29 research outputs found
3D radiative transfer: Continuum and line scattering in non-spherical winds from OB stars
Context: State of the art quantitative spectroscopy of OB-stars compares
synthetic spectra (calculated by means of 1D, spherically symmetric computer
codes) with observations. Certain stellar atmospheres, however, show strong
deviations from spherical symmetry, and need to be treated in 3D. Aims: We
present a newly developed 3D radiative transfer code, tailored to the solution
of the radiation field in rapidly expanding stellar atmospheres. We apply our
code to the continuum transfer in wind-ablation models, and to the UV resonance
line formation in magnetic winds. Methods: We have used a 3D finite-volume
method for the solution of the equation of radiative transfer, to study
continuum- and line-scattering problems. Convergence has been accelerated by a
non-local approximate Lambda-iteration scheme. Particular emphasis has been put
on careful (spherically symmetric) test cases. Results: Typical errors of the
source functions, when compared to 1D solutions, are of the order of 10-20 %,
and increase for optically thick continua. In circumstellar discs, the
radiation temperatures in the (optically thin) transition region from wind to
disc are quite similar to corresponding values in the wind. For MHD simulations
of dynamical magnetospheres, the line profiles, calculated with our 3D code,
agree well with previous solutions using a 3D-SEI method. When compared with
profiles resulting from the `analytic dynamical magnetosphere' (ADM) model,
significant differences become apparent. Conclusions: Due to similar radiation
temperatures in the wind and the transition region to the disc, the same
line-strength distribution can be applied within radiation hydrodynamic
calculations for circumstellar discs in `accreting high-mass stars'. To
properly describe the UV line formation in dynamical magnetospheres, the ADM
model needs to be further developed, at least in a large part of the outer
wind
Refinement in hybridised institutions
Hybrid logics, which add to the modal description of transition structures the ability to refer to specific
states, offer a generic framework to approach the specification and design of reconfigurable systems, i.e., systems
with reconfiguration mechanisms governing the dynamic evolution of their execution configurations in response
to both external stimuli or internal performance measures. A formal representation of such systems is through
transition structures whose states correspond to the different configurations they may adopt. Therefore, each
node is endowed with, for example, an algebra, or a first-order structure, to precisely characterise the semantics
of the services provided in the corresponding configuration. This paper characterises equivalence and refinement
for these sorts of models in a way which is independent of (or parametric on) whatever logic (propositional,
equational, fuzzy, etc) is found appropriate to describe the local configurations. A Hennessy–Milner like theorem
is proved for hybridised logics.This work is funded by ERDF-European Regional Development Fund, through the COMPETE Programme, and by National Funds through FCT within project FCOMP-01-0124-FEDER-028923 and by project NORTE-07-0124-FEDER-000060, co-financed by the North Portugal Regional Operational Programme (ON.2), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF). The work had also partial financial assistance by the project PEst-OE/MAT/UI4106/2014 at CIDMA, FCOMP-01-0124-FEDER-037281 at INESC TEC and the Marie Curie project FP7-PEOPLE-2012-IRSES (GetFun)
Dynamic logic with binders and its application to the development of reactive systems
Publicado em "Theoretical aspects of computing - ICTAC 2016: 13th International Colloquium, Taipei, Taiwan, ROC, October 24–31, 2016, Proceedings". ISBN 978-3-319-46749-8This paper introduces a logic to support the specification and
development of reactive systems on various levels of abstraction, from
property specifications, concerning e.g. safety and liveness requirements,
to constructive specifications representing concrete processes. This is
achieved by combining binders of hybrid logic with regular modalities
of dynamic logics in the same formalism, which we call D↓-logic. The
semantics of our logic focuses on effective processes and is therefore given
in terms of reachable transition systems with initial states. The second
part of the paper resorts to this logic to frame stepwise development of
reactive systems within the software development methodology proposed
by Sannella and Tarlecki. In particular, we instantiate the generic concepts
of constructor and abstractor implementations by using standard
operators on reactive components, like relabelling and parallel composition,
as constructors, and bisimulation for abstraction. We also study
vertical composition of implementations which relies on the preservation
of bisimularity by the constructions on labeleld transition systems.FCT individual grants SFRH/BPD/103004/2014 and SFRH/BSAB/113890/2015ERDF European Regional Development Fund through the Operational Programme for Competitiveness and Internationalisation - COMPETE 2020 Programme and by National Funds through the Portuguese funding agency, FCT - Fundação para a Cência e a Tecnologia within project POCI-01-0145-FEDER-016692 and UID/MAT/04106/2013 at CIDM
A 3D short-characteristics method for continuum and line scattering problems in the winds of hot stars
Context: Knowledge about hot, massive stars is usually inferred from
quantitative spectroscopy. To analyse non-spherical phenomena, the existing 1D
codes must be extended to higher dimensions, and corresponding tools need to be
developed. Aims: We present a 3D radiative transfer code that is capable of
calculating continuum and line scattering problems in the winds of hot stars.
