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Register a new userDirect Analysis of Spectra of the Peculiar Type Ia Supernova 2000cx
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The Type Ia SN 2000cx exhibited multiple peculiarities, including a lopsided B-band light-curve peak that does not conform to current methods for using shapes of light curves to standardize SN Ia luminosities. We use the parameterized supernova synthetic-spectrum code SYNOW to study line identifications in the photospheric-phase spectra of SN 2000cx. Previous work established the presence of Ca II infrared-triplet features forming above velocity about 20,000 km/s, much higher than the photospheric velocity of about 10,000 km/s. We find Ti II features forming at the same high velocity. High-velocity line formation is partly responsible for the photometric peculiarities of SN 2000cx: for example, B-band flux blocking by Ti II absorption features that decreases with time causes the B light curve to rise more rapidly and decline more slowly than it otherwise would. SN 2000cx contains an absorption feature near 4530 A that may be H-beta, forming at the same high velocity. The lack of conspicuous H-alpha and P-alpha signatures does not necessarily invalidate the H-beta identification if the high-velocity line formation is confined to a clump that partly covers the photosphere and the H-alpha and P-alpha source functions are elevated relative to that of resonance scattering. The H-beta identification is tentative. If it is correct, the high-velocity matter must have come from a nondegenerate companion star.
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A comparative study of optical spectra of Type Ia supernovae (SNe Ia) obtained near 1 week, 3 weeks, and 3 months after maximum light is presented. Most members of the four groups that were defined on the basis of maximum light spectra in Paper II (core normal, broad line, cool, and shallow silicon) develop highly homogeneous postmaximum spectra, although there are interesting exceptions. Comparisons with SYNOW synthetic spectra show that most of the spectral features can be accounted for in a plausible way. The fits show that 3 months after maximum light, when SN Ia spectra are often said to be in the nebular phase and to consist of forbidden emission lines, the spectra actually remain dominated by resonance scattering features of permitted lines, primarily those of Fe II. Even in SN 1991bg, which is said to have made a very early transition to the nebular phase, there is no need to appeal to forbidden lines at 3 weeks postmaximum, and at 3 months postmaximum the only clear identification of a forbidden line is [Ca II] 7291, 7324. Recent studies of SN Ia rates indicate that most of the SNe Ia that have ever occurred have been prompt SNe Ia, produced by young (100,000,000 yr) stellar populations, while most of the SNe Ia that occur at low redshift today are tardy, produced by an older (several Gyrs) population. We suggest that the shallow silicon SNe Ia tend to be the prompt ones.
We have conducted a systematic and comprehensive monitoring programme of the Type Ia supernova 2000cx at late phases using the VLT and HST. The VLT observations cover phases 360 to 480 days past maximum brightness and include photometry in the BVRIJH bands, together with a single epoch in each of U and Ks. While the optical bands decay by about 1.4 mag per 100 days, we find that the near-IR magnitudes stay virtually constant during the observed period. This means that the importance of the near-IR to the bolometric light curve increases with time. The finding is also in agreement with our detailed modeling of a Type Ia supernova in the nebular phase. In these models, the increased importance of the near-IR is a temperature effect. We note that this complicates late-time studies where often only the V band is well monitored. In particular, it is not correct to assume that any optical band follows the bolometric light curve at these phases, and any conclusions based on such assumptions, e.g., regarding positron-escape, must be regarded as premature. A very simple model where all positrons are trapped can reasonably well account for the observations. The nickel mass deduced from the positron tail of this light curve is lower than found from the peak brightness, providing an estimate of the fraction of late-time emission that is outside of the observed wavelength range. Our detailed models show the signature of an infrared catastrophe at these epochs, which is not supported by the observations.
We present optical and infrared photometry of the unusual Type Ia supernova 2000cx. With the data of Li et al. (2001) and Jha (2002), this comprises the largest dataset ever assembled for a Type Ia SN, more than 600 points in UBVRIJHK. We confirm the finding of Li et al. regarding the unusually blue B-V colors as SN 2000cx entered the nebular phase. Its I-band secondary hump was extremely weak given its B-band decline rate. The V minus near infrared colors likewise do not match loci based on other slowly declining Type Ia SNe, though V-K is the least ``abnormal. In several ways SN 2000cx resembles other slow decliners, given its B-band decline rate (Delta m_15(B) = 0.93), the appearance of Fe III lines and weakness of Si II in its pre-maximum spectrum, the V-K colors and post-maximum V-H colors. If the distance modulus derived from Surface Brightness Fluctuations of the host galaxy is correct, we find that the rate of light increase prior to maximum, the characteristics of the bolometric light curve, and the implied absolute magnitude at maximum are all consistent with a sub-luminous object with Delta m_15(B) ~ 1.6-1.7 having a higher than normal kinetic energy.
Spectroscopic analyses of Type Ia supernovae have shown there exist four spectroscopic groups---cools, broad line, shallow silicon, and core normal---defined by the widths of the Si II features at 5972 Angstroms and 6355 Angstroms. 1991bg-likes are classified as cools. Cools are dim, undergo a rapid decline in luminosity, and produce significantly less nickel than normal Type Ia supernovae. They also have an unusually deep and wide trough in their spectra around 4200 Angstroms and a relatively strong Si II absorption attributed to the line at 5972 Angstroms. We examine the spectra of supernova (SN) 1991bg and the cools SN 1997cn, SN 1999by, and SN 2005bl using the highly parameterized synthetic spectrum code SYNOW, and find general agreement with similar spectroscopic studies. Our analysis reveals that this group of supernovae is fairly homogeneous, with many of the blue spectral features well fit by Fe II. The nature of the spectroscopic commonalities and the variations in the class are discussed. Finally, we examine intermediates such as SN 2004eo and discuss the spectroscopic subgroup distribution of Type Ia supernovae.
Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared to observed photospheric-phase spectra of the Type Ic supernova 1994I. The observed optical spectra can be well matched by synthetic spectra that are based on the assumption of spherical symmetry. We consider the identification of the infrared absorption feature observed near 10,250 AA, which previously has been attributed to He I $lambda10830$ and regarded as strong evidence that SN 1994I ejected some helium. We have difficulty accounting for the infrared absorption with He I alone. It could be a blend of He I and C I lines. Alternatively, we find that it can be fit by Si I lines without compromising the fit in the optical region. In synthetic spectra that match the observed spectra, from 4 days before to 26 days after the time of maximum brightness, the adopted velocity at the photosphere decreases from 17,500 to 7000 kms. Simple estimates of the kinetic energy carried by the ejected mass give values that are near the canonical supernova energy of $10^{51}$ ergs. The velocities and kinetic energies that we find for SN 1994I in this way are much lower than those that we find elsewhere for the peculiar Type Ic SNe 1997ef and 1998bw, which therefore appear to have been hyper-energetic.
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