  This work presents a detailed quantitative spectroscopic analysis of, and the
determination of the distance to, the type II supernovae (SN) SN1999em with
CMFGEN (Dessart & Hillier 2005a), based on spectrophotometric observations at
eight dates up to 40 days after discovery. We use the same iron-group metal
content for the ejecta, the same power-law density distribution (with exponent
n~10), and a Hubble-velocity law at all times. We adopt a H/He/C/N/O abundance
pattern compatible with CNO-cycle equilibrium values for a RSG/BSG progenitor,
with C/O enhanced and N depleted at later times. Based on our synthetic fits to
spectrophotometric observations of SN1999em, we obtain a distance of 11.5Mpc,
similar to that of Baron et al. (2004) and the Cepheid distance to the galaxy
host of 11.7Mpc (Leonard et al. 2003). Similarly, based on such models, the
Expanding Photosphere Method (EPM) delivers a distance of 11.6Mpc, with
negligible scatter between photometric bandpass sets; there is thus nothing
wrong with the EPM as such. Previous determinations using the tabulated
correction factors of Eastman et al. (1996) all led to 30-50% underestimates:
we find that this is caused by 1) an underestimate of the correction factors
compared to the only other study of the kind by Dessart & Hillier (2005b), 2) a
neglect of the intrinsic >20% scatter of correction factors, and 3) the use of
the EPM at late times when severe line blanketing makes the method inaccurate.
The need of detailed model computations for reliable EPM distance estimates
thus defeats the appeal and simplicity of the method. However, detailed fits to
SN optical spectra, based on tailored models for individual SN observations,
offers a promising approach to obtaining distances with 10-20% accuracy, either
through the EPM or a la Baron et al. (2004).
