Investigating the Unusual Spectroscopic Time Evolution in SN 2012fr
The type Ia supernova (SN) 2012fr displayed an unusual combination of its Si II λλ5972, 6355 features. This includes the ratio of their pseudo-equivalent widths, placing it at the border of the shallow silicon (SS) and core normal (CN) spectral subtype in the Branch diagram, while the Si II λ6355 expansion velocities place it as a high-velocity (HV) object in the Wang et al. spectral type that most interestingly evolves slowly, placing it in the low-velocity gradient (LVG) typing of Benetti et al. Only 5% of SNe Ia are HV and located in the SS+CN portion of the Branch diagram, and fewer than 10% of SNe Ia are both HV and LVG. These features point toward SN 2012fr being quite unusual, similar in many ways to the peculiar SN 2000cx. We modeled the spectral evolution of SN 2012fr to see if we could gain some insight into its evolutionary behavior. We use the parameterized radiative transfer code SYNOW to probe the abundance stratification of SN 2012fr at pre-maximum, maximum, and post-maximum light epochs. We also use a grid of W7 models in the radiative transfer code PHOENIX to probe the effect of different density structures on the formation of the more »
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NSF-PAR ID:
10093383
Journal Name:
Astrophysical journal
Volume:
869
Page Range or eLocation-ID:
162
ISSN:
1538-4357
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Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from −14.5 to +14.5 d relative to B-band maximum detected an intrinsic continuum polarization (pcont) of 0.21 ± 0.09 per cent at the first epoch. Between days −11.5 and  +0.5, pcont remained ∼0 and by day +14.5 was again significant at 0.19 ± 0.10 per cent. Not considering the first epoch, the dominant axis of ${\rm Si\, {\small II}}$ λ6355 was roughly constant staying close the continuum until both rotated in opposite directions on day  +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch pcont ≈ 0.2 per cent nevertheless suggests a separate structure with an axis ratio ∼2 in the outer carbon-rich (3.5–4) × 10−3 M⊙. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the ${\rm Ca\, {\small II}}$ near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The pcont and the absorptions due to ${\rm Si\, {\small II}}$ λ6355 and ${\rm Ca\, {\small II}}$ NIR3 formmore »
Spectropolarimetry enables us to measure the geometry and chemical structure of the ejecta in supernova explosions, which is fundamental for the understanding of their explosion mechanism(s) and progenitor systems. We collected archival data of 35 Type Ia supernovae (SNe Ia), observed with Focal Reducer and Low-Dispersion Spectrograph (FORS) on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si ii λ6355 Å line ($p_{\rm Si\, \small {II}}$) as a function of time, which is seen to peak at a range of various polarization degrees and epochs relative to maximum brightness. We reproduced the $\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$ relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the $\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$ relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si ii λ6355 Å line before maximum brightness and the Si ii line velocity and suggest that this, along with the $\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$ relationship, may be explained in the context of a delayed-detonation model. In contrast, we compared our observations to numerical predictions in the $\Delta m_{15}\!-\!v_{\rm Si\, \small {II}}$ plane and found a dichotomy in the polarization propertiesmore »