Detection of vibrationally excited HC₇N and HC₉N in IRC+10216

Abstract

Observations of IRC +10216 with the Yebes 40m telescope between 31 and 50 GHz have revealed more than 150 unidentified lines. Some of them can be grouped into a new series of 26 doublets, harmonically related with integer quantum numbers ranging from J _up=54 to 80. The separation of the doublets increases systematically with J, i.e., as expected for a linear species in one of its bending modes. The rotational parameters resulting from the fit to these data are B = 290.8844 ± 0.0004 MHz, D = 0.88 ± 0.04 Hz, q = 0.1463 ± 0.0001 MHz. The rotational constant is very close to that of the ground state of HC₉N. Ab initio calculations show an excellent agreement between these parameters and those predicted for the lowest energy vibrationally excited state, ν ₁₉=1, of HC₉N. This is the first detection, and complete characterization in space, of vibrationally excited HC₉N. An energy of 41.5 cm⁻¹ is estimated for the ν ₁₉ state. In addition, 17 doublets of HC₇N in the ν ₁₅=1 state, for which laboratory spectroscopy is available, have been detected for the first time in IRC+10216. Several doublets of HC₅N in its ν ₁₁=1 state have been also observed. The column density ratio between the ground and the lowest excited vibrational states are ≈127, 9.5, and 1.5 for HC₅N, HC₇N, and HC₉N, respectively. We find that these lowest-lying vibrational states are most probably populated via infrared pumping to vibrationally excited states lying at ≈600 cm⁻¹. The lowest vibrationally excited states thus need to be taken into account to precisely determine absolute abundances and abundanceratios for long carbon chains. The abundance ratios N(HC₅N)/N(HC₇N) and N(HC₇N)/N(HC₉N) are 2.4 and 7.7 respectively.

1. Introduction

Sensitive line surveys are the best tool to unveil the molecular content of astronomical sources and to search for newmolecules. In order to carry out a detailed analysis of line surveys, a key ingredient is the availability of spectroscopic information of the already known species, their isotopologues, and their vibrationally excited states. The ability to identify as many lines as possible coming from them leaves the cleanest possible forest of unidentified lines, opening the opportunity to discover new molecules and to get insights into the chemistry and chemical evolution of the observed object. Lines from vibrationally excited states of long molecules have been observed in the carbonrich circumstellar envelope (CSE) IRC +10216, being C₄H and C₆H good examples of such emission (Guélin et al. 1987; Yamamoto et al. 1987; Cernicharo et al. 2008). The longer a linear molecule is, the lower in energy its vibrational bending modes are. Hence, even in relatively cold regions of CSEs, the lines from vibrationally excited states can be rather prominent in sensitive line surveys. Moreover, the correct determination of the abundance ratios between species of the same family (e.g., C_nH radicals, or cyanopolyynes HC_2n+1N), requires an estimation of the number of molecules in the vibrationally excited states if they are significantly populated.

Rotational lines from vibrationally excited levels of moderate energies provide useful information on the pumping mechanisms in the CSE and allow to assess the role of infrared pumping and its effect on intensity line variationswith the stellar phase (Cernicharo et al. 2008, 2014; Agúndez et al. 2017; Pardo et al. 2018). A different case occurs when these lines involve very high energy vibrational states as they trace the physical and chemical conditions of the innermost and warm regions of CSEs (Cernicharo et al. 2011, 2013; Patel et al. 2011). The main difference between the rotational lines from low and high energy vibrational states relies on the line profile. Transitions from low energy excited vibrational levels should show the same velocity extent and similar line profiles as those of the ground vibrational state, with C₄H and C₆H as clear examples (Guélin et al. 1987; Cernicharo et al. 2008). However, rotational lines from high energy excited vibrational states show Gaussian profiles and trace the kinematics of the gas in the innermost regions (Cernicharo et al. 2011, 2013; Patel et al. 2011).

