Non-LTE time-dependent spectroscopic modelling of type II-plateau supernovae from the photospheric to the nebular phase: case study for 15 and 25Msun progenitor stars

We present the first non-LTE time-dependent radiative-transfer simulations of supernovae (SNe) II-Plateau (II-P) covering both the photospheric and nebular phases, from ~10 to >~1000d after the explosion, and based on 1.2B piston-driven ejecta produced from a 15Msun and a 25Msun non-rotating solar-metallicity star. The radial expansion of the gradually cooling photosphere gives rise to a near-constant luminosity up to >~100d after explosion. The photosphere remains in the outer 0.5Msun of the ejecta for up to ~50d after explosion. As the photosphere reaches the edge of the helium core, the SN luminosity drops by an amount mitigated by the progenitor radius and the 56Ni mass. Synthetic light-curves exhibit a bell-shape morphology, evolving faster for more compact progenitors, and with an earlier peak and narrower width in bluer filters. UV and U-band fluxes are very sensitive to line-blanketing, the metallicity, and the adopted model atoms. During the recombination epoch synthetic spectra are dominated by HI and metal lines, and are largely insensitive to the differing H/He/C/N/O composition of our two progenitor stars. In contrast, synthetic nebular-phase spectra reveal a broader/stronger OI doublet line in the higher-mass progenitor model, reflecting the larger masses of oxygen and nickel that are ejected. Our simulations overestimate the typical luminosity and the visual rise time of standard SNe II-P, likely a consequence of our progenitor stars being too big and/or too hydrogen rich. Comparison of our simulations with photospheric-phase observations of SN1999em of the same color are satisfactory. Our neglect of non-thermal processes leads to a fast disappearance of continuum radiation and Balmer-line emission at the end of the plateau phase. With the exception of HI lines, our nebular spectra show a striking similarity to contemporaneous observations of SN1999em.Comment: Accepted to MNRA

Non-thermal excitation and ionization in supernovae

We incorporate non-thermal excitation and ionization processes arising from non-thermal electrons that result from \gamma-ray energy deposition, into our radiative transfer code CMFGEN. The non-thermal electron distribution is obtained by solving the Spencer-Fano equation using the procedure of Kozma & Fransson (1992). We applied the non-thermal calculations to the blue supergiant explosion model whose early evolution was studied in Dessart & Hillier (2010). Non-thermal processes generally increase excitation and ionization and decrease the temperature of the ejecta. We confirm that non-thermal processes are crucial for modeling the nebular spectra. Both optical HI and HeI lines are significantly strengthened. While optical HeI lines are not easily discerned in observational spectra due to severe blending with other lines, HeI 2.058 \mu m provides an excellent opportunity to infer the influence of non-thermal processes. We also discuss the processes controlling the formation of the HeI lines during the nebular epoch. Most lines of other species are only slightly affected. We also show that the inclusion of FeI has substantial line-blanketing effects on the optical spectra. Our model spectra and synthetic light curves are compared to the observations of SN 1987A. The spectral evolution shows broad agreement with the observations, especially H\alpha. The uncertainties of the non-thermal solver are studied, and are expected to be small. With this new addition of non-thermal effects in CMFGEN, we now treat all known important processes controlling the radiative transfer of a supernova ejecta, whatever the type and the epoch.Comment: 18 pages, 22 figures, accepted to MNRA

Determining the main-sequence mass of Type II supernova progenitors

We present radiation-hydrodynamics simulations of core-collapse supernova (SN) explosions, artificially generated by driving a piston at the base of the envelope of a rotating or non-rotating red-supergiant progenitor star. We search for trends in ejecta kinematics in the resulting Type II-Plateau (II-P) SN, exploring dependencies with explosion energy and pre-SN stellar-evolution model. We recover the trivial result that larger explosion energies yield larger ejecta velocities in a given progenitor. However, we emphasise that for a given explosion energy, the increasing helium-core mass with main-sequence mass of such Type II-P SN progenitors leads to ejection of core-embedded oxygen-rich material at larger velocities. We find that the photospheric velocity at 15d after shock breakout is a good and simple indicator of the explosion energy in our selected set of pre-SN models. This measurement, combined with the width of the nebular-phase OI6303-6363A line, can be used to place an upper-limit on the progenitor main-sequence mass. Using the results from our simulations, we find that the current, but remarkably scant, late-time spectra of Type II-P SNe support progenitor main-sequence masses inferior to ~20Msun and thus, corroborate the inferences based on the direct, but difficult, progenitor identification in pre-explosion images. The narrow width of OI6303-6363A in Type II-P SNe with nebular spectra does not support high-mass progenitors in the range 25-30Msun. Combined with quantitative spectroscopic modelling, such diagnostics offer a means to constrain the main-sequence mass of the progenitor, the mass fraction of the core ejected, and thus, the mass of the compact remnant formed.Comment: accepted to MNRA

