This chapter reviews the details of the photometric data acquired by the Hipparcos satellite, and the scientific results based on the large body of homogeneous photometric data, including variability, provided in the Hipparcos and Tycho Catalogues.

Hipparcos and Tycho photometric data

A description of the photometric measurements and results from Hipparcos is given by van Leeuwen et al (1997a), from where the following details of the data reduction methods used, the variability analysis, and the data products, are extracted. Further descriptions of all these aspects can be found in the Hipparcos and Tycho Catalogues (ESA, 1997), in particular Volume 1, Section 1.3 (description of the photometric data), Volume 3, Chapters 14 and 21 (description of the data reduction methods and the verification of the results), and Volume 4, Chapters 8 and 9 (the Tycho photometric data analysis).

The main Hipparcos passband, Hp, resulted purely from an attempt to maximise the number of photons gathered in the astrometric measurements, with no consideration of astrophysical features. Photometric information was contained in both the mean intensity and the modulation amplitude of the signal as the star image passed across the main modulating grid (Figure 1.2, left). The Hp passband was defined by the spectral response of the S20 photocathode of the image tube detector, combined with the transmission of the optics. The large width of the Hp passband results in significant systematic differences between Hp and standard V magnitudes, depending on effective temperature (or colour), metallicity and interstellar extinction.

The high-precision Hp magnitudes combined with homogeneous whole-sky coverage make the Hp magnitudes a very important mission product, with the multiepoch measurements providing an important database for variability studies (Table 4.3 below).

Introduction

In this contribution, we will take a broad view of active galactic nuclei (AGNs), but concentrating on emission-line phenonema. As a result, some interesting topics such as blazars and jets, connections with starbursts, and the environments of AGNs will receive little attention, although these are important topics and are discussed at length elsewhere.

It is useful to begin a discussion of AGNs with some history of the subject, partly because the history of how a scientific field is launched and develops over time is interesting and instructive, but primarily because it gives us some insight into the observed properties of AGNs, which is essential for distinguishing them from other objects in the sky. As we introduce the observational properties of these sources, at least the basis of the sometimes complicated taxonomy we use to describe active galaxies will become clear. An underlying theme throughout this discussion will be the principle of “AGN unification,” which posits that the diverse taxonomy of AGNs has more to do with observational circumstances, such as inclination and obscuration effects, than with intrinsic physical differences among various types of AGN. Considerable effort has been expended in attempts to explain the broadest range of phenomena with the most-limited range of physical mechanisms and structures.

The discovery and nature of active galaxies

The word “activity” in connection with phenomena in the nuclei of galaxies appears to have originated with Ambartsumian (1968).

Introduction

The imaging method which uses narrow-band filters is the first and most direct way to obtain information about the emission lines. In contrast to broad-band photometry, in which several emission lines are integrated within one filter, the narrow-band filters are designed to transmit only one emission line (in ideal cases).

This method has considerable advantages over spectroscopic methods: we are able to obtain spatial resolution, i.e. several objects fit in only one exposure frame; the required time for an observation run is shortened; and lastly, the observation procedures and reduction are less complicated than those of a spectroscopic survey. Usually during a run information for only a few lines (three or four at best) can be obtained, in contrast to the full spectrum obtained in spectroscopy: see, for example, the works of Belley & Roy (1992), where hundreds of HII regions are presented with fluxes in three emission lines using direct imaging, and van Zee et al. (1998), with only about a dozen HII regions in ten different emission lines employing optical spectroscopy.

Narrow-band imaging is a powerful tool for characterizing the physical properties of the star-forming regions in nearby galaxies. With a few lines, we are able to determine, for example, the abundance, temperature, or initial mass function of the ionizing stars, thus obtaining clues to the physical processes that occur in the core of the HII regions; see, for example, Cedrés & Cepa (2002). This can be seen in Figure 8.1, where the composite broad-band image (with the B, R, and K bands) and an Hα image of the galaxy UCM 2325+2318 can be compared. The star-forming regions are clearly defined in the narrow-band image.

Introduction

The goal of emission-line surveys is to identify a preselected class of astrophysical sources by means of emission lines in their spectral energy distribution. The techniques vary, depending on the sources, the spectral region and even the exact wavelength at which the observations are made, the desired sensitivity of the survey, and the volume of space that it aims to cover. But the basic methodology remains the same: exploiting the fact that emission lines in the spectrum of astronomical sources radiate significantly more luminosity over a relatively small wavelength interval than the continuum emission does over the same, or even larger, intervals in nearby portions of the spectrum. The excess luminosity, quantitatively expressed as the equivalent width of the line, enhances the signal-to-noise ratio of measures of the line flux for those sources whose emission lines satisfy the selection criteria of the survey, allowing the observer to cull them from otherwise similar sources.

Operationally, emission-line surveys exploit the presence of emission lines to substantially improve the detection rate of a particular class of astrophysical sources for which other methods of investigation would be significantly more inefficient or even outright impossible. The typical targets of these surveys are either sources that are too faint to detect by means of their continuum emission or sources that are rare and/or inconspicuous, and hence very difficult to recognize from all the other sources of similar apparent luminosity and/or morphology that crowd images made using the continuum emission.

The powerful outflows from pre-main-sequence stars are now understood as a general byproduct of disk accretion. The relation between mass accretion rates and mass loss rates now spans several orders of magnitude by connecting the T Tauri stars with the FU Ori objects. The bipolar nature of these outflows, which begin in the earliest stages of star formation, clearly points to a disk origin. With mass ejection rates of order 10% of the disk accretion rates, outflows represent perhaps as much as half of the energy released by disk accretion.

