Normal SNe Research Articles

First Stars – Type Ib Supernovae Connection

AbstractThe very peculiar abundance patterns observed in extremely metal-poor (EMP) stars can not be explained by conventional normal supernova nucleosynthesis but can be well-reproduced by nucleosynthesis in hyper-energetic and hyper-aspherical explosions, i.e., Hypernovae (HNe). Previously, such HNe have been observed only as Type Ic supernovae. Here, we examine the properties of recent Type Ib supernovae (SNe Ib). In particular, SN Ib 2008D associated with the luminous X-ray transient 080109 is found to be a more energetic explosion than normal core-collapse supernovae. We estimate that the progenitor's main sequence mass isMMS= 20 − 25M⊙with an explosion of kinetic energy ofEK~ 6.0 × 1051erg. These properties are intermediate between those of normal SNe and hypernovae associated with gamma-ray bursts. Therefore, such energetic SNe Ib could also make an important contribution to the chemical enrichment in the early Universe.

Detailed Spectral Modeling of a Three‐dimensional Pulsating Reverse Detonation Model: Too Much Nickel

We calculate detailed non-LTE synthetic spectra of a pulsating reverse detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae. While the hydro models are calculated in three dimensions, the spectra use an angle-averaged hydro model and thus some of the three-dimensional (3D) details are lost, but the overall average should be a good representation of the average observed spectra. We study the model at three epochs: maximum light, 7 days prior to maximum light, and 5 days after maximum light. At maximum the defining Si II feature is prominent, but there is also a prominent C II feature, not usually observed in normal SNe Ia near maximum. We compare to the early spectrum of SN 2006D, which did show a prominent C II feature, but the fit to the observations is not compelling. Finally, we compare to the postmaximum UV+optical spectrum of SN 1992A. With the broad spectral coverage it is clear that the iron-peak elements on the outside of the model push too much flux to the red and thus the particular PRD realizations studied would be intrinsically far redder than observed SNe Ia. We briefly discuss variations that could improve future PRD models.

A Three‐Dimensional Deflagration Model for Type Ia Supernovae Compared with Observations

A simulation of the thermonuclear explosion of a Chandrasekhar-mass C+O white dwarf, the most popular scenario of a Type Ia supernova (SN Ia), is presented. The underlying modeling is pursued in a self-consistent way, treating the combustion wave as a turbulent deflagration using well tested methods developed for laboratory combustion and based on the concept of large-eddy (LESs). Such consistency requires to capture the onset of the turbulent cascade on resolved scales. This is achieved by computing the dynamical evolution on a 10243 moving grid, which resulted in the best-resolved three-dimensional SN Ia simulation carried out thus far, reaching the limits of what can be done on present supercomputers. Consequently, the model has no free parameters other than the initial conditions at the onset of the explosion, and therefore it has considerable predictive power. Our main objective is to determine to which extent such a simulation can account for the observations of normal SNe Ia. Guided by previous simulations with less resolution and a less sophisticated flame model, initial conditions were chosen that yield a reasonably strong explosion and a sufficient amount of radioactive nickel for a bright display. We show that observables are indeed matched to a reasonable degree. In particular, good agreement is found with the light curves of normal SNe Ia. Moreover, the model reproduces the general features of the abundance stratification as inferred from the analysis of spectra. This indicates that it captures the main features of the explosion mechanism of SNe Ia. However, we also show that even a seemingly best-choice pure deflagration model has shortcomings that indicate the need for a different mode of nuclear burning at late times, perhaps the transition to a detonation at low density.

The Unique Type Ib Supernova 2005bf at Nebular Phases: A Possible Birth Event of a Strongly Magnetized Neutron Star

