How can double-barred galaxies be long-lived?

Double-barred galaxies account for almost one third of all barred galaxies, suggesting that secondary stellar bars, which are embedded in large-scale primary bars, are long-lived structures. However, up to now it has been hard to self-consistently simulate a disc galaxy that sustains two nested stellar bars for longer than a few rotation periods. N-body/hydrodynamical simulations including star formation recipes have been performed. Their properties have been compared with the most recent observational data in order to prove that they are representative of double-barred galaxies, even SB0. Overlaps in dynamical resonances and bar modes have been looked for using Fourier spectrograms. Double-barred galaxies have been successfully simulated with lifetimes as long as 7 Gyr. The stellar population of the secondary bar is younger on average than for the primary large-scale bar. An important feature of these simulations is the absence of any resonance overlap for several Gyr. In particular, there is no overlap between the primary bar ILR and the secondary bar corotation. Therefore, mode coupling cannot sustain the secondary bar mode. Star formation is identified here as possibly being responsible for bringing energy to the nuclear mode. Star formation is also responsible for limiting the amount of gas in the central region which prevents the orbits sustaining the secondary bar from being destroyed. Therefore, the secondary bar can dissolve but reappear after approx. 1 Gyr. When star formation is switched off the dynamical perturbation associated with the secondary bar needs several Gyr to fully vanish. Double-bars can be long-lived in numerical simulations with a gaseous component, even in the absence of overlap of resonances or mode coupling, provided that star formation remains active in the central region where the nuclear bar lies.Comment: 14 pages, 14 low resolution figures ; high resolution paper available at http://herve.wozniak.fr/DB.pdf (27 Mb

Consistent bilateral assignment

In the bilateral assignment problem, source a holds the amount ra of resource of type a, while sink i must receive the total amount xi of the various resources. We look for assignment rules meeting the powerful separability property known as Consistency: “every subassignment of a fair assignment is fair”. They are essentially those rules selecting the feasible flow minimizing the sum ∑i,aW(yia), where W is smooth and strictly convex

Attractions to radiation-like eras in superstring cosmologies

We review the cosmology induced by finite temperature and quantum effects on non-supersymmetric string models. We show the evolution is attracted to radiation-like solutions after the Hagedorn era and before the electroweak phase transition. This mechanism generates a hierarchy between the Planck mass and the supersymmetry breaking scale. A dynamical change of space-time dimension can take place.Comment: 8 page

A new criterion for the existence of KdV solitons in ferromagnets

The long-time evolution of the KdV-type solitons propagating in ferromagnetic materials is considered trough a multi-time formalism, it is governed by all equations of the KdV Hierarchy. The scaling coefficients of the higher order time variables are explicitly computed in terms of the physical parameters, showing that the KdV asymptotic is valid only when the angle between the propagation direction and the external magnetic field is large enough. The one-soliton solution of the KdV hierarchy is written down in terms of the physical parameters. A maximum value of the soliton parameter is determined, above which the perturbative approach is not valid. Below this value, the KdV soliton conserves its properties during an infinite propagation time

Is more investment needed in Solar & Biogas Energy Sources in Rwanda?

The Rwanda Third National Communication Under the United Nations Framework Convention on Climate Change mentions that Green House Gas (GHG) emissions per capita increased from 532.39kg (2006) to 676.23kg (2015) with an annual increase of 2.46%. As of 2015, the dominant emissions are from agriculture (70.4%) followed by energy (20.11%). Urea application in agriculture have increased from 1,246,400 to 2,559,000 tons of CO2 eq. Charcoal or wood being the primary source for cooking; emissions from domestic energy use has increased from 626,800 to 741,400 tons of CO2 eq. If such trends continue severe health issues, deforestation, soil erosion and droughts will be the heavy price Rwanda will have to pay. However, an investment in solar and biogas energy sources can be a solution in mitigating the afore-mentioned problems. In the developing world, Texas, USA is ranked by the Solar Energy Industries Association (SEIA) as the seventh in USA for cumulative solar capacity as of 2017. SEIA reports that Texas has 532 solar companies including 100 manufacturers. Additionally, around 210,000 Texas homes use solar power due to improved business models, distribution channels, service provider networks and low costs. Moreover, customers of solar systems can claim a 30% tax credit due to a federal government investment tax. The cost of solar is estimated at an average of $0.12/Kwh or 110RWF/Kwh while costs range from 89 – 189 RWF/Kwh (for residential customers) and from 189 – 192 RWF/Kwh (for non-residential customers) in Rwanda. According to Clean Energy Wire, Germany has 8,900 biogas power stations and 183 biomethane plants as of 2015. The biomethane plants feed pure methane into natural gas network to be used as a cooking fuel. The biogas technology also provides a liquid fertilizer which can directly be applied to agricultural fields or home gardens. Biogas power stations and biomethane plants have helped Germany avoid 156.1 million tons of CO2 eq. in 2015. However, Germany deploys heavy investments (e.g.: 15 billion euros in 2015) in renewable energy systems including biogas power stations and biomethane plants. In Rwanda, the National Domestic Biogas Programme (NDBP) has made tremendous efforts in disseminating biogas plants to interested citizens since 2009. To this day, some biogas plants are still operational and others are not due to construction flaws, lack of feedstocks and skilled labor to operate the biogas plants. The failure of some biogas systems doesn’t imply that the biogas technology is not feasible in Rwanda but better policies and better technologies are needed to improve this sector. According to Amy Yee, New York Times Journalist, the cost of biogas plants ($450 to $900) are significantly high for poor families in Rwanda with an average income of$70/month. However, the government of Rwanda is providing great incentives for all parties investing in renewable energies. As of 2019, the government has exempted import taxes of 23% and 18% value added tax paid by customers for all renewable energies. Therefore, it is safe to say that investments in renewable energies such as biogas and solar is worth it. Solar and biogas energy resources have the potential of creating jobs and providing a means of fighting deforestation, soil erosion, droughts and severe health issues. As Rwanda strives to be one of the most pristine places in the worldwide, an investment in solar and biogas is worthwhile

Minimizing the Worst Slowdown: Off-Line and On-Line

Minimizing the slowdown (expected sojourn time divided by job size) is a key concern of fairness in scheduling and queuing problems where job sizes are very heterogeneous. We look for protocols (service disciplines) capping the worst slowdown (called here liability) a job may face no matter how large (or small) the other jobs are. In the scheduling problem (all jobs released at the same time), allowing the server to randomize the order of service cuts almost in half the liability profiles feasible under deterministic protocols. The same statement holds if cash transfers are feasible and users have linear waiting costs.