Spectral Signatures of Gravitationally Confined Thermonuclear Supernova Explosions

We consider some of the spectral and polarimetric signatures of the gravitational confined detonation scenario for Type Ia supernova explosions. In this model, material produced by an off-center deflagration (which itself fails to produce the explosion) forms a metal-rich atmosphere above the white dwarf surface. Using hydrodynamical simulations, we show that this atmosphere is compressed and accelerated during the subsequent interaction with the supernova ejecta. This leads ultimately to the formation of a high-velocity pancake of metal-rich material that is geometrically detached from the bulk of the ejecta. When observed at the epochs near maximum light, this absorbing pancake produces a highly blueshifted and polarized calcium IR triplet absorption feature similar to that observed in several Type~Ia supernovae. We discuss the orientation effects present in our model and contrast them to those expected in other supernova explosion models. We propose that a large sample of spectropolarimetric observations can be used to critically evaluate the different theoretical scenarios.Comment: 4 pages, 3 figures. To appear in ApJ Letters. For higher resolution images and movies see http://panisse.lbl.gov/~dnkasen/gcd.htm

Two-Dimensional Hydrodynamic Models of Super Star Clusters with a Positive Star Formation Feedback

Using the hydrodynamic code ZEUS, we perform 2D simulations to determine the fate of the gas ejected by massive stars within super star clusters. It turns out that the outcome depends mainly on the mass and radius of the cluster. In the case of less massive clusters, a hot high velocity ($\sim 1000$ km s$^{-1}$) stationary wind develops and the metals injected by supernovae are dispersed to large distances from the cluster. On the other hand, the density of the thermalized ejecta within massive and compact clusters is sufficiently large as to immediately provoke the onset of thermal instabilities. These deplete, particularly in the central densest regions, the pressure and the pressure gradient required to establish a stationary wind, and instead the thermally unstable parcels of gas are rapidly compressed, by a plethora of re-pressurizing shocks, into compact high density condensations. Most of these are unable to leave the cluster volume and thus accumulate to eventually feed further generations of star formation. The simulations cover an important fraction of the parameter-space, which allows us to estimate the fraction of the reinserted gas which accumulates within the cluster and the fraction that leaves the cluster as a function of the cluster mechanical luminosity, the cluster size and heating efficiency.Comment: Accepted for publication in ApJ; 27 pages, 9 figures, 1 tabl

Crushing of interstellar gas clouds in supernova remnants II. X-ray emission

AIMS. We study and discuss the time-dependent X-ray emission predicted by hydrodynamic modeling of the interaction of a SNR shock wave with an interstellar gas cloud. The scope includes: 1) to study the correspondence between modeled and X-ray emitting structures, 2) to explore two different physical regimes in which either thermal conduction or radiative cooling plays a dominant role, and 3) to investigate the effects of the physical processes at work on the emission of the shocked cloud in the two different regimes. METHODS. We use a detailed hydrodynamic model, including thermal conduction and radiation, and explore two cases characterized by different Mach numbers of the primary shock: M = 30 in which the cloud dynamics is dominated by radiative cooling and M = 50 dominated by thermal conduction. From the simulations, we synthesize the expected X-ray emission, using available spectral codes. RESULTS. The morphology of the X-ray emitting structures is significantly different from that of the flow structures originating from the shock-cloud interaction. The hydrodynamic instabilities are never clearly visible in the X-ray band. Shocked clouds are preferentially visible during the early phases of their evolution. Thermal conduction and radiative cooling lead to two different phases of the shocked cloud: a cold cooling dominated core emitting at low energies and a hot thermally conducting corona emitting in the X-ray band. The thermal conduction makes the X-ray image of the cloud smaller, more diffuse, and shorter-lived than that observed when thermal conduction is neglected

3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2

The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still under debate. We present three-dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of G2, modeled as an outflow from a "compact source" moving on the observed orbit. The construction of mock position-velocity (PV) diagrams enables a direct comparison with observations and allow us to conclude that the observational properties of the gaseous component of G2 could be matched by a massive ($\dot{M}_\mathrm{w}=5\times 10^{-7} \;M_{\odot} \mathrm{yr^{-1}}$) and slow ($50 \;\mathrm{km \;s^{-1}}$) outflow, as observed for T Tauri stars. In order for this to be true, only the material at larger ($>100 \;\mathrm{AU}$) distances from the source must be actually emitting, otherwise G2 would appear too compact compared to the observed PV diagrams. On the other hand, the presence of a central dusty source might be able to explain the compactness of G2's dust component. In the present scenario, 5-10 years after pericentre the compact source should decouple from the previously ejected material, due to the hydrodynamic interaction of the latter with the surrounding hot and dense atmosphere. In this case, a new outflow should form, ahead of the previous one, which would be the smoking gun evidence for an outflow scenario.Comment: resubmitted to MNRAS after referee report, 16 pages, 11 figure

Crushing of interstellar gas clouds in supernova remnants. I. The role of thermal conduction and radiative losses

We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M = 30 and 50, corresponding to a post-shock temperature $T\approx 1.7\times 10^6$ K and $\approx 4.7\times 10^6$ K, respectively), we explore two very different physical regimes: for M = 30, the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M = 50, the thermal conduction dominates the evolution of the shocked cloud, which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries.Comment: 18 pages, 11 figures, A&A in press, full res. paper at http://www.astropa.unipa.it/Library/OAPA_preprints/orl_AA_2896.ps.g

Can Light Echoes Account for the Slow Decay of Type IIn Supernovae?

