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Register a new userModelling the early time behaviour of type Ia supernovae: effects of the $^{56}$Ni distribution
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M. R. Magee
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Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia) at early times and highlighted a need for a better understanding of the explosion physics as manifested by observations soon after explosion. To this end, we present a Monte Carlo code designed to model the light curves of radioactively driven, hydrogen-free transients from explosion to approximately maximum light. In this initial study, we have used a parametrised description of the ejecta in SNe Ia, and performed a parameter study of the effects of the $^{56}$Ni distribution on the observed colours and light curves for a fixed $^{56}$Ni mass of 0.6 $M_odot$. For a given density profile, we find that models with $^{56}$Ni extending throughout the entirety of the ejecta are typically brighter and bluer shortly after explosion. Additionally, the shape of the density profile itself also plays an important role in determining the shape, rise time, and colours of observed light curves. We find that the multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent with less extended $^{56}$Ni distributions, but show good agreement with models that incorporate $^{56}$Ni throughout the entire ejecta. We further demonstrate that comparisons with full $UVOIR$ colour light curves are powerful tools in discriminating various $^{56}$Ni distributions, and hence explosion models.
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Recent studies have shown how the distribution of $^{56}$Ni within the ejecta of type Ia supernovae can have profound consequences on the observed light curves. Observations at early times can therefore provide important details on the explosion physics in thermonuclear supernovae. We present a series of radiative transfer calculations that explore variations in the $^{56}$Ni distribution. Our models also show the importance of the density profile in shaping the light curve, which is often neglected in the literature. Using our model set, we investigate the observations that are necessary to determine the $^{56}$Ni distribution as robustly as possible within the current model set. We find that this includes observations beginning at least $sim$14 days before $B$-band maximum, extending to approximately maximum light with a high ($lesssim$3 day) cadence, and in at least one blue and one red band are required (such as $B$ and $R$, or $g$ and $r$). We compare a number of well-observed type Ia supernovae that meet these criteria to our models and find that the light curves of $sim$70-80% of objects in our sample are consistent with being produced solely by variations in the $^{56}$Ni distributions. The remaining supernovae show an excess of flux at early times, indicating missing physics that is not accounted for within our model set, such as an interaction or the presence of short-lived radioactive isotopes. Comparing our model light curves and spectra to observations and delayed detonation models demonstrates that while a somewhat extended $^{56}$Ni distribution is necessary to reproduce the observed light curve shape, this does not negatively affect the spectra at maximum light. Investigating current explosion models shows that observations typically require a shallower decrease in the $^{56}$Ni mass towards the outer ejecta than is produced for models of a given $^{56}$Ni mass.
An excess of flux (i.e. a bump) in the early light curves of type Ia supernovae has been observed in a handful of cases. Multiple scenarios have been proposed to explain this. It has been shown that for at least one object (SN~2018oh) the excess emission observed could be the result of a large amount of $^{56}$Ni in the outer ejecta ($sim$0.03~$M_{rm{odot}}$). We present a series of model light curves and spectra for ejecta profiles containing $^{56}$Ni shells of varying masses (0.01, 0.02, 0.03, and 0.04~$M_{rm{odot}}$) and widths. We find that even for our lowest mass $^{56}$Ni shell, an increase of textgreater2 magnitudes is produced in the bolometric light curve at one day after explosion relative to models without a $^{56}$Ni shell. We show that the colour evolution of models with a $^{56}$Ni shell differs significantly from those without and shows a colour inversion similar to some double-detonation explosions. Spectra of our $^{56}$Ni shell models show that strong suppression of flux between $sim$3,700 -- 4,000~$AA$ close to maximum light appears to be a generic feature for this class of model. Comparing our models to observations of SNe~2017cbv and 2018oh, we show that a $^{56}$Ni shell of 0.02 -- 0.04~$M_{rm{odot}}$ can match shapes of the early optical light curve bumps, but the colour and spectral evolution are in disagreement. This would indicate that an alternative origin for the flux excess is necessary. Based on existing explosion scenarios, producing such a $^{56}$Ni shell in the outer ejecta as required to match the light curve shape, without the presence of additional short-lived radioactive material, may prove challenging. Given that only a small amount of $^{56}$Ni in the outer ejecta is required to produce a bump in the light curve, such non-monotonically decreasing $^{56}$Ni distributions in the outer ejecta must be rare, if they were to occur at all.
