We combine the CfA3 supernova Type Ia (SN Ia) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. (2008) to form the Constitution set and, combined with a BAO prior, produces 1 + w = 0.013 +0.066 −0.068 (0.11 syst), consistent with the cosmological constant. The CfA3 addition makes the cosmologically-useful sample of nearby SN Ia between 2.6 and 2.9 times larger than before, reducing the statistical uncertainty to the point where systematics play the largest role. We use four light curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2 (R V = 3.1), and MLCS2k2 (R V = 1.7). SALT produces high-redshift Hubble residuals with systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2 overestimates the intrinsic luminosity of SN Ia with 0.7 < ∆ < 1.2. MLCS2k2 with R V = 3.1 overestimates host-galaxy extinction while R V ≈ 1.7 does not. Our investigation is consistent with no Hubble bubble. We also find that, after lightcurve correction, SN Ia in Scd/Sd/Irr hosts are intrinsically fainter than those in E/S0 hosts by 2σ, suggesting that they may come from different populations. We also find that SN Ia in Scd/Sd/Irr hosts have low scatter (0.1 mag) and equation of state, p = wρ, where the equation of state parameter, w, relates the dark energy density, ρ, to the dark energy pressure, p. In a Friedman universe, ρ depends on 1 + w and the scale factor of the universe, a, as ρ ∼ a −3(1+w) . The first question that arises is whether the dark energy density is constant (1 + w = 0, a cosmological constant) or not. We choose to use the notation, 1 + w, since it is then easier to think about values of w larger than −1 (1 + w > 0) or more negative than −1 (1 + w < 0). In the case of 1 + w < 0 the dark energy grows in density as the universe expands! The second question is whether the dark energy properties, as described by w, are constant in time or not.The first study on the equation of state produced a 95%-confidence limit of 1 + w < 0.3, assuming Ω M ∼ 0.2 and zero possibility of 1 + w < 0 (Garnavich et al. 1998). Knop et al. (2003) found 1 + w = −0.05 +0.15 −0.20 . Riess et al. (2005) reported 1 + w = −0.02 +0.13 −0.19 . The SNLS and ESSENCE surveys were designed to narrow the constraints on 1+w and their first reports showed significant improvement in statistical uncertainty over the previous values, bringing them down to the range where systematic uncertainties,which they try to reduce as well, are of roughly equal importance. Astier et al. (2006, A06, hereafter) found 1 + w = −0.02 ± 0.09 while Wood-Vasey et al. (2007, WV07, hereafter) found 1+w = −0.07±0.09. Most recently, Kowalski et al. (2008) (K08, hereafter) made a compilation of the literature SN Ia, plus several new nearby ones that they present, and found 1 + w = −0.01 ± 0.08 when using the same priors as A06 and WV07. All of these studies are consistent with a cosmological constant. On the time-evolution...

We analyze the Type II Plateau supernovae (SN II-P) 2005cs and 2006bp with the non-LTE model atmosphere code CMFGEN. We fit 13 spectra in the first month for SN 2005cs and 18 for SN 2006bp. Swift ultraviolet photometry and ground-based optical photometry calibrate each spectrum. Our analysis shows both objects were discovered less than 3 days after they exploded, making these the earliest SN II-P spectra ever studied. They reveal broad and very weak lines from highly-ionized fast ejecta with an extremely steep density profile. We identify He ii 4686Å emission in the SN 2006bp ejecta. Days later, the spectra resemble the prototypical Type II-P SN 1999em, which had a supergiant-like photospheric composition. Despite the association of SN 2005cs with possible X-ray emission, the emergent UV and optical light comes from the photosphere, not from circumstellar emission.We surmise that the very steep density fall-off we infer at early times may be a fossil of the combined actions of the shock wave passage and radiation driving at shock breakout. Based on tailored CMFGEN models, the direct-fitting technique and the Expanding Photosphere Method both yield distances and explosion times that agree within a few percent. We derive a distance to NGC 5194, the host of SN 2005cs, of 8.9±0.5 Mpc and 17.5±0.8 Mpc for SN 2006bp in NGC 3953. The luminosity of SN 2006bp is 1.5 times that of SN 1999em, and 6 times that of SN 2005cs. Reliable distances to Type II-P supernovae that do not depend on a small range in luminosity provide an independent route to the Hubble Constant and improved constraints on other cosmological parameters.

