Abstract
The computer-accessible Cologne Database for Molecular Spectroscopy (CDMS) contains a catalog of radio frequency and microwave to far-infrared spectral lines of atomic and molecular species that (may) occur in the interstellar or circumstellar medium or in planetary atmospheres. The catalog is continuously updated and is primarily intended to supplement existing data bases such as the JPL or NIST catalogs. Presently it contains entries for 111 species. Other sections of the database include information on the programs used for fitting spectra and creating the catalog entries, along with detailed information on some spectroscopy projects performed at the university of Cologne and a list of known inter- and circumstellar molecules. The database can be accessed online free of charge via http://www.ph1.uni-koeln.de/vorhersagen/ or alternatively via the short-cut http://www.cdms.de.

Key words: line: identification - molecular data - ISM: molecules - radio lines: ISM - catalogs

Ever since molecular lines have been detected from interstellar and circumstellar sources, their unequivocal assignment was substantially dependent on the availability of appropriately precise laboratory data. However, the availability of laboratory rest frequencies depends to a large extent on the frequency region of interest. Above 500 GHz the number density of measured transitions decreases fast. But, for example, around 663 GHz strong features of molecules as heavy as CH₃CN and even SO₂ reveal strong features towards the Orion A molecular cloud (Belov et al. 1998). More recently, Herpin & Cernicharo (2000) detected interstellar lines of CO, ¹³CO, HCN, HNC, H₂O, and OH between 80 and 197 $\mu$ m, which is equivalent to 4.2 and 1.7THz, respectively. These observations were performed towards CRL 618 using the long wavelength spectrometer of the Infrared Space Observatory, ISO. Accurate rest frequencies up to $\approx$ 2 THz for the first four species were provided by our laboratory (Winnewisser et al. 1997; Klapper et al. 2000; Maiwald et al. 2000; Thorwirth et al. 2000).

In addition to the more familiar pure rotational transitions, also vibration rotation transitions originating predominantly from low-lying bending modes of, e.g., carbon chain molecules can be detected in the far-infrared region, as demonstrated for the $\nu_2$ mode of the C₃ molecule (origin at 63.4cm^-1 or 1.9 THz) by Cernicharo et al. (2000) towards Sgr B2. Very recently, improved rest frequencies have been obtained by Giesen et al. (2001). Future missions such as the Stratospheric Observatory For Infrared Astronomy (SOFIA) and the Herschel Space Observatory, formerly known as FIRST, will extend high frequency observations by a substantial amount. This offers not only the possibility to observe (new) species at terahertz frequencies but also affords the knowledge of accurate rest frequencies.

One major goal of the Cologne Database for Molecular Spectroscopy (CDMS), which we have started to build recently, is to provide a catalog of pure rotational and ro-vibrational transition frequencies of atomic and molecular species from the radio frequency and microwave to the far-infrared regions (i.e. frequencies up to 14.3 THz or wavelengths longer than 21 $\mu$ m). This section is called New Line Predictions. In addition, information on programs to fit and predict spectra, on spectroscopic data obtained in the Cologne laboratories, and on recent changes in the CDMS along with a list of known inter- and circumstellar molecules is available in separate sections of the CDMS. These sections are called Fitting Spectra, Cologne Spectroscopy Data, What's New, and Molecules in Space.

2 New line predictions section

**Table 1:** Species in the CDMS catalog as of January 2001
$\begin{table} \par\includegraphics[width=18cm,clip]{table1.eps}\end{table}$

Currently, the catalog contains pure rotational transition frequencies of atomic and molecular species. Vibration-rotation spectral information in the far infrared region, such as the bending mode of C₃ mentioned above, will be included shortly. This section of the CDMS is intended to be used as a planning guide or to facilitate the identification and analysis of observed spectral features in the interstellar and circumstellar region as well as in planetary atmospheres.

Because of the great number of astrochemically and - physically relevant species and the various isotopomers as well as vibrationally excited states, the focus is presently on supplementing existing databases such as the Jet Propulsion Laboratory (JPL) catalog (Pickett et al. 1998) and the NIST recommended rest frequencies database (Lovas 1992). Currently, there are 111 atomic and molecular species in the CDMS catalog which are presented in Table 1. The majority of these are species that have been detected recently during astronomical or laboratory investigations. However, several entries are also present in one of the catalogs mentioned above, but generally with a less extensive data set.

The catalog part consists of three subsections: the new line predictions main page, the catalog entries page, and the catalog search page.

Some general information is given on the catalog main page. It includes a brief, color-coded description of the structure of the entries including units as well as some useful equations.

**Table 2:** Explanation of catalog entry, format (in parentheses), and units using the example of H₂¹³CO
$\begin{table} \par\includegraphics[width=18cm,clip]{table2.eps}\end{table}$

The catalog entries page gives the six-digit molecule tag tttttt, which is the sorting criterion, the chemical formula or name, the number of lines for this entry, the current version number, the catalog entry file having the name ctttttt.cat, the documentation file called etttttt.cat, and the date of the generation of the current entry version. The six-digit tag consists of the molecular weight in atomic mass units for the first three digits, a 5, and two digits for counting species with the same molecular weight.

