Context. Use of Type Ia supernovae (SNe Ia) as distance indicators has proven to be a powerful technique for measuring the darkenergy equation of state. However, recent studies have highlighted potential biases correlated with the global properties of their host galaxies, large enough to induce systematic errors into such cosmological measurements if not properly treated. Aims. We study the host galaxy regions in close proximity to SNe Ia in order to analyze relations between the properties of SN Ia events and environments where their progenitors most likely formed. In this paper we focus on local Hα emission as an indicator of young progenitor environments. Methods. The Nearby Supernova Factory has obtained flux-calibrated spectral timeseries for SNe Ia using integral field spectroscopy. These observations enabled the simultaneous measurement of the SN and its immediate vicinity. For 89 SNe Ia we measured or set limits on Hα emission, used as a tracer of ongoing star formation, within a 1 kpc radius around each SN. This constitutes the first direct study of the local environment for a large sample of SNe Ia with accurate luminosity, color, and stretch measurements. Results. Our local star formation measurements provide several critical new insights. We find that SNe Ia with local Hα emission are redder by 0.036 ± 0.017 mag, and that the previously noted correlation between stretch and host mass is driven entirely by the SNe Ia coming from locally passive environments, in particular at the low-stretch end. There is no such trend for SNe Ia in locally star-forming environments. Our most important finding is that the mean standardized brightness for SNe Ia with local Hα emission is 0.094 ± 0.031 mag fainter on average than for those without. This offset arises from a bimodal structure in the Hubble residuals, with one mode being shared by SNe Ia in all environments and the other one exclusive to SNe Ia in locally passive environments. This structure also explains the previously known host-mass bias. We combine the star formation dependence of this bimodality with the cosmic star formation rate to predict changes with redshift in the mean SN Ia brightness and the host-mass bias. The strong change predicted is confirmed using high-redshift SNe Ia from the literature. Conclusions. The environmental dependences in SN Ia Hubble residuals and color found here point to remaining systematic errors in the standardization of SNe Ia. In particular, the observed brightness offset associated with local Hα emission is predicted to cause a significant bias in current measurements of the dark energy equation of state. Recognition of these effects offers new opportunities to improve SNe Ia as cosmological probes. For instance, we note that the SNe Ia associated with local Hα emission are more homogeneous, resulting in a brightness dispersion of only 0.105 ± 0.012 mag. Key words. cosmology: observationsAppendix is available in electronic form at http://www.aanda.orgArticle published by EDP Sciences A66, page 1 of 17 A&A 560...

We present a sample of normal type Ia supernovae from the Nearby Supernova Factory dataset with spectrophotometry at sufficiently late phases to estimate the ejected mass using the bolometric light curve. We measure 56 Ni masses from the peak bolometric luminosity, then compare the luminosity in the 56 Co-decay tail to the expected rate of radioactive energy release from ejecta of a given mass. We infer the ejected mass in a Bayesian context using a semi-analytic model of the ejecta, incorporating constraints from contemporary numerical models as priors on the density structure and distribution of 56 Ni throughout the ejecta. We find a strong correlation between ejected mass and light curve decline rate, and consequently 56 Ni mass, with ejected masses in our data ranging from 0.9-1.4 M ⊙ . Most fast-declining (SALT2 x 1 < −1) normal SNe Ia have significantly sub-Chandrasekhar ejected masses in our fiducial analysis.

Previously we used the Nearby Supernova Factory sample to show that SNe Ia having locally star-forming environments are dimmer than SNe Ia having locally passive environments. Here we use the Constitution sample together with host galaxy data from GALEX to independently confirm that result. The effect is seen using both the SALT2 and MLCS2k2 lightcurve fitting and standardization methods, with brightness differences of 0.094 ± 0.037 mag for SALT2 and 0.155 ± 0.041 mag for MLCS2k2 with R V = 2.5. When combined with our previous measurement the effect is 0.094 ± 0.025 mag for SALT2. If the ratio of these local SN Ia environments changes with redshift or sample selection, this can lead to a bias in cosmological measurements. We explore this -2issue further, using as an example the direct measurement of H 0 . GALEX observations show that the SNe Ia having standardized absolute magnitudes calibrated via the Cepheid period-luminosity relation using HST originate in predominately star-forming environments, whereas only ∼ 50% of the Hubble-flow comparison sample have locally star-forming environments. As a consequence, the H 0 measurement using SNe Ia is currently overestimated. Correcting for this bias, we find a value of H corr 0 = 70.6 ± 2.6 km s −1 Mpc −1 when using the LMC distance, Milky Way parallaxes and the NGC 4258 megamaser as the Cepheid zeropoint, and 68.8 ± 3.3 km s −1 Mpc −1 when only using NGC 4258. Our correction brings the direct measurement of H 0 within ∼ 1 σ of recent indirect measurements based on the CMB power spectrum.B SF = 0.094 ± 0.031 mag 1 . Since the underlying connection is with star formation rather than the Hα emission itself, we refer to this effect as the star-formation bias, or SF bias for short.R13 connected the SF bias to the host-mass step by noting that few of the Ia in the SNfactory sample occur in low-mass hosts, leading to a shift in mean brightness with host mass that is driven by the changing fraction of star formation. However, this also implies that simply correcting for the host-mass step will not

The nearby supernova SN 2011fe can be observed in unprecedented detail. Therefore, it is an important test case for Type Ia supernova (SN Ia) models, which may bring us closer to understanding the physical nature of these objects. Here, we explore how available and expected future observations of SN 2011fe can be used to constrain SN Ia explosion scenarios. We base our discussion on three-dimensional simulations of a delayed detonation in a Chandrasekhar-mass white dwarf and of a violent merger of two white dwarfs-realizations of explosion models appropriate for two of the most widely-discussed progenitor channels that may give rise to SNe Ia. Although both models have their shortcomings in reproducing details of the early and near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory (SNfactory), the overall match with the observations is reasonable. The level of agreement is slightly better for the merger, in particular around maximum, but a clear preference for one model over the other is still not justified. Observations at late epochs, however, hold promise for discriminating the explosion scenarios in a straightforward way, as a nucleosynthesis effect leads to differences in the 55 Co production. SN 2011fe is close enough to be followed sufficiently long to study this effect.