Ethyl Cyanide

Next we consider ethyl cyanide to illustrate the impact of a more complex spectrum with many uncataloged lines, a more complicated lineshape, and optical thickness. Ethyl cyanide has a considerably denser and more complex spectrum than methyl cyanide.

Figure: QM Catalog and Experimental Spectra Comparison. A
comparison of the Complete Experimental Spectra (blue, upward going)
and a QM catalog (black, downward going).

where we combine the CES and astrophysical lineshapes to simulate the ALMA spectrum in this region.

**Figure: Intermediate Strength Ethyl Cyanide Spectrum.** A
region of intermediate strength ethyl cyanide lines near 242 GHz
that illustrates the fine detail in which the simulation based on the
CES and LTE reproduced the ALMA spectrum. The ALMA spectrum
is the upper black trace and the simulation is the lower blue trace.
The stick spectrum in blue is that of the CES at laboratory resolution.

We have chosen the region near 224 GHz shown in Figure: Ethyl Cyanide Spectrum to determine the intensities and optical thickness of ethyl cyanide. In this figure the ALMA spectrum is shown in black, in blue is the simulation (including effects of optical depth), and in dashed red is the simulation without the effects of optical depth. It is clear that optical thickness plays a major role in the ALMA spectrum of ethyl cyanide.

When lineshape and optical thickness effects are convolved with the 190 K simulation from the CES of

Figure: Ethyl Cyanide Spectrum. The bandhead of ethyl cya-
nide near 224 GHz. The ALMA spectrum is the middle black trace,
the simulation with optical depth effects included is lower blue tr-
ace, and the simulation without optical depth effects is the dashed
upper red trace for the stronger lines.

, this shows the importance of these contributions to showing the simple LTE model used here accounts for a wide range of rotational and vibrational states.

Figure: Ethyl and Vinyl Cyanide provides additional detail. The top trace (blue) is the difference between the ALMA spectrum and the CES and the second trace from the top (red) THIS IS THE WRONG FIGURE - SEE PAPER is the difference between the ALMA spectrum and a simulation based on the QM catalog. Positive difference can be attributed to lines other than those of the ethyl cyanide simulation. For a more direct comparison, below the ALMA spectrum are the simulations based on the QM catalog (red - dashed) and the CES simulation (blue). While the CES simulation is very good, we expect that when we do a more careful adjustment of intensities and lineshapes in a global fit that additional reductions in the residuals will occur, especially in the regions that include optically thick lines.