QM Catalog Completeness
Figure: Astrophysical Weeds.
As an illustration, we have completed experimental work for eight well known astrophysical weeds: ethyl cyanide methyl formate, acetaldehyde, dimethyl ether, methanol, sulfur dioxide, methyl cyanide, and vinyl cyanide in the 210-270 GHz region.
Figure: Astrophysical Weeds shows catalog completeness in a graphical format. In this figure, the observed experimental lines (which are presumably complete) are sorted according to intensity, as are the QM catalog lines. As long as the QM catalog is complete, these two plots will overlap, diverging at the intensity of the strongest line that is not included in the QM catalog.
For semi-rigid molecule the rotational structure in these excited states will be similar to that of the ground state. With this assumption, it would be expected that the strongest lines not included in the catalog would be reduced from the strongest lines in the spectrum by the vibrational Boltzmann factor for these vibrational states. Inspection of Figure: Astrophysical Weeds shows that this expectation is realized. For molecules with internal rotation, somewhat more complex considerations are required to account for degeneracies. Additionally for molecules such as methanol, which have spectra that is strongly torsional state dependent, the divergence will be much more gradual.
The location of this divergence as a function of temperature is an important astrophysical question. Although much of the interstellar medium is at low temperature, the ability of telescope arrays to resolve smaller hot cores has resulted in the observation of much higher temperatures. For example, it has been shown with the SMA and using methyl cyanide as a probe that the temperature measured for the Orion KL hot core has continually increased with increasing telescope angular resolution, with temperatures over 600 K observed in a recent study [1].
Figure: Methanol Spectrum. A small segment of the spectrum of methanol
near 625 GHz. The blue trace is a spectrum simulated from the QM catalog and
the red trace a simulation based on the CES.
Accordingly, Figure: Strong Lines shows these intensity factors over the 100 - 500 K range for the excited state of vinyl cyanide near 460 cm-1, as well as for a number of other astrophysical weeds. Unlike the case of ethyl cyanide, for which only an analysis of the unperturbed ground vibrational is included in the catalogs, the inclusion of even the truncated analyses of the excited states of vinyl cyanide significantly reduces the intensities of the lines which are not included in the catalogs.
Figure: Vinyl Cyanide Spectrum. A small segment
of the spectrum of vinyl cyanide at 300K - expanded
from top to bottom. The green trace is one of the ~
400 experimental scans. The black trace is the CES
simulation based on a frequency point-by-point anal-
ysis. The blue trace is the CES simulation based on a
catalog derived from the CES, and the red sticks the
QM catalog simulation. Note that near 227.86 GHz
the QM simulation is much smaller than the observed
spectral line, indicating an overlap of a weak predicted
line by a stronger uncataloged line.
Figure: Strong Lines. The intensity of the strongest line not |
Figure: Vinyl Cyanide Spectrum. A small segment |
References
- Kinetic temperatures of the dense gas clumps in the Orion KL molecular core Astrophys. J. 713, 1192-1206 (2010). Google Scholar
- High-resolution submillimeter multiline observations of G19.61-0.23: Small-scale chemistry Astrophys. J. 711, 399-416 (2010). Google Scholar