This is a report of a Discussion Meeting titled “Astronomy, Physics, and Chemistry of ” which was held on February 9 and 10, 2000 at the Royal Society in London.

The Far Ultraviolet Spectroscopic Explorer (FUSE) is presently producing high resolution (R ∼ 20,000) absorption-line spectra of astronomical objects ranging from Solar System planets to quasars. The 900-1200 Å spectral region observed by FUSE is exceedingly rich in atomic and molecular transitions arising out of the ground state. It is already clear from early FUSE observations that the atomic data (e.g., oscillator strengths) for some transitions are considerably different than those predicted by theoretical calculations. I briefly describe the most pressing oscillator strength needs in this wavelength range for studies of the interstellar medium.

With the launches of the Chandra X-ray Observatory and XMM-Newton, high resolution X-ray spectra of cosmic sources are broadening our understanding of the physical conditions, such as temperature, density, ionization state, and elemental abundances. X-ray emitting astrophysical plasmas can be generally classified by their dominant ionization mechanism, either collisional ionization or X-ray photoionization. The atomic data needs are significantly different for these two cases; however, for both cases it is important that we identify robust and accurate diagnostics and that we verify completeness of the broadband models. We discuss the status of the atomic data currently used in atomic databases for X-ray astronomy, in view of theoretical and experimental atomic physics considerations.

We report on an international collaboration, the FERRUM Project, which aims at extending the database of experimental oscillator strengths for iron group elements and possibly evaluating theoretical and astrophysical data. The selection of individual projects is made with respect to their relevance for abundance work and plasma diagnostics in astrophysics. In this paper we present recent measurements of gf-values in Fe II. We also report on the first measurements of forbidden lines of the same spectrum, [Fe II].

The complex organic chemistry involved in comets and planetary atmospheres can be better understood via comparison between observations and results obtained by models. In this paper, we point out some lacks in chemical schemes used in models that can be responsible for the discrepancies between observational and theoretical data. Then, we propose UV studies (coupling experimental and theoretical approaches) to be developed in order to determine fundamental parameters: absorption cross sections, quantum yields and rate constants which will be used to improve models as well as to better analyse and interpret observations.

Investigating the formation of organic molecules in hydrocarbon rich atmospheres of planets and their moons is of paramount importance to understand the chemical composition, mixing ratios, and the future evolution of carbon bearing species in Solar System environments. The diacetylene molecule (C4H2) as detected in the stratosphere of Saturn and its moon Titan is the largest polyatomic organic species observed so far in the outer Solar System. In addition the methyl radical (CH3) together with methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), methylacetylene (CH3CCH), propane (C3H8) and the CN bearing species hydrogen cyanide (HCN), dicyan (NCCN), methykyanide (CH3CN), cyanoacetylene (HCCCN), and presumably solid state dicyanoacetylene (NCC-CCN) were detected as well. Photochemical models of Jupiter, Saturn, Uranus, Neptune, Pluto and their satellites Titan and Triton predict further the presence of even more complex hydrocarbons up to triacetylene (C6H2) and tetraacetylene (C8H2). Previous experiments attempting to unravel the formation of these molecules in planetary environments employed and open reactors subjecting gas mixtures to high energy discharges or electron bombardments; often products were analyzed via gas chromatography coupled to a mass spectrometer (GS-MS). This approach can match the existence of major trace constituents. However, no information on radical intermediates could be supplied; further, possible open shell products and extremely labile products cannot be identified; hence involved reaction mechanism can only be guessed. However, the explicit identification of reaction intermediates, ALL reaction products, and the mechanism are a top priority to give a systematic picture on the chemistry in hydrocarbon rich atmospheres and to predict the formation of hitherto unobserved molecules.

