Christopher F. McKee

Dust Destruction in the Interstellar Medium

Grains are injected into the interstellar medium (ISM) from evolved stars and supernovae; in addition, supernova ejecta may condense onto pre-existing grains before becoming well-mixed with the interstellar gas. Once in the ISM, grains can grow by accretion, but are also subject to destruction by interstellar shocks, The current status of the theory of shock destruction of interstellar grains is reviewed briefly. Small grains are destroyed by thermal sputtering in fast, non-radiative shocks; large grains are destroyed by grain-grain collisions and eroded by nonthermal sputtering in radiative shocks. The dominant shocks in the ISM are from supernova remnants (SNRs), and the mass of grains destroyed is proportional to the energy of the SNR. In a multiphase ISM, these shocks destroy the grains at a rate proportional to the volume filling factor of the phase; since the density of the hot phase is too low for efficient grain destruction, most of the destruction occurs in the warm phase. Not all SNRs are effective at destroying grains, however: some are above the gas disk, and some —Type IPs in associations—are highly correlated in space and time. The galactic SN rate is observed to about 2.2 per century (van den Bergh, 1983), but the effective supernova rate for grain destruction is estimated to be only about 0.8 per century. As a result, the timescale for the destruction of a typical refractory grain in the ISM is inferred to be about 4 × 108 yr for either a two-phase or a three-phase ISM. Most of the refractory material in the ISM (other than carbon) is injected by supernovae, not evolved stars; the net injection timescale is estimated as about 1.5 x 109 yr. Comparison of the destruction and injection timescales indicates that the fraction of grains injected by stars which survive in the ISM is only about 20%. Most of the refractory material in interstellar grains must, therefore, have accreted onto the grains in the ISM. Nonetheless, a significant fraction of dust formed in stars survives in the ISM and may be detectable in meteorites and interplanetary dust particles.

The Dynamical Structure and Evolution of Giant Molecular Clouds

The interstellar medium (ISM) of galaxies contains gas that spans a wide range of physical conditions, from hot X-ray emitting plasma to cold molecular gas. The molecular gas is of particular importance because it is believed to be the site of all the star formation that occurs in galaxies. In the Milky Way, molecular gas constitutes about half the total mass of gas within the solar circle. Much of this gas is concentrated in large aggregations called giant molecular clouds (GMCs), which have masses M ≳ 104 M⊙. Smaller molecular clouds are also observed, such as the high latitude clouds discovered by Blitz et al. [9] and the small molecular clouds in the Galactic plane cataloged by Clemens & Barvainis [19]. GMCs have internal structure, and I shall follow the terminology of Williams et al. [114] in describing this: Clumps are coherent regions in longitude-latitude-velocity space that are generally identified from spectral line maps of molecular emission. Starforming clumps are the massive clumps out of which stellar clusters form. Finally, cores are the regions out of which single stars (or multiple stellar systems like binaries) are formed. These characteristics, together with the important observational properties of GMCs, are reviewed elsewhere in this volume by Blitz. In this review I shall first briefly summarize some of the key properties of GMCs and then attempt to account for the dynamical properties theoretically.

Infrared Spectroscopy of Interstellar Shocks

Infrared emission lines from interstellar shocks provide valuable diagnostics for violent events in the interstellar medium, such as supernova remnants and mass outflow from young stellar objects. There are two types of interstellar shocks: In J shocks, gas properties “jump” from their preshock to their postshock values in a shock front with a thickness 50 km/s) in molecular gas. In C shocks, gas is accelerated and heated by collisions between charged particles, which have a low concentration and are coupled to the magnetic field, and neutral particles; radiation is generated throughout the shock and is emitted almost entirely in infrared emission lines. Such shocks occur in weakly ionized molecular gas for shock velocities below about 50 km/s.

Gravitational Collapse and Fragmentation in Molecular Clouds with Adaptive Mesh Refinement Hydrodynamics

We describe a powerful methodology for numerical solution of 3-D self-gravitational hydrodynamics problems with extremely high resolution. This code utilizes the technique of local adaptive mesh refinement (AMR), employing multiple grids at multiple levels of resolution. These grids are automatically and dynamically added and removed as necessary to maintain adequate resolution. This technology allows for the solution of problems in a manner that is both more efficient and more versatile than other fixed and variable resolution methods. The application of this technique to simulate the collapse and fragmentation of a molecular cloud, a key step in star formation, is discussed. Such simulations involve many orders of magnitude of variation in length scale as fragments form. In this paper we briefly describe the methodology and present an illustrative application for nonisothermal cloud collapse. We describe the numerical Jeans condition, a criterion for stability of self-gravitational hydrodynamics problems. We show the first well-resolved nonisothermal evolutionary sequence that leads to the formation of a binary system consisting of protostellar cores surrounded by distinct protostellar disks. The scale of the disks, of order 100 AU, is consistent with observations of gaseous disks surrounding single T-Tauri stars and debris disks surrounding systems such as β Pictoris.

