Discovery of two new magnesium-bearing species in IRC+10216: MgC₃N and MgC₄H

Abstract

We report on the detection of two series of harmonically related doublets in IRC +10216. From the observed frequencies, the rotational constant of the first series is B = 1380.888 MHz and that of the second series is B = 1381.512 MHz. The two series correspond to two species with a ²Σ electronic ground state. After considering all possible candidates, and based on quantum chemical calculations, the first series is assigned to MgC₃N and the second to MgC₄H. For the latter species, optical spectroscopy measurements support its identification. Unlike diatomic metal-containing molecules, the line profiles of the two new molecules indicate that they are formed in the outer layers of the envelope, as occurs for MgNC and other polyatomic metal-cyanides. We also confirm the detection of MgCCH that was previously reported from the observation of two doublets. The relative abundance of MgC₃N with respect to MgNC is close to one while that of MgC₄H relative to MgCCH is about ten. The synthesis of these magnesium cyanides and acetylides in IRC +10216 can be explained in terms of a two-step process initiated by the radiative association of Mg⁺ with large cyanopolyynes and polyynes followed by the dissociative recombination of the ionic complexes.

1. Introduction

Metal-containing molecules are rarely observed in the interstellar medium, but are widely detected in circumstellar envelopes around evolved stars. Stable closed-shell diatomic molecules containing Al, Na, or K and a halogen atom have been detected in the prototypical carbon star envelope IRC +10216 (Cernicharo & Guélin 1987). These species are mostly formed in the hot inner parts of the envelope, close to the star. Metals like Na, K, Ca, Cr, and Fe are found to survive in the gas phase as neutral and/or ionized atoms in the outer layers of IRC +10216 (Mauron & Huggins 2010), which points to a very rich metal chemistry in the cool outer envelope. Various triatomic metal cyanides (M−NC or/and M−CN) containing Mg, Na, Al, Si, K, Fe, and Ca (most of which are open-shell radicals) have been detected in IRC +10216 (Kawaguchi et al. 1993; Turner et al. 1994; Ziurys et al. 1995, 2002; Guélin et al. 2000, 2004; Pulliam et al. 2010; Zack et al. 2011; Cernicharo et al. 2019). Guélin et al. (1993) studied the spatial distribution of MgNC and concluded that this radical arises from a shell located at 15″ from the star.

The only tetratomic metal-bearing species detected so far in IRC +10216 is HMgNC (Cabezas et al. 2013). A pentatomic species, MgC₃N, was postulated by Petrie et al. (2003) as a potential abundant species in IRC +10216. They suggested that a series of doublets with rotational constant around 1395 MHz, although initially assigned to an electronic excited state of C₆H by Aoki (2000), were in fact produced by MgC₃N. However, Cernicharo et al. (2008) have shown that these doublets certainly belong to the ²Σ state of the ν₁₁ bending mode of C₆H in its ²Π ground electronic state. Nevertheless, MgC₃N, together with MgC₄H, which could be formed in the cold outer layers of IRC +10216, are interesting molecules to be searched for in this object.

Sensitive line surveys can be used as the magic tool to reveal the molecular content of astronomical sources and to search for new molecules. Once we are able to identify the lines coming from the isotopologues and vibrationally excited states of known molecules, then a forest of unidentified lines appear that provide the opportunity to discover new molecules and insights into the chemistry and chemical evolution of the observed object.

In this Letter we report the discovery of two new series of doublets with very close rotational constants (1380.9 MHz and 1381.5 MHz) produced by two new molecular species having a ²Σ ground electronic state. On the basis of precise quantum chemical calculations and an exploration of all possible candidates, we assign these lines to MgC₃N and MgC₄H.

2. Observations

The observations at 3 mm presented in this paper were carried out with the IRAM 30m radio telescope and have been described in detail by Cernicharo et al. (2019). Briefly, the data in the 3 mm window correspond to observations acquired during the last 35 years and cover the 70-116 GHz domain with very high sensitivity (1-3 mK). Examples of these data can be found in Cernicharo et al. (2004, 2007, 2008, 2019) and Agúndez et al. (2008, 2014). The observing mode, where we wobbled the secondary mirror by ±90” at a rate of 0.5 Hz, ensured flat baselines. This observing method provides reference data free from emission from all molecules but CO (see Cernicharo et al. 2015). The emission of all other molecular species is restricted to a region ≤ 15-20″ from the star (see e.g. Guélin et al. 1993; Agúndez et al. 2015, 2017; Velilla Prieto et al. 2015; Quintana-Lacaci et al. 2016).