Numerical simulations of super-luminous supernovae of type IIn

We present numerical simulations that include 1-D Eulerian multi-group radiation-hydrodynamics, 1-D non-LTE radiative transfer, and 2-D polarised radiative transfer for super-luminous interacting supernovae (SNe). Our reference model is a ~10Msun inner shell with 10^51erg ramming into a ~3Msun cold outer shell (the circumstellar-medium, or CSM) that extends from 10^15cm to 2x10^16cm and moves at 100km/s. We discuss the light curve evolution, which cannot be captured adequately with a grey approach. In these interactions, the shock-crossing time through the optically-thick CSM is much longer than the photon diffusion time. Radiation is thus continuously leaking from the shock through the CSM, in disagreement with the shell-shocked model that is often invoked. Our spectra redden with time, with a peak distribution in the near-UV during the first month gradually shifting to the optical range over the following year. Initially Balmer lines exhibit a narrow line core and the broad line wings that are characteristic of electron scattering in the SNe IIn atmospheres (CSM). At later times they also exhibit a broad blue shifted component which arises from the cold dense shell. Our model results are broadly consistent with the bolometric light curve and spectral evolution observed for SN2010jl. Invoking a prolate pole-to-equator density ratio in the CSM, we can also reproduce the ~2% continuum polarisation, and line depolarisation, observed in SN2010jl. By varying the inner shell kinetic energy and the mass and extent of the outer shell, a large range of peak luminosities and durations, broadly compatible with super-luminous SNe IIn like 2010jl or 2006gy, can be produced.Comment: paper accepted to MNRA

Synthetic line and continuum linear-polarisation signatures of axisymmetric type II supernova ejecta

We present synthetic single-line and continuum linear-polarisation signatures due to electron scattering in axially-symmetric Type II supernovae (SNe) which we calculate using a Monte Carlo and a long-characteristic radiative-transfer code. Aspherical ejecta are produced by prescribing a latitudinal scaling or stretching of SN ejecta inputs obtained from 1-D non-LTE time-dependent calculations. We study polarisation signatures as a function of inclination, shape factor, wavelength, line identity, post-explosion time. At early times, cancellation and optical-depth effects make the polarisation intrinsically low, causing complicated sign reversals with inclination or continuum wavelength, and across line profiles. While the line polarisation is positive (negative) for an oblate (prolate) morphology at the peak and in the red wing, the continuum polarisation may be of any sign. These complex polarisation variations are produced not just by the asymmetric distribution of scatterers but also of the flux. Our early-time signatures are in contradiction with predictions for a centrally illuminated aspherical nebula, although this becomes a better approximation at nebular times. For a fixed asymmetry, our synthetic continuum polarisation is generally low, may evolve non-monotonically during the plateau phase, but it systematically rises as the ejecta become optically thin. Changes in polarization over time do not necessarily imply a change in the asymmetry of the ejecta. The SN structure (e.g., density/ionization) critically influences the level of polarisation. Importantly, a low polarisation (<0.5%) at early times does not necessarily imply a low degree of asymmetry as usually assumed. Asphericity influences line-profile morphology and the luminosity, which may compromise the accuracy of SN characteristics inferred from these.Comment: 25 pages, 23 figures, accepted to MNRA

