Spectroscopy. AST443, Lecture 14 Stanimir Metchev

Transcription

1 Spectroscopy AST443, Lecture 14 Stanimir Metchev

2 Administrative Homework 2: problem 5.4 extension: until Mon, Nov 2 Homework 3: problems 8.32, 8.41, 10.31, of Bradt due in class Mon, Nov 9 Reading: Howell, chapter 6 Tenagra data: see bottom of Assignments & Exams section on course website M11, M52, M37, HD b: all data taken Hyades: partial observations Astronomy Town Hall Wed, 12:30 in ESS450; free pizza RSVP to Prof. Michael Zingale 2

3 Outline Overview electromagnetic spectra stellar diagnostics Diffraction, spectrographs, spectroscopy 3

4 Examples of Continuum Spectra optically thin thermal radiation synchrotron radiation (non-thermal) blackbody (optically thick) thermal radiation 4

5 Electronic Transitions bound-free free-bound free-free (bremsstrahlung) 5

6 Radiative Transfer (again) The optical depth τ λ accounts for interaction between photospheric matter and radiation field. 6

7 Spectral Lines as Atmospheric Diagnostics chemical content and abundances mostly H and He, but heavier metals (Z > 2) + molecules are important sources of opacity photospheric temperature individual line strength line ratios photospheric pressure non-zero line width surface gravity g, mass M * stellar rotation Doppler broadening dp dr = " GM r# = "g# r 2 equation of hydrostatic equilibrium 7

8 Taking the Stellar Temperature individual line strengths N n " g n e #$ n kt g n statistical weight g n = 2n 2 for hydrogen line ratios N n = g n e # ( $ n #$ m ) kt N m g m 8

9 Taking the Stellar Temperature T eff (Fe II λ5317 / Fe I λ5328) line ratio decreases with decreasing T eff 9

10 Line Profiles Natural line width (Lorentzian [a.k.a., Cauchy] profile) Heisenberg uncertainty principle: ν = E/h Collisional broadening (Lorentzian profile) collisions interrupt photon emission process t coll < t emission ~ 10 9 s dependent on T, ρ Pressure broadening (~ Lorentzian profile) t interaction > t emission nearby particles shift energy levels of emitting particle Stark effect (n = 2, 4) van der Waals force (n = 6) dipole coupling between pairs of same species (n = 3) dependent mostly on ρ, less on T Thermal Doppler broadening (Gaussian profile) emitting particles have a Maxwellian distribution of velocities Rotational Doppler broadening (Gaussian profile) radiation emitted from a spatially unresolved rotating body Composite line profile: Lorentzian + Gaussian = Voigt profile # /2$ I " = I 0 (" %" 0 ) 2 + # 2 /4 # & Lorentzian FWHM " natural = #E i + #E f h /2$ " collisional = 2 #t coll = 1 #t i + 1 #t f " pressure % r &n ; n = 2,3,4,6 (" %" 0 ) 2 2$ 2 1 I " = 2#$ e% $ & Gaussian FWHM " thermal = # 0 kt mc 2 " rotational = 2# 0 u /c 10

11 Example: Surface Gravity Effects at Spectral Type A0 (figure: D. Gray) 11

12 Line Profiles: Equivalent Width (EW) EW = " $ 2 ( F ", cont # F ", line ) d" " 1 " $ 2 F ", cont d" " 1 λ 1 λ 2 12

13 Universal Curve of Growth the ratio of W to Doppler line width Δλ depends upon the product of N and a line s oscillator strength f in the same way for every spectral line (e.g. Unsöld 1955). " log W # $ % ' &! ( linear W " N flat square root W " ln N W " N "# = # v c = # c 2kT m m: absorber particle mass ( ) log Nf 13

14 Spectroscopic Binary (a) (d) (a) double-lined (SB2) spectra of both stars visible (b) (c) (b) (c) (d) single-lined (SB1) only spectrum of brighter star visible (d) 14

15 Example: SB1 15

16 Example: SB2 16

17 Outline Overview electromagnetic spectra stellar diagnostics Diffraction, spectrographs, spectroscopy 17

18 Diffraction multiple orders order overlap 18

19 Spectrographs Ebert Spectrograph flat grating combining collimator and focuser allows compact design 19

20 Spectrographs Wadsworth Spectrograph curved grating allows compact design 20

21 HST/STIS Spectrograph 21

22 Blazing Angle: Efficient m>0 Order Dispersion 22

23 Echelle Spectrographs: High Dispersion need to cross-disperse to avoid order overlap 23

24 Echelle Spectrographs: high blaze angle High Dispersion 24

25 Example: A Long-Slit Spectrum a continuum (telluric) calibrator (a white dwarf) 25

26 Example: a Long-Slit Spectrum a galaxy 26

27 Example: an Echelle Spectrum RU Lupi Å 27

28 Example: an Echelle Spectrum Sun Å 28

29 Multi-Object Spectroscopy use multiple slits one per science target 29

30 Multi-Object Spectroscopy use multiple slits one per science target 30

31 Multi-Object Spectroscopy extracted spectrum of an example target 31

32 Integral Field Spectroscopy 32

33 High Contrast Instrumentation: Lenslet IFS How it works 1. Focal Plane Image 2. Image on Lenslets 3. Pupil images 4. Pupil images dispersed 5. Extracted Data Cube λ y x λ (credit: UCLA IR lab) 33

34 Dispersing Lenslet Spots (credit: UCLA IR lab) 34

35 Dispersing Lenslet Spots (credit: UCLA IR lab) 35

36 OSIRIS (OH-Suppressing InfraRed Imaging Spectograph) Integral Field Spectrograph Spectra over a contiguous rectangular field. Spatial resolution at the Keck Diffraction Limit (< ) Spectral resolution (λ/δλ) ~ 3800 Full z, J, H, or K spectra in single exposure (16x64 lenslets) Integrated Data Reduction Pipeline Low Wavefront Error (< 25 nm) UCLA IR Lab y λ x (credit: UCLA IR lab) 36

37 Keck/OSIRIS Spectra of GQ Lup B GQ Lup 0.73 B J-band L5 L2 L0, 2 Gyr GQ Lup B integral field spectrograph behind Keck AO (PI: J. Larkin, UCLA) M9 GQ Lup B commissioning OSIRIS data (Aug 2005) J M6 H L0, 10 Myr (McElwain, Metchev et al., 2007) 37

38 Slitless Spectroscopy spectra of reentry of ESA s ATV-1 Jules Verne 38

39 Spectroscopic Calibration wavelength (dispersion solution) atmospheric (telluric) + instrumental transmission spectrophotometry 39

40 Wavelength Calibration He, Ar, Ne standard arc lamps each line has a known wavelength solve for λ/pix scale dispersion Ne: A 40

41 Transmission, Spectrophotometric Calibration stars with known spectral shapes, featureless continua B, A stars white dwarfs after calibration stars with well known F λ (spectral flux distributions) at each λ measure count rate [counts s 1 Å 1 ] get λ-dependent conversion factor [erg cm 2 count 1 ] need photometric conditions before calibration 41