1,629 research outputs found
Von der Evaluation von Lernsoftware zur Gestaltung von Unterricht
In der Forschung zur Wirkung der neuen Medien kann oft beobachtet werden, dass mediale und (unterrichts-)methodische Einflüsse nicht auseinander gehalten werden. Daher wurde bislang wenig systematisch untersucht, unter welchen instruktionalen Bedingungen der Einsatz einer bestimmten Software überhaupt effektiv Lernen unterstützt. Wir schlagen eine Evaluationsmethode vor, die vom Konstrukt der Lernumgebung ausgeht und plädieren dafür, Software unter der möglichst umfassenden Kontrolle der Parameter der jeweiligen Lernumgebung zu evaluieren. Auf diese Weise können Erkenntnisse darüber gewonnen werden, unter welchen Bedingungen das Potenzial einer bestimmten Software zur Geltung kommt und wie Software didaktisch begründet im Unterricht eingesetzt werden sollte
Probing the Canis Major stellar over-density as due to the Galactic warp
Proper-motion, star counts and photometric catalog simulations are used to
explain the detected stellar over-density in the region of Canis Major (CMa),
claimed to be the core of a disrupted dwarf galaxy (Martin et al. 2004,
Bellazzini et al. 2003), as due to the Galactic warp and flare in the external
disk. We compare the kinematics of CMa M-giant selected sample with surrounding
Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion
signature: CMa stars mimic thick disk kinematics. Moreover, when taking into
account the Galactic warp and flare of the disk, 2MASS star count profiles
reproduce the CMa stellar over-density. This star count analysis is confirmed
by direct comparison with synthetic color-magnitude diagrams simulated with the
Besancon models (Robin et al. 2003) that include the warp and flare of the
disk. The presented evidence casts doubt on the identification of the CMa
over-density as the core of a disrupted Milky Way satellite. This however does
not make clear the origin of over-densities responsible for the ring structure
in the anticenter direction of the Galactic halo (Newberg et al. 2002; Yanny et
al. 2003; Zaggia et al. 2004, in preparation).Comment: Accepted for publication in A&A Letters, 4 page
The Milky Way's external disc constrained by 2MASS star counts
Context. Thanks to recent large scale surveys in the near infrared such as
2MASS, the galactic plane that most suffers from extinction is revealed and its
overall structure can be studied. Aims. This work aims at constraining the
structure of the Milky Way external disc as seen in 2MASS data, and in
particular the warp. Methods. We use the Two Micron All Sky Survey (hereafter
2MASS) along with the Stellar Population Synthesis Model of the Galaxy,
developed in Besancon, to constrain the external disc parameters such as its
scale length, its cutoff radius, and the slope of the warp. In order to
properly interpret the observations, the simulated stars are reddened using a
three dimensional extinction map. The shape of the stellar warp is then
compared with previous results and with similar structures in gas and dust.
Results. We find new constraints on the stellar disc, which is shown to be
asymmetrical, similar to observations of HI. The positive longitude side is
found to be easily modelled with a S shape warp but with a slope significantly
smaller than the slope seen in the HI warp. At negative longitudes, the disc
presents peculiarities which are not well reproduced by any simple model.
Finally, comparing with the warp seen in the dust, it seems to follow a slope
intermediate between the gas and the stars.Comment: 9 pages. Accepted for publication in Astronomy and Astrophysic
Magnetic Field Measurement on the C/NOFS Satellite: Geomagnetic Storm Effects in the Low Latitude Ionosphere
The Vector Electric Field Investigation (VEFI) suite onboard the Communications/Navigation Outage Forecasting System (C/NOFS) spacecraft includes a sensitive fluxgate magnetometer to measure DC and ULF magnetic fields in the low latitude ionosphere. The instrument includes a DC vector measurement at 1 sample/sec with a range of +/- 45,000 nT whose primary objective is to provide direct measurements of both V x B and E x B that are more accurate than those obtained using a simple magnetic field model. These data can also be used for scientific research to provide information of large-scale ionospheric and magnetospheric current systems, which, when analyzed in conjunction with the C/NOFS DC electric field measurements, promise to advance our understanding of the electrodynamics of the low latitude ionosphere. In this study, we use the magnetic field data to study the temporal and local time variations of the ring currents during geomagnetic storms. We first compare the in situ measurements with the POMME (the POtsdam Magnetic Model of the Earth) model in order to provide an in-flight "calibration" of the data as well as compute magnetic field residuals essential for revealing large scale external current systems. We then compare the magnetic field residuals observed both during quiet times and during geomagnetic storms at the same geographic locations to deduce the magnetic field signatures of the ring current. As will be shown, the low inclination of the C/NOFS satellite provides a unique opportunity to study the evolution of the ring current as a function of local time, which is particularly insightful during periods of magnetic storms. This paper will present the initial results of this study
Galactic Warp in the overdensity of the Canis Major Region
Bellazzini et al. (2006b) claim that Lopez-Corredoira et al.'s (2002) warp
model is totally unable to reproduce the Canis Major structure in the red clump
stars. However, slight variations in the azimuth of the Lopez-Corredoira et al.
