1,652 research outputs found
Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. These lines serve as powerful probes for examining
the structure of inner regions of accretion disks. Previous studies of line
emission have considered geometrically thin disks only, where the gas moves
along geodesics in the equatorial plane of a black hole. Here we extend this
work to consider effects on line profiles from finite disk thickness, radial
accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution,
and find that within this framework, turbulent broadening is the dominant new
effect. The most prominent change in the skewed, double-horned line profiles is
a substantial reduction in the maximum flux at both red and blue peaks. The
effect is most pronounced when the inclination angle is large, and when the
accretion rate is high. Thus, the effects discussed here may be important for
future detailed modeling of high quality observational data.Comment: 21 pages including 8 figures; LaTeX; ApJ format; accepted by ApJ;
short results of this paper appeared before as a conference proceedings
(astro-ph/9711214
Kinematics of the swimming of Spiroplasma
\emph{Spiroplasma} swimming is studied with a simple model based on
resistive-force theory. Specifically, we consider a bacterium shaped in the
form of a helix that propagates traveling-wave distortions which flip the
handedness of the helical cell body. We treat cell length, pitch angle, kink
velocity, and distance between kinks as parameters and calculate the swimming
velocity that arises due to the distortions. We find that, for a fixed pitch
angle, scaling collapses the swimming velocity (and the swimming efficiency) to
a universal curve that depends only on the ratio of the distance between kinks
to the cell length. Simultaneously optimizing the swimming efficiency with
respect to inter-kink length and pitch angle, we find that the optimal pitch
angle is 35.5 and the optimal inter-kink length ratio is 0.338, values
in good agreement with experimental observations.Comment: 4 pages, 5 figure
Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center
We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars
traveling with such extreme velocities that dynamical ejection from a massive
black hole (MBH) is their only suggested origin. We also detect a population of
possibly bound HVSs. The significant asymmetry we observe in the velocity
distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <
-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main
sequence stars. Any population of hypervelocity post-main sequence stars should
contain stars falling back onto the Galaxy, contrary to the observations. The
spatial distribution of HVSs also supports the main sequence interpretation:
longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are
missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -
4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection
mechanisms and potential models. Depending on the mass function of HVSs, we
predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs
favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is
also allowed.Comment: 10 pages, 8 figures, accepted to ApJ, minor revision
In Situ Transfection by Controlled Release of Lipoplexes Using Acoustic Droplet Vaporization
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/133565/1/adhm201600008_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133565/2/adhm201600008.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133565/3/adhm201600008-sup-0001-S1.pd
Understanding Human and Ecosystem Dynamics in the Kola Arctic : A Participatory Integrated Study
The Lake Imandra watershed is located in one of the most developed regions in the Arctic - the Kola Peninsula of Russia. Approximately 300 000 people live on the roughly 27 000 km² watershed, making it one of the most densely populated areas of the Arctic. Most of the people are involved in large-scale mineral extraction and processing and the infrastructure needed to support this industry. This paper reports the results of a pilot project staged for the Lake Imandra watershed that has put human dynamics within the framework of ecosystem change to integrate available information and formulate conceptual models of likely future scenarios. The observation period is one of both rapid economic growth and human expansion, with an overall economic decline in the past decade. We are applying the Participatory Integrated Assessment (PIA) approach to integrate information, identify information gaps, generate likely future scenarios, and link scientific findings to the decision-making process. We found an increasingly vulnerable human population in varying states of awareness about their local environment and fully cognizant of their economic troubles, with many determined to attempt maintenance of relatively high population densities in the near future even as many residents of northern Russia migrate south. A series of workshops have involved the citizens and local decision makers in an attempt to tap their knowledge of the region and to increase their awareness about the linkages between the socioeconomic and ecological components.Le bassin hydrographique du lac Imandra est situé dans l'une des régions les plus développées de l'Arctique, soit la presqu'île de Kola, en Russie. Près de 300 000 personnes vivent dans la zone du bassin qui couvre environ 27 000 km², ce qui en fait l'une des régions les plus peuplées de l'Arctique. La plupart