This volume contains the texts of the invited lectures presented at the NATO Advanced Study Institute ‘Theory of Solar and Planetary Dynamos’ held at the Isaac Newton Institute for Mathematical Sciences in Cambridge from September 20 to October 2 1992. Its companion volume ‘Solar and Planetary Dynamos’, containing the texts of the contributed papers, has recently been published in the same series as the present one, and contains a full list of participants and their addresses. It is a measure of the recent growth of the subject that one volume has proved insufficient to contain all the material presented at the meeting: indeed, dynamo theory now acts as an interface between such diverse areas of mathematical interest as bifurcation theory, Hamiltonian mechanics, turbulence theory, large-scale computational fluid dynamics and asymptotic methods, as well as providing a forum for the interchange of ideas between astrophysicists, geophysicists and those concerned with the industrial applications of magnetohydrodynamics.

The topics of the lectures cover almost all the principal parts of the subject. Authors were asked to give reviews of a pedagogical nature. Earlier chapters cover relatively fundamental aspects of the subject; later chapters treat more specialised topics. Although each chapter is self-contained, there are cross-references to other lectures where appropriate; in addition, the Editors have striven to maintain uniformity of notation and style, in the hope that the resulting complete text will find favour as a unified work of reference, rather than as a disparate set of reviews.

INTRODUCTION

The aim of this chapter is to provide a link between observations of magnetic fields in the Sun and other active stars, and the theory that is presented elsewhere in this volume. I shall begin therefore by considering the observational background and the phenomenological picture that emerges from it. Then I shall go on to discuss a hierarchy of idealized dynamo models that help to explain different aspects of these observations. This material has been the subject of several recent reviews (Weiss 1989, Belvedere 1990, Brandenburg & Tuominen 1991, Stix 1991, DeLuca & Gilman 1991, Rosner & Weiss 1992, Schmitt 1993).

This treatment relies heavily on what has become known as the solarstellar connection. Figure 2.1 shows a Hertzsprung-Russell diagram, with the relative luminosity of the stars plotted as a function of their effective surface temperature (or, equivalently, their spectral type). The stars on the main sequence form a one-parameter family, with their positions determined by their masses. Some of the hot stars to the left of the vertical line have strong magnetic fields, which vary only as a consequence of the star's rotation. The fields in these magnetic stars (the Ap stars) are apparently fossil relics and will not be considered here. Stars to the right of the vertical line are sufficiently cool that hydrogen only becomes ionized beneath their visible surfaces; as a result, they have deep convective envelopes. The combination of convection and rotation is associated with magnetic activity in these cool stars. Their behaviour is similar to that found in the Sun, and it is with them that we are concerned.

PREFACE

Almost 400 hundred years ago Galileo noticed that the period of a pendulum is the same for all small amplitudes. Not long afterwards, Galileo and his contemporaries (see figure 8.1) proved that sunspots really were on the sun. So the same person was involved in discovering the paradigm of periodicity and establishing an exemplar of irregularity. But just how irregularly do sunspots behave? In modern terms, this question comes down to asking how many degrees of freedom are involved in the phenomenon. If the mechanism I am going to describe here, on/off intermittency, is operative, this question cannot be answered soon (Platt, Spiegel & Tresser 1993a). That I should begin this discussion by mentioning aperiodicity is a sign of where we are in the long saga of sunspot studies. Shortly after Galileo's discoveries, serious work on sunspots got under way. This was somewhat disappointing for a time because sunspots had become quite scarce, with only a few per year being detected. This intermission in solar activity lasted approximately throughout the life of Newton, being most extreme when he was in his prime and ending about a decade before his death (Eddy 1978). So the question of the changing level of solar activity must have been much on astronomers' minds at that time. By the time this puzzle was fadinga from memory, a new issue was raised in the middle of the nineteenth century, when it was noticed that the level of solar activity (as judged mainly by sunspots) was found to vary with some regularity.

