Maxclique and unit disk characterizations of strongly chordal graphs

Maxcliques (maximal complete subgraphs) and unit disks (closed neighborhoods of vertices) sometime play almost interchangeable roles in graph theory. For instance, interchanging them makes two existing characterizations of chordal graphs into two new characterizations. More intriguingly, these characterizations of chordal graphs can be naturally strengthened to new characterizations of strongly chordal graphs.Facultad de Ciencias Exacta

The evolution of interstellar clouds in a streaming hot plasma including heat conduction

To examine the evolution of giant molecular clouds in the stream of a hot plasma we performed two-dimensional hydrodynamical simulations that take full account of self-gravity, heating and cooling effects and heat conduction by electrons. We use the thermal conductivity of a fully ionized hydrogen plasma proposed by Spitzer and a saturated heat flux according to Cowie & McKee in regions where the mean free path of the electrons is large compared to the temperature scaleheight. Significant structural and evolutionary differences occur between simulations with and without heat conduction. Dense clouds in pure dynamical models experience dynamical destruction by Kelvin-Helmholtz (KH) instability. In static models heat conduction leads to evaporation of such clouds. Heat conduction acting on clouds in a gas stream smooths out steep temperature and density gradients at the edge of the cloud because the conduction timescale is shorter than the cooling timescale. This diminishes the velocity gradient between the streaming plasma and the cloud, so that the timescale for the onset of KH instabilities increases, and the surface of the cloud becomes less susceptible to KH instabilities. The stabilisation effect of heat conduction against KH instability is more pronounced for smaller and less massive clouds. As in the static case more realistic cloud conditions allow heat conduction to transfer hot material onto the cloud's surface and to mix the accreted gas deeper into the cloud.Comment: 19 pages, 12 figures, accepted in Astronomy and Astrophysic

Onset of Fast Magnetic Reconnection in Partially Ionized Gases

We consider quasi-stationary two-dimensional magnetic reconnection in a partially ionized incompressible plasma. We find that when the plasma is weakly ionized and the collisions between the ions and the neutral particles are significant, the transition to fast collisionless reconnection due to the Hall effect in the generalized Ohm's law is expected to occur at much lower values of the Lundquist number, as compared to a fully ionized plasma case. We estimate that these conditions for fast reconnection are satisfied in molecular clouds and in protostellar disks.Comment: 19 pages, 1 figure, 1 tabl

Evaporation and condensation of spherical interstellar clouds. Self-consistent models with saturated heat conduction and cooling

Shortened version: The fate of IS clouds embedded in a hot tenuous medium depends on whether the clouds suffer from evaporation or whether material condensates onto them. Analytical solutions for the rate of evaporative mass loss from an isolated spherical cloud embedded in a hot tenuous gas are deduced by Cowie & McKee (1977). In order to test the validity of the analytical results for more realistic IS conditions the full hydrodynamical equations must be treated. Therefore, 2D numerical simulations of the evolution of IS clouds %are performed with different internal density structures and surrounded by a hot plasma reservoir. Self-gravity, interstellar heating and cooling effects and heat conduction by electrons are added. Classical thermal conductivity of a fully ionized hydrogen plasma and saturated heat flux are considered. Using pure hydrodynamics and classical heat flux we can reproduce the analytical results. Heat flux saturation reduces the evaporation rate by one order of magnitude below the analytical value. The evolution changes totally for more realistic conditions when interstellar heating and cooling effects stabilize the self-gravity. Evaporation then turns into condensation, because the additional energy by heat conduction can be transported away from the interface and radiated off efficiently from the cloud's inner parts. I.e. that the saturated heat flux consideration is inevitable for IS clouds embedded in hot tenuous gas. Various consequences are discussed in the paper.Comment: 16 pages, 24 figures, accepted in Astronomy and Astrophysic

Task Shifting and Health System Design: Report of the Expert Panel on effective ways of investing in Health (EXPH)

