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Register a new userLine Identifications of Type I Supernovae: On the Detection of Si II for these Hydrogen-poor Events
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Here we revisit line identifications of type I supernovae and highlight trace amounts of unburned hydrogen as an important free parameter for the composition of the progenitor. Most 1-dimensional stripped-envelope models of supernovae indicate that observed features near 6000-6400 Ang in type I spectra are due to more than Si II 6355. However, while an interpretation of conspicuous Si II 6355 can approximate 6150 Ang absorption features for all type Ia supernovae during the first month of free expansion, similar identifications applied to 6250 Ang features of type Ib and Ic supernovae have not been as successful. When the corresponding synthetic spectra are compared to high quality time-series observations, the computed spectra are frequently too blue in wavelength. Some improvement can be achieved with Fe II lines that contribute red-ward of 6150 Ang, however the computed spectra either remain too blue, or the spectrum only reaches fair agreement when the rise-time to peak brightness of the model conflicts with observations by a factor of two. This degree of disagreement brings into question the proposed explosion scenario. Similarly, a detection of strong Si II 6355 in the spectra of broad-lined Ic and super-luminous events of type I/R is less convincing despite numerous model spectra used to show otherwise. Alternatively, we suggest 6000-6400 Ang features are possibly influenced by either trace amounts of hydrogen, or blue-shifted absorption and emission in Halpha, the latter being an effect which is frequently observed in the spectra of hydrogen-rich, type II supernovae.
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Type II supernovae (SNe II) show strong hydrogen features in their spectra throughout their whole evolution while type IIb supernovae (SNe IIb) spectra evolve from dominant hydrogen lines at early times to increasingly strong helium features later on. However, it is currently unclear whether the progenitors of these supernova (SN) types form a continuum in pre-SN hydrogen mass or whether they are physically distinct. SN light-curve morphology directly relates to progenitor and explosion properties such as the amount of hydrogen in the envelope, the pre-SN radius, the explosion energy and the synthesized mass of radioactive material. In this work we study the morphology of the optical-wavelength light curves of hydrogen-rich SNe II and hydrogen-poor SNe IIb to test whether an observational continuum exists between the two. Using a sample of 95 SNe (73 SNe II and 22 SNe IIb), we define a range of key observational parameters and present a comparative analysis between both types. We find a lack of events that bridge the observed properties of SNe II and SNe IIb. Light curve parameters such as rise times and post-maximum decline rates and curvatures clearly separate both SN types and we therefore conclude that there is no continuum, with the two SN types forming two observationally distinct families. In the V-band a rise time of 17 days (SNe II lower, SNe IIb higher), and a magnitude difference between 30 and 40 days post explosion of 0.4 mag (SNe II lower, SNe IIb higher) serve as approximate thresholds to differentiate both types.
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 FORS on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si II $lambda$6355 $AA$ line (p$_{rm Si II}$) 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$_{rm Si II}$ relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the $Delta$m$_{15}$-p$_{rm Si II}$ relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si II $lambda$6355 $AA$ line before maximum brightness and the Si II line velocity and suggest that this, along with the $Delta$m$_{15}$-p$_{rm Si II}$ 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$_{rm Si II}$ plane and found a dichotomy in the polarization properties between Chandrasekhar and sub-Chandrasekhar mass explosions, which supports the possibility of two distinct explosion mechanisms. A subsample of SNe display evolution of loops in the $q$-$u$ plane that suggests a more complex Si structure with depth. This insight, which could not be gleaned from total flux spectra, presents a new constraint on explosion models. Finally, we compared our statistical sample of the Si II polarization to quantitative predictions of the polarization levels for the double-detonation, delayed-detonation, and violent-merger models.
