arXiv subjects

D. John Hillier

Publications and source records attributed to D. John Hillier.

At least 37 records · Page 2Linked to original sources

arXiv2023

Photoionization and electron-ion recombination in astrophysical plasmas

Photoionization and its inverse, electron-ion recombination, are key processes that influence many astrophysical plasmas (and gasses), and the diagnostics that we use to analyse the plasmas. In this review we provide a brief overview of the importance of photoionization and recombination in astrophysics. We highlight how the data needed for spectral analyses, and the required accuracy, varies considerably in different astrophysical environments. We then discuss photoionization processes, highlighting resonances in their cross-sections. Next we discuss radiative recombination, and low and high temperature dielectronic recombination. The possible suppression of low temperature dielectronic recombination (LTDR) and high temperature dielectronic recombination (HTDR) due to the radiation field and high densities is discussed. Finally we discuss a few astrophysical examples to highlight photoionization and recombination processes.

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arXiv2023

A new study on a type Iax stellar remnant and its probable association with SN 1181

We report observations and modeling of the stellar remnant and presumed double-degenerate merger of Type~Iax supernova Pa30, which is the probable remnant of SN~1181~AD. It is the only known bound stellar SN remnant and the only star with Wolf-Rayet features that is neither a planetary nebula central star nor a massive Pop I progenitor. We model the unique emission-line spectrum with broad, strong O~{\sc vi} and O~{\sc viii} lines as a fast stellar wind and shocked, hot gas. Non-LTE wind modeling indicates a mass-loss rate of $\sim 10^{-6}\,\rm M_\odot\,yr^{-1}$ and a terminal velocity of $ \sim$15,000~km\,s$^{-1}$, consistent with earlier results. O~{\sc viii} lines indicate shocked gas temperatures of $T \simeq 4\,$MK. We derive a magnetic field upper limit of $B<2.5\,$MG, below earlier suggestions. The luminosity indicates a remnant mass of 1.0--1.65\,\rm M$_\odot$ with ejecta mass $0.15\pm0.05\,\rm M_\odot$. Archival photometry suggests the stellar remnant has dimmed by $\sim$0.5 magnitudes over 100 years. A low Ne/O$\,<0.15$ argues against a O-Ne white dwarf in the merger. A cold dust shell is only the second detection of dust in a SN Iax and the first of cold dust. Our ejecta mass and kinetic energy estimates of the remnant are consistent with Type Iax extragalactic sources.

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arXiv2022

The orbital kinematics of eta Carinae over three periastra with a possible detection of the elusive secondary's motion

The binary eta Carinae is the closest example of a very massive star, which may have formed through a merger during its Great Eruption in the mid-nineteenth century. We aimed to confirm and improve the kinematics using a spectroscopic data set taken with the CTIO 1.5 m telescope over the time period of 2008-2020, covering three periastron passages of the highly eccentric orbit. We measure line variability of H-alpha and H-beta, where the radial velocity and orbital kinematics of the primary star were measured from the H-beta emission line using a bisector method. At phases away from periastron, we observed the He II 4686 emission moving opposite the primary star, consistent with a possible Wolf-Rayet companion, although with a seemingly narrow emission line. This could represent the first detection of emission from the companion.

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arXiv2022

Using Shell Models to Investigate Clumping in the WN4 Star HD 50896

The spectra of Wolf-Rayet (WR) stars exhibit strong, broad emission lines that originate in the wind. These winds are radiatively driven and are susceptible to hydrodynamic instabilities that result in the formation of clumps. When modelling spectra of WR stars the volume-filling factor (VFF) approach is usually employed to treat clumpy winds. However, it is based on the assumption that the entire wind mass resides in optically thin clumps, which is not necessarily justifiable in dense winds. To test the validity of the VFF approach we use a previously described method of treating clumping, the ``Shell'' approach, to study line and continuum formation in the dense wind of the WN4 star, HD 50896. Our models indicate that fully intact spherical shells are in tension with observed spectra; a persistent ``dip'' in emission lines occurs at line centre. Removing this dip requires our models to use ``broken'' shells -- shells that are highly decoherent laterally. This insinuates that the wind of HD 50896, and by extension the winds of other WR stars, are comprised of small laterally confined and radially compressed clumps -- clumps smaller than the Sobolev length. We discuss some of the conditions necessary for the VFF approach to be valid.

