Results

  1. Molecules
    • n-butyl cyanide
      The detection of saturated n-propyl cyanide in Sagittarius B2(N) was first reported in 2009. The next larger unbranched alkyl cyanide is n-butyl cyanide. We assigned and predicted accurate rest frequency predictions beyond the millimeter wave range to search for this molecule in the Galactic center source Sagittarius B2(N) and facilitate its detection in space.
      The results are based on laboratory measurements of the rotational spectrum of n-butyl cyanide between 75 and 348 GHz. We identified more than one thousand rotational transitions in the laboratory for each of the three conformers for which only limited data had been obtained previously in a molecular beam microwave study. The quantum number range was greatly extended to J ≈ 120 or more and Ka > 35, resulting in accurate spectroscopic parameters and accurate rest frequency calculations up to about 500 GHz for strong to moderately weak transitions of the two lower energy conformers.
      With these predictions at hand, we searched for emission lines produced by the molecule in our sensitive IRAM 30m molecular line survey of Sagittarius B2(N), and were able to derive upper limits to the column densities of N = 3×1015 cm2 and 8×1015 cm2 towards Sagittarius B2(N) for the two lower-energy conformers, anti-anti and gauche-anti, respectively. Our present data will be helpful for identifying n-butyl cyanide at millimeter or longer wavelengths with radio telescope arrays such as ALMA, NOEMA, or EVLA. In particular, its detection in Sagittarius B2(N) with ALMA seems feasible.

      Fig. 1: Different conformers of a molecule with otherwise identical structure lead to increased complexity of rotational spectra. Depicted here are the three most abundant conformers of n-butyl cyanide, labeled with "A" meaning "anti" and "G" meaning "gauche", the latter denoting the angled forms of the molecule.

      Publications:

      • M. H. Ordu, H. S. P. Müller, A. Walters, M. Nuñez, F. Lewen, A. Belloche, K. M. Menten, and S.
        Schlemmer: The quest for complex molecules in space: Laboratory spectroscopy of n-butyl cyanide, n-C4H9CN, in the millimeter wave region and its astronomical search in Sagittarius B2(N). Astronomy and Astrophysics 541, A121 (2012)
      • Belloche, A., Garrod, R. T., Müller, H. S. P., Menten, K. M., Comito, C., & Schilke, P.: Increased complexity in interstellar chemistry: Detection and chemical modeling of ethyl formate and n-propyl cyanide in Sagittarius B2 (N). Astronomy and Astrophysics, 499 (1), 215-232 (2009)
         
    • Dimethyl ether-13C

      Dimethyl Ether is one of the 20 largest molecules detected in the interstellar medium. It is known to be very prominent in star forming regions. After the first interstellar detection in the Orion nebula by Snyder et al. in 1974, dimethyl ether was identified in the spectra of various high-mass star forming regions (e.g. Sutton et al. 1995, Nummelin et al. 1998, 2000). DME is detected as well towards low-mass star forming regions (e.g. Cazaux et al. 2003, Jorgensen et al. 2005, Bacmann et al. 2012, Persson et al. 2012). Due to its strong and dense spectrum covering a broad frequency region (see Fig. 1), dimethyl ether significantly contributes to spectral line confusion in astronomical observations. As can be seen in Fig. 2, a large fraction of the spectral lines that are detected in hot core regions are caused by only a few molecular species and their isotopologues.
      The main aim of our study on dimethyl ether was to extend the knowledge on its rotational spectrum to 13C isotopic species and to vibrationally excited states. Using the data obtained in the laboratory study, accurate predictions of the rotational transition frequencies and their intensities is possible and enables the identification these transitions in astronomical line surveys.

      Fig. 1: Stick spectrum of 13CH3O12CH3 in its vibrational ground state. The intensities of the transitions are calculated for a temperature of 100 K, which is taken as a typical temperature for molecular clouds. Under this condition, the most intense transitions are found at around 600 GHz, but the whole spectrum covers a broad frequency range.