Accretion onto magnetized neutron stars: Magnetospheric structure and stability

A summary is given of the magnetospheric physics of accreting magnetized neutron stars. The role of hydromagnetic instabilities is described, with particular attention to the formation of polar caps in spherical and disk on, and to the origin of spin down torques in a centrigugally driven stellar wind, when the accretion is from a disk. The most elementary consequences for formation of the emergent spectrum are also discussed.

A shock origin for interstellar H2O masers

We present a comprehensive model for the powerful H2O masers observed in starforming regions. In this model the masers occur behind dissociative shocks propagating in dense regions (preshock density n o ≈ 106 – 108 cm−3). This paper focuses on high-velocity (ν s ≳ 30 km/s) dissociative shocks in which the heat of H2 reformation on dust grains maintains a large column of ≈ 300 – 400 K gas, where the chemistry drives a considerable fraction of the oxygen not in CO to form H2O . The H2O column densities, the hydrogen densities, and the warm temperatures produced by these shocks are sufficiently high to enable powerful maser action, where the maser is excited by thermal collisions with H atoms and H2 molecules. A critical ingredient in determining the shock structure is the magnetic pressure, and the fields required by our models are in agreement with recent observations. The observed brightness temperatures (generally ≈ 1011 – 1014 K) are the result of coherent velocity regions which have dimensions in the shock plane that are 5 to 50 times the postshock thickness.

Magnetic Fields in Dense Regions

This paper concentrates on reviewing magnetic fields in dense regions; see Heiles (1988, 90) for a review of fields in diffuse regions. The past few years have increased our observational knowledge of magnetic fields in dense regions by an enormous factor—not because there are many measurements, but because we started from zero. The observable is polarization, which is small and subject to systematic errors. The fact that the advances have occurred only recently is a result of several factors: technological development; the interest and commitment of experimentally-minded astronomers; and the maturing of molecular and infrared astronomy to the point that really new results require either new insights or more difficult techniques

The Evolution of Supernova Remnants and the Structure of the Interstellar Medium

Supernova remnants (SNRs) play a key role in astrophysics. Young SNRs inject the nucleosynthesis products of supernova explosions into the interstellar medium (ISM). SNRs are thought to produce most of the cosmic rays and the nonthermal radio emission in the Galaxy (Blandford 1982). They are also one of the major energy sources for the medium and thereby affect its structure and ultimately the star formation process which leads to supernovae.

Supernova Remnant Shocks in an Inhomogeneous Interstellar Medium

The inhomogeneity of the interstellar medium (ISM) has a profound effect on the propagation of the interstellar shock generated by a supernova and on the appearance of the resulting supernova remnant (SNR). Low mass supernovae produce remnants that interact with the “pristine” ISM, which has density inhomogeneities (clouds) on a wide range of scales. The shock compresses and accelerates the clouds it encounters; inside the blast wave, the clouds are hydrodynamically unstable, and mass is injected from the clouds into the intercloud medium. Embedded clouds interact thermally with the shock also, adding mass to the hot intercloud medium via thermal evaporation or subtracting it via condensation and thermal instability. Mass injection into the hot intercloud medium, whether dynamical or thermal, leads to infrared emission as dust mixes with the hot gas and is thermally sputtered. The remnants of massive supernovae interact primarily with circumstellar matter and with interstellar material which has been processed by the ionizing radiation and wind of the progenitor star. After passing through any circumstellar material which may be present, the shock encounters a cavity which tends to “muffle” the SNR. The remnants of massive supernovae therefore tell us more about the late stages of the evolution of massive stars than about the ISM.

X-Ray Emission from Supernova Remnants in a Cloudy Medium

Young SNRs expand to much larger radii in a cloudy ISM than in a homogeneous medium, and they can have large variations in the pressure. The collision between supernova ejecta and an ambient cloud can result in an expanding high pressure region (a “secondary blast wave”). Observations of MSH 15–52 can be accounted for in this manner. X-ray emission from both young and older SNRs can provide an important probe for inferring the structure of the ISM.