In this Letter we report on the discovery of a new series of lines toward IRC +10216 that we attribute to the ν ₁₉=1 state of HC₉N, for which we determine accurate rotational constants. In addition, we report on the detection of HC₇N in the ν ₁₅=1 and 2 states, and of HC₅N in the ν ₁₁ = 1 state. The abundance ratio between members of this molecular family is revisited in the light of the high column densities observed in vibrationally excited states for some of them.

2. Observations

The Q-band observations (31.0-50.3 GHz) were carried out in spring 2019 with the 40m radiotelescope of the Centro Astronómico de Yebes (IGN, Spain), Yebes 40m throughout this letter. New receivers, built within the Nanocosmos project¹, and installed at the telescope, were used for these observations during its commissioning phase. The experimental setup will be described elsewhere (Tercero et al., in prep.). Briefly, the Q-band receiver consists of two HEMT cold amplifiers covering the 31.0-50.3 GHz band with horizontal and vertical polarizations. Receiver temperatures vary from 22 K at 32 GHz to 42 K at 50 GHz. The spectrometers are 16 × 2.5 GHz Fast Fourier Transform Spectrometers (FFTS) with a spectral resolution of 38.1 kHz providing the whole coverage of the Q-band in both polarizations. The observing mode was position switching with an off position at 300′′ in azimuth. The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz. The final spectra were smoothed to a resolution of 0.195 MHz, i.e. a velocity resolution of ≈1.9 & 1.2 km s⁻¹ at 31 and 50 GHz respectively. The sensitivity of the final spectra varies between 0.4 and 1 mK across the Q-band, which is a factor of ≈10 better than previous observations in the same frequency range with the Nobeyama 45m telescope (Kawaguchi et al. 1995).

The observations in the λ 3 mm band 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 beam size of the Yebes 40m in the Q-band is in the range 36-56′′ while that of the IRAM 30m telescope in the 3mm domain is 21-30′′. Pointing correctionswere 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 $\left(T_A^{\ast }\right)$, was corrected for atmospheric absorption using the ATM package (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties have been adopted to be 10 %. 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 have been analyzed using the GILDAS package².

3. Results

One of the most surprising results from the line survey in the Q-band is the presence of bright lines from vibrationally excited states of C₄H, C₆H, HC₅N, and HC₇N, and of two series of doublets assigned recently by Cernicharo et al. (2019) to MgCCCN and MgCCCCH (see Fig. 1, where the bottom panel shows one of the MgCCCN doublets).

Fig. 1.

Data from two frequency ranges within the Q-band observed with the Yebes 40m telescope towards IRC +10216. Each frequency range is shown through two panels with different limits in intensity. The vertical scale is antenna temperature in mK. The horizontal scale is the rest frequency in MHz. Labels for HC₇N features are in blue color, while those belonging to HC₉N are in violet. Other spectral features from known species, together with unidentified lines (labelled as “U” lines), are indicated in red. The N = 16 − 15 doublet of MgCCCN, a new species recently detected (Cernicharo et al. 2019), is shown in the bottom panels. Vibrationally excited lines from HC₇N, HC₉N and C₆H are nicely detected at these frequencies. Additional lines from vibrationally excited states of HC₅N, HC₇N, and HC₉N are shown in Fig. 2 and Figures A.1 and A.3

A new series of lines consisting of 26 doublets, with central frequencies in harmonic relation from J = 54 up to J = 80, has been found in the Q-band data. None of these frequencies could be identified in the public catalogs JPL (Pickett 1998), CDMS (Müller et al. 2005), or in the MADEX catalog (Cernicharo 2012). They belong to a new molecular species or to a new vibrationally excited state of a known molecule. The lines appear at frequencies slightly higher than those of HC₉N with the same quantum numbers. Line frequencies for this series of doublets have been determined by fitting them with a specific line profile typical of expanding envelopes (Cernicharo et al. 2018). Selected lines of this series are shown in Fig. 1 and 2. The typical profile of the lines with rather sharp edges permit to fit, even when signals are weak, their central frequencies with an accuracy of the order, or even better, than the spectral resolution of the data (Cernicharo et al. 2018). Some doublets of this series are blended with other features, but frequencies can still be derived for most of them by fixing the expanding velocity to 14.5 km s⁻¹ (Cernicharo et al. 2000, 2018). However, the integrated line intensities are rather uncertain in these cases. Observed and fitted line frequencies for these doublets are given in Table A.2. One of the components of the J = 64 − 63 doublet is fully blended with HC₇N J = 33 − 32, with a frequency difference between both features of ≈0.5 MHz. The other component of the same transition is blended with HCCC¹³CCN J = 14 − 13 with a frequency difference of 1.3 MHz. This is the only doublet missing in the series from J = 54 − 53 through J = 80 − 79.

