Transit Spectroscopy with HST/WFC3January 18, 2012Exoplanet Transit Spectroscopy
with HST/WFC3: Probing H2O with New Precision
Avi M. MandellNASA Goddard
A Host of Collaborators:
Transit Spectroscopy with HST/WFC3January 18, 2012
Wide Field Camera 3 on the Hubble Space Telescope
• New IR camera installed on the HST in May of 2009
• Two channels: optical (200 – 1000 nm) and IR (800 – 1700 nm)
•IR Channel: 1024 x 1024 pixels covering 2.3 x 2.1 arcmin
•Slitless grism spectroscopy provides two options for IR spectra
• G102: λ= 0.8 – 1.15 μm, R = 210• G141: λ= 1.1 – 1.7 μm, R = 130
0th Order 1st Order
Single Exposure, 512 x 512
Transit Spectroscopy with HST/WFC3January 18, 2012
1.1 – 1.7 Microns: Measuring Water Absorption
• Wavelength range samples both the strong water bands at 1.15 and 1.4 μm as well as continuum regions on either side
• Contrast between the water features and continuum regions can reveal both the chemical composition as well as information on the temperature structure
Transit Spectroscopy with HST/WFC3January 18, 2012
Multiple Exoplanet Campaigns with HST/WFC3
• Berta et al. 2011: Transmission spectrum of GJ 1214 b
• Examined instrument systematics in detail, identifying a characteristic single-orbit ramp in counts
• Combined three transits to produce a well-constrained spectrum showing no water absorption within uncertainties
• Heavy-element atmosphere or clouds?
•Other programs are ongoing…
• Gibson et al. (poster)• Swain et al.• Deming et al. (this talk, poster by Sukrit Ranjin)
Transit Spectroscopy with HST/WFC3January 18, 2012
Deming et al. Cycle 18 Program
• Large collaboration focused on hot giant exoplanets
• Sample of 16 objects
• Most planets in the sample have radii significantly larger than expected from formation models
• A number of planets may have upper-atmosphere temperature inversions
• This talk will focus on 3 interesting cases:
• WASP-17: Ultra-low density, retrograde orbit
• WASP-18: Very massive planet in a 0.94-hr orbit (extremely hot!)
• WASP-19: Shortest-period planet known (P ~ 19 hr) but no temperature inversion
List of Observed/Scheduled Planets
WASP-19 WASP-18
WASP-17
Transit Spectroscopy with HST/WFC3January 18, 2012
Extracting & Correcting the White Light curves
• Tested several extraction methods (direct extraction vs. the STScI pipeline algorithm aXe) and various extraction box sizes
• Similar to Berta et al., a ramp-up in measured counts is observed between each read-out of the buffer
• Due to persistent charge build-up
• However, characteristic shape of ramp is different for each object – unclear whether it is due to exposure level or differences in the buffer read-out options
• As noted by Berta et al., ramp pattern can be removed extremely well by dividing by a mean of the out-of-transit data (divide-oot method)
• Additional improvements can be made by subtracting a background-level spectrum derived from off-spectrum data
0.05 0.10 0.15 0.20 0.25 0.30 0.35
Days – Const.
WASP-17 Transit: Total Counts
0.05 0.10 0.15 0.20 0.25 0.30
Days – Const.
WASP-19 Transit: Total Counts
Transit Spectroscopy with HST/WFC3January 18, 2012
Fitting Light curves With MCMC
• We fit the data using a Markov Chain Monte Carlo (MCMC) analysis (Ford 2005)
• Light curve model from Mandel & Agol (2002)
• Priors for orbital and transit parameters taken from previous results
• Additional components for linear trend over multi-orbit observational period (e.g. Berta et al.) as well as a possible sinusoidal component due to thermal emission
• We compare our MCMC results with results from the TAP (Transit Analysis Package) program developed by Gazak et al.
• Uncertainties are estimated from distribution of MCMC parameters; we test several types of restrictions on priors
Transit Spectroscopy with HST/WFC3January 18, 2012
Preliminary Results• First: No Spectroscopic Results Yet! Sorry!
• Resolving some issues with spectral drift on the detector…
• White-light photometry for the three transits matches up well with previous data at optical wavelengths (as we expected) – and the uncertainties are extremely small!
• The white-light photometry for the WASP-19 eclipse data can be compared directly to models constrained by Spitzer (Anderson et al. 2012), and seems to be consistent with C/O ~ 1.0
Preliminary ResultsRemoved
Preliminary ResultsRemoved
Transit Spectroscopy with HST/WFC3January 18, 2012
Next Steps
• Finish correcting the data for motion across the chip, and fit binned sections of the spectra to determine the change in effective planetary radius across the spectrum
• Fit the spectral extractions with models -- Madhusudhan et al., Burrows et al., etc. – to constrain the molecular composition, C/O ratio, and the temperature structure
• Address the sample as a whole: what trends do we see in planets with similar equilibrium temperature, stellar type, mass/radius, stellar activity, etc?