Asphericity, Interaction, and Dust in the Type II-P/II-L Supernova 2013ej in Messier 74

SN 2013ej is a well-studied core-collapse supernova (SN) that stemmed from a directly identified red supergiant (RSG) progenitor in galaxy M74. The source exhibits signs of substantial geometric asphericity, X-rays from persistent interaction with circumstellar material (CSM), thermal emission from warm dust, and a light curve that appears intermediate between supernovae of Types II-P and II-L. The proximity of this source motivates a close inspection of these physical characteristics and their potential interconnection. We present multi-epoch spectropolarimetry of SN 2013ej during the first 107 days, and deep optical spectroscopy and ultraviolet through infrared photometry past ~800 days. SN 2013ej exhibits the strongest and most persistent continuum and line polarization ever observed for a SN of its class during the recombination phase. Modeling indicates that the data are consistent with an oblate ellipsoidal photosphere, viewed nearly edge-on, and probably augmented by optical scattering from circumstellar dust. We suggest that interaction with an equatorial distribution of CSM, perhaps the result of binary evolution, is responsible for generating the photospheric asphericity. Relatedly, our late-time optical imaging and spectroscopy shows that asymmetric CSM interaction is ongoing, and the morphology of broad H-alpha emission from shock-excited ejecta provides additional evidence that the geometry of the interaction region is ellipsoidal. Alternatively, a prolate ellipsoidal geometry from an intrinsically bipolar explosion is also a plausible interpretation of the data, but would probably require a ballistic jet of radioactive material capable of penetrating the hydrogen envelope early in the recombination phase (abridged).Comment: Post-proof edit. Accepted to ApJ on Nov. 23 2016; 21 pages, 16 figure

Recent Type II Radio Supernovae

We present the results of radio observations, taken primarily with the Very Large Array, of Supernovae 1993J, 2001gd, 2001em, 2002hh, 2004dj, and 2004et. We have fit a parameterized model to the multi-frequency observations of each supernova. We compare the observed and derived radio properties of these supernovae by optical classification and discuss the implications.Comment: 5 pages, 2 figures, 1 table Conference Proceedings: "Supernova 1987A: 20 Years After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W. Weiler, and R. McCra

Incorporating Uncertainties in Atomic Data Into the Analysis of Solar and Stellar Observations: A Case Study in Fe XIII

Information about the physical properties of astrophysical objects cannot be measured directly but is inferred by interpreting spectroscopic observations in the context of atomic physics calculations. Ratios of emission lines, for example, can be used to infer the electron density of the emitting plasma. Similarly, the relative intensities of emission lines formed over a wide range of temperatures yield information on the temperature structure. A critical component of this analysis is understanding how uncertainties in the underlying atomic physics propagates to the uncertainties in the inferred plasma parameters. At present, however, atomic physics databases do not include uncertainties on the atomic parameters and there is no established methodology for using them even if they did. In this paper we develop simple models for the uncertainties in the collision strengths and decay rates for Fe XIII and apply them to the interpretation of density sensitive lines observed with the EUV Imagining spectrometer (EIS) on Hinode. We incorporate these uncertainties in a Bayesian framework. We consider both a pragmatic Bayesian method where the atomic physics information is unaffected by the observed data, and a fully Bayesian method where the data can be used to probe the physics. The former generally increases the uncertainty in the inferred density by about a factor of 5 compared with models that incorporate only statistical uncertainties. The latter reduces the uncertainties on the inferred densities, but identifies areas of possible systematic problems with either the atomic physics or the observed intensities.Comment: in press at Ap

Identifying diffuse spatial structures in high-energy photon lists

Data from high-energy observations are usually obtained as lists of photon events. A common analysis task for such data is to identify whether diffuse emission exists, and to estimate its surface brightness, even in the presence of point sources that may be superposed. We have developed a novel non-parametric event list segmentation algorithm to divide up the field of view into distinct emission components. We use photon location data directly, without binning them into an image. We first construct a graph from the Voronoi tessellation of the observed photon locations and then grow segments using a new adaptation of seeded region growing, that we call Seeded Region Growing on Graph, after which the overall method is named SRGonG. Starting with a set of seed locations, this results in an over-segmented dataset, which SRGonG then coalesces using a greedy algorithm where adjacent segments are merged to minimize a model comparison statistic; we use the Bayesian Information Criterion. Using SRGonG we are able to identify point-like and diffuse extended sources in the data with equal facility. We validate SRGonG using simulations, demonstrating that it is capable of discerning irregularly shaped low surface-brightness emission structures as well as point-like sources with strengths comparable to that seen in typical X-ray data. We demonstrate SRGonG's use on the Chandra data of the Antennae galaxies, and show that it segments the complex structures appropriately

