A study of the color diversity around maximum light in Type Ia supernovae

We use a sample of 58 low-redshift (z <= 0.03) Type Ia supernovae (SNe Ia) having well-sampled light curves and spectra near maximum light to examine the behaviour of high-velocity features (HVFs) in SN Ia spectra. We take advantage of the fact that Si II 6355 is free of HVFs at maximum light in all SNe Ia, allowing us to quantify the strength of HVFs by comparing the structure of these two lines. We find that the average HVF strength increases with decreasing light-curve decline rate, and rapidly declining SNe Ia (dm_15(B) >= 1.4 mag) show no HVFs in their maximum-light spectra. Comparison of HVF strength to the light-curve colour of the SNe Ia in our sample shows no evidence of correlation. We find a correlation of HVF strength with the velocity of Si II 6355 at maximum light (v_Si), such that SNe Ia with lower v_Si have stronger HVFs, while those SNe Ia firmly in the "high-velocity" (i.e., v_Si >= 12,000 km/s) subclass exhibit no HVFs in their maximum-light spectra. While v_Si and dm_15(B) show no correlation in the full sample of SNe Ia, we find a significant correlation between these quantities in the subset of SNe Ia having weak HVFs. In general, we find that slowly declining (low dm_15(B)) SNe Ia, which are more luminous and more energetic than average SNe Ia, tend to produce either high photospheric ejecta velocities (i.e., high v_Si) or strong HVFs at maximum light, but not both. Finally, we examine the evolution of HVF strength for a sample of SNe Ia having extensive pre-maximum spectroscopic coverage and find significant diversity of the pre-maximum HVF behaviour.

The high-velocity features (HVFs) in optical spectra of type Ia supernovae (SNe Ia) are examined with a large sample including very early-time spectra (e.g., t < -7 days). Multiple Gaussian fits are applied to examine the HVFs and their evolutions, using constraints on expansion velocities for the same species (i.e., SiII 5972 and SiII 6355). We find that strong HVFs tend to appear in SNe Ia with smaller decline rates (e.g., dm15(B)<1.4 mag), clarifying that the finding by Childress et al. (2014) for the Ca-HVFs in near-maximum-light spectra applies both to the Si-HVFs and Ca-HVFs in the earlier phase. The Si-HVFs seem to be more common in fast-expanding SNe Ia, which is different from the earlier result that the Ca-HVFs are associated with SNe Ia having slower SiII 6355 velocities at maximum light (i.e., Vsi). This difference can be due to that the HVFs in fast-expanding SNe Ia usually disappear more rapidly and are easily blended with the photospheric components when approaching the maximum light. Moreover, SNe Ia with both stronger HVFs at early phases and larger Vsi are found to have noticeably redder B-V colors and occur preferentially in the inner regions of their host galaxies, while those with stronger HVFs but smaller Vsi show opposite tendencies, suggesting that these two subclasses have different explosion environments and their HVFs may have different origins. We further examine the relationships between the absorption features of SiII 6355 and CaII IR lines, and find that their photospheric components are well correlated in velocity and strength but the corresponding HVFs show larger scatter. These results cannot be explained with ionization and/or thermal processes alone, and different mechanisms are required for the creation of HVF-forming region in SNe Ia.

Statistical connections between the properties of Type Ia supernovae (SNe Ia) and the B--V colors of their parent galaxies are established. Compared to SNe Ia in blue galaxies [B-V$\la$0.75], SNe Ia in redder galaxies have (1) a wider dispersion in the blueshifts of their Si II $\lambda$6355 absorption features, ten days after maximum light; (2) more rapidly declining light curves; and (3) lower luminosities. Even when the spectroscopically peculiar, very subluminous SNe Ia such as SN 1991bg are disregarded, SNe Ia in red galaxies are less luminous than those in blue galaxies by about 0.3 magnitudes. When SNe Ia that are thought to have been significantly extinguished by dust in their parent galaxies are disregarded, those in blue galaxies have observational absolute--magnitude dispersions of only $\sigma_{obs}(M_B)$=0.20 and $\sigma_{obs}(M_V)$=0.17, which implies that their intrinsic absolute--magnitude dispersions are very small. We use six SNe Ia whose absolute magnitudes have been calibrated by means of Cepheids, which also indicate that the intrinsic absolute--magnitude dispersions of SNe Ia in blue galaxies are very small, to calibrate SNe Ia in blue galaxies and obtain $\rm H_0=57\pm4\ km\ s^{-1}\ Mpc^{-1}$. This value is in excellent agreement with that obtained by Saha et al. (1995b), in spite of the fact that they do not take into account any dependence of SN Ia absolute magnitude on the nature of the parent galaxy. Some implications of the statistical connections between SNe Ia and the colors of their parent galaxies, for identifying the progenitor binary systems of SNe Ia and for using high--redshift SNe Ia to measure q$_0$, are briefly discussed.