Late-phase nebular spectra and photometry of Type Ib Supernova (SN) 2005bf taken by the Subaru telescope at ~270 and ~310 days since the explosion are presented. Emission lines ([O I] ??6300, 6363; [Ca II] ??7291, 7324; and [Fe II] ?7155) show a blueshift of ~1500-2000 km s-1. The [O I] doublet shows a doubly peaked profile. The line luminosities can be interpreted as coming from a blob or jet containing only ~0.1-0.4 M?, in which ~0.02-0.06 M? is 56Ni synthesized at the explosion. To explain the blueshift, the blob should either be unipolar, moving at the center-of-mass velocity v ~ 2000-5000 km s-1, or suffer from self-absorption within the ejecta, as seen in SN 1990I. In both interpretations, the low-mass blob component dominates the optical output both at the first peak (~20 days) and at the late phase (~300 days). The low luminosity at the late phase (the absolute R magnitude MR ~ -10.2 mag at ~270 days) sets the upper limit for the mass of 56Ni 0.08 M?, which is in contradiction to the value necessary to explain the second, main peak luminosity (MR ~ -18.3 mag at ~40 days). Encountered by this difficulty in the 56Ni heating model, we suggest an alternative scenario in which the heating source is a newly born, strongly magnetized neutron star (a magnetar) with the surface magnetic field Bmag ~ 1014-1015 G and the initial spin period P0 ~ 10 ms. Then, SN 2005bf could be a link between normal SNe Ib/c and an X-ray flash associated SN 2006aj, connected in terms of Bmag and/or P0.

SN 2003du: 480 days in the life of a normal type Ia supernova

Aims. We present a study of the optical and near-infrared (NIR) properties of the Type Ia Supernova (SN Ia) 2003du. Methods. An extensive set of optical and NIR photometry and low-resolution long-slit spectra was obtained using a number of facilities. The observations started 13 days before B-band maximum light and continued for 480 days with exceptionally good time sampling. The optical photometry was calibrated through the S-correction technique. Results. The UBVRIJHK light curves and the color indices of SN 2003du closely resemble those of normal SNeIa. SN 2003du reached a B-band maximum of 13.49 ± 0.02 mag on JD2 452 766.38 ± 0.5. We derive a B-band stretch parameter of 0.988 ± 0.003, which corresponds to Δm 15 = 1.02 ± 0.05, indicative of a SN Ia of standard luminosity. The reddening in the host galaxy was estimated by three methods, and was consistently found to be negligible. Using an updated calibration of the V and JHK absolute magnitudes of SNeIa, we find a distance modulus μ = 32.79 ± 0.15 mag to the host galaxy, UGC 9391. We measure a peak uvoir bolometric luminosity of 1.35(±0.20) x 10 43 erg s -1 and Arnett's rule implies that M 56Nι ≃ 0.68 ± 0.14M ⊙ of 56 Ni was synthesized during the explosion. Modeling of the uvoir bolometric light curve also indicates M56 Ni in the range 0.6-0.8 M ⊙ . The spectral evolution of SN 2003du at both optical and NIR wavelengths also closely resembles normal SNe Ia. In particular, the Si II ratio at maximum R(Si II) = 0.22 ± 0.02 and the time evolution of the blueshift velocities of the absorption line minima are typical. The pre-maximum spectra of SN 2003du showed conspicuous high-velocity features in the Call H&K doublet and infrared triplet, and possibly in Si II λ6355, lines. We compare the time evolution of the profiles of these lines with other well-observed SNe Ia and we suggest that the peculiar pre-maximum evolution of SiII λ6355 line in many SNe Ia is due to the presence of two blended absorption components.

The Hubble Constant: A Summary of theHubble Space TelescopeProgram for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids

This is the final summary paper of our 15 yr program using the Hubble Space Telescope (HST) to determine the Hubble constant from the Cepheid-calibrated luminosity of Type Ia supernovae. Several recent developments have made it necessary to put the summary on H0 on a broader basis than we originally thought (see the four papers cited in the text). The new Cepheid distances of the subset of 10 galaxies, which were hosts of normal SNe Ia, give weighted mean luminosities in B, V, and I at maximum light of -19.49, -19.46, and -19.22, respectively. These calibrate the adopted SNe Ia Hubble diagram from Paper III to give H0 = 62.3 ± 1.3(random) ± 5.0(systematic) in units of km s-1 Mpc-1. This is a global value because it uses the Hubble diagram between redshift limits of 3000 and 20,000 km s-1, beyond the effects of any local random and streaming motions. Local values of H0 between 4.4 and 30 Mpc from Cepheids, SNe Ia, 21 cm line widths, and the tip of the red giant branch (TRGB) all agree within 5% of our global value. This agreement of H0 on all scales from ~4 to 200 Mpc finds its most obvious explanation in the smoothing effect of vacuum energy on the otherwise lumpy gravitational field due to the nonuniform distribution of the local galaxies. The present value of H0 is consistent with physical methods relying on the time delay of gravitational lenses and the Sunyaev-Zeldovich effect, and it is not in contradiction with the CMB data. Our value of H0 is 14% smaller than the value of H0 found by Freedman et al. because our independent Cepheid distances to the six SNe Ia-calibrating galaxies used in that analysis average 0.35 mag larger than those used earlier.

