Automatically ‘Verifying’ Discrete-Time Complex Systems through Learning, Abstraction and Refinement

Precisely modeling complex systems like cyber-physical systems is challenging, which often render model-based system verification techniques like model checking infeasible. To overcome this challenge, we propose a method called LAR to automatically `verify' such complex systems through a combination of learning, abstraction and refinement from a set of system log traces. We assume that log traces and sampling frequency are adequate to capture `enough' behaviour of the system. Given a safety property and the concrete system log traces as input, LAR automatically learns and refines system models, and produces two kinds of outputs. One is a counterexample with a bounded probability of being spurious. The other is a probabilistic model based on which the given property is `verified'. The model can be viewed as a proof obligation, i.e., the property is verified if the model is correct. It can also be used for subsequent system analysis activities like runtime monitoring or model-based testing. Our method has been implemented as a self-contained software toolkit. The evaluation on multiple benchmark systems as well as a real-world water treatment system shows promising results.Comment: Accepted by IEEE Transactions on Software Engineerin

A multi-wavelength study of star formation activity in the S235 complex

We have carried out an extensive multi-wavelength study to investigate the star formation process in the S235 complex. The S235 complex has a sphere-like shell appearance at wavelengths longer than 2 $\mu$m and harbors an O9.5V type star approximately at its center. Near-infrared extinction map traces eight subregions (having A$_{V}$ $>$ 8 mag), and five of them appear to be distributed in an almost regularly spaced manner along the sphere-like shell surrounding the ionized emission. This picture is also supported by the integrated $^{12}$CO and $^{13}$CO intensity maps and by Bolocam 1.1 mm continuum emission. The position-velocity analysis of CO reveals an almost semi-ring like structure, suggesting an expanding H\,{\sc ii} region. We find that the Bolocam clump masses increase as we move away from the location of the ionizing star. This correlation is seen only for those clumps which are distributed near the edges of the shell. Photometric analysis reveals 435 young stellar objects (YSOs), 59\% of which are found in clusters. Six subregions (including five located near the edges of the shell) are very well correlated with the dust clumps, CO gas, and YSOs. The average values of Mach numbers derived using NH$_{3}$ data for three (East~1, East~2, and Central~E) out of these six subregions are 2.9, 2.3, and 2.9, indicating these subregions are supersonic. The molecular outflows are detected in these three subregions, further confirming the on-going star formation activity. Together, all these results are interpreted as observational evidence of positive feedback of a massive star.Comment: 28 pages, 15 figures, 3 tables, Accepted for publication in The Astrophysical Journa

Thermodynamics of the Massive Gross-Neveu Model

We study the thermodynamics of massive Gross-Neveu models with explicitly broken discrete or continuous chiral symmetries for finite temperature and fermion densities. The large $N$ limit is discussed bearing attention to the no-go theorems for symmetry breaking in two dimensions which apply to the massless cases. The main purpose of the study is to serve as analytical orientation for the more complex problem of chiral transition in $4-$dimensional QCD with quarks. For any non-vanishing fermion mass we find, at finite densities, lines of first order phase transitions. For small mass values traces of would-be second order transitions and a tricritical point are recognizable. We study the thermodynamics of these models, and in the model with broken continuous chiral symmetry we examine the properties of the pion like state.Comment: 34 pages (+18 figures, available upon request to [email protected]), LATEX file, uses art12a.sty, macro included, UGVA-DPT 1994/06-85

Infrared photometric study of the massive star forming region S235 using Spitzer-IRAC and JHK observations

We present the {\it Spitzer}-IRAC images of the S235 star forming complex that includes the East~1 & 2, Central and S235 A & B regions. In addition, we present the near-infrared images of the S235 A & B regions. The IRAC photometry reveals on-going star formation, with 86 Class 0/I and 144 Class II YSOs in the entire S235 complex. Nearly 73% of these YSOs are present in clusters with a maximum surface density of 120 YSOs/pc$^{2}$ (in the vicinity of S235A & B regions). A few YSOs, possibly in an arc-like formation, are identified towards the south of S235A region, which may be speculated as an evidence for magnetically super-critical collapse. One of the sources in the arc-like formation, namely S235AB-MIR, seems to be a young, massive star that is still accreting matter. SED modeling of some of the newly identified YSOs confirms the classification made on the basis of IRAC colours. The IRAC ratio map of Ch2/Ch4 traces clearly the Br$\alpha$ emission associated with the HII region of S235A within the horse-shoe envelope. Outside the horse-shoe structure, the ratio map indicates shock-excited H$_{2}$ emission. Br$\alpha$ emission is also seen around S235B (from the ratio map). The ratio map of Ch2/Ch4 reveals that the source "e2s3" in the East~2 region may be associated with shock-excited H$_2$ emission outflow or jet. The SED modeling of this new source indicates that it is a very young massive star that is not yet able to drive an HII region.Comment: 19 pages; 13 figures; 8 tables. Accepted in MNRAS, Feb 201

ALMA observations of TiO2_2 around VY Canis Majoris

Titanium dioxide, TiO$_2$, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO$_2$ has been detected only in the complex environment of the red supergiant VY CMa. We aim to constrain the distribution and excitation of TiO$_2$ around VY CMa in order to clarify its role in dust formation. We analyse spectra and channel maps for TiO$_2$ extracted from ALMA science verification data. We detect 15 transitions of TiO$_2$, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO$_2$ emission likely traces gas exposed to the stellar radiation field. A roughly east-west oriented, accelerating bipolar-like structure is found, of which the blue component runs into and breaks up around a solid continuum component. A distinct tail to the south-west is seen for some transitions, consistent with features seen in the optical and near-infrared. We find that a significant fraction of TiO$_2$ remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.Comment: Accepted for publication in Astronomy & Astrophysics, 25 pages, 20 figure