  Despite its importance in the thermal-balance of the gas and in the
determination of primeval planetary atmospheres, the chemistry in
protoplanetary discs remains poorly constrained with only a handful of detected
species. We observed the emission from disc around the Herbig Be star HD 100546
with the PACS instrument in the spectroscopic mode on board the Herschel Space
Telescope as part of the Gas in Protoplanetary Systems (GASPS) programme and
used archival data from the DIGIT programme to search for the rotational
emission of CH+. We detected in both datasets an emission line centred at 72.16
micron that most likely corresponds to the J=5-4 rotational emission of CH+.
The J=3-2 and 6-5 transitions are also detected albeit with lower confidence.
Other CH+ rotational lines in the PACS observations are blended with water
lines. A rotational diagram analysis shows that the CH+ gas is warm at 323
(+2320/-151) K with a mass of 3e-14-5e-12 M_Sun. We modelled the CH+ chemistry
with the chemo-physical code ProDiMo using a disc density structure and grain
parameters that match continuum observations and near- and mid-infrared
interferometric data. The model suggests that CH+ is most abundant at the
location of the disc rim at 10-13 AU from the star where the gas is warm,
consistent with previous observations of hot CO gas emission.