Fig. 2.

Selected doublets of HC₉N ν ₁₉ = 1 in the 31-50 GHz domain. The left panels show lines of HC₉N in the ground vibrational state. The rotational quantum numbers are indicated in the top right side of each panel. The same transitions for the ν ₁₉ = 1 state are shown in the middle (e component) and right (f component) panels. The intensity scale is in antenna temperature in mK. The abscissa corresponds to LSR velocities in km s⁻¹. The vertical violet dotted line at −26.5 km s⁻¹ indicates the systemic velocity of the envelope (Cernicharo et al. 2000, 2018). Two additional ν ₁₉ doublets are shown in Fig. 1.

The line frequencies were fitted to the following expression for the energy of the rotational levels in a excited vibrational bending mode with ℓ=1:

$$E\left(J,p\right)=B\left(J\left(J+1\right)-1\right)-D{\left(J\left(J+1\right)-1\right)}^2\pm 1/2q_{t}J\left(J+1\right)$$ (1)

where the sign ± corresponds to the different parities of each doublet (f for + and e for -, assuming q _t is positive following the convection of Brown et al. (1975)). for a given value of J. Additional distortion terms were found unnecessary in the fitting process, the results of which are:

$$B=290.8844\pm 0.0003\text{MHz},$$ (2)

$$D=0.88\pm 0.04\text{Hz},$$ (3)

$$q_t=0.1463\pm 0.0001\text{MHz},$$ (4)

where the quoted uncertainties are 1 σ values. The standard deviation of the fit to the 52 observed lines is 105 kHz (roughly half a resolution element). The rotational constant, B, is just 0.37 MHz larger than that of the ground state of HC₉N, while the distortion constant, D, is practically the same: For the ground state of HC₉N, B = 290.51832 ± 0.00001 MHz and D = 0.860 ± 0.010 Hz (McCarthy et al. 2000). Hence, it is very likely that this series of doublets corresponds to a vibrational state of HC₉N with an excited bending mode. From the observed value of q _t and B it is possible to estimate the frequency of this bending mode using the relation ${\omega }_v\approx 2.6B_v^2/q_{vt}\approx 50.1{\text{cm}}^{-1}$ (Gordy & Cook 1984). To assign the lines to one of the bendingmodes of HC₉N we have performed ab initio calculations at different levels of theory (see appendix B). We unambiguously conclude that the lines belong to the lowest energy bending mode, ν ₁₉=1, of HC₉N. From the observed rotational constants we derive a first order vibration-rotation coupling constant α ₁₉ = −0.3661 ± 0.0004 MHz. Line intensities and other parameters for the observed HC₉N lines in the ground and its ν ₁₉ states are given in Table A.2.

For HC₅N, in addition to the lines of the ground vibrational state, all the doublets arising from its lowest vibrationally excited state ν ₁₁ have been detected in the 31-50 and 70-116 GHz domains. It has been previously detected towards CRL 618 (Cernicharo et al. 2001; Wyrowski et al. 2003). Its energy above the ground state has been estimated by Vichietti & Haiduke (2012) to be ≈111 cm⁻¹ (see also Appendix B). Details on the available laboratory spectroscopy for this bending mode of HC₅N are given in Appendix A. Selected lines are shown in Fig. A.1 and the derived parameters are given in Table A.4.