With a booming number of Type Ia supernovae (SNe Ia) discovered within a few days of their explosions, a fraction of SNe Ia that show luminosity excess in the early phase (early-excess SNe Ia) have been confirmed. In this article, we report early-phase observations of seven photometrically normal SNe Ia (six early detections and one deep non-detection limit) at the COSMOS field through a half-year transient survey as a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP). In particular, a blue light-curve excess was discovered for HSC17bmhk, a normal SN Ia with rise time longer than 18.8 days, during the first four days after the discovery. The blue early excess in optical wavelength can be explained not only by interactions with a non-degenerate companion or surrounding dense circumstellar matter but also radiation powered by radioactive decays of $^{56}$Ni at the surface of the SN ejecta. Given the growing evidence of the early-excess discoveries in normal SNe Ia that have longer rise times than the average and a similarity in the nature of the blue excess to a luminous SN Ia subclass, we infer that early excess discovered in HSC17bmhk and other normal SNe Ia are most likely attributed to radioactive $^{56}$Ni decay at the surface of the SN ejecta. In order to successfully identify normal SNe Ia with early excess similar to that of HSC17bmhk, early UV photometries or high-cadence blue-band surveys are necessary.
A two-dimensional hydrodynamics code for Type Ia supernovae (SNIa) simulations is presented. The code includes a fifth-order shock-capturing scheme WENO, detailed nuclear reaction network, flame-capturing scheme and sub-grid turbulence. For post-processing we have developed a tracer particle scheme to record the thermodynamical history of the fluid elements. We also present a one-dimensional radiative transfer code for computing observational signals. The code solves the Lagrangian hydrodynamics and moment-integrated radiative transfer equations. A local ionization scheme and composition dependent opacity are included. Various verification tests are presented, including standard benchmark tests in one and two dimensions. SNIa models using the pure turbulent deflagration model and the delayed-detonation transition model are studied. The results are consistent with those in the literature. We compute the detailed chemical evolution using the tracer particles histories, and we construct corresponding bolometric light curves from the hydrodynamics results. We also use a Graphics Processing Unit (GPU) to speed up the computation of some highly repetitive subroutines. We achieve an acceleration of 50 times for some subroutines and a factor of 6 in the global run time.
The increase in the number of Type Ia supernovae (SNe,Ia) has demonstrated that the population shows larger diversity than has been assumed in the past. The reasons (e.g. parent population, explosion mechanism) for this diversity remain largely unknown. We have investigated a sample of SNe,Ia near-infrared light curves and have correlated the phase of the second maximum with the bolometric peak luminosity. The peak bolometric luminosity is related to the time of the second maximum (relative to the {it B} light curve maximum) as follows : $L_{max}(10^{43} erg s^{-1}) = (0.039 pm 0.004) times t_2(J)(days) + (0.013 pm 0.106)$. $^{56}$Ni masses can be derived from the peak luminosity based on Arnetts rule, which states that the luminosity at maximum is equal to instantaneous energy generated by the nickel decay. We check this assumption against recent radiative-transfer calculations of Chandrasekhar-mass delayed detonation models and find this assumption is valid to within 10% in recent radiative-transfer calculations of Chandrasekhar-mass delayed detonation models. The $L_{max}$ vs. $t_2$ relation is applied to a sample of 40 additional SNe,Ia with significant reddening ($E(B-V) >$ 0.1 mag) and a reddening-free bolometric luminosity function of SNe~Ia is established. The method is tested with the $^{56}$Ni mass measurement from the direct observation of $gamma-$rays in the heavily absorbed SN 2014J and found to be fully consistent. Super-Chandrasekhar-mass explosions, in particular SN,2007if, do not follow the relations between peak luminosity and second IR maximum. This may point to an additional energy source contributing at maximum light. The luminosity function of SNe,Ia is constructed and is shown to be asymmetric with a tail of low-luminosity objects and a rather sharp high-luminosity cutoff, although it might be influenced by selection effects.
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