      Fig. 2: Part of the emission spectrum of the interstellar hot-core region Orion KL, observed with the HIFI spectrometer aboard the Herschel Space Observatory. Strong lines in both spectra are identified. As can be seen, a significant fraction of these spectral lines correspond to only a few different molecules such as methanol (CH3OH), sulfur dioxide (SO2), formaldehyde (H2CO), water (H2O), dimethyl ether (CH3OCH3), and their isotopic species. The figure is taken from Bergin et al., A&A 521, L20 (2010).

      Astrochemical interest

      Despite its abundance in star forming regions, the interstellar formation process of dimethyl ether and other complex molecules found in the interstellar medium is still unclear. Studies on the chemistry of hot cores (e.g. by Garrod and Herbst 2006, 2008) suggest that saturated molecules or their precursors are formed on the ice surfaces of dust grains during a colder era. Once star formation begins and the temperature gradually rises from 10 K to over 100 K, heavy radicals become mobile on grain surfaces triggering further reactions into more complex species. During warm-up, surfase molecules desorb into the gas and can be processed in addition by gas-phase reactions. For dimethyl ether, gas-phase synthesis via ion-molecule reactions from evaporated methanol proposed by Blake et al. (1987) was thought to be the dominant formation process in the interstellar medium (see e.g. Charnley et al. 2004, and Peeters et al. 2006). In this reaction, protonated methanol transfers the methyl cation to methanol and dimethyl ether forms after electron dissociative recombination:
      CH3OH2+ + CH3OH → CH3OCH4+ + H2O
      CH3OCH4+  + e → CH3OCH3 + H

      However, laboratory studies on methanol by Geppert et al. (2006) raised questions concerning the efficiency of dissociative recombinations of large saturated ions. In these studies just a fraction of about 3% of recombinations result in methanol, whereas more than 80% result in separate C and O groups.
      The most likely surface route to form dimethyl ether was proposed by Allen & Robinson (1977): 

      CH3 + CH3O → CH3OCH3

      The radical CH3O is formed via successive hydrogenation of CO, which is directly accreted from the gas phase. Current gas-grain chemical models by Garrod and Herbst (2006, 2008) combine surface and gas-phase reaction networks probing the abundances of molecular species for different timescales and temperature gradients for the warm-up phase of hot core evolution. These models reveal that dimethyl ether may be formed in the gas phase as well as on grain surfaces. The efficiency of each formation route strongly depends on the physical conditions and the evolution timescale of the hot-core region. The longer the timescale for protostellar switch-on, i.e. the longer the system remains at intermediate temperatures, the more important are the surface processes.
      An observational test for grain-surface formation of molecules was proposed by Charnley et al. (2004): Isotopic labelling “a posteriori”. It is based on the effect of efficient incorporation of 13C into CO at low temperatures. The ion-molecule process

      13C+ + 12CO → 13CO + 12C+

      with an exothermicity of 35 K results in an overabundance of 13C in CO accreting on cold grains and a 13C deficiency in other molecules forming from C+ in the gas phase (see Langer et al. 1984). If selective fractionation persists adsorption and desorbtion, then organic molecules that are formed from addition reactions to solid CO and HCO should have the same 12C/13C ratio as their parent molecules. Hence, the investigation of 12C/13C ratios in various molecular species could be used to distinguish between grain-surface formation from CO or synthesis by gas-phase reactions. Recently, Wirström et al. investigated the 12C/13C ratio of methanol and CO in various young stellar objects at an early stage of evolution. Their results are consistent with the formation of methanol from hydrogenation of CO on grain surfaces. Further studies on 13C isotopologues of larger organic molecules like dimethyl ether enable to test astrochemical networks, in which the interstellar formation processes of complex species are modelled. This can provide important constraints on the evolutionary cycle of star-forming regions.

