scholarly journals Theory Of Spiral Structure

1983 ◽  
Vol 100 ◽  
pp. 109-116
Author(s):  
Agris J. Kalnajs
Keyword(s):  
Time Scales ◽  
Spiral Structure ◽  
The Galaxy ◽  
Over Time

Spiral structure is associated with a slow redistribution of gas, which may already be quite significant over time scales short compared to the age of the galaxy. One has to worry about replacing the gas in order to keep the structure alive.

Density waves in disk galaxies

1967 ◽  
Vol 31 ◽  
pp. 313-317 ◽  
Author(s):  
C. C. Lin ◽  
F. H. Shu
Keyword(s):  
Mathematical Analysis ◽  
Spiral Structure ◽  
Stellar System ◽  
Collective Modes ◽  
Disk Galaxies ◽  
Spiral Pattern ◽  
Density Waves ◽  
The Galaxy ◽  
Detailed Mathematical Analysis

Density waves in the nature of those proposed by B. Lindblad are described by detailed mathematical analysis of collective modes in a disk-like stellar system. The treatment is centered around a hypothesis of quasi-stationary spiral structure. We examine (a) the mechanism for the maintenance of this spiral pattern, and (b) its consequences on the observable features of the galaxy.

scholarly journals Variability of L Dwarfs in the Near Infrared

2003 ◽  
Vol 211 ◽  
pp. 455-456 ◽  
Author(s):  
José A. Caballero ◽  
Víctor J. S. Béjar ◽  
Rafael Rebolo
Keyword(s):  
Time Scales ◽  
Near Infrared ◽  
Brown Dwarfs ◽  
L Dwarfs ◽  
Atmospheric Changes ◽  
Changes Over Time ◽  
Over Time

We have obtained series of images in the near infrared J and Ks bands for seven L-type dwarfs with a duration of 3 to 6 hours. We present results on: 1) the amplitude of variability associated with atmospheric changes over time scales from minutes to several hours; 2) the search for cool companions in wide orbits; 3) the search for transits of brown dwarfs and planetary companions in very close orbits.

scholarly journals On the maintenance of spiral structure

1979 ◽  
Vol 84 ◽  
pp. 151-153
Author(s):  
James W-K. Mark ◽  
Linda Sugiyama ◽  
Robert H. Berman ◽  
Giuseppe Bertin
Keyword(s):  
Spiral Structure ◽  
Travelling Waves ◽  
Integral Condition ◽  
Stimulated Emission ◽  
Stellar Disk ◽  
Wave System ◽  
Wave Amplification ◽  
Corotation Radius ◽  
The Galaxy ◽  
Central Regions

A concentrated nuclear bulge with about 30% of the galaxy mass is sufficient (Lin, 1975; Berman and Mark, 1978) to eliminate strong bar-forming instabilities which dominate the dynamics of the stellar disk. Weak bar-like or oval distortions might remain depending on the model. In such systems self-excited discrete modes give rise to global spiral patterns which are maintained in the presence of differential rotation and dissipation (cf. especially the spiral patterns in Bertin et al., 1977, 1978). These spiral modes are standing waves that are physically analyzable (Mark, 1977) into a superposition of two travelling waves propagating in opposite directions back and forth between galactic central regions and corotation (a resonator). Only a few discrete pattern frequencies are allowed. An interpretation is that the central regions and corotation radius must be sufficiently far apart so that a Bohr-Sommerfeld type of phase-integral condition is satisfied for the wave system of each mode. The temporal growth of these modes is mostly due to an effect of Wave Amplification by Stimulated Emission (of Rotating Spirals, abbrev. WASERS, cf. Mark 1976) which occurs in the vicinity of corotation. In some galaxies one mode might be predominent while other galaxies could exhibit more complicated spiral structure because several modes are present. Weak barlike or oval distortions hardly interfere with the structure of these modes. But they might nevertheless contribute partially towards strengthening the growth of one mode relative to another, as well as affecting the kinematics of the gaseous component.

scholarly journals Evidence for azimuthal variations of the oxygen-abundance gradient tracing the spiral structure of the galaxy HCG 91c

2017 ◽  
Vol 601 ◽  
pp. A61 ◽  
Author(s):  
F. P. A. Vogt ◽  
E. Pérez ◽  
M. A. Dopita ◽  
L. Verdes-Montenegro ◽  
S. Borthakur
Keyword(s):  
Spiral Structure ◽  
Oxygen Abundance ◽  
Abundance Gradient ◽  
The Galaxy

scholarly journals Cross-Correlations in Meat Prices in Brazil: A Non-Linear Approach Using Different Time Scales

Economies ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 133
Author(s):  
Derick Quintino ◽  
José Telo da Gama ◽  
Paulo Ferreira
Keyword(s):  
Time Scales ◽  
Cross Correlation ◽  
Positive Association ◽  
Recent Period ◽  
Linear Approach ◽  
Long Run ◽  
Short Run ◽  
Cross Correlations ◽  
Over Time ◽  
Different Time Scales

Brazil is one of the world’s largest producers and exporters of cattle, chicken and swine. Therefore, co-movements of Brazilian meat prices are important for both domestic and foreign stakeholders. We propose to analyse the cross-correlation between meat prices in Brazil, namely, cattle, swine and chicken, including also in the analysis information from some commodities, namely maize, soya beans, oil, and the Brazilian exchange rate. Our sample covers the recent period which coincided with extensive macroeconomic and institutional changes in Brazil, from 2011 to 2020, and is divided in two periods: (i) presidential pre-impeachment (P1), occurring in August 2016, and; (ii) post-impeachment (P2). Our results indicate that in P1, only the prices of swine and chicken showed a positive and strong correlation over time, and that cattle showed some positive correlation with chicken only in the short run. In P2, there was also a positive and consistent correlation between swine and chicken, and only a positive association with swine and cattle in the long run. For more spaced time scales (days), the changes in the degree of correlation were significant only in the long run for swine and cattle.

scholarly journals The predictability of a lake phytoplankton community, over time-scales of hours to years

2018 ◽  
Vol 21 (5) ◽  
pp. 619-628 ◽  
Author(s):  
Mridul K. Thomas ◽  
Simone Fontana ◽  
Marta Reyes ◽  
Michael Kehoe ◽  
Francesco Pomati
Keyword(s):  
Time Scales ◽  
Phytoplankton Community ◽  
Over Time

Epilogue: Paradigms, Problems, and Predictions

2017 ◽  
Author(s):  
John Chambers ◽  
Jacqueline Mitton
Keyword(s):  
Solar System ◽  
Planetary Systems ◽  
Planetary Science ◽  
Good Time ◽  
Science Program ◽  
The Galaxy ◽  
Recent Developments ◽  
Powerful Stimulus ◽  
Rapid Pace ◽  
Over Time

This concluding chapter talks about how astronomers and space agencies in dozens of countries are helping to see the solar system as never before, transforming points of light into real worlds, and even bringing samples of those worlds back to Earth. At the same time, the stunning discovery of hundreds of other planetary systems in the galaxy has provided a powerful stimulus to understand how planetary systems form and evolve, and to find out what makes one system different from another. Moreover, in 2010, NASA announced its latest science plan. One of the key goals for NASA's future planetary science program is to learn how the Sun's family began and how it has changed over time. The chapter argues that the rapid pace of recent developments makes now a good time to take stock of what scholars know, even though the story is still incomplete.

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