We propose STIS transit observations of the exoplanet HD189733b with the goal of measuring atmospheric atomic sodium. Our strategy is to repeat the observing meathods used for HD209458b, which resulted in a successful exoplanetary atmospheric sodium detection. Initial ground-based measurements suggest that the sodium signature on HD189733 could be up to three times larger than HD209458b, making a robust 8σ detection possible within a 12 orbit program observing three transits. Transit transmission spectra resulting from space-based measurements have the advantage of retaining absolute transit depths when features are measured, which will make it possible to provide an observational link between sodium and atmospheric haze detected with ACS. Such a link can break modeling degeneracies and providing stringent constraints on the overall atmospheric properties, making such atmospheric information as abundances and the temperature-pressure-altitude relation known. A successful measurement will also allow for comparative atmospheric exoplanetology, as an atmospheric feature will be measured with the same instrument in two separate planets.

We propose observe HD189733 with the STIS G750M during three transits each visit consisting of four consecutive orbits (12 total). The spirit of transit observations is to attempt to gather as many spectra as possible before, during, and after a transit event with each image as identical as possible. As a transit observation consists of measuring a drop in stellar flux during the event, identical exposures are resistant to uncertainties in flat-fielding and very high precisions are possible. Our strategy is to replicate the observing strategy used by T. Brown during HST cycle 8 (program ID 8789, Brown et al. 2001) when observing HD209458b with the G750M grating. As HD189733 and HD209458b have nearly identical V magnitudes (7.65 vs 7.67 respectively), the observed G750M flux will be nearly equal in our wavelength range and even identical exposure times to PID8789 can be used. Thus we will have 60 second exposure times with the G750M grating at the 6094 ? secondary tilt, covering a wavelength range of 5808 – 6380 ? at a resolution of R=5540, utilizing the 52”x2” slit to minimize slit losses. STIS has a duty cycle of 20 seconds (in sub-array mode) giving a cadence of 1 spectrum every 80 seconds. This set up will result in a total of 143 spectra during a single 4-orbit visit. The first visit is expected to have systematically low flux, as consistently observed in previous STIS programs likely due to telescope thermal relaxation, will likely not be utilized in our study, and is not included in our S/N calculations. In order to obtain the largest number of spectra, we will use the mode WAVECAL=NO in orbits 2, 3 and 4 of each visit. The wavelength calibration will come from visit 1 but could be obtained from the high S/N of the stellar spectrum itself. EXTRA-SOLAR PLANETS 12 false 0.0 0.0 false false false false false false AVAILABLE EXT-STAR K V-IV This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 20 00 43.7133 +22 42 39.07 J2000 STIS/CCD Spectroscopic No No G750M STIS/CCD Spectroscopic No No G750M STIS/CCD Spectroscopic No No G750M Phase windows are not calculated when the period is less than 7.0 days. Assuming no scheduling restrictions for Phase 0.926/0.936. The scheduling data was generated by CASM 17.2. 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