Laboratory spectroscopic study of the N isotopomers of cyanamide, HNCN, and a search for them toward IRAS 162932422 B††thanks: Transition frequencies from this work as well as related data from earlier work are given for each isotopic species as supplementary material. Given are also quantum numbers, uncertainties, and residuals between measured frequencies and those calculated from the final sets of spectroscopic parameters. The data are available at CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/
Key Words.:Molecular data – Methods: laboratory: molecular – Techniques: spectroscopic – Radio lines: ISM – ISM: molecules – Astrochemistry
Context:Cyanamide is one of the few interstellar molecules containing two chemically different N atoms. It was detected recently toward the solar-type protostar IRAS 162932422 B together with HNCN and HDNCN in the course of the Atacama Large Millemeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS). The detection of the N isotopomers or the determination of upper limits to their column densities was hampered by the lack of accurate laboratory data at the frequencies of the survey.
Aims:We wanted to determine spectroscopic parameters of the N isotopomers of cyanamide that are accurate enough for predictions well into the submillimeter region and to search for them in the PILS data.
Methods:We investigated the laboratory rotational spectra of HNCN and HNCN in the selected region between 192 and 507 GHz employing a cyanamide sample in natural isotopic composition. Additionally, we recorded transitions of HNCN.
Results:We obtained new or improved spectroscopic parameters for the three isotopic species. Neither of the N isotopomers of cyanamide were detected unambiguously in the PILS data. Two relatively clean lines can be tentatively assigned to HNCN. If confirmed, their column densities would imply a low N/N ratio for cyanamide toward this source.
Conclusions:The resulting line lists should be accurate enough for observations up to about 1 THz. More sensitive observations, potentially at different frequencies, may eventually lead to the astronomical detection of these isotopic species.
Cyanamide, HNCN, is one of the few interstellar molecules that contain two chemically inequivalent N atoms.111The other are NH, NO, aminoacetonitrile (NHCHCN), a heavy homolog of cyanamide, and -cyanomethanimine (-HNCHCN), see e.g., the Molecules in Space page (http://www.astro.uni-koeln.de/cdms/molecules) of the Cologne Database for Molecular Spectroscopy (Endres et al. 2016), or the list of Interstellar and Circumstellar Molecules of the Astrochymist page (http://www.astrochymist.org/astrochymist_ism.html). It was among the molecules detected early in the interstellar medium (ISM) by radio astronomical means (Turner et al. 1975). The detection was made toward the prolific high-mass star-forming region Sagittarius (Sgr) B2, which is close to the Galactic center. There is also a report on the detection of HNCN toward the nearby Orion-KL region (White et al. 2003) and the high-mass protostar IRAS 20126+410 (Palau et al. 2017). Cyanamide was also found in two nearby starburst galaxies NGC 253 (Martín et al. 2006) and M82 (Aladro et al. 2011).
Coutens et al. (2018) reported the first detection of HNCN toward two solar-type protostars; one was IRAS 162932422 B with the Atacama Large Millimeter/submillimeter Array (ALMA) in the framework of the Protostellar Interferometric Line Survey (PILS), and the other was NGC 1333 IRAS2A in the course of Plateau de Bure Interferometer (PdBI) observations. More recently, a tentative detection was also reported toward the low-mass protostellar source Barnard 1b (Marcelino et al. 2018).
Isotopic fractionation in the N isotopes has attracted considerable interest in recent years. Very different N/N ratios have been determined in star-forming regions which range from less than 100 to around 1000 (e.g., Milam & Charnley 2012; Bizzocchi et al. 2013; Wampfler et al. 2014), compared to the terrestrial ratio of 272 (Berglund & Wieser 2011). However, the reasons for the enrichment or depletion in N are not clear at present.
