An important and perhaps critical clue to the mechanism driving the explosion of massive stars as supernovae is provided by the accumulating evidence for asymmetry in the explosion. Indirect evidence comes from high pulsar velocities, associations of supernovae with long-soft gamma-ray bursts, and asymmetries in late-time emission-line profiles. Spectropolarimetry provides a direct probe of young supernova geometry, with higher polarization generally indicating a greater departure from spherical symmetry. Large polarizations have been measured for 'stripped-envelope' (that is, type Ic) supernovae, which confirms their non-spherical morphology; but the explosions of massive stars with intact hydrogen envelopes (type II-P supernovae) have shown only weak polarizations at the early times observed. Here we report multi-epoch spectropolarimetry of a classic type II-P supernova that reveals the abrupt appearance of significant polarization when the inner core is first exposed in the thinning ejecta (~90 days after explosion). We infer a departure from spherical symmetry of at least 30 per cent for the inner ejecta. Combined with earlier results, this suggests that a strongly non-spherical explosion may be a generic feature of core-collapse supernovae of all types, where the asphericity in type II-P supernovae is cloaked at early times by the massive, opaque, hydrogen envelope.Comment: Accepted for publication by Nature (results embargoed until 23 March 2006); 14 pages, 2 figure

We present spectropolarimetry of SN 2009ip throughout the evolution of its 2012 explosion. During the 2012a phase, when the spectrum exhibits broad P-Cygni lines, we measure a V -band polarization of P ≈ 0.9% at a position angle of θ ≈ 166 • , indicating substantial asphericity for the 2012a outflow. Near the subsequent peak of the 2012b phase, when the spectrum shows signs of intense interaction with circumstellar material (CSM), we measure P ≈ 1.7% and θ ≈ 72 • , indicating a separate component of polarization during 2012b, which exhibits a higher degree of asphericity than 2012a and an orthogonal axis of symmetry on the sky. Around 30 days past peak, coincident with a substantial bump in the declining light curve, we measure P ≈ 0.7% and another significant shift in θ. At this point, broad photospheric lines have again become prominent and exhibit significant variations in P relative to the continuum, particularly He i/Na I D. By 60 days past peak the continuum polarization has dropped below 0.2%, probably declining toward a low value of interstellar polarization. The results are consistent with a scenario in which a prolate (possibly bipolar) explosion launched during the 2012a phase impacts an oblate (toroidal) distribution of CSM in 2012b. Previous calculations that assumed spherical symmetry for the CSM have substantially underestimated the required explosion energy, since only a small fraction of the SN ejecta appears to have participated in strong CSM interaction. An ejecta kinetic energy of at least ∼ 10 51 ergs is difficult to avoid, supporting the interpretation that the 2012 outburst of SN 2009ip was the result of a core-collapse explosion.

The unusual flux variations of the pre-main-sequence binary star KH 15D have been attributed to occultations by a circumbinary disk. We test whether or not this theory is compatible with newly available data, including recent radial velocity measurements, CCD photometry over the past decade, and photographic photometry over the past 50 years. We find the model to be successful, after two refinements: a more realistic motion of the occulting feature, and a halo around each star that probably represents scattering by the disk. The occulting feature is exceptionally sharp-edged, raising the possibility that the dust in the disk has settled into a thin layer, and providing a tool for fine-scale mapping of the immediate environment of a T Tauri star. However, the window of opportunity is closing, as the currently visible star may be hidden at all orbital phases by as early as 2008.Comment: To appear in ApJ [16 pages, 13 figures

We present multi-epoch spectral and spectropolarimetric observations of the Type IIn supernova (SN ) 1997eg that indicate the presence of a flattened disklike concentration of circumstellar material surrounding aspherical ejecta, with which the disk is misaligned. The polarization across the broad H , H , and He i k5876 lines of SN 1997eg forms closed loops when viewed in the Stokes q-u plane. Such loops occur when the geometrical symmetry of one or both of the Stokes parameters across spectral lines is broken, in this case most likely by occultation of the ejecta by the equatorial circumstellar matter concentration. The polarization of the narrow Balmer lines possesses an intrinsic axis that differs by 12 from that of the elongated ejecta and probably indicates the orientation of the disklike circumstellar material. The existence of two different axes of symmetry in SN 1997eg suggests that neither rotation of the progenitor nor the influence of a companion star can be the sole mechanism creating a preferred axis within the supernova system. Our model supports the emerging hypothesis that the progenitors of some Type IIn supernovae are luminous blue variable stars, whose presupernova mass eruptions form the circumstellar shells that physically characterize the SN IIn subclass. These conclusions, which are independent of interstellar polarization effects, would have been unobservable with only a single epoch of spectropolarimetry.

