Susanne Pfalzner

ATLASGAL — towards a complete sample of massive star forming clumps ⋆

By matching infrared-selected, massive young stellar objects (MYSOs) and compact HII regions in the RMS survey to massive clumps found in the submillimetre ATLASGAL survey, we have identified ∼1000 embedded young massive stars between 280 ◦ <l< 350 ◦ and 10 ◦ <l< 60 ◦ with| b|< 1.5 ◦ . Combined with an existing sample of radio-selected methanol masers and compact HII regions, the result is a catalogue of∼1700 massive stars embedded within∼1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and HII-region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no sig nificant di fferences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of s tar formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and HII-regio n subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs asso ciated with the most massive clumps, probably due to very short pre-main sequence lifetimes in the most luminous sources.

Universality of young cluster sequences

Aims. Most stars do not form in isolation but as part of a cluster comprising anywhere between a few dozen to several million stars with stellar densities ranging from 0.01 to several 10 5 Mpc −3 . The majority of these clusters dissolve within 20 Myr. The general assumption is that clusters are born more or less over this entire density range. Methods. A new analysis of cluster observations is presented. Results. It demonstrates that, in fact, clustered star formation works under surprisingly tight constraints with respect to cluster size and density. Conclusions. The observed multitude of cluster densities simply results from snapshots of two sequences evolving in time along pre-defined tracks in the density-radius plane. This implies that the cluster size can actually be used to determine its age.

An introduction to inertial confinement fusion

FUNDAMENTALS OF INERTIAL CONFINEMENT FUSION What happens in the sun? Can one produce energy on Earth like in the sun? The two approaches - Magnetic vs. Inertial Confinement Stages in inertial confinement fusion Outline of the Book LASER DRIVERS FOR ICF Basics of laser physics Lasers for ICF applications Nd-glass lasers for ICF Alternatives to Nd-glass lasers BASIC PLASMA PHYSICS Debye length and plasma frequency Particle description Fluid description Plasma waves Plasma heating The ponderomotive force Shock waves Equation of state for dense plasmas ABSORPTION OF LASER LIGHT Coupling of the laser energy to the target Inverse Bremsstrahlung absorption Resonance absorption Parametric instabilities Indirect drive: coupling laser energy to the hohlraum ii Energy transport HYDRODYNAMIC COMPRESSION AND BURN Implosion of solid target Foil target Rocket Model and Ablation Compression wave - Shock front - Shock wave Compression phase Spherically convergent shock waves Isentropic compression Multiple shocks Burn RAYLEIGH-TAYLOR INSTABILITIES Basic concept RT in the ablation phase RT instabilities in the deceleration phase Consequences for target design Idealized RT instabilities vs. ICF situation Other dynamic instabilities ENERGY REQUIREMENTS AND GAIN Power balance Energy requirements Gain TARGETS Basic considerations for target design Direct and indirect-drive targets Direct-drive targets Indirect-drive targets Target fabrication ICF POWER PLANT Power plant design Plant efficiency Target chamber Target fabrication for power plant Safety issues HEAVY-ION DRIVEN FUSION Heavy-ion drivers Ion beam energy deposition Target design for heavy-ion drivers Heavy-ion power plant Light-ion drivers FAST IGNITOR Fast-ignitor vs. hot-spot concept Hole boring or laser cone guiding? O_-center ignition Status and Future ABC OF ICF APPENDIX Predicted energy consumption and resources Constants Formulae Abbreviations Constants of the semiempirical mass formula List of Codes used for Numerical Modelling References Bibliography

