arXiv subjects

David Branch

Publications and source records attributed to David Branch.

At least 55 records · Page 3Linked to original sources

arXiv2003

On the Geometry of the High-Velocity Ejecta of the Peculiar Type Ia Supernova 2000cx

High-velocity features in Type Ia supernova spectra provide a way to probe the outer layers of these explosions. The maximum-light spectra of the unique Type Ia supernova 2000cx exhibit interesting Ca II features with high-velocity components. The infrared triplet absorption is quadruply notched, while the H&K absorption is wide and flat. Stimulated by a three-dimensional interpretation of similar Ca II features in another Type Ia supernova (SN 2001el, Kasen et al. 2003), we present alternative spherically symmetric and three-dimensional ejecta models to fit the high-velocity (v > 16,000 km/s) Ca II features of SN 2000cx. We also present simple estimates of the high-velocity ejecta mass for a few trial compositions and discuss their implications for explosion modelling.

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arXiv2002

Determination of Primordial Metallicity and Mixing in the Type IIP Supernova 1993W

We present the results of a large grid of synthetic spectra and compare them to early spectroscopic observations of SN 1993W. This supernova was discovered close to its explosion date and at a recession velocity of 5400 km/s is located in the Hubble flow. We focus here on two early spectra that were obtained approximately 5 and 9 days after explosion. We parameterize the outer supernova envelope as a power-law density profile in homologous expansion. In order to extract information on the value of the parameters a large number of models was required. We show that very early spectra combined with detailed models can provide constraints on the value of the power law index, the ratio of hydrogen to helium in the surface of the progenitor, the progenitor metallicity and the amount of radioactive nickel mixed into the outer envelope of the supernova. The spectral fits reproduce the observed spectra exceedingly well. The spectral results combined with the early photometry predict that the explosion date was 4.7 \pm 0.7 days before the first spectrum was obtained. The ability to obtain the metallicity from early spectra make SN IIP attractive probes of chemical evolution in the universe and by showing that we have the ability to pin down the parameters of the progenitor and mixing during the supernova explosion, it is likely to make SN IIP useful cosmological distance indicators which are at the same time complementary to SNe Ia.

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arXiv2002

Detectibility of Hydrogen Mixing in Type Ia Supernova pre-Maximum Spectra

The presence of a small amount of hydrogen is expected in most single degenerate scenarios for producing a Type Ia supernova (SN Ia). While hydrogen may be detected in very early high resolution optical spectra, in early radio spectra, and in X-ray spectra, here we examine the possibility of detecting hydrogen in early low resolution spectra such as those that will be obtained by proposed large scale searches for nearby SNe Ia. We find that definitive detections will require both very early spectra (less than 5 days after explosion) and perhaps slightly higher amounts of hydrogen than are currently predicted to be mixed into the outer layers of SNe Ia. Thus, the non-detection of hydrogen so far does not in and of itself rule out any current progenitor models. Nevertheless, very early spectra of SNe Ia will provide significant clues to the amount of hydrogen present and hence to the nature of the SN Ia progenitor system. Spectral coverage in both the optical and IR will be required to definitively identify hydrogen in low resolution spectra.

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arXiv2002

Searching for Hydrogen in Type Ib Supernovae, and for Helium and Hydrogen in Type Ic

Identifying the progenitors of Type Ib and Type Ic supernovae requires knowing, among other things, whether SNe Ib eject hydrogen, and whether SNe Ic eject helium, and perhaps even hydrogen. Recently it has become clear that some SNe Ib do eject hydrogen, and it may be that all SNe Ib do. Two arguments that have been made in the past that SNe Ic eject helium are difficult to confirm, but I discuss other possible evidence that SNe Ic eject helium, as well as hydrogen. If so, these elements extend to lower ejection velocities than in SNe Ib. The spectroscopic differences between SNe Ib and SNe Ic may depend on the radial distributions of the helium and hydrogen as well as on the ejected masses of helium and hydrogen. We should consider the possibility that SNe Ic are more mixed up.

