arXiv subjects

E. Baron

Publications and source records attributed to E. Baron.

At least 127 records · Page 7Linked to original sources

arXiv2009

A 3D radiative transfer framework: VI. PHOENIX/3D example applications

We demonstrate the application of our 3D radiative transfer framework in the model atmosphere code PHOENIX/3D for a number of spectrum synthesis calculations for very different conditions. The 3DRT framework discussed in the previous papers of this series was added to our general-purpose model atmosphere code PHOENIX/1D and an extended 3D version PHOENIX/3D was created. The \phxT code is parallelized via the MPI library using a hierarchical domain decomposition and displays very good strong scaling. We present the results of several test cases for widely different atmosphere conditions and compare the 3D calculations with equivalent 1D models to assess the internal accuracy of the 3D modeling. In addition, we show the results for a number of parameterized 3D structures. With presently available computational resources it is possible to solve the full 3D radiative transfer (including scattering) problem with the same micro-physics as included in 1D modeling.

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arXiv2009

On the Hydrogen Recombination Time in Type II Supernova Atmospheres

NLTE radiative transfer calculations of differentially expanding supernovae atmospheres are computationally intensive and are almost universally performed in time-independent snapshot mode. The validity of the steady-state approximation in the rate equations has recently been questioned. We calculate the effective recombination time of hydrogen in SN II using our general purpose model atmosphere code PHOENIX. While we find that the recombination time for the conditions of SNe II at early times is increased over the classical value for the case of a simple hydrogen model atom with energy levels corresponding to just the first 2 principle quantum numbers, the classical value of the recombination time is recovered in the case of a multi-level hydrogen atom. We also find that the recombination time at most optical depths is smaller in the case of a multi-level atom than for a simple two-level hydrogen atom. We find that time dependence in the rate equations is important in the early epochs of a supernova's lifetime. The changes due to the time dependent rate equation (at constant input luminosity) are manifested in physical parameters such as the level populations which directly affects the spectra. The H-alpha profile is affected by the time dependent rate equations at early times. At later times time dependence does not significantly modify the level populations and therefore the H-alpha profile is roughly independent of whether the steady-state or time-dependent approach is used.

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arXiv2009

Comoving-frame radiative transfer in arbitrary velocity fields -- II. Large scale applications

A solution of the radiative-transfer problem in arbitrary velocity fields introduced in a previous paper, has limitations in its applicability. For large-scale applications, the methods described also require large memory sets that are commonly not available to state-of-the-art computing hardware. In this work, we modify the algorithm to allow the computation of large-scale problems. We reduce the memory footprint via a domain decomposition. By introducing iterative Gauss-Seidel type solvers, we improve the speed of the overall computation. Because of the domain decomposition, the new algorithm requires the use of parallel-computing systems. The algorithm that we present permits large-scale solutions of radiative-transfer problems that include arbitrary wavelength couplings. In addition, we discover a quasi-analytic formal solution of the radiative transfer that significantly improves the overall computation speed. More importantly, this method ensures that our algorithm can be applied to multi-dimensional Lagrangian radiative-transfer calculations. In multi-dimensional atmospheres, velocity fields are in general chaotic ensuring that the inclusion of arbitrary wavelength couplings are mandatory.

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arXiv2009

3D Radiative Transfer with PHOENIX

Using the methods of general relativity Lindquist derived the radiative transfer equation that is correct to all orders in v/c. Mihalas developed a method of solution for the important case of monotonic velocity fields with spherically symmetry. We have developed the generalized atmosphere code PHOENIX, which in 1-D has used the framework of Mihalas to solve the radiative transfer equation (RTE) in 1-D moving flows. We describe our recent work including 3-D radiation transfer in PHOENIX and particularly including moving flows exactly using a novel affine method. We briefly discuss quantitative spectroscopy in supernovae.

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arXiv2009

Time-dependent radiative transfer with PHOENIX

Aims. We present first results and tests of a time-dependent extension to the general purpose model atmosphere code PHOENIX. We aim to produce light curves and spectra of hydro models for all types of supernovae. Methods. We extend our model atmosphere code PHOENIX to solve time-dependent non-grey, NLTE, radiative transfer in a special relativistic framework. A simple hydrodynamics solver was implemented to keep track of the energy conservation of the atmosphere during free expansion. Results. The correct operation of the new additions to PHOENIX were verified in test calculations. Conclusions. We have shown the correct operation of our extension to time-dependent radiative transfer and will be able to calculate supernova light curves and spectra in future work.

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arXiv2009

A 3D radiative transfer framework: V. Homologous Flows

Observations and theoretical calculations have shown the importance of non-spherically symmetric structures in supernovae. Thus, the interpretation of observed supernova spectra requires the ability to solve the transfer equation in 3-D moving atmospheres. We present an implementation of the solution of the radiative transfer equation in 3-D homologously expanding atmospheres in spherical coordinates. The implementation is exact to all orders in v/c. We use a new affine method that makes use of the fact that photons travel on straight lines. We compare our results in 3-D for spherically symmetric test problems with high velocity fields and find excellent agreement. Our well-tested 1-D results are based on methods where the momentum directions vary along the characteristic (co-moving momentum directions). Thus, we are able to verify both the analytic framework and its numerical implementation. Additionally, we have been able to test the parallelization over characteristics. Using 512^2 momentum angles we ran the code on 16,384 Opteron processors and achieved excellent scaling. It is now possible to calculate synthetic spectra from realistic 3D hydro simulations. This should open an era of progress in hydro modeling, similar to that that occurred in the 1980s when 1-D models were confronted with synthetic spectra.