By considering spherically symmetric test models, we discuss potential error
sources, and indicate advantages and disadvantages by comparing different
solution methods. Further, we analyse the UV resonance line formation in the
winds of rapidly rotating O stars. Methods: We consider both a (simplified)
continuum model including scattering and thermal sources, and a UV resonance
line transition approximated by a two-level-atom. We applied the
short-characteristics (SC) method, using linear or monotonic B\'ezier
interpolations, to solve the equation of radiative transfer on a non-uniform
Cartesian grid. To calculate scattering dominated problems, our solution method
is supplemented by an accelerated -iteration scheme. Results: For the
spherical test models, the mean relative error of the source function is on the
level, depending on the applied interpolation technique and the
complexity of the considered model. All calculated line profiles are in
excellent agreement with corresponding 1D solutions. The predicted line
profiles from fast rotating stars show a distinct behaviour as a function of
rotational speed and inclination. This behaviour is tightly coupled to the wind
structure and the description of gravity darkening and stellar surface
distortion. Conclusions: Our SC methods are ready to be used for quantitative
analyses of UV resonance line profiles. When calculating optically thick
continua, both SC methods give reliable results, in contrast to the alternative
finite-volume method
Binary-object spectral-synthesis in 3D (BOSS-3D) -- Modelling H-alpha emission in the enigmatic multiple system LB-1
Context: To quantitatively decode the information stored within an observed
spectrum, detailed modelling of the physical state and accurate radiative
transfer solution schemes are required. In the analysis of stellar spectra, the
numerical model often needs to account for binary companions and 3D structures
in the stellar envelopes. The enigmatic binary (or multiple) system LB-1
constitutes a perfect example of such a complex multi-D problem. Aims: To
improve our understanding of the LB-1 system, we directly modelled the
phase-dependent H-alpha line profiles of this system. To this end, we developed
a multi-purpose binary-object spectral-synthesis code in 3D (BOSS-3D). Methods:
BOSS-3D calculates synthetic line profiles for a given state of the
circumstellar material. The standard pz-geometry commonly used for single stars
is extended by defining individual coordinate systems for each involved object
and by accounting for the appropriate coordinate transformations. The code is
then applied to the LB-1 system, considering two main hypotheses, a binary
containing a stripped star and Be star, or a B star and a black hole with a
disc. Results: Comparing these two scenarios, neither model can reproduce the
detailed phase-dependent shape of the H-alpha line profiles. A satisfactory
match with the observations, however, is obtained by invoking a disc around the
primary object in addition to the Be-star disc or the black-hole accretion
disc. Conclusions: The developed code can be used to model synthetic line
profiles for a wide variety of binary systems, ranging from transit spectra of
planetary atmospheres, to post-asymptotic giant branch binaries including
circumstellar and circumbinary discs and massive-star binaries with stellar
winds and disc systems. For the LB-1 system, our modelling provides strong
evidence that each object in the system contains a disc-like structure
Ultraviolet Line Profiles of Slowly Rotating Massive Star Winds Using the "Analytic Dynamical Magnetosphere" Formalism
Recent large-scale spectropolarimetric surveys have established that a small
but significant percentage of massive stars host stable, surface dipolar
magnetic fields with strengths on the order of kG. These fields channel the
dense, radiatively driven stellar wind into circumstellar magnetospheres, whose
density and velocity structure can be probed using ultraviolet (UV)
spectroscopy of wind-sensitive resonance lines. Coupled with appropriate
magnetosphere models, UV spectroscopy provides a valuable way to investigate
the wind-field interaction, and can yield quantitative estimates of the wind
parameters of magnetic massive stars. We report a systematic investigation of
the formation of UV resonance lines in slowly rotating magnetic massive stars
with dynamical magnetospheres. We pair the Analytic Dynamical Magnetosphere
(ADM) formalism with a simplified radiative transfer technique to produce
synthetic UV line profiles. Using a grid of models, we examine the effect of
magnetosphere size, the line strength parameter, and the cooling parameter on
the structure and modulation of the line profile. We find that magnetic massive
stars uniquely exhibit redshifted absorption at most viewing angles and
magnetosphere sizes, and that significant changes to the shape and variation of
the line profile with varying line strengths can be explained by examining the
