Andrew Szentgyorgyi, David Charbonneau, David Latham, Dimitar Sasselov

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The GMT-CFA Large Earth Finder (G-CLEF) A high
stability optical echelle spectrograph for
exoplanet and cosmology research
Andrew Szentgyorgyi, David Charbonneau, David
Latham, Dimitar Sasselov Guillermo
Torres CfA with Claire Cramer, Chih-Hao Li, David
Phillips Ronald Walsworth CfA Franz Kaertner
Andrew Benedick
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Science Missions for G-CLEF

• Find real earth mass, rocky planets in the
  habitable zone of a star exoearths
• Search for variation of fundamental constants on
  cosmological time scales
• Asteroseismology
• Directly measure the acceleration of the Universe
  (but this may be very hard )

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Exoplant Context

• Useful exoplanets are transit planets where the
  mass and radius are known.
• Today ( 1year) Kepler Corot massive
  space-based photometric surveys will find 10s of
  exoearth candidates.
• PRV required to measure mass PRV to identify
  real exoearths challenging.
• Important for TPFs and analogues

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Transit Planets Are Evidently Numerous
31 Known Transit Planets (last Thursday, 27
shown)

• Torres, Winn Holman arXiv0801.1841v3

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Transmission/Occultation Spectroscopy
Methane and water on HD189733b Swain, Vasisht
Tinetti arXiv0802.1030
Weather on HD189733b Knutson et al. Nature,
447, 183
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Gas Bags, Waterworlds and Earth Analogues
From Charbonneau Deming ExoPTF White Paper
arXiv0706.1047
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Interior Structure of Super-Earths with Kepler
HARPS-NEF
Valencia, Sasselov, OConnell (2006)
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Motivation for exoearth searches around FGK
dwarfs

• There is one rocky earth-sized planet in the
  habitable zone of a G star (that has surface
  water in liquid phase)
• Habitable zones are large occupancy probability
  high
• Rich structure of narrow lines that presently
  enable PRV to below 1 m/s
• These lines are in the optical where techniques
  are mature
• Well matched to currently calibration available
  techniques (I2 continuous ThAr)

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Motivation for exoearth searches around M
Dwarfs(see Charbonneau Deming ExoPTF White
Paper)

• Exoearths in HZ are easier to find around M
  dwarfs
• Reflex signal is large for exoearths.
• M stars are relatively numerous, so small HZ is
  compensated by large number of candidate.
• HZ orbits are small, so transits of exoearths in
  HZ are more probable.
• M dwarf transits are deeper for fixed exoplanet
  radius
• HZ transits are more frequent in smaller HZ
  orbits.
• Exoearths spend more time in eclipse in M dwarf
  HZ orbit.

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An Earth mass planet orbiting
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M5-M6 May Be the Sweet Spot

• Nutzman Charbonneau (2008)
• M5 M6 are
• Numerous
• Bright
• Often low V Sin i

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MEarth Transit Search

• 2 telescopes now, soon to be 8
• Survey 1,976 late M dwarfs in 3 years
• If occurance of planets in HZ is 10 expected
  yield is 2.6 planets to 2 Mearth

16 Telescope
Phil Nutzman Midget Graduate Student
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Technical consideration

• FGK dwarfs?
• Requires extreme PRV 10 cm/sec precision.
• No clear understanding what intrinsic PRV limits
  are.
• M dwarfs?
• Requires working in the red end of optical
  passband.
• No good calibrators exist today.

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Spectral Quality for Velocimetry
per Connes(1985), Bouchy et al.(2001)
Doppler Shift
Intensity Change
per pixel i
Weight
Quality
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Quality as a Figure of Merit
From Bouchy, Pepe Queloz 2001, A A, 374, 733
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Solar Spectrum
Intensity
Derivative
Kurucz Atlas Model, Convolved to R120,000
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Telluric Absorption
99 Mask
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Solar Quality, 99 Mask
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GJ 411, M4 Dwarf
Intensity
Derivative
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GJ 411 Quality 99
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GJ 411 Quality 99.9
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PRV Calibration Difficulties(The Searching Under
the Lamp Post)

• I2 cells
• Have usable bandpass in 500 600nm
• Poor efficiency
• ThAr Hollow Cathode Lamp
• Line spacing irregular, uneven brightness
• Hard to use longward of 660 nm
• Lamps have personality finite lifetime

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Laser Comb Capabilities

• Potentially have wavelength stability (to 11015)
• Potential for wavelength calibration in
  300-1100nm
• Regular comb of features
• Relatively uniform brightness

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Comb Laser Fundamentals Femtosecond Lasers
Ultraprecise Atomic Clocks
Ideal comb laser
Envelope of real comb laser output

• Real comb lasers
• Stability determined by stability of trigger
• To date, features too closely spaced feature
  unresolved

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Thinning Out the Comb

• Suppress all but every nth comb tine with an
  etalon
• Stabilize sidelobes with actively controlled
  etalon

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Broadening the Passband(100 nm at a time)

• 600-1000 nm
• Done ? TRES then MMT
• Doubling to 400-500 nm
• Soon ? HARPS - NEF
• Doubling to 500-600 nm
• Requires research ? G-CLEF

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G-Clef Parameters

• Fiber feed scrambling
• Red channel first, blue channel second
• Need gravity-invariant location for spectrograph
• Probably looks a lot life HARPS/UVES
• Lessons from PFS grating or whole spectrograph
  in vacuum?

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G-Clef Needs a Fixed Gravity Vector

• Fiber length should be short (gt20-30m)
• Optical feed should not be Baroque

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Optical Design for Fiber Feed Guide

• Feeds fibers at fixed point
• Preliminary reducer design
• Preliminary guide system design