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?