EDP Sciences
Free Access
Issue
A&A
Volume 587, March 2016
Article Number A152
Number of page(s) 6
Section Atomic, molecular, and nuclear data
DOI https://doi.org/10.1051/0004-6361/201526924
Published online 07 March 2016

© ESO, 2016

1. Introduction

This paper is a part of a series of studies conducted in PhLAM Lille (France) that are devoted to the investigations of the spectra of different isotopic species of astrophysical molecules (Demyk et al. 2007; Margulès et al. 2009a,b, 2015; Carvajal et al. 2009; Tercero et al. 2012; Bouchez et al. 2012; Coudert et al. 2012; Richard et al. 2012, 2013; Haykal et al. 2013; Kutsenko et al. 2013; Nguyen et al. 2013). In particular these works led to the first interstellar detection of HCOOCH2D (Coudert et al. 2013), HCOO13CH3 (Carvajal et al. 2009), HCO18OCH3, HC18OOCH3 (Tercero et al. 2012), CH2DOCH3 (Richard et al. 2013). In the current paper we focus our attention on the 13C isotopologue of the methylamine molecule. Interstellar methylamine was first detected toward Sgr B2 at 3.5 cm (Fourikis et al. 1974) and at 3 mm (Kaifu et al. 1974). Later surveys conducted by Turner (1989) toward Sgr B2(OH) and Nummelin et al. (1998) in Sgr B2(N) also found spectral lines of CH3NH2. Recently, methylamine has been detected in a spiral galaxy with a high redshift of 0.89, located in front of the quasar PKS 1830-211 (Muller et al. 2011). Also Fourikis et al. (1977) reported probable detection of deuterated methyl amine (CH3NHD) in the southern region of Sgr B2.

The rotational spectrum of the parent methylamine species has been the subject of a number of investigations. The latest paper on this issue (Motiyenko et al. 2014) covers the frequency range up to 2.6 THz and the range of rotational quantum number J up to 45, which provides reliable predictions of the methylamine spectrum in the frequency range up to 3 THz. Also a number of deuterated isotopologues were studied by means of microwave spectroscopy: CH3ND2 (Takagi & Kojima 1971), CD3NH2 (Sastry 1960; Kreglewski et al. 1990b), CD3ND2 (Lide 1954; Kreglewski et al. 1990a), CH3NHD (Ohashi et al. 1991), and CH2DNH2 (Tamagake & Tsuboi 1974). To our knowledge, there has been no previous spectroscopic study of 13CH3NH2 isotopologue of methylamine in the microwave range. In this context, we present here an experimental study of the 13CH3NH2 ground state rotational spectrum in the frequency range from 48 to 945 GHz. Based on the results obtained, a search for interstellar 13C methylamine was made using available data from our recent observations of methanimine (CH2NH) and methyl amine (CH3NH2) toward Sgr B2(N) (Halfen et al. 2013).

2. Experiments

The measurements in Lille were done with a 13C enriched sample of methylamine purchased from Sigma-Aldrich, which was used without further purification. The measurements in the frequency ranges 150–315, 400–630 and 775–945 GHz were covered using the Lille spectrometer with solid state multiplied sources. The frequency of the Agilent synthesizer (12.5–17.5 GHz) was used as a reference source of radiation, which was further multiplied using various active and passive multipliers. Estimated uncertainties for measured line frequencies are 30 kHz, 60 kHz, and 100 kHz, depending on the observed S/N ratio and the frequency range. The measurements in Kharkov were made using a sample with a natural abundance of 13C isotopologue of methylamine. The measurements were performed in the frequency range from 48 to 148 GHz using the automated spectrometer of Institute of Radio Astronomy of NASU (Alekseev et al. 2012). An initial search for the 13CH3NH2 ground state rotational transitions was made using the spectrum records from our previous study (Ilyushin et al. 2005). Later a number of transitions were remeasured with an increased signal accumulation to provide a better S/N ratio. The estimated uncertainties for the measured line frequencies are 30 kHz and 100 kHz, depending on the observed S/N ratio.

Table 1

Molecular parameters of the ground torsional state of 13CH3NH2.