INTRODUCTION

In the belief that only unkind gods would arrange two energy sources for planetary dynamos as equally important, this re-exploration of plausible sources seeks to eliminate rotational energy in favor of convection. Recent experiments and theory of the ‘elliptical’ instabilities in a rotating fluid due to precessional and tidal strains provide quantitative results for velocity fields and energy production. The adequacy of these flows to produce a. dynamo on both terrestrial and giant planets is assessed in the context of ‘strong field’ scaling. With little ambiguity it is concluded that Mercury, Venus, and Mars can not have a dynamo of tidal or precessional origin. The case for today's Earth is marginal. Here precessional strains (accidentally comparable to tidal strains) also are potential sources of inertial instabilities. The ancient Earth with its closer Moon, as well as all the giant planets, have tides well in excess of those needed to critically maintain dynamos. Hence the project proposed here proves to be successful only in part – an Earth in the distant future will not be able to sustain the geodynamo with its rotational energy. On the other hand, convection remains a possible dynamo energy source, with such a large number of undetermined processes and parameters that it is unfairly easy to establish conditions for its inadequacy. A large literature explores its adequacy. A brief review of this literature, in both a ‘strong-field’ and ‘weak-field’ context, advances several cautionary restraints to be employed on that day when the limits of validity of a quantitative dynamo-convection theory are to be determined.

INTRODUCTION

Many astrophysical bodies possess magnetic fields that arise from dynamo action. The case of the Earth is a unique one because the observational data available are much more detailed for the Earth than for any other astrophysical body, making possible a rather detailed comparison of geodynamo theory with observations. To meet this unique opportunity we therefore need a geodynamo theory that is very detailed. To develop the fully-fledged theory of such a complicated system as the geodynamo, even with the help of modern computers, it is however necessary to possess a qualitative understanding of its structure. This can be achieved by preliminary ‘scouting’ calculations of some artificially simplified models that are much simpler than the full geodynamo model but nevertheless help to understand it. A kinematic dynamo theory is the first step towards this goal. Kinematic models provide us with an understanding of its electrodynamics (the magnetic field generation process). The next necessary step is an understanding of its mechanics. The model-Z geodynamo emerges as a result of this step of scouting calculations. It may be considered as a specific case of a more general model that we call the nonlinear (pseudo-) axisymmetric dynamo model. This is a natural generalisation of the linear, nearly axisymmetric, kinematic dynamo model (Braginsky 1964a, b, c, d), and it is ‘intermediate’ between the kinematic and the complete theories of the geodynamo.

The nonlinear axisymmetric dynamo model aims at understanding the specific features of the main convective flow and the production of axisymmetric field in the core while the field generation due to the non-axisymmetric motion (a-effect) is considered as given. Another direction for an essential ‘intermediate’ investigation is to explore non-axisymmetric magnetoconvection.

This contribution summarises our studies on the emission line profiles from compact Supernova Remnant shells and how they might be related to the broad line profiles in active galaxies. The emphasis is on the theoretical problems associated with radiative transfer effects in spherical and irregularly shaped shells. Line profiles from systems containing many compact remnants are also calculated with the aim of comparing the results to luminous active nuclei, where several remnants are expected to coexist. The observed diversity of profile characteristics in QSOs and the consequences it has to the starburst model are discussed.

Introduction

Line profiles of any astrophysical object, from stellar atmospheres to the Broad Line Region of active galaxies, provide valuable information on the physical and dynamical conditions which may be used to constrain or even reject theoretical models for such objects. Our goal in this work is to develop models for the emission line profiles in compact Supernova Remnants (cSNR) and Active Galactic Nuclei (AGN). The presence of both cSNR and AGN in the same title can only mean that we are talking about the starburst model for AGN of Roberto Terlevich and collaborators (see Terlevich et al. 1992 as well as Franco's, Plewa's and R. Terlevich's papers in this volume). Indeed, the main idea here is to see how well the starburst model does regarding the broad lines in active galaxies.

Collisions of galaxies are often observed to produce increases in the far-IR flux and star formation rates, as compared to those seen in isolated galaxies. It is expected that the star formation taking place in these systems occurs under more violent circumstances than those found in the spiral arms of disk galaxies, for example. We will present some results from a study in which we have produced combined N-body/Smooth Particle Hydrodynamics simulations of collisions of galaxies, looking for regions in which shocks develop and where the gas density gets high enough that new star formation might be expected to take place. These results give us a preliminary idea of what the properties of the shocked, high density regions might be and provide a basis for the future inclusion of violent star formation into such calculations with a view to eventually explaining the observed enhancements.

