bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of inbreeding on potential and realized immune responses inTenebrio molitor
Authors: RANTALA MARKUS J. , VIITANIEMI HEIDI , ROFF DEREK A.
Published: April 2011

Although numerous studies on vertebrates suggest that inbreeding reduces their resistance against parasites and pathogens, studies in insects have found contradictory evidence. In this study we tested the effect of 1 generation of brother–sister mating (inbreeding) on potential and realized immune responses and other life-history traits inTenebrio molitor. We found that inbreeding reduced adult mass, pre-adult survival and increased development time, suggesting that inbreeding reduced the condition of the adults and thus potentially made them more susceptible to physiological stress. However, we found no significant effect of inbreeding on the potential immune response (encapsulation response), but inbreeding reduced the realized immune response (resistance against the entomopathogenic fungi,Beauveria bassiana). There was a significant family effect on encapsulation response, but no family effect on the resistance against the entomopathogenic fungi. Given that this latter trait showed significant inbreeding depression and that the sample size for the family-effect analysis was small it is likely that the lack of a significant family effect is due to reduced statistical power, rather than the lack of a heritable basis to the trait. Our study highlights the importance of using pathogens and parasites in immunoecological studies.
The effect of retirement and age at retirement on self-perceived health after three years of follow-up in Dutch 55–64-year-olds
Authors: RIJS KELLY J. , COZIJNSEN RABINA , DEEG DORLY J. H.
Published: April 2011

Health consequences of retirement have not been included in the current public debate about increasing the age at retirement, which might be due to the fact that studies aimed at health consequences of retirement show ambiguous results. The literature indicates that various contextual characteristics might explain conflicting results. The current study examines the effect of retirement and age at retirement (55–64 years) on self-perceived health. Characteristics tested for confounding and effect modification were: demographic, health, psychological, job, and retirement characteristics. Subjects were 506 participants in the Longitudinal Aging Study Amsterdam (LASA). After three years, 216 retired and 290 remained employed. Multinomial logistic regression analyses show no main effect for retirement compared to continued employment. Modal (59–60) retirees were more likely to attain excellent or good self-perceived health (less than good self-perceived health as the reference category). Early (55–58) and late (61–64) retirees were unaffected by retirement if they did not receive a disability pension. Early and late retirees who received a disability pension were less likely to attain excellent self-perceived health after retirement. Higher educated were less likely to attain excellent self-perceived health after retirement, especially at late retirement age, although health selection might explain this result. Finally, mastery possibly acts as an adjustment resource. The paper concludes with a discussion on explanations for the effect of retirement and age at retirement.
An exploration of the value and mechanisms of befriending for older adults in England
Authors: LESTER HELEN , MEAD NICKI , GRAHAM CAROLYN CHEW , GASK LINDA , REILLY SIOBHAN
Published: April 2011

Social isolation and loneliness in older adults are growing problems. Empirical research suggests that loneliness can lead to poorer health outcomes including higher mortality rates. Befriending has been shown to decrease loneliness and depression although the exact mechanisms of action are unclear. In this study we aimed to explore experiences and identify key ‘ingredients’ of befriending through interviews conducted with 25 older adults who had used five different befriending services across England. We used Berkman's theoretical model of how individual social networks impact on health to help interpret our data and explore the mechanisms of befriending for older adults. Findings suggest that befriending offers some compensation for loss of elective relationships from older adults’ social networks, providing opportunities for emotional support and reciprocal social exchange through development of safe, confiding relationships. Good conversational skills and empathy were the foundation of successful relationships within which commonalities were then sought. Befrienders broadened befriendees’ perspectives on life (particularly among older adults in residential care). Social engagement was a powerful mechanism of action, particularly in terms of connecting people back into the community, reinforcing meaningful social roles and connecting to a past life that had often been significantly disrupted by loss. Understanding key components and mechanisms of befriending for older adults may facilitate development of more effective and theoretically sound befriending services.
Modeling and control of biologically inspired flying robots
Authors: Couceiro Micael S. , Luz J. Miguel A. , Figueiredo Carlos M. , Ferreira N. M. Fonseca
Published: April 2011

This paper covers a wide knowledge of physical and dynamical models useful for building flying robots and a new generation of flying platform developed in the similarity of flying animals. The goal of this work is to develop a simulation environment and dynamic control using the high-level calculation tool MatLab and the modeling, simulation, and analysis of dynamic systems tool Simulink. Once created the dynamic models to study, this work involves the study and understanding of the dynamic stability criteria to be adopted and their potential use in the control of flying models.
Numerical criteria for integral dependence
Authors: ULRICH BERND , VALIDASHTI JAVID
Published: April 2011

We study multiplicity based criteria for integral dependence of modules or of standard graded algebras, known as ‘Rees criteria’. Rather than using the known numerical invariants, we achieve this goal with a more direct approach by introducing a multiplicity defined as a limit superior of a sequence of normalized lengths; this multiplicity is a non-negative real number that can be irrational.
Leading terms of Artin L-series at negative integers and annihilation of higher K-groups
Authors: NICKEL ANDREAS
Published: April 2011

