Photoevaporation of Circumstellar Disks due to External FUV Radiation in Stellar Aggregates

When stars form in small groups (N = 100 - 500 members), their circumstellar disks are exposed to little EUV radiation but a great deal of FUV radiation from massive stars in the group. This paper calculates mass loss rates for circumstellar disks exposed to external FUV radiation. Previous work treated large disks and/or intense radiation fields in which the disk radius exceeds the critical radius (supercritical disks) where the sound speed in the FUV heated layer exceeds the escape speed. This paper shows that significant mass loss still takes place for subcritical systems. Some of the gas extends beyond the disk edge (above the disk surface) to larger distances where the temperature is higher, the escape speed is lower, and an outflow develops. The evaporation rate is a sensitive function of the stellar mass and disk radius, which determine the escape speed, and the external FUV flux, which determines the temperature structure of the flow. Disks around red dwarfs are readily evaporated and shrink to disk radii of 15 AU on short time scales (10 Myr) when exposed to moderate FUV fields with $G_0$ = 3000. Although disks around solar type stars are more durable, these disks shrink to 15 AU in 10 Myr for intense FUV radiation fields with $G_0$ = 30,000; such fields exist in the central 0.7 pc of a cluster with N = 4000 stars. If our solar system formed in the presence of such strong FUV radiation fields, this mechanism could explain why Neptune and Uranus in our solar system are gas poor, whereas Jupiter and Saturn are gas rich. This mechanism for photoevaporation can also limit the production of Kuiper belt objects and can suppress giant planet formation in sufficiently large clusters, such as the Hyades, especially for disks associated with low mass stars.Comment: 49 pages including 12 figures; accepted to Ap

HST/WFPC2 and VLT/ISAAC observations of PROPLYDS in the giant HII region NGC 3603

We report the discovery of three proplyd-like structures in the giant HII region NGC 3603. The emission nebulae are clearly resolved in narrow-band and broad-band HST/WFPC2 observations in the optical and broad-band VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6,000 A.U. x 20,000 A.U. The nebulae share the overall morphology of the proplyds (``PROto PLanetarY DiskS'') in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. The striking similarity of the tadpole shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present 2D radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10^-5 Mo/yr are required in order to explain the size of the proplyds. (abbreviated)Comment: 10 pages, 4 Postscript figures, uses emulateapj.sty and psfig.tex. Astronomical Journal, in press (January 2000 issue

The nature of the Lyman-alpha emission region of FDF-4691

In order to study the origin of the strong Lyman-alpha emission of high-redshift starburst galaxies we observed and modeled the emission of the z = 3.304 galaxy FDF-4691 (rest-frame EW = 103 Angstroem). The observations show that FDF-4691 is a young starburst galaxy with a (for this redshift) typical metallicity. The broad, double-peaked profile of the Lyman-alpha emission line can be explained assuming a highly turbulent emission region in the inner part of the starburst galaxy, and a surrounding extended shell of low-density neutral gas with a normal dust/gas ratio and with Galactic dust properties. The detection of the Lyman-alpha emission line is explained by the intrinsic broad Lyman-alpha emission and a low HI column density of the neutral shell. A low dust/gas ratio in the neutral shell is not needed to explain the strong Lyman-alpha line.Comment: Accepted for publication in A&A Letter

Topology-aware equipartitioning with coscheduling on multicore systems

Over the last decade, multicore architectures have become omnipresent. Today, they are used in the whole product range from server systems to handheld computers. The deployed software still undergoes the slow transition from sequential to parallel. This transition, however, is gaining more and more momentum due to the increased availability of more sophisticated parallel programming environments, which replace the some-times crude results of ad-hoc parallelization. Combined with the ever increasing complexity of multicore architectures, this results in a scheduling problem that is different from what it has been, because features such as non-uniform memory access, shared caches, or simultaneous multithreading have to be considered. In this paper, we compare different ways of scheduling multiple parallel applications. Due to emerging parallel programming environments, we only consider malleable applications, i. e., applications where the parallelism degree can be changed on the fly. We propose a topology-aware scheduling scheme that combines equipartitioning and coscheduling. It does not suffer from the drawbacks of the individual concepts and also allows to run applications at different degrees of parallelisms without compromising fairness. We find that topology-awareness increases performance for all evaluated workloads. The combination with coscheduling is more sensitive towards the executed workloads. However, the gained versatility allows new use cases to be explored, which were not possible before

