arXiv subjects

D. Branch

Publications and source records attributed to D. Branch.

34 records · Page 2Linked to original sources

arXiv2000

Spectrum Analysis of the Type Ib Supernova 1999dn: Probable Identifications of C II and H-alpha

Low resolution spectra of SN 1999dn at early times are presented and compared with synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW. We find that the spectra of SN 1999dn strongly resemble those of SN 1997X and SN 1984L, and hence we classify it as a Type Ib event. Line-identifications are established through spectrum synthesis. Strong evidence of both H-alpha and C II 6580 is found. We infer that H-alpha appears first, before the time of maximum brightness, and then is blended with and finally overwhelmed by the C II line after maximum; this favors a thin high-velocity hydrogen skin in this Type Ib supernova.

astro-ph↗

arXiv1999

Supernova Resonance-Scattering Profiles in the Presence of External Illumination

We discuss a simple model for the formation of a supernova spectral line by resonance scattering in the presence of external illumination of the line-forming region by light from circumstellar interaction (toplighting). The simple model provides a clear understanding of the most conspicuous toplighting effect: a rescaling or, as we prefer, a ``muting'' of the line profile relative to the continuum. This effect would be present in more realistic models, but would be harder to isolate. An analytic expression for a muting factor for a P-Cygni line is derived that depends on the ratio E of the toplighting specific intensity to the specific intensity from the supernova photosphere. If E<1, the line profile is reduced in scale or ``muted''. If E=1, the line profile vanishes altogether. If E>1, the line profile flips vertically: then having an absorption component near the observer-frame line center wavelength and a blueshifted emission component.

astro-ph↗

arXiv1999

Direct Analysis of Spectra of the Type Ic Supernova 1994I

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 Å, 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.

astro-ph↗

arXiv1999

On the High--Velocity Ejecta of the Type Ia Supernova 1994D

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared to spectra of the Type Ia SN 1994D that were obtained before the time of maximum brightness. Evidence is found for the presence of two-component Fe II and Ca II features, forming in high velocity ($\ge 20,000$ \kms) and lower velocity ($\le 16,000$ \kms) matter. Possible interpretations of these spectral splits, and implications for using early--time spectra of SNe Ia to probe the metallicity of the progenitor white dwarf and the nature of the nuclear burning front in the outer layers of the explosion, are discussed.

astro-ph↗

arXiv1999

Direct Analysis of Spectra of Type Ic Supernovae

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared with observed photospheric-phase optical spectra of the normal Type Ic SN 1994I and the peculiar Type Ic SNe 1997ef and 1998bw. The observed spectra can be matched fairly well with synthetic spectra that are based on spherical symmetry and that include lines of just a few ions that are expected to appear on the basis of LTE calculations. Spectroscopic estimates of the mass and kinetic energy of the line-forming layers of the ejected matter give conventional values for SN 1994I but high kinetic energy ($\sim 30 \times 10^{51}$ erg) for SN 1997ef and even higher ($\sim 60 \times 10^{51}$ erg) for SN 1998bw. It is likely that even if SNe 1997ef and 1998bw were non-spherical, they also were hyper-energetic.

astro-ph↗

arXiv1999

Determination of the Hubble Constant Using a Two-Parameter Luminosity Correction for Type Ia Supernovae

In this paper, we make a comprehensive determination of the Hubble constant $H_0$ by using two parameters - the B-V color and the rate of decline $Δm_{15}$ - to simultaneously standardize the luminosities of all nearby Cepheid-calibrated type Ia supernovae (SNe Ia) and those of a larger, more distant sample of 29 SNe Ia. Each group is treated in as similar a manner as possible in order to avoid systematic effects. A simultaneous $χ^2$ minimization yields a standardized absolute luminosity of the Cepheid-calibrated supernovae as well as the Hubble constant obtained from the more distant sample. We find $H_0 = 62 km/s Mpc^{-1}$ and a standardized absolute magnitude of -19.46. The sensitivity of $H_0$ to a metallicity dependence of the Cepheid-determined distances is investigated. The total uncertainty $δH_0$, dominated by uncertainties in the primary Cepheid distance indicator, is estimated to be 5 km/s Mpc^{-1}.