In order to address the possible N-fractionation for cyanamide, reliable transition frequencies of these rarer isotopologs are critical. The laboratory rotational spectrum of the HNCN main isotopolog has been studied extensively since the fairly early days of microwave spectroscopy (Tyler et al. 1959), especially by Read et al. (1986) and by Birk et al. (1993). Only very limited information was available for minor isotopologs of cyanamide (Tyler et al. 1972; Brown et al. 1985) until fairly recently when Kraśnicki et al. (2011) reported millimeter and in part submillimeter transition frequencies for numerous isotopic species. Kisiel et al. (2013) presented updated accounts on HNCN, HDNCN, and DNCN. The line lists of the N isotopomers covered 135176 GHz ( to 8) besides some older, lower-accuracy data with or 1 (Kraśnicki et al. 2011). Therefore, we decided to record rotational transitions of HNCN and HNCN between 192 and 507 GHz which should be sufficient to search for these isotopic species throughout the entire presently available frequency range of ALMA up to almost 1 THz. In the course of our measurements, we also recorded some transitions of HNCN to improve its spectroscopic parameters somewhat.
2 Spectroscopic properties of cyanamide
Cyanamide is an asymmetric rotor with , very close to the symmetric limit of . A model of the molecule is shown in Fig 1. It is nonplanar with two equivalent configurations, albeit with a low barrier to planarity (Kisiel et al. 2013). Tunneling between the two minima lifts the degeneracy and creates tunneling substates and a which are separated by 1486 GHz (or 49.57 cm or 71.32 K). The symmetry of the planar transition state is with the -axis being the symmetry axis. The spin-statistics of the two equivalent H nuclei lead to ortho and para states with a spin-statistical weight ratio of 3 : 1. The ortho states are described by being even in and odd in . Transitions within the tunneling states obey -type selection rules with D and D (Brown et al. 1985). Tunneling occurs along the -inertial axis, hence the two tunneling states are connected by -type transitions with D (Brown et al. 1985).
Carbon has two stable isotopes with mass numbers 12 and 13; the terrestrial abundances of the latter is 1.11 % (Berglund & Wieser 2011). The corresponding abundance of N is 0.36 %, much less than the dominant N.
3 Laboratory spectroscopic details and observed lines
The investigations of isotopic cyanamide were carried out in a five-meter long double-path glass cell at room temperature and under slow flow conditions. A commercial sample from Sigma-Aldrich was used without further purification. The sample required slight heating to reach a sufficient vapor pressure which was maintained at about 0.5 Pa inside the cell. We employed three frequency multipliers (Virginia Diodes, Inc.) driven by an Agilent E8257D synthesizer as sources with appropriate Schottky diodes as detectors to cover frequencies between 192 and 507 GHz. Frequency modulation reduces standing waves, and the demodulation at twice the modulation frequency leads to approximately second-derivative line-shapes. Further details on the spectrometer system are available in Martin-Drumel et al. (2015).
We optimized the conditions, measurement time, pressure, and modulation deviation, by recording selected transitions of HNCN with values of 1 and 2 around 300 GHz which were well predicted by the available data (Kraśnicki et al. 2011). An integration time of 60 ms per measurement point of one scan upward in frequency and one downward was sufficient for these lines and most of the N isotopomers. Several scans were coadded for some weaker lines.
The available HNCN and HNCN transition frequencies obeyed -type selection rules ( and ). They covered to with and 4 for 0 and 0, respectively, from Kraśnicki et al. (2011), and some and transitions from Tyler et al. (1972) and Brown et al. (1985). The resulting spectroscopic parameters were good enough to find transitions with low values of , starting with around 195 GHz and low values of . We determined new spectroscopic parameters and covered several higher up to near 505 GHz. The range extended eventually to 7 and 6 for 0 and 0, respectively. The uncertainties ranged from 20 kHz for isolated lines with symmetric line shape and good signal-to-noise ratio to 100 kHz.
Subsequently, we recorded HNCN transitions to supplement earlier measurements by Kraśnicki et al. (2011). Their data included to and to with and 6 for 0 and 0, respectively, at frequencies of 119 to 161 and 578 to 641 GHz. Our data covered to and mostly higher values, 6 for 0 and 7 for 0 and 0, along with some lower- data. We assigned uncertainties of 20 or 30 kHz to our HNCN transition frequencies. Figure 2 displays the , transition of each of the three isotopologs.