Introduction

In seeking out places in the present Universe where star formation might be expected to occur under more violent circumstances than those that occur in the arms of spiral galaxies one is drawn to consider the interiors of gas-rich galaxies that are undergoing a collision with another galaxy. Although not extremely common at this epoch, such systems of galaxies have been detected and extensively studied during recent years. There are now considerable data available to suggest that these systems commonly experience increased star formation, this taking place most often in the nuclei of the disk galaxies, rather than in their spiral arms.

The star formation history in the nuclei of late-type spiral galaxies is compared between a sample in a high galaxy density medium (HDS) and a control sample (CS) of isolated galaxies. We have observed 20 HDS and 18 CS galaxies selected from a larger list generated by the application of a group-finding algorithm to the SSRS survey. Using equivalent widths of absorption lines and the continuum distribution, we determined the nuclear stellar population types, from those dominated by old populations to those containing star formation bursts of different ages and intensities. The HDS and CS stellar population type histograms are similar, suggesting that environmental influences, at least for the present samples, do not substantially affect the nuclear stellar population. However, the nuclear emission lines indicate that, in the BPT diagnostic diagrams, there is an excess of HDS galaxies located within or close to the AGN loci. For 6 HDS and 2 CS galaxies it was possible to determine Oxygen (O/H) and Nitrogen (N/H) abundances. The samples present similar (O/H) values, but in the CS galaxies the (N/O) ratio is lower at equal galaxy luminosity.

Introduction

Evidence of several environmental effects that affect galaxy properties have been reported recently: (a) the morphology-density relation (fractional increase of early-type galaxies towards regions of high concentration, Dressier 1980; Postman & Geller 1984; Giovanelli, Haynes & Chincarini 1986; Maia & da Costa 1990); (b) the morphology-clustercentric radius relation (Whitmore, Gilmore & Jones 1993).

The vertical distribution of molecular complexes along the Carina-Sagittarius arm has been studied on the basis of the giant molecular cloud (GMC) data compiled by Myers et al. (1986). The analysis indicates that the CO complexes are preferentially located below the formal Galactic plane. A separation of the sample into two groups: (a) GMCs associated with HII regions, and (b) GMCs without associated HII regions, establishes that group (a) shows, in average, larger z departure and mass than group (b). This result seems to suggest that the star formation activity in this major arm displays a vertical asymmetry which opens up interesting questions about the triggering mechanisms of star formation in spiral arms.

The density and location of young stars in major spiral arms and the relation to their parent molecular clouds are important to the understanding of how molecular clouds evolve and form stars in our Galaxy. In previous work (Alfaro et al. 1992, 1993) we analyzed the vertical structure of young open clusters (YOCs) along the optical segment of the Carina-Sagittarius arm, and its connection with the density of YOCs as representative of star formation activity. The main conclusions of that work can be summarized as follows:

1. A clear correlation between YOC density and z-departure from the formal Galactic plane is found when this density distribution is compared with the vertical structure. The cores of both supercomplexes are closely coincident with the two minima of the vertical profile, and the regions of lowest star-forming tracers appear associated with the relative maximum of z.

Super-associations and star complexes

The concept of a “superassociation” was first introduced by Baade (1963) in his Harvard lectures in 1958. He gave this name to a region about 500 pc across around the giant HII region 30 Dor in the LMC which is full of OB-associations. The same region was the first example of a super-association given by Ambartsumian (1964). Altogether, 19 OB-associations and young clusters here form a morphological unit of 1 kpc in diameter with evident hierarchical structure (Efremov 1988, 1989).

One may say that a “super-association” is the counterpart of a hydrogen emission nebula (HII region) in B, V etc. broad bands. Wray and de Vaucouleur (1980) have shown that in the B bandpass the continuum-to-emission ratio is always greater than 10:1. Thus in this band one deals mainly with the star population of a super-association.

