Multi-epoch multi-wavelength spectroscopic observations of photospheric-phase type II supernovae ( SN ) provide information on massive-star progenitor properties , the core-collapse mechanism , and distances in the Universe .
Following successes of recent endeavors ( Dessart & Hillier 2005ab ) with the non-LTE model atmosphere code CMFGEN ( Hillier & Miller 1998 ) , we present a detailed quantitative spectroscopic analysis of the type II SN1999em and , using the Expanding Photosphere Method ( EPM ) or synthetic fits to observed spectra , à la Baron et al .
( 2004 ) , we estimate its distance .
Selecting eight epochs , which cover the first 38 days after discovery , we obtain satisfactory fits to optical spectroscopic observations of SN1999em ( including the UV and near-IR ranges when available ) .
We use the same iron-group metal content for the ejecta , the same power-law density distribution ( with exponent n = 10 - 12 ) , 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 .
The overall evolution of the spectral energy distribution , whose peak shifts to longer wavelengths as time progresses , reflects the steady temperature/ionization-level decrease of the ejecta , associated non-linearly with a dramatic shift to ions with stronger line-blocking powers in the UV and optical ( Fe ii , Ti ii ) .
In the parameter space investigated , CMFGEN is very sensitive and provides photospheric temperatures and velocities , reddenings , and the H/He abundance ratio with an accuracy of \pm 500 K , \pm 10 % , 0.05 and 50 % , respectively .
Following Leonard et al .
( 2002 ) , and their use of correction factors from Hamuy et al .
( 2001 ) , we estimate an EPM distance to SN1999em that also falls 30 % short of the Cepheid distance of 11.7 Mpc to its host galaxy NGC 1637 ( Leonard et al .
2003 ) .
However , using the systematically higher correction factors of Dessart & Hillier ( 2005b ) removes the discrepancy .
A significant scatter , arising primarily from errors in the correction factors and derived temperatures , is seen in distances derived using different band passes .
However , adopting both correction factors and corresponding color-temperatures from tailored models to each observation leads to a good agreement between distance estimates obtained from different band passes .
The need for detailed model computations thus defeats the appeal and simplicity of the original EPM method , which uses tabulated correction factors and broadband fluxes , for distance determinations .
However , detailed fits to SN optical spectra , based on tailored models for individual SN observations , offers a promising approach to obtaining accurate distances , either through the EPM or via the technique of Baron et al .
( 2004 ) .
Our best distance-estimate to SN1999em is 11.5 \pm 1.0 Mpc .
We note that to achieve 10-20 % accuracy in such distance estimates requires multiple observations , covering preferentially a range of early epochs preceding the hydrogen-recombination phase .