LetL/Kbe a finite Galois extension of number fields with Galois groupG. We use leading terms of ArtinL-series at strictly negative integers to construct elements which we conjecture to lie in the annihilator ideal associated to the Galois action on the higher dimensional algebraicK-groups of the ring of integers inL. For abelianGour conjecture coincides with a conjecture of Snaith and thus generalizes also the well-known Coates–Sinnott conjecture. We show that our conjecture is implied by the appropriate special case of the equivariant Tamagawa number conjecture (ETNC) provided that the Quillen–Lichtenbaum conjecture holds. Moreover, we prove induction results for the ETNC in the case of Tate motivesh0(Spec(L))(r), whereris a strictly negative integer. In particular, this implies the ETNC for the pair (h0(Spec(L))(r),), whereLis totally real,r< 0 is odd andis a maximal order containing ℤ[]G, and will also provide some evidence for our conjecture.
Berge's distance 3 pairs of genus 2 Heegaard splittings
Authors: SCHARLEMANN MARTIN
Published: April 2011

Following an example discovered by John Berge [2], we show that there is a 4-component link L ⊂ (S1 × S2)#(S1 × S2) so that, generically, the result of Dehn surgery on L is a 3-manifold with two inequivalent genus 2 Heegaard splittings, and each of these Heegaard splittings is of Hempel distance 3.
A pointwise ergodic theorem for Fuchsian groups
Authors: BUFETOV ALEXANDER I. , SERIES CAROLINE
Published: April 2011

We use Series' Markovian coding for words in Fuchsian groups and the Bowen-Series coding of limit sets to prove an ergodic theorem for Cesàro averages of spherical averages in a Fuchsian group.
Plücker formulae for singular space curves
Authors: WALL C. T. C.
Published: April 2011

In [4] I extended the Plücker formulae to curves in Pn for any n, under fairly strong genericity hypotheses. In this paper we seek to weaken these hypotheses using the theory of excess intersections. The method applies to just one of the formulae (so the results are complete only for n=3), and for this we obtain a number which is a sum of local contributions, given by an explicit rule. We evaluate these contributions in the cases when they are determined by the terms of least order in the respective Taylor expansions, thus obtaining explicit formulae in numerous cases.
Evolution of supercooling under coastal Antarctic sea ice during winter
Authors: Leonard Gregory H. , Langhorne Patricia J. , Williams Michael J.M. , Vennell Ross , Purdie Craig R. , Dempsey David E. , Haskell Timothy G. , Frew Russell D.
Published: April 2011

Here we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m deep with a maximum supercooling of c. 25 mK located 1–2 m below the sea ice-water interface. Transitory in situ supercooling events were also observed, one lasting c. 17 hours and reaching a depth of 70 m. In spite of these very low temperatures the isotopic composition of the water was relatively heavy, suggesting little glacial melt. Further, the water's temperature-salinity signature indicates contributions to water mass properties from High Salinity Shelf Water produced in areas of high sea ice production to the north of McMurdo Sound. Our measurements imply the existence of a heat sink beneath the supercooled layer that extracts heat from the ocean to thicken and cool this layer and contributes to the thickness of the sea ice cover. This sink is linked to the circulation pattern of the McMurdo Sound.
Rafting in the Antarctic springtail,Gomphiocephalus hodgsoni
Authors: Hawes T.C.
Published: April 2011

Capture by water surfaces is a frequent but stochastic phenomenon for Antarctic collembolans (springtails), with implications for both the survival and dispersal of populations. This paper provides the first field description of the characteristics of rafting aggregations of the springtailGomphiocephalus hodgsoniand provides the first observation of the inclusion of the mite,Stereotydeus mollis, within these aggregations. Field observations of aggregations in meltwater pools are used to develop a picture of the ecological implications of rafting for Antarctic springtails. The potential for marine capture was also demonstrated by additional observations of two springtails aerially deposited on to sea ice as well as springtails floating in the tidewater cracks between the sea ice and land.
In vivo dosimetry; essential or unnecessary?
Authors: Leman Jenna L
Published: April 2011

Introduction: The radiotherapy profession has learned from errors made during treatment planning and delivery. Quality assurance in radiotherapy (QART) procedures are implemented to reduce the risk of an error occuring. The chief medical officer, along with others, has recommended that the QART of all departments includes in vivo dosimetry (IVD) to ensure that the delivered dose equals the planned dose.

Why we need IVD: A lot of effort goes into field verification and it is just as vital that dosimetry is verified. Overdose to normal tissue can cause devastating side effects, even death, whilst tumour underdose may compromise control. Without IVD, there is no way of knowing that a patient is receiving an overdose until it is too late. Underdoses are unlikely to manifest without IVD. IVD allows radiotherapists and physicists to correct for dose errors in a timely manner.

