Observed SNe Ia Research Articles

DEVIATIONS FROM AXISYMMETRY REVEALED BY LINE POLARIZATION IN THE NORMAL TYPE Ia SUPERNOVA 2004S

We present a single epoch of high signal-to-noise ratio spectropolarimetry of the Type Ia supernova (SN Ia) 2004S taken nine days after maximum light. The flux spectrum is normal, but with the additional presence of high-velocity (HV) line features in both Ca II and Fe II. The object shows continuum polarization at the 0.4% level in the red, a value which appears to be typical of SNe Ia. The continuum data are consistent with a ~10% global asphericity in an axisymmetric geometry. Unlike previous observations of other SNe Ia with HV features, the HV features in SN 2004S show no strong polarimetric signature, though this may be due to the timing of our observations. Instead, the object shows line polarization features (P < 0.5%) that are rotated with respect to the axis of symmetry of the continuum. The line features are visible in Si II, Fe II, and Ca II, and appear to be narrowly confined in velocity space just above the photosphere. These polarization features are a result of compositional inhomogeneities in the ejecta. They may represent newly synthesized elements whose clumpy spatial distribution within the ejecta is distinct from that of the globally aspherical ejecta as a whole.

Host galaxy extinction of Type Ia supernovae: co-evolution of interstellar medium structure and the extinction law with star formation

This paper presents a mechanism that may modify the extinction law for Type Ia supernovae (SNeIa) observed at higher redshift. Starting from the observations that (i) SNeIa occur predominantly in spiral galaxies, (ii) star formation ejects interstellar medium (ISM) out of the plane of spirals, (iii) star formation alters the extinction properties of the dust in the ISM, and (iv) there is substantially more star formation at higher redshift, I propose that spiral galaxies have a dustier halo in the past than they do now. The ejected material's lower value of RV will lead to a lower average value for SNeIa observed at higher redshift. Two relations in SNIa observations indicate evolution of the average RV: the relation of observed RV with inclination of the host galaxy at low redshift and the matching of the distribution of extinction values (AV) for SNeIa in different redshift intervals. The inclination effect does point to a halo with lower RV values. In contrast, the distributions of AV values match best for an evolution that mimics the relation of SNIa dimming with redshift attributed to the cosmological constant. However, even in the worse-case scenario, the evolution cannot fully explain the dimming of SNeIa: host galaxy extinction law evolution is not a viable alternative to account for the dimming of SNeIa. Future observations of SNeIa – multicolour light curves and spectra – will solve separately for values of AV and RV foreach SNIa. Solving for evolution of (and AV) with redshift will be important for the coming generation of cosmological SNIa measurements and has the bonus science of insight into the distribution of dust-rich ISM in the host galaxies in the distant past.

Detailed Spectral Modeling of a Three‐dimensional Pulsating Reverse Detonation Model: Too Much Nickel

We calculate detailed non-LTE synthetic spectra of a pulsating reverse detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae. While the hydro models are calculated in three dimensions, the spectra use an angle-averaged hydro model and thus some of the three-dimensional (3D) details are lost, but the overall average should be a good representation of the average observed spectra. We study the model at three epochs: maximum light, 7 days prior to maximum light, and 5 days after maximum light. At maximum the defining Si II feature is prominent, but there is also a prominent C II feature, not usually observed in normal SNe Ia near maximum. We compare to the early spectrum of SN 2006D, which did show a prominent C II feature, but the fit to the observations is not compelling. Finally, we compare to the postmaximum UV+optical spectrum of SN 1992A. With the broad spectral coverage it is clear that the iron-peak elements on the outside of the model push too much flux to the red and thus the particular PRD realizations studied would be intrinsically far redder than observed SNe Ia. We briefly discuss variations that could improve future PRD models.

