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

The upcoming radio interferometer Square Kilometre Array is expected to directly detect the redshifted 21-cm signal from the Cosmic Dawn for the first time. In this era temperature fluctuations from X-ray heating of the neutral intergalactic medium can impact this signal dramatically. Previously, in (Ross et al, 2017), we presented the first large-volume, 244 $h^{-1}$Mpc = 349 Mpc a side, fully numerical radiative transfer simulations of X-ray heating. This work is a follow-up where we now also consider QSO-like sources in addition to high mass X-ray binaries. Images of the two cases are clearly distinguishable at SKA1-LOW resolution and have RMS fluctuations above the expected noise. The inclusion of QSOs leads to a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm signal. We conclude that this increased non-Gaussianity is a promising signature of early QSOs.

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New simulation of QSO X-ray heating during the Cosmic Dawn

The status of numerical hydrodynamical models for Planetary Nebulae is reviewed. Since all of the numerical work is based on the interacting winds model, we start with a description of this model and give an overview of the early analytical and numerical models. Subsequently we address the numerical models which include radiation effects, first of all the ones which neglect any effects of stellar evolution. These `constant environment' models are shown to closely match typical observed nebulae, both in images and kinematic data. This shows that the basic generalized interacting winds model gives a good description of the situation in aspherical PNe. Next we discuss models that do include the effects of stellar and fast wind evolution. This introduces several new effects, the most important of which are the formation of a surrounding attached envelope, and the modification of the expansion of the nebula, which helps in creating aspherical PNe very early on in their evolution. The ionization of the slow wind also leads to a gradual smoothing out of its aspherical character, working against aspherical PNe forming in later stages. Finally we discuss some applications of the model to nebular problems.

Numerical models and our understanding of aspherical Planetary Nebulae

One of the explanations for the recent EDGES-LOW band 21-cm measurements of a strong absorption signal around 80~MHz is the presence of an excess radio background to the Cosmic Microwave Background (CMB). Such excess can be produced by the decay of unstable particles into small mass dark photons which have a non-zero mixing angle with electromagnetism. We use the EDGES-LOW band measurements to derive joint constraints on the properties of the early galaxies and the parameters of such a particle physics model for the excess radio background. A Bayesian analysis shows that a high star formation efficiency and an X-ray luminosity of $1-2 \times 10^{41} \rm erg ~s^{-1} ~ Mpc^{-3}$ are required along with a suppression of star formation in halos with virial temperatures $\lesssim 2\times 10^4$ K. The same analysis also suggests a 68 percent credible intervals for the mass of the decaying dark matter particles, it's lifetime, dark photon mass and the mixing angle of the dark and ordinary photon oscillation of $[10^{-3.5}, 10^{-2.4}]$ eV, $[10^{1.1}, 10^{2.7}]\times \tau_U$, $[10^{-12.2}, 10^{-10}]$ eV and $[10^{-7}, 10^{-5.6}]$ respectively. This implies an excess radio background which is $\approx 5.7$ times stronger than the CMB around 80~MHz. This value is a factor $\sim 3$ higher than the previous predictions which used a simplified model for the 21-cm signal.

Astrophysical information from the Rayleigh-Jeans Tail of the CMB

Using a radiation-hydrodynamics code I follow the formation of planetary nebulae around stars of different mass. Because a more massive central star evolves much faster than a lower mass one, it is to be expected that this will affect the formation of the PN. For the stars I use the evolutionary tracks for remnants with masses of 0.605 M0 and 0.836 M0, taken from Bloecker (1995). The AGB wind is assumed to be concentrated in a thin disk, which in models without evolving stars leads to the formation of a bipolar nebula. I find that in the case of the 0.836 M0 remnant the nebula indeed acquires a bipolar shape, whereas for the 0.605 M0 remnant the shape is more elliptical. The reason for this is the time it takes to ionize the AGB material; if this happens sufficiently slowly the density distribution in the AGB wind will be smoothed out, leading to more elliptical shapes. If it happens quickly, the original density distribution (in this case a thin disk) is hardly affected. This result suggests that lower mass central stars will less easily produce bipolar nebulae, which is supported by observations.

The formation of bipolar planetary nebulae

Context. The detection of the 21cm signal of neutral hydrogen from the Epoch of Reionization (EoR) is challenging due to bright foreground sources, radio frequency interference (RFI), and the ionosphere as well as instrumental effects. Even after correcting for these effects in the calibration step and applying foreground removal techniques, the remaining residuals in the observed 21cm power spectra are still above the thermal noise, which is referred to as the “excess variance.” Aims. We study a number of potential causes of this excess variance based on 13 nights of data obtained with the Low-Frequency Array (LOFAR). Methods. We focused on the impact of gain errors, the sky model, and ionospheric effects on the excess variance by correlating the relevant parameters such as the gain variance over time or frequency, local sidereal time (LST), diffractive scale, and phase structure– function slope with the level of excess variance. Results. Our analysis shows that the excess variance, at the current level, is neither strongly correlated with gain variance nor the ionospheric parameters. Rather, excess variance has an LST dependence, which is related to the power from the sky. Furthermore, the simulated Stokes I power spectra from bright sources and the excess variance show a similar progression over LST with the minimum power appearing at LST bin 6h to 9h. This LST dependence is also present in sky images of the residual Stokes I of the observations. In very-wide sky images based on one night of observation after direction-dependent calibration, we demonstrate that the extra power comes exactly from the direction of bright and distant sources Cassiopeia A and Cygnus A with the array beam patterns. Conclusions. These results suggest that the level of excess variance in the 21cm signal power spectra is related to sky effects and, hence, it depends on LST. In particular, very bright and distant sources such as Cassiopeia A and Cygnus A can dominate the effect. This is in line with earlier studies and offers a path forward toward a solution, since the correlation between the sky-related effects and the excess variance is non-negligible.

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Garrelt Mellema

Papers

New simulation of QSO X-ray heating during the Cosmic Dawn

Numerical models and our understanding of aspherical Planetary Nebulae

Astrophysical information from the Rayleigh-Jeans Tail of the CMB

The formation of bipolar planetary nebulae

Statistical analysis of the causes of excess variance in the 21 cm signal power spectra obtained with the Low-Frequency Array