Normal Type Ia Supernovae Research Articles

The Type Ia Supernova 2004S, a Clone of SN 2001el, and the Optimal Photometric Bands for Extinction Estimation

We present optical (UBVRI) and near-IR (YJHK) photometry of the normal Type Ia supernova (SN) 2004S. We also present eight optical spectra and one near-IR spectrum of SN 2004S. The light curves and spectra are nearly identical to those of SN 2001el. This is the first time we have seen optical and IR light curves of two Type Ia SNe match so closely. Within the one parameter family of light curves for normal Type Ia SNe, that two objects should have such similar light curves implies that they had identical intrinsic colors and produced similar amounts of 56Ni. From the similarities of the light-curve shapes we obtain a set of extinctions as a function of wavelength that allows a simultaneous solution for the distance modulus difference of the two objects, the difference of the host galaxy extinctions, and RV. Since SN 2001el had roughly an order of magnitude more host galaxy extinction than SN 2004S, the value of RV = 2.15 pertains primarily to dust in the host galaxy of SN 2001el. We have also shown via Monte Carlo simulations that adding rest-frame J-band photometry to the complement of BVRI photometry of Type Ia SNe decreases the uncertainty in the distance modulus by a factor of 2.7. A combination of rest-frame optical and near-IR photometry clearly gives more accurate distances than using rest-frame optical photometry alone.

Late Light Curves of Normal Type Ia Supernovae

We present late-epoch optical photometry (BVRI) of seven normally luminous and superluminous Type Ia supernovae: SN 2000E, SN 2000ce, SN 2000cx, SN 2001C, SN 2001V, SN 2001bg, and SN 2001dp. The photometry of these objects was obtained using a template subtraction method to eliminate galaxy light contamination during aperture photometry. We show the optical light curves of these supernovae out to epochs of up to ~640 days after the explosion of the supernova. We show a linear decline in these data during the epoch of 200-500 days after explosion, with the decline rate in the B, V, and R bands equal to about 1.4 mag per 100 days, but the decline rate of the I-band is much shallower at 0.94 mag per 100 days.

Optical and Near-Infrared Observations of the Peculiar Type ia Supernova 1999ac

We present 39 nights of optical photometry, 34 nights of IR photometry, and 4 nights of optical spectroscopy of the Type Ia supernova 1999ac. This supernova was discovered 2 weeks before maximum light, and observations were begun shortly thereafter. At early times its spectra resembled the unusual SN 1999aa and were characterized by very high velocities in the Ca II H and K lines but very low velocities in the Si II λ6355 line. The optical photometry showed a slow rise to peak brightness but, quite peculiarly, was followed by a more rapid decline from maximum. Thus, the B- and V-band light curves cannot be characterized by a single stretch factor. We argue that the best measure of the nature of this object is not the decline rate parameter Δm15(B). The B - V colors were unusual from 30 to 90 days after maximum light in that they evolved to bluer values at a much slower rate than normal Type Ia supernovae. The spectra and bolometric light curve indicate that this event was similar to the spectroscopically peculiar slow decliner SN 1999aa.

Beyond the Bubble Catastrophe of Type Ia Supernovae: Pulsating Reverse Detonation Models

We describe a mechanism by which a failed deflagration of a Chandrasekhar-mass carbon-oxygen white dwarf can turn into a successful thermonuclear supernova explosion, without invoking an ad hoc high-density deflagration-detonation transition. Following a pulsating phase, an accretion shock develops above a core of 1 M_sun composed of carbon and oxygen, inducing a converging detonation. A three-dimensional simulation of the explosion produced a kinetic energy of 1.05E51 ergs and 0.70 M_sun of 56Ni, ejecting scarcely 0.01 M_sun of C-O moving at low velocities. The mechanism works under quite general conditions and is flexible enough to account for the diversity of normal Type Ia supernovae. In given conditions the detonation might not occur, which would reflect in peculiar signatures in the gamma and UV-wavelengths

