ALBERT

Description: We present 1D non-local thermodynamic equilibrium time-dependent radiative-transfer simulations for supernovae (SNe) of Type IIb, Ib, and Ic that result from the terminal explosion of the mass donor in a close-binary system. Here, we select three ejecta with a total kinetic energy of 1.2  x  10 51  erg, but characterized by different ejecta masses (2–5 M ), composition, and chemical mixing. The Type IIb/Ib models correspond to the progenitors that have retained their He-rich shell at the time of explosion. The Type Ic model arises from a progenitor that has lost its helium shell, but retains 0.32 M of helium in a CO-rich core of 5.11 M . We discuss their photometric and spectroscopic properties during the first 2–3 months after explosion, and connect these to their progenitor and ejecta properties including chemical stratification. For these three models, Arnett's rule overestimates the 56 Ni mass by  50 per cent while the procedure of Katz et al., based on an energy argument, yields a more reliable estimate. The presence of strong C i lines around 9000Å prior to maximum is an indicator that the pre-SN star was underabundant in helium. As noted by others, the 1.08μm feature is a complex blend of C i , Mg ii , and He i lines, which makes the identification of He uncertain in SNe Ibc unless other He i lines can be identified. Our models show little scatter in ( V – R ) colour 10 d after R -band maximum. We also address a number of radiative transfer properties of SNe Ibc, including the notion of a photosphere, the inference of a representative ejecta expansion rate, spectrum formation, blackbody fits and ‘correction factors’.

Description: We have monitored the massive binary star Carinae with the CTIO/Small and Moderate Aperture Research Telescope System 1.5 m telescope and CHIRON spectrograph from the previous apastron passage of the system through the recent 2014.6 periastron passage. Our monitoring has resulted in a large, homogeneous data set with an unprecedented time-sampling, spectral resolving power, and signal to noise. This allowed us to investigate temporal variability previously unexplored in the system and discover a kinematic structure in the P Cygni absorption troughs of neutral helium wind lines. The features observed occurred prior to the periastron passage and are seen as we look through the trailing arm of the wind–wind collision shock cone. We show that the bulk of the variability is repeatable across the last five periastron passages, and that the absorption occurs in the inner 230 au of the system. In addition, we found an additional, high-velocity absorption component superimposed on the P Cygni absorption troughs that has been previously unobserved in these lines, but which bears resemblance to the observations of the He i 10830 Å feature across previous cycles. Through a comparison of the current smoothed particle hydrodynamical simulations, we show that the observed variations are likely caused by instabilities in the wind–wind collision region in our line of sight, coupled with stochastic variability related to clumping in the winds.

Description: We present a spectroscopic analysis of Hubble Space Telescope /Cosmic Origins Spectrograph observations of three massive stars in the low metallicity dwarf galaxies IC 1613 and WLM. These stars, were previously observed with Very Large Telescope (VLT)/X-shooter by Tramper et al., who claimed that their mass-loss rates are higher than expected from theoretical predictions for the underlying metallicity. A comparison of the far ultraviolet (FUV) spectra with those of stars of similar spectral types/luminosity classes in the Galaxy, and the Magellanic Clouds provides a direct, model-independent check of the mass-loss–metallicity relation. Then, a quantitative spectroscopic analysis is carried out using the non-LTE (NLTE) stellar atmosphere code cmfgen . We derive the photospheric and wind characteristics, benefiting from a much better sensitivity of the FUV lines to wind properties than Hα. Iron and CNO abundances are measured, providing an independent check of the stellar metallicity. The spectroscopic analysis indicates that Z/Z  = 1/5, similar to a Small Magellanic Cloud-type environment, and higher than usually quoted for IC 1613 and WLM. The mass-loss rates are smaller than the empirical ones by Tramper et al., and those predicted by the widely used theoretical recipe by Vink et al. On the other hand, we show that the empirical, FUV-based, mass-loss rates are in good agreement with those derived from mass fluxes computed by Lucy. We do not concur with Tramper et al. that there is a breakdown in the mass-loss–metallicity relation.