4 Determination of spectroscopic parameters
We used Pickett’s SPCAT and SPFIT programs (Pickett 1991) to predict and fit rotational spectra of the minor isotopic species of cyanamide employing the reduced axis system Hamiltonian (Pickett 1972), as was done previously (Read et al. 1986; Birk et al. 1993; Kraśnicki et al. 2011; Kisiel et al. 2013). We used Watson’s reduction of the rotational Hamiltonian, in contrast to the latest two publications but in agreement with earlier work. Noting that the ground vibrational state consists of the substates 0 and 0, we fit average spectroscopic parameters and , as proposed by Christen & Müller (2003). The model was tested for the rotational spectrum of NHD and applied successfully in an analysis of the rotational and rovibrational spectra of HDO (Müller et al. 2010). The advantage of being able to add parameters or their differences separately was particularly apparent in the case of ethanethiol (Müller et al. 2016). As the available experimental lines of the minor isotopic species are considerably smaller than those of the main species, we estimated several of the higher-order parameters of the former by scaling parameters of the main isotopolog by the ratios of appropriate powers of , , and , as done recently for HCO, HCO, and HCO (Müller & Lewen 2017). A similar approach was taken for fitting spectra of isotopic species of cyanamide (Kraśnicki et al. 2011).
|1460174||. (120)||1486116||. (118)||1486522||. (66)||1486004||.275 (185)|
|342||.505 (46)||335||.828 (47)||345||.963 (59)||346||.5304 (245)|
|1||.2756 (113)||1||.1048 (119)||1||.2514 (108)||1||.2009 (89)|
|307819||.7 (241)||308328||.7 (239)||308205||.1 (122)||308298||.054 (66)|
|9845||.11174 (75)||9794||.54528 (82)||10117||.07868 (93)||10121||.205258 (188)|
|8||.74933 (71)||8||.09439 (79)||8||.56394 (93)||8||.551540 (113)|
|9604||.18856 (76)||9555||.59774 (79)||9862||.34933 (114)||9866||.291773 (186)|
|0||.28904 (77)||0||.26534 (81)||0||.35051 (110)||0||.368180 (109)|
|368||.428 (80)||357||.939( 80)||377||.325 (101)||377||.705 (40)|
|19||.239 (44)||17||.766 (42)||18||.097 (83)||18||.494 (26)|
|3||.59457 (39)||3||.50397 (38)||3||.75968 (31)||3||.759689 (180)|
|0||.01490 (34)||0||.01741 (32)||0||.01813 (26)||0||.018511 (49)|
|126||.84 (48)||117||.29 (49)||129||.70 (36)||130||.2699 (176)|
|8||.89 (47)||9||.37 (49)||10||.88 (36)||10||.5633 (225)|
|25||.100 (283)||23||.484 (304)||27||.739 (219)||27||.2965 (45)|
|4||.931 (150)||4||.062 (206)||5||.516 (165)||5||.2987 (43)|
|219||.2 (50)||267||.7 (48)||196||.2 (45)||264||.41 (234)|
|71||.6 (12)||70||.6 (10)||82||.3 (7)||73||.64 (180)|
|0||.889 (73)||1||.115 (70)||0||.945 (44)||1||.0798 (167)|
|0||.250 (27)||0||.268 (23)||0||.297 (11)||0||.2879 (65)|
|4||.118 (88)||3||.439 (82)||5||.800 (54)||3||.681 (46)|
|17||.78 (223)||10||.10 (199)||17||.44 (94)||12||.47 (42)|
Our reanalysis of HNCN data started with the line list from Kisiel et al. (2013). The very extensive line list was not reproduced within experimental uncertainties on average, such that the authors omitted transitions with residuals between observed transition frequencies and those calculated from the final set of spectroscopic parameters exceeding three times the uncertainties. Initially, we weighted down these lines, but omitted most of them in the end because the residuals in our final fit were very similar to those of the previous fit. In fact, our final parameter set differed effectively only slightly from that obtained by Kisiel et al. (2013). We added fixed values of and to account better for transitions higher in , especially -type transitions between the tunneling states. The parameter values were obtained from a fit to the line list deposited in the JPL catalog archive (Pickett et al. 1998) which was based on Read et al. (1986) and Birk et al. (1993) and which extended to transitions considerably higher in . In addition, we did not use in the fit. Our parameter values were in good agreement with those of Kisiel et al. (2013), as was expected. The majority of their transition frequencies were obtained by far-infrared spectroscopy. They were reproduced on average to 0.00023 cm, slightly above the reported uncertainties of 0.00020 cm. Therefore, we increased the uncertainties to 0.00025 cm. The parameter values changed little, but now not only the frequencies determined with microwave accuracy were reproduced within uncertainties on average, but also the far-infrared transition frequencies.
