EDP Sciences
Free Access
Issue
A&A
Volume 552, April 2013
Article Number A117
Number of page(s) 10
Section Atomic, molecular, and nuclear data
DOI https://doi.org/10.1051/0004-6361/201220826
Published online 12 April 2013

© ESO, 2013

1. Introduction

Dimethyl ether (DME) is a large complex organic molecule (COM) detected for the first time in the interstellar medium (ISM) by Snyder et al. (1974). Abundant in the hot cores, which are the precursors of high-mass stars (Ikeda et al. 2001), DME is also one of the main COMs in hot corinos, forming low-mass stars, such as IRAS 16293-2422 (Cazaux et al. 2003; Bottinelli et al. 2004). In both types of sources, the commonly adopted scenario is that grain surface chemistry plays a crucial role in the formation of COMs in the early, cold prestellar stage of star formation; subsequently, during the warm up phase corresponding to the hot cores and hot corinos stages, the icy grain mantles evaporate and inject the products of grain surface chemistry into the molecular gas (Herbst & van Dishoeck 2009, and references therein). However, the relative importance of cold grain surface and post-evaporation warm gas-phase processes in the formation of DME is under debate (see e.g. Peeters et al. 2006; Brouillet et al. 2013). Furthermore, DME has been recently detected in a cold prestellar core, L1689B, (Bacmann et al. 2012), where the warm-up phase has not yet taken place. At least for DME, the COM formation scenario in protostars needs further investigations. Many hydrogenated molecules observed in hot corinos such as IRAS 16293-2422 show remarkably high D/H abundance ratios, significantly higher than observed in hot cores. This so-called super-deuteration phenomenon (Ceccarelli et al. 2007) is thought to be linked to molecular depletion on the grain surface during the cold prestellar phase. However, deuterium chemistry in hot cores might be significantly different as shown by the distinct methanol deuterated species relative abundances (Ratajczak et al. 2011). Thus, measuring the deuteration ratio of DME in different types of sources is likely to provide constraints on this species chemistry and formation history. Even though dimethyl ether is such an abundant interstellar molecule, up to now, the mono-deuterated isotopologue has not been observed.

thumbnail Fig. 1

Representation of the DME and of its electric dipole moment in the principal inertial axes. The dipole moment arrow is drawn from the negative to positive charge.

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DME is a near-prolate asymmetric top (κ =  −0.922 MHz) with only a b-dipole moment component, μb = 1.302 D (Blukis et al. 1963). Figure 1 represents the molecule and its electronic dipole moment pointing from the negative to the positive poles along the b-axis. Previous laboratory investigations have been conducted for DME up to 2.1 THz (Groner et al. 1998; Endres et al. 2009). Upon partial deuteration on one of the methyl groups, the DME molecule can exist in one of two different conformations, each with a single CH internal rotor. The symmetric conformation is characterized by the D atom located in the C-O-C plane, and thus by C symmetry. When the D atom is located outside of the C-O-C plane the corresponding conformation is called asymmetric. The asymmetric conformation has two equivalent configurations with a possible tunneling motion between them. In summary, one can expect to observe in the rotational spectrum of deuterated DME the typical A-E splittings due to the nonsubstituted methyl top internal rotation for both conformations, and additional doublet splitting of A and E lines of the asymmetric conformation due to tunneling of the CHD group.

The rotational spectrum of mono-deuterated DME in the vibrational ground-state (see simulated spectra at different temperatures in Fig. 2) was studied for the first time by Blukis et al. (1963). They measured a few transitions for several isotopic species in the centimeter-wave range (8.2–50 GHz) and determined the rotational constants. The starting point of our study was the analysis carried out in the same frequency range with better accuracy for almost all isotopic species by Niide & Hayashi (2003). In the present study, the set of assigned transitions was thus greatly extended to 1 THz, and data from this current investigation were combined with published data into a global fit for each conformer. At the same time, two new accurate sets of parameters were derived from the fits performed with the ERHAM code (Groner 1997, 2012). These new data are thus now precise enough to allow an astrophysical detection in the ISM.

thumbnail Fig. 2

Stick spectrum of mono-deuterated DME (symmetric conformer) in its vibrational ground state at 150 K (above) and 300 K (below). This figure illustrates the importance of the analysis around 1 THz. Although the dense ISM is generally colder, such temperatures exist in the warm inner regions of the collapsing protostars (see for instance Ceccarelli et al. 2000). The spectrum intensity scale is arbitrary.

