We present optical spectra of the nearby Type Ia supernova SN 2011fe at 100, 205, 311, 349 and 578 d post-maximum light, as well as an ultraviolet (UV) spectrum obtained with the Hubble Space Telescope at 360 d post-maximum light. We compare these observations with synthetic spectra produced with the radiative transfer code PHOENIX. The day +100 spectrum can be well fitted with models that neglect collisional and radiative data for forbidden lines. Curiously, including these data and recomputing the fit yields a quite similar spectrum, but with different combinations of lines forming some of the stronger features. At day +205 and later epochs, forbidden lines dominate much of the optical spectrum formation; however, our results indicate that recombination, not collisional excitation, is the most influential physical process driving spectrum formation at these late times. Consequently, our synthetic optical and UV spectra at all epochs presented here are formed almost exclusively through recombinationdriven fluorescence. Furthermore, our models suggest that the UV spectrum even as late as day +360 is optically thick and consists of permitted lines from several iron-peak species. These results indicate that the transition to the 'nebular' phase in Type Ia supernovae is complex and highly wavelength dependent.
Bibliographical noteFunding Information:
We thank Claes Fransson for helpful comments. This work has been supported in part by NASA/HST grant GO-12948.004-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Additional support was provided by NSF grant AST-0707704, NASA grant NNX16AB25G and DOE grant DE-SC0009956. The work of EB and PHH was also supported in part by SFB 676 and GRK 1354 from the DFG. RJF gratefully acknowledges support from NASA grant 14-WPS14-0048, NSF grant AST-1518052 and the Alfred P. Sloan Foundation. The workof KM is partly supported by JSPS KAKENHI Grant (26800100) and by the WPI Initiative, MEXT, Japan. JMS is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1302771. The supernova group of AVF at UC Berkeley has received generous financial assistance from the Christopher R. Redlich Fund, the TABASGO Foundation and NSF grant AST-1211916. The work of AVF was conducted in part at the Aspen Center for Physics, which is supported by NSF grant PHY-1066293; he thanks the Center for its hospitality during the black holes workshop in 2016 June and July. This research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the US Department of Energy under Contract No. DE-AC02-05CH11231; and the H?chstleistungs Rechenzentrum Nord (HLRN).We thank both these institutions for a generous allocation of computer time. Research at Lick Observatory is partially supported by a generous gift from Google. We thank the staffs of the various observatories and telescopes where data were taken, as well as observers who helped obtain some of the data.
© 2017 The Authors.
- Supernovae: general
- Supernovae: individual: SN 2011fe