Why IVD is unnecessary: Radiotherapy accidents are rare. Implementing IVD is expensive, time consuming and takes resources away from developing techniques which will improve patient outcomes. Current IVD methods are not suitable for modern techniques such as intensity modulated radiotherapy (IMRT).

Discussion: IVD appears to be a useful QART tool, particularly as dose escalation techniques develop allowing a higher dose to be delivered to the tumour. Departments may be unwilling to spend time and money on an IVD system that is costly and time consuming if it cannot perform IVD on modern techniques. Electronic portal imaging devices (EPIDs) can be utilised to perform IVD on complex techniques, such as IMRT and arc therapy, which current IVD methods cannot, however there is currently no EPID IVD system available commercially.

Conclusion: Ideally, all departments would conduct IVD on all new patients. IVD has proven to be an important QART tool, however, until technology is developed to allow EPID to include IVD, the procedure is not likely to be implemented countrywide.
Modeling the prestellar cores in Ophiuchus
Authors: Whitworth Ant , Stamatellos Dimitri , Walch Steffi
Published: April 2011

We present the first results from a project to model the prestellar cores in Ophiuchus, using initial conditions constrained as closely as possible by observation. The prestellar cores in Ophiuchus appear to be evolving in isolation — in the sense that the timescale on which an individual prestellar core collapses and fragments is estimated to be much shorter than the timescale on which it is likely to interact dynamically with another core. Therefore it is realistic to simulate individual cores separately, and this in turn makes it feasible (a) to perform multiple realisations of the evolution of each core (to allow for uncertainties in the initial conditions which persist, even for the most comprehensively observed cores), and (b) to do so at high resolution (so that even the smallest protostars are well resolved). The aims of this project are (i) to address how best to convert the observations into initial conditions; (ii) to explore, by means of numerical simulations, how the observed cores are likely to evolve in the future; (iii) to predict the properties of the protostars that they will form (mass function, multiplicity statistics, etc.); and (iv) to compare these properties with the properties of the observed pre-Main Sequence stars in Ophiuchus. We find that if the observed non-thermal velocities in the Ophiuchus prestellar cores are attributed to purely solenoidal turbulence, they do not fragment; they all collapse to form single protostars. If the non-thermal velocities are attributed to a mixture of solenoidal and compressive turbulence, multiple systems form readily. The turbulence first generates a network of filaments, and material then tends to flow along the filaments, at first into a primary protostar, and then onto a compact accretion disc around this protostar; secondary protostars condense out of the material flowing into the disc along the filaments. If the turbulence is purely solenoidal, but part of the non-thermal velocity dispersion is attributed to solid-body rotation, then again multiple systems form readily, but the pattern of fragmentation is quite different. A primary protostar forms near the centre of the core, and then an extended accretion disc forms around the primary protostar, and eventually becomes so massive that it fragments to produce low-mass secondaries; these frequently end up in hierarchical multiple systems.
Low-mass Star Formation: Observations
Authors: Evans Neal J.
Published: April 2011

I briefly review recent observations of regions forming low mass stars. The discussion is cast in the form of seven questions that have been partially answered, or at least illuminated, by new data. These are the following: where do stars form in molecular clouds; what determines the IMF; how long do the steps of the process take; how efficient is star formation; do any theories explain the data; how are the star and disk built over time; and what chemical changes accompany star and planet formation. I close with a summary and list of open questions.
Formation of massive stars
Authors: Beltrán Maria T.
Published: April 2011

The formation of high-mass stars represents a challenge from both a theoretical and an observational point of view. Here, we present an overview of the current status of the observational research on this field, outlining the progress achieved in recent years on our knowledge of the initial phases of massive star formation. The fragmentation of cold, infrared-dark clouds, and the evidence for star formation activity on some of them will be discussed, together with the kinematics of the gas in hot molecular cores, which can give us insights on the mechanism leading to the birth of an OB star.
Pre-main sequence disks
Authors: Duchêne Gaspard
Published: April 2011

In this contribution, I briefly review our empirical knowledge of disks around ≲2M⊙ pre-main sequence (T Tauri) stars, focusing first on the dichotomic question of their frequency before moving on to some more detailed disk properties (overall orientation, total mass, outer radius). Finally, I conclude with a brief discussion of disks around embedded protostars, which will play in the next few years a major role in testing star formation theory and simulations.
Fragmentation and dynamics in Massive Dense Cores in Cygnus-X
Authors: Csengeri T. , Bontemps S. , Schneider N. , Motte F.
Published: April 2011

A systematic, high angular-resolution study of IR-quiet Massive Dense Cores (MDCs) of Cygnus-X in continuum and high-density molecular tracers is presented. The results are compared with the quasi-static and the dynamical evolutionary scenario. We find that the fragmentation properties are not compatible with the quasi-static, monolithic collapse scenario, nor are they entirely compatible with the formation of a cluster of mostly low-mass stars. The kinematics of MDCs shows individual velocity components appearing as coherent flows, which indicate important dynamical processes at the scale of the mass reservoir around high-mass protostars.
The Luminosity Problem: Testing Theories of Star Formation
Authors: McKee Christopher F. , Offner Stella R. R.
Published: April 2011

