scholarly journals Long-Term Brightness Changes in Cool Pulsating Variables

1993 ◽  
Vol 139 ◽  
pp. 425-427
Author(s):  
John R. Percy
Keyword(s):  
Time Scales ◽  
Large Amplitude ◽  
Variable Star ◽  
American Association ◽  
Long Term Changes ◽  
Long Time ◽  
Red Giant

AbstractSeveral types of cool pulsating variables show unexplained long-term changes in brightness, typically on time scales of 10 to 20 times the basic (pulsational) period. The visual and photoelectric programs of the American Association of Variable Star Observers (AAVSO) are well-suited for detecting and studying these changes. Some examples are given here, including yellow hypergiants, RV Tauri stars, small- and large-amplitude red giant and super giant variables. The study of pulsating variables on long time scales provides “new perspectives” on their behavior.

The Spread of Pollutants from Harbour Sludge Depots

1984 ◽  
Vol 16 (3-4) ◽  
pp. 623-633
Author(s):  
M Loxham ◽  
F Weststrate
Keyword(s):  
Time Scales ◽  
Molecular Diffusion ◽  
Near Field ◽  
Parameter Analysis ◽  
Migration Patterns ◽  
Long Time ◽  
Dilution Effects ◽  
Short Time ◽  
Harbour Sludge

It is generally agreed that both the landfill option, or the civil techniques option for the final disposal of contaminated harbour sludge involves the isolation of the sludge from the environment. For short time scales, engineered barriers such as a bentonite screen, plastic sheets, pumping strategies etc. can be used. However for long time scales the effectiveness of such measures cannot be counted upon. It is thus necessary to be able to predict the long term environmenttal spread of contaminants from a mature landfill. A model is presented that considers diffusion and adsorption in the landfill site and convection and adsorption in the underlaying aquifer. From a parameter analysis starting form practical values it is shown that the adsorption behaviour and the molecular diffusion coefficient of the sludge, are the key parameters involved in the near field. The dilution effects of the far field migration patterns are also illustrated.

scholarly journals Long-Term Amplitude and Period Variations of δ Scuti Stars: A Sign of Chaos?

1993 ◽  
Vol 134 ◽  
pp. 173-180
Author(s):  
M. Breger
Keyword(s):  
Time Scales ◽  
Chaotic Behavior ◽  
Deterministic Chaos ◽  
Observational Evidence ◽  
Mode Interaction ◽  
Mode Switching ◽  
Completely Regular ◽  
Long Term Changes ◽  
Scuti Stars

AbstractOn short time-scales of under a year, the vast majority of δ Scuti stars studied in detail show completely regular multiperiodic pulsation. Nonradial pulsation is characterized by the excitation of a large number of modes with small amplitudes. Reports of short-term irregularity or nonperiodicity in the literature need to be examined carefully, since insufficient observational data can lead to an incorrect impression of irregularity. Some interesting cases of reported irregularities are examined.A few δ Scuti stars, such as 21 Mon, have shown stable variations with sudden mode switching to a new frequency spectrum. This situation might be an indication of deterministic chaos. However, the observational evidence for mode switching is still weak.One the other hand, the case for the existence of long-term amplitude and period changes is becoming quite convincing. Recently found examples of nonradial pulsators with long-term changes are 4 CVn, 44 Tau, τ Peg and HD 2724. (We note that other δ Scuti pulsators such as X Cae and θ2 Tau, have shown no evidence for amplitude variations over the years.) Neither the amplitude nor the period changes are periodic, although irregular cycles with time scales between a few and twenty years can be seen. While the amplitude changes can be very large, the period changes are quite small. This property is common in nonlinear systems which lead to chaotic behavior. There exists observational evidence for relatively sudden period jumps changing the period by about 10−5 and/or slow period changes near dP/dt ≤ 10−9. These period changes are an order of magnitude larger than those expected from stellar evolution.The nonperiodic long-term changes are interpreted in terms of resonances between different nonradial modes. It is shown that a large number of the nonradial acoustic modes can be in resonance with other modes once the mode interaction terms, different radial orders and rotational m-mode splitting are considered. These resonances are illustrated numerically by the use of pulsation model. Observational evidence is presented that these interaction modes exist in the low-frequency domain.

Simulated Local and Remote Biophysical Effects of Afforestation over the Southeast United States in Boreal Summer*

2012 ◽  
Vol 25 (13) ◽  
pp. 4511-4522 ◽  
Author(s):  
Guang-Shan Chen ◽  
Michael Notaro ◽  
Zhengyu Liu ◽  
Yongqiang Liu
Keyword(s):  
United States ◽  
Carbon Sequestration ◽  
Time Scales ◽  
Boreal Summer ◽  
Surface Cooling ◽  
Local Cooling ◽  
Southeast United States ◽  
Long Time ◽  
Biophysical Effects

