The Contribution of Molecular Line Emission to Broadband Flux Measurements at Millimeter and Submillimeter Wavelengths

Citation

Groesbeck, Todd Dickson (1995) The Contribution of Molecular Line Emission to Broadband Flux Measurements at Millimeter and Submillimeter Wavelengths. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rbaq-gb42. https://resolver.caltech.edu/CaltechETD:etd-03222006-103422

Abstract

We present measurements of the ratio of flux carried by molecular emission lines to the total flux observed from six astronomical sources in the 330-360 GHz frequency range, demonstrating that the integrated line emission can represent a significant fraction, and in some cases even the majority, of the total flux observed. Given the narrow widths of these emission lines for most sources in the interstellar medium, it has generally been assumed that the contribution of the integrated lines would be negligible when compared to the continuum flux measured by a very broadband instrument. We extend the previous work by Sutton et al. (1984), which demonstrated that line emission was responsible for a large part of the total flux observed from Orion-KL, by presenting observations at higher frequencies of Orion-KL and several additional sources. These observations provide the most comprehensive determination of the ratio of line flux to continuum flux yet made at submillimeter wavelengths.

We have performed spectral line surveys of the star-forming regions Orion-KL, Orion-S, and IRAS 16293-2422, and the evolved stars IRC +10216, VY Canis Majoris, and OH 231.8+4.2. Observations for Orion-KL and IRC +10216 give continuous frequency coverage over approximately the 330-360 GHz band; for the other sources the coverage ranges between 30 and 60% of the same frequency band. Comparisons of the line survey data with total flux measurements demonstrates that the contribution of the integrated line emission to the total flux at these frequencies ranges from as low as ~10% for Orion-S and IRAS 16293-2422 to at least ~60% for Orion-KL and IRC +10216. We also estimate the amount of flux carried in weak lines below our sensitivity limits based on the observed distributions of line intensities.

An improved deconvolution method has been used to obtain a single sideband spectrum from the double sideband observations. We have also developed computer versions of line catalogs which allow immediate line identification and analysis, assuming conditions of local thermodynamic equilibrium (LTE) prevail. The same catalog programs also permit simulation of emission line spectra for arbitrary molecular abundances and excitation temperatures, again assuming LIE. For each of the observed sources, we present simulations of the spectrum from 0 to 1000 GHz, using molecular parameters derived from our data and additional published observations. In all cases, the simulations indicate that the relative importance of the line emission decreases at higher frequencies (~ > 700 GHz). We discuss the significance of these findings regarding the determination of dust parameters from broadband flux measurements, concluding that such measurements must be corrected for possible line flux contributions before they can be reliably used.

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