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arXiv2022

Using LSST late-time photometry to constrain Type Ibc supernovae and their progenitors

Over its lifespan, the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will monitor millions of supernovae (SNe) from explosion to oblivion, yielding an unprecedented ugrizy photometric dataset on their late-time evolution. Here, we show that the photometric evolution of Type Ibc SNe can be used to constrain numerous properties of their ejecta, without the need for expensive spectroscopic observations. Using radiative-transfer simulations for explosions of He-star progenitors of different initial masses, we show that the g-band filter follows primarily the strength of the FeII emission, the r-band [OI]6300-6364A and [NII]6548-6583A, the i-band [CaII]7291,7323A, and the z-band the CaII NIR triplet, and hence provides information on nucleosynthetic yields. Information on weaker lines, which may be used, for example, to constrain clumping, is absent. However, this deficiency may eventually be cured by improving the physical realism of radiative-transfer simulations through a closer connection to physically consistent 3D explosion models, and by the judicial selection of a much smaller set of spectral observations. Degeneracies inherent to the SN radiation will affect the interpretation of photometric measures, but line fluxes from nebular-phase spectra are similarly compromised. Importantly, our ``family'' of Type Ibc SN models follows a distinct trajectory in color-color magnitude diagrams as the ejecta evolve from 100 to 450d, allowing one to disentangle different progenitors or explosions. This photometric procedure provides a promising approach to study statistical samples of SNe Ibc and to confront them to ever improving progenitor and explosion models, to capture the onset of late-time interaction with circumstellar material, or to identify events currently unknown.

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arXiv2022

Changes in the Na D$_1$ Absorption Components of $η$ Carinae Provide Clues on the Location of the Dissipating Central Occulter

The Na D absorption doublet in the spectrum of $η$ Carinae is complex, with multiple absorption features associated with the Great Eruption (1840s), the Lesser Eruption (1890s), and interstellar clouds. The velocity profile is further complicated by the P Cygni profile originating in the system's stellar winds and blending with the He I $λ$5876 profile. The Na D profile contains a multitude of absorption components, including those at velocities of $-$145 km s$^{-1}$, $-$168 km s$^{-1}$, and $+$87 km s$^{-1}$ that we concentrate on in this analysis. Ground-based spectra recorded from 2008 to 2021 show significant variability of the $-$145 km s$^{-1}$ absorption throughout long-term observations. In the high ionization phases of $η$ Carinae prior to the 2020 periastron passage, this feature disappeared completely but briefly reappeared across the 2020 periastron, along with a second absorption at $-$168 km s$^{-1}$. Over the past few decades, $η$ Car has been gradually brightening demonstrated to be caused by a dissipating occulter. The decreasing absorption of the $-$145 km s$^{-1}$ component, coupled with similar trends seen in absorptions of ultraviolet resonant lines, indicate that this central occulter was possibly a large clump associated with the Little Homunculus or another clump between the Little Homunculus and the star. We also report on a foreground absorption component at $+$87 km s$^{-1}$. Comparison of Na D absorption in the spectra of nearby systems demonstrates that this red-shifted component likely originates in an extended foreground structure consistent with a previous ultraviolet spectral survey in the Carina Nebula.

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arXiv2022

UV Spectropolarimetry with Polstar: Massive Star Binary Colliding Winds

The winds of massive stars are important for their direct impact on the interstellar medium, and for their influence on the final state of a star prior to it exploding as a supernova. However, the dynamics of these winds is understood primarily via their illumination from a single central source. The Doppler shift seen in resonance lines is a useful tool for inferring these dynamics, but the mapping from that Doppler shift to the radial distance from the source is ambiguous. Binary systems can reduce this ambiguity by providing a second light source at a known radius in the wind, seen from orbitally modulated directions. From the nature of the collision between the winds, a massive companion also provides unique additional information about wind momentum fluxes. Since massive stars are strong ultraviolet (UV) sources, and UV resonance line opacity in the wind is strong, UV instruments with a high resolution spectroscopic capability are essential for extracting this dynamical information. Polarimetric capability also helps to further resolve ambiguities in aspects of the wind geometry that are not axisymmetric about the line of sight, because of its unique access to scattering direction information. We review how the proposed MIDEX-scale mission Polstar can use UV spectropolarimetric observations to critically constrain the physics of colliding winds, and hence radiatively-driven winds in general. We propose a sample of 20 binary targets, capitalizing on this unique combination of illumination by companion starlight, and collision with a companion wind, to probe wind attributes over a range in wind strengths. Of particular interest is the hypothesis that the radial distribution of the wind acceleration is altered significantly, when the radiative transfer within the winds becomes optically thick to resonance scattering in multiple overlapping UV lines.