Abstract Afforestation has been proposed as a climate change mitigation strategy by sequestrating atmospheric carbon dioxide. With the goal of increasing carbon sequestration, a Congressional project has been planned to afforest about 18 million acres by 2020 in the Southeast United States (SEUS), the Great Lake states, and the Corn Belt states. However, biophysical feedbacks of afforestation have the potential to counter the beneficial climatic consequences of carbon sequestration. To assess the potential biophysical effects of afforestation over the SEUS, the authors designed a set of initial value ensemble experiments and long-term quasi-equilibrium experiments in a fully coupled Community Climate System Model, version 3.5 (CCSM3.5). Model results show that afforestation over the SEUS not only has a local cooling effect in boreal summer [June–August (JJA)] at short and long time scales but also induces remote warming over adjacent regions of the SEUS at long time scales. Precipitation, in response to afforestation, increases over the SEUS (local effect) and decreases over adjacent regions (remote effect) in JJA. The local surface cooling and increase in precipitation over SEUS in JJA are hydrologically driven by the changes in evapotranspiration and latent heat flux. The remote surface warming and decrease in precipitation over adjacent regions are adiabatically induced by anomalous subsidence. Our results suggest that the planned afforestation efforts should be developed carefully by taking account of short-term (local) and long-term (remote) biophysical effects of afforestation.

scholarly journals Infrared Light Curves of Carbon-Rich Variables

2000 ◽  
Vol 177 ◽  
pp. 179-190 ◽  
Author(s):  
Patricia Whitelock
Keyword(s):  
Large Amplitude ◽  
Light Curves ◽  
Infrared Light ◽  
Dust Shells ◽  
Long Term Changes ◽  
Long Term Trends

Long-term trends in the infrared (JHKL) light curves of various carbon variables are described. Some stars, e.g. the semi-regular variables R Scl and GM CMa, show multiple periodicities; others, particularly the Miras with moderately thick dust-shells, show more erratic long-term changes. The light curves for R For, which have been intensively monitored over 20 years, show a pattern which is reminiscent of that seen for R CrB stars. This pattern is superimposed on regular large-amplitude Mira pulsations. The multi-periodic and erratic behaviour of these stars is compared with the predictions from various models.

scholarly journals X-ray variability in LINERs

2013 ◽  
Vol 9 (S304) ◽  
pp. 399-402
Author(s):  
Josefa Masegosa ◽  
Lorena Hernández-García ◽  
Isabel Márquez ◽  
Omaira González-Martín
Keyword(s):  
Active Galactic Nuclei ◽  
Time Scales ◽  
Galactic Nuclei ◽  
Spectral Variability ◽  
X Ray ◽  
Line Region ◽  
Variability Pattern ◽  
Long Time ◽  
Public Archives

AbstractOne of the most important features in active galactic nuclei (AGN) is the variability of their emission. Variability has been discovered at X-ray, UV, and radio frequencies on time scales from hours to years. Among the AGN family and according to theoretical studies, Low-Ionization Nuclear Emission Line Region (LINER) nuclei would be variable objects on long time scales. Our purpose is to investigate spectral X-ray variability in LINERs and to understand the nature of these kinds of objects, as well as their accretion mechanism. Chandra and XMM–Newton public archives were used to compile X-ray spectra of LINER nuclei at different epochs with time scales of years. To search for variability we fit all the spectra from the same object with a set of models, in order to identify the parameters responsible for the variability pattern. We found that long term spectral variability is very common, with variations mostly related to hard energies (2-10 keV). These variations are due to changes in the soft excess, and/or changes in the absorber, and/or intrinsic variations of the source.

scholarly journals Optical variability of quasars: a damped random walk

2013 ◽  
Vol 9 (S304) ◽  
pp. 395-398 ◽  
Author(s):  
Željko Ivezić ◽  
Chelsea MacLeod
Keyword(s):  
Random Walk ◽  
Time Scales ◽  
Model Parameters ◽  
Optical Wavelength ◽  
Long Time ◽  
Asymptotic Variability ◽  
Short Time ◽  
Quasar Variability ◽  
Best Fit

AbstractA damped random walk is a stochastic process, defined by an exponential covariance matrix that behaves as a random walk for short time scales and asymptotically achieves a finite variability amplitude at long time scales. Over the last few years, it has been demonstrated, mostly but not exclusively using SDSS data, that a damped random walk model provides a satisfactory statistical description of observed quasar variability in the optical wavelength range, for rest-frame timescales from 5 days to 2000 days. The best-fit characteristic timescale and asymptotic variability amplitude scale with the luminosity, black hole mass, and rest wavelength, and appear independent of redshift. In addition to providing insights into the physics of quasar variability, the best-fit model parameters can be used to efficiently separate quasars from stars in imaging surveys with adequate long-term multi-epoch data, such as expected from LSST.

scholarly journals On the differences between two semi-empirical sea-level models for the last two millennia

2012 ◽  
Vol 8 (4) ◽  
pp. 3551-3581 ◽  
Author(s):  
M. Vermeer ◽  
S. Rahmstorf ◽  
A. Kemp ◽  
B. Horton
Keyword(s):  
North Carolina ◽  
Sea Level Rise ◽  
Sea Level ◽  
Time Scales ◽  
Long Time ◽  
Global Mean Sea Level ◽  
Semi Empirical ◽  
The Last Two Millennia ◽  
Global Mean