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arXiv2002

Synthetic Spectra from 3D Models of Supernovae

We describe Brute, a code for generating synthetic spectra from 3D supernova models. It is based on a Monte Carlo implementation of the Sobolev approximation. The current version is highly parameterized, but in the future it will be extended for full NLTE spectrum modelling of supernovae.

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arXiv2002

A Search for Radio Emission from Supernovae With Ages from About One Week to More Than 80 Years

We report VLA radio observations of 29 SNe with ages ranging from 10 days to about 90 years past explosion. These observations significantly contribute to the existing data pool on such objects. Included are detections of known radio SNe~1950B, 1957D, 1970G, 1983N, the suspected radio SN 1923A, and the possible radio SN 1961V. None of the remaining 23 observations resulted in detections, providing further evidence to support the observed trend that most SNe are not detectable radio emitters. To investigate the apparent lack of radio emission from the SNe reported here, we have followed standard practice and used Chevalier's ``standard model'' to derive (upper limits to) the mass-loss rates for the super nova progenitors. These upper limits to the fluxes are consistent with a lack of circumstellar material needed to provide detectable radio emission for SNe at these ages and distances. Comparison of the radio luminosities of these supernovae as a function of age past explosion to other well-observed radio SNe indicates that the Type II SNe upper limits are more consistent with the extrapolated light curves of SN 1980K than of SN 1979C, suggesting that SN 1980K may be a more typical radio emitter than SN 1979C. For completeness, we have included an appendix where the results of analyses of the non-SN radio sources are presented. Where possible, we make (tentative) identifications of these sources using various methods.

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arXiv2002

Detailed Spectroscopic Analysis of SN 1987A: The Distance to the LMC using the SEAM method

Supernova 1987A remains the most well-studied supernova to date. Observations produced excellent broad-band photometric and spectroscopic coverage over a wide wavelength range at all epochs. We model the observed spectra from Day 1 to Day 81 using a hydrodynamical model. We show that good agreement can be obtained at times up to about 60 days, if we allow for extended nickel mixing. Later than about 60 days the observed Balmer lines become stronger than our models can reproduce. We show that this is likely due to a more complicated distribution of gamma-rays than we allow for in our spherically symmetric calculations. We present synthetic light curves in UBVRIJHK and a synthetic bolometric light curve. Using this broad baseline of detailed spectroscopic models we find a distance modulus mu = 18.5 +/- 0.2 using the SEAM method of determining distances to supernovae. We find that the explosion time agrees with that of the neutrino burst and is constrained at 68% confidence to within +/- 0.9 days. We argue that the weak Balmer lines of our detailed model calculations casts doubt on the accuracy of the purely photometric EPM method. We also suggest that Type IIP supernovae will be most useful as distance indicators at early times due to a variety of effects.

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arXiv2002

Modelling the Hubble Space Telescope Ultraviolet and Optical Spectrum of Spot 1 on the Circumstellar Ring of SN 1987A

We report and interpret HST/STIS long-slit observations of the optical and ultraviolet (1150 - 10270 Angstrom) emission-line spectra of the rapidly brightening Spot 1 on the equatorial ring of SN 1987A between 1997 September and 1999 October (days 3869 -- 4606 after outburst). The emission is caused by radiative shocks created where the supernova blast wave strikes dense gas protruding inward from the equatorial ring. We measure and tabulate line identifications, fluxes and, in some cases, line widths and shifts. We compute flux correction factors to account for substantial interstellar line absorption of several emission lines. Nebular analysis shows that optical emission lines come from a region of cool (T_e ~ 10^4 K) and dense (n_e ~ 10^6 cm^-3) gas in the compressed photoionized layer behind the radiative shock. The observed line widths indicate that only shocks with shock velocities V_s < 250 km/s have become radiative, while line ratios indicate that much of the emission must have come from yet slower (V_s < 135 k/ms) shocks. We are able to fit the UV fluxes with an idealized radiative shock model consisting of two shocks (V_s = 135 and 250 km/s). The observed UV flux increase with time can be explained by the increase in shock surface areas as the blast wave overtakes more of the protrusion. The observed flux ratios of optical to highly-ionized UV lines are greater by a factor of ~ 2 -- 3 than predictions from the radiative shock models and we discuss the possible causes. We also present models for the observed H-alpha line widths and profiles, which suggests that a chaotic flow exists in the photoionized regions of these shocks. We discuss what can be learned with future observations of all the spots present on the equatorial ring.