Aims. We aim to present 70 spectra of 68 new high-redshift type Ia supernovae (SNe Ia) measured at ESO’s VLT during the final two years of operation (2006–2008) of the Supernova Legacy Survey (SNLS). This new sample complements the VLT three year spectral set. Altogether, these two data sets form the five year sample of SNLS SN Ia spectra measured at the VLT on which the final SNLS cosmological analysis will partly be based. In the redshift range considered, this sample is unique in terms of homogeneity and number of spectra. We use it to investigate the possibility of a spectral evolution of SNe Ia populations with redshift as well as SNe Ia spectral properties as a function of lightcurve fit parameters and the mass of the host-galaxy. Methods. Reduction and extraction are based on both IRAF standard tasks and our own reduction pipeline. Redshifts are estimated from host-galaxy lines whenever possible or alternatively from supernova features. We used the spectro-photometric SN Ia model SALT2 combined with a set of galaxy templates that model the host-galaxy contamination to assess the type Ia nature of the candidates. Results. We identify 68 new SNe Ia with redshift ranging from z = 0.207 to z = 0.98 for an average redshift of z = 0.62. Each spectrum is presented individually along with its best-fit SALT2 model. Adding this new sample to the three year VLT sample of SNLS, the final dataset contains 209 spectra corresponding to 192 SNe Ia identified at the VLT. We also publish the redshifts of other candidates (host galaxies or other transients) whose spectra were obtained at the same time as the spectra of live SNe Ia. This list provides a new redshift catalog useful for upcoming galaxy surveys. Using the full VLT SNe Ia sample, we build composite spectra around maximum light with cuts in color, the lightcurve shape parameter (“stretch”), host-galaxy mass and redshift. We find that high-z SNe Ia are bluer, brighter and have weaker intermediate mass element absorption lines than their low-z counterparts at a level consistent with what is expected from selection effects. We also find a flux excess in the range [3000–3400] Å for SNe Ia in low mass host-galaxies (M &lt; 1010M⊙) or with locally blue U–V colors, and suggest that the UV flux (or local color) may be used in future cosmological studies as a third standardization parameter in addition to stretch and color.

