On the presence of Silicon and Carbon in the pre-maximum spectrum of the Type Ia SN 1990N

The spectrum of the normal Type Ia SN 1990N observed very early on (14 days before B maximum) was analysed by Fisher et al (1997), who showed that the large width and the unusual profile of the strong line near 6000\AA can be reproduced if the line is assumed to be due to \CII 6578, 6583\AA and if Carbon is located in a high velocity shell. This line is one of the characterising features of SNe Ia, and is usually thought to be due to \SiII. A Monte Carlo spectrum synthesis code was used to investigate this suggestion further. The result is that if a standard explosion model is used the mass enclosed in the shell at the required high velocity (25,000--35,000 \kms) is too small to give rise to a strong \CII line. At the same time, removing Silicon has a negative effect on the synthetic spectrum at other wavelengths, and removing Carbon from the lower velocity regions near the photosphere makes it difficult to reproduce two weak lines which are naturally explained as \CII, one of them being the line which Fisher et al (1997) suggested is responsible for the strong 6000\AA feature. However, synthetic spectra confirm that although \SiII can reproduce most of the observed 6000\AA line, the red wing of the line extends too far to be compatible with a \SiII origin, and that the flat bottom of the line is also not easy to reproduce. The best fit is obtained for a normal SN Ia abundance mix at velocities near the photosphere (15,500-19,000 \kms) and an outer Carbon-Silicon shell beyond 20,000 \kms. This suggests that mixing is not complete in the outer ejecta of a SN Ia. Observations at even earlier epochs might reveal to what extent a Carbon shell is unmixed.Comment: 12 pages, (4 figures). MNRAS, in pres

3D Models for High Velocity Features in Type Ia Supernovae

Spectral synthesis in 3-dimensional (3D) space for the earliest spectra of Type Ia supernovae (SNe Ia) is presented. In particular, the high velocity absorption features that are commonly seen at the earliest epochs ($\sim 10$ days before maximum light) are investigated by means of a 3D Monte Carlo spectral synthesis code. The increasing number of early spectra available allows statistical study of the geometry of the ejecta. The observed diversity in strength of the high velocity features (HVFs) can be explained in terms of a ``covering factor'', which represents the fraction of the projected photosphere that is concealed by high velocity material. Various geometrical models involving high velocity material with a clumpy structure or a thick torus can naturally account for the observed statistics of HVFs. HVFs may be formed by a combination of density and abundance enhancements. Such enhancements may be produced in the explosion itself or may be the result of interaction with circumstellar material or an accretion disk. Models with 1 or 2 blobs, as well as a thin torus or disk-like enhancement are unlikely as a standard situation.Comment: 17 pages, 12 figures. Accepted for publication in the Astrophysical Journa

Multi-Dimensional Simulations for Early Phase Spectra of Aspherical Hypernovae: SN 1998bw and Off-Axis Hypernovae

Early phase optical spectra of aspherical jet-like supernovae (SNe) are presented. We focus on energetic core-collapse SNe, or hypernovae. Based on hydrodynamic and nucleosynthetic models, radiative transfer in SN atmosphere is solved with a multi-dimensional Monte-Carlo radiative transfer code, SAMURAI. Since the luminosity is boosted in the jet direction, the temperature there is higher than in the equatorial plane by ~ 2,000 K. This causes anisotropic ionization in the ejecta. Emergent spectra are different depending on viewing angle, reflecting both aspherical abundance distribution and anisotropic ionization. Spectra computed with an aspherical explosion model with kinetic energy 20 x 10^{51} ergs are compatible with those of the Type Ic SN 1998bw if ~ 10-20% of the synthesized metals are mixed out to higher velocities. The simulations enable us to predict the properties of off-axis hypernovae. Even if an aspherical hypernova explosion is observed from the side, it should show hypernova-like spectra but with some differences in the line velocity, the width of the Fe absorptions and the strength of the Na I line.Comment: 4 pages, 4 figures. Accepted for publication in The Astrophysical Journal Letter

Abundance stratification in Type Ia Supernovae - II: The rapidly declining, spectroscopically normal SN 2004eo