The delayed-detonation explosion mechanism applied to a Chandrasekhar-mass white dwarf offers a very attractive model to explain the inferred characteristics of Type Ia supernovae (SNe Ia). The resulting ejecta are chemically stratified, have the same mass and roughly the same asymptotic kinetic energy, but exhibit a range in 56 Ni mass. We investigate the contemporaneous photometric and spectroscopic properties of a sequence of delayed-detonation models, characterized by 56 Ni masses between 0.18 and 0.81 M ⊙ . Starting at 1 d after explosion, we perform the full non-LTE, time-dependent radiative transfer with the code CMFGEN, with an accurate treatment of line blanketing, and compare our results to SNe Ia at bolometric maximum. Despite the 1D treatment, our approach delivers an excellent agreement to observations. We recover the range of SN Ia luminosities, colours, and spectral characteristics from the near-UV to 1 µm, for standard as well as low-luminosity 91bg-like SNe Ia. Our models predict an increase in rise time to peak with increasing 56 Ni mass, from ∼ 15 to ∼ 21 d, yield peak bolometric luminosities that match Arnett's rule to within 10 % and reproduce the much smaller scatter in near-IR magnitudes compared to the optical. We reproduce the morphology of individual spectral features, the stiff dependence of the R(Si) spectroscopic ratio on 56 Ni mass, and the onset of blanketing from Ti II/Sc II in low-luminosity SNe Ia with a 56 Ni mass 0.3 M ⊙ . We find that ionization effects, which often dominate over abundance variations, can produce high-velocity features in Ca II lines, even in 1D. Distinguishing between different SN Ia explosion mechanisms is a considerable challenge but the results presented here provide additional support to the viability of the delayed-detonation model.

We compare models for Type Ia supernova (SN Ia) light curves and spectra with an extensive set of observations. The models come from a recent survey of 44 two‐dimensional delayed‐detonation models computed by Kasen et al., each viewed from multiple directions. The data include optical light curves of 251 SNe Ia, some of which have near‐infrared observations, and 2231 low‐dispersion spectra from the Center for Astrophysics, plus data from the literature. These allow us to compare a wide range of SN Ia models with observations for a wide range of luminosities and decline rates. The analysis uses standard techniques employed by observers, including MLCS2k2, SALT2 and SNooPy for light‐curve analysis, and the Supernova Identification (snid) code of Blondin & Tonry for spectroscopic comparisons to assess how well the models match the data. The ability to use the tools developed for observational data directly on the models marks a significant step forward in the realism of the models. We show that the models that match observed spectra best lie systematically on the observed width–luminosity relation. Conversely, we reject six models with highly asymmetric ignition conditions and a large amount (≳1 M⊙) of synthesized 56Ni that yield poor matches to observed SN Ia spectra. More subtle features of the comparison include the general difficulty of the models to match the U‐band flux at early times, caused by hot ionized ejecta that affect the subsequent redistribution of flux at longer wavelengths. The models have systematically higher velocities than the observed spectra at maximum light, as inferred from the Si ii λ6355 line. We examine ways in which the asymptotic kinetic energy of the explosion affects both the predicted velocity and velocity gradient in the Si ii and Ca ii lines. Models with an asymmetric distribution of 56Ni are found to result in a larger variation of photometric and spectroscopic properties with viewing angle, regardless of the initial ignition setup. We discuss more generally whether highly anisotropic ignition conditions are ruled out by observations, and how detailed comparisons between models and observations involving both light curves and spectra can lead to a better understanding of SN Ia explosion mechanisms.