The catalog entry itself has one line of 80 characters for each spectral feature as demonstrated in Table 2 for a detail of the H₂ ¹³CO entry. The intensity I(T) of the spectral feature is calculated according to

$\begin{displaymath}I(T)=(8\pi ^3/3hc)\,\nu \, S_g \, \mu_g ^2\,({\rm e}^{-E''/kT}-{\rm e}^{-E'/kT})/Q_{\mathrm{rs}}(T), \end{displaymath}$

(1)

The catalog entries have been generated by least-squares fitting of published spectral lines to accepted spectroscopic models. The predicted line frequencies and their uncertainties are derived from the resulting fitted parameters and their covariance. For the sake of transferability and comparability, mostly the same programs are used for the CDMS as for the JPL catalog (Pickett 1991), therefore, both formats are identical in general. As a consequence, most of the description of the JPL catalog given by Pickett et al. (1998) applies also to the catalog section of the CDMS.

The documentation gives the tag number, chemical formula, frequently the proper chemical name along with trivial names, the electronic or vibrational state, in particular if it is not a singlet state with v=0, the version number and the name of the creator of the entry, some information on the data used for the fit including special considerations. Furthermore, values for the number of lines, the highest frequency and J quantum numbers, the rotational constants, dipole components, the partition function at certain temperatures, intensity cut-offs, the energy of the state, and the (terrestrial) isotopic correction for the intensity are listed in the documentation.

The partition function ( Q_rs) in the documentation takes into account rotation, splitting caused by the electron or nuclear spin, and spin statistics. Care is taken to ensure proper calculation of the partition function even at high temperatures. Vibrational or electronic corrections are generally not considered mainly because of lacking data; exceptions are noted. For heavier molecules, the vibrational corrections to the partition function may be substantial even at fairly low temperatures. Thus, neglect of vibrational corrections to the partition function constitutes a possible major source of error in the calculation of the intensities. Additional large errors may stem from the fact that the dipole components or their centrifugal distortion corrections are not or only with moderate precision known experimentally.

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{hc3n.eps} \end{figure}$	Figure 1: Graphic output of a search for transitions of all available vibrational states of HCCCN between 336 and 350GHz at 300K
Open with DEXTER

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{alle.eps} \end{figure}$	Figure 2: Graphic output of a search for transitions of all species contained in the CDMS between 336 and 350GHz at 300K
Open with DEXTER

Finally, a search form offers the opportunity to scan for transition frequencies of certain species (including all) in desired frequency ranges. The output can be displayed in tabular or graphic form. In addition, intensities may be recalculated for temperatures of 225, 150, 75, 37.5, 18.75, and 9.375K besides the default temperature of 300K. Currently we are working on an option to calculate A-values instead of intensities. Figures 1 and 2 show graphic outputs for all available vibrational states of HC₃N and all molecules contained in the catalog, respectively, between 336 and 350GHz at 300K. It is intended to include online labeling of the graphic output.

3 Fitting spectra section

Another section of the CDMS gives information and examples to the programs that are used to fit spectra and to generate predictions of transition frequencies. At present, information is available only on the JPL spectroscopy programs (Pickett 1991). The examples include diatomics as well as linear, symmetric, and asymmetric polyatomic molecules. Singlet, doublet, and triplet molecules are included. For linear molecules, the different electronic states $\Sigma$ , $\Pi$ , and $\Delta$ are covered. Simple examples include CO ( $^1 \Sigma ^+$ ), CO⁺ ( $^2 \Sigma ^+$ ), C₄O ( $^3 \Sigma ^-$ ), NF (a $^1 \Delta$ ), TiO ( $^3 \Delta$ ), the symmetric top molecule CH₃CCH, the asymmetric H₂S, H₂CO, H₂COH⁺, and the asymmetric radical DSS. More complicated examples with nuclear (SOCl₂) or with electron spin interaction, such as SiC ( $^3 \Pi _{\mathrm{i}}$ ), or both, e.g. CH ( $^2 \Pi _{\mathrm{r}}$ ), NH₂ ( $^2 {\rm B}_1$ ), and OBrO ( $^2 {\rm B}_1$ ), are also included as are examples with vibration rotation interaction, such as ClNO₂, C₃H, and CH₂CHNH₂.

4 Cologne spectroscopy data section

Details related to spectroscopic projects performed in the Cologne laboratories are presented in this section for the convenience of other spectroscopists. Information that is too extensive to be published fully in a refereed journal can be included, for example the output of a fitting procedure which includes the full correlation matrix among other details.

The Cologne Database for Molecular Spectroscopy, CDMS

1 Introduction

2 New line predictions section

3 Fitting spectra section

4 Cologne spectroscopy data section

References