A Three‐Dimensional Deflagration Model for Type Ia Supernovae Compared with Observations

A simulation of the thermonuclear explosion of a Chandrasekhar-mass C+O white dwarf, the most popular scenario of a Type Ia supernova (SN Ia), is presented. The underlying modeling is pursued in a self-consistent way, treating the combustion wave as a turbulent deflagration using well tested methods developed for laboratory combustion and based on the concept of large-eddy (LESs). Such consistency requires to capture the onset of the turbulent cascade on resolved scales. This is achieved by computing the dynamical evolution on a 10243 moving grid, which resulted in the best-resolved three-dimensional SN Ia simulation carried out thus far, reaching the limits of what can be done on present supercomputers. Consequently, the model has no free parameters other than the initial conditions at the onset of the explosion, and therefore it has considerable predictive power. Our main objective is to determine to which extent such a simulation can account for the observations of normal SNe Ia. Guided by previous simulations with less resolution and a less sophisticated flame model, initial conditions were chosen that yield a reasonably strong explosion and a sufficient amount of radioactive nickel for a bright display. We show that observables are indeed matched to a reasonable degree. In particular, good agreement is found with the light curves of normal SNe Ia. Moreover, the model reproduces the general features of the abundance stratification as inferred from the analysis of spectra. This indicates that it captures the main features of the explosion mechanism of SNe Ia. However, we also show that even a seemingly best-choice pure deflagration model has shortcomings that indicate the need for a different mode of nuclear burning at late times, perhaps the transition to a detonation at low density.

Type Ia Supernova Light Curves

The diversity of Type Ia supernova (SN Ia) photometry is explored using a grid of 130 one-dimensional models. It is shown that the observable properties of SNe Ia resulting from Chandrasekhar-mass explosions are chiefly determined by their final composition and some measure of ``mixing'' in the explosion. A grid of final compositions is explored including essentially all combinations of 56Ni, stable ``iron'', and intermediate mass elements that result in an unbound white dwarf. Light curves (and in some cases spectra) are calculated for each model using two different approaches to the radiation transport problem. Within the resulting templates are models that provide good photometric matches to essentially the entire range of observed SNe Ia. On the whole, the grid of models spans a wide range in B-band peak magnitudes and decline rates, and does not obey a Phillips relation. In particular, models with the same mass of 56Ni show large variations in their light curve decline rates. We identify the physical parameters responsible for this dispersion, and consider physically motivated ``cuts'' of the models that agree better with the Phillips relation. For example, models that produce a constant total mass of burned material of 1.1 +/- Msun do give a crude Phillips relation, albeit with much scatter. The scatter is further reduced if one restricts that set to models that make 0.1 to 0.3 Msun of stable iron and nickel isotopes, and then mix the ejecta strongly between the center and 0.8 Msun. We conclude that the supernovae that occur most frequently in nature are highly constrained by the Phillips relation and that a large part of the currently observed scatter in the relation is likely a consequence of the intrinsic diversity of these objects.

Modeling repulsive gravity with creation

There is a growing interest in the cosmologists for theories with negative energy scalar fields and creation, in order to model a repulsive gravity. The classical steady state cosmology proposed by Bondi, Gold and Hoyle in 1948, was the first such theory which used a negative kinetic energy creation field to invoke creation of matter. We emphasize that creation plays very crucial role in cosmology and provides a natural explanation to the various explosive phenomena occurring in local (z<0.1) and extra galactic universe. We exemplify this point of view by considering the resurrected version of this theory - the quasi-steady state theory, which tries to relate creation events directly to the large scale dynamics of the universe and supplies more natural explanations of the observed phenomena. Although the theory predicts a decelerating universe at the present era, it explains successfully the recent SNe Ia observations (which require an accelerating universe in the standard cosmology), as we show in this paper by performing a Bayesian analysis of the data.