Neither thermal nor radiation pressures are able to drive the observed rates of mass loss. The inescapable conclusion is that these jets and winds are produced by magnetic acceleration; models show that magnetic fields rotating with the disk naturally produce the necessary collimation along the rotation axis. The precise manner in which this acceleration and collimation takes place is uncertain because the magnetic field structure in the inner disk is not known.

Magnetic fields also play an important role in accretion onto pre-main-sequence stars. The magnetic fields of T Tauri stars are apparently strong enough to hold off disks from the stellar surface; the accreting gas deviates from the disk plane as it falls in along the stellar magnetic field lines, eventually shocking at the stellar surface and producing the observed hot continuum radiation (Figure 8.1).

Emission lines are powerful means to detect faint objects and to study their composition and physical properties. Detecting and studying objects ranging from galactic sources to the most distant galaxies is made possible by using these lines. The aim of the XVIII Winter School is to give a thorough introduction to this emission-line Universe from both theoretical and observational points of view. For this reason, the Winter School contents include not only classical lectures, but also tutorials on data reduction and analysis. This structure enables young researchers to participate actively in current and future research projects, while serving also as a reference book for experienced researchers.

The subject of this School was motivated by the upcoming advent of a new generation of wide-field instruments for large telescopes, specifically optimized for observing emission-line objects in two dimensions. These instruments will boost the study of these kinds of objects by providing large amounts of data, whose digestion will require a theoretical basis as well as specific data-reduction techniques. These powerful facilities will enable the study of very faint emission lines of nearby objects, or conspicuous lines of very distant targets. The former will provide finer details on the chemical composition and characteristics of the gas, while the latter will furnish insight on structure formation and its evolution via scanning of large proper volumes of Universe.

Most cosmological surveys have been based on the continuum emission of the objects of the Universe via broadband imaging and their spectroscopic follow-up.

Virtually all stars are born with neighbors. Most stellar systems are multiple, and many if not most stars are born in groups, with a subset in clusters of substantial numbers of stars. An understanding of multiplicity is therefore an important part of theories of star formation.

The complex structure within molecular clouds, and in particular the asymmetries present in protostellar cores, may be essential to understanding the process of forming binaries. Whether the disks formed during protostellar collapse are capable of spawning multiple stellar companions rather than simply accreting onto the central object is an open question, perhaps requiring strong departures from axisymmetry in the infalling material. Observationally, we detect multiple protostellar systems which appear to lie within disks or toroids in Class I sources, indicating that stellar fragmentation occurs before infall to the disk is complete.

The early evolution of binary and multiple stellar systems is likely to be complex. The growth in mass of fragments formed early on will depend upon complicated accretion processes in disks and infalling envelopes. If more than two fragments are formed, the system can become dynamically unstable, resulting in dispersal or ejection of some members. Whether many or even any stars are initially formed as single objects, or instead were ejected from multiple systems, is currently unclear.

Probably most young stars are born in clusters or groups. Currently most stars in the solar neigborhood are born in modest-sized groups of order 10–100 members in regions of order a pc in size.

Introduction

Emission lines are observed almost everywhere in the Universe, from the Earth's atmosphere (see Wyse & Gilmore 1992 for a summary) to the most-distant objects known (quasars and galaxies), on all scales and at all wavelengths, from the radio domain (e.g. Lobanov 2005) to gamma rays (e.g. Diehl et al. 2006). They provide very efficient tools to explore the Universe, measure the chemical composition of celestial bodies and determine the physical conditions prevailing in the regions where they are emitted.

The subject is extremely vast. Here, we will restrict ourselves in wavelength, being mostly concerned with the optical domain, with some excursions to the infrared and ultraviolet domains and, occasionally, to the X-ray region.

We will mainly deal with the mechanisms of line production and with the interpretation of line intensities in various astrophysical contexts. We will discuss neither quasars and Seyfert galaxies, since those are the subject of Chapter 5, nor Lyman-α galaxies, which are extensively covered in Chapter 4 of this book. However, we will discuss diagnostic diagrams used to distinguish active galaxies from other emission-line galaxies and will mention some topics linked with H Lyα. Most of our examples will be taken from recent literature on planetary nebulae, H II regions and emission-line galaxies. Emission-line stars are briefly described in Chapter 7 and a more detailed presentation is given in the book The Astrophysics of Emission Line Stars by Kogure & Leung (2007).

As described in Chapters 4 and 5, the collapse of protostellar clouds with plausible amounts of angular momentum generally should result in the formation of disks as well as protostars. Disk formation during the collapse phase is then followed by a longer phase of disk accretion during which angular momentum is transferred to a small fraction of disk particles at large radial distances, permitting the accretion of most of the disk mass onto the central star (with some fraction possibly forming planets). The subsequent evolution of a star-disk system will be controlled by the rate at which angular momentum is transported in the disk.

Substantial progress has been made in understanding two likely mechanisms for angular momentum transport: magnetic turbulence and gravitational instability. Unfortunately, it has proved difficult to apply these mechanisms to the development of a predictive theory of disk evolution for young systems. The low ionization levels predicted for large regions of YSO disks make it unclear whether the magnetic field can couple sufficiently well to the gas for the so-called “magnetorotational instability” (MRI) to efficiently transport angular momentum. While it seems likely that the MRI operates in some regions of YSO disks, it is far from clear that it is the dominant factor in producing accretion. The effects of gravitational instability depend sensitively on disk energy balance, and local heating and cooling rates are difficult to determine.