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This paper presents our laboratory investigation and analysis of mono-deuterated DME, as well as a detection in the solar-type protostar IRAS 16293-2422 using the IRAM 30 m telescope.

2. Experimental details

2.1. Preparation of mono-deuterated dimethyl ether

The synthesis of mono-deuterated DME has already been reported by Shtarev et al. (1999) and has been modified as follows. Lithium aluminum deuteride (420 mg, 10 mol) and tetraglyme (20 mL) were introduced into a 100 mL two-necked flask equipped with a stirring bar, a stopcock, and a septum. The flask was fitted to a vacuum line equipped with two traps. The flask was immersed in a cold bath (−25  °C) and degassed. The stopcock was then closed. Bromomethyl methyl ether (2.5 g, 20 mmol) diluted in tetraglyme (5 mL) was added slowly with a syringe through the septum. At the end of the addition, the mixture was stirred for 30 min at room temperature. The first trap was then immersed in a  −80  °C cold bath and the second one in a liquid nitrogen bath (−196  °C). The stopcock of the cell was opened slowly. Residual bromomethyl methyl ether and high boiling impurities were condensed in the first trap, and mono-deuterated DME was selectively condensed in the second trap. The yield was 80% based on the starting brominated ether.

2.2. Lille – submillimeter wave spectrometer

The submillimeter-wave measurements were performed with the Lille spectrometer (150–990 GHz) (Motiyenko et al. 2010). The sources are only solid-state devices. The frequency of the Agilent synthesizer (12.5–17.5 GHz) was first multiplied by six and amplified by a Spacek active sextupler providing the output power of  + 15 dBm in the W-band range (75–110 GHz). This power is high enough to use passive Schottky multipliers (×2,  × 3,  × 5,  × 2 × 3,  × 3 × 3) from Virginia Diodes Inc. in the next stage of the frequency multiplication chain. As a detector we used an InSb liquid He-cooled bolometer from QMC Instruments Ltd. to improve the sensitivity of the spectrometer; the source was frequency-modulated at 10 kHz. The absorption cell was a stainless-steel tube (6 cm diameter, 220 cm long). The sample pressure during measurements was about 1.5 Pa (15 μbar), and the linewidth was limited by Doppler broadening. These measurements were performed at room temperature. The measurement accuracy for isolated lines is estimated to be better than 30 kHz up to 630 GHz and 50 kHz at higher frequencies owing to the Doppler effect. However, if the lines were blended or had a poor signal-to-noise ratio, they were assigned an uncertainty of 100 or even 200 kHz. The spectrum of mono-deuterated DME recorded at Lille appears in its entirety in Fig. 3.

thumbnail Fig. 3

Spectrum of mono-deuterated DME recorded at Lille is represented with each multiplier range ( × 2,  × 3,  × 5,  × 2 × 3,  × 3 × 3), which cover most of the frequencies up to 990 GHz. The spectrum intensity scale is arbitrary.

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3. Spectral analysis

This analysis of the spectrum of mono-deuterated DME was undertaken to extend the work of Niide & Hayashi (2003). Previous parameters and assigned transitions were used for the initial prediction with the ERHAM code (Groner 1997, 2012) in Watson’s A reduction (Watson 1977). Then, the prediction was improved step by step with the addition of new identified lines. The XIAM program (Hartwig & Dreizler 1996) was also used for comparisons, but owing limitations in the program for blended transitions, fits results were poorer so are not presented in this paper except for the barrier height for rotation of the methyl group, V3.