astro-ph↗

arXiv1998

Effect of anisotropy on universal transport in unconventional superconductors

We investigate the universal electronic transport for a mixed $d_{x^2-y^2}$+s-wave superconductor in the presence of an anisotropic elliptical Fermi surface. Similar to the universal low-temperature transport predicted in a $d_{x^2-y^2}$-wave superconductor with a circular Fermi surface, anisotropic universal features are found in the low-temperature microwave conductivity, and thermal conductivity in the anisotropic system. The effects of anisotropy on the penetration depth, impurity induced $T_c$ suppression, and the zero-frequency density of states are also considered. While a small amount of anisotropy can lead to a strong suppression of the effective scattering rate and hence the density of states at zero frequency, experimental data suggests that large effects are restored by a negative $s$-component gap admixture.

cond-mat.supr-con↗

arXiv1998

Ion Signatures in Supernova Spectra

A systematic survey of ions that could be responsible for features in the optical spectra of supernovae is carried out. Six different compositions that could be encountered in supernovae are considered. For each composition, the LTE optical depth of one of the strongest optical lines of each ion is plotted against temperature. For each ion that can realistically be considered as a candidate to produce identifiable features in supernova spectra, a sample synthetic spectrum is displayed. The optical depth plots and the synthetic spectra can provide guidance to studies of line identifications in the optical spectra of all types of supernovae during their photospheric phases.

astro-ph↗

arXiv1998

On the Spectrum and Nature of the Peculiar Type Ia Supernova 1991T

A parameterized supernova synthetic-spectrum code is used to study line identifications in the photospheric-phase spectra of the peculiar Type Ia SN 1991T, and to extract some constraints on the composition structure of the ejected matter. The inferred composition structure is not like that of any hydrodynamical model for Type Ia supernovae. Evidence that SN 1991T was overluminous for an SN Ia is presented, and it is suggested that this peculiar event probably was a substantially super-Chandrasekhar explosion that resulted from the merger of two white dwarfs.

astro-ph↗

arXiv1998

The Probable Detection of SN 1923A: The Oldest Radio Supernova?

Based upon the results of VLA observations, we report the detection of two unresolved radio sources that are coincident with the reported optical position of SN 1923A in M83. For the source closest to the SN position, the flux density was determined to be 0.30 +/- 0.05 mJy at 20 cm and 0.093 +/- 0.028 mJy at 6 cm. The flux density of the second nearby source was determined to be 0.29 +/- 0.05 at 20 cm and 0.13 +/- 0.028 at 6 cm. Both sources are non-thermal with spectral indices of alpha = -1.0 +/- 0.30 and -0.69 +/- 0.24, respectively. SN 1923A has been designated as a Type II-P. No Type II-P (other than SN 1987A) has been detected previously in the radio. The radio emission from both sources appears to be fading with time. At an age of approximately 68 years when we observed it, this would be the oldest radio supernova (of known age) yet detected.

astro-ph↗

arXiv1998

Type Ia Supernovae and the Hubble Constant

The focus of this review is the work that has been done during the 1990s on using Type Ia supernovae (SNe Ia) to measure the Hubble constant ($H_0$). SNe Ia are well suited for measuring $H_0$. A straightforward maximum-light color criterion can weed out the minority of observed events that are either intrinsically subluminous or substantially extinguished by dust, leaving a majority subsample that has observational absolute-magnitude dispersions of less than $σ_{obs}(M_B) \simeq σ_{obs}(M_V) \simeq 0.3$ mag. Correlations between absolute magnitude and one or more distance-independent SN Ia or parent-galaxy observables can be used to further standardize the absolute magnitudes to better than 0.2 mag. The absolute magnitudes can be calibrated in two independent ways --- empirically, using Cepheid-based distances to parent galaxies of SNe Ia, and physically, by light curve and spectrum fitting. At present the empirical and physical calibrations are in agreement at $M_B \simeq M_V \simeq -19.4$ or -19.5. Various ways that have been used to match Cepheid-calibrated SNe Ia or physical models to SNe Ia that have been observed out in the Hubble flow have given values of $H_0$ distributed throughout the range 54 to 67 km/s Mpc$^{-1}$. Astronomers who want a consensus value of $H_0$ from SNe Ia with conservative errors could, for now, use $60 \pm 10$ km/s Mpc^{-1}$.

astro-ph↗

arXiv1997

Extinction and the Radial Distribution of Supernova Properties in Their Parent Galaxies

