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 Quantitative spectroscopic analysis of and distance to SN1999em

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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.

Author: Luc Dessart; D. John Hillier

Source: https://archive.org/

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