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.

Spectral Modeling of SNe Ia Near Maximum Light: Probing the Characteristics of Hydrodynamical Models

We have performed detailed non-local thermodynamic equilibrium (NLTE) spectral synthesis modeling of two types of one-dimensional hydrodynamical models: the very highly parameterized deflagration model W7, and two delayed-detonation models. We find that, overall, both models do about equally well at fitting well-observed SNe Ia near maximum light. However, the Si II ?6150 feature of W7 is systematically too fast, whereas for the delayed-detonation models it is also somewhat too fast but significantly better than that of W7. We find that a parameterized mixed model does the best job of reproducing the Si II ?6150 line near maximum light, and we study the differences in the models that lead to better fits to normal SNe Ia. We discuss what is required of a hydrodynamical model to fit the spectra of observed SNe Ia near maximum light.

Determination of the Hubble Constant, the Intrinsic Scatter of Luminosities of Type Ia Supernovae, and Evidence for Nonstandard Dust in Other Galaxies

A sample of 109 type Ia supernovae (SNe Ia) with recession velocity < 30,000 km s^{-1}, is compiled from published SNe Ia light curves to explore the expansion rate of the local Universe. Based on the color parameter delta C_{12} and the decline rate delta m_{15}, we found that the average absorption to reddening ratio for SN Ia host galaxies to be R_{UBVI} = 4.37+/-0.25, 3.33+/-0.11, 2.30+/-0.11, 1.18+/-0.11, which are systematically lower than the standard values in the Milky Way. We investigated the correlations of the intrinsic luminosity with light curve decline rate, color index, and supernova environmental parameters. In particular, we found SNe Ia in E/S0 galaxies to be brighter close to the central region than those in the outer region, which may suggest a possible metallicity effect on SN luminosity. The dependence of SN luminosity on galactic environment disappears after corrections for the extinction and delta C_{12}. The Hubble diagrams constructed using 73 Hubble flow SNe Ia yield a 1-sigma scatter of <0.12 mag in BVI bands and ~0.16 mag in U band. The luminosity difference between normal SNe Ia and peculiar objects (including SN 1991bg-like and 1991T-like events) has now been reduced to within 0.15 mag via delta C_{12} correction. We use the same precepts to correct the nearby SNe Ia with Cepheid distances and found that the fully corrected absolute magnitudes of SNe Ia are: M_{B} = -19.33+/-0.06 mag, M_{V} = -19.27+/-0.05 mag. We deduced a value for the Hubble constant of H_{0} = 72 +/- 6 (total) km s^{-1} Mpc^{-1}.

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.