Mono-deuterated DME shows a very complex spectrum, and there are a lot of unassigned lines due to the excited torsional states about 200–240 cm above the ground state (values given by Endres et al. (2010) for the parent species). In addition, many lines are blended or distorted. As a rule, lines with residuals higher than 4σ were excluded from the fit. Since the principal axes in mono-deuterated DME have almost the same orientation with respect to the molecular frame as in normal DME, the dipole moment in mono-deuterated DME is almost parallel to the b axis; in consequence, the spectrum contains mainly b-transitions. However, Groner et al. (1998) report that “forbidden” c-transitions can occur when pseudo-quantum numbers Ka and Kc do not represent the wave functions very well in the case of mixing or level crossing. In our analysis, a few c-transitions have been assigned especially at high frequency. Transitions without intense torsional-rotational interaction obey b-type asymmetric top selection rules: ΔJ = 0, ± 1;ΔKa =  ± 1,3,...Kc =  ± 1,3,...    (Gordy & Cook 1984). The spin weight for both substates A and E is 4. The spectroscopic parameters and their uncertainties are presented in Table 1 for both conformers.

Table 1

Spectroscopic constants of the ground-vibrational state of mono-deuterated DME for the two different conformers.

For the symmetric conformer, the fit includes 1255 distinct lines (1219 new lines). The lines were fitted with J up to 54 and Ka up to 15. A total of 20 spectroscopic parameters were determined by the least-squares method and are listed in Table 1. The fifteen nontunneling parameters correspond to the rotational and distortion constants. In addition, two internal rotation parameters (ρ and β), one energy tunneling parameter (ϵ1) and two rotational constant tunneling parameters ( [A − (B + C)/2] 1 and  [(B − C)/4] 1) were determined. The notation used in this paper was developed in detail by Groner (1997). The weighted (dimensionless) standard deviation of the fit is of 1.06, while the rms (for the unweighted frequencies) is of 91 kHz.

Then, 1286 distinct lines (1251 new lines) were assigned and included in the fit for the asymmetric conformer. Three lines from the previous study (Niide & Hayashi 2003) were removed from the final fit because of higher residuals (more than 4σ). In addition, three other microwave lines originally assigned to the A state by Niide & Hayashi (2003) were attributed to the E state, and another tunneling coefficient was determined ( [(B + C)/2] 1). The total of the transitions was fitted with 0 ≤ J ≤ 55 and a Ka value up to 19. Table 1 presents the 21 parameters determined. The fit gives a weighted standard deviation of 1.12 and an rms of 104 kHz.

4. Discussion

The ground-vibrational state rotational spectrum of mono-deuterated DME (CHDOCH) was measured and analyzed in the frequency range up to 1 THz for both conformers. All experimental frequencies given in Tables A.1 and A.2 are available in their entirety in electronic form at the CDS. Only 17 and 19 frequencies for the symmetric and asymmetric conformers, respectively, deviated by more than 3σ. In both cases, lines with residuals greater than 4σ were assigned but not included in the fit, and these frequencies are reported in Tables A.1 and A.2 with an uncertainty of 0.

It seems that somewhat better results were obtained for symmetric conformer in the least-squares fit. Indeed, in the case of asymmetric conformer, tunneling between two equivalent configurations leads to Coriolis-type perturbations in the spectra that cannot be accounted for in the present ERHAM code. Moreover, some series of A and E lines of asymmetric conformer exhibit additional doublet structure due to the tunneling. This effect was particularly visible within the first members of Q branches and at low frequency, i.e., for the lines with low Ka (≤5) and J (≤15). Indeed, the splitting increases as J decreases as shown in Fig. 4. Another example of the quartets for R lines is given in Fig. 5. In all these cases, the center frequency of the doublet was entered as the transition frequency in the ERHAM fit, and an uncertainty of 0.1 MHz was assumed. This method has been used for 89 lines between 150 and 290 GHz. Fortunately for the present investigation, most of the lines showed doublets (twice as intense as for the symmetric conformer); therefore, it was possible to use the one-top approximation for the CH group as long as no quartets were included in the fit. The correct treatment of such additional doubling requires inclusion of new Coriolis-type terms in the model.

thumbnail Fig. 4

Rotational transitions JKa,Kc = 145,10 ← 144,11 and JKa,Kc = 125,8 ← 124,9 of asymmetric mono-deuterated DME in the vibrational ground state at 230 GHz. Stick spectrum below experimental lines represents the prediction given by ERHAM. The internal motion of the CHD group is observed through the A and E components, which are split into two substates. It is also noticeable that this separation increases as J decreases and it is more intense in the E component. The experimental measurements are peaked in the center of the doublet. The spectrum intensity scale is arbitrary.

