arXiv subjects

J. C. Wheeler

Publications and source records attributed to J. C. Wheeler.

134 records · Page 8Linked to original sources

arXiv1998

The QSE-reduced αNetwork

Examination of the process of silicon burning, the burning stage that leads to the production of the iron peak nuclei, reveals that the nuclear evolution is dominated by large groups of nuclei in mutual equilibrium. These quasi-equilibrium (QSE) groups form well in advance of the global Nuclear Statistical Equilibrium (NSE). We present an improved ``minimal'' nuclear network, which takes advantage of quasi-equilibrium in order to calculate the nuclear evolution and energy generation while further reducing the computational cost compared to a conventional α-chain network. During silicon burning, the resultant \emph{QSE-reduced} αnetwork is twice as fast as the full αnetwork it replaces and requires the tracking of only half as many abundance variables, without significant loss of accuracy. When the QSE-reduced αnetwork is used in combination with a conventional αnetwork stretching from He to Si, the combined \alpha7 network provides an accurate approximation for all of the burning stages from He burning to NSE, while tracking only 7 abundances. These reductions in computational cost and the number of species evolved make the \alpha7 network well suited for inclusion within hydrodynamic simulations, particularly those in multi-dimension.

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arXiv1997

Supernovae and the Hubble Constant (or Butthead's Revenge)

Physical models of Type Ia supernovae (SN Ia) that do not depend on secondary calibrators have indicated that the Hubble Constant must be about 65 km/s/Mpc for well over a decade with the range of uncertainty shrinking with the sophistication of the models. This estimate is in good agreement with those based on HST observations of Cepheid variables and with purely empirical methods based on SN Ia calibrated with Cepheids. The prospects of progress in understanding the physics of the explosion of SN Ia and of their application to measure other cosmological parameters is reviewed. Despite the rather complex nature of their atmospheres, SN Ia may give a more reliable estimate of distances than Type II. The latter depend on remaining uncertainties in the scattering atmospheres, the helium abundance and possible systematic effects due to distortions of the envelope that will not average out even in a large sample.

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arXiv1996

Maximum Brightness and Post-Maximum Decline of Light Curves of SN~Ia: A Comparison of Theory and Observations

We compare the observed correlations between the maximum brightness, postmaximum decline rate and color at maximum light of Type Ia supernovae (SN Ia) with model predictions. The observations are based on a total of 40 SN Ia with 29 SN of the Calan Tololo Supernova Search and 11 local SN which cover a range of 2 mag in the absolute visual brightness. The observed correlations are not tight, one dimensional relations. Supernovae with the same postmaximum decline or the same color have a spread in visual magnitude of about 0.7 mag. The dispersion in the color-magnitude relation may result from uncertainties in the distance determinations or the interstellar reddening within the host galaxy. The dispersion in the decline rate-magnitude relation suggests that an intrinsic spread in the supernova properties exists that cannot be accounted for by any single relation between visual brightness and postmaximum decline. Theoretical correlations are derived from a grid of models which encompasses delayed detonations, pulsating delayed detonations, the merging scenario and helium detonations. We find that the observed correlations can be understood in terms of explosions of Chandrasekhar mass white dwarfs. Our models show an intrinsic spread in the relations of about 0.5 mag in the maximum brightness and about 0.1 mag in the B-V color. Our study provides strong evidence against the mechanism of helium detonation for subluminous, red SN Ia.

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arXiv1996

Polarization of SN 1987A Revisited

The conventional picture for the origin of the polarization of a supernova is based on a model of Thomson or resonance scattering of photons traveling through an aspherical supernova atmosphere. Positive detection of intrinsic polarization in SN 1987A is then interpretated as evidence of an asymmetrical supernova atmosphere. We show here a different view based on the scattering of the supernova light by a dusty circumstellar material (CSM), or the ``light echo'' effect. At a given epoch after the explosion, the observed photons consist of both those propagating directly from the supernova and those scattered by dust particles in the CSM. Polarized light can be produced if the distribution of the dust particles is aspherical. The model can reproduce both the time evolution of the observed broad band polarization of SN 1987A and major features of the polarization spectra. It is also successful in providing a natural model for the early infrared light curve, in particular the observed 30 day delay of the IR maximum compared to the maximum of the bolometric light curve.

