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The past triennium has continued to see a huge influx of astrometric positions of small solar system bodies provided by near-Earth object (NEO) surveys. As a result, the size of the orbital databases of all populations of small solar system bodies continues to increase dramatically, and this in turn allows finer and finer analyses of the types of motion in various regions of the orbital elements space.
The WGNEO, a Working Group of Divisions I and III, was formed in the early 1990s to coordinate study of Earth-approaching asteroids and comets (NEOs) and provide timely advice to the General Secretary and officers of the IAU on discovery of any objects that threaten collision with the Earth. Since then, the WGNEO has steadily grown, reflecting increasing international interest and concern over impacts, especially from asteroids (which dominate over comets in their risk to Earth). In this triennium, the WGNEO had a membership of 49 (including the Organizing Committee of 17 members), plus 10 consultants. The Chair is David Morrison (USA), Vice-Chair Andrea Milani (Italy), Secretary Richard Binzel (USA), and Past-Chair Andrea Carusi (Italy).
As documented by the reports of the Presidents of Commission 28 and Commission 47 the fields of extra-galactic research and cosmology have experienced a higher and higher development leading to a vast harvest of results and discoveries. They range from the description of the overall structure of the universe to that of the individual properties of galaxies. The availability of very large telescopes and the coverage of regions of the sky with deep surveys, on the observational side, and the wide use of sophisticated numerical simulations on the theoretical one are starting to produce a satisfactory understanding of the physical processes taking place during the evolution of galaxies. Very often there is an profitable interplay between the subjects of the two Commissions without clearcut boundaries. This makes Division VIII, which is one of the largest of the IAU, counting 1373 members, very well balanced and deserving to remain without modifications for the future.
The Working Group on Extrasolar Planets (hereafter the WGESP) was created at a meeting of the IAU Executive Council in 1999 as a Working Group of IAU Division III and was renewed for three more years at the IAU General Assembly in 2003. The charge of the WGESP is to act as a focal point for international research on extrasolar planets. The membership of the WGESP has remained unchanged for the last three years.
The activities in scientific research related to Commission 19 are mostly developed in the different institutions that have sent their reports here enclosed, in the different meetings that have been organized in related themes, and in the WGs of the Division 1. An important additional activity has been developed in the frame of precession and nutation. This research has been initiated by the Descartes Prize received by the Nutation Consortium in 2003.
The working group “The Future Development of Ground-Based Astrometry” of the IAU Division 1, founded in 2000, continued its activity for the last triennium. Part of its results—meetings or programs—are presented here. The FDGBA web site is http://www.astro.ro/wg
The report of Commission 15 was prepared primarily by the chairpersons of its two working groups: the Minor Planet Working Group and the Comet Working Group. In particular, the Minor Planet section was created by A. Cellino with a little help from E. Tedesco and the Comet section by T. Yamamoto with the assistance of D. Bockelée-Morvan, W. Huebner, A. Bhardwaj, D. Biesecker, L. Jorda, H. Kawakita, H. U. Keller, H. Kimura, A. Kouchi, and D. Prialnik. E. Tedesco was responsible for the Introduction, final editing, and merging of the two reports.
This triennial report from Commission 8 covers astrometry-related matters for objects ranging from Solar system bodies out to Milky Way stars and QSOs at cosmological distances. This enormous range of distances is needed to establish, maintain, and improve the metric of the visible Universe--a very challenging effort since everything is moving. The progress of astrometry in the last three years (2002-2005) is reflected here. To locate the references, the reader is advised to check the NASA ADS Astronomy Abstract Service and the expanded report posted at URL http://www.pha.jhu.edu/iau_comm8/comm8.html
A Division 1 Working Group on “Nomenclature for Fundamental Astronomy” (NFA) was formed at the 25th IAU GA in 2003 in order to provide proposals for the new nomenclature associated with the implementation of the IAU 2000 resolutions on reference systems. This WG is also intended to make related educational efforts for addressing the issue to the large community of scientists. The activities of the NFA WG since October 2003 have consisted of newsletters, questionnaires, detailed e-mail discussion, and the preparation of WG recommendations and guidelines which are supported by explanatory documents. The NFA documents have been discussed during international meetings in 2004 and 2005. A NFA WG resolution proposal will be submitted to the IAU 2006 GA as a supplement to the IAU 2000 resolutions. The NFA material has been made available on the NFA web Bite at: http://syrte.obspm.fr/iauWGnfa/.
