The Distance to SN 1999em in NGC 1637 from the Expanding Photosphere Method

We present 30 optical spectra and 49 photometric epochs sampling the first\n517 days after discovery of supernova (SN) 1999em, and derive its distance\nthrough the expanding photosphere method (EPM). SN 1999em is shown to be a Type\nII-plateau (II-P) event, with a photometric plateau lasting until about 100\ndays after explosion. We identify the dominant ions responsible for most of the\nabsorption features seen in the optical portion of the spectrum during the\nplateau phase. We find the distance to SN 1999em to be D = 8.2 +/- 0.6 Mpc,\nwith an explosion date of 5.3 +/- 1.4 days before discovery.\n We examine potential sources of systematic error in EPM-derived distances,\nand find the most significant to result from uncertainty in the theoretical\nmodeling of the flux distribution emitted by the SN photosphere (i.e., the\n``flux dilution factor''). We compare previously derived EPM distances to 5 SNe\nII in galaxies for which a recently revised Cepheid distance exists from the\nHST Key Project and find D(Cepheids) / D(EPM) = 0.96 +/- 0.09. Finally, we\ninvestigate the possible use of SNe II-P as standard candles and find that for\n8 photometrically confirmed SNe II-P with previously derived EPM distances and\nSN 1999em, the mean plateau absolute brightness is M_V(plateau) = -16.4 +/- 0.6\nmag, implying that distances good to ~30% (1-sigma) may be possible without the\nneed for a complete EPM analysis. At M_V(plateau) = -15.9 +/- 0.2 mag, SN\n1999em is somewhat fainter than the average SN II-P. The general consistency of\nabsolute SNe II-P brightness during the plateau suggests that the standard\ncandle assumption may allow SNe II-P to be viable cosmological beacons at z >\n2.\n