The observations in the Q band (32-50 GHz) were performed in May and June 2019 with the 40m radiotelescope of the Centro Astronómico de Yebes (IGN, Spain). New receivers built for the European Research Council synergy project Nanocosmos were installed at the telescospe and used for these observations during its commissioning phase. The experimental set-up will be described in detail by Tercero et al. (2019). Briefly, the Q-band receiver consists of two HEMT cold amplifiers covering the 32-50 GHz band with horizontal and perpendicular polarizations. Receiver temperatures vary from 22 K at 32 GHz up to 42 K at 50 GHz. The spectrometers are 16 × 2.5 GHz FFTs with a spectral resolution of 38.1 kHz providing the whole coverage of the Q-band in both polarizations. The total observing time on-source for the observations presented here corresponds to ~ 24 h. The observing mode was position switching with an off position at 300″ in azimuth.

Pointing corrections were obtained by observing strong nearby quasars and the SiO masers of R Leo. Pointing errors were always within 2-3″. The intensity scale, antenna temperature ($T_A^{*}$), was corrected for atmospheric absorption using the ATM package (Cernicharo 1985; Pardo et al. 2001). The beam size of the IRAM 30m telescope in the 3mm domain is 21-30″, while for the 40m telescope the beam size in the Q band is in the range 36-56″. The main beam efficiency of the 40m telescope varies between 60 % at 32 GHz and 50 % at 50 GHz. Calibration uncertainties of 10% were adopted for data covering such a long observing period in the 3 mm domain. A similar uncertainty was assumed for the data in the Q band. Additional uncertainties could arise from the line intensity fluctuation with time induced by the variation of the stellar infrared flux (Cernicharo et al. 2014; Pardo et al. 2018). All data were analysed using the GILDAS package¹.

3. Results

In the IRAM 30m data we found a series of doublets (S 1) whose components are separated by ~4.3 MHz. The doublets are in harmonic relation with integer quantum numbers from N_up=26 to N_up=40. Some of these doublets show some overlap with other features, as shown in Figs. 1 and 2. As a first step we verified that no other harmonic relation can apply and the lines cannot be due to any known species. Once we were sure that the doublets belong to a new molecular species, we searched for the corresponding doublets in the Q band, which correspond to quantum numbers from N_up=12 to N_up=18. All them were detected and are shown in Fig. 1. Interestingly, none of these doublets are present in the previous survey of IRC +10216 in the Q band performed with the Nobeyama radiotelescope (Kawaguchi et al. 1995). This is due to the higher sensitivity of our data, less than 1 mK (see Fig. 1) versus 5 mK in the Nobeyama data. Finally, a total of 22 doublets (see Figs. 1 and 2) were found between the Q and the 3 mm bands. Their frequencies are given in Table A.1 of Appendix A. From a fit to the observed frequencies, we obtain the following rotational, distortion, and spin-rotation constants:

Fig. 1.

Observed doublets towards IRC+10216 (black histogram). The vertical scale is the antenna temperature in mK and the horizontal scale is the rest frequency in MHz. Shown are the lines of the series of doublets S 1 (labelled in red) and those of series S 2, with the same quantum numbers as S 1 (in magenta); unidentified (U) lines and lines arising from other species are also shown (in blue). When lines of the S 2 series are blended with other features, or are too weak to be detected (those below 50 GHz), their positions are indicated by dashed arrows.

Fig. 2.

Same as in Figure 1, but for the doublets in the 3 mm domain observed with the IRAM 30m radio telescope.