Low-mass protostars are less luminous than expected. This luminosity problem is important because the observations appear to be inconsistent with some of the basic premises of star formation theory. Two possible solutions are that stars form slowly, which is supported by recent data, and/or that protostellar accretion is episodic; current data suggest that the latter accounts for less than half the missing luminosity. The solution to the luminosity problem bears directly on the fundamental problem of the time required to form a low-mass star. The protostellar mass and luminosity functions provide powerful tools both for addressing the luminosity problem and for testing theories of star formation. Results are presented for the collapse of singular isothermal spheres, for the collapse of turbulent cores, and for competitive accretion.
Internal Structure of Stellar Clusters: Geometry of Star Formation
Authors: Alfaro Emilio J. , Sánchez Néstor
Published: April 2011

The study of the internal structure of star clusters provides important clues concerning their formation mechanism and dynamical evolution. There are both observational and numerical evidences indicating that open clusters evolve from an initial clumpy structure, presumably a direct consequence of the formation in a fractal medium, toward a centrally condensed state. This simple picture has, however, several drawbacks. There can be very young clusters exhibiting radial patterns maybe reflecting the early effect of gravity on primordial gas. There can be also very evolved clusters showing fractal patterns that either have survived through time or have been generated subsequently by some (unknown) mechanism. Additionally, the fractal structure of some open clusters is much clumpier than the average structure of the interstellar medium in the Milky Way, although in principle a very similar structure should be expected. Here we summarize and discuss observational and numerical results concerning this subject.
Submillimeter Array Observations of Magnetic Fields in Star Forming Regions
Authors: Rao R. , Girart J.-M. , Marrone D. P.
Published: April 2011

There have been a number of theoretical and computational models which state that magnetic fields play an important role in the process of star formation. Competing theories instead postulate that it is turbulence which is dominant and magnetic fields are weak. The recent installation of a polarimetry system at the Submillimeter Array (SMA) has enabled us to conduct observations that could potentially distinguish between the two theories. Some of the nearby low mass star forming regions show hour-glass shaped magnetic field structures that are consistent with theoretical models in which the magnetic field plays a dominant role. However, there are other similar regions where no significant polarization is detected. Future polarimetry observations made by the Submillimeter Array should be able to increase the sample of observed regions. These measurements will allow us to address observationally the important question of the role of magnetic fields and/or turbulence in the process of star formation.
Modeling High-Mass Star Formation and Ultracompact H ii Regions
Authors: Klessen Ralf S. , Peters Thomas , Banerjee Robi , Mac Low Mordecai-Mark , Galván-Madrid Roberto , Keto Eric R.
Published: April 2011

Massive stars influence the surrounding universe far out of proportion to their numbers through ionizing radiation, supernova explosions, and heavy element production. Their formation requires the collapse of massive interstellar gas clouds with very high accretion rates. We discuss results from the first three-dimensional simulations of the gravitational collapse of a massive, rotating molecular cloud core that include heating by both non-ionizing and ionizing radiation. Local gravitational instabilities in the accretion flow lead to the build-up of a small cluster of stars. These lower-mass companions subsequently compete with the high-mass star for the same common gas reservoir and limit its overall mass growth. This process is called fragmentation-induced starvation, and explains why massive stars are usually found as members of high-order stellar systems. These simulations also show that the H ii regions forming around massive stars are initially trapped by the infalling gas, but soon begin to fluctuate rapidly. Over time, the same ultracompact H ii region can expand anisotropically, contract again, and take on any of the observed morphological classes. The total lifetime of H ii regions is given by the global accretion timescale, rather than their short internal sound-crossing time. This solves the so-called lifetime problem of ultracompact H ii region. We conclude that the the most significant differences between the formation of low-mass and high-mass stars are all explained as the result of rapid accretion within a dense, gravitationally unstable flow.
Magnetic Diffusion in Star Formation
Authors: Basu Shantanu , Dapp Wolf B.
Published: April 2011

Magnetic diffusion plays a vital role in star formation. We trace its influence from interstellar cloud scales down to star-disk scales. On both scales, we find that magnetic diffusion can be significantly enhanced by the buildup of strong gradients in magnetic field structure. Large scale nonlinear flows can create compressed cloud layers within which ambipolar diffusion occurs rapidly. However, in the flux-freezing limit that may be applicable to photoionized molecular cloud envelopes, supersonic motions can persist for long times if driven by an externally generated magnetic field that corresponds to a subcritical mass-to-flux ratio. In the case of protostellar accretion, rapid magnetic diffusion (through Ohmic dissipation with additional support from ambipolar diffusion) near the protostar causes dramatic magnetic flux loss. By doing so, it also allows the formation of a centrifugal disk, thereby avoiding the magnetic braking catastrophe.
Dependence of star formation on initial conditions and molecular cloud structure
Authors: Bate Matthew R.
Published: April 2011

