331 research outputs found
Field #3 of the Palomar-Groningen Survey II. Near-infrared photometry of semiregular variables
Near-infrared photometry (JHKL'M) was obtained for 78 semiregular variables
(SRVs) in field #3 of the Palomar-Groningen survey (PG3, l=0, b=-10). Together
with a sample of Miras in this field a comparison is made with a sample of
field SRVs and Miras. The PG3 SRVs form a sequence (period-luminosity
& period-colour) with the PG3 Miras, in which the SRVs are the short period
extension to the Miras. The field and PG3 Miras follow the same P/(J--K)o
relation, while this is not the case for the field and PG3 SRVs. Both the PG3
SRVs and Miras follow the SgrI period-luminosity relation adopted from Glass et
al. (1995, MNRAS 273, 383). They are likely pulsating in the fundamental mode
and have metallicities spanning the range from intermediate to approximately
solar.Comment: 14 pages LaTeX (2 tables, 8 figures), to appear in A&A 338 (1998);
minor modifications in tex
Working Fluid Quantity Effect on Magnetic Field Control of Heat Pipes
This paper deals with heat pipes controlled by a static magnetic field and with an important side effect – the quantity of working fluid. Heat pipes are able to provide very effective heat transport. Several standard regulation methods are commonly used for this purpose. In previous experiments implemented in our laboratory, we have observed the significant influence of a magnetic field on the heat conductance of the selected heat pipe. A special heat pipe was manufactured for this purpose and pure oxygen was chosen as a working fluid, due to its suitable magnetic properties. The heat pipe operation and the magnetic field control depend on various parameters. This paper is focused on the influence of the quantity of working fluid. Some important results of our experiments are presented and discussed.
Synthetic seismograms for deep seismic sounding studies using asymptotic ray theory
In asymptotic ray theory, the solution for particle motion is assumed to be an infinite power series of inverse frequency and a vector amplitude, Ān(x, y, z), independent of frequency. A point source with any desired impulse response and radiation pattern is easily incorporated. A synthetic seismogram computer program has been written for a plane-layered homogeneous elastic media using the first or second terms of the expansion where necessary. Multiply converted refracted and reflected phases and also head waves at distances away from the critical angle are included. In addition, the phases are all identified and their amplitude-distance function plotted if desired. The synthetic seismograms are calculated for a model in southern Alberta and another in northwestern Ontario as obtained by deep seismic sounding programs. It is found that reflected phases dominate the seismograms and they are at least as important as head waves in the interpretation of experimental results
The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample
The mass-loss process from evolved stars is a key ingredient for our
understanding of many fields of astrophysics, including stellar evolution and
the chemical enrichment of the interstellar medium via stellar yields. One the
main unsolved questions is the geometry of the mass-loss process. Taking
advantage of the results from the Herschel Mass loss of Evolved StarS (MESS)
programme, we initiated a coordinated effort to characterise the geometry of
mass loss from evolved red giants at various spatial scales. For this purpose
we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner
envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this
contribution we present an overview of the interferometric data collected
within the frame of our Large Programme, and we also add archive data for
completeness. We studied the geometry of the inner atmosphere by comparing the
observations with predictions from different geometric models. Asymmetries are
detected for five O-rich and S-type, suggesting that asymmetries in the N band
are more common among stars with such chemistry. We speculate that this fact is
related to the characteristics of the dust grains. Except for one star, no
interferometric variability is detected, i.e. the changes in size of the shells
of non-mira stars correspond to changes of the visibility of less than 10%. The
observed spectral variability confirms previous findings from the literature.