3. Assignment and analysis of the spectrum

3.1. Theoretical model

In our current study, we used the phenomenological Hamiltonian model, which is based on the group-theoretical high-barrier tunneling formalism that was developed for methylamine by Ohashi & Hougen (1987). Since rather complete descriptions of this formalism and its implementation in the case of the methylamine molecule have been already presented several times in the literature (Ohashi & Hougen 1987; Ohashi et al. 1987; Ilyushin et al. 2005; Ilyushin & Lovas 2007; Motiyenko et al. 2014), we do not repeat a general description here. The formalism appeared to be the most successful for fitting the rotational spectrum of methylamine in the ground state, as well as in the first excited torsional state, and therefore it was chosen for the analysis of the 13CH3NH2 ground state rotational spectrum.

The Hamiltonian operator is defined as (1)where “higher order terms” represent ordinary centrifugal distortion terms as well as the J and K dependences of the various tunneling splitting parameters. The physical meaning of different parameters in Hamiltonian (1) was briefly discussed in Motiyenko et al. (2014). The computer fitting program used in the present analysis of the rotational spectrum of 13C methylamine was developed by N. Ohashi (Ohashi et al. 1987) and previously modified by V. Ilyushin, who added new Hamiltonian terms (Ilyushin et al. 2005; Motiyenko et al. 2014) and provided the line strength calculations (Ilyushin & Lovas 2007). As in the case of the main isotopologue (Ilyushin et al. 2005; Motiyenko et al. 2014) we have made a separate analysis of the quadrupole hyperfine structure of 13C methylamine transitions, which is present because of the nonzero nuclear quadrupole moment of nitrogen 14N. We used the following hyperfine energy expression, which permits the calculation of the hyperfine splittings for each rotational transition: (2)where χ+ = −χzz, χ = χyyχxx (Kreglewski et al. 1990a); , , , JxJz + JzJx are the expectation values of the operators in the axis system of the effective Hamiltonian (1) used, and f(I,J,F) is the Casimir function (Gordy & Cook 1984). As in the case of the main methylamine isotopologue, the analysis of observed hyperfine structure produced a set of “hyperfine free” rotational transition frequencies which were further analyzed using the effective Hamiltonian (1).

3.2. Assignment and analysis

We started our analysis of recorded spectra from a search of Ka = 0R-type transitions which, because of torsion-wagging large amplitude motion in the molecule, form a characteristic quartet of lines. This search was based on the assumption that the substitution of 13C will not affect significantly the tunneling splittings in the molecule (in comparison to the main methylamine isotopologue) and that the corresponding quartets of Ka = 0R-type transitions will have a similar appearance in 13CH3NH2 and 12CH3NH2 spectra. The tentative assignment of the first such quartet at 173.2 GHz that corresponds to the Ka = 0J = 4 ← 3 transitions gave us an opportunity to assign the whole series of this kind of transitions and get an initial approximation for the rotational constants of 13CH3NH2 (at this stage the tunneling parameters were kept fixed at the values of the main methylamine isotopologue, Motiyenko et al. 2014). Obtained in this way, the initial model allowed us to continue assigning the spectra in a usual iterative manner by adding new assigned lines to the fit, refining the Hamiltonian model, and producing new predictions.

The Doppler-limited resolution of our spectrometers provided an opportunity to resolve nuclear quadrupole hyperfine splittings for 898 rotational transitions. Typically, as for the main methylamine isotopologue, a resolved pattern of the hyperfine structure was observed as a doublet with an approximately 2-to-1 ratio in intensities. The stronger doublet component contains unresolved hyperfine transitions with selection rules F = J + 1 → F′ = J′ + 1 and F = J−1 → F′ = J′−1, whereas the weaker doublet component corresponds to the F = JF′ = J transition. At the same time, a number of hyperfine components with selection rules ΔF ≠ ΔJ were also assigned. To provide the hyperfine-free frequencies we used in our analysis of the rotation-torsion-wagging spectrum of the ground state of 13C methylamine, we fitted the frequencies of the individual hyperfine components using the model described above. The quadrupole hyperfine parameters χ+, and χ that we obtained from the fitting of the hyperfine patterns of the rotational transitions are presented in Table 1.

