  All far ultraviolet observations of HD209458 tend to support a scenario in
which the inflated hydrogen atmosphere of its planetary companion strongly
absorbs the stellar \lya flux during transit. However, it was not clear how the
transit absorption depends on the selected wavelength range in the stellar line
profile, nor how the atomic hydrogen cloud was distributed spatially around
HD209458b. Here we report a sensitivity study of observed time and spectral
variations of the stellar flux. In particular, the sensitivity of the
absorption depth during transit to the assumed spectral range in the stellar
line profile is shown to be very weak, leading to a transit depth in the range
$(8.4-8.9)%\pm 2.0%$ for all possible wavelength ranges, and thereby confirming
our initially-reported absorption rate. Taking the ratio of the line profile
during transit to the unperturbed line profile, we also show that the spectral
signature of the absorption by the exoplanetary hydrogen nebula is symmetric
and typical of a Lorentzian, optically thick medium. Our results question the
adequacy of models that require a huge absorption and/or a strong asymmetry
between the blue and red side of the absorption line during transit as no such
features could be detected in the HST FUV absorption profile. Finally, we show
that standard atmospheric models of HD209458b provide a good fit to the
observed absorption profile during transit. Other hybrid models assuming a
standard model with a thin layer of superthermal hydrogen on top remain
possible.