I review what has been learnt so far regarding the origin of stellar properties from numerical simulations of the formation of groups and clusters of stars. In agreement with observations, stellar properties are found to be relatively robust to variations of initial conditions in terms of molecular cloud structure and kinetics, as long as extreme initial conditions (e.g. strong central condensation, weak or no turbulence) and small-scale driving are avoided, but properties may differ between bound and unbound clouds. Radiative feedback appears crucial for setting stellar masses, even for low-mass stars, while magnetic fields can provide low star formation rates.
Simulations of the IMF in Clusters
Authors: Pudritz Ralph E.
Published: April 2011

We review computational approaches to understanding the origin of the Initial Mass Function (IMF) during the formation of star clusters. We examine the role of turbulence, gravity and accretion, equations of state, and magnetic fields in producing the distribution of core masses - the Core Mass Function (CMF). Observations show that the CMF is similar in form to the IMF. We focus on feedback processes such as stellar dynamics, radiation, and outflows can reduce the accreted mass to give rise to the IMF. Numerical work suggests that filamentary accretion may play a key role in the origin of the IMF.
Theories of the initial mass function
Authors: Hennebelle Patrick , Chabrier Gilles
Published: April 2011

We review the various theories which have been proposed along the years to explain the origin of the stellar initial mass function. We pay particular attention to four models, namely the competitive accretion and the theories based respectively on stopped accretion, MHD shocks and turbulent dispersion. In each case, we derive the main assumptions and calculations that support each theory and stress their respective successes and failures or difficulties.
Star Formation with Adaptive Mesh Refinement Radiation Hydrodynamics
Authors: Krumholz Mark R.
Published: April 2011

I provide a pedagogic review of adaptive mesh refinement (AMR) radiation hydrodynamics (RHD) methods and codes used in simulations of star formation, at a level suitable for researchers who are not computational experts. I begin with a brief overview of the types of RHD processes that are most important to star formation, and then I formally introduce the equations of RHD and the approximations one uses to render them computationally tractable. I discuss strategies for solving these approximate equations on adaptive grids, with particular emphasis on identifying the main advantages and disadvantages of various approximations and numerical approaches. Finally, I conclude by discussing areas ripe for improvement.
The role of accretion disks in the formation of massive stars
Authors: Kuiper R. , Klahr H. , Beuther H. , Henning Th.
Published: April 2011

We present radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. We treat frequency dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, it is possible for the first time to simulate the whole accretion phase of several 105 yr for the forming massive star and to perform a comprehensive scan of the parameter space. Our simulation series show evidently the necessity to incorporate the dust sublimation front to preserve the high shielding property of massive accretion disks. Our disk accretion models show a persistent high anisotropy of the corresponding thermal radiation field, yielding to the growth of the highest-mass stars ever formed in multi-dimensional radiation hydrodynamics simulations. Non-axially symmetric effects are not necessary to sustain accretion. The radiation pressure launches a stable bipolar outflow, which grows in angle with time as presumed from observations. For an initial mass of the pre-stellar host core of 60, 120, 240, and 480⊙ the masses of the final stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2⊙ respectively.
Embedded disks around low-mass protostars
Authors: Vorobyov Eduard I. , Basu Shantanu
Published: April 2011

The time evolution of protostellar disks in the embedded phase of star formation (EPSF) is reviewed based on numerical hydrodynamics simulations of the gravitational collapse of two cloud cores with distinct initial masses. Special emphasis is given to disk, stellar, and envelope masses and also mass accretion rates onto the star. It is shown that accretion is highly variable in the EPSF, in agreement with recent theoretical and observational expectations. Protostellar disks quickly accumulate mass upon formation and may reach a sizeable fraction of the envelope mass (~35%) by the end of the Class 0 phase. Systems with disk-to-star mass ratio ξ≈0.5 are common but systems with ξ≥1.0 are rare because the latter quickly evolve into binary or multiple systems. Embedded disks are characterized by radial pulsations, the amplitude of which increases with growing core mass.
The Effects of Radiation Feedback on Early Fragmentation and Stellar Multiplicity
Authors: Offner Stella S. R.
Published: April 2011

Forming stars emit a significant amount of radiation into their natal environment. While the importance of radiation feedback from high-mass stars is widely accepted, radiation has generally been ignored in simulations of low-mass star formation. I use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodynamics code, to model low-mass star formation in a turbulent molecular cloud. I demonstrate that including radiation feedback has a profound effect on fragmentation and protostellar multiplicity. Although heating is mainly confined within the core envelope, it is sufficient to suppress disk fragmentation that would otherwise result in low-mass companions or brown dwarfs. As a consequence, turbulent fragmentation, not disk fragmentation, is likely the origin of low-mass binaries.
Simulations of Massive Star Cluster Formation and Feedback in Turbulent Giant Molecular Clouds
Authors: Harper-Clark Elizabeth , Murray Norman
Published: April 2011

