astrophysics i, lecture slides – chapter 2, eth zurich · 2019. 9. 23. · spectral energy...
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Spectral energy distribution of the sun
400 600 800 1000 1200 [nm]
Astrophysics I, Lecture Slides – Chapter 2, ETH Zurich
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Wien's displacement lawBlack body curves for different temperatures
250 500 750 1000 1250 nm
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Low resolution spectra of stars
3500350 400 450 500 550wavelength [nm]
rel.
flux
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''Mindmap'' for plane-parallel atmosphere
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formal solution + Taylor expansion for source funct. I(τ) as function of B(τ)
~ U ~ F~aT4 ~σTeff
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isotr. anisotr.
B(τ): - grey atmosphere approximation- RT-equation (pp-atmosphere) rad.pressure ~ flux + Eddington approximation
approx.Temperature structure
U or B(τ) decreasewith τ
F or dB(τ)/dτ= const
approximation: very good good not so good
surface
τ=2/3τ>>1 τ>1
Comparison of detailed models with grey atmosphere and Eddington approximation
• Temperature structure as function of optical depths at 500nm, for two modern 1-D models (red and green) and the mean of a 3D-model (blue). The dashed lines give the rms variations of the 3-D model (because of surface structures). Asplund et al. 2009, ARA&A 47,481
Temperature structure T(τ) for the solar atmosphere
grey atmosphere withEddington approximationTeff=5780 K adopted
4 43 2( ) ( )4 3effT Tτ τ= +
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Limb darkening for the sun
from Choudhuri
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Limb darkening
Grey atmosphere + Eddington approximation
Two parameter description (for diff. λ )
from Wikipedia, Photosphäre, 6.10.2017
sun during Venus transit 2012
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Planet transits yield limb darkening for stars
from Knutson et al., ApJ 655, 564 (2007)
HST observations of the color dependence of the transit of the planet around HD 209458
HD 209458 G0V (Teff=6000 K)Planet, 0.7 MJ, 1.3 RJ, P=3.5 d, Teq=1000K
wavelengths from top to bottom:950nm, 870nm, 770nm, 680nm, 580nm540nm, 480nm, 430nm, 380nm, 330nm
(curves not corrected for finite size of planet)
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Line formation in stars
from Choudhuri
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Spectral atlas for the sun
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Fe I (Fe0) and FeII (Fe+)
The atomic transitions of FeI and FeII which are partly responsible for the many absorption lines in the solar spectrum
Energy given in cm-1
E/hc = 1/λ (hν=hc/λ)λ = wavelength of line
from F. Thevenin, T.P. Idiart,
arXiv:astro-ph/9906433
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Curve of growth (interstellar/intergalactic Lyα)
• Curve of grows for HI Lyα line
b: Doppler widths b/c=ΔνD/ν
Top: line profiles for b=23 km/s(with HI column density log x=12,13,…,21 in cm2)
Middle: curves of growthdifferent b-parameters
Bottom: lines profiles with different b-parameters
https://ned.ipac.Caltech.edu/level5/Charlton/Charlton1_1.html 12
Curve of growths analysis
Arcturus (K-giant , 4000K) relative to empirical curves from the sun
yields differential abundancese.g. NaIArcturus/NaIsun
(from Griffin and Griffin)
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Solar photospheric elemental abundances (by number) normalized to nH = 1012
Asplund et al. 2009, ARA&A 47,481 14
there are stars with very peculiar abundances
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Warning: sun and stars have complex structures
A monster quiescent prominence photographed through a SolarMax II 60 H-alpha scope on September 17, 2015. Two days later, when the prominence rotated onto the disk, it appeared as a prominent, dark filament. Bright white patches are "plages", described below.
Bob Antol / stargate4173.com 16
https://ase.tufts.edu/cosmos/print_images.asp?id=28
Outer structure of the solar atmosphere- Photosphere- Chromosphere- Transition region- Corona
Complex magneticfield processesheat thehigher layers
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