spectroscopy in modern astronomy: just another tool? · heterodyning in astronomy?! well developed...

Spectroscopy in Modern Astronomy:Just Another Tool? Hans Ulrich Käufl, ESO Garching, May. 19, 2010

Astro-Seminar 'Nuclei in the Cosmos, MPE Garching May 19th , 2010 Ulli Käufl, ESO slide # 2

What you – potentially – always wanted to know about Optical Spectroscopy, but never dared to ask ... :spectroscopy: its fundamental principlehow does a modern spectrograph look likehow do spectra form: here specificallyoverview of rotational-vibrational molecular spectraa bit about radiative transferwhere do the elements in the universe really come fromcool stars and stellar evolutionother hot topics and applications outlook and suggestions for reading

Outline:


Answers by the Physicist from the street:It something about the color of light!?It is measuring the frequency of light!It is measuring the energy of light / photons!

Take-home-message from this talk: optical spectrographs measure the temporal auto-correlation function of light!

What is Spectroscopy?


Measuring the Frequency:heterodyne detection


Basic Principle of Heterodyning

assume two sinusoidally varying electric fields

coherently superimposing these two electric fields on a power detector yields:

note: ω1 , ω2 , ω1 + ω2 is optical light ω1 - ω2 is a radio signal: “beat frequency”

E1 t = E01 cos 1 t E2 t = E02 cos 2 t

P= E1t E2 t 2

= E012 cos2 1 t E02

2 cos22 t E01E 02[cos 12 t ]E01E 02[cos 1−2 t ]


Heterodyning in Astronomy?!

Well developed standard technique in

Laser and other laboratory spectroscopy(Udem, Holzwart & Hänsch, 2002, Nature 414 233)

Radio Astronomy

allows to calibrate spectrographs to the time standard

but not in optical astronomy for at least the following reasons:

Orders of magnitude less sensitive than a direct detector

Very inflexible and narrow bandwidth coherently superimposing these two electric fields on a power detector yields:

Spectral resolution ν/Δν (or vDoppler

/ΔvDoppler

) way to high


Well established technique for γ and x-raysE

phot > 1 keV versus “ionisation energy” of detector material

(e.g. Si or Ge: ) yields at least 102 photo-electrons per γ for visible or infrared light this number is <10thus no energy resolution worth speaking off(ΔE/E = Δν/ν ≈ √n

e )

but, research is ongoing to use other materials, e.g. super-conductorsJosephson pair binding energy is 10-3 eV, panoramic detectors with energy resolution possible:thus stay tuned!

Measuring the Energy


possible spectrograph embodimentsfrom an old, but good text book(Pohl, Optik, 1940)

The Third Way ...


The Third Way ...


All spectrograph designs have as common feature:collimated light beam is split or segmentedpart of the beam is delayedbeams are then superimposed coherently

To have a non-zero intensity at the output, the light has to be superimposed coherently, spatially: restricts field-of-view and optics expensive spectrally: requires temporal coherence

What is in common?


ΔE · Δ t ≥ ħ /2in a spectrograph with a path length difference Δs this yields:

ΔE ≥ ħ / [2 (Δs / c) ] note: Δs is the only relevant parameter, the rest are technicalitiesoptical light originates normally from dipole radiationi.e., the approximation of a damped harmonic oscillatorapplies: A(t) = A

0 • e-iωt • e -t / τ

Fourier transformation of A(t) leads to the uncertainty principle

Heisenberg's Uncertainty Principle Enters the Scene

Astro-Seminar 'Nuclei in the Cosmos' MPE Garching May 19th , 2010 Ulli Käufl, ESO slide # 12

It has to measure the autocorrelation function:

device has to have uncompromised sensitivity combinations of gratings and prisms win

What are the requirements for the spectrograph

A x = ⟨∫0

x

E t ∗E txcdx ⟩

x=c∗

A = ⟨∫0

E t ∗E td ⟩

CRIRES, a worked example


SchematicsofCRIRES


`CRIRES without CRIRES'


