The modern science of networks has brought significant advances to our understanding of complex systems. One of the most relevant features of graphs representing real systems is community structure, or clustering, i. e. the organization of vertices in clusters, with many edges joining vertices of the same cluster and comparatively few edges joining vertices of different clusters. Such clusters, or communities, can be considered as fairly independent compartments of a graph, playing a similar role like, e. g., the tissues or the organs in the human body. Detecting communities is of great importance in sociology, biology and computer science, disciplines where systems are often represented as graphs. This problem is very hard and not yet satisfactorily solved, despite the huge effort of a large interdisciplinary community of scientists working on it over the past few years. We will attempt a thorough exposition of the topic, from the definition of the main elements of the problem, to the presentation of most methods developed, with a special focus on techniques designed by statistical physicists, from the discussion of crucial issues like the significance of clustering and how methods should be tested and compared against each other, to the description of applications to real networks.

/pdf/community-detection-in-graphs-34rogm3r6p.pdf

Community detection in graphs

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/~vdtak/radex/index.shtml . The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes. Note: Accepted by AA 18 A4 pages, 11 figures;

/pdf/a-computer-program-for-fast-non-lte-analysis-of-interstellar-17wsxkwvpj.pdf

A computer program for fast non-LTE analysis of interstellar line spectra

Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

/pdf/complex-organic-interstellar-molecules-sz3r2nq94z.pdf

Complex Organic Interstellar Molecules

Flattened, rotating disks of cool dust and gas extending for tens to hundreds of astronomical units are found around almost all low-mass stars shortly after their birth. These disks generally persist for several million years, during which time some material accretes onto the star, some is lost through outflows and photoevaporation, and some condenses into centimeter- and larger-sized bodies or planetesimals. Through observations mainly at IR through millimeter wavelengths, we can determine how common disks are at different ages; measure basic properties including mass, size, structure, and composition; and follow their varied evolutionary pathways. In this way, we see the first steps toward exoplanet formation and learn about the origins of the Solar System. This review addresses observations of the outer parts, beyond 1 AU, of protoplanetary disks with a focus on recent IR and (sub)millimeter results and an eye to the promise of new facilities in the immediate future.

http://www.ifa.hawaii.edu/users/cieza/WC2011_ARAA.pdf

Protoplanetary Disks and Their Evolution

The understanding of the formation process of massive stars (>8 Msun) is limited, due to theoretical complications and observational challenges. 
We investigate the physical structure of the large-scale (~10^4-10^5 AU) molecular envelope of the high-mass protostar AFGL2591 using spectral imaging in the 330-373 GHz regime from the JCMT Spectral Legacy Survey. Out of ~160 spectral features, this paper uses the 35 that are spatially resolved. 
The observed spatial distributions of a selection of six species are compared with radiative transfer models based on a static spherically symmetric structure, a dynamic spherical structure, and a static flattened structure. The maps of CO and its isotopic variations exhibit elongated geometries on scales of ~100", and smaller scale substructure is found in maps of N2H+, o-H2CO, CS, SO2, CCH, and methanol lines. A velocity gradient is apparent in maps of all molecular lines presented here, except SO, SO2, and H2CO. We find two emission peaks in warm (Eup~200K) methanol separated by 12", indicative of a secondary heating source in the envelope. 
The spherical models are able to explain the distribution of emission for the optically thin H13CO+ and C34S, but not for the optically thick HCN, HCO+, and CS, nor for the optically thin C17O. The introduction of velocity structure mitigates the optical depth effects, but does not fully explain the observations, especially in the spectral dimension. A static flattened envelope viewed at a small inclination angle does slightly better. 
We conclude that a geometry of the envelope other than an isotropic static sphere is needed to circumvent line optical depth effects. We propose that this could be achieved in envelope models with an outflow cavity and/or inhomogeneous structure at scales smaller than ~10^4 AU. The picture of inhomogeneity is supported by observed substructure in at least six species.

