The Cologne Database for Molecular Spectroscopy, CDMS: a useful tool for astronomers and spectroscopists

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Abstract

The general features of the internet browser-accessible Cologne Database for Molecular Spectroscopy (CDMS) and recent developments in the CDMS are described in the present article. The database consists of several parts; among them is a catalog of transition frequencies from the radio-frequency to the far-infrared region covering atomic and molecular species that (may) occur in the interstellar or circumstellar medium or in planetary atmospheres. As of December 2004, 280 species are present in this catalog. The transition frequencies were predicted from fits of experimental data to established Hamiltonian models. We present some examples to demonstrate how the combination of various input data or a compact representation of the Hamiltonian can be beneficial for the prediction of the line frequencies.

Introduction

Spectral features in various frequency regions of the electromagnetic radiation are used to indicate the presence of atomic or molecular species in stellar or planetary atmospheres, including the Earth, or in the interstellar (ISM) or circumstellar medium (CSM). The availability of transition frequencies and intensities with appropriate accuracies is required to identify and quantify the abundance of a certain species unambiguously. Observations are currently performed routinely in the microwave and millimeter wave regions (8–300 GHz). However, in recent years the submillimeter region (300–1000 GHz) has become increasingly important for radio-astronomers, and not only satellite-based, but even ground-based observations in the terahertz region (∼1–10 THz), also known as the far-infrared region (∼30–300 cm−1), have been performed, see e.g. Refs. [1], [2]. The submillimeter and terahertz regions will become more important with the Atacama Pathfinder Experiment (APEX), which is about to start routine observations, and future missions such as the Stratospheric Observatory For Infrared Astronomy (SOFIA), the HERSCHEL space observatory, and the Atacama Large Millimeter Array (ALMA).

More than 4 years ago, the internet browser-accessible Cologne Database for Molecular Spectroscopy (CDMS) has been created [3]. Its Catalog section contains mostly rotational transition frequencies, uncertainties, intensities, and a wealth of other information of atomic and molecular species that may occur in the ISM or CSM or in planetary atmospheres. An important incentive for the creation was that the relevant laboratory data on these molecules is scattered over many journals and very many issues, and the CDMS has the purpose to provide the relevant data to the astronomical community in a standardized way. Generally, emphasis has been put on creating new or updated entries taking into account particularly the submillimeter and terahertz regions. The basic features of this catalog have been described elsewhere [4]. Additional information, so far only provided in conference proceedings [5], will be included in the current article together with recent developments and suggestions for the near future. In Section 2, an update on the Catalog section of the CDMS is provided with the subsections General, Entries, and Search. Considerations for the choice of new entries will be described, and some suggestions for future laboratory work will be given. The new section Molecules in space will be presented briefly in Section 3, while Section 4 will give some information on the Fitting spectra section.

Section snippets

The Catalog

The Catalog section of the CDMS was formerly known as New line predictions section. The basic properties of the Entries and the Search subsections have been described previously [4]. Considerable background information is available on the internet [3] in the respective subsections and in the General subsection described below.

Molecules in space

A new section in the CDMS lists the approximately 120–130 molecules that have been detected in the ISM or CSM, in part with some background information on their detection. Species detected in stellar or planetary atmospheres, comets, or interstellar ices only are not (yet) included. Table 2 presents the status as of December 2004. The exact number depends very much on whether some doubtful or uncertain detections are included. An interesting example is the recent report on the detection of the

General considerations

Almost all of the fitting and prediction of the spectral features present in the CDMS catalog has been done with Pickett's spfit/spcat program suite [33]. It is a rather general program written to fit and predict spectra of asymmetric top molecules involving spin– and rotation–vibration interaction. Symmetric top or linear molecules are treated as special cases. Up to 9 spins with gI>1 and more than 100 states (e. g. vibrational) can be considered simultaneously. However, because of the very

Acknowledgements

HSPM thanks J.F. Ogilvie and P.F. Bernath for providing the BrF and CS data from Refs. [40], [51], respectively. The CDMS and the experimental investigations in Cologne are supported by the Deutsche Forschungsgemeinschaft (DFG) via grant SFB 494 and by the Ministry of Science and Technology of the Land Nordrhein-Westfalen (NRW).

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