Description: The X-ray emission of Carinae shows multiple features at various spatial and temporal scales. The central constant emission (CCE) component is centred on the binary and arises from spatial scales much smaller than the bipolar Homunculus nebula, but likely larger than the central wind–wind collision region between the stars as it does not vary over the ~2–3 month X-ray minimum when it can be observed. Using large-scale 3D smoothed particle hydrodynamics (SPH) simulations, we model both the colliding-wind region between the stars, and the region where the secondary wind collides with primary wind ejected from the previous periastron passage. The simulations extend out to one hundred semimajor axes and make two limiting assumptions (strong coupling and no coupling) about the influence of the primary radiation field on the secondary wind. We perform 3D radiative transfer calculations on the SPH output to synthesize the X-ray emission, with the aim of reproducing the CCE spectrum. For the preferred primary mass-loss rate $\dot{M}_A\approx 8.5\times 10^{-4}\,\mathrm{M}_{\odot }$  yr –1 , the model spectra well reproduce the observation as the strong- and no-coupling spectra bound the CCE observation for longitude of periastron 252°, and bound/converge on the observation for 90°. This suggests that Carinae has moderate coupling between the primary radiation and secondary wind, that both the region between the stars and the comoving collision on the backside of the secondary generate the CCE, and that the CCE cannot place constraints on the binary's line of sight. We also discuss comparisons with common X-ray fitting parameters.

Description: Using radiation hydrodynamics and radiative transfer simulations, we explore the origin of the spectral diversity of interacting supernovae (SNe) of Type IIn. We revisit SN 1994W and investigate the dynamical configurations that can give rise to spectra with narrow lines at all times. We find that a standard ~10 M 10 51  erg SN ejecta ramming into a 0.4 M dense circumstellar material is inadequate for SN 1994W, as it leads to the appearance of broad lines at late times. This structure, however, generates spectra that exhibit the key morphological changes seen in SN 1998S. For SN 1994W, we consider a completely different configuration, which involves the interaction at a large radius of a low-mass inner shell with a high-mass outer shell. Such a structure may arise in an 8–12 M star from a nuclear flash (e.g. of Ne) followed within a few years by core collapse. Our simulations show that the large mass of the outer shell leads to the complete braking of the inner shell material, the formation of a slow dense shell, and the powering of a luminous SN IIn, even for a low inner shell energy. Early on, our model line profiles are typical of SNe IIn, exhibiting narrow cores and broad electron-scattering wings. As observed in SN 1994W, they also remain narrow at late times. Our SN 1994W model invokes two low-energy ejections, both atypical of observed massive stars, and illustrates the diversity of configurations leading to SNe IIn. These results also highlight the importance of spectra to constrain the dynamical properties and understand the origin of SNe IIn.

Description: Eta Carinae, the closest, active, massive binary containing a highly unstable Luminous Blue Variable, exhibits expanding, compressed wind shells, seen in emission, that are spatially and spectrally resolved by Hubble Space Telescope / Space Telescope Imaging Spectrograph . Starting in 2009 June, these structures were mapped across its 5.54-yr, highly elliptical, binary orbit to follow temporal changes in the light of [Fe iii ] 4659 Å and [Fe ii ] 4815 Å. The emissions trace portions of fossil wind shells, that were formed by wind–wind interactions across each cycle. Over the high-ionization state, dense arcs, photoionized by far-ultraviolet radiation from the hot secondary, are seen in [Fe iii ]. Other arcs, ionized by mid-ultraviolet radiation from the primary star, are seen in [Fe ii ]. The [Fe iii ] structures tend to be interior to [Fe ii ] structures that trace extensive, less disturbed primary wind. During the brief periastron passage when the secondary plunges deep into the primary's extremely dense wind, on the far side of primary star, high-ionization [Fe iii ] structures fade and reappear in [Fe ii ]. Multiple fossil wind structures were traced across the 5.7-yr monitoring interval. The strong similarity of the expanding [Fe ii ] shells suggests that the wind and photoionization properties of the massive binary have not changed substantially from one orbit to the next over the past several orbital cycles. These observations trace structures that can be used to test 3D hydrodynamical and radiative-transfer models of massive, interacting winds. They also provide a baseline for following future changes in Car, especially of its winds and photoionization properties.