The spectroscopy of giant planets in the infrared range gives access to a remote sensing of many physical parameters. The composition, pressure/temperature structure, and the cloud structure all contribute to the spectrum, in solar reflected light below 3 micrometer as well as thermal emission above, from atmospheric levels ranging from the mesosphere down to the troposphere. Imaging spectroscopy revealing the variability of the atmosphere gives access to spatial and temporal evolution of these parameters, constraining the meteorological evolution of the planets.

Two Resolutions have been submitted to the 24th General Assembly of the IAU concerning the definition and use of the celestial pole of reference and the celestial origin. The aim of both resolutions is to provide new parameters for Earth rotation which are consistent with the properties of the International Celestial Reference System (ICRS), adopted from 1 January 1998 as the IAU celestial reference system. The definition of the parameters have also to be consistent with the precision and the temporal resolution of the current Earth rotation measurements as well as with the theory for nutation and polar motion at the microarcsecond level. This paper explains the basis of the resolutions as well as their practical application. One of the resolutions defines the “Celestial Intermediate Pole” (CIP) in order to replace the “Celestial Ephemeris Pole” (CEP) for the new IAU precession-nutation model; its specifies the way for taking into account the constant offset from the ICRS and the high frequency terms in polar motion and nutation. The other resolution recommends the use of the “non-rotating origin” (Guinot 1979) on the moving equator, for defining Earth rotation and UT1; it also recommends the use of the celestial and terrestrial coordinates of the CIP in the transformation from the celestial to the terrestrial systems.

Given was additional information to a previous report on the recent progress in the determinations of astronomical constants (Fukushima 2000), including the revision of Lg, Δp and the offsets of CEP. As a summary, presented was the (revised) IAU 2000 File of Current Best Estimates of astronomical constants, which is to replace the former 1994 version (Standish 1995).

The present (2000) set of primary “constants” and “current best estimates” is being reviewed. The emphasis within IUGG is on the consideration and numerical evaluation of temporal changes of relevant parameters. The accuracy of many parameters has been substantially improved, but many numerical values did not change significantly. Therefore, to avoid confusion IUGG/IAG are reluctant in introducing new official parameter sets but keep “current best estimates” up-to-date.

With respect to the publication in the Proceedings of the IAU Colloquium 180 (Dehant 2000), I have a few additional remarks that I would like to make. There were three competing models:

1. MHB2000 of Mathews et al. (2000), (see also Mathews 2000), constructed from a rigid Earth nutation series with about 1400 terms and a transfer function obeying the sum rules, based on the fit of a semianalytical nutation theory to VLBI observations, with resonances determined by geophysical parameters including those estimated from the fit, incorporating the atmospheric annual effect fitted to observations, including ocean tide effects based on admittances with frequency dependence due to the FCN (Free Core Nutation) resonance and to ocean dynamics (fitted to ocean tide data), including electromagnetic couplings of the fluid core, and with mutual consistency maintained in the treatments of nutations, solid Earth tides and ocean tides which influence one another;

2. GF2000 of Getino and Ferrándiz (2000), using a global Hamiltonian approach for 106 waves fitted to VLBI data, incorporating a resonance with global parameters fitted to observations such as the FCN free mode frequency, compliances, and dissipation coefficients, incorporating ocean corrections from Huang et al. (2000) with a frequency dependent resonance and fitted atmospheric corrections, and necessitating empirical corrections;

3. SF2000 of Shirai and Fukushima (2000a and 2000b) or Herring (2000, not published), empirical models, based on a simple resonance formula fitted to the VLBI observation.

Analyses of residuals between VLBI observations and combinations of nutation series show that the MHB 2000 nonrigid-Earth nutation model applied to the REN 2000 rigid-Earth model results in the best fit, and that amplitudes of any possible periodic terms remaining in the observed corrections to the MHB2000 theory could be expected to be less than 0.1 mas.

The recent theoretical developments have provided accurate series of nutations, which are close to the Very Long Baseline Interferometry (VLBI) data. At the milliarcsecond (mas) level, three series are available: MHB2000 (Mathews et al. 2000), FG2000 (Getino and Ferrándiz 2000), and SF2000 (Shirai and Fukushima 2000a,b) (see Dehant 2000, and in this volume, for more information and for a short description of these models).