      Spectroscopic Challenges

      Dimethyl ether is an asymmetric-top molecule. In addition to the asymmetry splitting of transitions, the structure of its rotational spectrum is further complicated by large amplitude motions of both methyl groups around the C-O bond, resulting in additional splitting of the rotational energy levels into torsional components. Due to the non-zero tunnelling probability between the nine equivalent configurations of the molecule, dimethyl ether cannot be described in the rigid rotor limit. Since rotational and torsional motion of the molecule are strongly coupled, the torsional splitting of transitions changes with the rotational excitation. Hence, models and fitting routines have to be employed which take the torsional motion of the molecule into account. For dimethyl ether and its 13C isotopologues, the effective rotational Hamiltonian ERHAM proposed by P. Groner (1997, 2012) for molecules with two large-amplitude motions was used. The transition frequencies obtained in the laboratory are fitted in a least-squares analysis to parameters of the effective rotational Hamiltonian which then allows to predict the rotational-torsional spectrum of dimethyl ether over a large frequency region.

      Results for (12CH3)2O:

      • Lowest vibrationally excited states v11=1 and v15=1

        Extensive measurements on dimethyl ether’s main isotopologue have been performed in Cologne and at the Jet Propulsion Laboratory (JPL) covering frequencies from 38 GHz up to 1.6 THz.
        The limited dataset of only 52 transition frequencies for vibrational states available in the literature so far has been greatly extended to more than 9500 transitions, which belong to more than 7500 different frequencies. Energy levels up to quantum numbers of J < 60 and K = 21 have been accessed for both states. Second order Coriolis perturbation between both vibrational states is observed for various rotational levels which stretches across a long range of J values. For the analysis two different fitting routines (ERHAM by P. Groner and SPFIT by H. M. Pickett) have been used to interpolate the spectroscopic data to an asymmetric rotor Hamiltonian. Both approaches have significant drawbacks: ERHAM currently does not account for the interaction between different vibrational states, and SPFIT is worse in describing the torsional splitting between rotational energy states. The analysis of the two torsional excited states reveals the limits of the current theoretical models. Although the dataset could not be reproduced within the experimental uncertainty, the derived frequency predictions should be sufficiently good for most astronomical purposes. Especially transitions with quantum numbers J ≤ 30 and K ≤ 10 should be accurate within 1 MHz.

        Using the predictions provided by the laboratory studies, transitions of dimethyl ether in its two lowest vibrational states v11=1 and v15=1 have been detected in space for the first time in an unbiased submillimeter line survey of the hot core toward the high-mass star forming region G327.3-0.6 performed with the APEX (Atacama Pathfinder Experiment) telescope. More than 100 lines have been reliably assigned in collaboration with S. Bisschop (Centre for Star and Planet Formation, Denmark) to rotational transitions within the v11=1 vibrational state, and more than 40 spectral features are identified to rotational transitions within the v15=1 vibrational state. Examples of detected lines are shown in Fig. 3. Analyzing the excitation of both states compared to the vibrational ground state enables information on the interaction of dimethyl ether with the infrared radiation field.


        Fig. 3: Detail of the line survey toward the hot core G327.3-0.6 performed with the APEX telescope. The observed spectrum is depicted in black color, and the model for dimethyl ether in its vibrational excited state v11 = 1 and v15 = 1, respectively, is overplotted in red. The general model including transitions of all previously identified molecules is given in blue color.

      • 13C-isotopologues of dimethyl ether

        New laboratory data on single-13C dimethyl ether (13CH3O12CH3) and double-13C dimethyl ether ((13CH3)2O) have been recorded from 34 GHz up to 1.5 THz. For each molecule about 1500 rotational lines have been measured and assigned. With this, the existing data on 13CH3O12CH3 by Niide & Hayashi (2003) have been substantially extended, and for (13CH3)2O the first rotational spectroscopic data are presented. Transitions involving energy levels up to J = 60 and K = 25 (K = 20 for double-13C dimethyl ether) have been analyzed and fitted to experimental uncertainty using the ERHAM effective rotational Hamiltonian.
        The laboratory analysis enables us to predict the complete ground state rotational spectra of 13CH3O12CH3 and (13CH3)2O up to 2 THz. For transitions not exceeding the range of quantum numbers covered by the laboratory analysis, the accuracy is expected to be better than 1 MHz. Hence, the data offer a very reliable basis for astronomical applications and makes an interstellar detection of these isotopolgues possible in line-rich hot-core sources. The range of predictions and their accuracy also fulfills the requirements for ALMA (Atacama Large Millimeter Array) with its exceptional sensitivity and spatial resolution which will possibly also be able to probe the weaker transitions of (13CH3)2O). The predictions for both isotopologues are available through the Cologne Database for Molecular Spectroscopy (CDMS).