Low Carbon Abundance in Type Ia Supernovae

We investigate the quantity and composition of unburned material in the outer layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr, and 2004bw. Pristine matter from a white dwarf progenitor is expected to be a mixture of oxygen and carbon in approximately equal abundance. Using near-infrared (NIR, 0.7-2.5 μm) spectra, we find that oxygen is abundant, while carbon is severely depleted with low upper limits in the outer third of the ejected mass. Strong features from the O I line at λrest = 0.7773 μm are observed through a wide range of expansion velocities ≈ × 103 km s-1. This large velocity domain corresponds to a physical region of the supernova with a large radial depth. We show that the ionization of C and O will be substantially the same in this region. C I lines in the NIR are expected to be 7-50 times stronger than those from O I, but there is only marginal evidence of C I in the spectra and none of C II. We deduce that for these three normal SNe Ia, oxygen is more abundant than carbon by factors of 102-103. Mg II is also detected in a velocity range similar to that of O I. The presence of O and Mg combined with the absence of C indicates that for these SNe Ia, nuclear burning has reached all but the extreme outer layers; any unburned material must have expansion velocities greater than 18 × 103 km s-1. This result favors deflagration to detonation transition (DD) models over pure deflagration models for SNe Ia.

SN 2005cg: Explosion Physics and Circumstellar Interaction of a Normal Type Ia Supernova in a Low‐Luminosity Host

We present the spectral evolution, light curve, and corresponding interpretation for the normal-bright Type Ia Supernova 2005cg discovered by ROTSE-IIIc. The host is a low-luminosity (M_r = -16.75), blue galaxy with strong indications of active star formation and an environment similar to that expected for SNe Ia at high redshifts. Early-time (t ~ -10 days) optical spectra obtained with the HET reveal an asymmetric, triangular-shaped Si II absorption feature at about 6100 \AA with a sharp transition to the continuum at a blue shift of about 24,000 km s^-1. By 4 days before maximum, the Si II absorption feature becomes symmetric with smoothly curved sides. Similar Si II profile evolution has previously been observed in other supernovae, and is predicted by some explosion models, but its significance has not been fully recognized. Although the spectra predicted by pure deflagration and delayed detonation models are similar near maximum light, they predict qualitatively different chemical abundances in the outer layers and thus give qualitatively different spectra at the earliest phases. The Si line observed in SN 2005cg at early times requires the presence of burning products at high velocities and the triangular shape is likely to be formed in an extended region of slowly declining Si abundance that characterizes delayed detonation models. The spectra show a high-velocity Ca II IR feature that coincides in velocity space with the Si II cutoff. This supports the interpretation that the Ca II is formed when the outer layers of the SN ejecta sweep up about 5 x 10^-3 M_sun of material within the progenitor system. (Abridged)

Early and late time VLT spectroscopy of SN 2001el - progenitor constraints for a type Ia supernova

We present early time high-resolution (VLT/UVES) and late time low-resolution (VLT/FORS) optical spectra of the normal type Ia supernova, SN 2001el. The high-resolution spectra were obtained at -9 and -2 days to allow the detection of narrow hydrogen and/or helium emission lines from the circumstellar medium of the SN. No such lines were detected, and we therefore use photoionisation models to derive upper limits of 9x10^-6 Msun/yr and 5x10^-5 Msun/yr for the mass loss rate from the progenitor system assuming velocities of 10 km/s and 50 km/s, respectively, for a wind extending to outside at least a few x 10^15 cm away from the SN explosion site. These limits exclude a symbiotic star in the upper mass loss rate regime from being the progenitor of SN 2001el. The low resolution spectrum was obtained in the nebular phase of the SN, 400 days after the maximum light, to search for any hydrogen rich gas originating from the SN progenitor system. However, we see no signs of Balmer lines in our spectrum. Therefore, we model the late time spectra to derive an upper limit of ~0.03 Msun for solar abundance material present at velocities lower than 1000 km/s within the SN explosion site. According to simulations of Marietta et al. (2000) this is less than the expected mass lost by a subgiant, red giant or main sequence secondary star at a small binary separation as a result of the SN explosion. Finally, we discuss the origin of high velocity Ca II lines. We see both the CaII IR triplet and the H&K lines in the -9 days spectrum at a very high velocity of up to 34000 km/s. The spectrum also shows a flat-bottomed Si II `6150 A' feature similar to the one previously observed in SN 1990N at -14 days. We compare these spectral features to those observed in SNe 1984A and 1990N at even higher velocities.