Structure and Dynamics in the Central 10 pc of the Galaxy

The Galactic center influences the current nature as well as the formation, evolution, and the fate of the Milky Way. The Galactic nucleus is representative of the galactic nuclei of other galaxies and provides an opportunity to study the environment around a supermassive black hole (SMBH) at high spatial resolution. The central 10 pc of the Galaxy, the Sgr A region, contains several principal components: the SMBH candidate (Sgr A*), the central cluster, the circumnuclear disk (CND), Sgr A West, a powerful supernova-like remnant (Sgr A East), and surrounding molecular clouds. Developing a consistent picture of the interactions between these components will improve our understanding of the Galaxy and the nature of galactic nuclei in general. Previous studies of the spatial and dynamical relationships between the various objects have been mostly based on indirect and qualitative evidence and leave unsolved many questions that require more robust evidence. Molecular hydrogen (H2) emission has been used as an excellent tracer and diagnostic for interactions between dense molecular clouds and hot, powerful sources. We observed the H2 (1 0) S(1) (l p 2.1218 mm) and H2 (2 1) S(1) (l p 2.2477 mm) emission-line spectra from the interaction regions between Sgr A East, the CND, and the surrounding molecular clouds. Using the long-slit Cooled Grating Spectrometer 4 (CGS4) with an echelle grating at the 3.8 m United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, Hawaii, we scanned 56 positions in the interaction regions. We reduced two-dimensional spectral images using IRAF and analyzed a three-dimensional data cube using MIRIAD. The data cube has the H2 information both in space (with a resolution of ∼2 ) and in velocity (with a resolution of ∼18 km s ). The H2 (1 0) S(1) data cube was directly compared with the NH3 (3, 3) data cube from McGary et al. (2001, ApJ, 559, 326) to investigate gas kinematics. Based on the H2 (1 0) S(1) and (2 1) S(1) line intensities and gas kinematics, we concluded that the H2 excitation can be explained by two mechanisms: a combination of fluorescence and C-shocks in very strong magnetic fields, or a mixture of slow C-shocks and fast J-shocks. We estimated shock velocities (∼100 km s ) of Sgr A East by comparing H2 line profiles with those of NH3. From the distribution of the shocked H2 emission, we determined the interacting boundary of Sgr A East in projection to be an ellipse, with the center at ∼1.5 pc offset from Sgr A*, and a dimension of 10.8 # 7.6 pc. We also determined the positional relationship between Sgr A East and the molecular clouds along the line of sight and suggested a revised model for the three-dimensional structure of the central 10 pc. From the estimated shock velocities, we deduced the initial explosion energy ([0.2–4] # 10 ergs) of Sgr A East. This extremely large energy excludes the hypothesis of a single, typical supernova (SN) for the origin of Sgr A East. We examined other hypotheses (tidal disruption of a star by the SMBH, multiple supernovae, and a hypernova) and we concluded that a hypernova (collapsar or microquasar) is the most probable origin of Sgr A East. Based on the energy, we investigated the influence of Sgr A East–like explosions (hypernovae) and normal SNe on the mass inflow to the Galactic nucleus. We suggest a scenario in which the continuous mass inflow into the Galactic nucleus makes it active by igniting the SMBH or stimulating a starburst every ∼108 yr, but each active phase continues for only 10 yr, since a large number of SNe resulting from newly born massive stars quickly stop the mass supply. The Galactic nucleus is likely to spend only about 1/10 of its life as active. As for the recent history of the central 10 pc, the mass inflow restarted several 10 yr ago after a quiescent phase, for ∼108 yr. In its usual schedule, the Galactic nucleus would continue its activity for a few 10 yr more before a huge number of SNe would occur. However, the active phase was unexpectedly stopped ∼104 yr ago by Sgr A East.

GRB 050509B: Constraints on Short Gamma-Ray Burst Models

We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509B. From V and R imaging, initiated ~2 days after the GRB trigger and lasting up to three weeks, we detect no variable object inside the small Swift/XRT X-ray error circle down to 2sigma limits of V = 26.5 and R = 25.1. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow. Simple models indicate that, unless the intrinsic energy in the outflow from GRB 050509B was << 10^51 erg, there was very little radioactive material with efficient decay timescales for generating a large luminosity. These limits strongly constrain progenitor models for this short GRB.

Classifications of the Host Galaxies of Supernovae, Set III

ABSTRACTA homogeneous sample comprising host galaxies of 604 recent supernovae, including 212 objects discovered primarily in 2003 and 2004, has been classified on the David Dunlap Observatory system. Most SN 1991bg–like SNe Ia occur in E and E/Sa galaxies, whereas the majority of SN 1991T–like SNe Ia occur in intermediate‐type galaxies. This difference is significant at the 99.9% level. As expected, all types of SNe II are rare in early‐type galaxies, whereas normal SNe Ia occur in all Hubble types. This difference is significant at the 99.99% level. A small number of SNe II in E galaxies might be due to galaxy classification errors or to a small young‐population component in these mainly old objects. No significant difference is found between the distributions over the Hubble type of SNe Ibc and SNe II. This confirms that both of these types of objects have similar (massive) progenitors. The present data show that in order to understand the dependence of supernova type on host‐galaxy population, it is more important to obtain accurate morphological classifications than it is to increase the size of the data sample.

