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

In this paper we explore the eect of radiative losses on purely hydrodynamic jet collimation models applicable to Young Stellar Objects (YSOs). In our models aspherical bubbles form from the interaction of a central YSO wind with an aspherical circumprotostellar density distribution. Building on a previous non-radiative study (Frank & Mellema 1996) we demonstrate that supersonic jets are a natural and robust consequence of aspherical wind-blown bubble evolution. The simulations show that the addition of radiative cooling makes the hydrodynamic collimation mechanisms studied by Frank & Mellema (1996) more eective. We nd a number of time-dependent processes contributing to the collimation whose relative strength depends on the age of the system and parameters characterising the wind and the environment. As predicted by Frank & Mellema (1996) the flow-focusing at an oblique inner shock becomes more eective when radiative cooling is included. An unexpected result of this is the production of cool ( T< 10 4 K), dense (n 10 4 cm 3 ) jets forming through conical converging flows at the poles of the bubbles. For steady winds the formation of these jets occurs early in the bubble evolution. At later times we nd that the dynamical and cooling time scales for the jet material become similar and the jet beam increases in temperature (T 10 6 K). The duration of the cool jet phase depends on the mass loss rate, _ Mw, and velocity, Vw, of the wind. High values of _ Mw and low values of Vw produce longer cool jet phases. Since observations of YSO jets show considerable variability in the jet beam we present a simple one-dimensional (1-D) model for the evolution of a variable wind interacting with an accreting environment. We nd that the accretion ram pressure can halt the expansion of the bubble on time scales comparable to the periodicity of the wind and length scales less than 100 AU, the approximate observed scale for YSO jet collimation. These models indicate that, in the presence of a varying protostellar wind, the hydrodynamic collimation processes studied in our simulations can produce cool jets with sizes and time scales consistent with observations.

Outflow collimation in young stellar objects

We explain the Hα emission of the cometary knots in the Helix Nebula (NGC 7293) with an analytical model that describes the emission of the head of the globules as a photoevaporated flow produced by the incident ionizing radiation of the central star. We compare these models with the Hα emission obtained from the Hubble Space Telescope (HST) archival images of the Helix Nebula. From a comparison of the Hα emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central star of about 5 × 1045 s-1, which is consistent with estimates based on the total Hβ flux of the nebula. We also model the tails of the cometary knots as a photoevaporated wind from a neutral shadow region produced by the diffuse ionizing photon field of the nebula. A comparison with the HST images allows us to obtain a direct determination of the value of the diffuse ionizing flux. We compare the ratio of diffuse to direct stellar flux as a function of radius inside an H II region with those obtained from the observational data through the analytical tail and head wind model. The agreement of this model with the values determined from the observations of the knots is excellent.

Photoevaporating Flows from the Cometary Knots in the Helix Nebula (NGC 7293)

The ability of the future low-frequency component of the Square Kilometre Array radio telescope to produce tomographic images of the redshifted 21-cm signal will enable direct studies of the evolut ...

Optimal identification of H ii regions during reionization in 21-cm observations

We investigate the fast (type III) migration regime of high-mass protoplanets orbiting in protoplanetary disks. This type of migration is dominated by corotational torques. We study the details of flow structure in the planet's vicinity, the dependence of migration rate on the adopted disc model, and the numerical convergence of models (independence of certain numerical parameters such as gravitational softening). We use two-dimensional hydrodynamical simulations with adaptive mesh refinement,based on the FLASH code with improved time-stepping scheme. We perform global disk simulations with sufficient resolution close to the planet, which is allowed to freely move throughout the grid. We employ a new type of equation of state in which the gas temperature depends on both the distance to the star and planet, and a simplified correction for self-gravity of the circumplanetary gas. We find that the migration rate in the type III migration regime depends strongly on the gas dynamics inside the Hill sphere (Roche lobe of the planet) which, in turn, is sensitive to the aspect ratio of the circumplanetary disc. Furthermore, corrections due to the gas self-gravity are necessary to reduce numerical artifacts that act against rapid planet migration. Reliable numerical studies of Type III migration thus require consideration of both the thermal andthe self-gravity corrections, as well as a sufficient spatial resolution and the calculation of disk-planet attraction both inside and outside the Hill sphere. With this proviso, we find Type III migration to be a robust mode of migration, astrophysically promising because of a speed much faster than in the previously studied modes of migration.

Numerical simulations of the type III migration:I. Disc model and convergence tests

Potential small-scale discrepancies in the picture of galaxy formation painted by the Lambda CDM paradigm have led to considerations of modi fied dark matter models. One such dark matter model that ...

Garrelt Mellema

Papers

Outflow collimation in young stellar objects

Photoevaporating Flows from the Cometary Knots in the Helix Nebula (NGC 7293)

Optimal identification of H ii regions during reionization in 21-cm observations

Numerical simulations of the type III migration:I. Disc model and convergence tests

Fuzzy Dark Matter at Cosmic Dawn: New 21-cm Constraints