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thumbnail Fig. 5

Rotational transition JKa,Kc = 63,3 ← 52,4 of asymmetric mono-deuterated DME in the vibrational ground state at 238 GHz. In the case of an R line, the splitting of the A and E states is nearly identical unlike in the Q line as represented in Fig. 4. The spectrum intensity scale is arbitrary.

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Unlike the symmetric conformation, the asymmetric conformer does not have a symmetry plane. Therefore at the beginning, α, the angle of the ρ axis with regard to ab principal plane, was set to an arbitrary value of 10°. The subsequent least-squares fit resulted in an extremely low value for α, much less than 1°. As a consequence, α was set to zero for the final fit.

Another interesting quantity reported in Table 1 is the barrier height V3, which was derived with the XIAM code and, therefore, not used in the fit of the free spectroscopic parameters listed in the table. Its value, either for the symmetric or the asymmetric configuration, is in good agreement with earlier work (Durig et al. 1976; Lovas et al. 1979) for the parent species.

Table 2

Rotational partition function for the symmetric and asymmetric conformers of mono-deuterated DME in the ground vibrational state computed for nine different temperatures.

5. Prediction

The newly derived sets of spectroscopic constants shown in Table 1 have permitted predictions of transition frequencies for the symmetric and asymmetric conformers up to 1.2 THz. Two short examples are provided in Tables B.1 and B.2 from 301.6 GHz to 303.5 GHz. The complete tables are available through the CDS. Calculated frequencies for both torsional substates A and E (symmetry numbers 0 and 1, respectively) are given with their line strength S for the μb component. To obtain the proper transition intensity, S must be multiplied by the square of μb and by the spin weight. Lines with S < 0.1, uncertainty  ≥ 0.2 MHz, and J > 60 were removed from the predictions in order to keep only those lines that could be relevant for an astrophysical detection. In addition, a modified version of ERHAM provided predictions in the format of the JPL catalog (Pickett 1991).

Numerical values of the overall partition function were computed for nine different temperatures and listed in Table 2 in order to derive column densities. The results for the asymmetric conformation are multiplied by two because of the additional degeneracy discussed in Sect. 4. However, the intensity calculation in ERHAM for the JPL catalog format file does not consider this additional degeneracy. To arrive at the proper overall intensity for the asymmetric conformer, one needs to multiply the catalog intensity by two or divide the quoted sum of states by two.

6. Observations

We have successfully searched for the deuterated DME lines in the nearby low-mass protostar IRAS 16293-2422 (hereinafter IRAS 16293). Located in the Ophiuchi complex at 120 pc from the Sun (Loinard et al. 2008), IRAS 16293 has played a similar role to a prototype for low-mass protostars in astrochemical studies, such as Sgr B2 or Orion KL for high-mass protostars. Many complex organic molecules (COMs) have been detected towards IRAS 16293 including DME (Cazaux et al. 2003; Caux et al. 2011), with abundances comparable to those found in high-mass protostars. Because the level of molecular deuteration is considerably higher in low-mass protostars than in their massive counterparts (Ceccarelli et al. 2007), IRAS 16293 is thus the best candidate to look for deuterated DME. The data presented here come from recent observations, performed in March 2012 in four selected frequency ranges at 3, 2, and 1 mm, with the new broad band EMIR receivers at the IRAM 30 m telescope. IRAS 16293 hosts in a common colder envelope, two hot corinos, A (southeast) and B (northwest), separated by about 4″ (Wooten 1989).

Our observations, performed in DBS (double-beam-switch) observing mode with a 90″ throw, were centered on the B component at α(2000.0) = 163222.6, δ(2000.0) =  − 242833. The pointing and focus were checked every two hours on nearby planets or on continuum radio sources (1741-038 or 1730-130). The pointing accuracy was better than 2″, and even at the highest frequencies, the A and B components were both inside the beam of our observations so that the observed emission includes the contributions from both cores. Interferometric observations show that the molecular lines emitted by core B are much narrower than those emitted by core A (Bottinelli et al. 2004) and that DME lines emitted by core B are at least as strong as DME lines emitted by core A (Jørgensen et al. 2011). The TIMASSS survey (The IRAS 16293-2422 Millimeter And Submillimeter Spectral Survey, Caux et al. 2011) performed with the IRAM 30 m telescope and JCMT telescopes confirms that narrow DME lines from B are easily detected when contributions from both sources are simultaneously observed. This represents an additional favorable factor for identifying of deuterated DME lines in the spectrum from IRAS 16293, since it reduces the risk of blending by nearby lines from other species.