Reddening, Absorption, and Decline Rate Corrections for a Complete Sample of Type Ia Supernovae Leading to a Fully Corrected Hubble Diagram tov&lt; 30,000 km s−1

Photometric (BVI) and redshift data corrected for streaming motions are compiled for 111 Branch-normal, four 1991T-like, seven 1991bg-like, and two unusual supernovae of Type Ia (SNe Ia). Color excesses E(B - V)host of normal SNe Ia, due to the absorption of the host galaxy, are derived by three independent methods, giving excellent agreement leading to the intrinsic colors at maximum of (B - V)00 = -0.024 ± 0.010 and (V - I)00 = -0.265 ± 0.016 if normalized to a common decline rate of Δm15 = 1.1. The strong correlation between redshift absolute magnitudes (based on an arbitrary Hubble constant of H0 = 60 km s-1 Mpc-1), corrected only for the extrinsic Galactic absorption, and the derived E(B - V)host color excesses leads to the well-determined yet abnormal absorption-to-reddening ratios of BVI = 3.65 ± 0.16, 2.65 ± 0.15, and 1.35 ± 0.21. Comparison with the canonical Galactic values of 4.1, 3.1, and 1.8 forces the conclusion that the law of interstellar absorption in the path length to the SN in the host galaxy is different from the local Galactic law, a result consistent with earlier conclusions by others. Improved correlations of the fully corrected absolute magnitudes (on the same arbitrary Hubble constant zero point) with host galaxy morphological type, decline rate, and intrinsic color are derived. We recover the result that SNe Ia in E/S0 galaxies are ~0.3 mag fainter than in spiral galaxies for possible reasons discussed in the text. The new decline rate corrections to absolute magnitudes are smaller than those by some authors for reasons explained in the text. The four spectroscopically peculiar 1991T-type SNe are significantly overluminous as compared to Branch-normal SNe Ia. The overluminosity of the seven 1999aa-like SNe is less pronounced. The seven 1991bg types in the sample constitute a separate class of SNe Ia, averaging in B 2 mag fainter than the normal Ia. New Hubble diagrams in B, V, and I are derived out to ~30,000 km s-1 using the fully corrected magnitudes and velocities, corrected for streaming motions. Nine solutions for the intercept magnitudes in these diagrams show extreme stability at the 0.02 mag level using various subsamples of the data for both low and high extinctions in the sample, proving the validity of the corrections for host galaxy absorption. We shall use the same precepts for fully correcting SN magnitudes for the luminosity recalibration of SNe Ia in the forthcoming final review of our Hubble Space Telescope Cepheid-SN experiment for the Hubble constant.

On the Light Curve and Spectrum of SN 2003dh Separated from the Optical Afterglow of GRB 030329

The net optical light curves and spectra of the supernova (SN) 2003dh are obtained from the published spectra of GRB 030329, covering about 6 days before SN maximum to about 60 days after. The bulk of the U-band flux is subtracted from the observed spectra using early-time afterglow templates, because strong line blanketing greatly depresses the UV and U-band SN flux in a metal-rich, fast-moving SN atmosphere. The blue-end spectra of the gamma-ray burst (GRB) connected hypernova SN 1998bw is used to determine the amount of subtraction. The subtraction of a host galaxy template affects the late-time results. The derived SN 2003dh light curves are narrower than those of SN 1998bw, rising as fast before maximum, reaching a possibly fainter maximum, and then declining ~ 1.2-1.4 times faster. We then build UVOIR bolometric SN light curve. Allowing for uncertainties, it can be reproduced with a spherical ejecta model of Mej ~ 7 ± 3 M☉, EK ~ 3.5 ± 1.5 × 1052 ergs, with EK/Mej ~ 5 following previous spectrum modeling, and M(56Ni) ~ 0.4 M☉. This suggests a progenitor main-sequence mass of ~25-40 M☉, lower than SN 1998bw but significantly higher than normal Type Ic SNe and the GRB-unrelated hypernova SN 2002ap.

The Diversity of Type Ia Supernovae: Evidence for Systematics?

The photometric and spectroscopic properties of 26 well observed Type Ia Supernovae (SNeIa) were analyzed with the aim to explore SNIa diversity. The sample includes (Branch-)normal SNe as well as extreme events like SNe 1991T and 1991bg, while the truly peculiar SNIa, SN2000cx and SN2002cx are not included in our sample . A statistical treatment reveals the existence of three different groups. The first group (FAINT) consists of faint SNeIa similar to SN1991bg, with low expansion velocities and rapid evolution of SiII velocity. A second group consists of ``normal'' SNeIa, also with high temporal velocity gradient (HVG), but with brighter mean absolute magnitude =-19.3 and higher expansion velocities than the FAINT SNe. The third group includes both ``normal'' and SN1991T-like SNeIa: these SNe populate a narrow strip in the SiII velocity evolution plot, with a small velocity gradient (SVG), but have absolute magnitudes similar to HVGs. While the FAINT and HVG SNeIa together seem to define a relation between RSi(II) and Dm15(B), the SVG ones either do not conform with that relation or define a new, looser one. The RSi(II) pre-maximum evolution of HVGs is strikingly different from that of SVGs. The impact of this evidence on the understanding of SNIa diversity, in terms of explosion mechanisms, degree of ejecta mixing, and ejecta-CSM interaction, is discussed.

