This is a comprehensive and richly illustrated textbook on the astrophysics of the interstellar and intergalactic medium--the gas and dust, as well as the electromagnetic radiation, cosmic rays, and magnetic and gravitational fields, present between the stars in a galaxy and also between galaxies themselves. Topics include radiative processes across the electromagnetic spectrum; radiative transfer; ionization; heating and cooling; astrochemistry; interstellar dust; fluid dynamics, including ionization fronts and shock waves; cosmic rays; distribution and evolution of the interstellar medium; and star formation. While it is assumed that the reader has a background in undergraduate-level physics, including some prior exposure to atomic and molecular physics, statistical mechanics, and electromagnetism, the first six chapters of the book include a review of the basic physics that is used in later chapters. This graduate-level textbook includes references for further reading, and serves as an invaluable resource for working astrophysicists. * Essential textbook on the physics of the interstellar and intergalactic medium * Based on a course taught by the author for more than twenty years at Princeton University * Covers radiative processes, fluid dynamics, cosmic rays, astrochemistry, interstellar dust, and more * Discusses the physical state and distribution of the ionized, atomic, and molecular phases of the interstellar medium * Reviews diagnostics using emission and absorption lines * Features color illustrations and detailed reference materials in appendices * Instructor's manual with problems and solutions (available only to teachers)

Physics of the Interstellar and Intergalactic Medium

A new, comprehensive set of low-temperature opacity data has been assembled. From this basic data set, Rosseland and Planck mean opacities have been computed for temperatures between 12,500 and 700 K. In addition to the usual continuous absorbers, atomic line absorption (with more than 8 million lines), molecular line absorption (with nearly 60 million lines), and grain absorption and scattering (by silicates, iron, carbon, and SiC) have been accounted for. The absorption due to lines is computed monochromatically and included in the mean with the opacity sampling technique. Grains are assumed to form in chemical equilibrium with the gas and to form into a continuous distribution of ellipsoids. Agreement of these opacities with other recent tabulations of opacities for temperatures above 5000 K is excellent. It is shown that opacities which neglect molecules become unreliable for temperatures below 5000 K. Triatomic molecules become important absorbers at 3200 K. Similarly, grains must be included in the computation for temperatures below 1700 K.

Low Temperature Rosseland Opacities.

Cloud environment is thought to play a critical role in determining the mechanism of formation of massive stars. In this contribution we review the physical characteristics of the environment around recently formed massive stars. Particular emphasis is given to recent high angular resolution obser- vations which have improved our knowledge of the physical conditions and kinematics of compact regions of ionized gas and of dense and hot molecular cores associated with luminous O and B stars. We will show that this large body of data, gathered during the last decade, has allowed signiÐcant progress in the understanding of the physical processes that take place during the formation and early evolution of massive stars.

https://www.jstor.org/stable/10.1086/316416

Massive Stars : Their Environment and Formation

New deep narrowband images of the Orion Nebula obtained with WFPC2 on the Hubble Space Telescope (HST) and spectra taken with the HIRES spectrometer at the Keck Observatory are presented. We report eight new circumstellar disks seen in silhouette against the background nebular light and about 30 dark disks embedded within the bright proplyds rimmed by ionization fronts. Deep narrowband λ6300 A images reveal skins of glowing [O I] emission associated with several disks embedded within bright proplyds. [O I] emission also surrounds one dark disk not surrounded by an ionization front; this object may be embedded within the photon-dominated, mostly neutral region behind the ionization front of the Orion Nebula. The intensity and morphology of the [O I] emission provides support for the photon-dominated–region models of externally irradiated circumstellar disks in which soft UV powers photoablation of the disk surface. Dozens of outflows powered by young stars have been discovered on the new images. More than 20 stellar jets emerge from the externally irradiated circumstellar disks or their associated young stars embedded within the Nebula. Most are one-sided (monopolar) subarcsecond-scale microjets, too small to be seen on ground-based images against the bright background nebular light. Additionally, wide-angle winds from 10 young stars in the outskirts of the Nebula power large-scale bow shocks facing the Trapezium OB stars. These shocks may be produced by wind-wind interactions where the T-Tauri winds interact with the outflow of plasma from the core of M42. The largest such structure, associated with the star LL Ori, contains a number of compact high–proper-motion clumps moving almost tangentially to the bow shock. The new data are combined with older HST images to determine proper motions for many nebular features. Neither the LL Ori type bow shocks in the outskirts of the nebula nor the Hα + [O III] arcs that surround many proplyds near the Trapezium show measurable proper motions and are therefore stationary structures. However, most other bow-shaped features not centered on young stars exhibit large proper motions, with velocities ranging from 50 to 300 km s-1. The sources of many of these moving features remain unknown. The proper-motion survey of the nebular core reveals the presence of about a dozen new large-scale (>0.1 pc) outflow complexes. Many of these new outflows originate from the vicinity of the high-luminosity OMC-1S infrared and submillimeter source complex located southwest of the Trapezium. These supersonic features provide evidence that stellar outflows inject large amounts of kinetic energy into the nebula. However, a quantitative analysis indicates that their total power is small compared with the power in the plasma flowing away from the main nebular ionization front.

