Monte Carlo techniques for time-dependent radiative transfer in 3-D supernovae

Monte Carlo techniques based on indivisible energy packets are described for computing light curves and spectra for 3-D supernovae. The radiative transfer is time-dependent and includes all effects of O(v/c). Monte Carlo quantization is achieved by discretizing the initial distribution of 56Ni into radioactive pellets. Each pellet decays with the emission of a single energy packet comprising gamma-ray photons representing one line from either the 56Ni or the 56Co decay spectrum. Subsequently, these energy packets propagate through the homologously-expanding ejecta with appropriate changes in the nature of their contained energy as they undergo Compton scatterings and pure absorptions. The 3-D code is tested by applying it to a spherically-symmetric SN in which the transfer of optical radiation is treated with a grey absorption coefficient. This 1-D problem is separately solved using Castor's co-moving frame moment equations. Satisfactory agreement is obtained. The Monte Carlo code is a platform onto which more advanced treatments of the interactions of matter and radiation can be added. Some of these have already been developed and tested in previous papers and are summarized here.Comment: 14 pages, 5 figures. Accepted by A&

Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis

We observed seven central stars of planetary nebulae (CSPN) in the Large Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE), and performed a model-based analysis of these spectra in conjunction with Hubble Space Telescope (HST) spectra in the UV and optical range to determine the stellar and nebular parameters. Most of the objects show wind features, and they have effective temperatures ranging from 38 to 60 kK with mass-loss rates of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One object (SMP LMC 61) is a [WC4] star, and we fit its spectra with He/C/O-rich abundances typical of the [WC] class, and find its atmosphere to be iron-deficient. Most objects have very hot (T ~> 2000 K) molecular hydrogen in their nebulae, which may indicate a shocked environment. One of these (SMP LMC 62) also displays OVI 1032-38 nebular emission lines, rarely observed in PN.Comment: 53 pages, 15 figures (11 color). Accepted for publication in Ap

Far-UV Spectroscopic Analyses of Four Central Stars of Planetary Nebulae

We analyze the Far-UV/UV spectra of four central stars of planetary nebulae with strong wind features -- NGC 2371, Abell 78, IC 4776 and NGC 1535, and derive their photospheric and wind parameters by modeling high-resolution FUSE (Far-Ultraviolet Spectroscopic Explorer) data in the Far-UV and HST-STIS and IUE data in the UV with spherical non-LTE line-blanketed model atmospheres. Abell 78 is a hydrogen-deficient transitional [WR]-PG 1159 object, and we find NGC 2371 to be in the same stage, both migrating from the constant-luminosity phase to the white dwarf cooling sequence with Teff ~= 120 kK, Mdot ~= 5x10^-8 Msun/yr. NGC 1535 is a ``hydrogen-rich'' O(H) CSPN, and the exact nature of IC 4776 is ambiguous, although it appears to be helium burning. Both objects lie on the constant-luminosity branch of post-AGB evolution and have Teff ~= 65 kK, Mdot ~= 1x10^-8 Msun/yr. Thus, both the H-rich and H-deficient channels of PN evolution are represented in our sample. We also investigate the effects of including higher ionization stages of iron (up to FeX) in the model atmosphere calculations of these hot objects (usually neglected in previous analyses), and find iron to be a useful diagnostic of the stellar parameters in some cases. The Far-UV spectra of all four objects show evidence of hot (T ~ 300 K) molecular hydrogen in their circumstellar environments.Comment: 38 pages, 8 figures (6 color). Accepted for publication in Ap

Particle kinetic analysis of a polar jet from SECCHI COR data

Aims. We analyze coronagraph observations of a polar jet observed by the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite onboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft. Methods. In our analysis we compare the brightness distribution of the jet in white-light coronagraph images with a dedicated kinetic particle model. We obtain a consistent estimate of the time that the jet was launched from the solar surface and an approximate initial velocity distribution in the jet source. The method also allows us to check the consistency of the kinetic model. In this first application, we consider only gravity as the dominant force on the jet particles along the magnetic field. Results. We find that the kinetic model explains the observed brightness evolution well. The derived initiation time is consistent with the jet observations by the EUVI telescope at various wavelengths. The initial particle velocity distribution is fitted by Maxwellian distributions and we find deviations of the high energy tail from the Maxwellian distributions. We estimate the jet's total electron content to have a mass between 3.2 \times 1014 and 1.8 \times 1015 g. Mapping the integrated particle number along the jet trajectory to its source region and assuming a typical source region size, we obtain an initial electron density between 8 \times 109 and 5 \times 1010 cm-3 that is characteristic for the lower corona or the upper chromosphere. The total kinetic energy of all particles in the jet source region amounts from 2.1 \times 1028 to 2.4 \times 1029 erg.Comment: A&A, in pres

First measurements of radar coherent scatter by the Radio Aurora Explorer CubeSat

