A high resolution UV absorption spectrum of supernova ejecta in SN1006

We report a high resolution, far-ultraviolet, STIS E140M spectrum of the strong, broad Si II, III, and IV features produced by the ejecta of SN1006 seen in absorption against the background Schweizer-Middleditch star. The spectrum confirms the extreme sharpness of the red edge of the redshifted Si II 1260 A feature, supporting the idea that this edge represents the location of the reverse shock moving into the freely expanding ejecta. The expansion velocity of ejecta at the reverse shock is measured to be 7026 +-3(relative) +-10(absolute) km/s. If the shock model is correct, then the expansion velocity should be decreasing at the observable rate of 2.7 +-0.1 km/s per year. The pre-shock velocity, post-shock velocity, and post-shock velocity dispersion are all measured from the Si II 1260 A feature, and consistency of these velocities with the shock jump conditions implies that there is little or no electron heating in this fast (2680 km/s) Si-rich shock.Comment: 9 pages, 5 embedded postscript fig

A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds

(ABRIDGED) We present a Far-Ultraviolet Spectroscopic Explorer survey of highly ionized high-velocity clouds (HVCs) in 66 extragalactic sight lines. We find a total of 63 high-velocity O VI absorbers, 16 with 21 cm-emitting H I counterparts and 47 ``highly ionized'' absorbers without 21 cm emission. 11 of these high-velocity O VI absorbers are positive-velocity wings (broad O VI features extending asymmetrically to velocities of up to 300 km/s). The highly ionized HVC population is characterized by =38+/-10 km/s and <log N_a(O VI)>=13.83+/-0.36. We find that 81% (30/37) of high-velocity O VI absorbers have clear accompanying C III absorption, and 76% (29/38) have accompanying H I absorption in the Lyman series. The lower average width of the high-velocity H I absorbers implies the H I lines arise in a separate, lower temperature phase than the O VI. We find that the shape of the wing profiles is well reproduced by a radiatively cooling, vertical outflow. However, the outflow has to be patchy and out of ionization equilibrium. An alternative model, consistent with the observations, is one where the highly ionized HVCs represent the low N(H I) tail of the HVC population, with the O VI formed at the interfaces around the embedded H I cores. Though we cannot rule out a Local Group explanation, we favor a Galactic origin. This is based on the recent evidence that both H I HVCs and the million-degree gas detected in X-ray absorption are Galactic phenomena. Since the highly ionized HVCs appear to trace the interface between these two Galactic phases, it follows that highly ionized HVCs are Galactic themselves. However, the non-detection of high-velocity O VI in halo star spectra implies that any Galactic high-velocity O VI exists at z-distances beyond a few kpc.Comment: 36 pages, 14 figures (3 in color), accepted to ApJS. Some figures downgraded to limit file siz

A practical fpt algorithm for Flow Decomposition and transcript assembly

The Flow Decomposition problem, which asks for the smallest set of weighted paths that "covers" a flow on a DAG, has recently been used as an important computational step in transcript assembly. We prove the problem is in FPT when parameterized by the number of paths by giving a practical linear fpt algorithm. Further, we implement and engineer a Flow Decomposition solver based on this algorithm, and evaluate its performance on RNA-sequence data. Crucially, our solver finds exact solutions while achieving runtimes competitive with a state-of-the-art heuristic. Finally, we contextualize our design choices with two hardness results related to preprocessing and weight recovery. Specifically, $k$-Flow Decomposition does not admit polynomial kernels under standard complexity assumptions, and the related problem of assigning (known) weights to a given set of paths is NP-hard.Comment: Introduces software package Toboggan: Version 1.0. http://dx.doi.org/10.5281/zenodo.82163

A High-Resolution Ultraviolet Absorption Spectrum of Supernova Ejecta in SN1006

We report a high-resolution, far-ultraviolet, Space Telescope Imaging Spectrograph (STIS) E140M spectrum of the strong, broad Siii, iii and iv features produced by the ejecta of the supernova of 1006 ad (SN1006) seen in absorption against the background Schweizer–Middleditch OB subdwarf star. The spectrum confirms the extreme sharpness of the red edge of the redshifted Si ii 1260 Å feature, supporting the idea that this edge represents the location of the reverse shock moving into the freely expanding Si-rich ejecta. The expansion velocity of ejecta at the reverse shock is measured to be 7026 ± 3(relative) ±10(absolute) km s−1. If the shock model is correct, then the expansion velocity should be decreasing at the observable rate of 2.7 ± 0.1 km s−1 yr−1. The pre-shock velocity, post-shock velocity and post-shock velocity dispersion are all measured from the Si ii 1260 Å feature, and consistency of these velocities with the shock jump conditions implies that there is little or no electron heating in this fast (2680 km s−1) Si-rich shock; the 3σ upper limit on the fraction of shock energy that goes into electron heating is 0.26

Dynamical instabilities of Bose-Einstein condensates at the band-edge in one-dimensional optical lattices

We report on experiments that demonstrate dynamical instability in a Bose-Einstein condensate at the band-edge of a one-dimensional optical lattice. The instability manifests as rapid depletion of the condensate and conversion to a thermal cloud. We consider the collisional processes that can occur in such a system, and perform numerical modeling of the experiments using both a mean-field and beyond mean-field approach. We compare our numerical results to the experimental data, and find that the Gross-Pitaevskii equation is not able to describe this experiment. Our beyond mean-field approach, known as the truncated Wigner method, allows us to make quantitative predictions for the processes of parametric growth and thermalization that are observed in the laboratory, and we find good agreement with the experimental results.Comment: v2: Added several reference