We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

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Theory of Star Formation

Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Monthly Notices of the Royal Astronomical Society

LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

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LOFAR: The LOw-Frequency ARray

Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques

The Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, has mapped out the Cosmic Microwave Background with unprecedented accuracy over the whole sky. Its observations have led to the establishment of a simple concordance cosmological model for the contents and evolution of the universe, consistent with virtually all other astronomical measurements. The WMAP first-year and three-year data have allowed us to place strong constraints on the parameters describing the ACDM model. a flat universe filled with baryons, cold dark matter, neutrinos. and a cosmological constant. with initial fluctuations described by nearly scale-invariant power law fluctuations, as well as placing limits on extensions to this simple model (Spergel et al. 2003. 2007). With all-sky measurements of the polarization anisotropy (Kogut et al. 2003; Page et al. 2007), two orders of magnitude smaller than the intensity fluctuations. WMAP has not only given us an additional picture of the universe as it transitioned from ionized to neutral at redshift z approx.1100. but also an observation of the later reionization of the universe by the first stars. In this paper we present cosmological constraints from WMAP alone. for both the ACDM model and a set of possible extensions. We also consider tlle consistency of WMAP constraints with other recent astronomical observations. This is one of seven five-year WMAP papers. Hinshaw et al. (2008) describe the data processing and basic results. Hill et al. (2008) present new beam models arid window functions, Gold et al. (2008) describe the emission from Galactic foregrounds, and Wright et al. (2008) the emission from extra-Galactic point sources. The angular power spectra are described in Nolta et al. (2008), and Komatsu et al. (2008) present and interpret cosmological constraints based on combining WMAP with other data. WMAP observations are used to produce full-sky maps of the CMB in five frequency bands centered at 23, 33, 41, 61, and 94 GHz (Hinshaw et al. 2008). With five years of data, we are now able to place better limits on the ACDM model. as well as to move beyond it to test the composition of the universe. details of reionization. sub-dominant components, characteristics of inflation, and primordial fluctuations. We have more than doubled the amount of polarized data used for cosmological analysis. allowing a better measure of the large-scale E-mode signal (Nolta et al. 2008). To this end we describe an alternative way to remove Galactic foregrounds from low resolution polarization maps in which Galactic emission is marginalized over, providing a cross-check of our results. With longer integration we also better probe the second and third acoustic peaks in the temperature angular power spectrum, and have many more year-to-year difference maps available for cross-checking systematic effects (Hinshaw et al. 2008).

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Five-year wilkinson microwave anisotropy probe * observations: likelihoods and parameters from the wmap data

At the end of their lives low mass stars such as our Sun lose most of their mass. The resulting planetary nebulae show a wide variety of shapes, from spherical to highly bipolar. According to the generalized interacting stellar winds model, these shapes are due to aninteraction between a very fast tenuous outflow, and a denser environment left over from an earlier slow phase of mass loss. Previous analytical and numerical work shows that this mechanismcan explain cylindrically symmetric nebulae very well. However, many circumstellar nebulae have a multipolar or point-symmetric shape. With two-dimensional calculations, Icke showed that these seemingly enigmatic forms can be easily reproduced by a two-wind model in which the confining disk is warped, as is expected to occur in irradiated disks. Here, we present the extension to fully three-dimensional adaptive mesh refinement simulations of such an interaction.

3D AMR Simulations of Point-Symmetric Nebulae

We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100/h Mpc)^3, while at the same time capturing the dwarf galaxies which are primarily responsible for reionization We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, C^2-Ray The resulting electron-scattering optical depth is in good agreement with the first-year WMAP polarization data We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exibit a large scatter about the mean and do not describe the global reionization history well The minimum reliable volume size for such predictions is ~30 Mpc We derive the power-spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcminute and larger scales We find two populations of HII regions according to their size, numerous, mid-sized (~10 Mpc) regions and a few, rare, very large regions tens of Mpc in size We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian (abridged)

Simulating Cosmic Reionization at Large Scales I: the Geometry of Reionization

Energy released by a small fraction of the baryons in the universe, which condensed out of while the IGM was cold, dark, and neutral,reheated and reionized it, exposing gas clouds within it to the glare of ionizing radiation. The first gas dynamical simulations of the photoevaporation of an intergalactic cloud by a quasar, including radiative transfer, are presented, along with a few observational diagnostics.

Reionization Feedback and the Photoevaporation of Intergalactic Clouds

The Letter ‘Observational constraints on supermassive dark stars’ was published in Mon. Not. R. Astron. Soc. 407, L74–L78 (2010). An error has been uncovered in the Letter. Owing to a numerical mistake, the formation rate of 1–2 × 108 M cold dark matter haloes used was too high by factors of ≈10–30. As a result, the observational constraints on f SMDS, the fraction of 1–2 × 108 M haloes that form 107 M supermassive dark stars (SMDS), should be relaxed accordingly. The corrected halo formation rate is presented as a function of redshift in Fig. 1. Because of the smaller number of haloes involved, the scatter between adjacent redshift bins is now considerably larger than in the original plot. By fitting a second-order polynomial (thick solid line) to the simulation data, we estimate that the formation rate of 1–2 × 108 M haloes is dn/dt ≈ 5 × 10−9 haloes per comoving Mpc3 and year at z = 10, and dn/dt ≈ 1 × 10−9 haloes per comoving Mpc3 and year at z = 15. This converts into Ṅ ≈ 580 haloes forming per unit redshift and arcmin2 at z = 10, and Ṅ ≈ 30 haloes forming per unit redshift and arcmin2 at z = 15. The resulting constraints on f SMDS, as a function of the SMDS lifetime τ , are plotted in Fig. 2 for our scenario A (where SMDS continue to form at z ≈ 10 rather than merely survive from previous

Erratum: Observational constraints on supermassive dark stars

The formation of planetary nebulae from the mass loss remnants of previous phases forms an interesting problem in both hydrodynamics and astrophysics. Not in the least because of the asphericity that most PNe show. The origin of this asphericity is still largely a mystery. Binarity is the most favoured explanation at the moment (Morris 1987) By assuming that the asphericity is introduced by some cause in the AGB phase, the work of Soker & Livio 1989, Mellema et al. 1991, and Icke et al. 1992 has shown that even if the effects of radiation from the central star and the gas are neglected (adiabatic case), many interesting features resembling observed aspherical PNe are found. See the paper of Icke at this conference for an overview of this. This paper reports on the work of including radiation effects in these models.

Garrelt Mellema

Papers

3D AMR Simulations of Point-Symmetric Nebulae

Simulating Cosmic Reionization at Large Scales I: the Geometry of Reionization

Reionization Feedback and the Photoevaporation of Intergalactic Clouds

Erratum: Observational constraints on supermassive dark stars

Modelling PN Formation from Hydrodynamics and Radiation