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.Comment: 7 pages, Astronomy & Astrophysics accepte