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arXiv2022

NICER X-ray Observations of Eta Carinae During its Most Recent Periastron Passage

We report high-precision X-ray monitoring observations in the 0.4-10 keV band of the luminous, long-period colliding-wind binary Eta Carinae up to and through its most recent X-ray minimum/periastron passage in February 2020. Eta Carinae reached its observed maximum X-ray flux on 7 January 2020, at a flux level of $3.30 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$, followed by a rapid plunge to its observed minimum flux, $0.03 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$ near 17 February 2020. The NICER observations show an X-ray recovery from minimum of only $\sim$16 days, the shortest X-ray minimum observed so far. We provide new constraints of the "deep" and "shallow" minimum intervals. Variations in the characteristic X-ray temperature of the hottest observed X-ray emission indicate that the apex of the wind-wind "bow shock" enters the companion's wind acceleration zone about 81 days before the start of the X-ray minimum. There is a step-like increase in column density just before the X-ray minimum, probably associated with the presence of dense clumps near the shock apex. During recovery and after, the column density shows a smooth decline, which agrees with previous $N_{H}$ measurements made by SWIFT at the same orbital phase, indicating that changes in mass-loss rate are only a few percent over the two cycles. Finally, we use the variations in the X-ray flux of the outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta of $\sim 10^{41}$ ergs s$^{-1}$ near the time of the "Great Eruption'.

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arXiv2022

Eta Carinae: an evolving view of the central binary, its interacting winds and its foreground ejecta

FUV spectra of Eta Car, recorded across two decades with HST/STIS, document multiple changes in resonant lines caused by dissipating extinction in our line of sight. The FUV flux has increased nearly ten-fold which has led to increased ionization of the multiple shells within the Homunculus and photo-destruction of molecular hydrogen. Comparison of observed resonant line profiles with CMFGEN model profiles allows separation of wind-wind collision and shell absorptions from the primary wind, P Cygni profiles.The dissipating occulter preferentially obscured the central binary and interacting winds relative to the very extended primary wind. We are now able to monitor changes in the colliding winds with orbital phase. High velocity transient absorptions occurred across the most recent periastron passage, indicating acceleration of the primary wind by the secondary wind which leads to a downstream, high velocity bowshock that is newly generated every orbital period. There is no evidence of changes in the properties of the binary winds.

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arXiv2022

WO-Type Wolf-Rayet Stars: the Last Hurrah of Massive Star Evolution

Are WO-type Wolf Rayet (WR) stars in the final stage of massive star evolution before core-collapse? Although WC- and WO-type WRs have very similar spectra, WOs show a much stronger O VI $λλ$3811,34 emission-line feature. This has usually been interpreted to mean that WOs are more oxygen rich than WCs, and thus further evolved. However, previous studies have failed to model this line, leaving the relative abundances uncertain, and the relationship between the two types unresolved. To answer this fundamental question, we modeled six WCs and two WOs in the LMC using UV, optical, and NIR spectra with the radiative transfer code CMFGEN in order to determine their physical properties. We find that WOs are not richer in oxygen; rather, the O VI feature is insensitive to the abundance. However, the WOs have a significantly higher carbon and lower helium content than the WCs, and hence are further evolved. A comparison of our results with single-star Geneva and binary BPASS evolutionary models show that while many properties match, there is more carbon and less oxygen in the WOs than either set of evolutionary model predicts. This discrepancy may be due to the large uncertainty in the $^{12}$C+$^4$He$\rightarrow^{16}$O nuclear reaction rate; we show that if the Kunz et al. rate is decreased by a factor of 25-50%, then there would be a good match with the observations. It would also help explain the LIGO/VIRGO detection of black holes whose masses are in the theoretical upper mass gap.