Supernovae from White Dwarfs Near Black Holes

We describe a new thermonuclear explosion model for Type I (or Type II) supernovae whereby relativistic terms enhance the self gravity of a carbon-oxygen white dwarf (or red-giant core) as it passes or orbits near a black hole. This relativistic compression can cause the central density to exceed the threshold for pycnonuclear or thermonuclear reactions so that an explosion ensues. We have considered three possible environments: 1) white dwarfs orbiting a low-mass (∼ 10 – 20 M ⊙ ) black hole; 2) white dwarfs encountering a massive (∼ 1 – 3 × 10 3 M ⊙ ) black hole in a dense globular cluster; and 3) white dwarfs passing a supermassive (∼ 10 6 – 10 9 M ⊙ ) black hole in a dense galactic core. We estimate the rate at which such events could occur to be significantly less than the rate of normal Type Ia supernovae for all three classes. Nevertheless, they should be frequent enough to warrant a search for this new class of supernova. We show results of three-dimensional thermonuclear burn calculations of white dwarfs or red-giant cores ignited near a supermassive black hole. Such an event might have produced the observed mixed-morphology Sgr A East supernova remnant (SNR) in the Galactic core.

Modelling Turbulent Deflagrations in Type Ia Supernovae

We present an overview of the current state of multidimensional modelling of type Ia supernovae and an example for surprising consequences of impoving the physics of the model. In this case, an improved handling of subgrid scale turbulence gives rise to lower burning rates and a decreased global energy release. While this result is too preliminary to be interpreted quantitatively, it shows that much work remains to be done before the turbulent deflagration model can be declared to be understood and/or insufficient to explain normal type Ia supernovae.

Abundance stratification in Type Ia supernovae - I. The case of SN 2002bo

The abundance stratification in the ejecta of the normal Type Ia supernova 2002bo is derived by fitting a series of spectra obtained at close time intervals. A Monte Carlo code, modified to include abundance stratification, is used to compute synthetic spectra at 13 epochs in the photospheric phase, starting 13 d before B maximum. A description of the abundance distribution above 7600 km s −1 is thus obtained. Abundances in deeper layers, down to zero velocity, are derived from models of two nebular-phase spectra. Elements synthesized in different stages of burning are significantly, but not completely, mixed in the ejecta. A total 56 Ni mass of 0.52 M� is derived. Evidence for intermediate-mass elements at high velocities (18 000 km s −1 )i s found, most clearly in Si II 6355 A, in Ca II H&K ,and in the Ca II infrared triplet. Carbon lines are not seen at any velocity, with possible implications for the progenitor/explosion scenario. A synthetic bolometric light curve computed using the inferred abundance distribution is in very good agreement with the observed one, providing an independent check. In particular, the fast rise of the light curve is reproduced very well. This is due to outward mixing of 56 Ni, which is clearly well determined by the spectroscopic modelling. Ke yw ords: supernovae: general ‐ supernovae: individual: SN 2002bo.

Spectroscopy of the type Ia supernova SN 2002er: Days –11 to +215

We present an extensive set of optical spectroscopy of the nearby type Ia supernova, SN 2002er, with 24 epochs spanning -11 to +34 days. Its spectral evolution is fairly typical of a type Ia supernova although it suffers high extinction. Nevertheless, there are differences in the spectral evolution when compared to coeval spectra of other normal type Ia supernova with comparable decline-rate parameters. Modelling of the photospheric phase spectra using a homogeneous abundance distribution in the atmosphere provides a fair match to the observations, but only by pushing the adopted distance and risetime close to the observational limits. Future improvements here will require models with a more realistic stratified abundance distribution. From simple modelling of a nebular spectrum obtained at +215 d, we infer a 56Ni mass of 0.69 , consistent with that derived from the light curve.

Type Ia supernova SN 2003du: Optical observations

UBVRI photometry and optical spectra of type Ia supernova SN 2003du obtained at the Indian Astronomical Observatory for nearly a year since discovery are presented. The apparent magnitude at maximum was B=13.53 +/- 0.02 mag, and the colour (B-V) = -0.08 +/- 0.03 mag. The luminosity decline rate, Delta(m_{15}(B)) = 1.04 +/- 0.04 mag indicates an absolute B magnitude at maximum of M_B = -19.34 +/- 0.3 mag and the distance modulus to the parent galaxy as mu=32.89 +/- 0.4.The light curve shapes are similar, though not identical, to those of SNe 1998bu and 1990N, both of which had luminosity decline rates similar to that of SN 2003du and occurred in spiral galaxies. The peak bolometric luminosity indicates that 0.9 Msun mass of 56Ni was ejected by the supernova. The spectral evolution and the evolution of the Si II and Ca II absorption velocities closely follows that of SN 1998bu, and in general, is within the scatter of the velocities observed in normal type Ia supernovae. The spectroscopic and photometric behaviour of SN 2003du is quite typical for SNe Ia in spirals. A high velocity absorption component in the Ca II (H & K) and IR-triplet features, with absorption velocities of ~20,000 km/s and ~22,000 km/s respectively, is detected in the pre-maximum spectra of days -11 and -7.