Imagine a health professional in any European country who fell asleep in 1960 and awoke in a health facility in 2019. Much of what the observer saw would be quite different. There would be many more patients who were surviving into old age thanks to advances in therapy. Many of the treatments that they were receiving would be much more complex, involving radically new techniques such as laparoscopic or even robotic surgery, and they would be amazed by the advances in diagnostic capacity. Yet, in many health systems, some things would have changed very little. Among them would be the traditional roles of different types of health worker, with responsibility for certain task being reserved for those with particular qualifications based on custom and practice rather than on evidence. This opinion argues that this situation must change. There is now an impressive body of evidence that things can often be done differently. This does not mean that they should be. Change is only appropriate where it helps to achieve the goals of the health system and allows it to provide better care in ways that are more responsive to the needs of users. Tasks can be shifted from health workers to patients and their carers, to machines, and to other health workers. Where these shifts have been evaluated, they often, but not always, are associated with outcomes that are as good or even better than with the status quo. However, the results are often context dependent, and it cannot be assumed that what works in one situation will apply equally to another. What matters is the evidence, rather than traditional, but often obsolete rules. If a health system can ensure that tasks are being undertaken by those most appropriate to do them, it will enhance patient care. However, change is often difficult. Those involved must be convinced of the rationale for change and must be supported in implementing it. This should recognise that any change in roles will have implication for their status and thus existing hierarchies. It may also be necessary to challenge outdated legislative or regulatory barriers. Finally, it is essential the changes are evaluated, results are documented, and lessons are learned, both in relation to what works and in what circumstances. Task shifting, where it is based on robust evidence and implemented effectively, can make a major contribution to health outcomes and to the sustainability of health systems. It is not, however, a panacea for all of the challenges health systems face.Představte si zdravotnického odborníka v kterékoli evropské zemi, který usnul v roce 1960 a probudil se ve zdravotnickém zařízení v roce 2019. Hodně z toho, co by pozorovatel viděl, by bylo úplně jiné. Bylo by mnohem více pacientů, kteří přežili do stáří díky pokrokům v terapii. Mnoho z ošetření, které dostávali, by bylo mnohem složitější, zahrnovalo radikálně nové techniky, jako je laparoskopická nebo dokonce robotická chirurgie, a byl by ohromen pokrokem v diagnostickém rozsahu. Přesto by se v mnoha zdravotnických systémech některé věci změnily jen velmi málo. Mezi nimi by byly tradiční role různých typů zdravotnických pracovníků, přičemž odpovědnost za určitý úkol by byla vyhrazena těm, kteří mají zvláštní kvalifikaci založenou spíše na zvyklostech a praxi než na důkazech. Toto stanovisko tvrdí, že se tato situace musí změnit. Nyní existuje impozantní soubor důkazů, že věci lze často dělat jinak. To neznamená, že by měli být jinak dělány. Změna je vhodná pouze tam, kde pomáhá dosahovat cílů zdravotnického systému a umožňuje jí poskytovat lepší péči způsoby, které lépe reagují na potřeby uživatelů. Úkoly lze převádět ze zdravotnických pracovníků na pacienty a jejich pečovatele, na stroje a další zdravotnické pracovníky. Tam, kde byly tyto posuny vyhodnoceny, jsou často, ale ne vždy, spojeny s výsledky, které jsou stejně dobré nebo dokonce lepší než za současného stavu. Výsledky jsou však často závislé na kontextu a nelze předpokládat, že to, co funguje v jedné situaci, se bude vztahovat stejně na jiné. Důležitý je důkaz spíše než tradiční, ale často zastaralá pravidla. Pokud zdravotní systém může zajistit, že osoby, které jsou pro ně nejvhodnější, plní správně alokované úkoly, zlepší se péče o pacienty. Změna je však často obtížná. Zúčastněné strany musí být přesvědčeny o důvodech změny a musí být podporovány při jejich provádění. Musí uznat, že každá změna rolí bude mít dopad na jejich stav, a tedy na existující hierarchie. Může být také třeba změnit zastaralé legislativní nebo regulační překážky. Nakonec je nezbytné, aby byly změny vyhodnoceny, výsledky zdokumentovány a vedly k ponaučení, co funguje a za jakých okolností. Posun úkolů, činností a kompetencí, pokud je založen na spolehlivých důkazech a je účinně prováděn, může významně přispět k lepším výsledkům v oblasti zdraví a k udržitelnosti zdravotních systémů. Nejedná se však o všelék na všechny výzvy, kterým zdravotnické systémy čelí.Imagine a health professional in any European country who fell asleep in 1960 and awoke in a health facility in 2019. Much of what the observer saw would be quite different. There would be many more patients who were surviving into old age thanks to advances in therapy. Many of the treatments that they were receiving would be much more complex, involving radically new techniques such as laparoscopic or even robotic surgery, and they would be amazed by the advances in diagnostic capacity. Yet, in many health systems, some things would have changed very little. Among them would be the traditional roles of different types of health worker, with responsibility for certain task being reserved for those with particular qualifications based on custom and practice rather than on evidence. This opinion argues that this situation must change. There is now an impressive body of evidence that things can often be done differently. This does not mean that they should be. Change is only appropriate where it helps to achieve the goals of the health system and allows it to provide better care in ways that are more responsive to the needs of users. Tasks can be shifted from health workers to patients and their carers, to machines, and to other health workers. Where these shifts have been evaluated, they often, but not always, are associated with outcomes that are as good or even better than with the status quo. However, the results are often context dependent, and it cannot be assumed that what works in one situation will apply equally to another. What matters is the evidence, rather than traditional, but often obsolete rules. If a health system can ensure that tasks are being undertaken by those most appropriate to do them, it will enhance patient care. However, change is often difficult. Those involved must be convinced of the rationale for change and must be supported in implementing it. This should recognise that any change in roles will have implication for their status and thus existing hierarchies. It may also be necessary to challenge outdated legislative or regulatory barriers. Finally, it is essential the changes are evaluated, results are documented, and lessons are learned, both in relation to what works and in what circumstances. Task shifting, where it is based on robust evidence and implemented effectively, can make a major contribution to health outcomes and to the sustainability of health systems. It is not, however, a panacea for all of the challenges health systems face