view Abstract Citations (59) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry of Three Types 1a Supernovae: V,R, and I Light Curves Ford, Charles H. ; Herbst, William ; Richmond, Michael W. ; Baker, Michael L. ; Filippenko, Alexei V. ; Treffers, Richard R. ; Paik, Young ; Benson, Priscilla J. Abstract Cousins system VRI photometry has been obtained for the Type Ia supernova 1992G with CCDs attached to small telescopes at Leuschner Observatory (LO), Van Vleck Observatory (VVO), and Whitin Observatory (WO). Similar equipment, observing and reduction procedures have been adopted at all three sites and there is generally good agreement among the data sets. Two other Type Ia supernovae, 1991M and 1991T, were observed only at VVO. The shapes of the light curves of all three supernovae, including the spectroscopically peculiar SN 1991, are quite similar, and the combined light curve in V matches Leibundgut's [Ph.D. thesis, University of Basel (1988)] template reasonably well. Since very little R and I photometry of supernovae has been done previously, our data provide the first real definition of SN Ia templates in those bands. The I template is of particular interest since it shows a secondary maximum 23+/-1 days after the primary peak, similar to what is seen in normal Type Ia supernovae at J, H, and K wavelengths. The behavior in R is intermediate between V and I. The existence of substantial structure in the R and I light curves of normal Type Ia supernovae (and its absence in the peculiar Type Ia SN 1991bg) enhances their utility as cosmological probes and provides an observational challenge to models of the event. The observed colors of SN 1991M and SN 1991T as a function of time are quite similar, indicating that they suffer about the same amount of reddening. SN 1992G is clearly more heavily reddened than these two, by about 0.1 mag in R -I. The intrinsic colors of these Type Ia supernovae at V_max_ are V-R =0.0 and R -I= -0.3, with an uncertainty of around 0.05 mag, if the reddening of SN 1991T is E(B-V) = 0.13. If Type Ia supernovae are assumed to be good standard candles, then NGC 4527, the parent galaxy of SN 1991T, must have a peculiar velocity of 1200+/-150 km s^-1^ relative to the Hubble flow and directed away from the Galaxy. If the peculiar velocity of NGC 4527 is less than this, then SN 1991T must be an overluminous SN Ia. Tully & Shaya (1984) regard NGC 4527 as a near-side member of the Virgo cluster falling in at approximately 1000 km s^-1^. Our data alone yield a Hubble constant of 75+/-15 km s^-1^ Mpc^-1^ if M_V_ at maximum light is -18.7 mag, and 50+/-10 km s^-1^ Mpc^-1^ if it is -19.6 mag. Publication: The Astronomical Journal Pub Date: September 1993 DOI: 10.1086/116708 Bibcode: 1993AJ....106.1101F Keywords: Astronomical Photometry; Charge Coupled Devices; Light Curve; Supernovae; Astronomical Observatories; Interstellar Extinction; Peculiar Stars; Stellar Color; Stellar Magnitude; Astrophysics; SUPERNOVAE: INDIVIDUAL: 1992G; SUPERNOVAE: INDIVIDUAL: 1992M; SUPERNOVAE: INDIVIDUAL: 1992T full text sources ADS | data products SIMBAD (10) NED (6)

We study the spectral diversity of Type Ia supernovae (SNe Ia) at maximum light using high signal-to-noise spectrophotometry of 173 SNe Ia from the Nearby Supernova Factory. We decompose the diversity of these spectra into different extrinsic and intrinsic components, and we construct a nonlinear parameterization of the intrinsic diversity of SNe Ia that preserves pairings of “twin” SNe Ia. We call this parameterization the “Twins Embedding.” Our methodology naturally handles highly nonlinear variability in spectra, such as changes in the photosphere expansion velocity, and uses the full spectrum rather than being limited to specific spectral line strengths, ratios, or velocities. We find that the time evolution of SNe Ia near maximum light is remarkably similar, with 84.6% of the variance in common to all SNe Ia. After correcting for brightness and color, the intrinsic variability of SNe Ia is mostly restricted to specific spectral lines, and we find intrinsic dispersions as low as ∼0.02 mag between 6600 and 7200 Å. With a nonlinear three-dimensional model plus one dimension for color, we can explain 89.2% of the intrinsic diversity in our sample of SNe Ia, which includes several different kinds of “peculiar” SNe Ia. A linear model requires seven dimensions to explain a comparable fraction of the intrinsic diversity. We show how a wide range of previously established indicators of diversity in SNe Ia can be recovered from the Twins Embedding. In a companion article, we discuss how these results can be applied to the standardization of SNe Ia for cosmology.

We present the second and final release of optical spectroscopy of Type Ia supernovae (SNe Ia) obtained during the first and second phases of the Carnegie Supernova Project (CSP-I and CSP-II). The newly released data consist of 148 spectra of 30 SNe Ia observed in the course of CSP-I and 234 spectra of 127 SNe Ia obtained during CSP-II. We also present 216 optical spectra of 46 historical SNe Ia, including 53 spectra of 30 SNe Ia observed by the Calán/Tololo Supernova Survey. We combine these observations with previously published CSP data and publicly available spectra to compile a large sample of measurements of spectroscopic parameters at maximum light, consisting of pseudo-equivalent widths and expansion velocities of selected features for 232 CSP and historical SNe Ia (including more than 1000 spectra). Finally, we review some of the strongest correlations between spectroscopic and photometric properties of SNe Ia. Specifically, we define two samples: one consisting of SNe Ia discovered by targeted searches (most of them CSP-I objects) and the other composed of SNe Ia discovered by untargeted searches, which includes most of the CSP-II objects. The analyzed correlations are similar for both samples. We find a larger incidence of SNe Ia belonging to the cool and broad-line Branch subtypes among the events discovered by targeted searches, shallow-silicon SNe Ia are present with similar frequencies in both samples, while core normal SNe Ia are more frequent in untargeted searches.