The details for determining spectroscopic parameters were the same for the two N isotopomers of cyanamide and for HNCN. The parameter set of the main isotopic species was taken as a start. Low-order spectroscopic parameters were fit until the transition frequencies were reproduced within around five times the experimental uncertainties. At each step, we searched for the parameter which improved the quality of the fit the most. Subsequently, parameters still kept fixed were scaled with appropriate powers of the ratios of and . Since was correlated with , scaling with appropriate powers of the ratios of was done subsequently; was scaled with the ratio of ; the distortion corrections to were scaled in addition with the ratios of the . In the next step, parameters of higher order were released until the qualities of the fits no longer significantly improved. In a final step, was scaled again with the ratio of . Kraśnicki et al. (2011) assigned uncertainties of 50 kHz to their transition frequencies. These were appropriate in the case of the N isotopomers. In the case of HNCN, they were slightly pessimistic at lower frequencies (119161 GHz) and slightly optimistic at higher frequencies (578644 GHz). Uncertainties of 30 and 60 kHz, respectively, turned out to be appropriate. Uncertainties of 100 kHz were used for the few older transition frequencies (Tyler et al. 1972; Brown et al. 1985). The transition frequencies of each isotopic species as well as subsets thereof were reproduced mostly within and always close to the experimental uncertainties on average. The resulting spectroscopic parameters are given in Table 1 together with those of the main isotopic species. The experimental line lists have been deposited at the Centre de Données astronomiques de Strasbourg (CDS) as three separate ascii files. A portion is shown for one isotopolog in Table 2 of Appendix A.
The line lists are quite similar in quantum number coverage among the three minor isotopic species and even more so among the two N isotopomers. Therefore, it is not so surprising that the parameters determined in the fits are the same and their respective uncertainties are quite similar. Some smaller uncertainties of specific HNCN parameters compared to those of the N species are a consequence of lines with higher values of (Kraśnicki et al. 2011). Our initial fits of the old data are summarized in Table 3 of Appendix B together with our new parameters for HNCN. The uncertainties of all parameters improved slightly upon addition of our experimental data. The main aspect in case of the two N isotopomers is the increase of determined spectroscopic parameters from ten to 22 because of the increase in from 9 to 26 and in from 5 to 7. The spectroscopic parameters included in the fit are equivalent to those of Kraśnicki et al. (2011); similar to that work, values for , , and so on were taken from the main isotopic species and the remaining parameters were estimated as described above.
It is also not surprising that the NH tunneling splitting is not affected within uncertainties by substituting the distant cyano-N atom, only marginally affected by substitution of the closer cyano-C atom, and somewhat more by the substitution of the amino-N atom. Replacing one or both of the H atoms by D affects the tunneling splitting much more (Kisiel et al. 2013). The heavy atoms are all located close to the -axis, see for example Fig. 3 in Kraśnicki et al. (2011), hence heavy atom substitutions do not change the rotational parameter much. In addition, the C atom is quite close to the center of mass of the molecule such that its substitution changes and only slightly whereas replacing one of the N atoms cause larger changes. Changes in higher order spectroscopic parameters often reflect the changes in and , but exceptions occur, especially for some of the higher order parameters.