Encounter-triggered Disk Mass Loss in the Orion Nebula Cluster

The relevance of encounters on the destruction of protoplanetary disks in the Orion Nebula cluster (ONC) is investigated by combining two different types of numerical simulation. First, star-cluster simulations are performed to model the stellar dynamics of the ONC, the results of which are used to investigate the frequency of encounters, the mass ratio and separation of the stars involved, and the eccentricity of the encounter orbits. The results show that interactions that could influence the star-surrounding disk are more frequent than previously assumed in the core of the ONC, the so-called Trapezium cluster. Second, a parameter study of star-disk encounters is performed to determine the upper limits of the mass loss of the disks in encounters. For simulation times of ~1-2 Myr (the likely age of the ONC) the results show that gravitational interaction might account for a significant disk mass loss in dense clusters. Disk destruction is dominated by encounters with high-mass stars, especially in the Trapezium cluster, where the fraction of disks destroyed due to stellar encounters can reach 10%-15%. These estimates are in accord with recent observations of Lada et al., who determined a stellar disk fraction of 80%-85%. Thus, it is shown that in the ONC—a typical star-forming region—stellar encounters do have a significant effect on the mass of protoplanetary disks and thus affect the formation of planetary systems.

Weighing the cusp at the Galactic Centre

As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years. Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole (MBH) with a mass of MBH = 3–4 × 106M⊙ implying an insignificant cusp mass. However, it has also been shown that the central black hole resides in a ∼1″ diameter stellar cluster of a priori unknown mass. In a spherical potential which is neither Keplerian nor harmonic, orbits will precess resulting in inclined rosetta shaped trajectories on the sky. In this case, the assumption of non-Keplerian orbits is a more physical approach. It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members. This paper presents the first exemplary modelling efforts in this direction. Using positional and radial data of star S2, we find that there could exist an unobserved extended mass component of several 105M⊙ forming a so-called ‘cusp’ centered on the black hole position. Considering only the fraction of the cusp mass M within the apo-center of the S2 orbit we find as an upper limit that M/(MBH + M) ≤ 0.05. A large extended cusp mass, if present, is unlikely to be composed of sub-solar mass constituents, but could be explained rather well by a cluster of high M/L stellar remnants, which we find to form a stable configuration. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stellar interactions in dense and sparse star clusters

Context. Stellar encounters potentially affect the evolution of the protoplanetary discs in the Orion Nebula Cluster (ONC). However, the role of encounters in other cluster environments is less known. Aims. We investigate the effect of the encounter-induced disc-mass loss in different cluster environments. Methods. Starting from an ONC-like cluster we vary the cluster size and density to determine the correlation of the collision time scale and disc-mass loss. We use the nbody6++ code to model the dynamics of these clusters and analyse the disc-mass loss due to encounters. Results. We find that the encounter rate strongly depends on the cluster density but remains rather unaffected by the size of the stellar population. This dependency translates directly into the effect on the encounter-induced disc-mass loss. The essential outcome of the simulations are: i) even in clusters of four times lower density than the ONC, the effect of encounters is still apparent; ii) the density of the ONC itself marks a threshold: in less dense and less massive clusters it is the massive stars that dominate the encounter-induced disc-mass loss, whereas in denser and more massive clusters the low-mass stars play the major role for the disc-mass removal. Conclusions. It seems that in the central regions of young dense star clusters – the common sites of star formation – stellar encounters do affect the evolution of the protoplanetary discs. With higher cluster density low-mass stars become more heavily involved in this process. These results can also be applied to extreme stellar systems: in the case of the Arches cluster one would expect stellar encounters to destroy the discs of most of the low- and high-mass stars in several hundred thousand years, whereas intermediate mass stars are able to retain their discs to some extent even under these harsh environmental conditions.

The expansion of massive young star clusters – observation meets theory

Most stars form as part of a star cluster. The most massive clusters in the Milky Way exist in two groups - loose and compact clusters - with significantly different sizes at the end of the star formation process. After their formation both types of clusters expand up to a factor 10-20 within the first 20 Myr. Gas expulsion at the end of the star formation process is usually regarded as only possible process that can lead to such an expansion.We investigate the effect of gas expulsion by a direct comparison between numerical models and observed clusters concentrating on clusters with masses >10^3 M(sun). For these clusters the initial conditions before gas expulsion, the characteristic cluster development, its dependence on cluster mass, and the star formation efficiency (SFE) are investigated. We perform N-body simulations of the cluster expansion process after gas expulsion and compare the results with observations. We find that the expansion processes of the observed loose and compact massive clusters are driven by completely different physical processes. As expected the expansion of loose massive clusters is largely driven by the gas loss due to the low SFE of ~30%. One new revelation is that all the observed massive clusters of this group seem to have a very similar size of 1-3 pc at the onset of expansion. It is demonstrated that compact clusters have a much higher effective SFE of 60-70% and are as a result much less affected by gas expulsion. Their expansion is mainly driven by stellar ejections caused by interactions between the cluster members. The reason why ejections are so efficient in driving cluster expansion is that they occur dominantly from the cluster centre and over an extended period of time. Thus during the first 10 Myr the internal dynamics of loose and compact clusters differ fundamentally.