Robust Dark Energy Constraints from Supernovae, Galaxy Clustering, and 3 yrWilkinson Microwave Anisotropy ProbeObservations

Type Ia supernova (SN Ia), galaxy clustering, and cosmic microwave background anisotropy (CMB) data provide complementary constraints on the nature of the dark energy in the universe. We find that the three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations give a CMB shift parameter of R = (\Omega_m H_0^2)^{1/2} \int_0^{z_{CMB}} dz'/H(z')= 1.70 \pm 0.03. Using this new measured value of the CMB shift parameter, together with the baryon acoustic oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS), and SN Ia data from the HST/GOODS program and the first year Supernova Legacy Survey, we derive model-independent constraints on the dark energy density rho_X(z) and the cosmic expansion rate H(z). We also derive constraints on the dark energy equation of state w_X(z)=w_0+w'z (with cutoff at z=2) and w_X(a)=w_0+(1-a)w_a. We find that current data provide slightly tighter constraints on rho_X(z) and H(z) as free functions in redshift, and roughly a factor of two improvement in constraining w_X(z). A cosmological constant remains consistent with data, however, uncertainties remain large for model-independent constraints of dark energy. Significant increase in the number of observed SNe Ia between redshifts of 1 and 2, complemented by improved BAO and weak lensing cosmography measurements (as expected from the JEDI mission concept for the Joint Dark Energy Mission), will be required to dramatically tighten model-independent dark energy constraints.

Secondary Maximum in the Near‐Infrared Light Curves of Type Ia Supernovae

We undertake a theoretical study of the near-infrared (NIR) light curves of Type Ia supernovae (SNe Ia). In these bands, the light curves are distinguished by a secondary maximum occurring roughly 20-30 days after the initial one. Using time-dependent multigroup radiative transfer calculations, we calculate the UBVRIJHK-band light curves of model SN Ia ejecta structures. Our synthetic NIR light curves show distinct secondary maxima and provide favorable fits to observed SNe Ia. We offer a detailed explanation of the origin of the NIR secondary maximum, which is shown to relate directly to the ionization evolution of iron group elements in the ejecta. This understanding provides immediate insight into the dependence of the NIR light curves on the physical properties of the ejecta and in particular explains why brighter supernovae have a later and more prominent secondary maximum. We demonstrate the dependence of the NIR light curves on the mass of 56Ni, the degree of 56Ni mixing, the mass of electron capture elements, the progenitor metallicity, and the abundance of intermediate-mass elements (especially calcium). The secondary maximum is shown to be a valuable diagnostic of these important physical parameters. The models further confirm that SNe Ia should be excellent standard candles in the NIR, with a dispersion of 0.2 mag even when the physical properties of the ejecta are varied widely. This study emphasizes the consummate value of NIR observations in probing the structure of SNe Ia and in furthering their cosmological utility.

Spectral Modeling of SNe Ia Near Maximum Light: Probing the Characteristics of Hydrodynamical Models

We have performed detailed non-local thermodynamic equilibrium (NLTE) spectral synthesis modeling of two types of one-dimensional hydrodynamical models: the very highly parameterized deflagration model W7, and two delayed-detonation models. We find that, overall, both models do about equally well at fitting well-observed SNe Ia near maximum light. However, the Si II ?6150 feature of W7 is systematically too fast, whereas for the delayed-detonation models it is also somewhat too fast but significantly better than that of W7. We find that a parameterized mixed model does the best job of reproducing the Si II ?6150 line near maximum light, and we study the differences in the models that lead to better fits to normal SNe Ia. We discuss what is required of a hydrodynamical model to fit the spectra of observed SNe Ia near maximum light.

Two populations of progenitors for Type Ia supernovae?