Table 3

Observational parameters.

thumbnail Fig. 6

Some observed transitions of DME and DME-1D (black) and the computed LTE model (red) using the CASSIS software. E values are the upper energy level of the observed lines. The notation E refers to the notation used in Table 1. The LTE model has been computed in bins of the same spectral resolution as the observations. Other transitions from other species are also present in these spectra. Panels a) to d): some observed transitions of DME. Panels e) to h): all detected transitions of DME-1D-sym. Panel e) DME-1D-sym (91,9 − 80,8) E and A lines. The blended lines are 1: OCS (14 − 13) and 2: CHOH (10 − 9). Panel f) DME-1D-sym (121,12 − 110,11) E and A lines. The blended lines are 3: HCOOCH (17 − 16) and 4, 5, 6: CHCHO (11 − 10) transitions. Panel g) DME-1D-sym (131,13 − 121,12) E and A lines. Panel h) DME-1D-sym (82,7 − 71,6) E and A lines. The blended lines are 7: HCOOCH (18 − 17), 8: HCO (31,2 − 21,1) and one unidentified line (9: around 13 km s). Panels i) to n): All detected transitions of DME-1D-asym. Panel i) DME-1D-asym (51,5 − 40.4) E and A lines. Panel j) DME-1D-asym (70,7 − 61,6) E and A lines. Panel k) DME-1D-asym (90,9 − 81,8) E and A lines. The blended line is 10: HC(O)NH (74,4 − 64,3). Panel l) DME-1D-asym (100,10 − 91,9) E and A lines. Panel m) DME-1D-asym (91,9 − 80,8) E and A lines. Panel n) DME-1D-asym (121,12 − 110,11) E and A lines.

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Comparison of the line intensities with those of the TIMASSS survey (Caux et al. 2011) shows that the calibration is accurate within 15%. Table 3 summarizes the observed bands and the details of the observations. Several representative spectra are plotted in Fig. 6.

Table 4

Mono-deuterated DME observed lines for both conformations.

7. Results: mono-deuterated DME identification in IRAS 16293

Thanks to the spectral resolution and the sensitivity of our IRAM 30 m observations, we were able to identify the 20 brightest lines of both forms of mono-deuterated DME, eight lines for the symmetric conformer and 12 lines for the asymmetric one (see Table 4) in the 27 GHz wide frequency range covered by our spectra. As for most complex molecules, mono-deuterated DME shows a very large number of transitions between 100 GHz and 220 GHz, and many of them were expected to be very faint. The identification was therefore checked using thresholds on both the upper energy level of the lines, restricted to 100 K and their Einstein coefficient, restricted to Aij  ≥ 10-5.

The lines listed in Table 4 are detected with an S/N higher than 5 (see Fig. 6). Other rotational transitions from both forms of mono-deuterated DME lie in the observed frequency range. According to the predictions of the LTE (local thermodynamic equilibrium) model based on the detected lines (see below), the intensities of these transitions are weak, and their non-detection is coherent with the noise of our observations.

In the same frequency range, several transitions of the main DME isotopomer are present as well (see Fig. 6). To derive the DME main and deuterated isotopomers column densities, we have assumed that emission from all three species were in LTE.

As mentioned above, both source A and source B contribute to the observed emission. The ALMA interferometric observations obtained during the Science Verification program, allow the central velocity Vlsr and the linewidth FWHM of each contribution to be estimated precisely (Pineda et al. 2012), and we used these values as fixed parameters in our LTE modeling of the DME lines.

