Spectroscopy
AST443, Lecture 14
Stanimir Metchev
Administrative
•
Homework 2:
– problem 5.4 extension: until Mon, Nov 2
•
Homework 3:
– problems 8.32, 8.41, 10.31, 11.32 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 209458b: all data taken
– Hyades: partial observations
•
Astronomy Town Hall
– Wed, 12:30 in ESS450; free pizza
– RSVP to Prof. Michael Zingale
2
Outline
• Overview
– electromagnetic spectra
– stellar diagnostics
• Diffraction, spectrographs,
spectroscopy
3
Examples of Continuum Spectra
• optically thin thermal radiation
• synchrotron radiation (non-thermal)
• blackbody (optically thick) thermal
radiation
4
Electronic Transitions
• bound-free
• free-bound
• free-free (bremsstrahlung)
5
Radiative Transfer (again)
The optical depth τλ accounts for interaction between photospheric
matter and radiation field.
6
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
dP
GM r #
= " 2 = "g#
dr
r
equation of hydrostatic equilibrium
– Doppler broadening
!
7
Taking the Stellar Temperature
• individual line strengths
N n " gn e# $ n kT
gn – statistical weight
gn = 2n2 for hydrogen
• line ratios
N n gn #( $ n # $ m ) kT
=
e
N m gm
8
Taking the Stellar Temperature
Teff
•
(Fe II λ5317 / Fe I λ5328) line ratio decreases with decreasing Teff
9
Line Profiles
•
Natural line width (Lorentzian [a.k.a., Cauchy] profile)
–
•
Heisenberg uncertainty principle: ∆ν =∆E/h
Collisional broadening (Lorentzian profile)
–
–
–
•
∆tinteraction > ∆temission
nearby particles shift energy levels of emitting particle
•
•
•
–
•
!
" collisional = 2 #t coll
" pressure % r
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
&n
; n = 2,3,4,6
(" % " 0 ) 2
%
1
2
I" =
e 2$
2# $
$ & Gaussian FWHM
emitting particles have a Maxwellian distribution of!velocities
Rotational Doppler broadening (Gaussian profile)
–
•
#E i + #E f
1
1
" natural =
=
+
h /2$
#t i #t f
Thermal Doppler broadening (Gaussian profile)
–
# /2$
2
(" % " 0 ) + # 2 /4
# & Lorentzian FWHM
collisions interrupt photon emission process
∆tcoll < ∆temission ~ 10–9 s
dependent on T, ρ
Pressure broadening (~ Lorentzian profile)
–
–
•
I" = I0
radiation emitted from a spatially unresolved rotating body
Composite line profile: Lorentzian + Gaussian = Voigt profile
!
kT
mc 2
"rotational = 2# 0 u /c
"thermal = # 0
10
!
Example: Surface Gravity Effects
at Spectral Type A0
(figure: D. Gray)
11
Line Profiles: Equivalent Width (EW)
λ1
"2
EW =
$ (F
$
"1
", cont
"2
"1
λ2
# F", line )d"
F", cont d"
12
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).
1
re
a
qu
s
flat
t
v # 2kT
o
o
W
"
#
r
"# = # =
log $
0
%
c c m
W" N
W " ln N
r
' &! (
a
e
lin
1
W "N
!
!
1
0
m: absorber particle mass
1
!
2
3
!
4
log (Nf )
13
Spectroscopic
Binary
(a)
• double-lined (SB2)
– spectra of both stars visible
(d)
(a)
(b)
(b)
(c)
(c)
(d)
(d)
• single-lined (SB1)
– only spectrum of brighter star visible
14
Example: SB1
15
Example: SB2
16
Outline
• Overview
– electromagnetic spectra
– stellar diagnostics
• Diffraction, spectrographs,
spectroscopy
17
Diffraction
• multiple orders
order overlap
18
Spectrographs
• Ebert Spectrograph
– flat grating
– combining collimator and focuser allows compact design
19
Spectrographs
• Wadsworth Spectrograph
– curved grating allows compact design
20
HST/STIS Spectrograph
21
Blazing Angle: Efficient m>0
Order Dispersion
22
Echelle Spectrographs:
High Dispersion
• need to cross-disperse to avoid order
overlap
23
Echelle Spectrographs:
High Dispersion
• high blaze angle
24
Example: A Long-Slit Spectrum
• a continuum (telluric) calibrator (a white
dwarf)
25
Example: a Long-Slit Spectrum
• a galaxy
26
Example: an Echelle Spectrum
• RU Lupi
• 1100–1700 Å
27
Example: an Echelle Spectrum
• Sun
• 4000–7000 Å
28
Multi-Object Spectroscopy
• use multiple
slits
• one per
science target
29
Multi-Object Spectroscopy
• use multiple
slits
• one per
science target
30
Multi-Object Spectroscopy
• extracted
spectrum of
an example
target
31
Integral Field Spectroscopy
32
High Contrast Instrumentation:
Lenslet IFS – How it works
1. Focal Plane Image 2. Image on Lenslets
3. Pupil images
4. Pupil images dispersed
λ
5. Extracted Data Cube
y
x
(credit: UCLA IR lab)
λ
33
Dispersing Lenslet Spots
(credit: UCLA IR lab)
34
Dispersing Lenslet Spots
(credit: UCLA IR lab)
35
OSIRIS (OH-Suppressing
InfraRed Imaging Spectograph)
 Integral Field Spectrograph
 Spectra over a contiguous rectangular field.





Spatial resolution at the Keck Diffraction Limit (< 0.050”)
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)
y
λ
UCLA IR Lab
x
(credit: UCLA IR lab)
36
Keck/OSIRIS Spectra of GQ Lup B
GQ Lup
B
0.73”
0.73”
L5
J-band
L0, 2 Gyr
L2
GQ Lup B
•
integral field spectrograph
behind Keck AO
(PI: J. Larkin, UCLA)
GQ Lup B
M9
M6
•
commissioning OSIRIS data
(Aug 2005)
J
(McElwain, Metchev et al., 2007)
L0, 10 Myr
H
37
Slitless Spectroscopy
• spectra of reentry
of ESA’s ATV-1
“Jules Verne”
38
Spectroscopic Calibration
• wavelength (dispersion solution)
• atmospheric (telluric) + instrumental
transmission
• spectrophotometry
39
Wavelength Calibration
• He, Ar, Ne standard arc lamps
• each line has a known wavelength
• solve for λ/pix scale
“dispersion”
• Ne: 6000–7500 A
40
Transmission, Spectrophotometric
Calibration
•
stars with known spectral
shapes, featureless
continua
after calibration
– B, A stars
– white dwarfs
•
stars with well known Fλ
(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