CRIRES main characteristics spectral coverage: ν ~ 58 000 – 310 000 GHz

( λ ~ 950 – 5200 nm )spectral resolution: ν / Δν (λ / Δλ) ≈105 or Δv

Doppler ≈ 3km/s

(2 pixel Nyquist sampling)array detector mosaic:4 x 1024 x 512 Aladdin III InSb mosaic

instantaneous frequency - coverage > 2.0 %☞ pixel scale 0.1”/pixinfrared slit viewer (Aladdin III) with J,H & K-filtersprecision for calibration and stability ~ 75m/si.e. 1/20th of a pixel or 5 mas tracking errorPiezo-electric actuator in pre-disperser collimator for vernier adjustment of spectrum on detectorusing sky-lines or fiber-injected light as reference


CRIRES main characteristics (cont)spectrograph intrinsic stability << 75m/s preference in design was given to stability

gas cells for high precision radial velocity work☞ curvature sensing Adaptive Optics

0.05” spatial resolution per pixel in☞ slit viewer camera right: composite JHK false color image of

the Jovian satellite Io (dia 1.1”) spectro-polarimetry in lines: magnetic fields

goal to measure all 4 Stokes parameter cold kinematic MgF

2 Wollaston prism

already in cryogenic fore-optics in preparation: λ / 4 Fresnel rhomb and λ / 2 plate in rotary mounts plus calibration at the gas-cell slide


◄ left: one of the four hybrids ▼complete assembly of mosaic

4 Aladdin III arrays, hybridized gap reduced to 286 pixel use a band of eight 512x512 arrays detector upgrade envisaged

spectrograph focal plane assembly


Physik Department E12 Garching May 7, 2008 Ulli Käufl, ESO slide # 20

Why CRIRES ?

a (sneak) preview: Sun spots contain spectral sequence from a GV to a magnetic MV star excellent preview for CRIRES


Why CRIRES ?

Molecular Rotational Vibrational Spectra: all molecules with N atoms can be approximated as a system of n=3N-6 or n=3N-5 coupled harmonic oscillators with a certain rotational energyErot << Evib << Eelectronic

transitions between two energy states are possible if the molecule has

a permanent dipole moment: e.g. CO, OH-, H2O, H

3O+, NH

3

or an induced dipole moment: e.g. CO2

, CH4

note: H2, N

2 or O

2 have no dipole moment

for an angular momentum of j there are (2j+1) sub-states

E 1n j = ∑i=1

n

ℏ∗ii 1/2 ℏ2∗ j∗ j1

2∗ ,


Rotational-Vibrational Molecular Spectra II

selection rules for transitions: symmetric tops Δ j = + 1 asymmetric tops Δ j = + 1, 0 if Δ j = 0 then Δ k= + 1

naming convention: Δ j = + 1 are called P and R -branch Δ j = 0 is called a Q-branch

if no change in vibrational state ☞ radio/sub-mm astronomy|Δ ν i| = 1 is called fundamental band |Δ ν i| = 2, 3 ... are called overtone bands|Δ ν i| = n and |Δ ν i| = m are called combination bands

and if the lower state is not the vibrational ground state then transitions are called hot bands

note: Θ and ν i are functions of j, k, and ν j = 1 ... n

and at this point we have neglected electrons .... and nuclear spins ... (e.g. H2O, NH3, H3

+)


Rotational-Vibrational Molecular Spectra III

to remember: for one molecular species typically several hundred transitions can be expected which all have a different optical depth and which thus sample- different zones spatially- different physical conditions, e.g. T

right: telluric N2Oν3-fundamental band at 76 000 GHz (aka 3900 nm)j = 0 ... 45 sampled !!!equivalent to one CRIRES exposure (data Solar FTS)


Rotational-Vibrational Molecular Spectra IV

Isotope Ratios:isotopic shifts scale with the reduced mass:

e.g. for 28Si16O vs 29Si16O : ΔMred ≈ 1.0 %

left: example of a stellar low resolution spectrum of the

bandheads of 1st overtone transitions of SiO (from Aringer

et al. 1999 A&A 342); all structure is statistically significant;

~ 100 lines each merge into one bandhead ...

