122 research outputs found
Directional tuning of human motion adaptation as reflected by the motion VEP
AbstractMotion onset evoked visual potentials are dominated by a negativity (N2) at occipital electrodes and a positivity (P2) at the vertex. The degree of true motion processing reflected by N2 and P2 was estimated from the direction specificity of motion adaptation. Adapting stimuli moved to the right and test stimuli (random dot patterns of 26° diameter; 10% contrast; 10.5°/s velocity) moved in one of eight directions, which differed by 45°. VEPs were recorded from occipito/temporal and central sites in eight subjects. Two adaptation effects were observed for N2 (P<0.01): a global amplitude reduction by 47% and a direction-specific reduction by a further 28%. For P2, only the global effect (54%; P<0.01) was observed. The global adaptation effect could also be induced by pattern reversal and pattern-onset adaptation, i.e. stimuli containing ambiguous or very little motion energy, respectively. We conclude that at least 28% of the N2 amplitude reflects the activity of direction-specific elements, whereas P2 does not at all
Quantitative atomic spectroscopy for primary thermometry
Quantitative spectroscopy has been used to measure accurately the
Doppler-broadening of atomic transitions in Rb vapor. By using a
conventional platinum resistance thermometer and the Doppler thermometry
technique, we were able to determine with a relative uncertainty of
, and with a deviation of from the
expected value. Our experiment, using an effusive vapour, departs significantly
from other Doppler-broadened thermometry (DBT) techniques, which rely on weakly
absorbing molecules in a diffusive regime. In these circumstances, very
different systematic effects such as magnetic sensitivity and optical pumping
are dominant. Using the model developed recently by Stace and Luiten, we
estimate the perturbation due to optical pumping of the measured value
was less than . The effects of optical pumping on atomic and
molecular DBT experiments is mapped over a wide range of beam size and
saturation intensity, indicating possible avenues for improvement. We also
compare the line-broadening mechanisms, windows of operation and detection
limits of some recent DBT experiments
Comments on alternative calculations of the broadening of spectral lines of neutral sodium by H-atom collisions
With the exception of the sodium D-lines recent calculations of line
broadening cross-sections for several multiplets of sodium by Leininger et al
(2000) are in substantial disagreement with cross-sections interpolated from
the tables of Anstee and O'Mara (1995) and Barklem and O'Mara (1997). The
discrepancy is as large as a factor of three for the 3p-4d multiplet. The two
theories are tested by using the results of each to synthesize lines in the
solar spectrum. It is found that generally the data from the theory of Anstee,
Barklem and O'Mara produce the best match to the observed solar spectrum. It is
found, using a simple model for reflection of the optical electron by the
potential barrier between the two atoms, that the reflection coefficient is too
large for avoided crossings with the upper states of subordinate lines to
contribute to line broadening, supporting the neglect of avoided ionic
crossings by Anstee, Barklem and O'Mara for these lines. The large
discrepancies between the two sets of calculations is a result of an
approximate treatment of avoided ionic crossings for these lines by Leininger
et al (2000).Comment: 18 pages, 5 ps figures included, to appear in J Phys B: At. Mol. Opt.
Phy
The Exotic Eclipsing Nucleus of the Ring Planetary Nebula SuWt2
SuWt2 is a planetary nebula (PN) consisting of a bright ionized thin ring
seen nearly edge-on. It has a bright (V=12) central star, too cool to ionize
the PN, which we discovered to be an eclipsing binary. A spectrum from IUE did
not reveal a UV source. We present extensive ground-based photometry and
spectroscopy of the central binary collected over the ensuing two decades,
resulting in the determination that the orbital period of the eclipsing pair is
4.9 d, and consists of two nearly identical A1 V stars, each of mass ~2.7
M_sun. The physical parameters of the A stars, combined with evolutionary
tracks, show that both are in the short-lived "blue-hook" evolutionary phase
that occurs between the main sequence and the Hertzsprung gap, and that the age
of the system is about 520 Myr. One puzzle is that the stars' rotational
velocities are different from each other, and considerably slower than
synchronous with the orbital period. It is possible that the center-of-mass
velocity of the eclipsing pair is varying with time, suggesting that there is
an unseen third orbiting body in the system. We propose a scenario in which the
system began as a hierarchical triple, consisting of a ~2.9 M_sun star orbiting
the close pair of A stars. Upon reaching the AGB stage, the primary engulfed
the pair into a common envelope, leading to a rapid contraction of the orbit
and catastrophic ejection of the envelope into the orbital plane. In this
picture, the exposed core of the initial primary is now a white dwarf of ~0.7
M_sun, orbiting the eclipsing pair, which has already cooled below the
detectability possible by IUE at our derived distance of 2.3 kpc and a
reddening of E(B-V)=0.40. The SuWt2 system may be destined to perish as a Type
Ia supernova. (Abridged)Comment: 60 pages, 11 figure, to appear in the Astronomical Journa
C, N and O abundances in red clump stars of the Milky Way
The Hipparcos orbiting observatory has revealed a large number of
helium-core-burning "clump" stars in the Galactic field. These low-mass stars
exhibit signatures of extra-mixing processes that require modeling beyond the
first dredge-up of standard models. The 12C/13C ratio is the most robust
diagnostic of deep mixing, because it is insensitive to the adopted stellar
parameters. In this work we present 12C/13C determinations in a sample of 34
Galactic clump stars as well as abundances of nitrogen, carbon and oxygen.
Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5
A. The wavelength interval 7980-8130 A with strong CN features was analysed in
order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen
abundances were determined from the [O I] line at 6300 A. Compared with the Sun
and dwarf stars of the Galactic disk, mean abundances in the investigated clump
stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by
0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to
evolutionary models show that the stars fall into two groups: the one is of
first ascent giants with carbon isotope ratios altered according to the first
dredge-up prediction, and the other one is of helium-core-burning stars with
carbon isotope ratios altered by extra mixing. The stars investigated fall to
these groups in approximately equal numbers.Comment: 8 pages 6 figures Accepted for publication in MNRA
A grid of NLTE corrections for magnesium and calcium in late-type giant and supergiant stars: application to Gaia
We investigate NLTE effects for magnesium and calcium in the atmospheres of
late-type giant and supergiant stars. The aim of this paper is to provide a
grid of NLTE/LTE equivalent width ratios W/W* of Mg and Ca lines for the
following range of stellar parameters: Teff in [3500, 5250] K, log g in [0.5,
2.0] dex and [Fe/H] in [-4.0, 0.5] dex. We use realistic model atoms with the
best physics available and taking into account the fine structure. The Mg and
Ca lines of interest are in optical and near IR ranges. A special interest
concerns the lines in the Gaia spectrograph (RVS) wavelength domain [8470,
8740] A. The NLTE corrections are provided as function of stellar parameters in
an electronic table as well as in a polynomial form for the Gaia/RVS lines.Comment: 27 pages, 19 figures and 11 tables. Also on-line data; MNRAS (2011)
000
Atmospheres from very low-mass stars to extrasolar planets
Within the next few years, several instruments aiming at imaging extrasolar
planets will see first light. In parallel, low mass planets are being searched
around red dwarfs which offer more favorable conditions, both for radial
velocity detection and transit studies, than solar-type stars. We review recent
advancements in modeling the stellar to substellar transition. The revised
solar oxygen abundances and cloud models allow to reproduce the photometric and
spectroscopic properties of this transition to a degree never achieved before,
but problems remain in the important M-L transition characteristic of the
effective temperature range of characterizable exoplanets.Comment: submitted to Memorie della Societa Astronomica Italian
CRIRES-POP: A library of high resolution spectra in the near-infrared
New instrumental capabilities and the wealth of astrophysical information
extractable from the near-infrared wavelength region have led to a growing
interest in the field of high resolution spectroscopy at 1-5 mu. We aim to
provide a library of observed high-resolution and high signal-to-noise-ratio
near-infrared spectra of stars of various types throughout the
Hertzsprung-Russell diagram. This is needed for the exploration of spectral
features in this wavelength range and for comparison of reference targets with
observations and models.
High quality spectra were obtained using the CRIRES near-infrared
spectrograph at ESO's VLT covering the range from 0.97 to 5.3 mu at high
spectral resolution. Accurate wavelength calibration and correction for of
telluric lines were performed by fitting synthetic transmission spectra for the
Earth's atmosphere to each spectrum individually. We describe the observational
strategy and the current status and content of the library which includes 13
objects. The first examples of finally reduced spectra are presented. This
publication will serve as a reference paper to introduce the library to the
community and explore the extensive amount of material.Comment: accepted for publication in A&A; see also the project webpage
http://www.univie.ac.at/crirespo
Manganese Abundances in the Globular Cluster Omega Centauri
We present manganese abundances in 10 red-giant members of the globular
cluster Omega Centauri; 8 stars are from the most metal-poor population (RGB MP
and RGB MInt1) while two targets are members of the more metal rich groups (RGB
MInt2 and MInt3). This is the first time Mn abundances have been studied in
this peculiar stellar system. The LTE values of [Mn/Fe] in Omega Cen overlap
those of Milky Way stars in the metal poor Omega Cen populations ([Fe/H] ~ -1.5
to -1.8), however unlike what is observed in Milky Way halo and disk stars,
[Mn/Fe] declines in the two more metal-rich RGB MInt2 and MInt3 targets.
Non-LTE calculations were carried out in order to derive corrections to the LTE
Mn abundances. The non-LTE results for Omega Cen in comparison with the non-LTE
[Mn/Fe] versus [Fe/H] trend obtained for the Milky Way confirm and strengthen
the conclusion that the manganese behavior in Omega Cen is distinct. These
results suggest that low-metallicity supernovae (with metallicities < -2) of
either Type II or Type Ia dominated the enrichment of the more metal-rich stars
in Omega Cen. The dominance of low-metallicity stars in the chemical evolution
of Omega Cen has been noted previously in the s-process elements where
enrichment from metal-poor AGB stars is indicated. In addition, copper, which
also has metallicity dependent yields, exhibits lower values of [Cu/Fe] in the
RGB MInt2 and MInt3 Omega Cen populations.Comment: Accepted for publication in the Ap
Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics
The atmospheric greenhouse effect, an idea that many authors trace back to
the traditional works of Fourier (1824), Tyndall (1861), and Arrhenius (1896),
and which is still supported in global climatology, essentially describes a
fictitious mechanism, in which a planetary atmosphere acts as a heat pump
driven by an environment that is radiatively interacting with but radiatively
equilibrated to the atmospheric system. According to the second law of
thermodynamics such a planetary machine can never exist. Nevertheless, in
almost all texts of global climatology and in a widespread secondary literature
it is taken for granted that such mechanism is real and stands on a firm
scientific foundation. In this paper the popular conjecture is analyzed and the
underlying physical principles are clarified. By showing that (a) there are no
common physical laws between the warming phenomenon in glass houses and the
fictitious atmospheric greenhouse effects, (b) there are no calculations to
determine an average surface temperature of a planet, (c) the frequently
mentioned difference of 33 degrees Celsius is a meaningless number calculated
wrongly, (d) the formulas of cavity radiation are used inappropriately, (e) the
assumption of a radiative balance is unphysical, (f) thermal conductivity and
friction must not be set to zero, the atmospheric greenhouse conjecture is
falsified.Comment: 115 pages, 32 figures, 13 tables (some typos corrected
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