Sarah M. Fortman

A rigorous detection of interstellar CH3NCO: An important missing species in astrochemical networks

The recent analysis of the composition of the frozen surface of comet 67P/Churyumov-Gerasimenko has revealed a significant number of complex organic molecules. Methyl isocyanate (CH3NCO) is one of the more abundant species detected on the comet surface. In this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the Orion clouds where 399 lines of the molecule have been unambiguously identified. We find that the limited mm-wave laboratory data reported prior to our work require some revision. The abundance of CH3NCO in Orion is only a factor of ten below those of HNCO and CH3CN. Unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in Orion are only weakly correlated with those measured on the comet surface. We also compare our abundances with those derived recently for this molecule towards Sgr B2 (Halfen et al. 2015). A more accurate abundance of CH3NCO is provided for this cloud based on our extensive laboratory work.

A NEW APPROACH TO ASTROPHYSICAL SPECTRA: THE COMPLETE EXPERIMENTAL SPECTRUM OF ETHYL CYANIDE (CH3CH2CN) BETWEEN 570 AND 645 GHZ

There is a general consensus that many of the unidentified features in astrophysical spectra are due to low lying excited vibrational and torsional states of a few molecules—commonly referred to as the astrophysical weeds. This is a challenging spectroscopic problem not only because there are many such states, but also because these states are often highly perturbed and difficult to analyze. We have previously described an alternative approach based on experimental, intensity-calibrated spectra taken at many temperatures. In this paper, we describe the procedures and results obtained with this approach for ethyl cyanide, strategies for archiving and disseminating these results, and the prospects for using these results to reduce the confusion limit in the powerful new observatories that are coming online.

HOW COMPLETE ARE ASTROPHYSICAL CATALOGS FOR THE MILLIMETER AND SUBMILLIMETER SPECTRAL REGION

With the growth in sensitivity and angular resolution of millimeter and submillimeter telescopes, the number of unidentified molecular spectral lines in surveys of the interstellar medium has grown rapidly. While some of these unidentified lines are due to as yet unidentified astrophysical species, it is the general consensus that most are due to lines from a limited number of well-known interstellar species, the interstellar weeds. These unidentified lines do not appear in astrophysical line catalogs, which are based on quantum mechanical models and are incomplete primarily because of the difficulty of performing the usual bootstrap assignment and analysis process in their often highly perturbed low-lying vibrational states. To address this problem, we have proposed and demonstrated an alternative catalog approach that is based on the analysis of intensity-calibrated spectra taken over a range of temperatures in the laboratory. These analyses also make it possible to quantitatively address the astrophysical completeness of existing catalogs. In this Letter, we use extensive new experimental data in the 210-270 GHz window to address this question for eight molecules that are considered to be the leading candidates for astronomical weeds—methyl formate, methanol, dimethyl ether, acetaldehyde, sulfur dioxide, methyl cyanide, vinyl cyanide, and ethyl cyanide. Additionally, for each of the eight molecules, we use these results and knowledge of the molecular vibrational/torsional energy levels to predict completeness as a function of astronomical source temperature.

THE COMPLETE, TEMPERATURE-RESOLVED EXPERIMENTAL SPECTRUM OF ETHYL CYANIDE (CH{sub 3}CH{sub 2}CN) BETWEEN 210 AND 270 GHz

This paper reports the extension of a previously reported experimental method for the identification and characterization of astrophysical weeds in millimeter and submillimeter spectra to the widely used 210-270 GHz atmospheric window. At 300 K, these spectra contain contributions from approximately 40 vibrational states in addition to the cataloged ground state. The quantum mechanical analysis of such a large number of states would be a formidable challenge due to the complex interactions among these dense vibrational states. A new heterodyne receiver-based system is reported, as well as its intensity calibration. Results are presented in the standard astrophysical catalog format as well as our previously described point-by-point format that is effective for the characterization of blends. We also describe and validate an additional spectral synthesis approach, based on the much smaller line list catalog, which is useful in the blended line limit.

The Complete, Temperature Resolved Experimental Spectrum of Methanol (CH3OH) between 560 and 654 GHz

The complete spectrum of methanol (CH{sub 3}OH) has been characterized over a range of astrophysically significant temperatures in the 560.4-654.0 GHz spectral region. Absolute intensity calibration and analysis of 166 experimental spectra recorded over a slow 248-398 K temperature ramp provide a means for the simulation of the complete spectrum of methanol as a function of temperature. These results include contributions from v{sub t} = 3 and other higher states that are difficult to model via quantum mechanical (QM) techniques. They also contain contributions from the {sup 13}C isotopologue in terrestrial abundance. In contrast to our earlier work on semi-rigid species, such as ethyl cyanide and vinyl cyanide, significant intensity differences between these experimental values and those calculated by QM methods were found for many of the lines. Analysis of these differences shows the difficulty of the calculation of dipole matrix elements in the context of the internal rotation of the methanol molecule. These results are used to both provide catalogs in the usual line frequency, linestrength, and lower state energy format, as well as in a frequency point-by-point catalog that is particularly well suited for the characterization of blended lines.

