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Register a new userEmpirically determined dilution factors of stripped-envelope, core-collapse SNe: Paper I - Method & Progenitor constraints
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Zach Cano
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In this work, the empirically derived dilution/correct factors of a sample stripped-envelope, core-collapse supernovae (SE-SNe), including five SNe IIb, four SNe Ib, six SNe Ic and two relativistic broad-lined type Ic supernovae (SNe IcBL) are presented. The ultimate goal of this project is to derive model-free distances to the host galaxy of one or more gamma-ray burst supernova (GRB-SN), and to exploit their observed luminosity$-$decline relationship by employing them as cosmological probes. In the first part of a two-paper analysis, I present my method for deriving the dilution factors of the SE-SN sample, which were chosen on the basis that cosmological-model-independent distances exist to their host galaxies, and each has a sufficient dataset that allows for host-subtracted, dereddened rest-frame $BVI$ LCs to be constructed, and time-series spectra. A Planck function was fit to the data to derive the blackbody radius and blackbody temperature as a function of time, while the blueshifted velocity of either Si II $lambda$6355 or Fe II $lambda$5169 was used a proxy of the photospheric velocity, and hence photospheric radius. The ratio of these empirically derived radii was taken as the dilution/correct factor. I then compared the empirically derived dilution factors with synthetic values obtained from radiative transfer models calculated for SE-SNe arising from binary systems. It is seen that the empirical dilution factors of the SNe Ic and GRB-SNe, the latter which were derived based on luminosity distances calculated from their spectroscopic redshift, are very similar. It is found that the dilution factors of the two relativistic SN IcBL are very different to those of the GRB-SNe, meaning that these engine-driven events may arise from fundamentally different progenitor systems.
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The aim of this work is to use gamma-ray burst supernovae (GRB-SNe) as cosmological probes to measure the Hubble constant, $H_0$, in the local Universe. In the context of the Expanding Photosphere Method (EPM), I use empirically derived dilution factors of a sample of nearby SNe Ic, which were derived in Paper I of a two-paper series, as a proxy for the dilution factors of GRB-SNe. It is seen that the dilution factors as a function of temperature in $VI$ display the least amount of scatter, relative to $BVI$ and $BV$. A power-law function is fit to the former, and is used to derive model dilution factors which are then used to derive EPM distances to GRB-SNe 1998bw and 2003lw: $36.7pm9.6$ and $372.2pm137.1$ Mpc, respectively. In linear Hubble diagrams in filters $BVR$, I determine the offset of the Hubble ridge line, and armed with the peak absolute magnitudes in these filters for the two aforementioned GRB-SNe, I find a (weighted average) Hubble constant of $bar{H_{0,rm w}} = 61.9pm12.3$ km s$^{-1}$ Mpc$^{-1}$ for GRB-SNe located at redshifts $zle0.1$. The 20% error is consistent with the value of $H_0$ calculated by Planck and SNe Ia within 1$sigma$. I tested the fitting method on five nearby SNe Ic, and found that their EPM distances varied by 18-50%, with smaller errors found for those SNe which had more numerous usable observations. For SN 2002ap, its EPM distance was overestimated by 18%, and if the distance to SN 1998bw was similarly over-estimated by the same amount, the resultant value of the Hubble constant is $H_0 = 72$ km s$^{-1}$ Mpc$^{-1}$, which perfectly matches that obtained using SNe Ia. [abridged]
We present 645 optical spectra of 73 supernovae (SNe) of Types IIb, Ib, Ic, and broad-lined Ic. All of these types are attributed to the core collapse of massive stars, with varying degrees of intact H and He envelopes before explosion. The SNe in our sample have a mean redshift <cz> = 4200 km/s. Most of these spectra were gathered at the Harvard-Smithsonian Center for Astrophysics (CfA) between 2004 and 2009. For 53 SNe, these are the first published spectra. The data coverage range from mere identification (1-3 spectra) for a few SNe to extensive series of observations (10-30 spectra) that trace the spectral evolution for others, with an average of 9 spectra per SN. For 44 SNe of the 73 SNe presented here, we have well-determined dates of maximum light to determine the phase of each spectrum. Our sample constitutes the most extensive spectral library of stripped-envelope SNe to date. We provide very early coverage (as early as 30 days before V-band max) for photospheric spectra, as well as late-time nebular coverage when the innermost regions of the SNe are visible (as late as 2 years after explosion, while for SN1993J, we have data as late as 11.6 years). This data set has homogeneous observations and reductions that allow us to study the spectroscopic diversity of these classes of stripped SNe and to compare these to SNe associated with gamma-ray bursts. We undertake these matters in follow-up papers.
The velocity of the inner ejecta of stripped-envelope core-collapse supernovae (CC-SNe) is studied by means of an analysis of their nebular spectra. Stripped-envelope CC-SNe are the result of the explosion of bare cores of massive stars ($geq 8$ M$_{odot}$), and their late-time spectra are typically dominated by a strong [O {sc i}] $lambdalambda$6300, 6363 emission line produced by the innermost, slow-moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped-envelope CC-SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the line width as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s$^{-1}$. Observations ($< 100$ days) of stripped-envelope CC-SNe have revealed a subclass of very energetic SNe, termed broad-lined SNe (BL-SNe) or hypernovae, which are characterised by broad absorption lines in the early-time spectra, indicative of outer ejecta moving at very high velocity ($v geq 0.1 c$). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC-SNe. This might indicate asphericity of the inner ejecta of BL-SNe, a possibility we investigate using synthetic three-dimensional nebular spectra.
We present modelling of line polarization to study multi-dimensional geometry of stripped-envelope core-collapse supernovae (SNe). We demonstrate that a purely axisymmetric, two-dimensional geometry cannot reproduce a loop in the Stokes Q-U diagram, i.e., a variation of the polarization angles along the velocities associated with the absorption lines. On the contrary, three-dimensional (3D) clumpy structures naturally reproduce the loop. The fact that the loop is commonly observed in stripped-envelope SNe suggests that SN ejecta generally have a 3D structure. We study the degree of line polarization as a function of the absorption depth for various 3D clumpy models with different clump sizes and covering factors. Comparison between the calculated and observed degree of line polarization indicates that a typical size of the clump is relatively large, >~ 25 % of the photospheric radius. Such large-scale clumps are similar to those observed in the SN remnant Cassiopeia A. Given the small size of the observed sample, the covering factor of the clumps is only weakly constrained (~ 5-80 %). The presence of large-scale clumpy structure suggests that the large-scale convection or standing accretion shock instability takes place at the onset of the explosion.
We report optical and near-infrared observations of SN 2012ca with the Public ESO Spectroscopy Survey of Transient Objects (PESSTO), spread over one year since discovery. The supernova (SN) bears many similarities to SN 1997cy and to other events classified as Type IIn but which have been suggested to have a thermonuclear origin with narrow hydrogen lines produced when the ejecta impact a hydrogen-rich circumstellar medium (CSM). Our analysis, especially in the nebular phase, reveals the presence of oxygen, magnesium and carbon features. This suggests a core collapse explanation for SN2012ca, in contrast to the thermonuclear interpretation proposed for some members of this group. We suggest that the data can be explained with a hydrogen and helium deficient SN ejecta (Type I) interacting with a hydrogen-rich CSM, but that the explosion was more likely a Type Ic core-collapse explosion than a Type Ia thermonuclear one. This suggests two channels (both thermonuclear and stripped envelope core-collapse) may be responsible for these SN 1997cy-like events.
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