The detection of dust in our sample follows the location of the AGBs in the
IRAS colour-colour diagram: more dust is detected around oxygen-rich stars in
region II and in the carbon stars in region VII. The SiC dust feature does not
appear in the visibility spectrum of UAnt and SSct, which are two carbon stars
with detached shells. This finding has implications for the theory of SiC dust
formation.Comment: 43 pages, 31 figures; accepted for publication in Astronomy &
Astrophysics. Abstract shortened for compilation reasons. Metadata correcte
Grown-up stars physics with MATISSE
MATISSE represents a great opportunity to image the environment around
massive and evolved stars. This will allow one to put constraints on the
circumstellar structure, on the mass ejection of dust and its reorganization ,
and on the dust-nature and formation processes. MATISSE measurements will often
be pivotal for the understanding of large multiwavelength datasets on the same
targets collected through many high-angular resolution facilities at ESO like
sub-millimeter interferometry (ALMA), near-infrared adaptive optics (NACO,
SPHERE), interferometry (PIONIER, GRAVITY), spectroscopy (CRIRES), and
mid-infrared imaging (VISIR). Among main sequence and evolved stars, several
cases of interest have been identified that we describe in this paper.Comment: SPIE, Jun 2016, Edimbourgh, Franc
Fostering collaborative knowledge construction with visualization tools
This study investigates to what extent collaborative knowledge construction can be fostered by providing students with visualization tools as structural support. Thirty-two students of Educational Psychology took part in the study. The students were subdivided into dyads and asked to solve a case problem of their learning domain under one of two conditions: 1) with content-specific visualization 2) with content-unspecific visualization. Results show that by being provided with a content-specific visualization tool, both the process and the outcome of the cooperative effort improved. More specifically, dyads under that condition referred to more adequate concepts, risked more conflicts, and were more successful in integrating prior knowledge into the collaborative solution. Moreover, those learning partners had a more similar individual learning outcome
Supersymmetric Fokker-Planck strict isospectrality
I report a study of the nonstationary one-dimensional Fokker-Planck solutions
by means of the strictly isospectral method of supesymmetric quantum mechanics.
The main conclusion is that this technique can lead to a space-dependent
(modulational) damping of the spatial part of the nonstationary Fokker-Planck
solutions, which I call strictly isospectral damping. At the same time, using
an additive decomposition of the nonstationary solutions suggested by the
strictly isospectral procedure and by an argument of Englefield [J. Stat. Phys.
52, 369 (1988)], they can be normalized and thus turned into physical
solutions, i.e., Fokker-Planck probability densities. There might be
applications to many physical processes during their transient periodComment: revised version, scheduled for PRE 56 (1 August 1997) as a B
The geometry of the close environment of SV Psc as probed by VLTI/MIDI
Context. SV Psc is an asymptotic giant branch (AGB) star surrounded by an
oxygen-rich dust envelope. The mm-CO line profile of the object's outflow shows
a clear double-component structure. Because of the high angular resolution,
mid-IR interferometry may give strong constraints on the origin of this
composite profile.
Aims. The aim of this work is to investigate the morphology of the
environment around SV Psc using high-angular resolution interferometry
observations in the mid-IR with the Very Large Telescope MID-infrared
Interferometric instrument (VLTI/MIDI).
Methods. Interferometric data in the N-band taken at different baseline
lengths (ranging from 32-64 m) and position angles (73- 142{\deg}) allow a
study of the morphology of the circumstellar environment close to the star. The
data are interpreted on the basis of 2-dimensional, chromatic geometrical
models using the fitting software tool GEM-FIND developed for this purpose.
Results. The results favor two scenarios: (i) the presence of a highly
inclined, optically thin, dusty disk surrounding the central star; (ii) the
presence of an unresolved binary companion at a separation of 13.7 AU and a
position angle of 121.8{\deg} NE. The derived orbital period of the binary is
38.1 yr. This detection is in good agreement with hydrodynamic simulations
showing that a close companion could be responsible for the entrainment of the
gas and dust into a circumbinary structure.Comment: 10 pages, 12 figure
Formation of convective cells in the scrape-off layer of the CASTOR tokamak
Understanding of the scrape-off layer (SOL) physics in tokamaks requires
diagnostics with sufficient temporal and spatial resolution. This contribution
describes results of experiments performed in the SOL of the CASTOR tokamak
(R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the
whole poloidal cross section. The individual probes measure either the ion
saturation current of the floating potential with the spatial resolution up to
3 mm. Experiments are performed in a particular magnetic configuration,
characterized by a long parallel connection length in the SOL, L_par ~q2piR. We
report on measurements in discharges, where the edge electric field is modified
by inserting a biased electrode into the edge plasma. In particular, a complex
picture is observed, if the biased electrode is located inside the SOL. The
poloidal distribution of the floating potential appears to be strongly
non-uniform at biasing. The peaks of potential are observed at particular
poloidal angles. This is interpreted as formation of a biased flux tube, which
emanates from the electrode along the magnetic field lines and snakes q times
around the torus. The resulting electric field in the SOL is 2-dimensional,
having the radial as well as the poloidal component. It is demonstrated that
the poloidal electric field E_pol convects the edge plasma radially due to the
E_pol x B_T drift either inward or outward depending on its sign. The
convective particle flux is by two orders of magnitude larger than the
fluctuation-induced one and consequently dominates.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
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