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arXiv2009

A 3D radiative transfer framework: IV. spherical & cylindrical coordinate systems

We extend our framework for 3D radiative transfer calculations with a non-local operator splitting methods along (full) characteristics to spherical and cylindrical coordinate systems. These coordinate systems are better suited to a number of physical problems than Cartesian coordinates. The scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a full characteristics method. The approximate $Λ$ operator is constructed considering nearest neighbors exactly. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter for the different coordinate systems. The results are directly compared to 1D plane parallel tests. The 3D results agree very well with the well-tested 1D calculations.

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arXiv2009

Type Ia supernova science 2010-2020

In the next decade Type Ia supernovae (SNe Ia) will be used to test theories predicting changes in the Dark Energy equation of state with time. Ultimately this requires a dedicated space mission like JDEM. SNe Ia are mature cosmological probes --- their limitations are well characterized, and a path to improvement is clear. Dominant systematic errors include photometric calibration, selection effects, reddening, and population-dependent differences. Building on past lessons, well-controlled new surveys are poised to make strides in these areas: the Palomar Transient Factory, Skymapper, La Silla QUEST, Pan-STARRS, the Dark Energy Survey, LSST, and JDEM. They will obviate historical calibrations and selection biases, and allow comparisons via large subsamples. Some systematics follow from our ignorance of SN Ia progenitors, which there is hope of determining with SN Ia rate studies from 0<z<4. Aside from cosmology, SNe Ia regulate galactic and cluster chemical evolution, inform stellar evolution, and are laboratories for extreme physics. Essential probes of SNe Ia in these contexts include spectroscopy from the UV to the IR, X-ray cluster and SN remnant observations, spectropolarimetry, and advanced theoretical studies. While there are an abundance of discovery facilities planned, there is a deficit of follow-up resources. Living in the systematics era demands deep understanding rather than larger statistics. NOAO ReSTAR initiative to build 2-4m telescopes would provide necessary follow-up capability. Finally, to fully exploit LSST, well-matched wide-field spectroscopic capabilities are desirable.

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arXiv2009

Analyzing SN2003Z with PHOENIX

We present synthetic spectra around maximum for the type II supernova SN 2003Z, which was first detected on January 29.7 2003. Comparison with observed spectra aim at the determination of physical parameters for SN 2003Z. Synthetic spectra are calculated with our stellar atmosphere code PHOENIX. It solves the special relativistic equation of radiative transfer, including large NLTE-calculations and line blanketing by design, in 1-dimensional spherical symmetry. The observed spectra were obtained at the 3.5 meter telescope at Calar Alto. The TWIN instrument was used so that a spectral range from about 3600 to 7500 Angstroem was covered. The spectra were taken on Feb. 4, 5, 9, and 11, 2003. The physical parameters of the models give the luminosities, a range of possible velocity profiles for the SN, an estimate of the colour excess, and the observed metalicity.

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arXiv2009

A new formal solution of the radiative transfer in arbitrary velocity fields

We present a new formal solution of the Lagrangian equation of radiative transfer that is useful in solving the equation of radiative transfer in the presence of arbitrary velocity fields. Normally a term due to the inclusion of the wavelength derivative in the Lagrangian equation of radiative transfer is associated with a generalised opacity. In non-monotonic velocity fields, this generalised opacity may become negative. To ensure that the opacity remains positive, this term of the derivative is included in the formal solution of the radiative transfer problem. The new definition of the generalised opacity allows for a new solution of the equation of radiative transfer in the presence of velocity fields. It is especially useful for arbitrary velocity fields, where it effectively prevents the occurrences of negative generalised opacities and still allows the explicit construction of the Lambda-operator of the system needed for an accelerated Lambda-iteration. We performed test calculations, where the results of old, established solutions were compared with the new solution. The relative deviations never exceeded 1% and so the new solution is indeed suitable for use in radiative-transfer modelling. Non-monotonic velocity fields along photon paths frequently occur in three-dimensional hydrodynamical models of astrophysical atmospheres. Therefore, the formal solution will be of use for multidimensional radiative transfer and has immediate applications in the modelling of pulsating stars and astrophysical shock fronts.

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arXiv2009

Comparative Direct Analysis of Type Ia Supernova Spectra. V. Insights from A Larger Sample and Quantitative Subclassification

A comparative study of optical spectra of Type Ia supernovae (SNe Ia) is extended, in the light of new data. The discussion is framed in terms of the four groups defined in previous papers of this series: core normal (CN); broad line (BL); cool (CL); and shallow silicon (SS). Emerging features of the SN Ia spectroscopic diversity include evidence (1) that extreme CL SN 1991bg-likes are not a physically distinct subgroup and (2) for the existence of a substantial number of SN 1999aa-like SSs that are very similar to each other and distinguishable from CN even as late as three weeks after maximum light. SN 1999aa-likes may be relatively numerous, yet not a physically distinct subgroup. The efficacy of quantitative spectroscopic subclassification of SNe Ia based on the equivalent widths of absorption features near 5750 A and 6100 A near maximum light is discussed. The absolute magnitude dispersion of a small sample of CNs is no larger than the characteristic absolute magnitude uncertainty.

