  The mid-IR detection rate of water lines in disks around Herbig stars disks
is about 5\%, while it is around 50\% for disks around TTauri stars. The reason
for this is still unclear. In this study, we want to find an explanation for
the different detection rates between low mass and high mass pre-main-sequence
stars (PMSs) in the mid-IR regime. We run disk models with stellar parameters
adjusted to spectral types B9 through M2, using the radiation thermo-chemical
disk modeling code ProDiMo. We produce convolved spectra at the resolution of
Spitzer IRS, JWST MIRI and VLT VISIR spectrographs. We apply random noise
derived from typical Spitzer spectra for a direct comparison with observations.
The strength of the mid-IR water lines correlates directly with the luminosity
of the central star. We explored a small parameter space around a standard disk
model, considering dust-to-gas mass ratio, disk gas mass, mixing coefficient
for dust settling, flaring index, dust maximum size and size power law
distribution index. The models show that it is possible to suppress the water
emission, however, current observations are not sensitive enough to detect
mid-IR lines in disks for most of the explored parameters. The presence of
noise in the spectra, combined with the high continuum flux (noise level is
proportional to the continuum flux), is the most likely explanation for the non
detections towards Herbig stars. Mid-IR spectra with resolution higher than
20000 are needed to investigate water in protoplanetary disks. Intrinsic
differences in disk structure, e.g. inner gaps, gas-to-dust ratio, dust size
and distribution, and inner disk scale height, between Herbig and TTauri star
disks are able to explain a lower water detection rate in disks around Herbig
stars.