des habitants travaillent dans l'extraction et le traitement miniers à grande échelle ainsi que dans l'infrastructure qui soutient cette industrie. Le présent article rapporte les résultats d'un projet pilote mis sur pied pour le bassin du lac Imandra, projet qui a placé la dynamique humaine dans le cadre du changement des écosystèmes, afin d'intégrer l'information disponible et de formuler des modèles conceptuels de scénarios probables dans l'avenir. La période d'observation en est une à la fois de croissance économique et d'expansion démographique rapides, suivie d'un déclin général au cours de la dernière décennie. On a recours à la méthode d'évaluation participative intégrée (EPI) pour intégrer l'information, y dégager des lacunes, générer des scénarios probables dans l'avenir et établir un lien entre résultats de la recherche et processus décisionnel. On a trouvé qu'il y avait une population humaine de plus en plus vulnérable qui était sensibilisée à divers degrés aux problèmes locaux de l'environnement et pleinement consciente des difficultés économiques, population dont une bonne part était fermement décidée à tenter de maintenir à brève échéance des densités de population relativement élevées, alors même que les résidents du nord de la Russie migrent en grand nombre vers le Sud. On a tenu une série d'ateliers avec citoyens et décideurs locaux pour chercher à capter leurs connaissances de la région et à accroître leur sensibilisation aux liens existant entre les composantes socio-économiques et écologiques
Convective suppression before and during the United States Northern Great Plains flash drought of 2017
Flash droughts tend to be disproportionately destructive because they
intensify rapidly and are difficult to prepare for. We demonstrate that the
2017 US Northern Great Plains (NGP) flash drought was preceded by a
breakdown of land–atmosphere coupling. Severe drought conditions in the NGP
were first identified by drought monitors in late May 2017 and rapidly
progressed to exceptional drought in July. The likelihood of convective
precipitation in May 2017 in northeastern Montana, however, resembled that of
a typical August when rain is unlikely. Based on the lower tropospheric
humidity index (HIlow), convective rain was suppressed by the
atmosphere on nearly 50 % of days during March in NE Montana and central
North Dakota, compared to 30 % during a normal year. Micrometeorological
variables, including potential evapotranspiration (ETp), were neither anomalously
high nor low before the onset of drought. Incorporating convective likelihood
to drought forecasts would have noted that convective precipitation in the
NGP was anomalously unlikely during the early growing season of 2017. It may
therefore be useful to do so in regions that rely on convective
precipitation.</p
The relativistic shift of narrow spectral features from black-hole accretion discs
Transient spectral features have been discovered in the X-ray spectra of
Active Galactic Nuclei, mostly in the 5--7 keV energy range. Several
interpretations were proposed for the origin of these features. We examined a
model of Doppler boosted blue horns of the iron line originating from a spot in
a black hole accretion disc, taking into account different approximations of
general relativistic light rays and the resulting shift of energy of photons.
We provide a practical formula for the blue horn energy of an intrinsically
narrow line and assess its accuracy by comparing the approximation against an
exact value, predicted under the assumption of a planar accretion disc. The
most accurate approximation provides excellent agreement with the spot orbital
radius down to the marginally stable orbit of a non-rotating black hole.Comment: Accepted for publication in A&A; 8 pages, 5 figure
Chaotic Scattering in Heavy--Ion Reactions
We discuss the relevance of chaotic scattering in heavy--ion reactions at
energies around the Coulomb barrier. A model in two and three dimensions which
takes into account rotational degrees of freedom is discussed both classically
and quantum-mechanically. The typical chaotic features found in this
description of heavy-ion collisions are connected with the anomalous behaviour
of several experimental data.Comment: 35 pages in RevTex (version 3.0) plus 27 PostScript figures
obtainable by anonymous ftp from VAXFCT.CT.INFN.IT in directory kaos. Fig. 1
upon request to the authors. To be published in the October Focus issue on
chaotic scattering of CHAO
Anomalous Lattice Vibrations of Single and Few-Layer MoS2
Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated
on SiO2/Si substrate and characterized by Raman spectroscopy. The number of
S-Mo-S layers of the samples was independently determined by contact-mode
atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive
thickness dependence, with the frequency of the former decreasing and that of
the latter increasing with thickness. The results provide a convenient and
reliable means for determining layer thickness with atomic-level precision. The
opposite direction of the frequency shifts, which cannot be explained solely by
van der Waals interlayer coupling, is attributed to Coulombic interactions and
possible stacking-induced changes of the intralayer bonding. This work
exemplifies the evolution of structural parameters in layered materials in
changing from the 3-dimensional to the 2-dimensional regime.Comment: 14 pages, 4 figure
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