In contrast, the source sizes, as determined by the interferometric observations, cannot directly be used to model single-dish observations because a fraction of the extended emission collected in the single-dish spectra is partly lost in the interferometric observations. We therefore adjusted the size of the two components in the LTE modeling, and we estimate that the uncertainties on the sizes of the two components adjusted in the LTE modeling are  ~50% so that it introduces an uncertainty of about a factor 2 on the derived column densities. In the following, we used the CASSIS software1 and computed synthetic spectra over a large grid of column densities and rotation temperatures [N, Trot] for each of the two cores assuming LTE2. We then performed a χ2 minimization of the model line profiles using the corresponding [N, Trot] values, compared to the observed lines, as (1)where nlin is the number of lines i, nchan the number of channels j for each line, Iobs,ij and Imod,ij the intensities observed and predicted by the model respectively in the channel j of the line i, and rms the rms of the line i (Coutens 2012). This minimization therefore gives the best-fit column densities and rotational temperature, as well as their corresponding uncertainties, which best reproduce our observations of DME. These values are listed in Table 5.

Table 5

Physical parameters of the DME species adopted and derived from the LTE modeling.

For the mono-deuterated DME species, we assumed, for each core, the same linewidth, central velocity, source size, and rotation temperature as for the main DME species and we have only adjusted the column densities to obtain the best fit to the observed lines. The resulting column densities are listed in Table 5.

The asymmetric conformation of mono-deuterated DME appears to be two times more abundant than the symmetric conformation of mono-deuterated DME on both sources. This result is simply the consequence of the statistical redistribution of D atoms due to a substitution of an H atom by a D atom in the main DME isotopomer. The total DME deuteration ratio is given by the fraction  [Na + Ns] /N. It is  ~ 15% for both A and B components, so much higher than the cosmic D/H value of 1.5    ×    10-5 (Linsky 2003) and comparable to “super-deuteration” ratios measured in IRAS 16293 for HCO and CHOH (Loinard et al. 2000; Parise et al. 2002). This result represents a strong constraint for the chemical modeling of the DME formation and deuteration processes.

8. Conclusion

The torsion-rotational spectrum of mono-deuterated dimethyl ether (CHOCHD) was observed in the laboratory up to 1 THz. More than 2500 distinct lines were assigned to the symmetric and asymmetric conformers. The spectroscopic parameters given in Table 1 were determined for both species, and it allowed us to reproduce measurements with a standard deviation better than 105 kHz. They also have permitted transitions to be predicted up to 1.2 THz. Thanks to these frequency predictions, the symmetric and the asymmetric conformers of mono-deuterated dimethyl ether have been detected in the solar-type binary protostar IRAS 16293-2422. From an LTE modeling of these lines, together with lines from the main isotopomer, we concluded that dimethyl ether is highly deuterated in this source, with a D/H abundance ratio  ~15%, as high as observed for methanol and formaldehyde, two species known to play important roles in the COMs formation history. Comparison of these species deuteration in hot cores might also contribute to a better understanding of the cold grain surface and warm gas-phase processes in the DME chemistry. A detailed and comparative study of the DME deuteration ratio in the two hot corinos of IRAS 16293, which are likely to present somewhat different evolutions due to their different masses, would provide crucial information on the chemical and physical history of the sources. With the high spatial resolution and sensitivity provided by the ALMA interferometer, such ambitious goals can be reached and will represent an important step towards understanding the history of solar-type systems like our own.


1

The CASSIS software was developed by IRAS-UPS/CNRS (http://cassis.irap.omp.eu/)

2

The complete LTE formalism can be found on the CASSIS webpage.

Acknowledgments

This work was supported by the CNES and the Action sur Projets de l’INSU, “Physique et Chimie du Milieu Interstellaire” and by the ANR-08-BLAN-0225 contracts.

References

Appendix A: Experimental frequencies

Table A.1

Experimental frequencies measured in laboratory up to 1 THz for the symmetric conformer.

Table A.2

Experimental frequencies measured in laboratory up to 1 THz for the asymmetric conformer.

Appendix B: Predicted frequencies

Table B.1

Predicted transition frequencies of mono-deuterated DME in the ground-vibrational state for the symmetric conformer up to 1.2 THz.

Table B.2

Predicted transition frequencies of mono-deuterated DME in the ground-vibrational state for the asymmetric conformer up to 1.2 THz.

All Tables

Table 1

Spectroscopic constants of the ground-vibrational state of mono-deuterated DME for the two different conformers.

Table 2

Rotational partition function for the symmetric and asymmetric conformers of mono-deuterated DME in the ground vibrational state computed for nine different temperatures.