Description: The light curves of Type Ia supernovae (SNe Ia) are powered by the radioactive decay of 56 Ni to 56 Co at early times, and the decay of 56 Co to 56 Fe from ~60 d after explosion. We examine the evolution of the [Co iii ] 5893 emission complex during the nebular phase for SNe Ia with multiple nebular spectra and show that the line flux follows the square of the mass of 56 Co as a function of time. This result indicates both efficient local energy deposition from positrons produced in 56 Co decay and long-term stability of the ionization state of the nebula. We compile SN Ia nebular spectra from the literature and present 21 new late-phase spectra of 7 SNe Ia, including SN 2014J. From these we measure the flux in the [Co iii ] 5893 line and remove its well-behaved time dependence to infer the initial mass of 56 Ni ( M Ni ) produced in the explosion. We then examine 56 Ni yields for different SN Ia ejected masses ( M ej – calculated using the relation between light-curve width and ejected mass) and find that the 56 Ni masses of SNe Ia fall into two regimes: for narrow light curves (low stretch s ~ 0.7–0.9), M Ni is clustered near M Ni 0.4 M and shows a shallow increase as M ej increases from ~1 to 1.4 M ; at high stretch, M ej clusters at the Chandrasekhar mass (1.4 M ) while M Ni spans a broad range from 0.6 to 1.2 M . This could constitute evidence for two distinct SN Ia explosion mechanisms.

Description: We present numerical simulations that include 1D Eulerian multigroup radiation-hydrodynamics, 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer, and 2D polarized radiative transfer for superluminous interacting supernovae (SNe). Our reference model is a ~10 M inner shell with 10 51  erg ramming into an ~3 M cold outer shell (the circumstellar medium, or CSM) that extends from 10 15 to 2 10 16  cm and moves at 100 km s –1 . We discuss the light-curve evolution, which cannot be captured adequately with a grey approach. In this type of 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. This configuration is distinct from the shell-shocked model. 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 SN 2010jl. Invoking a prolate pole-to-equator density ratio in the CSM, we can also reproduce the ~2 per cent continuum polarization, and line depolarization, observed in SN 2010jl. 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 superluminous SNe IIn like 2010jl or 2006gy, can be produced.

Description: The high metal content and fast expansion of supernova (SN) Ia ejecta lead to considerable line overlap in their optical spectra. Uncertainties in composition and ionization further complicate the process of line identification. In this paper, we focus on the 5900 Å emission feature seen in SN Ia spectra after bolometric maximum, a line which in the last two decades has been associated with [Co iii ] 5888 Å or Na i D. Using non-LTE time-dependent radiative-transfer calculations based on Chandrasekhar-mass delayed-detonation models, we find that Na i D line emission is extremely weak at all post-maximum epochs. Instead, we predict the presence of [Co iii ] 5888 Å after maximum in all our SN Ia models, which cover a range from 0.12 to 0.87 M of 56 Ni. We also find that the [Co iii ] 5888 Å forbidden line is present within days of bolometric maximum, and strengthens steadily for weeks thereafter. Both predictions are confirmed by observations. Rather than trivial taxonomy, these findings confirm that it is necessary to include forbidden-line transitions in radiative-transfer simulations of SNe Ia, both to obtain the correct ejecta cooling rate and to match observed optical spectra.

Description: We explore the physics of Type Ia supernova (SN Ia) light curves and spectra using the 1D non-local thermodynamic equilibrium (non-LTE) time-dependent radiative-transfer code cmfgen . Rather than adjusting ejecta properties to match observations, we select as input one ‘standard’ 1D Chandrasekhar-mass delayed-detonation hydrodynamical model, and then explore the sensitivity of radiation and gas properties of the ejecta on radiative-transfer modelling assumptions. The correct computation of SN Ia radiation is not exclusively a solution to an ‘opacity problem’, characterized by the treatment of a large number of lines. We demonstrate that the key is to identify and treat important atomic processes consistently. This is not limited to treating line blanketing in non-LTE. We show that including forbidden-line transitions of metals, and in particular Co, is increasingly important for the temperature and ionization of the gas beyond maximum light. Non-thermal ionization and excitation are also critical since they affect the colour evolution and the M 15 decline rate of our model. While impacting little the bolometric luminosity, a more complete treatment of decay routes leads to enhanced line blanketing, e.g. associated with 48 Ti in the U and B bands. Overall, we find that SN Ia radiation properties are influenced in a complicated way by the atomic data we employ, so that obtaining converged results is a real challenge. Nonetheless, with our fully fledged cmfgen model, we obtain good agreement with the golden standard Type Ia SN 2005cf in the optical and near-IR, from 5 to 60 d after explosion, suggesting that assuming spherical symmetry is not detrimental to SN Ia radiative-transfer modelling at these times. Multi-D effects no doubt matter, but they are perhaps less important than accurately treating the non-LTE processes that are crucial to obtain reliable temperature and ionization structures.