For HC₇N, two series of lines from the ν ₁₅ and 2ν ₁₅ have been detected in addition to those from the ground vibrational state. Fig. 1 shows a couple of doublets from these two vibrationally excited states. Additional lines are shown in Fig. A.3. In the 70-116 GHz domain only lines from the ν ₁₅ state up to J _up=72 are detected. Line parameters for HC₇N are given in Table A.3. This is the first time that this state is analyzed in detail in space. Nevertheless, the ν ₁₅ and 2ν ₁₅ states are reported, but not discussed, in the figures of the by Pardo et al. (2004); Pardo & Cernicharo (2007); Pardo et al. (2008). Several unidentified features in the 1.3 cm line survey of IRC +10216 by Gong et al. (2015) can be also assigned to the ν ₁₅ state of HC₇N, namely U21458.8 (J = 19 − 18e), U23732.7 (J = 21 − 20 f), U24862.7 (J = 22 − 21 f), U25976.2 and U25992.8 (J = 23 − 22 e and f components).

4. Discussion

Vibrationally excited cyanopolyynes show a clear trend in which the line intensities, relative to those of the ground vibrational state, increase with increasing chain length. For HC₅N, the observed intensity ratio between the ground and the sum of the two components (e and f) of its ν ₁₁=1 state is ≈30 (see Fig. A.1), while for HC₇N and its ν ₁₅=1 state the ratio is ≈8, and for HC₉N and its ν ₁₉=1 state it is ≈1. From the observed line shapes for these species, and taking into account the half power beam of the two telescopes, the emission arises from the external layers of the envelope where UV photons drive a rich photochemistry (Agúndez et al. 2017). The kinetic temperature is this zone of the circumstellar envelope is ≈20-30 K (Agúndez et al. 2017). Hence, it is difficult to believe that the ν ₁₁=1 state of HC₅N, which is at ≈160 K above the ground (Vichietti & Haiduke 2012), is pumped by collisions alone. The same applies to the ν ₁₅=1 and ν ₁₉=1 states of HC₇N and HC₉N respectively, which lie at ≈92 K (Vichietti & Haiduke 2012) and ≈72 K (this work) above the ground. Probably, pumping through infrared photons coming from the internal regions of the envelope plays an important role in the excitation of these species.

We have analyzed the data of HC₅N, HC₇N, and HC₉N by constructing rotational diagrams for their ground and low-lying vibrational states. We have assumed a radius for the source of 15′′ and convolved it with the main beam of both telescopes, depending on the line. For HC₉N, we find a rotational temperature for the ν ₁₉=1 state very similar to that of the ground vibrational state, ≈23.5 K, and a column density ratio between the ground and the ν ₁₉=1 state of 1.45 ± 0.50 (see Fig. 3). The vibrational partition function at 23.5 K (see Table A.1) is ≈1.15. If the ν ₁₉=1 state is populated by collisions and thermalized with the ground state at this temperature, then its expected column density would be 8% that of the ground state. In fact, the derived vibrational temperature for the ν ₁₉=1 is close to 80 K. Hence, an efficient mechanism must exist to pump molecules into this excited vibrational state. The column densities of HC₉N in the ground and the ν ₁₉ = 1 states are (4.5±0.5)×10¹³ cm⁻² and (3.1±0.5)×10¹³ cm⁻², respectively. The total column density of this species is, hence, (7.6±1.4)×10¹⁴ cm⁻². We have searched for possible lines that could be assigned to the ν ₁₉=2 state without success. Hence, contribution from other vibrational levels is expected to be marginal.

Fig. 3.

Rotational diagram of HC₉N in its ground (red) and ν ₁₉ = 1 (blue) vibrational states. The derived rotational temperatures and column densities are indicated in the plot.