We use recent observations of the evolution of the Type Ia supernova (SN Ia) rate with redshift, the dependence of the SN Ia rate on the colours of the parent galaxies, and the enhancement of the SN Ia rate in radio-loud early-type galaxies to derive on robust empirical grounds, the delay time distribution (DTD) between the formation of the progenitor star and its explosion as an SN. Our analysis finds: (i) delay times as long as 3–4 Gyr, derived from observations of SNe Ia at high redshift, cannot reproduce the dependence of the SN Ia rate on the colours and on the radio-luminosity of the parent galaxies, as observed in the local Universe; (ii) the comparison between observed SN rates and a grid of theoretical ‘single-population’ DTDs shows that only a few of them are possibly consistent with observations. The most successful models are all predicting a peak of SN explosions soon after star formation and an extended tail in the DTD, and can reproduce the data but only at a modest statistical confidence level; (iii) present data are best matched by a bimodal DTD, in which about 50 per cent of SNe Ia (dubbed ‘prompt’ SNe Ia) explode soon after their stellar birth, in a time of the order of 108 yr, while the remaining 50 per cent (‘tardy’ SNe Ia) have a much wider distribution, well described by an exponential function with a decay time of about 3 Gyr. The presence in the DTD of both a strong peak at early times and a prolonged exponential tail, coupled with the well-established bimodal distribution of the decay rate (Δm15) and the systematic difference observed in the expansion velocities of the ejecta of SNe Ia in ellipticals and spirals, suggests the existence of two classes of progenitors. We discuss the cosmological implications of this result and make simple predictions, which are testable with future instrumentation.

Cosmological constraints on a power law universe

Linearly coasting cosmology is comfortably concordant with a host of cosmological observations. It is surprisingly an excellent fit to SNe Ia observations and constraints arising from age of old quasars. In this Letter we highlight the overall viability of an open linear coasting cosmological model. The model is consistent with the latest SNe Ia “gold” sample and accommodates a very old high-redshift quasar, which the standard cold–dark model fails to do.

Unified One‐Dimensional Simulations of Gamma‐Ray Line Emission from Type Ia Supernovae

The light curves of Type Ia supernovae (SNe Ia) are powered by gamma rays emitted by the decay of radioactive elements such as 56Ni and its decay products. These gamma rays are downscattered, absorbed, and eventually reprocessed into the optical emission that makes up the bulk of all SN observations. Detection of the gamma rays that escape the expanding star provide the only direct means to study this power source for SN Ia light curves. Unfortunately, disagreements between calculations for the gamma-ray lines have made it difficult to interpret any gamma-ray observations. Here we present a detailed comparison of the major gamma-ray line transport codes for a series of one-dimensional SN Ia models. Discrepancies in past results were due to errors in the codes, and the corrected versions of the seven different codes yield very similar results. This convergence of the simulation results allows us to infer more reliable information from the current set of gamma-ray observations of SNe Ia. The observations of SN 1986G, SN 1991T, and SN 1998bu are consistent with explosion models based on their classification: subluminous, superluminous, and normally luminous, respectively.

Could There Be a Hole in Type Ia Supernovae?

In the favored progenitor scenario, Type Ia supernovae arise from a white dwarf accreting material from a non-degenerate companion star. Soon after the white dwarf explodes, the ejected supernova material engulfs the companion star; two-dimensional hydrodynamical simulations by Marietta et. al. show that, in the interaction, the companion star carves out a conical hole of opening angle 30-40 degrees in the supernova ejecta. In this paper we use multi-dimensional Monte Carlo radiative transfer calculations to explore the observable consequences of an ejecta-hole asymmetry. We calculate the variation of the spectrum, luminosity, and polarization with viewing angle for the aspherical supernova near maximum light. We find that the supernova looks normal from almost all viewing angles except when one looks almost directly down the hole. In the latter case, one sees into the deeper, hotter layers of ejecta. The supernova is relatively brighter and has a peculiar spectrum characterized by more highly ionized species, weaker absorption features, and lower absorption velocities. The spectrum viewed down the hole is comparable to the class of SN 1991T-like supernovae. We consider how the ejecta-hole asymmetry may explain the current spectropolarimetric observations of SNe Ia, and suggest a few observational signatures of the geometry. Finally, we discuss the variety currently seen in observed SNe Ia and how an ejecta-hole asymmetry may fit in as one of several possible sources of diversity.