Using the AMR code ENZO we are simulating the formation of massive star clusters within turbulent Giant Molecular Clouds (GMCs). Here we discuss the simulations from the first stages of building realistic turbulent GMCs, to accurate star formation, and ultimately comprehensive feedback. These simulations aim to build a better understanding of how stars affect GMCs, helping to answer the questions of how long GMCs live and why only a small fraction of the GMC gas becomes stars.
Radiation Driven Implosion and Triggered Star Formation
Authors: Bisbas T. G. , Whitworth A. P. , Wünsch R. , Hubber D. A. , Walch S.
Published: April 2011

We present simulations of stable isothermal clouds exposed to ionizing radiation from a discrete external source, and identify the conditions that lead to Radiatively Driven Implosion and Star Formation. We use the Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We find that the incident ionizing flux is the critical parameter determining the evolution; high fluxes disperse the cloud, whereas low fluxes trigger star formation. We find a clear connection between the intensity of the incident flux and the parameters of star formation.
Action of Winds Inside and Outside of Star Clusters
Authors: Palouš Jan , Dale Jim , Wünsch Richard , Silich Sergiy , Tenorio-Tagle Guillermo , Whitworth Anthony
Published: April 2011

The feedback form pre-main sequence and young stars influences their vicinity. The stars are formed in clusters, which implies that the winds of individual stars collide with each other. Inside of a star cluster, winds thermalize a fraction of their kinetic energy, forming a very hot medium able to escape from the cluster in the form of a large-scale wind. Outside of the cluster, the cluster wind forms a shock front as it interacts with the ambient medium which is accreted onto the expanding shell. A variety of instabilities may develop in such shells, and in some cases they fragment, triggering second generation of star formation. However, if the cluster surpasses a certain mass (depending on the radius and other parameters) the hot medium starts to be thermally unstable even inside of the cluster, forming dense warm clumps. The formation of next generations of stars may start if the clumps are big enough to self-shield against stellar radiation creating cold dense cores.
Theory of Feedback in Clusters and Molecular Cloud Turbulence
Authors: Vázquez-Semadeni Enrique
Published: April 2011

I review recent numerical and analytical work on the feedback from both low- and high-mass cluster stars into their gaseous environment. The main conclusions are that i) outflow driving appears capable of maintaing the turbulence in parsec-sized clumps and retarding their collapse from the free-fall rate, although there exist regions within molecular clouds, and even some examples of whole clouds, which are not actively forming stars, yet are just as turbulent, so that a more universal turbulence-driving mechanism is needed; ii) outflow-driven turbulence exhibits specific spectral features that can be tested observationally; iii) feedback plays an important role in reducing the SFR; iv) nevertheless, numerical simulations suggest feedback cannot completely prevent a net contracting motion of clouds and clumps. Therefore, an appealing source for driving the turbulence everywhere in GMCs is the accretion from the environment, at all scales. In this case, feedback's most important role may be to prevent a fraction of the gas nearest to newly formed stars from actually reaching them, thus reducing the SFE.
The theory of young cluster disruption
Authors: Goodwin Simon P.
Published: April 2011

Most stars seem to form in clusters, but the vast majority of these clusters do not seem to survive much beyond their embedded phase. The most favoured mechanism for the early destruction of star clusters is the effect of the removal of residual gas by feedback which dramatically changes the cluster potential. The effects of feedback depend on the ratio of the masses of stars and gas, and the velocity dispersion of the stars at the onset of gas removal. As gas removal is delayed by a few Myr from star formation these crucial parameters can change significantly from their initial values. In particular, in dynamically cool and clumpy clusters, the stars will collapse to a far denser state and if they decouple from the gas then gas removal may be far less destructive than previously thought. This might well help explain the survival of very massive clusters, such as globular clusters, without the need for extremely high star formation efficiencies or initial masses far greater than their current masses.
Ionisation Feedback in Star and Cluster Formation Simulations
Authors: Ercolano Barbara , Gritschneder Matthias
Published: April 2011

Feedback from photoionisation may dominate on parsec scales in massive star-forming regions. Such feedback may inhibit or enhance the star formation efficiency and sustain or even drive turbulence in the parent molecular cloud. Photoionisation feedback may also provide a mechanism for the rapid expulsion of gas from young clusters' potentials, often invoked as the main cause of ‘infant mortality’. There is currently no agreement, however, with regards to the efficiency of this process and how environment may affect the direction (positive or negative) in which it proceeds. The study of the photoionisation process as part of hydrodynamical simulations is key to understanding these issues, however, due to the computational demand of the problem, crude approximations for the radiation transfer are often employed.

We will briefly review some of the most commonly used approximations and discuss their major drawbacks. We will then present the results of detailed tests carried out using the detailed photoionisation code mocassin and the SPH+ionisation code iVINE code, aimed at understanding the error introduced by the simplified photoionisation algorithms. This is particularly relevant as a number of new codes have recently been developed along those lines.