ABSTRACTWe analyze the standardizability of Type Ia supernovae (SNe Ia) in the near-infrared (NIR) by investigating the correlation between observed peak NIR (YJH) absolute magnitude and postmaximum B-band decline rate [Δm15(B)]. A sample of 27 low-redshift SNe Ia with well-observed NIR light curves observed by the Carnegie Supernova Project (CSP) between 2004 and 2007 is used. All 27 objects have premaximum coverage in optical bands, with a subset of 13 having premaximum NIR observations as well; coverage of the other 14 begins shortly after NIR maximum brightness. We describe the methods used to derive light-curve parameters (absolute peak magnitudes and decline rates) from both spline- and template-fitting procedures, and we confirm prior findings that fitting templates to SNe Ia light curves in the NIR is problematic due to the diversity of postmaximum behavior of objects that are characterized by similar Δm15(B) values, especially at high decline rates. Nevertheless, we show that NIR light curves can be reasonably fit with a template, especially if the observations begin within 5 days after NIR maximum. SNe Ia appear to be better “standardizable candles” in the NIR bands than in the optical bands. For the subset of 13 objects in our data set that excludes the highly reddened and fast-declining SNe Ia and includes only those objects for which NIR observations began prior to 5 days after maximum light, we find modest (1.7σ) evidence for a peak-luminosity versus decline-rate relation in Y, and stronger evidence (2.8σ) in J and H. Using RV values differing from the canonical value (RV = 3.1) is shown to have little effect on the results. A Hubble diagram is presented for the NIR bands and the B band. The resulting scatter for the combined NIR bands is 0.13 mag, while the B band produces a scatter of 0.22 mag. Finally, we find evidence for a bimodal distribution in the NIR absolute magnitudes of fast-declining SNe Ia [Δm15(B) > 1.7]. These data suggest that applying a correction to SNe Ia peak luminosities for decline rate is likely to be beneficial in the J and H bands to make SNe Ia more precise distance indicators, but of only marginal importance in the Y band.

view Abstract Citations (77) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Type IA Supernovae as Standard Candles Branch, David ; Miller, Douglas L. Abstract The distribution of absolute blue magnitudes among Type Ia supernovae (SNs Ia) is studied using supernovae whose apparent magnitudes at maximum light have been well determined and whose parent galaxies have had their relative distances determined by the Tully-Fisher or D_n_ - σ techniques. Most supernovae in the sample appear to have been normal SNs Ia that were essentially unextinguished by dust. These supernovae have a mean absolute blue magnitude M_B_ = -19.72+/-0.06 + 5 log (H_0_/50), with an observational dispersion about the mean of only σ(M_B_) = 0.36, which is comparable to the expected combined errors in distance, apparent magnitude, and extinction. The mean (B - V) color at maximum light is 0.03 +/- 0.04, with a dispersion σ(B - V) = 0.20, which may be due primarily to observational error. The Cepheid-based distance to IC 4182 (Sandage et al. 1992), the parent galaxy of the normal and unextinguished Type Ia SN 1937C, leads to a Hubble constant of H_0_ = 51+/-12 km s^-1^ Mpc^-1^. The sample also includes a few SNs Ia that appear to have been intrinsically normal but reddened and dimmed by dust in their parent galaxies (e.g., SN 1989B), and several having peculiar spectra or light curves which tend to be intrinsically subluminous and red (e.g., SN 199 Ibg). Considering the strong observational selection against subluminous supernovae, and the fact that even when they are discovered they are betrayed by their spectroscopic or photometric peculiarities, their existence does not seriously compromise the use of SNs Ia as distance indicators. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/186752 Bibcode: 1993ApJ...405L...5B Keywords: Blue Stars; Distance; Interstellar Extinction; Standards; Stellar Magnitude; Supernovae; Dispersion; Hubble Constant; Stellar Color; Stellar Spectra; Astrophysics; COSMOLOGY: DISTANCE SCALE; STARS: SUPERNOVAE: GENERAL full text sources ADS | data products SIMBAD (80) NED (80)

