The distance to SN 1999em from the expanding photosphere method

We present optical and infrared spectroscopy of the first 2 months of evolu tion of the Type II supernova SN 1999em. We combine these data with high-qu ality optical/infrared photometry beginning only 3 days after shock breakou t, in order to study the performance of the " expanding photosphere method II (EPM) in the determination of distances. With this purpose, we develop a technique to measure accurate photospheric velocities by cross-correlating observed and model spectra. The application of this technique to SN 1999em shows that we can reach an average uncertainty of 11% in velocity from an individual spectrum. Our analysis shows that EPM is quite robust to the eff ects of dust. In particular, the distances derived from the VI filters chan ge by only 7% when the adopted visual extinction in the host galaxy is vari ed by 0.45 mag. The superb time sampling of the BV IZJHK light curves of SN 1999em permits us to study the internal consistency of EPM and test the di lution factors computed from atmosphere models for Type II plateau supernov ae. We find that, in the first week since explosion, the EPM distances are up to 50% lower than the average, possibly because of the presence of circu mstellar material. Over the following 65 days, on the other hand, our tests lend strong credence to the atmosphere models, and confirm previous claims that EPM can produce consistent distances without having to craft specific models to each supernova. This is particularly true for the V I filters, w hich yield distances with an internal consistency of 4%. From the whole set of BV IZJHK photometry, we obtain an average distance of 7.5 +/- 0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of the interna l precision of the method obtained from the analysis of the first 70 days o f the supernova evolution.