White Dwarfs near Black Holes: A New Paradigm for Type I Supernovae

We present calculations indicating the possibility of a new class of Type I supernovae. In this new paradigm relativistic terms enhance the self gravity of a carbon-oxygen white dwarf as it passes or orbits near a black hole. This relativistic compression can cause the central density to exceed the threshold for pycnonuclear reactions so that athermonuclear runaway ensues. We consider three possible environments: 1) white dwarfs orbiting a low-mass black hole; 2) white dwarfs encountering a massive black hole in a dense globular cluster; and 3) white dwarfs passing a supermassive black hole in a dense galactic core. We estimate the rate at which such events could occur out to a redshift of z = 1. Event rates are estimated to be significantly less than the rate of normal Type Ia supernovae for all three classes. Nevertheless, such events may be frequent enough to warrant a search for this new class of supernova. We propose several observable signatures which might be used to identify this type of event and speculate that such an event might have produced the observed mixed-morphology Sgr A East supernova remnant in the Galactic core.

Supernova 2002bo: inadequacy of the single parameter description

We present optical/near-infrared photometry and spectra of the type Ia SN 2002bo spanning epochs from -13 days before maximum B-band light to +102 days after. The pre-maximum optical coverage is particularly complete. In some respects, SN 2002bo behaves as a typical Branch type Ia supernova (SN Ia) at optical and IR wavelengths. We find a B-band risetime of 17.9+-0.5 days, a Dm_{15}(B) of 1.13+-0.05, and a M_B=-19.41+-0.42. However, comparison with other type Ia supernovae having similar Delta m_{15}(B) values indicates that in other respects SN 2002bo is unusual. While the optical spectra of SN 2002bo are very similar to those of SN 1984A, lower velocities and a generally more structured appearance are found in SNe 1990N, 1994D and 1998bu. For supernovae having Dm_(15)(B) > 1.2, we confirm the variation of R(SiII) (Nugent et al. 1995) with Dm_(15)(B). However, for supernovae such as SN2002bo, with lower values of Dm_(15)(B) the relation breaks down. Moreover, the evolution of R(SiII) for SN 2002bo is strikingly different from that shown by other type Ia supernovae. The velocities of SN 2002bo and 1984A derived from SII 5640A, SiII 6355A and CaII H&K lines are either much higher and/or evolve differently from those seen in other normal SNe Ia events. We suggest that the unusually low temperature, the presence of high-velocity intermediate-mass elements and the low abundance of carbon at early times indicates that burning to Si penetrated to much higher layers than in more normal type Ia supernovae. This may be indicative of a delayed-detonation explosion.

Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High‐Velocity Ejecta

SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic polarization signal. In addition, during the epochs prior to maximum light, the Ca II IR triplet absorption is seen distinctly and separately at both normal photospheric velocities and at very high velocities. The high-velocity triplet absorption is highly polarized, with a different polarization angle than the rest of the spectrum. The unique observation allows us to construct a relatively detailed picture of the layered geometrical structure of the supernova ejecta: in our interpretation, the ejecta layers near the photosphere (v ? 10,000 km s-1) obey a nearly axial symmetry, while a detached, high-velocity structure (v ? 18,000-25,000 km s-1) with high Ca II line opacity deviates from the photospheric axisymmetry. By partially obscuring the underlying photosphere, the high-velocity structure causes a more incomplete cancellation of the polarization of the photospheric light and so gives rise to the polarization peak and rotated polarization angle of the high-velocity IR triplet feature. In an effort to constrain the ejecta geometry, we develop a technique for calculating three-dimensional synthetic polarization spectra and use it to generate polarization profiles for several parameterized configurations. In particular, we examine the case in which the inner ejecta layers are ellipsoidal and the outer, high-velocity structure is one of four possibilities: a spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The synthetic spectra rule out the spherical shell model, disfavor a toroid, and find a best fit with the clumped shell. We show further that different geometries can be more clearly discriminated if observations are obtained from several different lines of sight. Thus, assuming that the high-velocity structure observed for SN 2001el is a consistent feature of at least a known subset of Type Ia supernovae, future observations and analyses such as these may allow one to put strong constraints on the ejecta geometry and hence on supernova progenitors and explosion mechanisms.