Disks, Microjets, Windblown Bubbles, and Outflows in the Orion Nebula*

The SILCC (SImulating the Life-Cycle of molecular Clouds) project aims to self-consistently understand the small-scale structure of the interstellar medium (ISM) and its link to galaxy evolution. We simulate the evolution of the multiphase ISM in a (500 pc)2 × ±5 kpc region of a galactic disc, with a gas surface density of ΣGAS=10M⊙pc−2. The flash 4 simulations include an external potential, self-gravity, magnetic fields, heating and radiative cooling, time-dependent chemistry of H2 and CO considering (self-) shielding, and supernova (SN) feedback but omit shear due to galactic rotation. We explore SN explosions at different rates in high-density regions (peak), in random locations with a Gaussian distribution in the vertical direction (random), in a combination of both (mixed), or clustered in space and time (clus/clus2). Only models with self-gravity and a significant fraction of SNe that explode in low-density gas are in agreement with observations. Without self-gravity and in models with peak driving the formation of H2 is strongly suppressed. For decreasing SN rates, the H2 mass fraction increases significantly from <10 per cent for high SN rates, i.e. 0.5 dex above Kennicutt–Schmidt, to 70–85 per cent for low SN rates, i.e. 0.5 dex below KS. For an intermediate SN rate, clustered driving results in slightly more H2 than random driving due to the more coherent compression of the gas in larger bubbles. Magnetic fields have little impact on the final disc structure but affect the dense gas (n ≳ 10 cm−3) and delay H2 formation. Most of the volume is filled with hot gas (∼80 per cent within ±150 pc). For all but peak driving a vertically expanding warm component of atomic hydrogen indicates a fountain flow. We highlight that individual chemical species populate different ISM phases and cannot be accurately modelled with temperature-/density-based phase cut-offs.

The SILCC (SImulating the LifeCycle of molecular Clouds) project – I. Chemical evolution of the supernova-driven ISM

We study the evolution of the interstellar and circumstellar media around massive stars (M {>=} 40 M{sub sun}) from the main sequence (MS) through to the Wolf-Rayet (WR) stage by means of radiation-hydrodynamic simulations. We use publicly available stellar evolution models to investigate the different possible structures that can form in the stellar wind bubbles around WR stars. We find significant differences between models with and without stellar rotation, and between models from different authors. More specifically, we find that the main ingredients in the formation of structures in the WR wind bubbles are the duration of the red supergiant (or luminous blue variable) phase, the amount of mass lost, and the wind velocity during this phase, in agreement with previous authors. Thermal conduction is also included in our models. We find that MS bubbles with thermal conduction are slightly smaller, due to extra cooling which reduces the pressure in the hot, shocked bubble, but that thermal conduction does not appear to significantly influence the formation of structures in post-MS bubbles. Finally, we study the predicted X-ray emission from the models and compare our results with observations of the WR bubbles S 308, NGC 6888, and RCW 58. We findmore » that bubbles composed primarily of clumps have reduced X-ray luminosity and very soft spectra, while bubbles with shells correspond more closely to observations.« less

https://iopscience.iop.org/article/10.1088/0004-637X/737/2/100/pdf

Radiation-hydrodynamic models of the evolving circumstellar medium around massive stars