5 Astronomical search
We used the ALMA PILS data to search for the N isotopomers of cyanamide. PILS is an unbiased molecular line survey of the Class 0 protostellar binary IRAS 162932422 carried out in band 7 of ALMA and covering the spectral range 329.15362.90 GHz at 0.244 MHz resolution (project-id: 2013.1.00278.S, PI: Jes K. Jørgensen333http://youngstars.nbi.dk/PILS/). Details of the survey have been presented by Jørgensen et al. (2016). The spectral sensitivity is also very high, 710 mJy beam channel or 45 mJy beam km s such that line confusion is reached in considerable parts of the survey. This high sensitivity is very important for searching for less abundant molecules or for minor isotopic species of somewhat more abundant molecules. Many of our analyses focused on source B because of its smaller lines widths of 1 km s compared to around 3 km s for source A. Among the most exciting results was the detection of methyl chloride toward both sources as the first interstellar organohalogen compound (Fayolle et al. 2017) besides numerous isotopologs containing D or C, many of them for the first time in space (Coutens et al. 2016; Jørgensen et al. 2016; Persson et al. 2018; Coutens et al. 2018; Calcutt et al. 2018; Jørgensen et al. 2018).
Coutens et al. (2018) analyzed column densities of cyanamide isotopologs toward a position offset by 0.5 from the continuum peak of source B (, ). This position was chosen to minimize the effects of line self-absorption, which was particularly prominent for the main isotopolog of cyanamide. The emission lines were modeled under the assumption of local thermodynamic equilibrium, which is reasonable at the high densities and high temperatures of source B, with an excitation temperature K, a source size of 0.5, and a full line width at half maximum (FWHM) of 1 km s. A column density of cm was obtained for the HNCN isotopolog. Assuming a standard C/C ratio, the abundance of the main isotopolog is about cm.
As a result of our spectroscopic investigation, we can now search for the N isotopomers of cyanamide with confidence. The molecules have several lines in the range of PILS, but most of them were well below the limit of detection or, in some cases, blended with bright lines of other species. We find two lines that can potentially be assigned to HNCN (see Fig. 3) and none for HNCN. We carefully checked that the two potential lines of HNCN were not due to any other species already identified toward this source and that no line expected to be observed was missing. Using the same position, , source size, and as for HNCN, we obtain a good model for a column density of cm (see Fig. 3). As the spectra of IRAS 162932422 are very line-rich, more transitions need to be detected to claim a secure detection. For now, this column density can only be considered as an upper limit. A 3 upper limit of cm is derived for HNCN. This corresponds to N/N ratios of 80 for HNCN and 160 for HNCN. For comparison in the same source, the N/N ratio of CHCN is about 250 (Calcutt et al. 2018), while a similar lower limit of 100 was derived for NHCHO (Coutens et al. 2016). Single-dish observations of two and one transitions of HCN and HNC isotopologs led to ratios of , , and , respectively (Wampfler et al. 2014).
More sensitive observations are required to confirm the detection of HNCN and possibly identify HNCN in IRAS 162932422. Such observations may be more promising at lower frequencies because line blending and issues due to absorption are likely smaller. In addition, observations are, for the most part, easier at lower frequencies. On the other hand, the intensities of cyanamide emission drop fast at lower frequencies. Therefore, we inspected a synthetic spectrum of the 200320 GHz region for bright lines of the N isotopomers of cyanamide that are not blended with lines of species in a synthetic spectrum which includes most of the species identified in the PILS data with certainty. About 20 lines of HNCN and less than 10 lines of HNCN are currently predicted to be free of blending. Since most of the blends in the PILS Band 7 data are caused by known species, it may well be that a fair fraction of the about 30 lines of cyanamide containing N will eventually turn out to be unblended.
6 Conclusion and outlook
Our studies of isotopic cyanamide yielded greatly extended experimental line lists of HNCN and HNCN up to 507 GHz. The resulting spectroscopic parameters should permit sufficiently accurate extrapolation in up to 1 THz, that is, throughout all presently available ALMA bands. Extrapolation in may be questionable because of vibration-rotation interaction, but the covered range should be sufficient for all minor isotopic species. Transitions between the tunneling states, which obey -type selection rules, cannot be predicted with meaningful accuracy. These transitions are quite weak and have, to the best of our knowledge, not been identified in space even for the main isotopic species. The line list of HNCN has been extended somewhat in the course of the present investigation, and the spectroscopic parameters have been improved.