(2002) warp model, justified by the uncertainties in the parameter as well as
the local irregularities with respect to the average model, derive a result
much closer to the observations of the overdensity south/north. The bump of red
clump stars with m_K=13-13.5 around l=241 deg., b=-8.5 deg. and the depth of
the Canis Major structure are also explainable in terms of the warp with an
appropriate extrapolation of constant height between galactocentric radii of 13
and 16 kpc, as observed roughly in the southern warp, instead of a
monotonically increasing height like the northern warp; and the observed
velocity distribution of stars cannot exclude the warp possibility. A warp
model is therefore still a possible explanation of the Canis Major overdensity,
and the hypothesis of the existence of a dwarf galaxy is unnecessary, although
still a possibility too.Comment: 6 pages, 3 figures, accepted to be published in MNRA
Using Schumann Resonance Measurements for Constraining the Water Abundance on the Giant Planets - Implications for the Solar System Formation
The formation and evolution of the Solar System is closely related to the abundance of volatiles, namely water, ammonia, and methane in the protoplanetary disk. Accurate measurement of volatiles in the Solar System is therefore important to understand not only the nebular hypothesis and origin of life but also planetary cosmogony as a whole. In this work, we propose a new, remote sensing technique to infer the outer planets water content by measuring Tremendously and Extremely Low Frequency (TLF-ELF) electromagnetic wave characteristics (Schumann resonances) excited by lightning in their gaseous envelopes. Schumann resonance detection can be potentially used for constraining the uncertainty of volatiles of the giant planets, mainly Uranus and Neptune, because such TLF-ELF wave signatures are closely related to the electric conductivity profile and water content
A simple model for the evolution of disc galaxies: The Milky Way
A simple model for the evolution of disc galaxies is presented. We adopt
three numbers from observations of the Milky Way disc, the local surface mass
density, the stellar scale length (of the assumedly exponential disc) and the
amplitude of the (assumedly flat) rotation curve, and physically, the (local)
dynamical Kennicutt star formation prescription, standard chemical evolution
equations assuming and a model for spectral evolution of stellar populations.
We can determine the detailed evolution of the model with only the addition of
standard cosmological scalings with time of the dimensional parameters. A
surprising wealth of detailed specifications follows from this prescription
including the gaseous infall rate as a function of radius and time, the
distribution of stellar ages and metallicities with time and radius, surface
brightness profiles at different wavelengths, colours etc. At the solar
neighbourhood stars start to form ago at an increasing rate
peaking 4 billion years ago and then slowly declining in good agreement with
observations. The mean age of long lived stars at the solar neighbourhood is
about . The local surface density of the stars and gas are 35 and , respectively. The metallicity distribution of the stars at
the solar radius is narrow with a peak at .Both a
Salpeter IMF and a Chabrier IMF are consistent with observations. Comparisons
with the current and local fossil evidence provides support for the model which
can then be used to assess other local disc galaxies, the evolution of disc
galaxies in deep optical surveys and also for theoretical investigations such
as simulations of merging disc galaxies (abbreviated).Comment: acceppted for publication in MNRA
The truncation of stellar discs: the magnetic hypothesis
We propose a hypothesis of the truncation of stellar discs based on the
magnetic model of the rotation curve of spiral galaxies. Once the disc had
formed and acquired its present structure, approximately, three balanced forces
were acting on the initial gas: gravity and magnetic forces, inwards, and
centrifugal force. When stars are formed from this gas, the magnetic force is
suddenly suppressed. Gravitation alone cannot retain the newly-formed stars and
at birth places beyond a certain galactocentric radius they escape to
intergalactic space. This radius is the so-called ``truncation radius'', which
is predicted to be at about 4-5 disc radial scale lengths, in promising
agreement with observations.Comment: 6 pages, accepted for publication in Astronomy and Astrophysics
Replaced using new A&A styl
Temporally-Controlled Site-Specific Recombination in Zebrafish
Conventional use of the site-specific recombinase Cre is a powerful technology in mouse, but almost absent in other vertebrate model organisms. In zebrafish, Cre-mediated recombination efficiency was previously very low. Here we show that using transposon-mediated transgenesis, Cre is in fact highly efficient in this organism. Furthermore, temporal control of recombination can be achieved by using the ligand-inducible CreERT2. Site-specific recombination only occurs upon administration of the drug tamoxifen (TAM) or its active metabolite, 4-hydroxy-tamoxifen (4-OHT). Cre-mediated recombination is detectable already 4 or 2 hours after administration of TAM or 4-OHT, demonstrating fast recombination kinetics. In addition, low doses of TAM allow mosaic labeling of single cells. Combined, our results show that conditional Cre/lox will be a valuable tool for both, embryonic and adult zebrafish studies. Furthermore, single copy insertion transgenesis of Cre/lox constructs suggest a strategy suitable also for other organisms
K0s K0s Final State in Two-Photon Collisions and Implications for Glueballs
The K0s K0s final state in two-photon collisions is studied with the L3
detector at LEP. The mass spectrum is dominated by the formation of the
f_2'(1525) tensor meson in the helicity-two state with a two-photon width times
the branching ratio into K Kbar of 76 +- 6 +- 11 eV. A clear signal for the
formation of the f_J(1710) is observed and it is found to be dominated by the
spin-two helicity-two state. No resonance is observed in the mass region around
2.2 GeV and an upper limit of 1.4 eV at 95% C.L. is derived for the two-photon
width times the branching ratio into K0s K0s for the glueball candidate
xi(2230)
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