The variation of properties of Type Ia supernovae, the thermonuclear explosions of Chandrasekhar-mass carbon-oxygen white dwarfs, is caused by different nucleosynthetic outcomes of these explosions, which can be traced from the distribution of abundances in the ejecta. The composition stratification of the spectroscopically normal but rapidly declining SN2004eo is studied performing spectrum synthesis of a time-series of spectra obtained before and after maximum, and of one nebular spectrum obtained about eight months later. Early-time spectra indicate that the outer ejecta are dominated by oxygen and silicon, and contain other intermediate-mass elements (IME), implying that the outer part of the star was subject only to partial burning. In the inner part, nuclear statistical equilibrium (NSE) material dominates, but the production of 56Ni was limited to ~0.43 \pm 0.05 Msun. An innermost zone containing ~0.25 Msun of stable Fe-group material is also present. The relatively small amount of NSE material synthesised by SN2004eo explains both the dimness and the rapidly evolving light curve of this SN.Comment: 12 pages, 7 figures. Accepted for publication in MNRA

The (54Fe+58Ni)/56Ni ratio as a second parameter for Type Ia supernova properties

A variation of the relative content of (54Fe+58Ni) versus 56Ni may be responsible for the observed scatter of Type Ia Supernovae (SNe Ia) about a mean relation between their intrinsic brightness and the shape of their light curve. Synthetic light curves are computed of parametrised Chandrasekhar-mass explosion models of constant kinetic energy, where the ejecta are divided into an inner NSE zone, composed of (54Fe+58Ni) inside and 56Ni outside, an outer zone with Intermediate Mass Elements and a CO zone. Both the size of the NSE zone and the fraction of (54Fe+58Ni) v. 56Ni are varied systematically. Models with the same original NSE content but different (54Fe+58Ni)/56Ni ratios reach different peak brightness but have similar light curve shapes. Synthetic spectra indicate that the V-band decline rate is not affected by the (54Fe+58Ni)/56Ni ratio. While the 56Ni mass and the total NSE mass are the dominant parameters determining the peak luminosity and the shape of the light curve, respectively, a variation in the (54Fe+58Ni)/56Ni ratio, which may depend on the metallicity of the progenitor (Timmes, Brown & Truran 2003) is likely to account for a significant part of the observed scatter of local SNe Ia about the mean brightness--decline rate relation.Comment: 7 pages, 2 figures; accepted by MNRA

The Outermost Ejecta of Type Ia Supernovae

The properties of the highest velocity ejecta of normal Type Ia supernovae (SNe Ia) are studied via models of very early optical spectra of 6 SNe. At epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II 6355 line velocity (HVG) have a larger photospheric velocity than SNe with a slowly evolving Si II 6355 line velocity (LVG). Since the two groups have comparable luminosities, the temperature at the photosphere is higher in LVG SNe. This explains the different overall spectral appearance of HVG and LVG SNe. However, the variation of the Ca II and Si II absorptions at the highest velocities (v >~ 20,000 km/s) suggests that additional factors, such as asphericity or different abundances in the progenitor white dwarf, affect the outermost layers. The C II 6578 line is marginally detected in 3 LVG SNe, suggesting that LVG undergo less intense burning. The carbon mass fraction is small, only less than 0.01 near the photosphere, so that he mass of unburned C is only <~ 0.01 Msun. Radioactive 56Ni and stable Fe are detected in both LVG and HVG SNe. Different Fe-group abundances in the outer layers may be one of the reasons for spectral diversity among SNe Ia at the earliest times. The diversity among SNe Ia at the earliest phases could also indicate an intrinsic dispersion in the width-luminosity relation of the light curve.Comment: 13 pages, 10 figures, Accepted for publication in The Astrophysical Journa

Spectral luminosity indicators in SNe Ia - Understanding the R(SiII) line strength ratio and beyond

SNe Ia are good distance indicators because the shape of their light curves, which can be measured independently of distance, varies smoothly with luminosity. This suggests that SNe Ia are a single family of events. Similar correlations are observed between luminosity and spectral properties. In particular, the ratio of the strengths of the SiII \lambda 5972 and \lambda 6355 lines, known as R(SiII), was suggested as a potential luminosity indicator. Here, the physical reasons for the observed correlation are investigated. A Monte-Carlo code is used to construct a sequence of synthetic spectra resembling those of SNe with different luminosities near B maximum. The influence of abundances and of ionisation and excitation conditions on the synthetic spectral features is investigated. The ratio R(SiII) depends ssentially on the strength of SiII \lambda 5972, because SiII \lambda 6355 is saturated. In less luminous objects, SiII \lambda 5972 is stronger because of a rapidly increasing SiII/SiIII ratio. Thus, the correlation between R(SiII) and luminosity is the effect of ionisation balance. The SiII \lambda 5972 line itself may be the best spectroscopic luminosity indicator for SNe Ia, but all indicators discussed show scatter which may be related to abundance distributions.Comment: 10 pages, 16 figures. Accepted for publication in MNRA