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arXiv2022

Modeling the signatures of interaction in Type II supernovae: UV emission, high-velocity features, broad-boxy profiles

Because mass loss is a fundamental phenomenon in massive stars, interaction with circumstellar material (CSM) should be universal in core-collapse supernovae (SNe). Leaving aside the extreme CSM density, extent, or mass typically encountered in Type IIn SNe, we investigate the diverse long-term radiative signatures of interaction between a Type II SN ejecta and CSM corresponding to mass loss rates up to 10$^{-3}$ $M_{\odot}$ yr$^{-1}$. Because these CSM are relatively tenuous and optically-thin to electron-scattering beyond a few stellar radii, radiation hydrodynamics is not essential and one may treat the interaction directly as an additional power source in the non-local thermodynamic equilibrium radiative transfer problem. The CSM accumulated since shock breakout forms a dense shell in the outer ejecta and leads to high-velocity absorption features in spectral lines, even for negligible shock power. Besides Balmer lines, such features may appear in NaID, HeI lines etc. A stronger interaction strengthens the continuum flux (preferentially in the UV), quenches the absorption of P-Cygni profiles, boosts the MgII $λλ$ $2795,2802$ doublet, and fosters the production of a broad boxy H$α$ emission component. The rise in ionization in the outer ejecta may quench some lines (e.g., the CaII near-infrared triplet). The interaction power emerges preferentially in the UV, in particular at later times, shifting the optical color to the blue, but increasing modestly the optical luminosity. Strong thermalization and clumping seem to be required to make an interaction superluminous in the optical. The UV range contains essential signatures that provide critical constraints to infer the mass loss history and inner workings of core-collapse SN progenitors at death.

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arXiv2022

Stable nickel production in Type Ia supernovae: A smoking gun for the progenitor mass?

At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit (MCh ~ 1.4 Msun) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between MCh and sub-MCh events. In particular, it is thought that the higher-density ejecta of MCh WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni II] lines in late-time spectra. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from MCh and sub-MCh progenitors. We explore the potential for lines of [Ni II] at 7378 Å and 1.94 microns in late-time spectra to serve as a diagnostic of the exploding WD mass, using nonlocal thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code. We find that the radiative proton-capture reaction 57Co(p,gamma)58Ni is the dominant production mode for 58Ni in both MCh and sub-MCh models, while the alpha-capture reaction on 54Fe has a negligible impact on the final 58Ni yield. More importantly, we demonstrate that the lack of [Ni II] lines in late-time spectra of sub-MCh events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong [Ni II] lines predicted in our 1D MCh models are completely suppressed when 56Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. [Ni II] lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among MCh and sub-MCh progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni II] lines would most likely result from a Chandrasekhar-mass progenitor. [Abridged]

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arXiv2022

New hydrodynamic solutions for line-driven winds of hot massive stars using Lambert $W$-function

Hot massive stars present strong stellar winds that are driven by absorption, scattering and re\-emission of photons by the ions of the atmosphere (\textit{line-driven winds}). A better comprehension of this phenomenon, and a more accurate calculation of hydrodynamics and radiative acceleration is required to reduce the number of free parameters in spectral fitting, to determine accurate wind parameters such as mass-loss rates and velocity profiles. We use the non-LTE model-atmosphere code CMFGEN to numerically solve the radiative transfer equation in the stellar atmosphere and to calculate the radiative acceleration $g_\text{rad}(r)$. Under the assumption that the radiative acceleration depends only on the radial coordinate, we solve analytically the equation of motion by means of the Lambert $W$-function. An iterative procedure between the solution of the radiative transfer and the equation of motion is executed in order to obtain a final self-consistent velocity field that is no longer based on any $β$-law. We apply the Lambert-procedure to three O supergiant stars ($ζ$-Puppis, HD~165763 and $α$-Cam) and discuss the Lambert-solutions for the velocity profiles. It is found that, even without recalculation of the mass-loss rate, the Lambert-procedure allows the calculation of consistent velocity profiles that reduce the number of free parameters when a spectral fitting using CMFGEN is performed. Synthetic spectra calculated from our Lambert-solutions show significant differences compared to the initial $β$-law CMFGEN models. The results indicate the importance of consistent velocity profile calculation in the CMFGEN code and its usage in a fitting procedure and interpretation of observed spectra.

