Late-time spectral line formation in Type IIb supernovae, with application to SN 1993J, SN 2008ax, and SN 2011dh

摘要

We investigate line formation processes in Type IIb supernovae (SNe) from 100to 500 days post-explosion using spectral synthesis calculations. The modelingidentifies the nuclear burning layers and physical mechanisms that produce themajor emission lines, and the diagnostic potential of these. We compare themodel calculations with data on the three best observed Type IIb SNe to-date -SN 1993J, SN 2008ax, and SN 2011dh. Oxygen nucleosynthesis depends sensitivelyon the main-sequence mass of the star and modeling of the [O I] 6300, 6364lines constrains the progenitors of these three SNe to the M_ZAMS=12-16 M_sunrange (ejected oxygen masses 0.3-0.9 M_sun), with SN 2011dh towards the lowerend and SN 1993J towards the upper end of the range. The high ejecta massesfrom M_ZAMS >= 17 M_sun progenitors give rise to brighter nebular phaseemission lines than observed. Nucleosynthesis analysis thus supports a scenarioof low/moderate mass progenitors for Type IIb SNe, and by implication an originin binary systems. We demonstrate how oxygen and magnesium recombination linesmay be combined to diagnose the magnesium mass in the SN ejecta. For SN 2011dh,a magnesium mass of of 0.02-0.14 M_sun is derived, which gives a Mg/Oproduction ratio consistent with the solar value. Nitrogen left in the Heenvelope from CNO-burning gives strong [N II] 6548, 6583 emission lines thatdominate over H-alpha emission in our models. The hydrogen envelopes of TypeIIb SNe are too small and dilute to produce any noticeable H-alpha emission orabsorption after ~150 days, and nebular phase emission seen around 6550 A is inmany cases likely caused by [N II] 6548, 6583. Finally, the influence ofradiative transport on the emergent line profiles is investigated...(abridged)