Free Access
Issue
A&A
Volume 612, April 2018
Article Number A107
Number of page(s) 8
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201732346
Published online 07 May 2018

© ESO 2018

1 Introduction

Cyanamide (NH2CN) is one of the rare interstellar molecules that contain two atoms of nitrogen. This species is thought to be relevant for prebiotic chemistry: in liquid water it may convert into urea, an important molecule in biological processes (Kilpatrick 1947). Its isomer carbodiimide (HNCNH) can be formed from NH2CN in photochemically and thermally induced reactions in interstellar ice analogs (Duvernay et al. 2005). Molecules with the carbodiimide moiety (–NCN–) find use in various biological processes, among which the assembly of amino acids into peptides (see Williams & Ibrahim 1981 for an overview). Although this molecule has been detected in other galaxies, such as NGC 253 and M82 (Martín et al. 2006; Aladro et al. 2011), only two detections are mentioned in our Galaxy, in the massive star-forming regions Sgr B2 (Turner et al. 1975) and Orion KL (White et al. 2003).

Molecules formed early during the star formation process may be incorporated into comets or asteroids and delivered to planets during heavy bombardment periods, similar to that experienced by the young Earth (Raymond et al. 2004). Here, we report the detection of cyanamide towards two solar-type protostars, IRAS 16293–2422 (hereafter IRAS 16293) and NGC 1333 IRAS2A (hereafter IRAS2A). These two low-mass protostars are known to harbor a very rich chemistry in their inner regions (e.g., Bottinelli et al. 2004; Jørgensen et al. 2005). This can be explained by the thermal desorption of the numerous and complex species formed in the icy mantles of the grains. The detection presented in this work complements the list of molecules of prebiotic interest such as glycolaldehyde, formamide, and methyl isocyanate discovered in low-mass protostars (Jørgensen et al. 2012; Kahane et al. 2013; Maury et al. 2014; Coutens et al. 2015; Ligterink et al. 2017; Martín-Doménech et al. 2017). We also detected the deuterated form of cyanamide, NHDCN, for the first time in the interstellar medium.

2 Observations

To search for cyanamide, we used data obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA) for IRAS 16293 and with the IRAM Plateau de Bure Interferometer (PdBI) for IRAS2A.

The ALMA data are part of the Protostellar Interferometric Line Survey (PILS) program1, a large spectral survey of IRAS 16293observed in Cycle 2 between 329.1 and 362.9 GHz at a spatial resolution of about 0.5′′ and a spectral resolution of ~0.2 km s−1. The observations and their reduction are presented in Jørgensen et al. (2016). The data reach a sensitivity of about 4–5 mJy beam−1 km s−1.

The PdBI data of the low-mass protostar IRAS2A were obtained with the WIDEX correlator in the framework of several programs (V010, V05B, W00A, and X060). The data reduction of each dataset is described in Coutens et al. (2014, 2015) and Persson et al. (2014). They cover the spectral ranges 223.5–227.1, 240.2–243.8, and 315.5–319.1 GHz with a spectral resolution of 1.95 MHz (~1.8–2.6 km s−1). The angular resolution is about 1.2′′ × 1.0′′ at 225 GHz, 1.4′′ × 1.0′′ at 242 GHz, and 0.9′′ × 0.8′′ at 317 GHz. The rms are ~5–6 mJy beam−1 km s−1 or lower.

The spectroscopic data used here for cyanamide and its 13 C isotopolog come from the spectroscopic catalogs Jet Propulsion Laboratory (JPL, Pickett et al. 1998; Read et al. 1986) and Cologne Database for Molecular Spectroscopy (CDMS, Müller et al. 2001, 2005; Kraśnicki et al. 2011). The spectroscopy of NHDCN was studied by Kisiel et al. (2013). Carbodiimide spectroscopic information comes from the CDMS (Birk et al. 1989; Jabs et al. 1997).

3 Results

The CASSIS2 software was used to search for and identify the lines of NH2CN and its isotopologs towards IRAS 16293 and IRAS2A. Synthetic spectra were produced and compared with the observations to identify the lines. Potential blending with other species from the CDMS or JPL catalogs was checked. Column densities were determined assuming local thermodynamic equilibrium (LTE), which is reasonable for the inner regions of low-mass protostellar envelopes owing to their very high densities (≳1010 cm−3, Jørgensen et al. 2016).