/pdf/the-jcmt-spectral-legacy-survey-physical-structure-of-the-1pbo382vkr.pdf

The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591

The effects of Wheeler's quantum foam on black hole growth are explored from an astrophysical perspective. Quantum fluctuations in the form of mini (10^-5 g) black holes can couple to macroscopic black holes and allow the latter to grow exponentially in mass on a time scale of ~10^9 years. Consequently, supermassive black holes can acquire a lot of their mass through these quantum contributions over the life time of the universe. This alleviates the need for very efficient forms of baryonic matter accretion more recent than a redshift z~6. Sgr A* in the Milky Way center is a candidate to verify this quantum space-time effect, with a predicted mass growth rate of 4x10^-3 Mo yr^-1. A few comments on the possibility and consequences of dark matter as quantum grown black holes are made, with a big crunch fate of the universe.

On Quantum Contributions to Black Hole Growth

We present models of the radiative transfer in circumstellar OH masers. In particular we model the 1612 MHz maser line in OH/IR stars. The coupled radiative transfer in expanding circumstellar shells can be simulated by means of a Monte Carlo technique (Spawns & Van Langevelde 1992). We show that such an approach is relatively advantageous for the radiative transport in masers. The combined effects of saturation and beaming can be observed in such models. In the geometry of expanding shells spectra can be obtained that resemble the average spectra in OH/IR stars quite well.

Monte Carlo models of 1612 MHz OH masers in circumstellar shells

Context: Sticking of colliding dust particles through van der Waals forces is the first stage in the grain growth process in protoplanetary disks, eventually leading to the formation of comets, asteroids and planets. A key aspect of the collisional evolution is the coupling between dust and gas motions, which depends on the internal structure (porosity) of aggregates. Aims: To quantify the importance of the internal structure on the collisional evolution of particles, and to create a new coagulation model to investigate the difference between porous and compact coagulation in the context of a turbulent protoplanetary disk. Methods: We have developed simple prescriptions for the collisional evolution of porosity of grain-aggregates in grain-grain collisions. Three regimes can then be distinguished: `hit-and-stick' at low velocities, with an increase in porosity; compaction at intermediate velocities, with a decrease of porosity; and fragmentation at high velocities. (..) Results: (..) We can discern three different stages in the particle growth process (..) We find that when compared to standard, compact models of coagulation, porous growth delays the onset of settling, because the surface area-to-mass ratio is higher, a consequence of the build-up of porosity during the initial stages. As a result, particles grow orders of magnitudes larger in mass before they rain-out to the mid-plane. Depending on the turbulent viscosity and on the position in the nebula, aggregates can grow to (porous) sizes of ~ 10 cm in a few thousand years. We also find that collisional energies are higher than in the limited PCA/CCA fractal models, thereby allowing aggregates to restructure. It is concluded that the microphysics of collisions plays a key role in the growth process.

/pdf/dust-coagulation-in-protoplanetary-disks-porosity-matters-3gqktkzbtg.pdf

Dust coagulation in protoplanetary disks: porosity matters

We show that ALMA is the first telescope that can probe the dust-obscured central region of quasars at z > 5 with a maximum resolution of ~ 30 pc employing the 18 km baseline.We explore the possibility of detecting the first quasars with ALMA (Schleicher, Spaans, & Klessen 2009). For this purpose, we adopt the Seyfert 2 galaxy NGC 1068 as a reference system and calculate the expected fluxes if this galaxy were placed at high redshift. This choice is motivated by the detailed observations available for this system and the absence of any indication for an evolution in metallicity in high-redshift quasars. It is a conservative choice due to the moderate column densities in NGC 1068, leading to moderate fluxes.

/pdf/detecting-the-first-quasars-with-alma-3g7cohmp2d.pdf

Marco Spaans

Papers

The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591

On Quantum Contributions to Black Hole Growth

Monte Carlo models of 1612 MHz OH masers in circumstellar shells

Dust coagulation in protoplanetary disks: porosity matters

Detecting the First Quasars with ALMA