Studies Of High Velocity Features In Spectra Of Type Ⅰa Supernovae

This dissertation reports our studies on the “high velocity feature”(HVF)in early spectra(t <-7 days)of SNe Ⅰa,which appears as an extra gaussian profile(velocity>16,000 km s-1)blending with the photospheric(PHO)absorption(velocity <15,000 km s-1).We use a large sample including very early-time spectra(e.g.,t <-7 days)to investigate the velocity(V)and line strength(quantified by pseudo-equivalent width,pEW)of the HVFs.To improve the performances of SiⅡ λ 6355 fittings,we invented multiple Gaussian fits with constraints on expansion velocities for the same species(i.e.,SiⅡ λs 5972,6355).Strong HVFs of SiⅡ λ 6355,CaⅡ NIR were found more likely appear in SNe Ⅰa with smaller decline rates(?m15(B)<1.4 mag)and they tend to be associated with fastexpanding SNe Ⅰa.However,no evident correlation was found between the HVFs and the properties of host galaxies.The velocities and pEWs of SiⅡ λ 6355 and CaⅡ NIR are found to be positively linearly correlated in both PHO and HVF components.This may imply that Si and Ca are produced from similar burning process.On the other hand,the HVFs of SiⅡ λ 6355 are found to have much lower velocities(by about 4,000 km s-1)and smaller pEWs(by about 83%)than that of CaⅡ NIR.This may be caused by difference in excitation energy(Eex = 8.12 ev and 1.7 ev for SiⅡ λ 6355 and CaⅡ NIR respectively).A strong reverse-correlation is found between the line strength of HVFs of O I λ7773 and that of the SiⅡ λ 6355(and CaⅡ NIR),which means the Si-(/Ca-)absorbing materials would increase if more O are burnt.Moreover,a strong correlation between absorption strengths of HVF and PHO component of O indicates that the material producing the HVFs is intrinsic to the SNe.We thus conclude that the HVF is an effect of O burning that may be initiated by either flames of an asymmetric detonation or He burning near the WD surface.It is worthwhile to point out that SNe with evident CⅡ λ 6580 were found to have a much tighter V-pEW correlation of O I λ 7773 in comparison with those without showing prominent carbon signature in early-time spectra.This is in line with the speculation that these C-detected SNe are from a population with more uniform progenitor system.Our result also sheds light on the still-debated explosion mechanism by providing a strong constraint that the nuclear burning must proceed even at the outermost layer of the exploding WD.Also,we found the delayed detonation model is most plausible model to explain the correlations we had found.Also,we find a new way to estimate the photometric parameters such as ?m15(B)and the B-V color based on the spectroscopic-photometric correlation we found for SNe Ⅰa.SNe Ⅰa with similar spectral properties are expected to have more homogeneous peak luminosity.We tested our sample with spectral constraints such as the pEW ratio of O-PHO to Si-PHO,and signature of carbon absorption,and found that the dispersion of the Hubble diagram can be significantly reduced by applying the above cut from the early-time spectra.This may help further improve the precision of SNe Ⅰa as distance indicators.In conclusion,the results from our measurements reveal statistically for the first time the distribution of burned(Si and Ca)and unburned(C and O)species in the ejecta of exploding white dwarf.Our finding also puts strong constraints on the origins of the HVFs and the explosion mechanism of SNe.We also provide selection criteria to select more uniform subclass of SNe Ⅰa for the purpose of cosmology.