14,302 research outputs found
Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)
Seven of ten candidate H-alpha emission-line stars found in an objective
grism survey of a 1 square degree region in MBM 18, were observed
spectroscopically. Four of these have weak H-alpha emission, and 6 out of 7
have spectral types M1-M4V. One star is of type F7-G1V, and has H-alpha in
absorption. The spectra of three of the M-stars may show an absorption line of
LiI, although none of these is an unambiguous detection. For the six M-stars a
good fit is obtained with pre-main-sequence isochrones indicating ages between
7.5 and 15Myr. The molecular cloud mass, derived from the integrated 12CO(1-0)
emission, is 160Mo (for a distance of 120pc), much smaller than the virial mass
(10^3Mo), and the cloud is not gravitationally bound. Nor are the individual
clumps we identified through a clump-finding routine. Considering the relative
weakness or absence of the H-alpha emission, the absence of other emission
lines, and the lack of clear LiI absorption, the targets are not T Tauri stars.
With ages between 7.5 and 15Myr they are old enough to explain the lack of
lithium in their spectra. Based on the derived distances (60-250pc), some of
the stars may lie inside the molecular cloud (120-150pc). From the fact that
the cloud as a whole, as well as the individual clumps, are not gravitationally
bound, in combination with the ages of the stars we conclude that it is not
likely that (these) stars were formed in MBM 18.Comment: Accepted for publication in Astronomy & Astrophysics (20 pages
Shocked H2 and Fe+ Dynamics in the Orion Bullets
Observations of H2 velocity profiles in the two most clearly defined Orion
bullets are extremely difficult to reconcile with existing steady-state shock
models. We have observed [FeII] 1.644um velocity profiles of selected bullets
and H2 1-0 S(1) 2.122um velocity profiles for a series of positions along and
across the corresponding bow-shaped shock fronts driven into the surrounding
molecular cloud. Integrated [FeII] velocity profiles of the brightest bullets
are consistent with theoretical bow shock predictions. However, observations of
broad, singly-peaked H2 1-0 S(1) profiles at a range of positions within the
most clearly resolved bullet wakes are not consistent with molecular shock
modelling. A uniform, collisionally broadened background component which
pervades the region in both tracers is inconsistent with fluorescence due to
the ionizing radiation of the Trapezium stars alone.Comment: 20 pages including 18 figures, published in MNRA
Directed Explicit Model Checking with HSF-SPIN
We present the explicit state model checker HSF-SPIN which is based on the model checker SPIN and its Promela modeling language. HSF-SPIN incorporates directed search algorithms for checking safety and a large class of LTL-specified liveness properties. We start off from the A* algorithm and define heuristics to accelerate the search into the direction of a specified failure situation. Next we propose an improved nested depth-first search algorithm that exploits the structure of Promela Never-Claims. As a result of both improvements, counterexamples will be shorter and the explored part of the state space will be smaller than with classical approaches, allowing to analyze larger state spaces. We evaluate the impact of the new heuristics and algorithms on a set of protocol models, some of which are real-world industrial protocols
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