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arXiv2001

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.

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arXiv2001

Optical Spectra of Supernovae

Supernova flux and polarization spectra bring vital information on the geometry, physical conditions, and composition structure of the ejected matter. For some supernovae the circumstellar matter is also probed by the observed spectra. Some of this information can be inferred directly from the observed line profiles and fluxes, but because of the Doppler broadening and severe line blending, interpretation often involves the use of synthetic spectra. The emphasis in this Chapter is on recent results obtained with the help of synthetic spectra.

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arXiv2001

Coping with Type Ia Supernova "Evolution" When Probing the Nature of the Dark Energy

Observations of high-redshift Type Ia supernovae (SNe Ia) have provided strong evidence that the dark energy is real, and making further accurate observations of high-redshift SNe Ia is the most promising way to probe the nature of the dark energy. We discuss one of the concerns about such a project - that of coping with SN Ia evolution. We emphasize that SN Ia evolution differs in an important respect from the kind of evolution that has foiled some past projects in observational cosmology, and we outline empirical strategies that will take it into account. The supporting role of physical models of SNe Ia also is discussed. Our conclusion is that systematic errors due to SN Ia evolution will be small.

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arXiv2001

A Complete Analytic Inversion of Supernova Lines in the Sobolev Approximation

We show that the shape of P-Cygni line profiles of photospheric phase supernova can be analytically inverted to extract both the optical depth and source function of the line -- i.e. all the physical content of the model for the case when the Sobolev approximation is valid. Under various simplifying assumptions, we derive formulae that give S(r) and tau(r) in terms of derivatives of the line flux with respect to wavelength. The transition region between the minimum and maximum of the line profile turns out to give especially interesting information on the optical depth near the photosphere. The formulae give insights into the relationship between line shape and physical quantities that may be useful in interpreting observed spectra and detailed numerical calculations.

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arXiv2001

Spectral Consequences of Deviation from Spherical Composition Symmetry in Type Ia Supernovae

We investigate the prospects for constraining the maximum scale of clumping in composition that is consistent with observed Type Ia supernova flux spectra. Synthetic spectra generated without purely spherical composition symmetry indicate that gross asymmetries make prominent changes to absorption features. Motivated by this, we consider the case of a single unblended line forming in an atmosphere with perturbations of different scales and spatial distributions. Perturbations of about 1% of the area of the photodisk simply weaken the absorption feature by the same amount independent of the line of sight. Conversely, perturbations of about 10% of the area of the photodisk introduce variation in the absorption depth which does depend on the line of sight. Thus, 1% photodisk area perturbations may be consistent with observed profile homogeneity but 10% photodisk area perturbations can not. Based on this, we suggest that the absence of significant variation in the depths of Si II 6355 absorption features in normal Type Ia spectra near maximum light indicates that any composition perturbations in these events are quite small. This also constrains future three-dimensional explosion models to produce ejecta profiles with only small scale inhomogeneities.

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arXiv2001

Direct Analysis of Spectra of Type Ib Supernovae

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared to photospheric-phase spectra of Type Ib supernovae (SNe Ib). Although the synthetic spectra are based on many simplifying approximations, including spherical symmetry, they account well for the observed spectra. Our sample of SNe Ib obeys a tight relation between the velocity at the photosphere, as determined from the Fe II features, and the time relative to that of maximum light. From this we infer that the masses and the kinetic energies of the events in this sample were similar. After maximum light the minimum velocity at which the He I features form usually is higher than the velocity at the photosphere, but the minimum velocity of the ejected helium is at least as low as 7000 kms. Previously unpublished spectra of SN 2000H reveal the presence of hydrogen absorption features, and we conclude that hydrogen lines also were present in SNe 1999di and 1954A. Hydrogen appears to be present in SNe Ib in general, although in most events it becomes too weak to identify soon after maximum light. The hydrogen-line optical depths that we use to fit the spectra of SNe 2000H, 1999di, and 1954A are not high, so only a mild reduction in the hydrogen optical depths would be required to make these events look like typical SNe Ib. Similarly, the He I line optical depths are not very high, so a moderate reduction would make SNe Ib look like SNe Ic.