Table 3

Observational parameters.

Table 4

Mono-deuterated DME observed lines for both conformations.

Table 5

Physical parameters of the DME species adopted and derived from the LTE modeling.

Table A.1

Experimental frequencies measured in laboratory up to 1 THz for the symmetric conformer.

Table A.2

Experimental frequencies measured in laboratory up to 1 THz for the asymmetric conformer.

Table B.1

Predicted transition frequencies of mono-deuterated DME in the ground-vibrational state for the symmetric conformer up to 1.2 THz.

Table B.2

Predicted transition frequencies of mono-deuterated DME in the ground-vibrational state for the asymmetric conformer up to 1.2 THz.

All Figures

thumbnail Fig. 1

Representation of the DME and of its electric dipole moment in the principal inertial axes. The dipole moment arrow is drawn from the negative to positive charge.

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In the text
thumbnail Fig. 2

Stick spectrum of mono-deuterated DME (symmetric conformer) in its vibrational ground state at 150 K (above) and 300 K (below). This figure illustrates the importance of the analysis around 1 THz. Although the dense ISM is generally colder, such temperatures exist in the warm inner regions of the collapsing protostars (see for instance Ceccarelli et al. 2000). The spectrum intensity scale is arbitrary.

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In the text
thumbnail Fig. 3

Spectrum of mono-deuterated DME recorded at Lille is represented with each multiplier range ( × 2,  × 3,  × 5,  × 2 × 3,  × 3 × 3), which cover most of the frequencies up to 990 GHz. The spectrum intensity scale is arbitrary.

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In the text
thumbnail Fig. 4

Rotational transitions JKa,Kc = 145,10 ← 144,11 and JKa,Kc = 125,8 ← 124,9 of asymmetric mono-deuterated DME in the vibrational ground state at 230 GHz. Stick spectrum below experimental lines represents the prediction given by ERHAM. The internal motion of the CHD group is observed through the A and E components, which are split into two substates. It is also noticeable that this separation increases as J decreases and it is more intense in the E component. The experimental measurements are peaked in the center of the doublet. The spectrum intensity scale is arbitrary.

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In the text
thumbnail Fig. 5

Rotational transition JKa,Kc = 63,3 ← 52,4 of asymmetric mono-deuterated DME in the vibrational ground state at 238 GHz. In the case of an R line, the splitting of the A and E states is nearly identical unlike in the Q line as represented in Fig. 4. The spectrum intensity scale is arbitrary.

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In the text
thumbnail Fig. 6

Some observed transitions of DME and DME-1D (black) and the computed LTE model (red) using the CASSIS software. E values are the upper energy level of the observed lines. The notation E refers to the notation used in Table 1. The LTE model has been computed in bins of the same spectral resolution as the observations. Other transitions from other species are also present in these spectra. Panels a) to d): some observed transitions of DME. Panels e) to h): all detected transitions of DME-1D-sym. Panel e) DME-1D-sym (91,9 − 80,8) E and A lines. The blended lines are 1: OCS (14 − 13) and 2: CHOH (10 − 9). Panel f) DME-1D-sym (121,12 − 110,11) E and A lines. The blended lines are 3: HCOOCH (17 − 16) and 4, 5, 6: CHCHO (11 − 10) transitions. Panel g) DME-1D-sym (131,13 − 121,12) E and A lines. Panel h) DME-1D-sym (82,7 − 71,6) E and A lines. The blended lines are 7: HCOOCH (18 − 17), 8: HCO (31,2 − 21,1) and one unidentified line (9: around 13 km s). Panels i) to n): All detected transitions of DME-1D-asym. Panel i) DME-1D-asym (51,5 − 40.4) E and A lines. Panel j) DME-1D-asym (70,7 − 61,6) E and A lines. Panel k) DME-1D-asym (90,9 − 81,8) E and A lines. The blended line is 10: HC(O)NH (74,4 − 64,3). Panel l) DME-1D-asym (100,10 − 91,9) E and A lines. Panel m) DME-1D-asym (91,9 − 80,8) E and A lines. Panel n) DME-1D-asym (121,12 − 110,11) E and A lines.

Open with DEXTER
In the text

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