individual wind components described in the ADM formalism. Finally, we show
that the cooling parameter has a negligible effect on the line profiles.Comment: 16 pages, 15 figures, accepted to MNRA
Extended ML: Past, present and future
An overview of past, present and future work on the Extended ML formal program development framework is given, with emphasis on two topics of current active research: the semantics of the Extended ML specification language, and tools to support formal program development
Ultraviolet line profiles of slowly rotating massive star winds using the 'analytic dynamical magnetosphere' formalism
peer reviewedRecent large-scale spectropolarimetric surveys have established that a small but significant percentage of massive stars host stable, surface dipolar magnetic fields with strengths on the order of kG. These fields channel the dense, radiatively driven stellar wind into circumstellar magnetospheres, whose density and velocity structure can be probed using ultraviolet (UV) spectroscopy of wind-sensitive resonance lines. Coupled with appropriate magnetosphere models, UV spectroscopy provides a valuable way to investigate the wind-field interaction, and can yield quantitative estimates of the wind parameters of magnetic massive stars. We report a systematic investigation of the formation of UV resonance lines in slowly rotating magnetic massive stars with dynamical magnetospheres. We pair the analytic dynamical magnetosphere (ADM) formalism with a simplified radiative transfer technique to produce synthetic UV line profiles. Using a grid of models, we examine the effect of magnetosphere size, the line strength parameter, and the cooling parameter on the structure and modulation of the line profile. We find that magnetic massive stars uniquely exhibit redshifted absorption at most viewing angles and magnetosphere sizes, and that significant changes to the shape and variation of the line profile with varying line strengths can be explained by examining the individual wind components described in the ADM formalism. Finally, we show that the cooling parameter has a negligible effect on the line profiles
Testing data types implementations from algebraic specifications
Algebraic specifications of data types provide a natural basis for testing
data types implementations. In this framework, the conformance relation is
based on the satisfaction of axioms. This makes it possible to formally state
the fundamental concepts of testing: exhaustive test set, testability
hypotheses, oracle. Various criteria for selecting finite test sets have been
proposed. They depend on the form of the axioms, and on the possibilities of
observation of the implementation under test. This last point is related to the
well-known oracle problem. As the main interest of algebraic specifications is
data type abstraction, testing a concrete implementation raises the issue of
the gap between the abstract description and the concrete representation. The
observational semantics of algebraic specifications bring solutions on the
basis of the so-called observable contexts. After a description of testing
methods based on algebraic specifications, the chapter gives a brief
presentation of some tools and case studies, and presents some applications to
other formal methods involving datatypes
A modern guide to quantitative spectroscopy of massive OB stars
Quantitative spectroscopy is a powerful technique from which we can extract
information about the physical properties and surface chemical composition of
stars. In this chapter, I guide the reader through the main ideas required to
get initiated in the learning process to become an expert in the application of
state-of-the-art quantitative spectroscopic techniques to the study of massive
OB stars.
NB: This chapter is intended to serve to young students as a first approach
to a field which has attracted my attention during the last 20 years. I should
note that, despite its importance, at present, the number of real experts in
the field around the world is limited to less than 50 people, and about one
third of them are close to retirement. Hence, I consider that this is a good
moment to write a summary text on the subject to serve as guideline for the
next generations of students interested in joining the massive star crew. If
you are one of them, please, use this chapter as a first working notebook. Do
not stop here. Dig also, for further details, into the literature I quote along
the text. And, once there, dig even deeper to find all the original sources
explaining in more detail the physical and technical concepts that are
presently incorporated into our modern (almost) automatized tools.Comment: Accepted for publication in the book "Reviews in Frontiers of Modern
Astrophysics: From Space Debris to Cosmology" (eds Kabath, Jones and Skarka;
publisher Springer Nature) funded by the European Union Erasmus+ Strategic
Partnership grant "Per Aspera Ad Astra Simul" 2017-1-CZ01-KA203-03556