We will finally propose a new approach that should allow to efficiently and self-consistently treat the photoionisation problem for complex radiation and density fields.
Pillars, Jets and Dynamical Features
Authors: Gritschneder Matthias , Burkert Andreas , Naab Thorsten , Walch Stefanie
Published: April 2011

We present high resolution simulations on the impact of ionizing radiation on turbulent molecular clouds. The combination of hydrodynamics, gravitational forces and ionization in the tree-SPH code iVINE naturally leads to the formation of elongated filaments and clumps, which are in excellent agreement with the pillars observed around HII regions. Including gravity the formation of a second generation of low-mass stars with surrounding protostellar disks is triggered at the tips of the pillars, as also observed. A parameter study allows us to determine the physical conditions under which irregular structures form and whether they resemble large pillars or a system of small, isolated globules.
Scaling Relations between Gas and Star Formation in Nearby Galaxies
Authors: Bigiel Frank , Leroy Adam , Walter Fabian
Published: April 2011

High resolution, multi-wavelength maps of a sizeable set of nearby galaxies have made it possible to study how the surface densities of H i, H2 and star formation rate (ΣHI, ΣH2, ΣSFR) relate on scales of a few hundred parsecs. At these scales, individual galaxy disks are comfortably resolved, making it possible to assess gas-SFR relations with respect to environment within galaxies. ΣH2, traced by CO intensity, shows a strong correlation with ΣSFR and the ratio between these two quantities, the molecular gas depletion time, appears to be constant at about 2 Gyr in large spiral galaxies. Within the star-forming disks of galaxies, ΣSFR shows almost no correlation with ΣHI. In the outer parts of galaxies, however, ΣSFR does scale with ΣHI, though with large scatter. Combining data from these different environments yields a distribution with multiple regimes in Σgas – ΣSFR space. If the underlying assumptions to convert observables to physical quantities are matched, even combined datasets based on different SFR tracers, methodologies and spatial scales occupy a well define locus in Σgas – ΣSFR space.
Theory of the Star Formation Rate
Authors: Padoan Paolo , Nordlund Åke
Published: April 2011

This work presents a new physical model of the star formation rate (SFR), tested with a large set of numerical simulations of driven, supersonic, self-gravitating, magneto-hydrodynamic (MHD) turbulence, where collapsing cores are captured with accreting sink particles. The model depends on the relative importance of gravitational, turbulent, magnetic, and thermal energies, expressed through the virial parameter, αvir, the rms sonic Mach number, S,0, and the ratio of mean gas pressure to mean magnetic pressure, β0. The SFR is predicted to decrease with increasing αvir (stronger turbulence relative to gravity), and to depend weakly on S,0 and β0, for values typical of star forming regions (S,0≈4-20 and β0≈1-20). The star-formation simulations used to test the model result in an approximately constant SFR, after an initial transient phase. Both the value of the SFR and its dependence on the virial parameter found in the simulations agree very well with the theoretical predictions.
Collisions of supersonic clouds
Authors: McLeod Andrew , Palouš Jan , Whitworth Anthony
Published: April 2011

We present simulations of supersonic collisions between molecular clouds of mass 500 M⊙ and radius 2.24 pc. The simulations are performed with the SEREN SPH code. The code treats the energy equation and the associated transport of heating and cooling radiation. The formation of protostars is captured by introducing sink particles. Low velocity collisions form a shock-compressed layer which fragments to form stars. For high-velocity collisions, υrel ⪆ 5 km s−1, the non-linear thin shell instability strongly disrupts the shock-compressed layer, and may inhibit the formation of stars.
Galactic scale star formation: Interplay between stellar spirals and the ISM
Authors: Wada Keiichi , Baba Junichi , Fujii Michiko , Saitoh Takayuki R.
Published: April 2011

Spiral structures in the disk galaxies have been extensively studied by many theoretical papers, but conventional steady-state models are not consistent with what we observe in time-dependent, multi-dimensional numerical simulations and also in real galaxies. Here we review recent progress in numerical modeling of stellar and gas spirals in disk galaxies. The spiral arms excited in a stellar disk can last for 10 Gyrs without the ISM, but each spiral arm is short-lived and is recurrently formed. The stellar spirals are not waves propagating with a single pattern speed. The ISM is concentrated in local potential minima, which roughly follow the galactic rotation together with the stellar arms, therefore galactic dust lanes are not the classic ‘galactic shocks’.
GRAPE Accelerators
Authors: Makino Junichiro
Published: April 2011

I'll overview the past, present, and future of the GRAPE project, which started as the effort to design and develop specialized hardware for gravitational N-body problem. The current hardware, GRAPE-DR, has an architecture quite different from previous GRAPEs, in the sense that it is a collection of small, but programmable processors, while previous GRAPEs had hardwired pipelines. I'll discuss pros and cons of these two approaches, comparisons with other accelerators and future directions.
Future Trends in Computing
Authors: Elmegreen Bruce G.
Published: April 2011

