characterizing atmospheres with jwst: optimizing multi-instrument observations via simulations and...

Characterizing atmospheres with JWST: Optimizing multi-instrument observations via

simulations and retrievals

Tom Greene (NASA ARC)Michael Line (UCSC / Hubble Fellow / ARC),

Jonathan Fortney (UCSC)November 16, 2015

Also see the posterBy Everett Schlawin

3JWST Transit Characterization

Some progress from transit (spectroscopy)

16 Nov 2015

• Molecules & atoms identified in exoplanet atmospheres– H2O, CO (CH4, CO2), Na, other alkali, HI, CII, OI,…

• Measured temperature-pressure profiles from hot Jupiter emission spectra– Few with high confidence T inversions

• Some Neptune-sized planets have been diagnosed– HAT-P-11 (Fraine+ 2014) and GJ 436b (Knutson+, etc.)

• Sub-Neptunes and super-Earths have been difficult– GJ 1214b: flat absorption, no sec. eclipse (many people…)

– Promise of cooler planets like K2-3 (Crossfield+ 2015) and K2-18b (Montet+ 2015): T = 300 – 500K, different clouds?

Questions about exoplanet atmospheres• What are their compositions?

– Elemental abundances • C/O and [Fe/H]: Both are formation diagnostics

– Molecular components and chemical processes• Identify equilibrium & disequilibrium chemistry:

– Vertical mixing, photochemistry, ion chemistry…

– 3-D effects: spatial variations• Energy budget and transport

– 1-D structure: measure profiles, inversions present?– Dynamical transport: day/night differences

• Clouds– Cloud composition, particle sizes, vertical & spatial distribution– Remove cloud effects to determine bulk properties

• Anything about low mass / small r< ~2Re planet atmospheres

• Trends with bulk parameters (mass, insolation, host stars, …)– Requires a population of diverse planets16 Nov 2015 JWST Transit Characterization 4

5JWST Transit Characterization16 Nov 2015

We clearly can use all ~10 years of JWST time to observe transiting planets!

But what should we do if that doesn’t happen?

6JWST Transit Characterization

New JWST Simulation / Retrieval Assessment

16 Nov 2015

Model some known planet types, simulate spectra, assess information & constraints

• Select archetypal planets from known system parameters– Hot Jupiter, warm Neptune, warm sub-Neptune, cool super-Earth

• Create model transmission and emission spectra (M. Line)

• Simulate JWST spectra using performance models (TG)– Simulate slitless modes with large bandpasses & good bright limits: NIRISS

SOSS, NIRCam grisms, MIRI LRS slitless 1 – 11 mm

– Code based on instrument models & data, detector parameters, JWST background models, random & systematic noise

– 1 transit or eclipse per spectrum

• Perform atmospheric retrievals (M. Line) to assess uncertainties in molecules, abundances, T-P profiles– Focus on uncertainties, not absolute parameters

• Identify what wavelengths give most useful information for what planets

7JWST Transit Characterization

• Use 1-D forward models– Emission: Line+(2013a), Diamond-Lowe+(2014), Stevenson+(2014)– Transmission: Line+(2013b) Swain+(2014), Kreidberg+(2014, 2015)

• Transmission model has 11 free parameters– T(SH), R(P=10b),hard clouds (Pc, s0, b), H2O, CH4, CO, CO2, NH3, N2

absorbers, constant with altitude

• Emission model has 1D T-P profile & 10 free params– H2O, CH4, CO, CO2, NH3, 5 gray atm parameters for T-P (Line+ 2013a)

• CHIMERA Bayesian retrieval suite (Line+ 2013a,b)– Updated with emcee MCMC– Uniform & Jeffreys priors

16 Nov 2015

Forward models & retrievals

Simulated Planet Signals (Sl) & Noise (Nl)

16 Nov 2015 8JWST Transit Characterization

l (mm) Noise floor

1.0 – 2.5 20 ppm

2.5 – 5.0 30 ppm

5.0 - 12 50 ppm

NIRISS

NIRCam

MIRI LRS

Selected Model Systems

16 Nov 2015 9JWST Transit Characterization

Model Transmission Spectra

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Simulated JWST Trans Spectra (1 transit)

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Model Emission Spectra

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Simulated JWST Emission Spectra (1 eclipse)

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Retrieval Birds & Bees

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Benneke & Seager (2012)

Retrieval Results: Hot Jupiter Gasses

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Priors

Retrieval Results: Warm Neptune Gasses

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Priors

Retrieval: Warm Sub-Neptune Gasses

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Priors

Retrieval Result: Cool Super-Earth Gasses

16 Nov 2015 18JWST Transit CharacterizationPriors

Emission retrievals: T-P Profiles

16 Nov 2015 19JWST Transit Characterization

Dashed: True valueSolid line: Retrieved mean valueShaded: 1 sigma

Detect inversion at 4 sigma with NIRISS only (red)

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Mass - Metallicity

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Transmission spectra Adapted from Kreidberg+ 2014b

JWST Transit Characterization 24

Summary / Conclusions (1)• NIRISS (1 – 2.5 mm) transmission spectra alone

sometimes constrain mixing ratios of dominant molecules in clear solar atmosphere planets (H2O, CH4, NH3)– C/O and [Fe/H] sometimes constrained with only NIRISS– l ≥ 5 mm spectra needed in a number of cases

• Cloudy solar atmospheres are often constrained (~ 1 dex or better mixing ratios) with l = 1 - 11 mm spectra – Transmission is better than emission for warm sub-Neptune– Hot Jupiter and warm Neptune do better with emission

• Need sufficient Fp and high Fp/F* for useful emission spectra

• High MMW atmospheres can be identified by high [Fe/H]• C/O is constrained to 0.2 dex for hot Jupiters with l = 1 –

5+ mm spectra. Also: C/O for hot planets with H2O + Teq• s[Fe/H] < 0.5 dex for warm, clear planets (l = 1-5+ mm tr)

16 Nov 2015

JWST Transit Characterization 25

Summary / Conclusions (2)• Non-equilibrium vertical mixing cannot be detected via

molecular mixing ratios– Photochemistry could show unexpected spectral features

• Observing 5 planets from Uranus to Jupiter mass should measure [Fe/H] vs. Log (M) slope to 1s = 0.13– l = 1-5+ mm transmission spectra– More than adequate (~20s) for detecting Solar System slope

• These results are for observations of single transits or eclipses. We will not know the actual JWST data quality – and how noise will decrease with co-adding – until after launch

• Many more retrieval issues to be explored (binning, Bayesian estimators, priors, 3D, parameterization), but will largely be driven by future data16 Nov 2015

JWST Transit Characterization 26

The End

16 Nov 2015