We use a Monte Carlo technique and assumed spatial distributions of dust and supernova (SN) progenitors in a simple model of a characteristic SN--producing disk galaxy to explore the effects of extinction on the radial distributions of SN properties in their parent galaxies. The model extinction distributions and projected radial number distributions are presented for various SN types. Even though the model has no core-collapse SNe within three kpc of the center, a considerable fraction of the core-collapse SNe are projected into the inner regions of inclined parent galaxies owing to their small vertical scale height. The model predicts that because of extinction, SNe projected into the central regions should on average appear dimmer and have a much larger magnitude scatter than those in the outer regions. In particular, the model predicts a strong deficit of bright core-collapse events inside a projected radius of a few kpc. Such a deficit is found to be present in the observations. It is a natural consequence of the characteristic spatial distributions of dust and core-collapse SNe in galaxies, and it leads us to offer an alternative to the conventional interpretation of the Shaw effect.

astro-ph↗

arXiv1996

Synthetic Spectra of Hydrodynamic Models of Type Ia Supernovae

We present detailed NLTE synthetic spectra of hydrodynamic SNe Ia models. We make no assumptions about the form of the spectrum at the inner boundary. We calculate both Chandrasekhar-mass deflagration models and sub-Chandrasekhar ``helium detonators.'' Gamma-ray deposition is handled in a simple, accurate manner. We have parameterized the storage of energy that arises from the time dependent deposition of radioactive decay energy in a reasonable manner, that spans the expected range. We find that the Chandrasekhar-mass deflagration model W7 of Nomoto etal shows good agreement with the observed spectra of SN 1992A and SN 1994D, particularly in the UV, where our models are expected to be most accurate. The sub-Chandrasekhar models do not reproduce the UV deficit observed in normal SNe Ia. They do bear some resemblance to sub-luminous SNe Ia, but the shape of the spectra (i.e. the colors) are opposite to that of the observed ones and the intermediate mass element lines such as Si II, and Ca II are extremely weak, which seems to be a generic difficulty of the models. Although the sub-Chandrasekhar models have a significant helium abundance (unlike Chandrasekhar-mass models), helium lines are not prominent in the spectra near maximum light and thus do not act as a spectral signature for the progenitor.

astro-ph↗

arXiv1996

NLTE Effects in Modeling of Supernovae near Maximum Light

Supernovae, with their diversity of compositions, velocities, envelope masses, and interactions are good testing grounds for probing the importance of NLTE in expanding atmospheres. In addition to treating H, He, Li I, O I, Ne I, Na I, and Mg II in NLTE, we use a very large model atom of Fe II to test the importance of NLTE processes in both SNe Ia and II. Since the total number of potential line transitions that one has to include is enormous ($\approx 40$ million), approximations and simplifications are required to treat the problem accurately and in finite computer time. With our large Fe II model atom (617 levels, 13,675 primary NLTE line transitions) we are able to test several assumptions for treating the background opacity that are needed to obtain correct UV line blanketing which determines the shape of near-maximum light supernova spectra. We find that, due to interactions within the multiplets, treating the background lines as pure scattering (thermalization parameter $ε= 0$) is a poor approximation and that an overall mean value of $ε\sim 0.05 - 0.10 $ is a far better approximation. This is true even in SNe Ia, where the continuum absorption optical depth at 5000 Å ($\equiv τ_{\rm std}$) is $<< 1$. We also demonstrate that a detailed treatment of NLTE effects is required to properly determine the ionization states of both abundant and trace elements.

astro-ph↗

arXiv1995

The Effects of Inhomogeneities on Evaluating the Deceleration Parameter q$_0$

Analytic expressions for distance-redshift relations which have been corrected for the effects of some inhomogeneities in the Friedmann-Robertson-Walker (FRW) mass density are used to illustrate the significance of inhomogeneities on a determination of $q_0$ made by using Type Ia supernovae. The value of $q_0$ inferred from a given set of observations depends on the fractional amount of matter in inhomogeneities and is up to 50 percent larger than that obtained by using the standard Mattig $m$-$z$ result for pure dust FRW models.

astro-ph↗

arXiv1995

Preliminary Spectral Analysis of SN 1994I

We present optical spectra of the Type Ic supernova 1994I in M51 and preliminary non-LTE analysis of the spectra. Our models are not inconsistent with the explosions of C+O cores of massive stars. While we find no direct evidence for helium in the optical spectra, our models cannot rule out small amounts of helium. More than 0.1~\msol\ of helium seems unlikely.

astro-ph↗