Time Dependent Radiative Transfer Calculations for Supernovae

In previous papers we discussed results from fully time-dependent radiative transfer models for core-collapse supernova (SN) ejecta, including the Type II-peculiar SN 1987A, the more "generic" SN II-Plateau, and more recently Type IIb/Ib/Ic SNe. Here we describe the modifications to our radiative modeling code, CMFGEN, which allowed those studies to be undertaken. The changes allow for time-dependent radiative transfer of SN ejecta in homologous expansion. In the modeling we treat the entire SN ejecta, from the innermost layer that does not fall back on the compact remnant out to the progenitor surface layers. From our non-LTE time-dependent line-blanketed synthetic spectra, we compute the bolometric and multi-band light curves: light curves and spectra are thus calculated simultaneously using the same physical processes and numerics. These upgrades, in conjunction with our previous modifications which allow the solution of the time dependent rate equations, will improve the modeling of SN spectra and light curves, and hence facilitate new insights into SN ejecta properties, the SN progenitors and the explosion mechanism(s). CMFGEN can now be applied to the modeling of all SN typesComment: 20 pages, 10 figures, to appear in MNRA

2D Simulations of the Line-Driven Instability in Hot-Star Winds: II. Approximations for the 2D Radiation Force

We present initial attempts to include the multi-dimensional nature of radiation transport in hydrodynamical simulations of the small-scale structure that arises from the line-driven instability in hot-star winds. Compared to previous 1D or 2D models that assume a purely radial radiation force, we seek additionally to treat the lateral momentum and transport of diffuse line-radiation, initially here within a 2D context. A key incentive is to study the damping effect of the associated diffuse line-drag on the dynamical properties of the flow, focusing particularly on whether this might prevent lateral break-up of shell structures at scales near the lateral Sobolev angle of ca. $1^{\rm o}$. We first explore nonlinear simulations that cast the lateral diffuse force in the simple, local form of a parallel viscosity. Second, to account for the lateral mixing of radiation associated with the radial driving, we next explore models in which the radial force is azimuthally smoothed over a chosen scale. Third, to account for both the lateral line-drag and the lateral mixing in a more self-consistent way, we explore further a method first proposed by Owocki (1999), which uses a restricted 3-ray approach that combines a radial ray with two oblique rays set to have an impact parameter $p < R_{\ast}$ within the stellar core. From numerical simulations, we find that, compared to equivalent 1-ray simulations, the high-resolution 3-ray models show systematically a much higher lateral coherence.... (Full abstract in paper)Comment: Accepted by A&A, 12 pages, 7 figures, 3 only shown in version available at http://www.mpa-garching.mpg.de/~luc/2778.ps.g

Time-dependent Effects in Photospheric-Phase Type II Supernova Spectra

Spectroscopic modeling of Type II supernovae (SNe) generally assumes steady-state. Following the recent suggestion of Utrobin & Chugai, but using the 1D non-LTE line-blanketed model atmosphere code CMFGEN, we investigate the effects of including time-dependent terms that appear in the statistical and radiative equilibrium equations. We base our discussion on the ejecta properties and the spectroscopic signatures obtained from time-dependent simulations, investigating different ejecta configurations, and covering their evolution from one day to six weeks after shock breakout. Compared to equivalent steady-state models, our time-dependent models produce SN ejecta that are systematically over-ionized, affecting helium at one week after explosion, but ultimately affecting all ions after a few weeks. While the continuum remains essentially unchanged, time-dependence effects on observed spectral lines are large. At the recombination epoch, HI lines and NaID are considerably stronger and broader than in equivalent steady-state models, while CaII8500A is weakened. If time dependence is allowed for, the HeI lines at 5875A and 10830A appear about 3 times stronger at one week, and HeI10830A persists as a blue-shifted absorption feature even at 6 weeks after explosion. Time dependence operates through the energy gain from changes in ionization and excitation, and, perhaps more universally across SN types, from the competition between recombination and expansion, which in-turn, can be affected by optical-depth effects. Our time-dependent models compare well with observations of the low-luminosity low-velocity SN 1999br and the more standard SN 1999em, reproducing the Halpha line strength at the recombination epoch, and without the need for setting unphysical requirements on the magnitude of nickel mixing.Comment: 19 pages, 18 figures, accepted for publication in MNRAS, high-resolution of the paper at http://hermes.as.arizona.edu/~luc/pap_ddt/pap_ddt.ps.g