B = 1380.888 ± 0.001 MHz

D = 0.0760 ± 0.0005 kHz

γ = 4.35 ± 0.04 MHz

After a careful examination of the data we realized that some unidentified features located on the right side of the S 1 doublets are in fact harmonically related doublets with integer quantum numbers (S 2). They are indicated by magenta arrows in Figs. 1 and 2. The S 2 doublets have the same quantum numbers as S 1 and correspond to a ²Σ molecule. The frequencies of S 2, given in Table A.2 of Appendix A, are fitted with the following constants:

B = 1381.512 ± 0.004 MHz

D = 0.074 ± 0.002 kHz

γ = 4.7 ± 0.1 MHz

A detailed analysis of possible carriers for S 1 and S 2 is given in Appendix B. After discarding all plausible molecular species, we conclude that our best candidates are metal-bearing species attached to a C₃N or C₄H group. Species containing Na or Al attached to C₃N or C₄H groups have singlet ground electronic states and therefore cannot be the carriers of S 1 and S 2. In fact, AlC₃N and NaC₃N have been observed in the laboratory (Cabezas et al. 2014, 2019) and their rotational constants are too low with respect to those quoted above for S 1 and S 2. In contrast, molecular species containing Mg or Ca present ²Σ electronic states and they are postulated as the candidates responsible for the S 1 and S 2 rotational transitions. In a previous paper, Cabezas et al. (2019) reported ab initio calculations for both MgC₃N and CaC₃N species. The calculated rotational constant for MgC₃N indicates that this molecule could be one of the carriers of S 1 or S 2. On the other hand, the rotational constant of CaC₃N is much smaller than those of S 1 and S 2, discarding Ca as a candidate. Hence, we performed high-level quantum chemical calculations for both MgC₄H and MgC₃N (see Appendix C). For MgC₃N we obtain B = 1376.5 MHz, D = 0.065 kHz, and γ = 4.1 MHz. For MgC₄H we obtained a slightly higher value of B, 1377.4 MHz, D = 0.061 kHz, and γ = 4.4 MHz. All these results are given in Appendix C.

High-resolution optical spectroscopy of the A²Π − X²Σ electronic transition of MgC₄H were performed by Ding et al. (2008) and Chasovskikh et al. (2008). They obtain a rotational constant of B = 1384.7 ± 6 MHz. Subsequent observations of the same electronic transition of MgC₄H by Forthomme et al. (2010) provide a much more accurate value for the rotational constant, B = 1380.9 ± 0.2 MHz. The spectral resolution of these observations is 0.002 cm⁻¹ (~60 MHz) and near 250 transitions have been observed with upper quantum numbers up to N = 42. Hence, the derived rotational constant and its uncertainty are highly confident. Using the relation D = B³/ω², these authors estimated the distortion constant to be 0.077 kHz. Hence, either of the two series S 1 or S 2 may be assigned to MgC₄H. The calculated dipole moment of MgC₃N is 6.3 D and that of MgC₄H 2.1 D (see Appendix C). Consequently, both species will harbour a different line intensity distribution with quantum number N. We consider that MgC₄H is responsible for the S 2 series as its dipole moment of 2.1 D produces rotational transitions in IRC +10216 peaking in the 3 mm domain with weak emission below 50 GHz. Consequently, the S 1 series is assigned to MgC₃N because with its high dipole moment its rotational transitions are strong in the Q band and will start to decline in intensity for high values of N in the 3mm band (similar to the behaviour of HC₅N). Moreover, all quantum chemical calculations indicate that the rotational constant of MgC₄H is slightly larger than that of MgC₃N, as observed for S 2 and S 1. Consequently, we conclude that we have detected, and fully characterized spectroscopically in space, the magnesium-bearing species MgC₃N (S 1) and MgC₄H (S 2). The definitive confirmation will require laboratory rotational spectroscopy. We also present new observations (see Appendix E) that confirm the tentative detection of MgCCH in IRC +10216 reported by Agúndez et al. (2014).