Cosmological Implications of the Second Parameter of Type Ia Supernovae

Theoretical models predict that the initial metallicity of the progenitor of a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This can cause a deviation from the standard light curve calibration employed when using SNe Ia as standardizable distance candles and, if there is a systematic evolution of the metallicity of SN Ia progenitors, could affect the determination of cosmological parameters. Here we show that this metallicity effect can be substantially larger than has been estimated previously, when the neutronisation in the immediate pre-explosion phase in the CO white dwarf is taken into account, and quantitatively assess the importance of metallicity evolution for determining cosmological parameters. We show that, in principle, a moderate and plausible amount of metallicity evolution could mimic a lambda-dominated, flat Universe in an open, lambda-free Universe. However, the effect of metallicity evolution appears not large enough to explain the high-z SN Ia data in a flat Universe, for which there is strong independent evidence, without a cosmological constant. We also estimate the systematic uncertainties introduced by metallicity evolution in a lambda-dominated, flat Universe. We find that metallicity evolution may limit the precision with which Omega_m and w can be measured and that it will be difficult to distinguish evolution of the equation of state of dark energy from metallicity evolution, at least from SN Ia data alone.Comment: 10 pages, 6 figures, constructive comments welcom

The Type Ic Hypernova SN 2003dh/GRB 030329

The spectra of SN 2003dh, identified in the afterglow of GRB030329, are modeled using radiation transport codes. It is shown that SN 2003dh had a high explosion kinetic energy ($\sim 4 \times 10^{52}$ erg in spherical symmetry), making it one of the most powerful hypernovae observed so far, and supporting the case for association between hypernovae and Gamma Ray Bursts. However, the light curve derived from fitting the spectra suggests that SN 2003dh was not as bright as SN 1998bw, ejecting only \sim 0.35\Msun of \Nifs. The spectra of SN 2003dh resemble those of SN 1998bw around maximum, but later they look more like those of the less energetic hypernova SN 1997ef. The spectra and the inferred light curve can be modeled adopting a density distribution similar to that used for SN 1998bw at $v > 25,000$\kms but more like that of SN 1997ef at lower velocities. The mass of the ejecta is \sim 8\Msun, somewhat less than in the other two hypernovae. The progenitor must have been a massive star (M \sim 35-40\Msun), as for other hypernovae. The need to combine different one-dimensional explosion models strongly indicates that SN 2003dh was an asymmetric explosion.Comment: 11 pages, 1 table and 5 figures. To appear in the Astrophysical Journal (Letters). Revised version taking referee's comments into account, minor change

The Type Ic SN 2007gr: a census of the ejecta from late-time optical-infrared spectra

Nebular spectra of Supernovae (SNe) offer an unimpeded view of the inner region of the ejecta, where most nucleosynthesis takes place. Optical spectra cover most, but not all of the emitting elements, and therefore offer only a partial view of the products of the explosion. Simultaneous optical-infrared spectra, on the other hand, contain emission lines of all important elements, from C and O through to the Intermediate Mass Elements (IME) Mg, Si, S, Ca, and to Fe and Ni. In particular, Si and S are best seen in the IR. The availability of IR data makes it possible to explore in greater detail the results of the explosion. SN\,2007gr is the first Type Ic SN for which such data are available. Modelling the spectra with a NLTE code reveals that the inner ejecta contain \sim 1 \Msun of material within a velocity of $\approx 4500$\,\kms. %The spectrum is powered by \Nifs, in an amount (0.076 \Msun) consistent with that %derived from the early-time data. The same mass of \Nifs\ derived from the light curve peak (0.076 \Msun) was used to power the spectrum, yielding consistent results. Oxygen is the dominant element, contributing \sim 0.8 \Msun. The C/O ratio is $< 0.2$. IME account for \sim 0.1 \Msun. This confirms that SN\,2007gr was the explosion of a low-mass CO core, probably the result of a star of main-sequence mass \approx 15 \Msun. The ratios of the \CaII\ lines, and of those of \FeII, are sensitive to the assumed degree of clumping. In particular, the optical lines of [\FeII] become stronger, relative to the IR lines, for higher degrees of clumping