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95350/1/grl29342.pd

Wind modelling of very massive stars up to 300 solar masses

Some studies have claimed a universal stellar upper-mass limit of 150 Msun. A factor that is often overlooked is that there might be a difference between the current and initial masses of the most massive stars, as a result of mass loss. We present Monte Carlo mass-loss predictions for very massive stars in the range 40-300 Msun, with large luminosities and Eddington factors Gamma. Using our new dynamical approach, we find an upturn in the mass-loss vs. Gamma dependence, at the point where the winds become optically thick. This coincides with the location where wind efficiency numbers surpass the single-scattering limit of Eta = 1, reaching values up to Eta = 2.5. Our modelling suggests a transition from common O-type winds to Wolf-Rayet characteristics at the point where the winds become optically thick. This transitional behaviour is also revealed with respect to the wind acceleration parameter beta, which starts at values below 1 for the optically thin O-stars, and naturally reaches values as high as 1.5-2 for the optically thick Wolf-Rayet models. An additional finding concerns the transition in spectral morphology of the Of and WN characteristic He II line at 4686 Angstrom. When we express our mass-loss predictions as a function of the electron scattering Gamma_e (=L/M) only, we obtain a mass-loss Gamma dependence that is consistent with a previously reported power-law Mdot propto Gamma^5 (Vink 2006) that was based on our semi-empirical modelling approach. When we express Mdot in terms of both Gamma and stellar mass, we find Mdot propto M^0.8 Gamma^4.8 for our high Gamma models. Finally, we confirm that the Gamma-effect on the mass-loss predictions is much stronger than that of an increased helium abundance, calling for a fundamental revision in the way mass loss is incorporated in evolutionary models of the most massive stars.Comment: minor language changes (Astronomy & Astrophysics in press - 11 pages, 10 figures

Mass-loss rates of Very Massive Stars

We discuss the basic physics of hot-star winds and we provide mass-loss rates for (very) massive stars. Whilst the emphasis is on theoretical concepts and line-force modelling, we also discuss the current state of observations and empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the Local Universe", Springer, Ed. Jorick S. Vin

Trans-Relativistic Blast Waves in Supernovae as Gamma-Ray Burst Progenitors

We investigate the acceleration of shock waves to relativistic velocities in the outer layers of exploding stars. By concentrating the explosion energy in the outermost ejecta, such trans-relativistic blast waves can serve as the progenitors of gamma-ray bursts (GRBs); in particular, the ``baryon-loading'' problem that plagues many models of GRBs is circumvented. We present physically motivated and numerically validated analytic expressions to describe trans-relativistic blast waves in supernovae. We find that relativistic ejecta are enhanced in more centrally condensed envelopes, e.g., for radiative envelopes, when the luminosity approaches the Eddington limit. We present convenient formulae for estimating the relativistic ejecta from a given progenitor. We apply our analytic and numerical methods to a model of SN 1998bw, finding significantly enhanced relativistic ejecta compared to previous studies. We propose that GRB 980425 is associated with SN 1998bw and may have resulted from an approximately spherical explosion producing ~10^-6 M_sun of mildly relativistic ejecta with mean Lorentz factor ~2, which then interacted with a dense circumstellar wind with mass loss rate ~few x 10^-4 M_sun/yr. A highly asymmetric explosion is not required. An extreme model of ``hypernova'' explosions in massive stars is able to account for the energetics and relativistic ejecta velocities required by many of the observed cosmological GRBs. We present simplified models of explosions resulting from accretion-induced collapse of white dwarfs and phase transitions of neutron stars. While we find increased energies in relativistic ejecta compared to previous studies, these explosions are unlikely to be observed at cosmological distances with current detectors. (abridged)Comment: Accepted to ApJ; minor changes from previous version; 41 pages (including 12 figures

Climate change impacts on banana yields around the world

This is the author accepted manuscript. The final version is available from Nature Research via the DOI in this r4ecordData availability: All data used are publicly available and open access. All banana production data sources are listed in Supplementary Table 1. All climatic and topographic data sources are listed in the Methods.Nutritional diversity is a key element of food security1,2,3. However, research on the effects of climate change on food security has, thus far, focused on the main food grains4,5,6,7,8, while the responses of other crops, particularly those that play an important role in the developing world, are poorly understood. Bananas are a staple food and a major export commodity for many tropical nations9. Here, we show that for 27 countries—accounting for 86% of global dessert banana production—a changing climate since 1961 has increased annual yields by an average of 1.37 t ha−1. Past gains have been largely ubiquitous across the countries assessed and African producers will continue to see yield increases in the future. However, global yield gains could be dampened or disappear, reducing to 0.59 t ha−1 and 0.19 t ha−1 by 2050 under the climate scenarios for Representative Concentration Pathways 4.5 and 8.5, respectively, driven by declining yields in the largest producers and exporters. By quantifying climate-driven and technology-driven influences on yield, we also identify countries at risk from climate change and those capable of mitigating its effects or capitalizing on its benefits.Biotechnology and Biological Sciences Research Council (BBSRC)European Union Horizon 202

A New Connection between Central Engine Weak Physics and the Dynamics of Gamma-Ray Burst Fireballs

We demonstrate a qualitatively new aspect of the dynamics of Gamma-Ray Burst (GRB) fireballs: the development of a substantial dispersion in the proton component in fireballs in which neutron decoupling occurs and is sufficiently pronounced. This effect depends sensitively on the neutron to proton ratio in the fireball, becoming more dramatic with increasing neutron excess. Simple physical arguments and transport calculations indicate that the dispersion in Lorentz factor of the protons can be of order the final mean Lorentz factor of the fireball. We show how plasma instabilities could play an important role in the evolution of the fireball and how they might ultimately govern the development of such a velocity dispersion in the proton component. The role of these instabilities in setting/diminishing a proton Lorentz factor dispersion represents a new and potentially important venue for the study of plasma instabilities. Significant dispersion in the proton velocities translates into fewer protons attaining the highest Lorentz factors. This is tantamount to a reduction in the total energy required to attain a given Lorentz factor for the highest energy protons. As well, a proton component dispersion can have consequences for the electromagnetic and neutrino signature of GRBs.Comment: Added discussion of plasma instabilities and the requirement of charge neutrality. 6 pages, 4 figure