Table 2

Main rotational and torsion-wagging splitting parameters (all in MHz except of ρ which is unitless) of the ground torsional state of 13C and 12C methylamines.

thumbnail Fig. 1

Predicted (in red) and observed (in blue) rotational spectrum of 13C methylamine between 882 and 908 GHz dominated by cQ-type series of transitions with Ka = 6 ← 5. A slight inconsistency between predicted and observed spectrum, which may be visible for some strong lines, is due to source power and detector sensitivity variations. Weak unassigned lines are due to higher excited vibrational states.

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In total, the dataset for the rotational spectra of 13CH3NH2 in the ground vibrational state includes 2721 rotational transitions with an upper value of J = 43. This set of rotational transitions was fitted to the effective Hamiltonian of the group-theoretical high-barrier tunneling formalism that was developed for methylamine by Ohashi & Hougen (1987). The fit adopted in the present study as the “best” achieved the root-mean-square (rms) deviation of 0.039 MHz that corresponds to the weighted rms deviation of 0.73. The Hamiltonian model includes 75 parameters. The values of the molecular parameters obtained from the final fit are presented in Table 1. In Table 2, the main rotational and torsion-wagging splitting parameters for 13CH3NH2 and 12CH3NH2 (Motiyenko et al. 2014) are compared. From the comparison we see that the values of the main tunneling splitting parameters in both isotopologues of methylamine are quite close. Since the is slightly larger in 13CH3NH2 than in 12CH3NH2, the strong Q-type series of lines in 13CH3NH2 are shifted up in frequency compared with the main isotopologue as it is seen, for example, from Fig. 1, which presents the same Ka = 6 ← 5cQ series of transitions as in Fig. 1 of Motiyenko et al. (2014). A portion of the observed rotational spectrum of 13C methylamine around 890 GHz, compared in Fig. 1 to the predicted rotational spectrum in the ground vibrational state, illustrates an overall very good correspondence between experimental and theoretical spectra. The majority of strong lines are assigned (and correspond mainly to cQ-type transitions with Ka = 6 ← 5) and are well predicted by our current model, although a number of rather strong unassigned lines, presumably belonging to the higher excited states, may still be found in the experimental spectrum.

Table 3

Measured rotational transitions of 13CH3NH2 in the ground vibrational state.

Table 4

Predicted transitions of 13C methylamine in the ground vibrational state.

The list of measured rotational transitions of the ground vibrational state of 13C methylamine is presented in Table 3. In the first six columns of Table 3, the quanta for each spectral line are given: J, Ka, and symmetry label Γ. In the following columns we provide the observed transition frequencies, measurement uncertainties, and residuals from the fit. Only hyperfine free rotational frequencies that were used in the final fit are presented in Table 3 for the observed transitions. Table 4 gives the prediction of the ground state rotational spectrum of 13C methylamine up to 950 GHz. The spectrum was calculated taking nuclear quadrupole hyperfine coupling into account. Therefore each energy level in Table 4 is labeled by four quantum numbers: J, Ka, Γ, and total angular momentum F. The quantum numbers are followed by the columns with calculated transition frequencies and corresponding uncertainties. The next two columns contain the product μ2S (where μ is the dipole moment of the molecule and S is transition linestrength), and the nuclear spin statistical weight which is equal to 1 for A1, A2, and E2 species and equal to 3 for B1, B2, and E1 species. The next column represents the energy of the lower state. In our calculations of the 13CH3NH2 spectrum, the values for the dipole moment components were taken to be equal to the corresponding values of the parent methylamine isotopologue μa = −0.307 D and μc = 1.258 D (Motiyenko et al. 2014). Owing to their significant sizes, the complete versions of Tables 3 and 4 are presented at the CDS. Here only parts of Tables 3 and 4 are provided for illustration purposes.

In our predictions of the ground state rotational spectrum, we have adopted the limitation of Jmax = 50, Ka,max = 20. In the calculation, we have included the rotational transitions with rotational selection rules ΔJ = 0, ± 1 and ΔKa = 0, ± 1, ± 2, ± 3. In Table 4, those transitions that match the frequency range requirement (from 1 GHz to 950 GHz), whose predicted uncertainties are less than 1 MHz and whose line strength exceeds the limit of 0.01, are included. Also, to limit the size of Table 4, we have only presented the most intense hyperfine quadrupole components for which the relative intensities exceeded 0.1% of the total intensity of the rotation transition (i.e., mainly with the selection rule ΔF = ΔJ). In addition, we provide the rotational part of the partition function Qr(T) of 13C methylamine calculated from first principles (Table 5), i.e. via direct summation over the rotational-tunneling states. The maximum value of the J quantum number for the energy levels taken for calculating the partition function is 100. The vibrational part Qv(T) may be estimated in the harmonic approximation using the normal modes reported by Schimanouchi (1972) for the main methylamine isotopologue. Simple formulas for calculating Qv(T) can be found elsewhere (see, for example Gordy & Cook 1984).