Division V, “Variable Stars”, consists of Commission 27, also called “Variable Stars” and Commission 42, “Close Binaries”. Thus the former deals with stars whose variations are intrinsic, whereas in the latter the variations are caused by the interactions between the components in the binary. It is evident that the definition of the Division is predominantly observational, and there may be cases where the assignment of an object to one of the two commissions might be in doubt (a recent somewhat related example was the first detection of an extra-solar planet, in 54 Pegasi, where intrinsic variability of the star in the form of high-order g modes was also initially suspected).
The primary aim of continuum measurements is to obtain the photospheric temperature scale, but numerous other uses range from gravity measurement, chemical composition studies, the detection of companion stars and disks, through properties of broad-band photometric systems, and bolometric corrections. In the hotter stars, the shape of the continuum is molded by the bound–free absorption of neutral hydrogen. From the ground, we can measure only a small part of the Balmer continuum (912–3647 Å) long-ward of the ozone cut-off ∼3400 Å, the complete Paschen continuum (3647–8207 Å), and some of the Brackett continuum (8207–14 588 Å) which is badly cut up by terrestrial molecular absorption. The Balmer and Paschen discontinuities are useful as pressure diagnostics for late A and F stars, but they also depend on temperature. In cooler stars, where the negative hydrogen ion dominates, all continuum characteristics depend almost exclusively on temperature.
The spectrum measured with low resolution, e.g., 10–50 Å, is often called the “energy distribution.” In such situations, the spectral lines are included, and then measurements of the fraction of light removed by spectral lines is needed to regain accurate information on the position of the continuum.
Absorption lines in stellar spectra show differences in shape and strength according to the physical conditions in the star's atmosphere. Some of the toughest and most fascinating problems arise in the study of the interplay of the line absorption with the temperature, pressure, radiation, and magnetic and velocity fields of the gas. We are not yet able to calculate the full interlinking of these variables. On the other hand, the panoramic view of spectral-line behavior can be understood in relatively simple terms which we shall presently describe.
The most fundamental point to bear in mind is that the strength of line absorption depends on the number of absorbers producing that absorption. Thus the atomic level populations are of primary concern. But since the number of absorbers also means along the line of sight through the visible depths of the atmosphere, the path length is equally important. Specifically, if the continuous absorption is strong, the path length will be short and vice versa. In effect, the depth of the atmosphere changes with the amount of continuous absorption. In this way, the ratio of the line absorption to the continuous absorption is seen to be the main factor to consider.
One of the goals of stellar atmosphere studies is to understand the various line profiles and line strengths shown by stars. Another is to use our knowledge of line behavior to interpret the fundamental properties of stars, for example, the measurement of effective temperature, surface gravity, radii, and chemical composition. Our attention is directed to the first of these goals in this chapter, while the others are taken up in subsequent chapters.
As a precursor to calculating the transfer of radiation through a model stellar photosphere, we now look at the continuous absorption coefficient. The wavelength dependence of the continuous absorption coefficient shapes the continuous spectrum emitted by the star: more absorption, less light. The strength of spectral lines also depends on the continuous absorption; more continuous absorption means a thinner photosphere with fewer atoms to make spectral lines. Consequently, we must know kν in order to match model-computed spectra to real stellar spectra. However, before we can compute the theoretical spectrum, we need to compute the model on which it is based, and that too can require kν. Specifically, if we invoke radiative equilibrium to find T(T0), then kν is needed from the very beginning. Furthermore, it then becomes particularly important to know kν in those spectral regions carrying most of the flux. In hot stars, this means the ultraviolet region, in cool stars, the infrared region. We escape this step if T(T0) is determined empirically, say by scaling the solar temperature distribution; the main need for kν reverts to the computation of the spectrum. Since scaled temperature distributions are emphasized in this book, kν in the visible window is the main focus of this chapter.
What is a model photosphere, and why build one? It would seem logical to take our stellar observations and deduce from them the physical conditions existing in the atmosphere of the star – somewhat like a parallax measurement yields the distance to a star. Alas, the formation of the stellar spectrum involves many physical variables, and a rigorous deductive interpretation cannot generally be made. Instead we hypothesize a model through which we organize and relate the information conveyed in the starlight. The model resembles a scientific theory in that it is constructed on the basis of our observations and known physical laws. We may then gather additional observations to test our model much like we would test a theory. The model is modified and improved as information is added. When our model closely reproduces all the available observations, we begin to feel that our model is worthy of some trust. Then properties associated with the model, or deduced from further application of the model, are associated with the star, properties such as effective temperature, surface gravity, radius, chemical composition, or rate of rotation.
The model photosphere consists of a table of numbers giving the source function and the pressure as a function of optical depth for an assumed chemical composition. Additional columns may be added to the table depending on the use intended for the model.