SN 1984A and Delayed‐Detonation Models of Type Ia Supernovae

SN 1984A shows unusually large expansion velocities in lines from freshly synthesized material, relative to typical Type Ia supernovae (SNe Ia). SN 1984A is an example of a group of SNe Ia which have very large blueshifts of the P Cygni features but otherwise normal spectra. We have modeled several early spectra of SN 1984A with the multipurpose non-LTE (NLTE) model atmosphere and spectrum synthesis code PHOENIX. We have used as input two delayed-detonation models: CS15DD3 (Iwamoto et al.) and DD21c (Hoflich, Wheeler, & Thielemann). These models show line-expansion velocities which are larger than that for a typical deflagration model like W7, which we have previously shown to fit the spectra of normal SNe Ia quite well. We find these delayed-detonation models to be reasonable approximations to large absorption feature blueshift SNe Ia, like SN 1984A. Higher densities of newly synthesized intermediate-mass elements at higher velocities, v > 15,000 km s-1, are found in delayed-detonation models than in deflagration models. We find that this increase in density at high velocities is responsible for the larger blueshifts in the synthetic spectra. We show that the variations in line width in observed SNe Ia are likely due to density variations in the outer, high-velocity layers of their atmospheres.

The Case for an Accelerating Universe from Supernovae

ABSTRACTThe unexpected faintness of high‐redshift Type Ia supernovae (SNe Ia), as measured by two teams, has been interpreted as evidence that the expansion of the universe is accelerating. We review the current challenges to this interpretation and seek to answer whether the cosmological implications are compelling. We discuss future observations of SNe Ia which could offer extraordinary evidence to test acceleration.

New constraints from high redshift supernovae and lensing statistics upon scalar field cosmologies

We explore the implications of gravitationally lensed quasistellar objects and high-redshift SNe Ia observations for spatially flat cosmological models in which a classically evolving scalar field currently dominates the energy density of the Universe. We consider two representative scalar field potentials that give rise to effective decaying $\ensuremath{\Lambda}$ (``quintessence'') models: pseudo Nambu-Goldstone bosons $[V(\ensuremath{\varphi}{)=M}^{4}(1+\mathrm{cos}(\ensuremath{\varphi}/f))]$ and an inverse power-law potential $[V(\ensuremath{\varphi}{)=M}^{4+\ensuremath{\alpha}}{\ensuremath{\varphi}}^{\ensuremath{-}\ensuremath{\alpha}}].$ We show that a large region of parameter space is consistent with current data if ${\ensuremath{\Omega}}_{m0}&gt;0.15.$ On the other hand, a higher lower bound for the matter density parameter suggested by large-scale galaxy flows, ${\ensuremath{\Omega}}_{m0}&gt;0.3,$ considerably reduces the allowed parameter space, forcing the scalar field behavior to approach that of a cosmological constant.

A New Method for the Determination of the Hubble Parameter

By chance, the slope of the MV(max) versus (B - V)max relation for recent theoretical models of Type Ia supernovae (SNe Ia) by Hoflich & Khokhlov is indistinguishable from the slope of a reddening line in the V versus B - V plane. This coincidence allows one to determine a parameter M*V(max) for SNe Ia that is independent of both supernova detonation model and interstellar reddening. Calibrating M*V(max) with observations of SNe Ia by Hamuy and coworkers yields values of the Hubble parameter H0 in the range 55-60 km s-1 Mpc-1. The discrepancy between this result and values of H0 recently obtained from observations of Cepheids in the Virgo Cluster suggests that either (1) the Cepheid distance scale is wrong, (2) the SN Ia models of Hoflich & Khokhlov are too bright by ~0.75 mag near maximum light, or (3) their models are too red by ~0.25 mag in B - V.