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arXiv1996

Analysis of the Polarization and Flux Spectra of SN1993J

Synthetic polarization and flux spectra are presented for aspherical, electron scattering-dominated photospheres of Type II supernovae (SN II) in general and the specific case of SN 1993J using a Monte Carlo scheme. The observed luminosity of a Type II supernova depends on the unknown inclination angle. Spectral analysis alone will fail to detect even strong deviations from spherical symmetry. Line scattering depolarizes incident polarized light, but the residual intrinsic polarization does not completely vanish because of electron scattering effects that depend on the electron density distribution. By combining results on the polarization and velocity structure of the emission lines, and the degree of polarization in the continuum, we place strong constraints on the degree of asphericity, the inclination of the system and the electron density distribution. Both the flux spectra and the percentage of polarization as a function of wavelength can be reproduced by an aspherical model with an axis ratio of 0.6, radial electron densities proportional to r**-5 and an effective temperature of 4800 K. In an oblate model, SN1993J is seen almost equator on. The line forming region is still within the hydrogen rich part of the envelope after 3 weeks, although the continuum may form in deeper layers. Although the residual polarization across Halpha is not zero, the interstellar component derived herein is consistent with that deduced by Trammell et al.(1993). The power of this technique for investigating the structure of other SN II, as a method for independently deriving the interstellar polarization, and the implications of these results on the use of SN II to determine distances through the Baade- Wesselink method are discussed.

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arXiv1996

Explosion Models, Light Curves, Spectra and Ho

From the spectra and light curves it is clear that SNIa are thermonuclear explosions of white dwarfs. However, details of the explosion are highly under debate. Here, we present detailed models which are consistent with respect to the explosion mechanism, the optical and infrared light curves (LC), and the spectral evolution. This leaves the description of the burning front and the structure of the white dwarf as the only free parameters. The explosions are calculated using one-dimensional Lagrangian codes including nuclear networks. Subsequently, optical and IR-LCs are constructed. Detailed NLTE-spectra are computed for several instants of time using the density, chemical and luminosity structure resulting from the LCs. The general methods and critical tests are presented (sect. 2). Different models for the thermonuclear explosion are discussed including detonations, deflagrations, delayed detonations, pulsating delayed detonations (PDD) and helium detonations (sect.3). Comparisons between theoretical and observed LCs and spectra provide an insight into details of the explosion and nature of the progenitor stars (sect. 4/5). We try to answer several related questions. Are subluminous SNe Ia a group different from `normal' SN Ia (sect. 5)? Can we understand observed properties of the LCs and spectra (sect. 4)? What do we learn about the progenitor evolution and its metallicity (sect. 3, Figs. 4,5)? Do successful SN~Ia models depend on the type of the host galaxy (Table 2)? Using both the spectral and LC information, theoretical models allow for a determination of the Hubble constant independent from `local' distance indicators such as delta-Cephei stars. Ho is found to be 67 +- 9km/s/Mpc and, from SN1988U, qo equals 0.7 +- 1. within 95 percent confidence levels.

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arXiv1996

New Perspectives on Type Ic Supernovae

The Type Ic supernovae are probably "nearly-nude" core collapse events. They are thus of special interest in terms of their evolutionary origin, but also as the source of potential clues to the explosion process that may not be obtained from other supernovae. Study of Type Ic shows that they are distinct from Type Ia, even those of the dimmer, redder variety. Type Ic also suggest that the historical classification scheme based on spectra cannot encompass the physical features of these and closely related events. The Type Ic probably all have a very small outer layer of helium, but not enough to make them merely a version of Type Ib. They probably require another evolutionary channel. Similar spectra near maximum light belie a variety of late-time light curve behavior. Some events classified as Type Ic, eg SN 1962L, 1983V, 1990B, and 1990U, follow a relatively slow evolution after maximum light, decaying about 3.0 magnitudes in V during the first 150 days after maximum. These events seem to belong to a general category that includes SN 1993J and the Type Ib SN 19983N. Their light curves decline more rapidly than the expected late-time Co-56 decay with full trapping, but less rapidly than Type Ia. The late time light curve of these events seems to be independent of the velocity of the ejecta and hence implies the trapping of gamma-rays in a manner that is independent of the mass that participates in the homologous expansion. Other Type Ic, SNe 1983I, 1987M, and 1994I, show steeper decline than even Type Ia.

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arXiv1996

Three-dimensional Combustion in Type Ia Supernovae

Turbulent combustion is three-dimensional. Turbulence in a Type Ia supernova is driven on large scales by the buoyancy of burning products. The turbulent cascade penetrates down to very small scales, and makes the rate of deflagration independent of the microphysics. The competition between the turbulent cascade and the freeze-out of turbulent motions due to stellar expansion determines the largest scale participating in the cascade. This sets the bulk rate of a deflagration in a supernova. The freeze-out limits the bulk rate of deflagration to a value that makes a powerful explosion impossible. Two-dimensional simulations cannot capture these essential elements of turbulent combustion, even in principle. A powerful delayed detonation explosion can take place if the burning makes a transition to a detonation. A deflagration to detonation transition (DDT) can occur in a layer of mixed cold fuel and hot burning products created either inside an active turbulent burning region by a high intensity of turbulence, or by mixing cold fuel with ashes of a dead deflagration front during the global pulsation of a star, or by both.

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