4. Astronomical observations

The spectral records used to search for 13CH3NH2 here are a part of a complete spectral-line survey of the 1, 2, and 3 mm windows toward Sgr B2(N). The data were recorded during the period September 2002 to March 2013 using the Arizona Radio Observatory (ARO) 12 m telescope on Kitt Peak and the Submillimeter Telescope (SMT) on Mount Graham.

The receivers employed were dual-polarization, SIS mixers covering the 3 and 2 mm bands (68–116 and 130–172 GHz). The single-sideband mixers were tuned to reject the image sideband of a level typically 18 dB. Data were also obtained with a new dual-polarization receiver using ALMA Band 3 (83–116 GHz) sideband-separating (SBS) mixers. With these devices, the image rejection was usually 16 dB, attained within the mixer architecture. The temperature scale was determined by the chopper wheel method, corrected for forward spillover losses, and is given as . The radiation temperature TR, assuming the source only fills the main beam, is , where ηc is the main beam efficiency, corrected for forward spillover losses. The spectrometer backend utilized for the measurements was a millimeter autocorrelator (MAC) with either 390 kHz or 781 kHz resolution, and a bandwidth of 600 MHz channel-1. The spectra were smoothed using a cubic spline routine to a 1 MHz resolution.

At the SMT, observations in the frequency range 210–280 GHz (1 mm) were taken with a dual-polarization receiver, utilizing ALMA Band 6 SBS mixers with rejection of at least 16 dB of the image sideband. The temperature scale was determined by the chopper wheel method, and is given as . The radiation temperature TR is , where ηb is the main beam efficiency. A 2048 channel filter bank with 1 MHz resolution was utilized as the spectrometer backend, split into parallel mode (2×1024). The beam size ranged from 87′′ to 37′′ at the 12 m and 36′′ to 23′′ at the SMT. All observations were conducted toward Sgr B2(N) (α = 17h44m09.5s; δ = −28°21′20′′; B1950.0, or α = 17h47m19.2s; δ = −28°22′22′′; J2000.0: NED1) in position switching mode with a +30′ OFF position in azimuth. A 10–20 MHz local oscillator shift and direct observation of the image sideband were employed to identify any image contamination. The pointing accuracy is estimated to be ± 5′′−10′′ at the 12 m and ± 2′′ at the SMT. The telescope pointing was determined by observations of planets.

thumbnail Fig. 2

Results of an interstellar search for the selected rotational transitions of 13CH3NH2.

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Table 5

Rotation part Qr(T) of the total internal partition function Q(T) = Qv(T) × Qr(T), calculated from first principles using the parameter set of Table 1.

Table 6

Uncontaminated transitions of 13C methylamine in Sgr B2(N).

5. Column density upper limit toward Sgr B2(N)

We searched for the transitions of 13CH3NH2 in our complete spectral-line survey of the 1, 2, and 3 mm windows toward Sgr B2(N). The search was conducted in a systematic way by modeling the expected emission in the local thermodynamic equilibrium (LTE) approximation, which is certainly valid for the rotational transitions in the vibrational ground state. There are a number of non-LTE effects which may affect our results. We may overestimate a total column density of 13CH3NH2 gas if the density is not sufficient to thermally populate the vibrationally excited states via collisions (the partition function of 13C methylamine includes the contribution of excited vibrational states in the LTE approximation). At the same time, we may underestimate the total column density because infrared excitation may also contribute to the population of the vibrationally excited states (e.g., Nummelin & Bergman 1999). Also we cannot exclude that radiative decay, involving rovibrational transitions, may affect the rotational populations of the vibrational ground state. Since there is no easy way to correct for these possible non-LTE effects, we restrained our analysis to the LTE assumption. Within the frequency ranges of our survey, we have looked for 69 transitions of 13C methylamine that are predicted to be the most prominent spectral features of the molecule under the physical parameters of our model. We assumed the same source size, temperature, linewidth, and systemic velocity as for parent isotopic species of methylamine (Halfen et al. 2013). The physical parameters of the 13CH3NH2 model are Trot = 159 K, ΔV1 / 2 = 15 km s-1, and VLSR = 64 km s-1 with the assumption that the source fills the beam. Among checked transitions, only eight are found to be relatively free of contamination, and these are listed in Table 6. All other transitions of 13CH3NH2 are heavily blended with transitions of other species and cannot be detected with our single-dish data. The LTE modeled spectrum for the four selected contamination-free transitions of 13CH3NH2 is compared to the observed spectrum in Fig. 2. It is seen that, at the typical noise level of this survey, we have the apparent non-detection of these 13CH3NH2 transitions toward Sgr B2(N). Therefore only an upper limit on the column density of 13CH3NH2 can be deduced from our current observational data which, for the assumed physical parameters of our model, is <6.5 × 1014 cm-2.

