Measuring the L-T transition for a warm Saturn exoplanet

0 Assigned ID: 816 ----- Attempting Submission 1 (Fri Apr 10 20:56:44 GMT 2015) ----- HST Phase I Proposal 816 (proposal.apt) successfully submitted. Receipt: # 816-1 ----- Attempting Submission 2 (Fri Apr 10 21:03:43 GMT 2015) ----- As noted in the Proposal Description: As in our previous Programs 12473 and 11740, we will override the default wavelength calibrations at the end of each exposure (WAVECAL=NO). This is done to improve the duty cycle and to allow the thermal breathing trends to be removed, which is only possible with an un-interrupted photometric time series. A separate wavelength calibration image will be taken at the end of each visit. Also: The phase-constraints have been optimised for obtaining in-transit data. There is some room to relax these if necessary for scheduling, but this would likely be at a cost of reducing the precision of our measurement (by reducing the number of in-transit data points we obtain). Hopefully there are enough observing opportunities that the phase constraints can be left as they are currently. -1433716691 ----- Attempting Submission 1 (Thu Jul 23 15:10:35 GMT 2015) ----- Measuring the L-T transition for a warm Saturn exoplanet The transition between the L and T spectral types in brown dwarf atmospheres is characterized by a reduction in high-altitude clouds and the appearance of strong methane absorption features. Models predict a similar transition for gas giant atmospheres across the 1000-1200K temperature range. To test this theory, we propose to measure a detailed transmission spectrum for HAT-P-18b, a Saturn-mass gas giant with an equilibrium temperature of 850K. Our observations will span the 0.3-1.7 micron wavelength range, with the near-infrared wavelengths allowing us to constrain methane and water absorption, and the optical wavelengths probing alkali metal absorption and high-altitude Rayleigh scattering processes. The latter will allow us to determine whether or not aerosols are present in the atmosphere, addressing a key aspect of atmospheric chemistry. Our measurement for HAT-P-18b will be the first transmission spectrum for a gas giant exoplanet that is predicted to have a high methane abundance, providing unique constraints for atmosphere models. Our observations will provide a near-UV to near-IR transmission spectrum for the transiting exoplanet HAT-P-18b. This broad wavelength coverage will allow us to distinguish between fundamental atmosphere classes: the optical provides leverage for distinguishing between a clear or hazy atmosphere, while the near-IR contains strong CH4 absorption features, providing leverage for distinguishing between an L-type or T-type atmosphere. **Observing strategy: To construct the transmission spectrum, we require three separate transits of HAT-P-18b to be observed: one with STIS+G430L, one with STIS+G750L, and one with WFC3+G141. All observations will be made in spectroscopic mode. The STIS observations will be made with the 52x2 arcsec slit to avoid slit losses. At the start of each visit, a standard phase-constrained acquisition image will be taken. The subsequent observing strategy consists of taking repeated exposures for the duration of the transit, plus time before and after the transit to establish the out-of-transit baseline flux. For the STIS observations, we will use stare mode with individual exposure times of 278 sec. We will read out a subarray size of 128 pixels to reduce overheads (SIZEAXIS2=128). As in our previous Programs 12473 and 11740, we will override the default wavelength calibrations at the end of each exposure (WAVECAL=NO). This is done to improve the duty cycle and to allow the thermal breathing trends to be removed, which is only possible with an un-interrupted photometric time series. A separate wavelength calibration image will be taken at the end of each visit. For the G750L visit, we will also take fringe flat images to correct for fringing longward of ~720 nm. For the WFC3 observations, we will use the forward spatial scanning mode with a scan rate of 0.03 arcsec/sec and individual exposure times of 90 sec (i.e. resulting in a scan across 2.7 arcsec = 20 pixels per exposure). This will leave plenty of room on the detector to estimate the background flux. We will read out a subarray size of 256 pixels to reduce overheads. Each transit of HAT-P-18b lasts for 163 minutes. We therefore require 5 consecutive HST orbits per visit. The first orbit serves to allow the telescope to thermally relax into its new pointing position, which is a standard procedure adopted by all HST exoplanet observations. The remaining orbits will provide good phase coverage during transit, and allow for sufficient time (2 HST orbits - one before transit, one after transit) to establish the baseline stellar flux necessary to accurately measure the transit depth. For