        In collaboration with S. Bisschop (Centre for Star and Planet Formation, Denmark) transitions of 13CH3O12CH3 have been detected in the interstellar space for the first time. More than 100 previously unidentified lines have been assigned to transitions of single-13C dimethyl ether in a large submillimeter line-survey of the hot core toward the high-mass star forming region G327.3-0.6 performed with the APEX (Atacama Pathfinder Experiment) telescope. An example is given in Fig. 4. A preliminary analysis of the 12C/13C abundance ratio for dimethyl ether in this interstellar source yields a value of (25 +/- 5) which indicates the gas-phase formation route to be dominant for dimethyl ether.
         


        Fig. 4: Detail of the APEX line survey toward the hot core G327.3-0.6. The observed spectrum is depicted in black color, and the model for 13CH3O12CH3 is overplotted in red. The general model including transitions of all previously identified molecules is given in blue color. The spectroscopic assignment of transitions of single-13C dimethyl ether is given in the upper left of each individual panel. In the first panel, the torsional splitting of the dimethyl ether transition is indicated as well.

        Publications:

        • S. E. Bisschop, P. Schilke, F. Wyrowski, A. Belloche, C. Brinch, C. P. Endres, R. Güsten, H. Hafok, S. Heyminck, J. K. Jorgensen, H. S. P. Müller, K. M. Menten, R. Rolffs, and S. Schlemmer: Dimethyl ether in its ground state, v = 0, and lowest two torsionally excited states, v11 = 1 and v15 = 1, in the high-mass star-forming region G327.3-0.6. Astronomy & Astrophysics 552, A122 (2013)

        • C. P. Endres: Terahertz Spectroscopy of Dimethyl Ether. PhD thesis, Universität zu Köln, 2010

        • M. Koerber, S. E. Bisschop, C. P. Endres, M. Kleshcheva, R. W. H. Pohl, A. Klein, F. Lewen, and S. Schlemmer: Laboratory rotational spectra of the dimethyl ether 13C isotopologues up to 1.5 THz. Submitted to Astronomy & Astrophysics

        • M. Koerber: The rotational spectrum of oxatrisulfane and dimethyl ether 13C isotopologues. PhD thesis, Universität zu Köln, 2012

    • Ethanol-13C
      Ethanol is a complex organic molecule (COM), observed principally in hot core regions in the interstellar medium (e.g. Sgr B2, W51M, Orion KL, G34.3+0.15). The 13C isotopologues have not been identified in the ISM and prior to this work only scarce low-frequency laboratory data were available. Absorption spectra of both 13C isotopologues of ethanol were recorded at Cologne. We measured around 350 lines for the trans configuration of each of the two 13C isotopologues: CH313CH2OH and 13CH3CH3OH. Measurements were taken in the range 80-600 GHz and a few lines between 700-800 GHz. A comparison between the abundance of the 12C and both 13C species in the ISM could give valuable clues as to the formation of this COM. Furthermore, ethanol-13C is a potential line pollutant in particular for high-sensitivity instruments such as ALMA. We are currently investigating possible candidates for an astronomical detection of these species.

      Publications:

      • Aurelia Bouchez, Adam Walters, Holger S.P. Müller, Matthias H. Ordu, Frank Lewen, Monika Koerber, Sandrine Bottinelli, Christian P. Endres, and Stephan Schlemmer: Millimetre-wave spectrum of anti-13C1 and 13C2 isotopologues of ethanol. Journal of Quantitative Spectroscopy and Radiative Transfer 113, 11, 1148 (2012)
         
  2. Spectrometers
    • BWO spectrometer with LHe-cooled InSb HEB in operation (high sensitivity, small scan widths)
    • THz spectrometer up to 2.5 THz with photoconductor (VDI multipliers and superlattices)