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arXiv2001

NLTE Synthetic Spectral Fits to the Type Ia Supernova 1994D in NGC 4526

We have fit the normal, well observed, Type Ia Supernova (SN Ia) SN 1994D with non-LTE spectra of the deflagration model W7. We find that well before maximum luminosity W7 fits the optical spectra of SN 1994D. After maximum brightness the quality of the fits weakens as the spectrum forms in a core rich in iron peak elements. We show the basic structure of W7 is likely to be representative of the typical SN Ia. We have shown that like W7, the typical SN Ia has a layer of unburned C+O composition at v > 15000 \kmps, followed by layers of C-burned and O-burned material with a density structure similar to W7. We present UVOIR (UBVRIJKH) synthetic photometry and colors and compare with observation. We have computed the distance to the host galaxy, NGC 4526, obtaining a distance modulus of μ= 30.8 \pm 0.3. We discuss further application of this direct measurement of SNe Ia distances. We also discuss some simple modifications to W7 that could improve the quality of the fits to the observations.

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arXiv2001

Nickel Mixing in the Outer Layers of SN 1987A

Supernova 1987A remains the most well-observed and well-studied supernova to date. Observations produced excellent broad-band photometric and spectroscopic coverage over a wide wavelength range at all epochs. Here, we focus on the very early spectroscopic observations. Only recently have numerical models been of sufficient detail to accurately explain the observed spectra. In SN 1987A, good agreement has been found between observed and synthetic spectra for day one, but by day four, the predicted Balmer lines become much weaker than the observed lines. We present the results of work based on a radiation-hydrodynamic model by Blinnikov and collaborators. Synthetic non-LTE spectra generated from this model by the general radiation transfer code PHOENIX strongly support the theory that significant mixing of nickel into the outer envelope is required to maintain strong Balmer lines. Preliminary results suggest a lower limit to the average nickel mass of 1.0 \times 10^{-5} solar masses is required above 5000 \kmps by day four. PHOENIX models thus have the potential to be a sensitive probe for nickel mixing in the outer layers of a supernova.

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arXiv2000

Type Ia Supernovae: Toward the Standard Model?

In this short review I suggest that recent developments support the conjecture that Type Ia supernovae (SNe Ia) are the complete disruptions of Chandrasekhar-mass carbon-oxygen white dwarfs in single-degenerate binary systems. The causes of the observational diversity of SNe Ia within the context of this standard model, and the implications of the model for young remnants of SNe Ia, are briefly discussed.

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arXiv2000

Preliminary Spectral Analysis of the Type II Supernova 1999em

We have calculated fast direct spectral model fits to two early-time spectra of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The first is an extremely early blue optical spectrum and the second a combined HST and optical spectrum obtained one week later. Spectroscopically this supernova appears to be a normal Type II and these fits are in excellent agreement with the observed spectra. Our direct analysis suggests the presence of enhanced nitrogen. We have further studied these spectra with the full NLTE general model atmosphere code PHOENIX. While we do not find confirmation for enhanced nitrogen (nor do we rule it out), we do require enhanced helium. An even more intriguing possible line identification is complicated Balmer and He I lines, which we show falls naturally out of the detailed calculations with a shallow density gradient. We also show that very early spectra such as those presented here combined with sophisticated spectral modeling allows an independent estimate of the total reddening to the supernova, since when the spectrum is very blue, dereddening leads to changes in the blue flux that cannot be reproduced by altering the ``temperature'' of the emitted radiation. These results are extremely encouraging since they imply that detailed modeling of early spectra can shed light on both the abundances and total extinction of SNe II, the latter improving their utility and reliability as distance indicators.

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