4. Discussion

The observed line intensities of MgC₃N can be fit to two components, a cold one with a rotational temperature of 15 ± 2 K and a column density of (5.7 ± 1.0) × 10¹² cm⁻², and a warm one with T_rot = 34 ± 6 K and N = (3.6 ± 0.6) × 10¹² cm⁻². For MgC₄H we derive a relatively high rotational temperature (36±6 K), which is consistent with the line intensity distribution quoted above, and a column density of (2.2 ± 0.5) × 10¹³ cm⁻². The determination of column densities can be quite uncertain if infrared pumping (IR) plays a role in the excitation of the rotational levels (see Appendix D). These effects could be particularly important for molecules having low-frequency bending modes such as HC₃N, HC₅N, C₄H, MgC₃N, and MgC₄H (see Appendix D). Nevertheless, our two magnesium-bearing species have similar low-frequency bending modes (see Tables C.3 and C.4) and we could expect to have similar effects in the infrared pumping of their rotational levels. Therefore, the ratio of column densities MgC₄H/MgC₃N ~2.5 should not be greatly affected by IR pumping effects. On the other hand, the abundance ratio MgC₄H/MgCCH ~10 may be overestimated if IR pumping is important for MgC₄H (see Appendices D and E). It is true that it seems counterintuitive to have a higher abundance for MgC₄H than for MgCCH, but in the chemical model discussed below everything depends on the branching ratios of the different fragmentation channels in the dissociative recombination of the ${\text{MgC}}_{2n}{\text{H}}_2^{+}$ complexes, which are difficult to anticipate (see below). Usually, within a given chemical family, larger molecules tend to be less abundant in IRC+10216. However, the anions C_nH⁻, for which their synthesis also relies on the radiative association of C_nH radicals and electrons, show an increase in abundance with the number of carbon atoms.

In order to investigate the formation of the various magnesium cyanides and acetylides detected in IRC +10216 (MgNC, MgCN, HMgNC, MgC₃N, MgCCH, and MgC₄H) we carried out chemical model calculations based on the model of the outer envelope by Agúndez et al. (2017). As in previous models dealing with the formation of metal-bearing molecules in the outer shells of IRC +10216 (Millar 2008; Cabezas et al. 2013; Cernicharo et al. 2019), the synthesis relies on a two-step mechanism starting with the radiative association of Mg⁺ with large cyanopolyynes and polyynes and the subsequent dissociative recombination of the corresponding Mg⁺/NC_2n+1H and Mg⁺/C_2nH₂ complexes with electrons (Petrie 1996).

The rate coefficients for the radiative associations of Mg⁺ with polyynes and cyanopolyynes were taken from the calculations by Dunbar & Petrie (2002). We consider that open-shell metal-containing species (MgNC, MgCN, MgC₃N, MgCCH, and MgC₄H) react with electrons and H atoms, while closed-shell molecules (HMgNC) do not. The branching ratios of the different fragmentation channels in the dissociative recombination of the Mg⁺/NC_2n+1H and Mg⁺/C_2nH₂ ions are not known. We have therefore adjusted them to obtain column density ratios in agreement with those derived from observations. We find that in order to reproduce the relative column densities of the magnesium cyanides, the branching ratios of the channels yielding HMgNC, MgCN, and MgC₃N should respectively be 0.01, 0.05, and 0.7 of that yielding MgNC. In the case of the magnesium acetylides, the branching ratio of the channel producing MgCCH should be 0.1 of that yielding MgC₄H. It is curious that while MgNC is slightly more abundant than MgC₃N, in the case of magnesium acetylides the larger molecule MgC₄H is significantly more abundant than MgCCH. If the conclusion of MgC₄H/MgCCH > 1 is not in error due to IR pumping effects, this should be a consequence of the different behaviour in the fragmentation of Mg⁺/C_2nH₂ complexes with respect to Mg⁺/NC_2n+1H.

The absolute column densities of the Mg-bearing molecules are approximately reproduced (calculated column densities for MgC₃N and MgC₄H are 5.4 × 10¹² and 3.6 × 10¹³ cm⁻², respectively) adopting an initial abundance of Mg relative to H of 1.5 × 10⁻⁶, which is about 4 % of the cosmic abundance of Mg. This value is higher than that used by Cabezas et al. (2013), mainly because in this model we include more metals (Mg, Na, Al, and Ca) and the new and abundant detected molecule MgC₃N must be accounted for. Unfortunately, Mauron & Huggins (2010) did not observe atomic Mg in the outer layers of IRC +10216, and therefore we do not have constraints on the abundance of atomic Mg. The results from the chemical model are shown in Fig. 3. In general, the two-step mechanism of Petrie (1996) can account for the formation of Mg cyanides and acetylides in IRC +10216. The main uncertainty is related to the branching ratios of the different fragmentation channels in the dissociative recombination of the cationic metal complexes, something that would be worth investigating in detail in the future.

Fig. 3.

Abundances calculated with the chemical model for the different Mg-bearing molecules detected in IRC +10216.