3.1 Analyses of cyanamide and carbodiimide

For the binary IRAS 16293, 11 unblended lines of the main cyanamide isotopolog are detected towards source B (see Fig. 2) at the full-beam offset position analyzed in previous studies (Coutens et al. 2016; Lykke et al. 2017; Ligterink et al. 2017). No clear detection could be obtained towards IRAS 16293 A, where the lines are broader (≥2 km s−1). Maps indicate that the emission of this species arises from the warm inner regions around the B component (see Fig. 1), similarly to other complex organic molecules (Baryshev et al. 2015; Jørgensen et al. 2016). This species also appearsto be strongly affected by absorption against the strong continuum similarly to formamide (Coutens et al. 2016, see their Fig. 1). The deep absorptions at the continuum peak and half-beam offset positions are clearly seen for all the lines (see Fig. B.1). Although a LTE model with a lower excitation temperature of 100 K is in relatively good agreement with the observations (see Fig. 2), a temperature of 300 K is a more appropriate fit. This is consistent with the temperature derived for other species with high binding energies such as glycolaldehyde, ethylene glycol, and formamide (see discussion in Jørgensen et al. 2018). At the full-beam offset position, a column density of ≥ 7 × 1013 cm−2 is derived for an excitation temperature of 300 K (≥ 5× 1013 cm−2 for 100 K) and a source size of 0.5′′. It should benoted that this column density can only be considered as a lower limit because of the absorption components that could lower the emission contribution of the line profile. The higher value of the column density of NH2 CN is confirmed by the analysis of the 13C isotopolog (see Sect. 3.2).

Three bright and unblended lines of cyanamide are also detected towards NGC 1333 IRAS2A (see Fig. 3). An excitation temperature of ~130 K and a source size of ~0.5′′ were derived from the analysis of other complex organics towards this source by Coutens et al. (2015). Based on these parameters, a LTE model with a column density of 2.5 ×1014 cm−2 is in very good agreement with the observations. An excitation temperature of 300 K does not properly reproduce the lines. No other line is missing in the spectral range covered by our data. Based on this model, we can confirm the detection of this molecule by comparing our predictions with the list of lines observed in the spectral range 216.8–220.4 GHz covered by the CALYPSO program (Maury et al. 2014). Among the six brightest lines, five of them are in agreement with the presence of unidentified lines detected by Maury et al. (2014; see Table A.5). The last one is blended with a NH2 CHO line. Maps confirm that the molecule is emitting within the warm inner regions of the source (see Fig. 1).

We also searched for carbodiimide, HNCNH, which is not detected with an upper limit of 3 × 1015 cm−2 towards IRAS 16293 B. Based on the non-detection of the HNCNH line at 223.7918 GHz, we derived a similar upper limit for IRAS2A. The non-detection of carbodiimide is not really surprising since the upper limits are high and this isomer is known to be less stable than cyanamide.

thumbnail Fig. 1

Integrated intensity maps of two transitions of NH2CN detected towards IRAS 16293 B (left panel, from 3 to 5σ by step of 1σ) and IRAS2A (right panel, from 5 to 25σ by step of 5σ). The position of the continuum peak position is indicated with a red triangle, while the position analyzed for IRAS 16293 B (full-beam offset) is indicated with a red circle. The beam size is shown in gray in the bottom right corner.

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3.2 Analyses of the deuterated and 13C isotopologs of cyanamide

The 13C and deuterated isotopologs of NH2CN were searched for towards both sources. Eight unblended lines of NHDCN were identified towards the full-beam offset position of IRAS 16293 B (see Fig. 4). This marks the first detection of this isotopolog in the interstellar medium. Although the lines are faint, we can confirm that all the features are real after checking the spectra at the half-beam position (where the lines are brighter). Additional lines are present, but are not included here as they are blended with other species. A few other lines do not appear as bright as expected due to the presence of absorptions produced by other species at the same frequency. A column density of 7 × 1013 cm−2 is obtained for an excitation temperature of 300 K. The detection of NH2 13CN is less straightforward. Three lines can be attributed to this isotopolog (see Fig. 5). One of them (361.997 GHz) may be blended with an unknown species since its flux is higher than predicted by the LTE model. Based on these lines, a column density of 3 × 1013 cm−2 is derived showing that the NH2CN column density is underestimated due to the contribution of the absorptions. The D/H of NH2 CN (corrected for statistics, i.e., divided by 2) is 1.7%, assuming a standard 12 C/13C of 68 (Milam et al. 2005). This value is very similar to the deuteration of formamide (~2%; Coutens et al. 2016) and also within the range of the D/H of other COMs (~1–8%, Jørgensen et al. 2018; Persson et al. 2018). In the case of a lower 12 C/13C of 30, whichwas found for a few COMs in this source (Jørgensen et al. 2016, 2018), the D/H ratio of NH2 CN would be about 4%.

