On NIR HST Spectro-photometry of Transiting Exo-planets

1. On NIR HST Spectro-photometry of Transiting Exo-planets Vasisht, Swain, Deroo, Chen, Wayne (JPL), Tinetti (UCL), Yung (CIT), Angerhausen (PIUK), Bouwman (MPIA), Deming (GSFC) Paris 2008

2. Outline • Motivation for NIR objective mode, time-resolved spectroscopy • Instrumental issues > General issues confronting shot-noise limited spectroscopy > Hubble specific limitations • Modeling and removal of instrumental limitations • Spectroscopy of the emergent flux from HD189733b (Swain, Vasisht, Tinetti, Deroo, Yung et al., accepted ApJL) Paris 2008

3. Spectroscopy with NICMOS • SlitlessGrism spectrograph between 0.8-2.5 microns • Camera 3 • Coverage by 3 grisms located in filter wheel • R = 200 native spectral resolution • Camera 3 is severely under-sampled at 0.2”/pixel (~λ/D @ 2 um, 52” FOV) • Exoplanet datasets have now been acquired with all grisms • G141 on HD 209548b (Brown et al. in 2005, unpublished) Paris 2008

4. Scientific Rationale • NIR Emission Spectroscopy (λ ~1-2.5 um; Spitzer 3-30 um) • Observable: Falling but favorable flux contrast (< 3 um) • Energetically Important: Maximum νFν (for emergent flux) • Decreased stellar shot-noise • NIR photosphere at greater pressure depths (0.1-1 bar) • Molecular activity: ro-vib bands of major species • Again some of the same advantages apply for transmission spectroscopy • Reduced opacity from small particle scattering Paris 2008

5. Hot, Cold or Cloudy Seager et al. 2005 Hot T = 1750 K dayside reradiation Paris 2008 Homogenous clouds

6. Active (common) C,N,O molecules Lodders & Fegley 2002 Paris 2008

7. Hubeny & Burrows 2008 Paris 2008

8. Molecular spectroscopy -> atmospheric physics • Atmospheres are a window to planetary composition, may have clues to evolutionary history • History of the planet can give rise to a range in core sizes, heavy element abundances, and abundance ratios • Relative fractions of refractory and volatile materials should reflect upon • Parent star abundances, history of formation, migration (?) Paris 2008

9. Part II – Photometry with HST • Detector anomalies • Optical anomalies Photometric systematic noise Paris 2008

10. NICMOS Detector Effects • Stress induced structure in the response • Pixel-to-Pixel stochastic response variations • Intrapixel structure in the response • T-dependence Figer et al. 2002 Paris 2008

11. Large scale structure NIC-3 is under-sampled PAM Defocus provides some “Immunity” This sets R ~ 40 Watch for structure under spectrum. Flats can remove some of this power Paris 2008

12. Small-scale structure and MTF Finger et al. 2000 Stiavelli et al. Paris 2008

13. Relative Photometry Evaluate in some statistical fashion Paris 2008

14. Relative Photometry k-space • Variance is integral over spatial frequencies of • Power spectrum of the detector response apodised by • 1. Power spectrum of the illumination • 2. 1-cos() high pass filter Paris 2008

15. Diffraction PSF Intrapixel gain Defocused PSF by Ray Tracing: Note this is a PSD 1-cos(k dx), dx = 0.1 pix Paris 2008

16. Implications • Significant substructure in the psf (ILS) • At spatial frequencies of D/λ, D/2λ etc • Due to diffraction • D/λ ~ 1/pixel • Mostly preserved in cross-dispersion axis • Varies with wavelength • For shorter λ, higher spatial frequencies • Can interact with sub-pixel structure Paris 2008

17. Beam wander • In x (spatial) and y (spectral) • Repositioning errors • Filter wheel positioning • Rot. about un-deviated ray • Orbital phase PSF modulation • Proxy (Gaussian FWHM) • Array response variations • QE with temperature • ~ 1%/K (2.5 micron), 3%/K (1.5 micron) DISCRETE OFFSETS X, Y, θ, T PERIODIC σ Paris 2008