Towards Energy-Aware Multi-Core Scheduling

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.There are two major current trends that can easily be identified in computer industry: (i) the shift towards massively parallelization fostered by multi-core technology resulting in growing numbers of cores per processor and (ii) the increasing importance of energy-awareness in computing due to rising energy costs and environmental awareness. In current operating systems, these two issues are often addressed independently: one component, the scheduler, assigns processes to cores and a second component manages the power states of individual cores in time. In this paper, we explain why this orthogonal treatment can lead to problems such as a considerably degraded system performance in case of on-demand processor power state management. Furthermore, we present an approach to energy-aware multi-core scheduling at the operating system level avoiding the performance penalty while still saving energy. To corroborate our argumentation and to illustrate the applicability of the presented approach, we give numbers from experiments based on Linux and current multi-core processors

a multiple linear regression model

The link between the indices of twelve atmospheric teleconnection patterns (mostly Northern Hemispheric) and gridded European temperature data is investigated by means of multiple linear regression models for each grid cell and month. Furthermore index-specific signals are calculated to estimate the contribution to temperature anomalies caused by each individual teleconnection pattern. To this extent, an observational product of monthly mean temperature (E-OBS), as well as monthly time series of teleconnection indices (CPC, NOAA) for the period 1951–2010 are evaluated. The stepwise regression approach is used to build grid cell based models for each month on the basis of the five most important teleconnection indices (NAO, EA, EAWR, SCAND, POLEUR), which are motivated by an exploratory correlation analysis. The temperature links are dominated by NAO and EA in Northern, Western, Central and South Western Europe, by EAWR during summer/autumn in Russia/Fenno-Scandia and by SCAND in Russia/Northern Europe; POLEUR shows minor effects only. In comparison to the climatological forecast, the presented linear regression models improve the temperature modelling by 30–40 % with better results in winter and spring. They can be used to model the spatial distribution and structure of observed temperature anomalies, where two to three patterns are the main contributors. As an example the estimated temperature signals induced by the teleconnection indices is shown for February 2010

TACO: A scheduling scheme for parallel applications on multicore architectures

While multicore architectures are used in the whole product range from server systems to handheld computers, the deployed software still undergoes the slow transition from sequential to parallel. This transition, however, is gaining more and more momentum due to the increased availability of more sophisticated parallel programming environments. Combined with the ever increasing complexity of multicore architectures, this results in a scheduling problem that is different from what it has been, because concurrently executing parallel programs and features such as non-uniform memory access, shared caches, or simultaneous multithreading have to be considered. In this paper, we compare different ways of scheduling multiple parallel applications on multicore architectures. Due to emerging parallel programming environments, we primarily consider applications where the parallelism degree can be changed on the fly. We propose TACO, a topology-aware scheduling scheme that combines equipartitioning and coscheduling, which does not suffer from the drawbacks of the individual concepts. Additionally, TACO is conceptually compatible with contention-aware scheduling strategies. We find that topology-awareness increases performance for all evaluated workloads. The combination with coscheduling is more sensitive towards the executed workloads and NUMA effects. However, the gained versatility allows new use cases to be explored, which were not possible before

Time-Evolution of Viscous Circumstellar Disks due to Photoevaporation by FUV, EUV and X-ray Radiation from the Central Star

We present the time evolution of viscously accreting circumstellar disks as they are irradiated by ultraviolet and X-ray photons from a low-mass central star. Our model is a hybrid of a 1D time-dependent viscous disk model coupled to a 1+1D disk vertical structure model used for calculating the disk structure and photoevaporation rates. We find that disks of initial mass 0.1M_o around 1M_o stars survive for 4x10^6 years, assuming a viscosity parameter $\alpha=0.01$, a time-dependent FUV luminosity $L_{FUV}~10^{-2}-10^{-3}$ L_o and with X-ray and EUV luminosities $L_X \sim L_{EUV} ~ 10^{-3}$L_o. We find that FUV/X-ray-induced photoevaporation and viscous accretion are both important in depleting disk mass. Photoevaporation rates are most significant at ~ 1-10 AU and at >~ 30 AU. Viscosity spreads the disk which causes mass loss by accretion onto the central star and feeds mass loss by photoevaporation in the outer disk. We find that FUV photons can create gaps in the inner, planet-forming regions of the disk (~ 1-10 AU) at relatively early epochs in disk evolution while disk masses are still substantial. EUV and X-ray photons are also capable of driving gaps, but EUV can only do so at late, low accretion-rate epochs after the disk mass has already declined substantially. Disks around stars with predominantly soft X-ray fields experience enhanced photoevaporative mass loss. We follow disk evolution around stars of different masses, and find that disk survival time is relatively independent of mass for stars with M ~ 3M_o the disks are short-lived(~10^5 years).Comment: Accepted to ApJ, Main Journa