This is the first release of optical spectroscopic data of low-redshift Type\nIa supernovae (SNe Ia) by the Carnegie Supernova Project including 604\npreviously unpublished spectra of 93 SNe Ia. The observations cover a range of\nphases from 12 days before to over 150 days after the time of B-band maximum\nlight. With the addition of 228 near-maximum spectra from the literature we\nstudy the diversity among SNe Ia in a quantitative manner. For that purpose,\nspectroscopic parameters are employed such as expansion velocities from\nspectral line blueshifts, and pseudo-equivalent widths (pW). The values of\nthose parameters at maximum light are obtained for 78 objects, thus providing a\ncharacterization of SNe Ia that may help to improve our understanding of the\nproperties of the exploding systems and the thermonuclear flame propagation.\nTwo objects, namely SNe 2005M and 2006is, stand out from the sample by showing\npeculiar Si II and S II velocities but otherwise standard velocities for the\nrest of the ions. We further study the correlations between spectroscopic and\nphotometric parameters such as light-curve decline rate and color. In agreement\nwith previous studies, we find that the pW of Si II absorption features are\nvery good indicators of light-curve decline rate. Furthermore, we demonstrate\nthat parameters such as pW2(SiII4130) and pW6(SiII5972) provide precise\ncalibrations of the peak B-band luminosity with dispersions of ~0.15 mag. In\nthe search for a secondary parameter in the calibration of peak luminosity for\nSNe Ia, we find a ~2--3-sigma correlation between B-band Hubble residuals and\nthe velocity at maximum light of S II and Si II lines.\n

We studied the spectral features of Si ii λλ4130, 5972, 6355 and the S ii W-trough for a large sample of Type Ia supernovae (SNe Ia). We found that in the NV (normal-velocity) subclass of SNe Ia, these features tend to reach a maximum line strength near maximum light, except for Si ii λ5972. Spectral features with higher excitation energy, such as the S ii W-trough, are relatively weak and have a relatively low velocity. SNe Ia with larger Δm15(B) tend to have lower velocities, especially at phases after maximum light. NV SNe show a trend of increasing line strength with increasing Δm15(B), while 91T/99aa-like SNe show an opposite trend. Near maximum light, the absorption depth of Si ii λ5972 shows the strongest correlation with Δm15(B), while at early times the sum of the depths of Si ii λλ4130 and 5972 shows the strongest correlation with Δm15(B). The overall correlation between velocity and line strength is positive, but within NV SNe the correlation is negative or unrelated, depending on the phase. In normal SNe Ia (including the high-velocity SNe), the velocity difference and depth ratio of a longer-wavelength feature to a shorter-wavelength feature tend to increase with increasing Δm15(B). These results are mostly well explained with atomic physics, but some puzzles remain, possibly related to the effects of saturation, line competition or other factors.

We report measurements of proper motion, radial velocity, and elemental composition for 14 compact X-ray-bright knots in Kepler’s supernova remnant (SNR) using archival Chandra data. The knots with the highest speed show both large proper motions (μ ∼ 0.″11–0.″14 yr−1) and high radial velocities (v ∼ 8700–10,020 km s−1). For these knots the estimated space velocities (9100 km s−1 ≲ v 3D ≲ 10,400 km s−1) are similar to the typical Si velocity seen in supernovae (SNe) Ia near maximum light. High-speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The proper motions of five knots extrapolate back over the age of Kepler’s SNR to a consistent central position. This new kinematic center agrees well with previous determinations, but is less subject to systematic errors and denotes a location about which several prominent structures in the remnant display a high degree of symmetry. These five knots are expanding at close to the free expansion rate (expansion indices of 0.75 ≲ m ≲ 1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through regions of relatively low density (n H &lt; 0.1 cm−3) in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance, and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for models of SNe Ia. Other knots show lower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock. Our new accurate location for the explosion site has well-defined positional uncertainties, allowing for a great reduction in the area to be searched for faint surviving donor stars under non-traditional single-degenerate SNe Ia scenarios; because of the lack of bright stars in the search area the traditional scenario remains ruled out.