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arXiv2021

The Physical Parameters of Four WC-type Wolf-Rayet Stars in the Large Magellanic Cloud: Evidence of Evolution

We present a spectral analysis of four LMC WC-type Wolf-Rayet (WR) stars (BAT99-8, BAT99-9, BAT99-11, and BAT99-52) to shed light on two evolutionary questions surrounding massive stars. The first is: are WO-type WR stars more oxygen enriched than the WC-type stars, indicating further chemical evolution, or are the strong high-excitation oxygen lines in the WO-type stars an indication of higher temperatures. This study will act as a baseline for answering the question of where WO-type stars fall in WR evolution. Each star's spectrum, extending from 1100~Å to 25000~Å, was modeled using \cmfgen\ to determine the star's physical properties such as luminosity, mass-loss rate, and chemical abundances. The oxygen abundance is a key evolutionary diagnostic, and with higher resolution data and an improved stellar atmosphere code, we found the oxygen abundance to be up to a factor of 5 lower than previous studies. The second evolutionary question revolves around the formation of WR stars: Do they evolve by themselves or is a close companion star necessary for their formation? Using our derived physical parameters, we compared our results to the Geneva single-star evolutionary models and the BPASS binary evolutionary models. We found that both the Geneva solar metallicity models and BPASS LMC metallicity models are in agreement with the four WC-type stars, while the Geneva LMC metallicity models are not. Therefore, these four WC4 stars could have been formed either via binary or single-star evolution.

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arXiv2021

A High-Velocity Scatterer Revealed in the Thinning Ejecta of a Type II Supernova

We present deep, nebular-phase spectropolarimetry of the Type II-P/L SN 2013ej, obtained 167 days after explosion with the European Southern Observatory's Very Large Telescope. The polarized flux spectrum appears as a nearly perfect (92% correlation), redshifted (by ~4,000 km/sec) replica of the total flux spectrum. Such a striking correspondence has never been observed before in nebular-phase supernova spectropolarimetry, although data capable of revealing it have heretofore been only rarely obtained. Through comparison with 2D polarized radiative transfer simulations of stellar explosions, we demonstrate that localized ionization produced by the decay of a high-velocity, spatially confined clump of radioactive 56-Ni -- synthesized by and launched as part of the explosion with final radial velocity exceeding 4,500 km/sec -- can reproduce the observations through enhanced electron scattering. Additional data taken earlier in the nebular phase (day 134) yield a similarly strong correlation (84%) and redshift, whereas photospheric-phase epochs that sample days 8 through 97, do not. This suggests that the primary polarization signatures of the high-velocity scattering source only come to dominate once the thick, initially opaque hydrogen envelope has turned sufficiently transparent. This detection in an otherwise fairly typical core-collapse supernova adds to the growing body of evidence supporting strong asymmetries across Nature's most common types of stellar explosions, and establishes the power of polarized flux -- and the specific information encoded by it in line photons at nebular epochs -- as a vital tool in such investigations going forward.

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arXiv2021

The explosion of 9$-$29$M_\odot$ stars as Type II supernovae : results from radiative-transfer modeling at one year after explosion

We present a set of nonlocal thermodynamic equilibrium steady-state calculations of radiative transfer for one-year old type II supernovae (SNe) starting from state-of-the-art explosion models computed with detailed nucleosynthesis. This grid covers single-star progenitors with initial masses between 9 and 29$M_{\odot}$, all evolved with KEPLER at solar metallicity and ignoring rotation. The [OI]$λλ$$6300,6364$ line flux generally grows with progenitor mass, and H$α$ exhibits an equally strong and opposite trend. The [CaII]$λλ$$7291,\,7323$ strength increases at low $^{56}$Ni mass, low explosion energy, or with clumping. This CaII doublet, which forms primarily in the explosively-produced Si/S zones, depends little on the progenitor mass, but may strengthen if Ca$^+$ dominates in the H-rich emitting zones or if Ca is abundant in the O-rich zones. Indeed, Si-O shell merging prior to core collapse may boost the CaII doublet at the expense of the OI doublet, and may thus mimic the metal line strengths of a lower mass progenitor. We find that the $^{56}$Ni bubble effect has a weak impact, probably because it is too weak to induce much of an ionization shift in the various emitting zones. Our simulations compare favorably to observed SNe II, including SN2008bk (e.g., 9$M_{\odot}$ model), SN2012aw (12$M_{\odot}$ model), SN1987A (15$M_{\odot}$ model), or SN2015bs (25$M_{\odot}$ model with no Si-O shell merging). SNe II with narrow lines and a low $^{56}$Ni mass are well matched by the weak explosion of 9$-$11$M_{\odot}$ progenitors. The nebular-phase spectra of standard SNe II can be explained with progenitors in the mass range 12$-$15$M_{\odot}$, with one notable exception for SN2015bs. In the intermediate mass range, these mass estimates may increase by a few $M_{\odot}$ with allowance for clumping of the O-rich material or CO molecular cooling.