For IRAS2A, an upper limit of 5 × 1013 cm−2 is derived forNHDCN, leading to a D/H of ≤10%. The 13C isotopolog also presents an upper limit of 5 × 1013 cm−2, which is equivalent to a 12 C/13C of ≥5.

thumbnail Fig. 2

Unblended lines of NH2CN detected with ALMA towards IRAS 16293 B at the full-beam offset position. The best-fit model for Tex = 300 K and 100 K are in red and blue, respectively. The Eup values are indicated in green in the bottom right corner of each panel.

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thumbnail Fig. 3

Unblended lines of NH2CN detected with PdBI towards IRAS2A. The best-fit model is shown in red. The Eup values are indicated in green in the top left corner of each panel.

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3.3 Abundances of cyanamide

Based on the analysis of the NH2 13CN isotopolog and the lower limit of the H2 column density derived by Jørgensen et al. (2016), for IRAS 16293 B we get an abundance of NH2CN with respect to H2 of ≲ 2 × 10−10 at 60 AU from source B. The NH2CN/NH2CHO abundance ratio is about 0.2.

Using the H2 column density derived by Taquet et al. (2015, 5 × 1024 cm−2) for IRAS2A, the abundance with respect to H2 is about 5 × 10−11. A simple analysis of the most optically thin NH2CHO lines covered by our data suggests a column density of about 1.2 × 1016 cm−2 assuming a similar excitation temperature of 130 K. This is in agreement with the value determined by Taquet et al. (2015). The ratio of NH2CN-to-NH2CHO is consequently about 0.02, an order of magnitude lower than for IRAS 16293 B.

The NH2CN-to-NH2CHO ratios derived for the low-mass protostars are similar to the range of values found in Sgr B2 (Belloche et al. 2013, see Table 1). The NH2CN-to-NH2CHO ratio in Orion KL seems to be higher (~0.4–1.5, White et al. 2003). This value is, however, more uncertain since it was obtained using only one line of NH2CHO and assuming different excitation temperatures for the two molecules.

4 Discussion

The formation routes of NH2CN have only been marginally explored. According to the Kinetic Database for Astrochemistry3 (KIDA, Wakelam et al. 2012), there are no known gas-phase mechanisms capable of its production. While the reaction CN + NH3 → NH2CN + H has been proposed (Smith et al. 2004), the theoretical study of Talbi & Smith (2009) suggests that the production of NH2CN involves large internal barriers, with HCN and NH2 being the likely products. An experimental study by Blitz et al. (2009) confirms that this reaction proceeds exclusively to HCN + NH2. Electronic recombination of NH2CNH+ may produce NH2CN, but the only apparent way to form this ion is through protonation of NH2CN itself. An alternative source of NH2CN is thus required to explain our observations. Cyanamide could be formed on grain surfaces through the addition of NH2 and CN radicals. The possible formation of formamide from the same precursor NH2 (Fedoseev et al. 2016) could explain the similarity of these two species in terms of deep absorption against the strong continuum and the similar deuteration of the two species towards IRAS 16293 B.

To test this hypothesis we ran MAGICKAL, a three-phase chemical kinetics model (Garrod 2013), modified with the grain-surface back-diffusion correction of Willis & Garrod (2017). The model uses a network based on that of Belloche et al. (2017), in which dissociative recombination of NH2CNH+ was assumed to produce NH2CN in 5% of cases. The reaction NH2 + CN → NH2CN was added to the grain/ice chemical network, and the gas-phase reaction between CN and NH3 was adjusted per Talbi & Smith (2009) and Blitz et al. (2009). The physical model used here is very similar to that described by Belloche et al. (2017), in which a cold collapse to high density is followed by a temperature increase to 400 K; here, a final density nH = 6 × 1010 cm−3 was assumed to better represent the density structure of IRAS 16293 B (Jørgensen et al. 2016).