According to a Top500.org compilation, large computer systems have been doubling in sustained speed every 1.14 years for the last 17 years. If this rapid growth continues, we will have computers by 2020 that can execute an Exaflop (1018) per second. Storage is also improving in cost and density at an exponential rate. Several innovations that will accompany this growth are reviewed here, including shrinkage of basic circuit components on Silicon, three-dimensional integration, and Phase Change Memory. Further growth will require new technologies, most notably those surrounding the basic building block of computers, the Field Effect Transistor. Implications of these changes for the types of problems that can be solved are briefly discussed.
Implementing and comparing sink particles in AMR and SPH
Authors: Federrath Christoph , Banerjee Robi , Seifried Daniel , Clark Paul C. , Klessen Ralf S.
Published: April 2011

We implemented sink particles in the Adaptive Mesh Refinement (AMR) code FLASH to model the gravitational collapse and accretion in turbulent molecular clouds and cores. Sink particles are frequently used to measure properties of star formation in numerical simulations, such as the star formation rate and efficiency, and the mass distribution of stars. We show that only using a density threshold for sink particle creation is insufficient in case of supersonic flows, because the density can exceed the threshold in strong shocks that do not necessarily lead to local collapse. Additional physical collapse indicators have to be considered. We apply our AMR sink particle module to the formation of a star cluster, and compare it to a Smoothed Particle Hydrodynamics (SPH) code with sink particles. Our comparison shows encouraging agreement of gas and sink particle properties between the AMR and SPH code.
Convergence of SPH and AMR simulations
Authors: Hubber David A. , Falle Sam A. E. G. , Goodwin Simon P.
Published: April 2011

We present the first results of a large suite of convergence tests between Adaptive Mesh Refinement (AMR) Finite Difference Hydrodynamics and Smoothed Particle Hydrodynamics (SPH) simulations of the non-linear thin shell instability and the Kelvin-Helmholtz instability. We find that the two methods converge in the limit of high resolution and accuracy. AMR and SPH simulations of the non-linear thin shell instability converge with each other with standard algorithms and parameters. The Kelvin-Helmholtz instability in SPH requires both an artificial conductivity term and a kernel with larger compact support and more neighbours (e.g. the quintic kernel) in order converge with AMR. For purely hydrodynamical problems, SPH simulations take an order of magnitude longer than the grid code when converged.
Radiative Transfer in Molecular Clouds
Authors: Juvela M.
Published: April 2011

Information of astronomical objects is obtained mainly through their radiation. Thus, the radiative transfer problem has a central role in all astrophysical research. Basic radiative transfer analysis or more complex modeling is needed both to interpret observations and to make predictions on the basis of numerical models. In this paper I will discuss radiative transfer in the context of interstellar molecular clouds where the main scientific questions involve the structure and evolution of the clouds and the star formation process. The studies rely on the analysis of spectral line and dust continuum observations. After a discussion of the corresponding radiative transfer methods, I will examine some of the current challenges in the field. Finally, I will present three studies where radiative transfer modeling pays a central role: the polarized dust emission, the Zeeman effect in emission lines, and the continuum emission from dense cloud cores.
Adaptable Radiative Transfer Innovations for Submillimeter Telescopes (ARTIST)
Authors: Padovani Marco , Jørgensen Jes K. , Bertoldi Frank , Brinch Christian , Frau Pau , Girart Josep Miquel , Hogerheijde Michiel , Juhasz Attila , Kuiper Rolf , Schaaf Reinhold , Vlemmings Wouter H. T.
Published: April 2011

Submillimeter observations are a key for answering many of the big questions in modern-day astrophysics, such as how stars and planets form, how galaxies evolve, and how material cycles through stars and the interstellar medium. With the upcoming large submillimeter facilities ALMA and Herschel a new window will open to study these questions. ARTIST is a project funded in context of the European ASTRONET program with the aim of developing a next generation model suite for comprehensive multi-dimensional radiative transfer calculations of the dust and line emission, as well as their polarization, to help interpret observations with these groundbreaking facilities.
Gas dynamics in whole galaxies: SPH
Authors: Dobbs Clare
Published: April 2011

I review the progress of SPH calculations for modelling galaxies, and resolving gas dynamics on GMC scales. SPH calculations first investigated the response of isothermal gas to a spiral potential, in the absence of self gravity and magnetic fields. Surprisingly though, even these simple calculations displayed substructure along the spiral arms. Numerical tests indicate that this substructure is still present at high resolution (100 million particles, ~10 pc), and is independent of the initial particle distribution. One interpretation of the formation of substructure is that smaller clouds can agglomerate into more massive GMCs via dissipative collisions. More recent calculations have investigated how other processes, such as the thermodynamics of the ISM, and self gravity affect this simple picture. Further research has focused on developing models with a more realistic spiral structure, either by including stars, or incorporating a tidal interaction.