SN Hunt 248: a super-Eddington outburst from a massive cool hypergiant

We present photometric and spectroscopic observations of SN Hunt 248, a new supernova (SN) impostor in NGC 5806, which began a multi-stage outburst in 2014 May. The initial '2014a' discovery brightening exhibited an absolute magnitude of M~-12 and the spectral characteristics of a cool dense outflow, including P-Cygni lines of Fe II, H I, Na I, and strong line blanketing from metals. The source rapidly climbed and peaked near M~-15 mag after two additional weeks. During this bright '2014b' phase the spectrum became dominated by Balmer emission and a stronger blue continuum, similar to the SN impostor SN 1997bs. Archival images from the Hubble Space Telescope between 1997 and 2005 reveal a luminous (4e5 Lsun) variable precursor star. Its location on the Hertzsprung-Russell diagram is consistent with a massive (M_init~30 Msun) cool hypergiant having an extremely dense wind and an Eddington ratio just below unity. At the onset of the recent 2014a brightening, however, the object became super-Eddington. The subsequent boost in luminosity during the 2014b phase probably resulted from circumstellar interaction. SN Hunt 248 provides the first case of a cool hypergiant undergoing a giant eruption reminiscent of outbursts from luminous blue variable (LBV) stars. This lends support to the hypothesis that some cool hypergiants, such as Rho Cas, could be LBVs masquerading under a pseudo-photosphere created by their extremely dense winds. Moreover, SN Hunt 248 demonstrates that eruptions stemming from such stars can rival in peak luminosity the giant outbursts of much more massive systems like Eta Car.Comment: Accepted to MNRAS on 2014 Dec 1. Post-proof version. 14 pages, 9 figure

On the nonlinear dynamics of self-sustained limit-cycle oscillations in a flapping-foil energy harvester

The nonlinear dynamics of an airfoil at Reynolds number Re = 10,000 constrained by two springs and subject to a uniform oncoming flow is studied numerically. The studies are carried out using open source computational fluid dynamics toolbox OpenFOAM. Under certain conditions related to aerodynamic flutter, this two-degree-of-freedom system undergoes self-sustained limit-cycle oscillations (LCOs) with potential application as an energy harvester. When the system is given a small initial perturbation, it is seen that the response of the system decays to zero at flow velocities below the flutter velocity, or oscillates in a limit cycle at velocities greater than the flutter velocity. The flutter velocity at Re = 10,000 is shown to deviate significantly from the theoretical prediction (which is derived with an assumption of infinite Reynolds number) owing to the effect of viscosity. The LCOs at freestream velocities higher than the flutter velocity result in unsteady flows that are heavily influenced by leading-edge vortex shedding as well as trailing-edge flow separation. The influence of different system parameters on the onset of flutter and on the limit-cycle response characteristics is investigated in this research. This is done by defining a baseline case and examining the effects of varying aerodynamic parameters such as freestream velocity, and structural parameters such as the pitch-to-plunge frequency ratio and the type of spring stiffnesses. The conditions corresponding to the lowest flutter velocities (and consequently the lowest “cut-in” speeds for power extraction) and the parameter space that provide single-period, single-amplitude and harmonic LCOs (ideal for power extraction) are identified. Calculation of instantaneous and time-averaged power is presented by modeling the extraction of energy through a viscous damper. The highest power coefficients and efficiencies are obtained at velocities just higher than the flutter velocity. Introduction of positive cubic stiffening in the system springs is seen to make the system more stable, LCOs more harmonic and single-period, and to potentially increase power extraction efficiency of the system

A Spitzer Space Telescope Study of SN 2002hh: An Infrared Echo from a Type IIP Supernova

We present late-time (590-994 days) mid-IR photometry of the normal but highly reddened Type IIP supernova SN 2002hh. Bright, cool, slowly fading emission is detected from the direction of the supernova. Most of this flux appears not to be driven by the supernova event but instead probably originates in a cool, obscured star formation region or molecular cloud along the line of sight. We also show, however, that the declining component of the flux is consistent with an SN-powered IR echo from a dusty progenitor CSM. Mid-IR emission could also be coming from newly condensed dust and/or an ejecta/CSM impact, but their contributions are likely to be small. For the case of a CSM-IR echo, we infer a dust mass of as little as 0.036 M☉ with a corresponding CSM mass of 3.6(0.01/rdg) M☉, where rdg is the dust-to-gas mass ratio. Such a CSM would have resulted from episodic mass loss whose rate declined significantly about 28,000 years ago. Alternatively, an IR echo from a surrounding, dense, dusty molecular cloud might also have been responsible for the fading component. Either way, this is the first time that an IR echo has been clearly identified in a Type IIP supernova. We find no evidence for or against the proposal that Type IIP supernovae produce large amounts of dust via grain condensation in the ejecta. However, within the CSM-IR echo scenario, the mass of dust derived implies that the progenitors of the most common of core-collapse supernovae may make an important contribution to the universal dust content