南方基地知识库

The absorption feature OI lambda 7773 is an important spectral indicator for type Ia supernovae (SNe Ia) that can be used to trace the unburned material in outer layers of the exploding white dwarf (WD). In this work, we use a large sample of SNe Ia to examine this absorption at early phases (i.e., -13 day less than or similar to t less than or similar to -7 day) and make comparisons with the absorption features of Si II lambda 6355 and the Ca II near-infrared triplet. We show that for a subgroup of spectroscopically normal SNe with normal photospheric velocities (i.e., v(si) less than or similar to 12,500 km s(-1) at optical maximum), the line strength of the high velocity feature (HVF) of O I is inversely correlated with that of Si II (or Ca II), and this feature also shows a negative correlation with the luminosity of SNe Ia. This finding, together with other features we find for the O I HVF, reveal that for this subgroup of SNe Ia, explosive oxygen burning occurs in the outermost layer of the SN. Differences in the oxygen burning could lead to the observed diversity, which is in remarkable agreement with the popular delayed-detonation model of Chandrasekhar mass WDs.