Abstract
Close-in extrasolar giant planets are expected to cool their thermospheres by producing H3+ emission in the near-infrared (NIR), but simulations predict H3+ emission intensities that differ in the resulting intensity by several orders of magnitude. We want to test the observability of H3+ emission with CRIRES at the Very Large Telescope (VLT), providing adequate spectral resolution for planetary atmospheric lines in NIR spectra. We search for signatures of planetary H3+ emission in the L` band, using spectra of HD 209458 obtained during and after secondary eclipse of its transiting planet HD 209458 b. We searched for H3+ emission signatures in spectra containing the combined light of the star and, possibly, the planet. With the information on the ephemeris of the transiting planet, we derive the radial velocities at the time of observation and search for the emission at the expected line positions and search for planetary signals and use a shift and add technique combining all observed spectra taken after sec. eclipse to calculate an upper emission limit. We do not find signatures of atmospheric H3+ emission in the spectra containing the combined light of HD 209458 and planet b. We calculate the emission limit for the H3+ line at 3953.0 nm (Q(1, 0)) to be 8.32 E18W and a limit of 5.34E18 W for the line at 3985.5 nm (Q(3, 0)). Comparing our emission limits to the theoretical predictions suggests that we lack 1 to 3 magnitudes of sensitivity to measure H3+ emission in our target object. We show that under more favorable weather conditions the data quality can be improved significantly, reaching 5 E16W for star-planet systems that are close to Earth. We estimate that pushing the detection limit down to 1E15W will be possible with ground-based observations with future instrumentation, for example, the E-ELT.
Highlights
Methods of exoplanet detection have become more and more successful in recent years and various kinds of exoplanets, from hot Jupiters to earth-sized planets, have recently been detected
The known ephemeris enables us to use a shift and add approach to search for H+3 emission: all observed combined light spectra are shifted in wavelength with respect to their apparent radial velocity of the planet at the time of observation
A rough estimation of the differential planetary radial velocity would be sufficient to search for the radial velocity shift that would occur in case of a detection
Summary
Methods of exoplanet detection have become more and more successful in recent years and various kinds of exoplanets, from hot Jupiters to earth-sized planets, have recently been detected. EGPs are derived by Koskinen et al (2007): With their model of a coupled thermosphere and ionosphere they performed threedimensional, self-consistent global simulations for different orbital distances of EGPs around a sunlike host star They conclude, that EGPs are cooled efficiently by H+3 inside 0.2–1 AU orbits and state that thermal dissociation and dissociative photoionization of H2 hampers the emission for orbits closer than 0.1 AU. Maillard & Miller (2011) suggested an observation strategy using high spectral resolution and possibly occultation spectroscopy in order to differentiate planetary and stellar flux They stated that the growing sample of known exoplanets offers more possible candidates for the search This work investigates the feasibility of H+3 for H+3 emission. The CRIRES spectrograph provides the highest resolution in the IR that is available today
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