Abstract. We compare hindcasts of global mean sea level over the past millennium obtained using two semi-empirical models linking temperature and sea-level rise. The models differ in that one of them includes a term for a very long-term sea-level rise component unfolding over many millennia. On short (century) time scales, both models give very similar results. Proxy sea-level reconstructions from the northern (North Carolina) and southern (New Zealand and Tasmania) hemispheres are used to test the ability of both models to reproduce the longer-term sea-level evolution. In both comparisons the model including the second term produces a markedly better fit from 1000 AD to the present. When both models are used for generating sea-level projections, they behave similarly out to 2100 AD. Further out, to 2300–2500 AD, the projections differ significantly, in no small part due to different values for the sea-level response time scale τ obtained. We conclude that careful model validation on long time scales is important before attempting multi-century projections.

scholarly journals Neuropixels 2.0: A miniaturized high-density probe for stable, long-term brain recordings

Science ◽  
2021 ◽  
Vol 372 (6539) ◽  
pp. eabf4588
Author(s):  
Nicholas A. Steinmetz ◽  
Cagatay Aydin ◽  
Anna Lebedeva ◽  
Michael Okun ◽  
Marius Pachitariu ◽  
...  
Keyword(s):  
Time Scales ◽  
Small Animals ◽  
High Quality ◽  
Site Density ◽  
Long Time ◽  
Free Behavior ◽  
Post Hoc ◽  
Density Probe ◽  
Data Processing Methods

Measuring the dynamics of neural processing across time scales requires following the spiking of thousands of individual neurons over milliseconds and months. To address this need, we introduce the Neuropixels 2.0 probe together with newly designed analysis algorithms. The probe has more than 5000 sites and is miniaturized to facilitate chronic implants in small mammals and recording during unrestrained behavior. High-quality recordings over long time scales were reliably obtained in mice and rats in six laboratories. Improved site density and arrangement combined with newly created data processing methods enable automatic post hoc correction for brain movements, allowing recording from the same neurons for more than 2 months. These probes and algorithms enable stable recordings from thousands of sites during free behavior, even in small animals such as mice.

scholarly journals Synchronized flutter of two slender flags

2016 ◽  
Vol 801 ◽  
pp. 652-669 ◽  
Author(s):  
Jérôme Mougel ◽  
Olivier Doaré ◽  
Sébastien Michelin
Keyword(s):  
Large Amplitude ◽  
Axial Flow ◽  
Coupled Dynamics ◽  
Hydrodynamic Coupling ◽  
Close Proximity ◽  
Long Time ◽  
Dynamical Properties ◽  
Body Theory ◽  
Selection Of

The interactions and synchronization of two parallel and slender flags in a uniform axial flow are studied in the present paper by generalizing Lighthill’s elongated body theory (EBT) and Lighthill’s large-amplitude elongated body theory (LAEBT) to account for the hydrodynamic coupling between flags. The proposed method consists of two successive steps, namely the reconstruction of the flow created by a flapping flag within the LAEBT framework and the computation of the fluid force generated by this non-uniform flow on the second flag. In the limit of slender flags in close proximity, we show that the effect of the wakes has little influence on the long-time coupled dynamics and can be neglected in the modelling. This provides a simplified framework extending LAEBT to the coupled dynamics of two flags. Using this simplified model, both linear and large-amplitude results are reported to explore the selection of the flapping regime as well as the dynamical properties of two side-by-side slender flags. Hydrodynamic coupling of the two flags is observed to destabilize the flags for most parameters, and to induce a long-term synchronization of the flags, either in-phase or out-of-phase.

scholarly journals 3.6. AGN variability studies: an agenda for the next millenium?

1998 ◽  
Vol 184 ◽  
pp. 99-101
Author(s):  
M.-H. Ulrich
Keyword(s):  
Data Base ◽  
Accretion Disk ◽  
Time Scales ◽  
Large Amplitude ◽  
Rare Events ◽  
The Future ◽  
Optical Flux ◽  
Factor 20

Data which are obtained today can be re-acquired better and faster in the future. Their only enduring value, after they have been interpreted, is as historical markers. With electronic archiving one can envision archives covering decades, centuries and even longer periods of observations of selected objects or classes of objects. Such a data base would allow the study of phenomena with low temporal frequencies, and of rare events.First hints of such phenomena in AGN have been discovered recently: 1) The Seyfert 1s, the best studied AGN at present, exhibit flux variations on time scales ranging from hours to several years (see Ulrich et al. 1997). The faster variations (up to months) have been extensively observed, and they are best understood as being caused by magnetic flares in the corona above the central part of the accretion disk (Blandford and Payne 1982). The origin of the long term and large amplitude UV/optical flux variations (factor 20 in several years as observed in NGC4151 and F9), however, remain essentially unknown.

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