M red=m1⋅m2

m1m2


Radiative Transfer in a Nut Shell

convenient coordinate system: optical depth τν Kirchhof's law: τ

ν ≈ 0 F☞

ν ≈ 0

the contribution ΔFν of a slice of atmosphere:

=>

=

thus, basically the Fν observed from outside samples the

atmosphere to a depth equivalent “τ(x)ν ≈ 1”

each transition from the 100s of individual lines of a molecular band samples a different zone in the atmosphere

altitude resolved observations are feasible! ☞

F = e− x ⋅d I

d x⋅ x = e−⋅

d I

d x⋅

dxd

⋅

F = ∫0

∞

e−⋅d I

d d

∫0

1

e−⋅d I

d d ∫

1

∞

e−⋅d I

d d


Heavy Elements: Where do they come from?

potential sites:

Super Novae (Crab Nebula)

Planetary Nebulae (Mz3 aka Ant Nebula)


Heavy Element Formation?


Heavy Element Formation? Technetium ?


Example: Fluorine (1)only one stable isotope 19F :

n-breading: much easier destroyed than producedpotentially AGB envelopes provide for the only possible siteto produce Fluorine; even ν-breading following SN-explosions has been proposed:- thermonuclear models (Woosley & Weaver 1995):

spallation of 20Ne with μ and τ-neutrinos suggested:

20Ne ( νx , ν'

x p ) 19F

observational problem:- Fluorine has only few weak optical transitions- stable and abundant molecule, HF, only in cool stars however, if present easy to observe


Example: Fluorine (2)

IR-spectra of H19F fundamental band (R14 to R23, λ ~ 2300nm): with precise atmospheric model (grey line; parameters from spectrum) precise abundances can be derived; here a low abundance is found as expected (Uttenthaler et al. 2008)


Stellar Abundances

- Sulfur triplet in the metal poor star G29-23: [Fe/H] = -1.8 [S/H] = -1.5 graphics/analysis Nissen et al 2007

- S/N ~ 330

- Sulfur is an α - element;hence very important to understandnucleosynthesis


more abundances: galactic bulge

- the H-band, an extremely promising domain for stellar abundances Ryde et al. astro-ph 0701916 note: only one of four detectors shown


Nuclear Spins: ortho and para configurations

I = 1 I = 0

ortho: 2I +1 = 3para: 2l +1 = 1

OPR = 3 e-ΔE/kT

ΔE/kB ≈ 35K

Mumma et al. 1987; 1989; 1993


Nuclear Spins: ortho and para H20

Dello Russo et al. 2003

Water in C/1999 H1 Lee OPR = 2.5 Tspin = 30 K


interstellar medium: galactic center

Infrared absorption spectra of H3

+ R(1,1)' topnote: j=0, k=0 does not exist (Pauli)

and CO R(1) bottom

towards the Quintuplet cluster GCS 3--2 and observed with: IRCS at the Subaru (R=20,000)

Phoenix at Gemini South (R=75,000)and

CRIRES at the VLT (R=100,000)

Note: higher velocity resolution of CRIRES and “deeper” absorption features (CRIRES SV, Goto et al.)



What I did not talk about: Emission line spectroscopyLong-slit spectroscopy and spectro-astrometryElectronic spectra, i.e. mostly UV and optical radiation Physics of line formation: LTE and non-LTESpectral calibrationFlux calibrationConversion of line strengths into abundancesSpectro-polarimetry and the Zeeman effect And much more ...

Final take-home-messageThey say a picture tells you more than 1000 wordsAnd for an astrophysicist this continues as:a spectrum tells you more than 1000 images !

Instead of Conclusions:

Suggested Reading Robert H. Kingston:Detection of Optical and Infrared RadiationSpringer

Anne Thorne et al.:Spectrophysics: Principles and ApplicationsSpringer

Thomas Udem et al. :Optical Frequency MetrologyNature 416 p 233, 2002

Wolfgang Demtröder :Laser SpectroscopySpringer

This talk: www.eso.org/~hukaufl/REPRINTS/spectro_tuto_2010.pdf

Käufl, H.U.:

VLT-CRIRES: “Good Vibrations” Astron. Nachr. 331, 549 2010

UVES and CRIRES user's manuals via www.eso.org/sciAstro-Seminar 'Nuclei in the Cosmos, MPE Garching May 19th , 2010 Ulli Käufl, ESO slide # 38

spectroscopy in modern astronomy: just another tool? · heterodyning in astronomy?! well developed...

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