Future time-domain surveys for transient events in the near- and midinfrared bands will significantly extend our understanding about the physics of the early universe. In this paper we study the implications of a deep (∼27 mag), long-term (∼3 yr), observationally inexpensive survey with the James Webb Space Telescope (JWST) within its Continuous Viewing Zone, aimed at discovering luminous supernovae beyond z ∼ 2 redshift. We explore the possibilities for detecting superluminous supernovae (SLSNe) as well as SNe Ia at such high redshifts and estimate their expected numbers within a relatively small (∼0.1 deg2) survey area. It is found that we can expect ∼10 new SLSNe and ∼50 SNe Ia discovered in the 1 &lt; z &lt; 4 redshift range. We show that it is possible to get relatively accurate (σ z ≲ 0.25) photometric redshifts for SNe Ia by fitting their Spectral Energy Distributions, redshifted into the observed near-IR bands, with SN templates. We propose that SNe Ia occupy a relatively narrow range on the JWST F220W−F440W versus F150W−F356W color–color diagram between ±7 rest-frame days around maximum light, which could be a useful classification tool for such types of transients. We also study the possibility of extending the Hubble-diagram of SNe Ia beyond redshift 2 up to z ∼ 4. Such high-z SNe Ia may provide new observational constraints for their progenitor scenario.

Context.Type Ia supernovae (SNe Ia) are widely used to measure the expansion of the Universe. Improving distance measurements of SNe Ia is one technique to better constrain the acceleration of expansion and determine its physical nature.Aims.This document develops a new SNe Ia spectral energy distribution (SED) model, called the SUpernova Generator And Reconstructor (SUGAR), which improves the spectral description of SNe Ia, and consequently could improve the distance measurements.Methods.This model was constructed from SNe Ia spectral properties and spectrophotometric data from the Nearby Supernova Factory collaboration. In a first step, a principal component analysis-like method was used on spectral features measured at maximum light, which allowed us to extract the intrinsic properties of SNe Ia. Next, the intrinsic properties were used to extract the average extinction curve. Third, an interpolation using Gaussian processes facilitated using data taken at different epochs during the lifetime of an SN Ia and then projecting the data on a fixed time grid. Finally, the three steps were combined to build the SED model as a function of time and wavelength. This is the SUGAR model.Results.The main advancement in SUGAR is the addition of two additional parameters to characterize SNe Ia variability. The first is tied to the properties of SNe Ia ejecta velocity and the second correlates with their calcium lines. The addition of these parameters, as well as the high quality of the Nearby Supernova Factory data, makes SUGAR an accurate and efficient model for describing the spectra of normal SNe Ia as they brighten and fade.Conclusions.The performance of this model makes it an excellent SED model for experiments like the Zwicky Transient Facility, the Large Synoptic Survey Telescope, or the Wide Field Infrared Survey Telescope.

More than 3000 spectroscopically confirmed Type Ia supernovae (SNe Ia) are presented in the second data release (DR2) of the Zwicky Transient Facility survey. In this paper we detail the spectral properties of 482 SNe Ia near maximum light, up to a redshift limit of z leq 0.06. We measured the velocities and pseudo-equivalent widths (pEW) of key spectral features ( and and investigated the relation between the properties of the spectral features and the photometric properties from the SALT2 light-curve parameters as a function of spectroscopic sub-class. We discuss the non-negligible impact of host galaxy contamination on SN Ia spectral classifications, and we investigate the accuracy of spectral template matching of the DR2 sample. We define a new subclass of underluminous SNe Ia (04gs-like) that lie spectroscopically between normal SNe Ia and transitional 86G-like SNe Ia (stronger than normal SNe Ia, but significantly weaker features than 86G-like SNe). We model these 04gs-like SN Ia spectra using the radiative-transfer spectral synthesis code tardis and show that cooler temperatures alone are unable to explain their spectra; some changes in elemental abundances are also required. However, the broad continuity in spectral properties seen from bright (91T-like) to faint normal SN Ia, including the transitional and 91bg-like SNe Ia, suggests that variations within a single explosion model may be able to explain their behaviour.