Eclipsing systems of massive stars allow one to explore the properties of their components in great detail. We perform a multi-wavelength, non-LTE analysis of the three components of the massive multiple system δ Ori A, focusing on the fundamental stellar properties, stellar winds, and X-ray characteristics of the system.The primary's distance-independent parameters turn out to be characteristic for its spectral type (O9.5 II), but usage of the Hipparcos parallax yields surprisingly low values for the mass, radius, and luminosity. Consistent values follow only if δ Ori lies at about twice the Hipparcos distance, in the vicinity of the σ-Orionis cluster. The primary and tertiary dominate the spectrum and leave the secondary only marginally detectable. We estimate the V-band magnitude difference between primary and secondary to be ∆V ≈ 2. m 8. The inferred parameters suggest the secondary is an early B-type dwarf (≈ B1 V), while the tertiary is an early B-type subgiant (≈ B0 IV). We find evidence for rapid turbulent velocities (∼ 200 km s −1 ) and wind inhomogeneities, partially optically thick, in the primary's wind. The bulk of the X-ray emission likely emerges from the primary's stellar wind (log L X /L Bol ≈ −6.85), initiating close to the stellar surface at R 0 ∼ 1.1 R * . Accounting for clumping, the mass-loss rate of the primary is found to be logṀ ≈ −6.4 [M ⊙ yr −1 ] , which agrees with hydrodynamic predictions, and provides a consistent picture along the X-ray, UV, optical and radio spectral domains.

The spectroscopic and spectropolarimetric variability of the peculiar variable V838 Monocerotis during the brighter phases of its multiple outbursts in 2002 is presented. Significant line profile variability of H and Si ii 6347.10 and 6371.36 Å occurred in spectra obtained between 2002 February 5 and March 14, and a unique secondary absorption component was observed near the end of this time period. Our observations also suggest that multiple shifts in ionization states occurred during the outbursts. Spectropolarimetric observations reveal that V838 Mon exhibited both intrinsic and interstellar polarization components during the initial stages of the second outburst, indicating the presence of an asymmetric geometry; however, the intrinsic component had declined significantly by February 14. We determine the interstellar polarization to be P max ¼ 2:746% AE 0:011%, max ¼ 5790 AE 37 G, and P:A: ¼ 153=43 AE 0=12, and we find the integrated intrinsic V-band polarization on February 5 to be P ¼ 0:983% AE 0:012% at a position angle of 127=0 AE 0=5. The implications of these observations for the nature of V838 Monocerotis, its distance, and its ejecta are discussed.

We present ultraviolet and visual spectropolarimetry of the interacting binary star b Lyrae, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment and the HPOL spectropolarimeter at Pine Blu † Observatory. Our observations span 3 years and cover the wavelength range from 1400 to 10500 Ó, with a resolution of 7.5È16Detailed broad-and narrowband spectropolarimetric analysis allows us to Ó. begin to decompose the complex spectrum of b Lyr : by examining the polarization behavior of a line or continuum, we can determine which component scatters the light and, ultimately, from which component that light originates. After removing interstellar polarization from our data and rotating the results to the apparent intrinsic position angle of the system, we Ðnd that the polarization of the hydrogen Balmer and vacuum ultraviolet "" UV bump ÏÏ emission lines, as well as that of the near-UV continuum, remains mostly constant with phase and is oriented at 90¡ to the visible polarization, indicating that the scattering plane of the light in these three spectral components is perpendicular to the scattering plane of the visible light. We propose that the UV bump, Balmer emission, and near-UV continuum polarization is produced by electron scattering within a bipolar outÑow in the b Lyr system. The intrinsic visible polarization of b Lyr shows eclipses that associate it with material near the accretion disk. We Ðnd conÑicting evidence regarding both the origin of this visible light and the scattering surface that polarizes it : continuum evidence points toward the secondary object as the illuminator and the accretion disk edge as the scatterer, while line analysis suggests that light from the loser scatters o † material between it and the disk. The presence of material away from the orbital plane may help resolve this contradiction.

We present results from a study of the eclipsing, colliding-wind binary V444 Cyg that uses a combination of X-ray and optical spectropolarimetric methods to describe the 3D nature of the shock and wind structure within the system. We have created the most complete X-ray light curve of V444 Cyg to date using 40 ks of new data from Swift, and 200 ks of new and archived XMM-Newton observations. In addition, we have characterized the intrinsic, polarimetric phase-dependent behavior of the strongest optical emission lines using data obtained with the University of Wisconsin's Half-Wave Spectropolarimeter. We have detected evidence of the Coriolis distortion of the wind-wind collision in the X-ray regime, which manifests itself through asymmetric behavior around the eclipses in the system's X-ray light curves. The large opening angle of the X-ray emitting region, as well as its location (i.e. the WN wind does not collide with the O star, but rather its wind) are evidence of radiative braking/inhibition occurring within the system. Additionally, the polarimetric results show evidence of the cavity the wind-wind collision region carves out of the Wolf-Rayet star's wind.