We investigate whether simple models of a photoevaporated flow from an externally ionized neutral clump or wind can reproduce the observed Hα intensity profiles of the proplyds in the inner Orion Nebula. We find that models fitted to the bright "cusp" at the head of each proplyd successfully predict the brightness distribution of the extended diffuse "atmosphere" between the proplyd and the ionizing star of the nebula (θ1 Ori C). This is strong evidence that the "atmospheres" are not confined but are freely expanding photoevaporated flows, with implied mass-loss rates of up to 10-7 M⊙ yr-1. The only way to reconcile such high mass-loss rates with the age of the nebula, the high fraction of stars that are proplyds, and the low extinction to these stars is for the neutral reservoir to be in the form of a flattened disk. The model fits imply that the initial Mach number of the flow from the ionization front is close to unity, which is consistent with theoretical predictions that such fronts should be approximately D-critical. The dust-to-gas ratio in the flow is not tightly constrained by the models. The distribution of projected radii of sources in the inner Trapezium cluster indicates that (1) both the proplyds and the nonproplyd stars in the cluster are strongly peaked about θ1 Ori C on a 0.02 pc scale, much smaller than has previously been suggested, and (2) the fraction of low-mass stars that are proplyds probably decreases smoothly with increasing physical distance from θ1 Ori C. Both these statements are supported by the distribution of inclination angles of the best-fit proplyd models, which allow the three-dimensional distribution of proplyds to be constrained.

Modeling the Brightness Profiles of the Orion Proplyds

The three-dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and newly established data. We find that the widely accepted model of a concave blister of ionized material needs to be altered in the southwest direction from the Trapezium, where we find that the Orion-S feature is a separate cloud of very optically thick molecules within the body of ionized gas, which is probably the location of the multiple embedded sources that produce the optical and molecular outflows that define the Orion-S star formation region. Evidence for this cloud comes from the presence of H2CO lines in absorption in the radio continuum and discrepancies in the extinction derived from radio-optical and optical-only emission. We present an equilibrium Cloudy model of the Orion-S Cloud, which successfully reproduces many observed properties of this feature, including the presence of gas-phase H2CO in absorption. We also report the discovery of an open-sided shell of [O III] surrounding the Trapezium stars, revealed through emission-line ratio images and the onset of radiation shadows beyond some proplyds. We show that the observed properties of the shell are consistent with it being a stationary structure, produced by shock interactions between the ambient nebular gas and the high-velocity wind from θ1 Ori C. We examine the implications of the recently published evidence for a large blueshifted velocity of θ1 Ori C with respect to the Orion molecular cloud, which could mean that this star has only recently begun to photoionize the Orion Nebula. We show that current observations of the nebula do not rule out such a possibility, so long as the ionization front has propagated into a pre-existing low-density region. In addition, a young age for the nebula would help explain the presence of nearby proplyds with a short mass-loss timescale to photoablation.

/pdf/the-three-dimensional-dynamic-structure-of-the-inner-orion-sci9wqnec0.pdf

The three-dimensional dynamic structure of the inner orion nebula*

Radiation-hydrodynamic Models of the Evolving Circumstellar Medium around Massive Stars

We present hydrodynamical simulations of the photoevaporation of a cloud with large-scale density gradients, giving rise to an ionized, photoevaporation flow. The flow is found to be approximately steady during the large part of its evolution, during which it can resemble a "champagne flow" or a "globule flow" depending on the curvature of the ionization front. The distance from source to ionization front and the front curvature uniquely determine the structure of the flow, with the curvature depending on the steepness of the lateral density gradient in the neutral cloud. We compare these simulations with both new and existing observations of the Orion nebula and find that a model with a mildly convex ionization front can reproduce the profiles of emission measure, electron density, and mean line velocity for a variety of emitting ions on scales of 10^{17} to 10^{18} cm. The principal failure of our model is that we cannot explain the large observed widths of the [O I] 6300 Angstrom line that forms at the ionization front.

Sarah Jane Arthur

Papers

Radiation-hydrodynamic models of the evolving circumstellar medium around massive stars

Modeling the Brightness Profiles of the Orion Proplyds

The three-dimensional dynamic structure of the inner orion nebula*

Radiation-hydrodynamic Models of the Evolving Circumstellar Medium around Massive Stars

Photoevaporation Flows in Blister HII Regions: I. Smooth Ionization Fronts and Application to the Orion Nebula