2006A&A...447..691D

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, 2005A&A...437..667D; 2005A&A...439..671D) with the non-LTE model atmosphere code CMFGEN (Hillier & Miller, 1998ApJ...496..407H), 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, a la Baron et al. (2004ApJ...616L..91B), 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 (FeII, TiII). 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 ±500K, ±10%, 0.05 and 50%, respectively. Following Leonard et al. (2002PASP..114...35L), and their use of correction factors from Hamuy et al. (2001ApJ...558..615H), we estimate an EPM distance to SN1999em that also falls 30% short of the Cepheid distance of 11.7Mpc to its host galaxy NGC1637 (Leonard et al., 2003ApJ...594..247L). However, using the systematically higher correction factors of Dessart & Hillier (2005A&A...439..671D) 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. (2004ApJ...616L..91B). Our best distance-estimate to SN1999em is 11.5 ±1.0Mpc. 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.