An Initial Mass Function for Individual Stars in Galactic Disks: I. Constraining the Shape of the IMF

We derive a semi-empirical galactic initial mass function (IMF) from observational constraints. We assume that the star formation rate in a galaxy can be expressed as the product of the IMF, $\psi (m)$, which is a smooth function of mass $m$ (in units of \msun), and a time- and space-dependent total rate of star formation per unit area of galactic disk. The mass dependence of the proposed IMF is determined by five parameters: the low-mass slope $\gamma$, the high-mass slope $-\Gamma$, the characteristic mass $m_{ch}$ (which is close to the mass $m_{\rm peak}$ at which the IMF turns over), and the lower and upper limits on the mass, $m_l$ (taken to be 0.004) and $m_u$ (taken to be 120). The star formation rate in terms of number of stars per unit area of galactic disk per unit logarithmic mass interval, is proportional to $m^{-\Gamma} \left\{1-\exp\left[{-(m/m_{ch})^{\gamma +\Gamma}}\right]\right\}$, where $\cal N_*$ is the number of stars, $m_l<m<m_u$ is the range of stellar masses. The values of $\gamma$ and \emch are derived from two integral constraints: i) the ratio of the number density of stars in the range $m=0.1-0.6$ to that in the range $m=0.6-0.8$ as inferred from the mass distribution of field stars in the local neighborhood, and ii) the ratio of the number of stars in the range $m=0.08 - 1$ to the number of brown dwarfs in the range $m=0.03-0.08$ in young clusters. The IMF satisfying the above constraints is characterized by the parameters $\gamma=0.51$ and \emch=0.35 (which corresponds to $m_{\rm peak}=0.27$). This IMF agrees quite well with the Chabrier (2005) IMF for the entire mass range over which we have compared with data, but predicts significantly more stars with masses $< 0.03\, M_\odot$; we also compare with other IMFs in current use.Comment: 46 pages, 8 figures, submitted to Ap

Coronal winds powered by radiative driving

A two-component phenomenological model developed originally for zeta Puppis is revised in order to model the outflows of late-type O dwarfs that exhibit the weak-wind phenomenon. With the theory's standard parameters for a generic weak-wind star, the ambient gas is heated to coronal temperatures ~ 3 x 10^{6}K at radii > 1.4 R, with cool radiativly-driven gas being then confined to dense clumps with filling factor ~ 0.02. Radiative driving ceases at radius ~ 2.1R when the clumps are finally destroyed by heat conduction from the coronal gas. Thereafter, the outflow is a pure coronal wind, which cools and decelerates reaching infinity with terminal velocity ~ 980$ km/ s.Comment: 10 pages, 4 figure

On the viability of holistic cosmic-ray source models

We consider the energy spectrum of cosmic-rays (CRs) from a purely phenomenological point of view and investigate the possibility that they all be produced by the same type of sources with a single power-law spectrum, in E^{-x}, from thermal to ultra-high energies. We show that the relative fluxes of the Galactic (GCR) and extra-galactic (EGCR) components are compatible with such a holistic model, provided that the index of the source spectrum be x \simeq 2.23\pm 0.07. This is compatible with the best-fit indices for both GCRs and EGCRs, assuming that their source composition is the same, which is indeed the case in a holistic model. It is also compatible with theoretical expectations for particle acceleration at relativistic shocks.Comment: 5 pages, 1 figure, Accepted for publication in Astronomy and Astrophysic

Density functional theory study of (OCS)2^-

The structural and electronic properties of the carbonyl sulfide dimer anion are calculated using density functional theory within a pseudopotential method. Three geometries are optimized and investigated: C2v and C2 symmetric, as well as one asymmetric structure. A distribution of an excess charge in three isomers are studied by the Hirshfeld method. In an asymmetric (OCS)2^- isomer the charge is not equally divided between the two moieties, but it is distributed as OCS^{-0.6} OCS^{-0.4}. Low-lying excitation levels of three isomers are compared using the time-dependent density functional theory in the Casida approach.Comment: pdf (included all figures): http://www.phy.hr/~goranka/Research/ocs.pd