The Complete, Temperature-resolved Experimental Spectrum of Ethyl Cyanide (CH3CH2CN) between 210 and 270 GHz

This paper reports the extension of a previously reported experimental method for the identification and characterization of astrophysical weeds in millimeter and submillimeter spectra to the widely used 210-270 GHz atmospheric window. At 300 K, these spectra contain contributions from approximately 40 vibrational states in addition to the cataloged ground state. The quantum mechanical analysis of such a large number of states would be a formidable challenge due to the complex interactions among these dense vibrational states. A new heterodyne receiver-based system is reported, as well as its intensity calibration. Results are presented in the standard astrophysical catalog format as well as our previously described point-by-point format that is effective for the characterization of blends. We also describe and validate an additional spectral synthesis approach, based on the much smaller line list catalog, which is useful in the blended line limit.

The Complete, Temperature Resolved Experimental Spectrum of Methanol (CH3OH) between 214.6 and 265.4 GHz

The spectrum of methanol (CH{sub 3}OH) has been characterized between 214.6 and 265.4 GHz for astrophysically significant temperatures. Four hundred and eighty-six spectra with absolute intensity calibration recorded between 240 and 389 K provided a means for the calculation of the complete experimental spectrum (CES) of methanol as a function of temperature. The CES includes contributions from v{sub t} = 3 and other higher states that are difficult to model quantum mechanically (QM). It also includes the spectrum of the {sup 13}C isotopologue in terrestrial abundance. In general the QM models provide frequencies that are within 1 MHz of their experimental values, but there are several outliers that differ by tens of MHz. As in our recent work on methanol in the 560-654 GHz region, significant intensity differences between our experimental intensities and cataloged values were found. In this work these differences are explored in the context of several QM analyses. The experimental results presented here are analyzed to provide a frequency point-by-point catalog that is well suited for the simulation of crowded and overlapped spectra. Additionally, a catalog in the usual line frequency, line strength, and lower state energy format is provided.

How Can We Use Complete Experimental Catalogs in the Complex Spectra Limit

There is a broad consensus that many, if not most, of the unidentified spectral lines in astrophysical spectra are due to transitions in excited vibrational states of a relatively small number of molecules, the astrophysical weeds. For these unidentified lines, it is somewhat less well understood that the spectroscopic effort required to characterize them in the traditional quantum mechanical catalog approach is substantially larger because of significant perturbations. We have previously discussed a new experimental approach that addresses this challenge. This approach is based on the analysis of many complete, intensity-calibrated spectra taken over a range of temperatures. However, the spectroscopic completeness of this approach results in a much larger database. These data can be transfer to the astrophysical community in a variety of ways, but because an order of magnitude larger number of lines is included, consideration must be given to implementation strategies.

THE COMPLETE, TEMPERATURE RESOLVED EXPERIMENTAL SPECTRUM OF METHYL FORMATE (HCOOCH3) BETWEEN 214.6 AND 265.4 GHz

Because methyl formate (HCOOCH3) is abundant in the interstellar medium and has a strong, complex spectrum, it is a major contributor to the list of identified astrophysical lines. Because of its spectral complexity, with many low lying torsional and vibrational states, the quantum mechanical (QM) analysis of its laboratory spectrum is challenging and thus incomplete. As a result it is assumed that methyl formate is also one of the major contributors to the lists of unassigned lines in astrophysical spectra. This paper provides a characterization, without the need for QM analysis, of the spectrum of methyl formate between 214.6 and 265.4 GHz for astrophysically significant temperatures. The experimental basis for this characterization is a set of 425 spectra, with absolute intensity calibration, recorded between 248 and 408 K. Analysis of these spectra makes possible the calculation of the Complete Experimental Spectrum of methyl formate as a function of temperature. Of the 7132 strongest lines reported in this paper, 2523 are in the QM catalogs. Intensity differences of 5%–10% from those calculated via QM models were also found. Results are provided in a frequency point-by-point catalog that is well suited for the simulation of overlapped spectra. The common astrophysical line frequency, line strength, and lower state energy catalog is also provided.

THE COMPLETE, TEMPERATURE RESOLVED SPECTRUM OF METHANOL BETWEEN 214 AND 265 GHZ

We have studied methanol, one of the so-called ‘astronomical weeds’, in the 215–265 GHz band. We have gathered a set of intensity calibrated, complete, experimental, and temperature resolved spectra from across the temperature range of 240–389 K. A number of low lying transitions, including the νt = 3 , have not been produced by available catalogs. Using our previously reported method of analysisa we were able generate a line list that contains lower state energies and linestrengths, for all of the observed lines in the band. This line list includes those lines which have no quantum mechanical assignment. In addition to this line list we provide a point by point method capable of generating the complete spectrum at an arbitrary temperature. The sensitivity of the point by point analysis is such that we are able to identify lines which would not have manifest in a single scan across the band. The consequence has been to reveal not only a number of new methanol lines, but also trace amounts of contaminants. We show how the intensities from the contaminants can be removed with indiscernible impact on the signal from methanol. To do this we use the point by point results from our previous studies of these contaminants. The efficacy of this process serves as strong proof of concept for usage of our point by point results on the problem of the weeds. The success of this approach for dealing with the weeds has also previously been reportedb.