Carole Haswell, The Open University

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HST observations of transiting exoplanets

• Carole Haswell, The Open University

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Talk Outline

• Why study transiting exoplanets?
• Recent theoretical developments ( limitations
  thereof)
• How we can test these with the new HST
• What it might mean for the big picture

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Why study transiting exoplanets?
The only planets outside the solar system for
which we can measure mass and radius

• Density varies by 5
• High precision photometry precise radius
• constrain theories of planetary formation,
  evolution
• Parameters jointly governing giant planet size
• Mass, age,
• stellar irradiation,
• atmospheric composition circulation
• presence mass of inner core
• convert Spitzer flux into brightness temperature

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Exoplanet atmospheric chemistry

• Transmission spectroscopy depth(?)
• Line opacity and silhouette radius
• characterise atmospheric physics and chemistry

From Brown (2001) ApJ 553 1006
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Giant Exoplanet atmospheric modellingdayside
nightside contrast

• Many planets are really hot Jupiters Teql
  1500K
• Intense heating drives global-scale atmospheric
  dynamics
• predictions of supersonic irradiation-driven
  winds cross hemisphere in 105 s (Cooper and
  Showman 2006)
• Disequilibrium chemistry?
• Fast reactions on hot dayside
• Frozen-in molecular abundances on cool night
  side?
• Spitzer observations of ?And b and HD 179949b
• show little sign of redistribution of energy (at
  24µm and 8 µm photospheric depth)
• predictions of supersonic irradiation-driven
  winds cross hemisphere in 105 s
• Vertical mixing too, poorly understood

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Example 2D, high resolution simulation
Dynamical flow tracer (PV) in simulation of HD
209458b (Cho et al. 2003)
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Giant Exoplanet atmospheric modelling

• Neutral atmosphere around cloud decks,
  responsible for most emission
• Coupling with interior not understood, even for
  Jupiter
• coupled atmosphere-interior model a target
• Many models,
• various simplifications
• Wide range of predictions!
• Field will be driven by observations for
  forseeable future!

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Giant Exoplanet atmospheric modelling

• Upper atmosphere can be modelled by simplified
  general circulation model (GCM)
• Assumes quasi hydrostatic-equilibrium, but still
  expensive!
• Can study atmospheric flow regimes and heat
  transport under different parameters
• predict spectra
• 3D hydro treatment (Dobbs-Dixon Lin 2007)
• High opacities ? large day/night T contrast
• Low opacities ? low day/night T contrast

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Why HST?

• Until recently HST alone had detected exoplanet
  atmospheric chemistry (Redfield et al
  arXiv0712.0761)
• Transit durations few hours
• timescale of telluric atmospheric extinction
  effects
• extinction correction is never perfect
• dominant source of systematic error (Bakos et al
  2006)
• HST above earths atmosphere stable platform
• well-supported observatory
• instruments exceed spec
• expert program coordinators, instrument
  scientists (Tony Roman)
• HST likely to continue to lead in this area.

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2 Classes of Hot Jupiters
pM Class hot enough for gaseous TiO and VO pL
Class TiO and VO condensed
Fortney et al 2007
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pM and pL class planets
Prediction hot stratosphere for pM class 6
enhancement in transit depth

• radius(?)

Fortney et al 2007
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pM and pL class planets
Prediction 6 enhancement in transit depth
distinct radius(?) curves Test with NICMOS
(now), STIS, WFC3? and/or ACS? Possible advances
i) cold-trap not well-understood
ii) atmospheres not iterated to
radiative- convective equilibrium
- may affect emergent spectrum - how
important is disequilibrium chemistry?

Fortney et al 2007
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Dust Cloud Formation Alkali lines
Alkali lines strongly dependent on model used for
dust formation
DUST MODEL
line profile model
Jonas et al arXiv08013544
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Dust Cloud Formation Alkali lines

• Calculation of formation and composition of dust
  grains
• Alters elemental abundances
• Alters atmospheric temperature profile
• Changes gas phase chemistry
• Alters line profiles

Alkali lines strongly dependent on model used for
dust formation
Jonas et al arXiv08013544
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Dust Cloud Formation Alkali lines

• Measure line profiles with STIS
• Correlate line profiles with T,P, convective
  velocity, chemistry
• Refine the models

Alkali lines strongly dependent on model used for
dust formation
Jonas et al arXiv08013544
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Dust Cloud Formation Alkali lines

• Measure line profiles with STIS
• Correlate line profiles with T,P, convective
  velocity, chemistry
• Refine the models

Alkali lines strongly dependent on model used for
dust formation
Jonas et al arXiv08013544
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COS looks like great new instrument!

• 10x previous UV sensitivity
• Detailed transmission spectroscopy of atmospheres
  possible
• Probe composition, vertical structure, mass loss
• Probe atmospheric dynamics?

See next talk!
HD 209458b Ly a silhouette
Vidal-Madjar et al (2003) Nature 422, 123
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COS looks like great new instrument!

• HD 209458b ACS/PRISM observations
• UV transit radius gt optical transit radius
• Detailed COS transmission spectroscopy to reveal
  why
• r(?)

Désert et al (2003) poster outside
See next talk!
HD 209458b Ly a silhouette
Vidal-Madjar et al (2003) Nature 422, 123
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COS looks like great new instrument!

• 10x previous UV sensitivity
• Activity enhancements induced by closely orbiting
  planets?
• Need to characterise spectral line variability to
  underpin transit transmission spectroscopy
• L dwarfs show Ly a and CIV emission
• Detect auroral emissions from planet?
• Measure orbital RV curve of unseen planet?
  (c.f. Bowen fluorescence in XRBs)

Moutou et al (2007) AA 473, 651
Gizis et al (2005) Froning, this conference
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Interior structure of giant exoplanets

• Host star constrains planet age
• As planets cool they contract
• ½ of known hot Jupiters are systematically
  larger than expected from standard cooling
  calculations

Hansen Barman 2007 Dotted traditional
radius solid transit radius
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Oversized giant exoplanets

• Physical reason still unclear need
• additional heating (e.g. tidal forcing)
• or less efficient heat loss (irradiation
  ineffective convection)
• or non-standard composition due to formation
  history
• or ?

Chabrier Baraffe 2007 Molecular weight gradient
mimics semi-convection Red, blue layered
convection Black traditional, adiabatic
convection, with and without core

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Interior Structure meets observations?

• Tint intrinsic Teff (i.e. as if no
  irradiation) affects T-P profile
• hence affects emergent spectrum particularly pL
  pM border
• hence gives a handle on planets (cooling) age

Fortney et al 2007
Tint 250K,150K
Charbonneau et al 2006
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How does this fit into a big picture?

• Close-in extrasolar giant planets one extreme
  of planetary demographics
• Formation, Migration
• Models nascent
• Structure huge variation in density
• Only partly explained by age
• Many parameters jointly govern radius
• Need large sample
• Understand as an ensemble
• Much theoretical activity
• Various approximations and approaches
• Many disparate predictions
• Observations will drive the field

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Refurbished HST should make a huge contribution

• radius determinations
• constrain structure models
• Measurements of atmospheric chemistry
• influence of dust, dynamics
  disequilibrium chemistry on
  atmospheric structure
• coupling of atmosphere with interior
• intrinsic temperature, planetary age?