A Comparative Study of the Absolute Magnitude Distributions of Supernovae

The Asiago Supernova Catalog is used to carry out a comparative study of supernova absolute-magnitude distributions. An overview of the absolute magnitudes of the supernovae in the current observational sample is presented, and the evidence for subluminous and overluminous events is examined. The fraction of supernovae that are underluminous (M_B > -15) appears to be higher (perhaps much higher) than one fifth but it remains very uncertain. The fraction that are overluminous (M_B < -20) is lower (probably much lower) than 0.01. The absolute-magnitude distributions for each supernova type, restricted to events within 1 Gpc, are compared. Although these distributions are affected by observational bias in favor of the more luminous events, they are useful for comparative studies. We find mean absolute blue magnitudes (for H_0=60) of -19.46 for normal Type Ia supernovae (SNe Ia), -18.04 for SNe Ibc, -17.61 and -20.26 for normal and bright SNe Ibc considered separately, -18.03 for SNe II-L, -17.56 and -19.27 for normal and bright SNe II-L considered separately, -17.00 for SNe II-P, and -19.15 for SNe IIn.

Can Differences in the Nickel Abundance in Chandrasekhar‐Mass Models Explain the Relation between the Brightness and Decline Rate of Normal Type Ia Supernovae?

The use of Type Ia supernovae as distance indicators relies on the determination of their brightness. This is not constant, but it can be calibrated using an observed relation between the brightness and the properties of the optical light curve (decline rate, width, shape), which indicates that brighter SNe have broader, slower light curves. However, the physical basis for this relation is not yet fully understood. Among possible causes are different masses of the progenitor white dwarfs or different opacities in Chandrasekhar-mass explosions. We parameterize Chandrasekhar-mass models, which synthesize different amounts of 56Ni, and compute bolometric light curves and spectra at various epochs. Since opacity in SNe Ia is mostly a result of spectral lines, it should depend on the mass of Fe-peak elements synthesized in the explosion, and on the temperature in the ejecta. Bolometric light curves computed using these prescriptions for the optical opacity reproduce the relation between brightness and decline rate. Furthermore, when spectra are calculated, the change in color between maximum and 2 weeks later allows the observed relation between MB(max) and Δm15(B) to be reproduced quite nicely. Spectra computed at various epochs compare well with corresponding spectra of spectroscopically normal SNe Ia selected to cover a similar range of Δm15(B) values.

RR Lyrae Stars and Type Ia Supernovae: Discovery and Calibration of Astronomical Standard Candles

