8,898 research outputs found
Double layers on auroral field lines
Time-stationary solutions to the Vlasov-Poisson equation for ion holes and double layers were examined along with particle simulations which pertain to recent observations of small amplitude (e phi)/t sub e approx. 1 electric field structures on auroral field lines. Both the time-stationary analysis and the simulations suggest that double layers evolve from holes in ion phase space when their amplitude reaches (e phi)/t sub e approx. 1. Multiple small amplitude double layers which are seen in long simulation systems and are seen to propagate past spacecraft may account for the acceleration of plasma sheet electrons to produce the discrete aurora
The nature of the red disk-like galaxies at high redshift: dust attenuation and intrinsically red stellar populations
We investigate which conditions of dust attenuation and stellar populations
allow models of dusty, continuously star-forming, bulge-less disk galaxies at
0.8<z<3.2 to meet the different colour selection criteria of high-z ``red''
galaxies (e.g. Rc-K>5.3, Ic-K>4, J-K>2.3). As a main novelty, we use stellar
population models that include the thermally pulsating Asymptotic Giant Branch
(TP-AGB) phase of stellar evolution. The star formation rate of the models
declines exponentially as a function of time, the e-folding time being longer
than 3 Gyr. In addition, we use calculations of radiative transfer of the
stellar and scattered radiation through different dusty interstellar media in
order to explore the wide parameter space of dust attenuation. We find that
synthetic disks can exhibit red optical/near-infrared colours because of
reddening by dust, but only if they have been forming stars for at least about
1 Gyr. Extremely few models barely exhibit Rc-K>5.3, if the inclination i=90
deg and if the opacity 2*tauV>6. Hence, Rc-K-selected galaxies at 1<z<2 most
probably are either systems with an old, passively evolving bulge or
starbursts. Synthetic disks at 1<z<2 exhibit 4<Ic-K<4.8, if they are seen edge
on (i.e. at i about 90 deg) and if 2*tauV>0.5. This explains the large fraction
of observed, edge-on disk-like galaxies with Ks4. Finally,
models with 2<z<3.2 exhibit 2.3<J-K<3, with no bias towards i about 90 deg and
for a large range in opacity (e.g. 2*tauV>1 for i about 70 deg). In conclusion,
red disk-like galaxies at 0.8<z<3.2 may not necessarily be dustier than nearby
disk galaxies (with 0.5<2*tauV<2) and/or much older than about 1 Gyr. This
result is due both to a realistic description of dust attenuation and to the
emission contribution by TP-AGB stars... (Abridged)Comment: 16 pages, 8 ps figures, accepted for publication in MNRA
Peter the Great’s Intermezzo with G. W. Leibniz and G. Delisle: The Development of Geographical Knowledge in Russia
The article was submitted on 22.11.2017.During his second trip through Western Europe, Russian Tsar Peter the Great (1672‒1725) met the German philosopher Gottfried Wilhelm Leibniz (1646‒1716) shortly before the death of the latter in 1716. Peter was fascinated by Leibniz’s ideas and started bringing a new system of education and academic life to Russia. Leibniz was deeply interested in the issue of a land connection between Asia and America, and discussed it with Peter. After meeting the famous French geographer Guillaume Delisle (1675‒1726) in Paris in the same year , the tsar began thinking about the usefulness of mapping his country. His encounters with the German and French scientists inspired Peter the Great to found the Academy of Sciences in St Petersburg and introduce astronomy and geography as scientific disciplines. After he returned to his newly founded capital St Petersburg, the tsar started organising large-scale expeditions to investigate and map his empire, including Siberia and Kamchatka during the First and Second Kamchatka Expeditions.Во время своей второй поездки по Западной Европе император Петр Великий (1672—1725) встречался с немецким философом Готфридом Вильгельмом Лейбницем (1646—1716) незадолго до смерти последнего в 1716 г. Петра I вдохновили идеи Лейбница, что способствовало его желанию утвердить новую систему образования и науки в России. Лейбница интересовало установление сухопутного сообщения между Азией и Америкой, которое он обсуждал с Петром I. После встречи с Гийомом Делилем (1675—1726), которая состоялась в том же году в Париже, царь стал размышлять о создании подробной карты своей страны. Его встречи с немецким и французским учеными вдохновили Петра на учреждение Академии наук в Санкт-Петербурге и придание статуса научных дисциплин в России астрономии и географии. По возращении в свою новую столицу Санкт-Петербург царь организовал несколько масштабных экспедиций, в том числе Первую и Вторую Камчатские экспедиции, целью которых было подробное изучение и картографирование всей империи, включая Сибирь и Камчатку
Narrative and persistence
