The Origin of Large-scale HI structures in the Magellanic Bridge

We investigate the formation of a number of key large-scale HI features in the ISM of the Magellanic Bridge using dissipationless numerical simulation techniques. This study comprises the first direct comparison between detailed HI maps of the Bridge and numerical simulations. We confirm that the SMC forms two tidal filaments: a near arm, which forms the connection between the SMC and LMC, and a counterarm. We show that the HI of the most dense part of the Bridge can become arranged into a bimodal configuration, and that the formation of a "loop" of HI, located off the North-Eastern edge of the SMC can be reproduced simply as a projection of the counter-arm, and without invoking localised energy-deposition processes such as SNe or stellar winds.Comment: 5 Pages, 4 Figures, Accepted - MNRAS let

Cluster Algebras of Grassmannians are Locally Acyclic

Considered as commutative algebras, cluster algebras can be very unpleasant objects. However, the first author introduced a condition known as "local acyclicity" which implies that cluster algebras behave reasonably. One of the earliest and most fundamental examples of a cluster algebra is the homogenous coordinate ring of the Grassmannian. We show that the Grassmannian is locally acyclic. Morally, we are in fact showing the stronger result that all positroid varieties are locally acyclic. However, it has not been shown that all positroid varieties have cluster structure, so what we actually prove is that certain cluster varieties associated to Postnikov's alternating strand diagrams are locally acylic. Moreover, we actually establish a slightly stronger property than local acyclicity, which we term the Louise property, that is designed to facilitate proofs involving the Mayer-Vietores sequence.Comment: 14 pages, 8 figures, minor edits from previous version, added references to recent work of LeCler

Gamma-Ray Light Curves and Spectra of Models for Type-Ia Supernovae

Based on detailed Monte Carlo calculations, we present gamma-ray energy deposition functions, gamma-ray light curves, and gamma-ray spectra for a large set of theoretical models of Type Ia supernovae including ''classical'' detonation and deflagration, delayed detonation, explosions of low mass white dwarfs, and tamped detonation scenarios. Our computations show that models for Type Ia supernovae can be discriminated and the absolute amount of Ni-56 synthesized in the event can be determined on the basis of the gamma-ray light curves and spectra if gamma-ray measurements are combined with observations at other wavelengths, e.g., in the optical band. We discuss at which times gamma-ray observations are most suitable and needed from the theoretical point of view. The implication of the upper limit in the gamma-ray flux by CGRO experiment for our understanding of SN 1991 T is discussed. We find that this limit is consistent with both the optical light curve and the implied distance (12.5 Mpc), i.e., several models can be ruled out by the gamma-ray observations.Astronom