Accurate apparent and intrinsic luminosities of objects are very important in astronomy, for without them we cannot calculate accurate distances or scale the energy budget of physical models. This dissertation deals with observations of two of the most widely used astronomical standard (or standardizable) candles. The Sloan Digital Sky Survey (SDSS) is an important survey presently being carried out, which will provide photometry of roughly 10 objects over a 10,000 square degree area of sky (D. G. York et al. 2000, AJ, 120, 1579). One million spectra of galaxies will be obtained, and many high-redshift quasars will be discovered (see, e.g., X. Fan et al. 1999, AJ, 118, 1). Many new candidate variable stars in our Galaxy will also be identified, amongst them RR Lyrae stars (roughly one per square degree). Distances and velocities of a large number of such objects will allow us to refine models of the Galaxy’s halo and to measure the Galaxy’s mass. We observed a number of types of objects of interest in SDSS filters (RR Lyrae stars, Cepheids, cataclysmic variables, carbon stars, and asteroids) and showed to what extent these objects can be identified solely on the basis of their SDSS colors. Details can be found in a paper already published in this journal (K. Krisciunas, B. Margon, & P. Szkody 1998, PASP, 110, 1342). In particular, Figure 12 of that paper shows the color locus of bona fide RR Lyrae stars. We have observed one dozen RR Lyrae candidates identified from SDSS commissioning data and proved beyond a shadow of a doubt that all are RR Lyrae stars. Light curves of six candidates identified by B. Willman and S. Anderson are to be found in Z. Ivezic et al. (2000, AJ, 120, 963). Five of the other confirmed RR Lyrae stars are objects 5, 9, 134, 141, and 144 from Table 2 of Ivezic et al. Figure 1 shows the folded light curve of star 144. Owing to their luminosities ( at maximum), Type M ≈ 19.4 V Ia supernovae (SNe) are the most important standardizable candle used in extragalactic astronomy because they can be seen halfway across the visible universe. We use the word “standardizable” because, unlike RR Lyrae stars of identical metallicity, Type Ia SNe do not all have the same intrinsic luminosity. Those which rise faster and subsequently decline faster are intrinsically fainter at maximum compared to those which rise more slowly and decline more slowly. See M. M. Phillips et al. (1999, AJ, 118, 1766). As to an explosion mechanism, the favorite scenario is the deflagration of a carbonoxygen white dwarf which reaches the Chandrasekhar limit due to mass infall from a companion star (M. Livio 2000, preprint, astro-ph/0005344). Deflagration is a type of rapid, but subsonic, combustion in which the flame front is driven by turbulent heat exchange. It is also possible that some Type Ia SNe are due to a sub–Chandrasekhar-mass detonation of an accreted surface helium layer on a C-O white dwarf (P. Pinto, R. G. Eastman, & T. Rogers 2000, preprint, astro-ph/0008330). One of the potentially significant sources of error in determining the distance to a Type Ia SN is the contribution of extinction by dust in the host galaxy. What we need is a “wellbehaved” color index with the largest possible wavelength baseline. Since V) for standard Galactic dust, the A ≈ 3.1 # E(B V uncertainties in the color excess and the scale factor can lead to significant uncertainties in the extinction correction. J. H. Elias et al. (1985, ApJ, 296, 379) already showed that Type Ia SNe exhibit some degree of uniform V K color evolution, while B H photometry such as that of B. Leibundgut (1988, Ph.D. dissertation, Univ. Basel) is not as well behaved. Since K), the potential for A ≈ 1.21 # E(V H) ≈ 1.13 # E(V V smaller uncertainties for using V minus near-infrared colors AV was encouraging. New optical and near-infrared (JHK) photometry of Type Ia SNe, especially data obtained before B-band maximum, was needed. Y. D. Mayya et al. (1998, IAU Circ. 6907), S. Jha et al. (1999, ApJS, 125, 73), and P. Meikle (2000, MNRAS, 314, 782) have published data for the spectroscopically normal SN 1998bu, the distance of the host of which has also been measured with HST using Cepheids. We have obtained optical and infrared data for the spectroscopically normal Type Ia SNe 1999cl, 1999cp, 2000bk, and 2000ce and for the overluminous, slowly declining, spectroscopically peculiar SNe 1999aa, 1999gp, and 2000cx. Data for three of these have already been published (K. Krisciunas et al. 2000, ApJ, 539, 658).

The Type Ia Supernova 1998bu in M96 and the Hubble Constant

We present optical and near-infrared photometry and spectroscopy of the type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8 +/- 0.8) with B = 12.22 +/- 0.03 and V = 11.88 +/- 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of A_V = 0.94 +/- 0.15 mag, and indicates the supernova was of average luminosity compared to other normal type Ia supernovae. Using the HST Cepheid distance modulus to M96 (Tanvir et al. 1995) and the MLCS fit parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, M_V = -19.42 +/- 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. (1999). Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H_0 = 64^{+8}_{-6} km/s/Mpc, where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.

The Type I[CLC]a[/CLC] Supernova 1997[CLC]br[/CLC] in ESO 576-G40

The peculiar type Ia supernova SN 1997br in ESO 576-G40 was extensively observed at Beijing Astronomical Observatory and Lick Observatory. In this paper, we present and discuss the BVRI photometry and the spectra collected over 3 months, beginning 9 days before maximum brightness. The light curves of SN 1997br are similar to those of SN 1991T, with slow decline rates after the B maximum. Well-sampled data before the B maximum show unambiguously that SN 1997br rises more slowly and has a wider peak than normal type Ia supernovae. The optical color evolution of SN 1997br is also similar to that of SN 1991T. We estimate the extinction of SN 1997br to be E(B-V) = 0.35 ± 0.10 mag by comparing its BVRI light curves with those of SN 1991T and by measuring the equivalent width of interstellar Na I D absorption lines. We have conducted a thorough comparison of the spectroscopic evolution of SN 1997br, SN 1991T, and SN 1994D. Although SN 1997br is generally very similar to SN 1991T, it shows some interesting differences at various epochs. Spectra of SN 1997br seem to indicate an earlier transition to the dominant phase of Fe-peak elements after the B maximum. Si II lines in SN 1997br show a very short duration after the B maximum. We discuss the implications of our observations of SN 1997br for models of type Ia supernovae. Specifically, we suggest that some SNe Ia may result from decelerated detonations of white dwarfs.