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arXiv2021

Multiepoch VLT-FORS spectro-polarimetric observations of supernova 2012aw reveal an asymmetric explosion

We present VLT-FORS spectropolarimetric observations of the type II supernova (SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to 120d after explosion. We correct for the interstellar polarization by postulating that the SN polarization is naught near the rest wavelength of the strongest lines - this is later confirmed by our modeling. SN2012aw exhibits intrinsic polarization, with strong variations across lines, and with a magnitude that grows in the 7000A line-free region from 0.1% at 16d up to 1.2% at 120d. This behavior is qualitatively similar to observations gathered for other type II SNe. A suitable rotation of Stokes vectors places the bulk of the polarization in q, suggesting the ejecta of SN2012aw is predominantly axisymmetric. Using an upgraded version of our 2D polarized radiative transfer code, we model the wavelength- and time-dependent polarization of SN2012aw. The key observables may be explained by the presence of a confined region of enhanced 56Ni at ~4000km/s, which boosts the electron density in a cone having an opening angle of ~50deg and an observer's inclination of ~70deg to the axis of symmetry. With this fixed asymmetry in time, the observed evolution of the SN2012aw polarization arises from the evolution of the ejecta optical depth, ionization, and the relative importance of multiple versus single scattering. However, the polarization signatures exhibit numerous degeneracies. Cancellation effects at early times imply that a low polarization may occur for ejecta with a large asymmetry. An axisymmetric ejecta with a latitude-dependent explosion energy can also yield similar polarization signatures as an asymmetric 56Ni distribution. Despite these uncertainties, SN2012aw provides additional evidence for the generic asymmetry of type II SN ejecta, of which VLT-FORS spectropolarimetric observations are a decisive and exquisite probe.

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arXiv2021

Polarization signatures of a high-velocity scatterer in nebular-phase spectra of Type II supernovae

Type II supernovae (SNe) often exhibit a linear polarization, arising from free-electron scattering, with complicated optical signatures, both in the continuum and in lines. Focusing on the early nebular phase, at a SN age of 200d, we conduct a systematic study of the polarization signatures associated with a 56Ni `blob' that breaks spherical symmetry. Our ansatz, supported by nonLTE radiative transfer calculations, is that the primary role of such a 56Ni blob is to boost the local density of free electrons, which is otherwise reduced following recombination in SNe II. Using 2D polarized radiation transfer modeling, we explore the influence of such an electron-density enhancement, varying its magnitude N_e_fac, its velocity location V_blob, and its spatial extent. For plausible N_e_fac values of a few tens, a high-velocity blob can deliver a continuum polarization P_cont of 0.5-1.0% at 200d. Our simulations reproduce the analytic scalings for P_cont, and in particular the linear growth with the blob radial optical depth. The most constraining information is, however, carried by polarized line photons. For a high V_blob, the polarized spectrum appears as a replica of the full spectrum, scaled down by a factor 100 to 1000 (i.e., 1/P_cont), and redshifted by an amount V_blob(1-cos(alpha_los)), where alpha_los is the line of sight angle. As V_blob is reduced, the redshift decreases and the replication deteriorates. Lines whose formation region overlap with the blob appear weaker and narrower in the polarized flux. Because of its dependence on inclination (~ sin^2 alpha_los), the polarization preferentially reveals asymmetries in the plane perpendicular to the line of sight. With the adequate choice of electron-density enhancement, some of these results may apply to asymmetric explosions in general, or to the polarization signatures from newly-formed dust in the outer ejecta. [abridged]

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