18. Biggest headache is image motion • Repositioning errors (Monte Carlo) • δx, δy ~ 0.1 pixel; linear perturbations • δx, δy > 0.25 pixels; large higher order errors (> 10-4) • Generally few usable orbits per visit • Adding 2nd order terms to expansion is problematic dx, dy, dθ Σ dI dT dσ Paris 2008

19. Paris 2008

20. Paris 2008

21. Orbit 1 Paris 2008

22. Other Systematics • Optical effects • Flux-migration between grating-orders • Response of interference filter • Geometrical shadowing by grooves • Woods anomalies Paris 2008

23. Part III – Observations of HD 189733b Paris 2008

24. State-Variables HD189733b angle position defocus temperature Paris 2008

25. Iterative Multivariate Fits Noise Light curve Design Matrix Model vector

26. Data Modeling-III Raw periodogram Post-fit residuals Paris 2008

27. Lightcurves Broadband 1.5 To 2.5 um K band K band with Common mode Noise removed Final K band Lightcurve Paris 2008

28. HD 189733 (Basic Data) J. Schneider, Ex. Enc. • HD 189733 (K1-K2V) • T ~ 5000 K • 19.3 pc • > 0.6 Gyr • Metallicity -0.03 +/- 0.04 • HD 189733b (Bouchy et al. 2005) • 1.144 MJ, 1.138 RJ • Circular 0.03 AU orbit (2.22 d) • Secondary eclipse observations • Barnes et al. 2007 (dC ~ 4x10-4) Paris 2008

29. Spectral Modeling • Retrieval using RT models (Goody & Yung 1989) • Disk-averaged radiative transfer models developed originally for Earthshine, Mars • (Tinetti et al. 2006, 2007) • P-T profiles (Barman et al. 2008, Burrows et al. 2008) • Photochemistry (Yung, Liang) • Layer-by-layer (log P between -6 and 0) • Input T-P profiles • Chemical profiles (simple constant VMR) • Opacities (T, ρ); Cloudless. Paris 2008

30. HD189733b NIR Contrast Spectrum Paris 2008

31. Contrast Spectrum Components

32. Comparison with radiation-hydrodynamics models Showman et al. 2008 Planet brightest away from anti-stellar point Knutson et al. 2007 Paris 2008

33. Retrieval Results • Dayside emission (subsolar) • Water (0.1-1 10-4) • Carbon monoxide (thermochemically very stable at these P,Ts; CO=CH4 T=1100K at 1 bar) • Also inferred from IRAC photometry (Charbonneau et al. 2008) • 10-4 • Carbon dioxide (trace concentration 10-6) • CO+H2O <=> CO2+H2 (thermochemical in a CO field; Lodders & Fegley 2002) • CO+OH <=> CO2+H (photochemical pathway) • Methane upper limit (10-7) • Significant residuals at the blue end of the spectrum Paris 2008

34. Abundances • C/O is high and not well constrained (cloudless model) • 0.5 to 10 • Solar 0.48 (Anders & Grevesse 1989) Favor lower values because high C/O implies disappearing water in CO field • Terminator (Swain, Vasisht, Tinetti 2008) • Lower pressure depths • Methane abundance is higher (CO < CH4) • Water 5.10-4 Paris 2008

35. In Summary Little evidence for … Hot Jovians not as “hot” as … good hot Curry !. Paris 2008

36. Chemistry • Hot less dense atmospheres are more likely to show abundant CO (and CO2 at lower T), while cooler, denser ones show more abundant methane. • At 1 bar the CO=CH4 boundary is at T = 1125 K. • C/O atomic ratio is 0.48 (solar) Exeter Exoplanet Workshop

37. Paris 2008

38. Paris 2008 Pont et al. 2008

39. Paris 2008 F. Pont et al. 2008

40. Carbon & Oxygen Chemistry • Major carbon bearing gases in a solar composition gas of given metallicity are generally CH4, CO and/or CO2 depending on T and P. Exeter Exoplanet Workshop