Calculated line lists of these three isotopic species, including information on their intensities and their calculated uncertainties, will be provided or updated in the catalog section444http://www.astro.uni-koeln.de/cdms/entries of the Cologne Database for Molecular Spectroscopy, CDMS (Endres et al. 2016). Files containing the experimental lines or parameters along with auxiliary files are available in the data section of the CDMS555http://www.astro.uni-koeln.de/cdms/daten/Cyanamide.
A first attempt to search for the N isotopologs of cyanamide in the PILS data turned out to be negative. The two lines assignable to HNCN would imply a low N/N ratio if their identity were to be confirmed. Future, more sensitive observations of IRAS 162932422 are required to improve the upper limits or even lead to an unambiguous detection of at least one of the isotopic species.
Acknowledgements.The measurements in Köln were supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the collaborative research grant SFB 956, project B3. A.C.’s postdoctoral grant is funded by the ERC Starting Grant 3DICE (grant agreement 336474) under the European Unionâs Horizon 2020 research and innovation program. A.C. is grateful to COST (European Cooperation in Science and Technology) for a STSM grant from the COST Action CM1401 “Our Astro-Chemical History”. O.Z. is funded by the DFG via the Gerätezentrum “Cologne Center for Terahertz Spectroscopy”. The research of J.K.J. is supported by the European Research Council through the ERC Consolidator Grant âS4Fâ (grant agreement No 646908) and Centre for Star and Planet Formation funded by the Danish National Research Foundation. This paper makes use of the following ALMA data: ADS/JAO.ALMA2013.1.00278.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. Our research benefited from NASA’s Astrophysics Data System (ADS).
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Appendix A. Supplementary material
|1||0||1||0||0||0||0||0||19458||.1||0.1||0.15777||Tyler et al. (1972)|
|2||0||2||0||1||0||1||0||38916||.282||0.100||0.00717||Brown et al. (1985)|
|7||0||7||0||6||0||6||0||136194||.32870||0.05000||0.00263||Kraśnicki et al. (2011)|
|8||0||8||0||7||0||7||0||155645||.83281||0.05000||0.01287||Kraśnicki et al. (2011)|
|9||0||9||0||8||0||8||0||175095||.43308||0.05000||0.00786||Kraśnicki et al. (2011)|
Appendix B. Fits of previous data
|1462540||. (106)||1486091||. (133)||1486783||. (96)||1486522||. (66)|
|342||.74 (23)||335||.42 (41)||345||.999 (69)||345||.963 (59)|
|1||.156||1||.125||1||.2388 (130)||1||.2514 (108)|
|308298||.||308298||.||308209||.3 (182)||308205||.1 (122)|
|9845||.1075 (35)||9794||.5437 (55)||10117||.07913 (141)||10117||.07868 (93)|
|8||.7495 (36)||8||.0946 (61)||8||.56355 (140)||8||.56394 (93)|
|9604||.1941 (33)||9555||.5997 (44)||9862||.34845 (140)||9862||.34933 (114)|
|0||.28904 (77)||0||.2699 (49)||0||.35121 (136)||0||.35051 (110)|
|367||.767 (92)||358||.168( 109)||376||.911 (169)||377||.325 (101)|
|19||.186 (139)||17||.628 (168)||17||.972 (102)||18||.097 (83)|
|3||.6056 (207)||3||.4887 (218)||3||.76075 (44)||3||.75968 (31)|
|0||.0165 (204)||0||.0405 (221)||0||.01861 (34)||0||.01813 (26)|
|119||.81||118||.22||130||.23 (44)||129||.70 (36)|
|9||.71||9||.59||10||.54 (44)||10||.88 (36)|
|24||.383||23||.984||27||.119 (280)||27||.739 (219)|
|4||.733||4||.656||5||.484 (167)||5||.516 (165)|
|257||.||256||.||235||.5 (118)||196||.2 (45)|
|72||.||71||.||84||.4 (32)||82||.3 (7)|
|1||.020||1||.012||1||.122 (86)||0||.945 (44)|
|0||.273||0||.270||0||.267 (26)||0||.297 (11)|
|3||.58||3||.56||4||.953 (239)||5||.800 (54)|
|11||.8||11||.7||12||.21 (266)||17||.44 (94)|