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arXiv2008

Stellar Population Models and Individual Element Abundances II: Stellar Spectra and Integrated Light Models

The first paper in this series explored the effects of altering the chemical mixture of the stellar population on an element by element basis on stellar evolutionary tracks and isochrones to the end of the red giant branch. This paper extends the discussion by incorporating the fully consistent synthetic stellar spectra with those isochrone models in predicting integrated colors, Lick indices, and synthetic spectra. Older populations display element ratio effects in their spectra at higher amplitude than younger populations. In addition, spectral effects in the photospheres of stars tend to dominate over effects from isochrone temperatures and lifetimes, but, further, the isochrone-based effects that are present tend to fall along the age-metallicity degeneracy vector, while the direct stellar spectral effects usually show considerable orthogonality.

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arXiv2008

A 3D radiative transfer framework: III. periodic boundary conditions

We present a general method to solve radiative transfer problems including scattering in the continuum as well as in lines in 3D configurations with periodic boundary conditions. he scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a full characteristics method. The approximate $Λ$ operator is constructed considering nearest neighbors exactly. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter and two choices for the temperature structure. The results are directly compared to 1D plane parallel tests. The 3D results agree very well with the well-tested 1D calculations.

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arXiv2008

GALEX Spectroscopy of SN 2005ay suggests a UV spectral uniformity among type II-P supernovae

We present the first results from our GALEX program designed to obtain ultraviolet (UV) spectroscopy of nearby core-collapse supernovae (SNe). Our first target, SN 2005ay in the nearby galaxy NGC 3938, is a typical member of the II-P SN subclass. Our spectra show remarkable similarity to those of the prototypical type II-P event SN 1999em, and resemble also Swift observations of the recent type II-P event SN 2005cs. Taken together, the observations of these three events trace the UV spectral evolution of SNe II-P during the first month after explosion, as required in order to interpret optical observations of high-redshift SNe II-P, and to derive cross-filter K-corrections. While still highly preliminary, the apparent UV homogeneity of SNe II-P bodes well for the use of these events as cosmological probes at high redshift.

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arXiv2008

Multi-layered Spectral Formation in SNe Ia Around Maximum Light

We use the radiative transfer code PHOENIX to study the line formation of the wavelength region 5000-7000 Angstroms. This is the region where the SNe Ia defining Si II feature occurs. This region is important since the ratio of the two nearby silicon lines has been shown to correlate with the absolute blue magnitude. We use a grid of LTE synthetic spectral models to investigate the formation of line features in the spectra of SNe Ia. By isolating the main contributors to the spectral formation we show that the ions that drive the spectral ratio are Fe III, Fe II, Si II, and S II. While the first two strongly dominate the flux transfer, the latter two form in the same physical region inside of the supernova. We also show that the naive blackbody that one would derive from a fit to the observed spectrum is far different than the true underlying continuum.

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arXiv2008

The Dartmouth Stellar Evolution Database

The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from -2.5 to +0.5, [alpha/Fe] from -0.2 to +0.8 (for [Fe/H] <=0) or +0.2 (for [Fe/H] >0), and initial He mass fractions from Y=0.245 to 0.40. Stellar evolution tracks were computed for masses between 0.1 and 4 Msun, allowing isochrones to be generated for ages as young as 250 Myr. For the range in masses where the core He flash occurs, separate He-burning tracks were computed starting from the zero age horizontal branch. The tracks and isochrones have been transformed to the observational plane in a variety of photometric systems including standard UBV(RI)c, Stromgren uvby, SDSS ugriz, 2MASS JHKs, and HST ACS-WFC and WFPC2. The Dartmouth Stellar Evolution Database is accessible through a website at http://stellar.dartmouth.edu/~models/ where all tracks, isochrones, and additional files can be downloaded.

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arXiv2008

A Self-Consistent NLTE-Spectra Synthesis Model of FeLoBAL QSOs

We present detailed radiative transfer spectral synthesis models for the Iron Low Ionization Broad Absorption Line (FeLoBAL) active galactic nuclei (AGN) FIRST J121442.3+280329 and ISO J005645.1-273816. Detailed NLTE spectral synthesis with a spherically symmetric outflow reproduces the observed spectra very well across a large wavelength range. While exact spherical symmetry is probably not required, our model fits are of high quality and thus very large covering fractions are strongly implied by our results. We constrain the kinetic energy and mass in the ejecta and discuss their implications on the accretion rate. Our results support the idea that FeLoBALs may be an evolutionary stage in the development of more ``ordinary'' QSOs.

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arXiv2007

Comparative Direct Analysis of Type Ia Supernova Spectra. IV. Postmaximum

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.

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