Many philosophers say that the nature of personal identity has to do with narratives: the stories we tell about ourselves. While different narrativists address different questions of personal identity, some propose narrativist accounts of personal identity over time. The paper argues that such accounts have troubling consequences about the beginning and end of our lives, lead to inconsistencies, and involve backwards causation. The problems can be solved, but only by modifying the accounts in ways that deprive them of their appeal
Infrared Photometry and Dust Absorption in Highly Inclined Spiral Galaxies
We present JHK surface photometry of 15 highly inclined, late-type (Sab-Sc)
spirals and investigate the quantitative effects of dust extinction. Using the
(J - H, H - K) two-color diagram, we compare the color changes along the minor
axis of each galaxy to the predictions from different models of radiative
transfer. Models in which scattering effects are significant and those with
more than a small fraction of the light sources located near the edge of the
dust distribution do not produce enough extinction to explain the observed
color gradients across disk absorption features. The optical depth in dust near
the plane as deduced from the color excess depends sensitively on the adopted
dust geometry, ranging from tau = 4 to 15 in the visual band. This suggests
that a realistic model of the dust distribution is required, even for infrared
photometry, to correct for dust extinction in the bulges of nearly edge-on
systems.Comment: Accepted for publication in the March 1996 AJ. LaTex source which
generates 27 pages of text and tables (no figures). Complete (text + figs)
compressed Postscript preprint is also available at
ftp://bessel.mps.ohio-state.edu/pub/terndrup/inclined.ps.Z (854 Mbyte
A Characteristic Planetary Feature in Double-Peaked, High-Magnification Microlensing Events
A significant fraction of microlensing planets have been discovered in
high-magnification events, and a significant fraction of these events exhibit a
double-peak structure at their peak. However, very wide or very close binaries
can also produce double-peaked high-magnification events, with the same gross
properties as those produced by planets. Traditionally, distinguishing between
these two interpretations has relied upon detailed modeling, which is both
time-consuming and generally does not provide insight into the observable
properties that allow discrimination between these two classes of models. We
study the morphologies of these two classes of double-peaked high-magnification
events, and identify a simple diagnostic that can be used to immediately
distinguish between perturbations caused by planetary and binary companions,
without detailed modeling. This diagnostic is based on the difference in the
shape of the intra-peak region of the light curves. The shape is smooth and
concave for binary lensing, while it tends to be either boxy or convex for
planetary lensing. In planetary lensing this intra-peak morphology is due to
the small, weak cusp of the planetary central caustic located between the two
stronger cusps. We apply this diagnostic to five observed double-peaked
high-magnification events to infer their underlying nature. A corollary of our
study is that good coverage of the intra-peak region of double-peaked
high-magnification events is likely to be important for their unique
interpretation.Comment: 6 pages, 3 figure
The Flux Ratio Method for Determining the Dust Attenuation of Starburst Galaxies
The presence of dust in starburst galaxies complicates the study of their
stellar populations as the dust's effects are similar to those associated with
changes in the galaxies' stellar age and metallicity. This degeneracy can be
overcome for starburst galaxies if UV/optical/near-infrared observations are
combined with far-infrared observations. We present the calibration of the flux
ratio method for calculating the dust attenuation at a particular wavelength,
Att(\lambda), based on the measurement of F(IR)/F(\lambda) flux ratio. Our
calibration is based on spectral energy distributions (SEDs) from the PEGASE
stellar evolutionary synthesis model and the effects of dust (absorption and
scattering) as calculated from our Monte Carlo radiative transfer model. We
tested the attenuations predicted from this method for the Balmer emission
lines of a sample starburst galaxies against those calculated using radio
observations and found good agreement. The UV attenuation curves for a handful
of starburst galaxies were calculated using the flux ratio method, and they
compare favorably with past work. The relationship between Att(\lambda) and
F(IR)/F(\lambda) is almost completely independent of the assumed dust
properties (grain type, distribution, and clumpiness). For the UV, the
relationship is also independent of the assumed stellar properties (age,
metallicity, etc) accept for the case of very old burst populations. However at
longer wavelengths, the relationship is dependent on the assumed stellar
properties.Comment: accepted by the ApJ, 18 pages, color figures, b/w version at
http://mips.as.arizona.edu/~kgordon/papers/fr_method.htm
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