The model results (for an intermediate warm-up timescale) are shown in Fig. 6 for both NH2CN and NH2CHO. NH2CNis seen to be produced at a temperature of ~30 K on the grainsurfaces, desorbing into the gas at higher temperatures. The model underproduces the amount of gas-phase NH2CN, showing a peak fractional abundance with respect to H2 of ~ 6.7 × 10−12 that is nevertheless well maintained to a temperature of 300 K and beyond. The low NH2CN abundance in the gas-phase is caused primarily by underproduction on the dust grains; at the high density used in the model, the rapid accretion of H and H2 onto grain surfaces makes hydrogenation of the NH2 and CN radicals much more competitive with the reaction that produces NH2CN. This competition becomes important for gas densities greater than ~109 cm−3. We therefore also present a model with a lower final density of nH = 1.6 × 107 cm−3 (corresponding to the density of the envelope between the two protostars in IRAS 16293, Jacobsen et al. 2018), intended to represent the approximate conditions of the gas while at a temperature of 30 K (see Fig. 6). This model produces an NH2CN fractional abundance of 3.7 × 10−10, a value very close to the detected value. However, the resulting NH2CN:NH2CHO peak abundance ratio of 0.0011 is still lower than the observed values in both IRAS2A (~0.02), and IRAS 16293 (~0.2). This may be due to the possible overproduction of NH2CHO, related to uncertainties in the efficiency of formation of that molecule, which is still a matter of debate, particularly for the gas-phase mechanism (e.g., Barone et al. 2015; Song & Kästner 2016); our model assumes only a grain-surface/ice formation route. The difficulty in reproducing the observed NH2CN abundance at the high densities determined for the source highlights the necessity for future models of hot-core/corino chemistry to treat the rising density and temperature in such cores concurrently, rather than as a two-stage process, so that the gas densities are appropriate at the key temperatures at which many molecules are formed.

In conclusion, the detection of cyanamide towards IRAS 16293 B and IRAS2A indicates that this species can be formed early in solar-type protostars. If it survives during the star formation process until its incorporation into comets or asteroids, these objects could then deliver it to planets, which may enable the development of life. Searches for this species in the coma of comets could shed further light on this possibility. Theoretical and experimental studies as well as more detailed chemical models are needed to confirm the formation of NH2CN through the grain-surface pathway NH2 + CN. It would also be interesting to investigate whether this mechanism is sufficient to explain the large-scale emission of NH2CN in galaxies.

thumbnail Fig. 4

Unblended lines of NHDCN detected with ALMA towards IRAS 16293 B at the full-beam offset position. The best-fit model for Tex = 300 K is in red. The Eup values are indicated in green in the bottom right corner of each panel.

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thumbnail Fig. 5

Lines of NH2 13CN detected with ALMA towards IRAS 16293 B at the full-beam offset position. The best-fit model for Tex = 300 K is in red. The Eup values are indicated in green in the bottom right corner of each panel.

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Table 1

Abundances of NH2CN with respect to H2 and NH2 CHO.

thumbnail Fig. 6

Chemical model abundances for the warm-up stage of a hot-core type model with a final collapse density of nH = 6 × 1010 cm−3 (high-density model, black lines for NH2CN and green lines for NH2CHO). Solid lines denote gas-phase molecules; dotted lines indicate the same species on the grains. The red lines correspond to the abundance profiles of gas-phase and grain-surface NH2 CN for the lower density model, run at nH = 1.6 × 107 cm−3.

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Acknowledgements

This paper makes use of the ALMA data ADS/JAO.ALMA#2013.1.00278.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This work is based on observations carried out under project numbers V010, V05B, W00A, and X060 with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement No. 283393 (RadioNet3). A.C.’s postdoctoral grant is funded by the ERC Starting Grant 3DICE (grant agreement 336474). The group of J.K.J. acknowledges support from ERC Consolidator Grant “S4F” (grant agreement 646908). Research at the Centre for Star and Planet Formation is funded by the Danish National Research Foundation. The group of E.v.D. acknowledges ERC Advanced Grant “CHEMPLAN” (grant agreement 291141). M.V.P.’s postdoctoral position is funded by the ERC consolidator grant 614264. M.N.D. acknowledges the financial support of the Center for Space and Habitability (CSH) Fellowship and the IAU Gruber Foundation Fellowship.

