- 29 Aug 2006
ALMA Working Group on Spectral Line Frequencies (AWGSLF)
NRAO Spectral Line Catalog (Splatalogue)
The next generation of powerful radio and millimeter/submillimeter observatories (e.g. EVLA, ALMA, & Herschel) require extensive resources to help identify spectral line transitions. We describe the compilation of a spectral line catalog. The Splatalogue is a comprehensive transition-resolved compilation of observed, measured and calculated spectral lines. Extending the JPL and CDMS lists, and updating the Lovas/NIST list of observed astrophysical transitions, it adds atomic and recombination lines, template spectra, and is completely VO-compliant, queryable under the IVOA SLAP standard.
The splatalogue is an attempt to collate, rationalize and extend existing spectroscopic resources for use by the astronomical community. The Jet Propulsion Laboratory (JPL) database, the Cologne Database of Molecular Spectroscopy (CDMS) and the Lovas/National Institute of Standards and Technology (NIST) database provide an enormous amount of data – collecting the data in all three online databases together generates over 3.5 million transition data entries across almost 700 molecular species. As part of the ALMA and EVLA scientific plans, one of the main goals of telescope operations is that each new instrument be easy to use by a “novice” observer. Specifically, in the ALMA Project Plan v2.0, it is stated clearly that:
"The final major scientific requirement affects the diverse community that will use and benefit from the scientific capabilities that ALMA brings to extend their research endeavors: ALMA should be “easy to use” by novices and experts alike. Astronomers certainly do not need to be experts in aperture synthesis to use ALMA. Automated image processing will be developed and applied to most ALMA data, with only the more intricate experiments requiring expert intervention."
To that end, we interpret the above statement to also apply to an observer interested in spectral line astrophysics. A spectral line database needs to be available that is descriptive as possible in the way it represents molecular, atomic and recombination line transitions.
Currently, the commonly used databases that are used for this search, the JPL and CDMS databases, do not describe transitions in a user-friendly way, and where the catalogs overlap, the descriptions have to be compared and resolved to be consistent. Furthermore, the Lovas/NIST database tabulates only observed interstellar transitions, but it does provide the user with a much better representation of molecular transitions by using a full description of the molecular transition.
One of the main goals of splatalogue is to update the procedure by which a user searches for spectral lines - the splatalogue will contain at least one example of every detected line. All the linelists on which the splatalogue builds are primarily ordered by species then frequency. The splatalogue is different – it is ordered by species then transition, which is more sensible. In this way, every observation, calculation or measurement is cross-referenced against that table. So you can ask for e.g. CH3
CN 4(3)-3(3) and see all the entries for that species/transition, be they observation, measurement or calculation.
In order to undertake such an effort and provide the astronomical community with the tools they need to conduct their research in spectral line astrophysics, we outline the following steps that will be taken by the ALMA Working Groups on Spectral Line Frequencies (AWGSLF):
1. “Resolution” of the 3 online spectral line catalogs within Splatalogue.
a) Each catalog writes the spectral line transitions and line strengths in a way that is unique to that catalog. The members of the AWGSLF will ensure each listed transition and line strength of all three catalogs are consistent and descriptive enough for the novice user.
b) The “resolution” of the catalog should be completed by December 31, 2006
2. “Reconciliation” of the 3 online spectral line catalogs within Splatalogue.
a) Currently, within each online catalog there exist some discrepancies where different frequencies are assigned for the same transition or similar frequencies are assigned for different transitions. This may be due to simple typographical errors or may be part of a large problem of misidentification of a transition at a specific frequency. The members of the AWGSLF will ensure that the frequencies and transitions of a given species within each online catalog are consistent.
b) The “reconciliation” of the catalog should be completed by December 31, 2007
3. Updates to Splatalogue.
a) The members of the AWGSLF will be in constant contact with the laboratory astrophysics community and when transitions and frequencies of a new species has been determined or a the transitions and frequencies of an already known species have been updates, splatalogue will be updated to account for those new observations.
b) Updates to splatalogue will be ongoing after its formal release to the astronomical community. In general a new species or a re-measured species will be added no later than one week upon receipt of the new data.
4. NRAO decision of the “best” frequencies.
a) Currently, transitions of known molecules between the 3 online catalogs may have frequencies that are not consistent – meaning the 3 online databases are not consistent in assigning a frequency to a unique transition. There are 2 ways we are planning to resolve this issue.
i) First, the members of the AWGSLF will provide the astronomical community a prioritized list of frequencies for the most common molecules and their transitions based on what NRAO believes to the most accurate measurement of that transition. This assessment may be based on simply the error in the frequency reported by the spectroscopists or new astronomical observations.
ii) Second, working in conjunction with laboratory spectroscopists and the prioritized list described above, the members of the AWGSLF will provide the astronomical community a database of transitions and frequencies that has been chosen by experts in the field as the “best” known frequency of that transition to use in astronomical observations. This list will eventually be part of the ALMA observing tool and can easily be added to the observing tool and proposal submission tool other current and upcoming radio and millimeter instruments.
b) NRAO’s release of the best frequencies will take place after the “reconciliation” of the catalog and should be completed no later than December 31, 2008.