both STIS and WFC3, we will obtain 10 exposures per HST orbit, coming to a total of 20 in-transit and 20 out-of-transit exposures for each instrument. **Signal-to-noise estimates: HAT-P-18b is a bright target in the conventional sense (V=12.8mag, J=10.8mag). The challenge with exoplanet transmission spectroscopy is to measure subtle changes in the system brightness across different wavelength channels, via variations in the transit depth. We used the HST Exposure Time Calculator with the observational setup described above to calculate the following SNRs per resolution element per exposure: 112 for G430L, 246 for G750L, and 1059 for G141. Note that the G141 flux will be spread over ~20 pixel columns due to the spatial scan (see above), so there is no risk of saturation. EXTRA-SOLAR PLANETS 12 false false false false AVAILABLE STAR EXTRA-SOLAR PLANETARY SYSTEM K V-IV This object was generated by the targetselector and retrieved from the SIMBAD database. GSC-02594-00646 J2000 J2000 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G750L 7751 WFC3/IR Spectroscopic No No G141 1437663586058 The scheduling data was generated by CASM 23.2. CASM was run in Phase II mode. HAT-P-18. G430L. STIS visit. It is essential that the five orbits be scheduled in a coninuous block, free of the SAA. 1447603530000 29.627402939839413 (Tic-Visit :proposal-id 14099 :visit-id "01" :proposal-version -1 :trans-version "20150609" :date "23-Jul-2015 15:59:44.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 139264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14099 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14099 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2912 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3018 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3018 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3018 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "01" :orbit-number 5 :pcs-time 222 :si-vis-utilization 3018 :al-overhead-at-end 123 :internal-dur-at-end 337 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) false 5 0 Short exposure to minimize instrument systematic of first exposure of orbit. Note that it is longer than first exposures in subsequent orbits, due to acquisition overhead and to ensure all time is used. Short exposure to minimize instrument systematic of first exposure in each orbit. Short exposure to minimize instrument systematic of first exposure in each orbit. Short exposure to minimize instrument systematic of first exposure in each orbit. Short exposure to minimize instrument systematic of first exposure in each orbit. Explicit WAVECAL, auto-waves disabled 1446573826286 The scheduling data was generated by CASM 23.2. CASM was run in Phase II mode. /private/var/folders/zc/ljhf1lgn10b1yd9ytmwlp4jr00070l/T/OPVisit3796044456194295440.xml HAT-P-18. G750L. STIS visit. It is essential that the five orbits be scheduled in a coninuous block, free of the SAA. 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Note that it is longer than first exposures in subsequent orbits, due to acquisition overhead and to ensure all time is used. Short exposure to minimize instrument systematic of first exposure in each orbit. Short exposure to minimize instrument systematic of first exposure in each orbit. Short exposure to minimize instrument systematic of first exposure in each orbit. Short exposure to minimize instrument systematic of first exposure in each orbit. Explicit WAVECAL, auto-waves disabled The scheduling data was generated by CASM 23.2. CASM was run in Phase II mode. 1437664190094 HAT-P-18. G141. WFC3 visit. It is essential that the five orbits be scheduled in a coninuous block, free of the SAA. We will use the spatial scanning mode to lengthen the spectrum along the slit during the exposures, to avoid saturation on relatively long exposures. This mode greatly increases the efficiency. 1446174896000 50.23645796901201 (Tic-Visit :proposal-id 14099 :visit-id "04" :proposal-version -1 :trans-version "20150609" :date "23-Jul-2015 16:09:48.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 715328 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14099 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14099 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2774 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3017 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "04" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3017 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "04" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3017 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14099 :visit-id "04" :orbit-number 5 :pcs-time 222 :si-vis-utilization 3017 :al-overhead-at-end 0 :internal-dur-at-end 475 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) false 5 0 Direct image for wavelength calibration. Phase constrained. Spatial scan.