RADIATIVE TRANSFER AND STELLAR ATMOSPHERES

Institute for AstronomySpring 2020

Dan Huber (huberd@hawaii.edu)

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