Nevertheless, the diameter of the 30 Dor HII region is only 250 pc and it occupies less than 0.1 of the total area of the super-association, the remaining HII regions here being much smaller. This may well also be the case for extragalactic super-associations – giant HII regions. In bright star cloud NGC 206 in M31, named by Baade (1963) as a real super-association, there are only a few small HII regions, not seen at all on the B plates.

In many respects super-associations (SAs) are similar to common star complexes (Efremov 1978, 1993), the main difference being the richness of an SA in HII gas and OB stars that causes the high total luminosity.

As part of the GEFE collaboration, observations of star-forming regions with high spectral resolution and long-slit sampling are being undertaken. 2D maps of physical parameters like density, excitation, extinction…etc. have been produced with 1″ spatial resolution and 2″ spatial sampling. Some preliminary results on the giant HII Region NGC 5471 and the irregular galaxy NGC 4214 are presented. Very high velocity components have been detected at some particular positions on the nebulae, as well as other peculiar kinematical structures (redshifted secondary emission peaks, line splitting etc.). The whole emitting area of NGC 5471 behaves as a unique entity with respect to excitation, with no correlation with the emitting knots. On the other hand, differentiated star-forming regions can be identified on NGC 4214. Finally, the distribution of dust particles seems to be rather inhomogeneous and anticorrelated with the distribution of emission-line intensities.

Introduction: aim and targets

The ultimate aim of the GEFE collaboration is to determine which are the physical parameters that control the formation of a violent burst of star formation. Within this framework and in order to fulfil this main objective it is important to know the physical properties of star-forming regions with high enough spatial resolution as to determine variations of the measurable parameters within the emitting nebulae. We aim to use measurements of age, excitation degree, velocity dispersion and chemical composition to know whether we are dealing with single star-forming regions or with well differentiated physical entities within a patch of ionized gas, which cause misinterpretation in our understanding of the “physical object” (Muñoz-Tuñón et al. 1993).

There are several questions in the field of the study of giant extragalactic HII regions (GEHRs) that remain controversial. The origin of the observed supersonic motions and the validity of the relationship between size and velocity dispersion obtained on the base of single-aperture observations are still debated. From a purely observational point of view, the spatial extent of kinematical features within the nebula, the sense in assuming Gaussian profiles as representative of the variety of emision lines found in GEHR, and finally how lawful the assignment of a single value (σ, R) as a defining kinematical parameter of a GEHR are also questioned. The energy input required to provide the supersonic motions observed in GEHRs has been the subject of several scenarios proposed in the literature and some of the points in favour and/or against them are mentioned in this contribution.

Bidimensional spectroscopy with good spatial and spectral resolution, sampling of a particular emision line over the whole emitting area, is shown to be the most suitable observational technique for understanding the global kinematics of GEHRs. In NGC 604 for instance, features like loops and filaments although clearly seen in deep exposures and obviously resulting from stellar winds and SN explosions, do not dominate the bulk of the emission that comes from smaller but much brighter areas. Thus the total flux from split or non-Gaussian profiles that arise from shells and/or filaments is much lower than that obtained from bright knots where line profiles are well fitted by Gaussians.

We present long-slit optical and near-infrared spectroscopy of the giant HII region NGC 4236III. We have found broad emission lines at 4686 Å attributed to WR stars. We have derived the physical conditions and chemical composition of the nebula.

Introduction

HII regions are one of the most useful tools to study the properties of massive stars as well as the physical conditions and chemical composition of the interstellar medium. One of the target of the GEFE programme is the giant HII region NGC 4236III located in the outskirts of the SBd galaxy NGC 4236. The object was observed with the 4.2-m WHT telescope in La Palma, using the blue and the red arms of the ISIS spectrograph and an EEV CCD in each arm. The dispersion was 1.4 Åpix−1, and the spatial sampling 0.33 arcsec pix−1.

Results

The emission of the region is extended over 15 arcsec. Three different spectra were extracted (A, B, C). These spectra are typical of high-excitation HII regions. In B (Figure la), where most of the continuum emission is concentrated, we have detected a broad emission bump at 4686 Å which is attributed to WR stars.