In the first part of our work we have compared these models with the (VLBI) observations (Ma et al. 2000) by computing rms of the residuals for several time intervals of measurements. We have concluded that these series have comparable precision.

In our work (Xu et al. 2000) for the first time a complete and closed set of post-Newtonian dynamical equations for elastically deformable, rotating astronomical bodies has been deduced. These bodies are assumed to differ only slightly from corresponding stationary and axisymmetric fluid reference bodies and show no internal dissipation, i.e., they are composed of a perfectly elastic medium. For such bodies a set of perturbation equations is derived by means of the Carter and Quintana formalism in a suitably defined rotating coordinate system. The post-Newtonian dynamical equation for the displacement field is the central result of this paper together with the various equations that define the quantities appearing therein. It is the post-Newtonian version of the well-known Jeffreys-Vicente (J-V) equation (Jeffreys & Vicente 1957) that forms the basis of any local theory of global geodynamics. PN J-V equation reads:

where sa is the covariant displacement field, (ρ is the density of tolal mass-energy, p is the isotropic pressure), WG a relativistic geopotential in rotating coordinate, Θ the volume dilation, δWG Euler variation of WG, K the compression modulus, is the shear tensor, W and Wα are the scalar and vector potential in rotating coordinate.

This Joint Discussion (JD) will consider the birth processes of massive stars. While similar phenomena (e.g., accretion discs, outflows, etc.) are found in low mass star formation, additional physics must be considered given the ionization of the interstellar environment by Lyman continuum photons, stellar winds from the hot star(s), and their deeper gravitational potentials. This JD will bring together experts from several disparate astronomical communities: stellar astrophysics, interstellar medium, radio astronomy, and stellar dynamics. The concept is to contrast observations of very young stars and star formation regions over various wavelengths with theoretical expectations.

Massive stars form in clusters within self-gravitating molecular clouds. The size scale of these clusters is sufficiently large that non-thermal, or turbulent, motions of the gas must be taken into account when considering their formation. Millimeter wavelength radio observations of the gas and dust in these clouds reveal a complex, self-similar structure that reflects the turbulent nature of the gas. Differences are seen, however, towards dense bound cores in proto-clusters. Examination of the kinematics of gas around such cores suggests that dissipation of turbulence may be the first step in the star formation process. Newly formed stars, on the other hand, replenish turbulence through their winds and outflows. In this way, star formation may be self-regulated. Observations and simulations are beginning to demonstrate the key role that cloud turbulence plays in the formation and evolution of stellar groups.

Millimeter and submillimeter wavelength images of massive star-forming regions are uncovering the natal material distribution and revealing the complexities of their circumstellar environments on size scales from parsecs to 100’s of AU. Progress in these areas has been slower than for low-mass stars because massive stars are more distant, and because they are gregarious siblings with different evolutionary stages that can co-exist even within a core. Nevertheless, observational goals for the near future include the characterization of an early evolutionary sequence for massive stars, determination if the accretion process and formation sequence for massive stars is similar to that of low-mass stars, and understanding of the role of triggering events in massive star formation.

Recent chemical studies of high-mass star-forming regions at submillimeter and infrared wavelengths reveal large variations in the abundances depending on evolutionary state. Such variations can be explained by freezing out of molecules onto grains in the cold collapse phase, followed by evaporation and high-temperature chemical reactions when the young star heats the envelope. Thus, the chemical composition can be a powerful diagnostic tool. A detailed study of a set of infrared-bright massive young stars reveals systematic increases in the gas/solid ratios and abundances of evaporated molecules with temperature. This ‘global heating’ plausibly results from the gradual dispersion of the envelopes. We argue that these objects form the earliest phase of massive star formation, before the ‘hot core’ and ultracompact H II region phase.