectroscopic look“L
ook-
back
tim
e”
Christlieb 2003
Gal
actic
che
mic
al e
volu
tion[Fe/H]= 0.0
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[Fe/H] = log(NFe/NH)* − log(NFe/NH)!Abundances are derived from
integrated absorption line strengths
Taking a spectroscopic look“L
ook-
back
tim
e”
Christlieb 2003
Gal
actic
che
mic
al e
volu
tion[Fe/H]= 0.0
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[Fe/H] = log(NFe/NH)* − log(NFe/NH)!Abundances are derived from
integrated absorption line strengths
Taking a spectroscopic look“L
ook-
back
tim
e”
Christlieb 2003
Gal
actic
che
mic
al e
volu
tion[Fe/H]= 0.0
[Fe/H] = log(NFe/NH)* − log(NFe/NH)!Abundances are derived from
integrated absorption line strengths
[Fe/H]= -4.0
[Fe/H]= -5.3
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Taking a spectroscopic look“L
ook-
back
tim
e”
Christlieb 2003
Gal
actic
che
mic
al e
volu
tion[Fe/H]= 0.0
[Fe/H]= -4.0
[Fe/H]= -5.3
?
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[Fe/H] = log(NFe/NH)* − log(NFe/NH)!Abundances are derived from
integrated absorption line strengths
High-resolution (R~60,000) Subaru/HDS spectrum (7h exposure time! taken by W. Aoki)
Extremely weak iron absorption lines detected ⇒ “Hyper iron-poor” star
Tiny littleIron Wiggles
hyper iron-poor
extremely iron-poor
extremely iron-poor
hyper iron-poor
[Fe/H]= −3.2
[Fe/H]= −5.4
[Fe/H]= −3.2
[Fe/H]= −5.4
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TransientsScience Example:
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Filippenko 1997
Time Evolution of Type Ia Supernovae
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Time Evolution of Neutron Star Mergers
Shappee et al. 2017, Science
Flux-calibrated spectroscopy (spectrophotometry) allows measurement of change in temperature after merger (~2000K in ~1 hour!)
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Shappee et al. 2017, Science
Spectral energy distributions allow tests of theoretical models for merger. No single model fits all the data!
Lanthanide-rich red Kilonova
Lanthanide-poor blue Kilonova
Disk-wind model
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Extragalactic Astronomy with Supergiants
Science Example:
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Study of metallicities A supergiants
NGC300
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Metallicity: spectral window
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[Z] = -1.3 … +0.3
[Z] = log Z/Zsun!63
Spectral window 4497-4607Å
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Spectral window 4497-4607Å
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χi spectral window 4497-4607Å
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Stellar metallicity gradient in NGC300
Kudritzki, Urbaneja, Bresolin, Przybilla, Gieren, Pietrzynski, 2008, ApJ 681, 269
angular galactocentric distance
ρ0 = 9.75 arcmin ≈ 5.7kpc
■ B0 – B3 supergiants
● B8 – A4 supergiants
--- log{Z/Z_sun} = -0.03 – 0.08•d/kpc
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NGC4625
NGC1512
NGC3621
M83
spirals with extended disks:
flat metallicity profiles
Bresolin et al. 2012, ApJ 750, 122
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Blue supergiants as distance indicators
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Flux weighted Gravity – Luminosity Relationship (FGLR)Kudritzki, Bresolin, Przybilla, ApJ Letters, 582, L83 (2003)
B1-A4
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Flux weighted Gravity – Luminosity Relationship (FGLR)Kudritzki, Bresolin, Przybilla, ApJ Letters, 582, L83 (2003)
B1-A4
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AG
200
9
Mbol = 3.41{ log(g/T4eff,4) -1.5} – 8.02
σ = 0.32 mag
Kudritzki, Urbaneja, Bresolin et al., 2008, ApJ 681, 269
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Galactic ArcheologyScience Example:
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How did the components of our galaxy form?
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Abundances as tracers for galaxy formation
Idea: abundances of stars change little, and hence trace their origin -> “chemical tagging”
Problem: star clusters disperse rapidly, kinematics are often ambiguous population tracers
Challenge: we need *lots* of spectra!
De Silva et al. 2007
HR1614 Moving Group
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Sloan 2.5m at Apache Point Observatory
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H-band spectroscopy of ~10^5+ stars in the galaxy
~15+ individual abundances for each star!!77
Solar Neighborhood
metal-poor metal-rich
alph
a-ric
hal
pha-
poor
alpha elements = C, O, Ne, Mg, Si, S, Ar, Ti …!78