On the Progenitor of the Type II Supernova 2004et in NGC 694611Based on data obtained at the Canada‐France‐Hawaii Telescope.

ABSTRACTSupernova (SN) 2004et is the eighth historical SN in the nearby spiral galaxy NGC 6946. Here we report on early photometric and spectroscopic monitoring of this object. SN 2004et is a Type II event, exhibiting a plateau in its light curves, but its spectral and color evolution appear to differ significantly from those of other, more normal Type II plateau (II‐P) SNe. We have analyzed Canada‐France‐Hawaii Telescope images of the host galaxy taken prior to the SN explosion, identifying a candidate progenitor for the SN. The star’s absolute magnitude and intrinsic color imply that it was a yellow, rather than red, supergiant star, with an estimated zero‐age main‐sequence mass of 15+5-2 M⊙. Although this mass estimate is consistent with estimates and upper limits for the progenitors of other, more normal SNe II‐P, the SN 2004et progenitor’s unusual color could further imply a preexplosion evolutionary history analogous to, but less extreme than, that for the progenitors of the peculiar Type II‐P SN 1987A or the Type IIb SN 1993J. The identity of the progenitor candidate needs to be verified when the SN has significantly dimmed.

Reading the Spectra of the Most Peculiar Type Ia Supernova 2002cx

ABSTRACTIn spite of the apparent lack of Si ii and S ii features in its spectra, SN 2002cx was classified as a peculiar Type Ia supernova (SN Ia) on the basis of its overall photometric and spectroscopic behavior. Spectra obtained near maximum light contained Fe iii features, as in SN 1991T–like events, but the blueshifts of the Fe iii absorptions were exceptionally low. The luminosity was also low. We use the supernova synthetic‐spectrum code SYNOW to study line identifications in SN 2002cx. We find that the maximum‐light spectra appear to contain weak features of Si ii, S ii, Si iii, and Ca ii, which strengthens the connection with SN 1991T–like events. We show that later spectra obtained 12, 25, and 56 days after maximum consist of P Cygni resonance‐scattering features due to permitted Fe ii and Co ii lines. SN 2002cx had been thought to have made the transition from a permitted‐line to a forbidden‐line spectrum between 25 and 56 days. Owing to the low expansion velocities, the postmaximum spectral features are narrower and easier to identify than they are in other SNe Ia. SN 2002cx will lead to improved line identifications in other SNe Ia and will clarify when the transition from a permitted‐ to a forbidden‐line spectrum occurs. In the context of current SN Ia explosion models, we suggest that the properties of SN 2002cx might be consistent with three‐dimensional deflagration models, which are not favored for normal SNe Ia.

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.

SN 2002cx: The Most Peculiar Known Type Ia Supernova

ABSTRACTWe present photometric and spectroscopic observations of supernova (SN) 2002cx, which reveal it to be unique among all observed Type Ia supernovae (SNe Ia). SN 2002cx exhibits an SN 1991T–like premaximum spectrum, an SN 1991bg–like luminosity, and expansion velocities roughly half those of normal SNe Ia. Photometrically, SN 2002cx has a broad peak in the R band and a plateau phase in the I band, and slow late‐time decline. The B−V color evolution is nearly normal, but the V−R and V−I colors are very red. Early‐time spectra of SN 2002cx evolve very quickly and are dominated by lines from Fe‐group elements; features from intermediate‐mass elements (Ca, S, Si) are weak or absent. Mysterious emission lines are observed around 7000 Å at about 3 weeks after maximum brightness. The nebular spectrum of SN 2002cx is also unique, consisting of narrow iron and cobalt lines. The observations of SN 2002cx are inconsistent with the observed spectral/photometric sequence and provide a major challenge to our understanding of SNe Ia. No existing theoretical model can successfully explain all observed aspects of SN 2002cx.