Spectroscopy in Astrophysics:

Radial Velocity

Seminar talk

Andreas Hasenfratz

19.12.2014

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Content

1. IntroductionRadial velocity, geometrical description

2. Observed objectsBinaries, planets, sun

3. Measurement techniquesEchelle spectrographs, accuracies, calibration methods; interferometer measurements

4. Exemple instrumentsHARPS, GONG, MDI

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Introduction

Radial velocity (RV):

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Introduction

Doppler effect:•blueshift of approaching/•redshift of recessing objects

λobs = λem 1+ β cosθ

•spectral lines serve as reference

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Introduction

periodic motion → periodicity in RV

→ importance of long term observationmostly just relative RV is interesting

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Introduction

Why are radial velocities interesting for physicists?

•Binary systems, ~10 km/s

•Planets, ~10 km/s – 1 m/s

•Astero- and helio-Seismology, ~km/s – m/s

cosmological redshift: not caused by radial velocity,due to expansion of space

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Spectroscopic binaries

Spectroscopic binary system with line-separation:

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Exoplanets

Typical RV-curve of a star orbited by a planet:

Motion around the centre of mass:

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Helioseismology

Dopplergram → analyse in sperical harmonics → internal structure of the sun

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Measurement technique

Interferometry•Spacially resolved observation•Single line observed•Several pictures for RV-measurement•Used in helioseismology

Echelle spectrographs•Observes point-like objects•Spectrum taken in one measurement•Observation of thousands of lines•Planet search

Resonant-scattering spectrometers•No spacial resolution•Uses resonant scattering in a gas cell•Very fast (e.g. BiSON ~4 sec)

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Echelle spectrographs

•cross dispersed echelle spectrographs•very high precision spectra•need for good calibration

1 Collimator2 Echelle grating3 Cross disperser4 Detector

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Echelle spectrographs

Instrumental requirements:•Mechanical stability•Wavelenght calibration•Thermal stability

To be taken into account:•Refractive index of air =/ 1•Time variation of earth rotation•(Gravitational redshift•Stellar surface effects)

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Echelle spectrographs

•Early methods (1970s) about 1 km/s accuracies•Calibration lamps•Not simultaneous•Position read off

•Significantly improved accuracies by telluric water vapour lines•Simultaneous read off•limited amount of lines of suitable strength (spectral range)•Systematic errors from path length and winds (~20m/s)

Todays calibration methods:•Gas cells•Th-Ar lamps•Infrared wavelength calibration•Laser frequency comb

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Echelle spectrographsmodern calibration methods:•Gas cells

•Captive gases (e.g. HF, I2) placed in the light path•sharp absorption lines superimposed on stellar spectrum•information about the spectrographs point-spread function•20-30% loss of light•Best only in the range 500-620 nm, difficult e.g. for redder dwarfs

•Th-Ar lamps•Emission lamp•Led to spectrograph by optical fibre •Wide optical to infrared range•Large number of strong lines•No intensity loss like through gas cell•Calibration accuracies of ~60 cm/s

•Laser frequency comb LFC•New technique•Uniform spacing and intensity•Calibration accuracies of ~1 cm/s possible

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Echelle spectrographs

Mode locked femtosecond laser

•waves with L=q λ/2 interfere constructively in a resonator of length L

•q = usually 105 - 106

•Δν = c/2L, between q and q+1•Equally spaced lines in frequencies

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Echelle spectrographs

Comparison of LFC (dotted lines) with Th-Ar lamp (less populated rows below)

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Interferometry

Fabry-Perot interferometer orMichelson interferometer

Used as tunable narrow band filter

Intensity scan over frequencies→ accurate position of a single line→ spacial resolved measurements

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some instruments

Echelle spectrographs•HARPS (High Accuracy Radial Velocity Planet Searcher) (3.6 m, ThAr)•ESPRESSO (Echelle SPectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations) (4 x 8.2 m, LFC)•CODEX (Cosmic Dynamics Experiment)(39.6 m telescope)

Interferometer•GONG (Global Oscillation Network Group) (ground based)•SDO-HMI (Helioseismic and Magnetic Imager) (space)•SOHO-MDI (Michelson Doppler Imager) (space)

Resonant-scattering spectrometer•BiSON (ground based)•SOHO-GOLF (space)

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HARPS

•ESO observatory La Silla (Chile) •3.6 m telescope•Cross dispersed echelle spectrograph in vacuum vessel•Fed by two fibres (observed object, reference spectrum)•RVs accurate to 1 m/s with simultaneous ThAr reference•Echelle grating of 31.6 gr/mm,

blaze angle 75°•Cross disperser grism with 257.17 gr/mm•68 refractive orders •Specral range of 378 – 691 nm•R = 120,000•2 CCDs(2k x 4k px each):

orders 89-114 and 116-161 (530-533 nm lost in the gap)

•4.1 px per FWHM, 17.3 px between the fibres

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GONG

•6 identical observatorys•Michelson interferometer•2.8 cm effective aperture•Hybrid pre-filter 1 Å passband•Ni I line (6768 Å)•temperature stabilized to the

order of 0.00001 K•CCD, 1k x 1k px

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SOHO-MDI

•Michelson interferometer•1 Å bandpass•Ni I line (6768 Å)•CCD, 1k x 1k px•1 measurement every 60 sec

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Thanks for your attention.

Are there questions?

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References http://eo.ucar.edu/staff/dward/sao/exoplanets/methods.htm

http://www.astro.ucla.edu/~wright/doppler.htm

http://astronomy.swin.edu.au/cosmos/B/Binary+Star

http://www.planetary.org/multimedia/space-images/charts/radial-velocity-graph-51-pegasi.html

http://hubblesite.org/explore_astronomy/black_holes/graphics/alberio_visual.jpg

http://www.popsci.com/science/article/2010-06/scientists-confirm-first-direct-photo-exoplanet

http://sepwww.stanford.edu/public/docs/sep109/paper_html/node9.html

http://www.uni-tuebingen.de/de/4258

http://upload.wikimedia.org/wikipedia/commons/d/d2/Wiki_Spect_Binaries_v2.gif

http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html

http://www.astrobio.net/topic/solar-system/meteoritescomets-and-asteroids/astrobiology-top-10-discovery-of-habitable-alien-world/

M. T. Murphy et al.: High-precision wavelength calibration of astronomical spectrographs with laser frequency combs (2007)

G. L. Curto et al.: Along the path towards extremely precise radial velocity measurements. (2010)

K. Glogowski: Bestimmung der solaren meridionalen Strömung und ihrer zeitlichen und räumlichen Variation durch Fourier-Legendre-Analyse (2011)

C. Lovis et al.: The exoplanet hunter HARPS: unequalled accuracy and perspectives toward 1 cm s-1 precision (2006)

http://bison.ph.bham.ac.uk/index.php?page=bison,operations

http://gong.nso.edu/instrument/

http://soi.stanford.edu/science/obs_prog.html