The distribution of the emission lines Hα, [OIII], [OII], [NII] and [SIII] are quite similar; they show a maximum at 1.5 arcsec to the North of the peak of the continuum distribution.

We studied the content of massive stars in the nearby HII region 30 Doradus and in the distant nuclear starburst NGC 7552. Ultraviolet imaging and ultraviolet spectroscopy with the Hubble Space Telescope and the International Ultraviolet Explorer have been obtained. The observational data have been compared with evolutionary population synthesis models in an attempt to constrain the star-formation history and mass spectrum of the two starburst regions. Despite the different physical and chemical conditions, the high-mass end of the mass functions in 30 Dor and NGC 7552 are remarkably similar.

30 Doradus and NGC 7552 — basic properties

30 Doradus in the Large Magellanic Cloud is the closest giant extragalactic HII region (GEHR). The combination of its relative proximity and its location outside the plane of our Milky Way makes 30 Dor a prime laboratory for the study of the formation and evolution of the most massive stars. Walborn (1991) reviewed the basic properties of 30 Dor, designating it the “Starburst Rosett”. 30 Dor is metal-deficient with respect to the Sun (Z = 0.3 Z⊙) and has only a moderate interstellar reddening of E(B − V) = 0.4. At a distance of 0.05 Mpc, 1″ corresponds to a linear distance of 0.25 pc. The most massive stars are concentrated within a few arcsec of the center of 30 Dor.

Recent observations of X-ray spectra of AGNs can be used to compare the “classical” black-hole type model for such objects with the starburst scenario emphasized in this meeting. The most important aspects of the observations are the commonly observed soft X-ray absorption, the Kα line profile and intensity, the X-ray variability and the hard X-ray cut-off. Modelling the observed X-ray spectrum requires an understanding of the absorption, emission and scattering properties of neutral and ionized gases. Examples from new calculations, including all these components, are shown and compared with the observations. Progress made on the observational and theoretical sides seem to give at least some satisfactory answers to previously open questions of the black-hole model. It remains to be seen whether the starburst model can come up with an equally good explanation.

Introduction

The purpose of this review is to summarize some well known facts, as well as new X-ray observations of various types of AGNs, and to confront them with suggested models. Since X-ray properties involve emission, absorption and scattering it is useful to stop, at each sub-section, and ask the question, What are the physical properties of the emitting, absorbing and scattering material? These are related to the location of the various components, their motion and other properties. They can provide an answer to the central question posed in this meeting, about the relationship between “classical” AGNs, hosting a giant black-hole, and violent star-forming events in nuclei of galaxies.

Near-infrared spectral imaging observations of the starburst galaxy NGC 1808 and of the Seyfert galaxy NGC 1068 are briefly discussed.

Introduction

Most of the presentations at this meeting have focussed on optical, ultraviolet, and X-ray observations of starburst galaxies (SBGs) and active galactic nuclei (AGN) and their interpretation. In this contribution I draw attention to the utility of infrared array spectroscopy and millimeter-wave interferometry to the study of energetic galaxy nuclei.

Infrared spectral observations are useful because they probe objects with large internal or foreground extinctions. Many interstellar sources such as photon-dominated regions in molecular clouds or non-dissociative shocks release energy at primarily infrared wavelengths. Millimeter spectroscopy provides information about the molecular medium which is not readily observable at optical, UV or X-ray wavelengths.

In this article I discuss infrared observations of the starburst galaxy NGC 1808 and of the Seyfert galaxy NGC 1068 carried out by members of the MPE group (Blietz et al. 1994; Krabbe, Sternberg & Genzel 1994; Taconni et al. 1994). Most of this work was carried out using the MPE infrared array spectrometer FAST (Krabbe et al. 1993).

NGC 1808

NGC 1808 is a nearby (10.9 Mpc, for H0 = 75 km s−1 Mpc−1) morphologically peculiar spiral galaxy (Sersic and Pastoriza 1965). Optical images show that several dust filaments protrude from the nucleus out into the galactic halo.