We present the light curves of the hydrogen-poor superluminous supernovae (SLSNe-I) PTF12dam and iPTF13dcc, discovered by the (intermediate) Palomar Transient Factory. Both show excess emission at early times and a slowly declining light curve at late times. The early bump in PTF12dam is very similar in duration (~10 days) and brightness relative to the main peak (2-3 mag fainter) compared to those observed in other SLSNe-I. In contrast, the long-duration (>30 days) early excess emission in iPTF13dcc, whose brightness competes with that of the main peak, appears to be of a different nature. We construct bolometric light curves for both targets, and fit a variety of light-curve models to both the early bump and main peak in an attempt to understand the nature of these explosions. Even though the slope of the late-time light-curve decline in both SLSNe is suggestively close to that expected from the radioactive decay of $^{56}$Ni and $^{56}$Co, the amount of nickel required to power the full light curves is too large considering the estimated ejecta mass. The magnetar model including an increasing escape fraction provides a reasonable description of the PTF12dam observations. However, neither the basic nor the double-peaked magnetar model is capable of reproducing the iPTF13dcc light curve. A model combining a shock breakout in an extended envelope with late-time magnetar energy injection provides a reasonable fit to the iPTF13dcc observations. Finally, we find that the light curves of both PTF12dam and iPTF13dcc can be adequately fit with the circumstellar medium (CSM) interaction model.
We present ultraviolet line identifications of near maximum-light HST observations of SN 2011fe using synthetic spectra generated from both SYNOW and $texttt{PHOENIX}$. We find the spectrum to be dominated by blends of iron group elements Fe, Co, and Ni (as expected due to heavy line blanketing by these elements in the UV) and for the first time identify lines from C IV and Si IV in a supernova spectrum. We also find that classical delayed detonation models of Type Ia supernovae are able to accurately reproduce the flux levels of SN 2011fe in the UV. Further analysis reveals that photionization edges play an important role in feature formation in the far-UV, and that temperature variations in the outer layers of the ejecta significantly alter the Fe III/Fe II ratio producing large flux changes in the far-UV and velocity shifts in mid-UV features. SN 2011fe is the best observed core-normal SNe Ia, therefore analysis its of UV spectra shows the power of UV spectra in discriminating between different metalicities and progenitor scenarios of Type Ia supernovae, due to the fact that the UV probes the outermost layers of the Type Ia supernova, which are most sensitive to metalicity and progenitor variations.
The ejecta velocity is a very important parameter in studying the structure and properties of Type Ia supernovae (SNe Ia). It is also a candidate key parameter in improving the utility of SNe Ia for cosmological distance determinations. Here we study the velocity distribution of a sample of 311 SNe Ia from the kaepora database. The velocities are derived from the Si II $lambda$6355 absorption line in optical spectra measured at (or extrapolated to) the time of peak brightness. We statistically show that the observed velocity has a bimodal Gaussian distribution consisting of two groups of SNe Ia: Group I with a lower but narrower scatter ($mu_1 = 11000 text{km s}^{-1}$, $sigma_1 = 700 text{km s}^{-1}$), and Group II with a higher but broader scatter ($mu_2 = 12300 text{km s}^{-1}$, $sigma_2 = 1800 text{km s}^{-1}$). The population ratio of Group I to Group II is 201:110 (65%:35%). There is substantial degeneracy between the two groups, but for SNe Ia with velocity $v > 12000 text{km s}^{-1}$, the distribution is dominated by Group II. The true origin of the two components is unknown, though there could be that naturally there exist two intrinsic velocity distributions as observed. However, we try to use asymmetric geometric models through statistical simulations to reproduce the observed distribution assuming all SNe Ia share the same intrinsic distribution. In the two cases we consider, 35% of SNe Ia are considered to be asymmetric in Case 1, and all SNe Ia are asymmetric in Case 2. Simulations for both cases can reproduce the observed velocity distribution but require a significantly large portion ($>35%$) of SNe Ia to be asymmetric. In addition, the Case 1 result is consistent with recent polarization observations that SNe Ia with higher Si II $lambda$6355 velocity tend to be more polarized.
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