6. Conclusions

The first study of the rotational spectra of the 13C isotopologue of methylamine molecule was carried out in the frequency range from 48 to 945 GHz. Using the effective Hamiltonian model based on the group-theoretical high-barrier tunneling formalism developed for methylamine by Ohashi & Hougen (1987), we were able to fit the available data within experimental accuracy. The results of the present study allow us to produce reliable predictions of rotational spectra in the ground vibrational state of 13CH3NH2 for astrophysical purposes in the frequency range up to 950 GHz for 0 <J< 50 and 0 <Ka< 20. An attempt to detect interstellar 13CH3NH2 toward Sgr B2(N) using available observational data from a spectral-line survey of the 1, 2, and 3 mm windows using the Arizona Radio Observatory (ARO) 12 m telescope on Kitt Peak and the Submillimeter Telescope (SMT) on Mount Graham was not successful and resulted in the determination of the 13CH3NH2 column density upper limit of <6.5 × 1014 cm-2. The ratio of 12CH3NH2 to 13CH3NH2, using the column density of 12CH3NH2 from Halfen et al. (2013), is >7.7. This value can be compared to those from other species, such as HC3N and CH2CHCN, in Sgr B2(N) of ~20 (Belloche et al. 2013, Halfen et al., in prep.). Thus, the upper limit for 13CH3NH2 in Sgr B2(N) could be approximately 2.5 × 1014 cm-2, about a factor of three less than the current limit, and below the current detection limit. We hope that the new observations toward Sgr B2(N) using the very sensitive ALMA telescope will provide an opportunity to detect 13CH3NH2 in the interstellar medium.


Acknowledgments

The authors are indebted to Dr. N. Ohashi for providing his fitting program for methylamine. This work was done with the support of the Ukrainian-French CNRS-PICS 6051 project. This research was supported by NSF grants AST-1140030 and AST-1211502. The SMT and Kitt Peak 12 m are operated by the Arizona Radio Observatory (ARO), Steward Observatory, University of Arizona, with support from the NSF University Radio Observatories program (URO: AST-1140030).

References

All Tables

Table 1

Molecular parameters of the ground torsional state of 13CH3NH2.

Table 2

Main rotational and torsion-wagging splitting parameters (all in MHz except of ρ which is unitless) of the ground torsional state of 13C and 12C methylamines.

Table 3

Measured rotational transitions of 13CH3NH2 in the ground vibrational state.

Table 4

Predicted transitions of 13C methylamine in the ground vibrational state.

Table 5

Rotation part Qr(T) of the total internal partition function Q(T) = Qv(T) × Qr(T), calculated from first principles using the parameter set of Table 1.

Table 6

Uncontaminated transitions of 13C methylamine in Sgr B2(N).

All Figures

thumbnail Fig. 1

Predicted (in red) and observed (in blue) rotational spectrum of 13C methylamine between 882 and 908 GHz dominated by cQ-type series of transitions with Ka = 6 ← 5. A slight inconsistency between predicted and observed spectrum, which may be visible for some strong lines, is due to source power and detector sensitivity variations. Weak unassigned lines are due to higher excited vibrational states.

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In the text
thumbnail Fig. 2

Results of an interstellar search for the selected rotational transitions of 13CH3NH2.

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In the text

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