Early evolution of the birth cluster of the solar system

Context. The solar system was most likely born in a star cluster containing at least 1000 stars. It is highly probable that this cluster environment influenced various properties of the solar system like its chemical composition, size and the orbital parameters of some of its constituting bodies. Aims. In the Milky Way, clusters with more than 2000 stars only form in two types - starburst clusters and leaky clusters - each following a unique temporal development in the mass-radius plane. The aim is here to determine the encounter probability in the range relevant to solar system formation for starburst or leaky cluster environments as a function of cluster age. Methods. N-body methods are used to investigate the cluster dynamics and the e ect of gravitational interactions between cluster members on young solar-type stars surrounded by discs. Results. Using the now available knowledge of the cluster density at a given cluster age it is demonstrated that in starburst clusters the central densities over the first 5Myr are so high (initially> 10 5 M pc 3 ) that hardly any discs with solar system building potential would survive this phase. This makes a starburst clusters an unlikely environment for the formation of our solar system. Instead it is highly probable that the solar system formed in a leaky cluster (often classified as OB association). It is demonstrated that an encounter determining the characteristic properties existing in our solar systems most likely happened very early on (< 2Myr) in its formation history and that after 5Myr the likelihood of a solar-type star experiencing such an encounter in a leaky cluster is negligible even if it was still part of the bound remnant. This explains why the solar system could develop and maintain its high circularity later in its development.

Local-density-driven clustered star formation

Context. A positive power-law trend between the local surface densities of molecular gas, Σgas, and young stellar objects, Σ� ,i n molecular clouds of the solar neighbourhood has recently been identified. How it relates to the properties of embedded clusters, in particular to the recently established radius-density relation, has so far not been investigated. Aims. We model the development of the stellar component of molecular clumps as a function of time and initial local volume density. Our study provides a coherent framework able to explain both the molecular-cloud and embedded-cluster relations quoted above. Methods. We associate the observed volume density gradient of molecular clumps to a density-dependent free-fall time. The molecular clump star formation history is obtained by applying a constant star formation efficiency per free-fall time, � ff. Results. For the volume density profiles typical of observed molecular clumps (i.e. power-law slope �− 1.7), our model gives a stargas surface-density relation of the form Σ� ∝ Σ 2, which agrees very well with the observations. Taking the case of a molecular ,

Weighing the cusp at the Galactic Center

As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years. Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole (MBH) with a mass of MBH = 3–4 × 106M⊙ implying an insignificant cusp mass. However, it has also been shown that the central black hole resides in a ∼1″ diameter stellar cluster of a priori unknown mass. In a spherical potential which is neither Keplerian nor harmonic, orbits will precess resulting in inclined rosetta shaped trajectories on the sky. In this case, the assumption of non-Keplerian orbits is a more physical approach. It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members. This paper presents the first exemplary modelling efforts in this direction. Using positional and radial data of star S2, we find that there could exist an unobserved extended mass component of several 105M⊙ forming a so-called ‘cusp’ centered on the black hole position. Considering only the fraction of the cusp mass M within the apo-center of the S2 orbit we find as an upper limit that M/(MBH + M) ≤ 0.05. A large extended cusp mass, if present, is unlikely to be composed of sub-solar mass constituents, but could be explained rather well by a cluster of high M/L stellar remnants, which we find to form a stable configuration. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)