Appendix A: Tables with detected lines of NH2CN and its isotopologs

Table A.1

Detected lines of NH2CN towards IRAS 16293 B with ALMA.

Table A.2

Detected lines of NH2CN towards IRAS2A with PdBI.

Table A.3

Detected lines of NHDCN towards IRAS 16293 B with ALMA.

Table A.4

Detected lines of NH2 13CN towards IRAS 16293 B with ALMA.

Table A.5

Unidentified lines from (Maury et al. 2014; their Table 1) that can be assigned to NH2 CN. These data have a spectral resolution of 3.9 MHz (~5.3 km s−1).

Appendix B: Spectra of NH2CN observed with ALMA at different positions towards IRAS 16293 B

thumbnail Fig. B.1

Spectra of the unblended lines of NH2CN detected with ALMA towards IRAS 16293 B at the full-beam offset position (left), at the half-beam offset position (middle), and at the peak continuum position (right). The best-fit model for Tex = 300 K at the full-beam offset position is shown in red in the left panels. The identification of the other lines is indicated in blue. The dashed lines in the middle and right panels indicate the average velocity of the absorptions, 3.8 km s−1. The dotted lines show the level 0.

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2

CASSIS has been developed by IRAP-UPS/CNRS (http://cassis.irap.omp.eu/).

All Tables

Table 1

Abundances of NH2CN with respect to H2 and NH2 CHO.

Table A.1

Detected lines of NH2CN towards IRAS 16293 B with ALMA.

Table A.2

Detected lines of NH2CN towards IRAS2A with PdBI.

Table A.3

Detected lines of NHDCN towards IRAS 16293 B with ALMA.

Table A.4

Detected lines of NH2 13CN towards IRAS 16293 B with ALMA.

Table A.5

Unidentified lines from (Maury et al. 2014; their Table 1) that can be assigned to NH2 CN. These data have a spectral resolution of 3.9 MHz (~5.3 km s−1).

All Figures

thumbnail Fig. 1

Integrated intensity maps of two transitions of NH2CN detected towards IRAS 16293 B (left panel, from 3 to 5σ by step of 1σ) and IRAS2A (right panel, from 5 to 25σ by step of 5σ). The position of the continuum peak position is indicated with a red triangle, while the position analyzed for IRAS 16293 B (full-beam offset) is indicated with a red circle. The beam size is shown in gray in the bottom right corner.

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In the text
thumbnail Fig. 2

Unblended lines of NH2CN detected with ALMA towards IRAS 16293 B at the full-beam offset position. The best-fit model for Tex = 300 K and 100 K are in red and blue, respectively. The Eup values are indicated in green in the bottom right corner of each panel.

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In the text
thumbnail Fig. 3

Unblended lines of NH2CN detected with PdBI towards IRAS2A. The best-fit model is shown in red. The Eup values are indicated in green in the top left corner of each panel.

Open with DEXTER
In the text
thumbnail Fig. 4

Unblended lines of NHDCN detected with ALMA towards IRAS 16293 B at the full-beam offset position. The best-fit model for Tex = 300 K is in red. The Eup values are indicated in green in the bottom right corner of each panel.

Open with DEXTER
In the text
thumbnail Fig. 5

Lines of NH2 13CN detected with ALMA towards IRAS 16293 B at the full-beam offset position. The best-fit model for Tex = 300 K is in red. The Eup values are indicated in green in the bottom right corner of each panel.

Open with DEXTER
In the text
thumbnail Fig. 6

Chemical model abundances for the warm-up stage of a hot-core type model with a final collapse density of nH = 6 × 1010 cm−3 (high-density model, black lines for NH2CN and green lines for NH2CHO). Solid lines denote gas-phase molecules; dotted lines indicate the same species on the grains. The red lines correspond to the abundance profiles of gas-phase and grain-surface NH2 CN for the lower density model, run at nH = 1.6 × 107 cm−3.

Open with DEXTER
In the text
thumbnail Fig. B.1

Spectra of the unblended lines of NH2CN detected with ALMA towards IRAS 16293 B at the full-beam offset position (left), at the half-beam offset position (middle), and at the peak continuum position (right). The best-fit model for Tex = 300 K at the full-beam offset position is shown in red in the left panels. The identification of the other lines is indicated in blue. The dashed lines in the middle and right panels indicate the average velocity of the absorptions, 3.8 km s−1. The dotted lines show the level 0.

Open with DEXTER
In the text

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