The spectra of type IIn supernovae indicate the presence of apre-existing slow, dense circumstellar wind (CSW). If the CSW extends sufficiently far from the progenitor star, then dust formation should occur in the wind. The light from the supernova explosion will scatter off this dust and produce a light echo. Continuum emission seen after the peak will have contributions from both this echo as well as from the shock of the ejecta colliding with the CSW, with a fundamental question of which source dominates the continuum. We calculate the brightness of the light echo as a function of time for a range of dust shell geometries, and use our calculations to fit to the light curves of SN 1988Z and SN 1997ab, the two slowest declining IIn supernovae on record. We find that the light curves of both objects can be reproduced by the echo model. However, their rate of decay from peak, color at peak and their observed peak absolute magnitudes when considered together are inconsistent with the echo model. Furthermore, when the observed values of M$_{B}$ are corrected for the effects of dust scattering, the values obtained imply that these supernovae have unrealistically high luminosities. We conclude that light echoes cannot properly account for the slow decline seen in some IIn's, and that the shock interaction is likely to dominate the continuum emission.Comment: 15 pages, 9 figure

The Post-Pericenter Evolution of the Galactic Center Source G2

In early 2014 the fast-moving near-infrared source G2 reached its closest approach to the supermassive black hole Sgr A* in the Galactic Center. We report on the evolution of the ionized gaseous component and the dusty component of G2 immediately after this event, revealed by new observations obtained in 2015 and 2016 with the SINFONI integral field spectrograph and the NACO imager at the ESO VLT. The spatially resolved dynamics of the Br$\gamma$ line emission can be accounted for by the ballistic motion and tidal shearing of a test-particle cloud that has followed a highly eccentric Keplerian orbit around the black hole for the last 12 years. The non-detection of a drag force or any strong hydrodynamic interaction with the hot gas in the inner accretion zone limits the ambient density to less than a few 10$^3$ cm$^{-3}$ at the distance of closest approach (1500 $R_s$), assuming G2 is a spherical cloud moving through a stationary and homogeneous atmosphere. The dust continuum emission is unresolved in L'-band, but stays consistent with the location of the Br$\gamma$ emission. The total luminosity of the Br$\gamma$ and L' emission has remained constant to within the measurement uncertainty. The nature and origin of G2 are likely related to that of the precursor source G1, since their orbital evolution is similar, though not identical. Both object are also likely related to a trailing tail structure, which is continuously connected to G2 over a large range in position and radial velocity.Comment: 17 pages, 12 figures; accepted for publication in Ap

Rayleigh-Taylor instability with self-generated magnetic field and thermal conduction in 2D

High energy density laboratory experiments on Rayleigh-Taylor instability (RTI) [1] in nonlinear regime show the plasma behavior significantly different from classical simulation results. We include the effects of self-generated magnetic field and heat conduction in simulations aiming to improve agreement with experiments. We find maximum magnetic fields generated ~11MG (β=0.091) without heat conduction (κ=0), field growth saturated by t=20ns; and ~1.7 MG with heat conduction taken into account. Strong magnetic fields in κ=0 simulations affect flow dynamics, new modes are generated. Effect of weaker magnetic fields in simulations with physical values of κ is insignificant; the main difference with classical RTI simulations is suppressed small scale features. In none of the simulations are mass extensions observed.В экспериментах неустойчивости Рэлея-Тэйлора (НРТ) в лабораториях высоких плотностей энергии [1] поведение жидкости существенно отличается от классических результатов численного моделирования. С целью улучшить согласие с экспериментами мы включили в моделирование эффекты самогенерирующегося магнитного поля и теплопроводности. Максимальное магнитное поле получено ~11 MG (β=0.091) в отсутствие теплопроводности (κ=0), рост поля насыщается к t=20 ns; и ~1.7 MG при учтённой теплопроводности. Сильное магнитное поле в модели с κ=0 меняет динамику неустойчивости, генерируются новые моды. Эффект более слабого поля в моделировании с физическими значениями κ несуществен; основное отличие от классической НРТ заключается в подавлении мелкомасштабных структур. Удлинения РТ структур в моделях не наблюдалось.У експериментах нестійкості Релея-Тейлора (НРТ) в лабораторіях великих щільностей енергії [1] поведінка рідини суттєво відрізняється від класичних результатів чисельного моделювання. Для узгодження з експериментами ми включили в моделювання ефекти магнітного поля, що самогенерується, та теплопровідності. Максимальне магнітне поле одержано ~11 MG (β=0.091) за відсутностю теплопровідності (κ=0), зростання поля насичується до t=20 ns; та ~1.7 MG, коли теплопровідність врахована. Сильне магнітне поле в моделі з κ=0 змінює динаміку нестійкості, генеруються нові моди. Ефект більш слабкого поля при моделюванні з фізичними значеннями κ несуттєвий; головною відмінністю від класичної НРТ є нерозвиненість дрібномасштабних структур. Подовження РТ структур в моделях не помічалося