For HC₅N and HC₇N, the rotational diagram analysis indicates the presence of a cold and a warm regime for the ground and the first vibrationally excited state (see sections A.1 and A.2). For these two components of HC₅N (T_rot ≃10 K and 25 K respectively) we derive N(ground)/N(ν ₁₁=1) ≈127 and 93, respectively (see Appendix A.1). The total column density of HC₅N is thus (8.3±0.7)×10¹⁴ cm⁻², with a negligible contribution of the ν ₁₁ state. For the cold (T_rot ≃17 K) and warm (T_rot ≃32 K) HC₇N components we derive N(ground)/N(ν ₁₅=1) ≈9.5 and 1.5, respectively (see Appendix A.2). The total column density of HC₇N, including the contribution of its ν ₁₅ state, is 3.5±0.7×10¹⁴ cm⁻². Hence, N(HC₅N)/N(HC₇N)≃2.4 and N(HC₇N)/N(HC₉N)≃7.7. The values obtained for these ratios by Gong et al. (2015), without any correction for the vibrational states, are 1.24 and 14.8, respectively. We note that without the contribution of the ν ₁₉ state, the abundance ratio N(HC₇N)/N(HC₉N) will be a factor ≃2 higher. The trend in the total abundance ratio between consecutive cyanopolyynes is similar to that found in cold molecular clouds, ≃3-4, but for long members of this molecular family the low energy bendingmodes population can be as high as that of the ground state.

Infrared pumping for polyatomic molecules can proceed through different paths. Stretching and bending modes harbor a large number of states and bands providing a plethora of radiative paths to pump the different levels of the molecule. The simple case of the triatomic molecule HNC, which has two stretching and one bending modes, was studied by Cernicharo et al. (2014). Excitation through each vibrational mode has a different effect on the line intensities of the ground vibrational state. In the case of HNC, the frequency of the bending mode is high and its infrared intensity corresponds to large Einstein coefficients, i.e., the molecule in the bending mode decays mainly to the ground vibrational state. However, for long linear molecules the frequencies of their bending modes and their infrared intensities decrease with increasing chain length. Hence, Einstein coefficients of pure rotational transitions within the bending mode are similar to those of ro-vibrational transitions between the bending mode and the ground vibrational state. This allows to maintain a significant population in the excited bending mode.

While in HNC there are a few radiative paths that redistribute the population of the ground vibrational state between the different vibrationally excited states, in the case of HC₉N there are ten stretching and nine bending modes. Hence, the number of transitions from the ground to excited vibrational states and the subsequent radiative de-excitation cascades is huge. Nevertheless, infrared pumping will also depend on the flux of the source at the wavelengths of the different infrared bands (see, e.g., Fonfría et al. 2008; Agúndez et al. 2017). In the case of IRC +10216 it is well known that the infrared emission peaks around 10 µm (Cernicharo et al. 1999). Hence, we could expect to have a dominant path through vibrational bands with large infrared intensities and frequencies around 1000 cm⁻¹.

To investigate these effects on the population of the excited vibrational states of cyanopolyynes in IRC +10216 we have carried out excitation and radiative transfer calculations for HC₅N, HC₇N, and HC₉N. The physical structure of the envelope and the radial abundance profiles are taken from the chemical model of IRC +10216 by Agúndez et al. (2017). We consider rotational levels within the ground vibrational state, within the lowest-lying vibrational state (ν =1 for HC N, ν =1 for HC N, and ν =1 to that of HC₉N and will reduce the intensity of the rotational lines of the ground state by a factor of two. Hence, detecting HC₁₁N will be at the sensitivity limit of present instruments. Infrared pumping has also important consequences on the possibility to detect other long chain molecules such as C₉H, C₁₀H, and C₁₁H. Their rotational frequencies are well known (see Gottlieb et al. 1998 and references therein). We have searched for them in our Q-band data without success. These species, as it is the case for C₆H (Cernicharo et al. 2008), have very low energy bending modes that could be highly populated decreasing the chances to detect them in their ground vibrational states.

In Fig. 4 we show the calculated line profile for one selected rotational transition of HC₅N, HC₇N, and HC₉N lying in the Q-band in both the ground and the lowest-lying vibrational state. It is seen that the intensity of the line belonging to the lowestlying vibrational state approaches the intensity of the line in the ground vibrational state as the size on the cyanopolyyne increases. The calculated line intensity ratio between the ground and lowest-lying vibrational state is ∼3.9, ∼2.2, and ∼1.9 for HC₅N, HC₇N, and HC₉N, respectively. These values differ from those observed. For example, they overestimate the relative population of the ν ₁₁=1 state of HC₅N and the ν ₁₅=1 state of HC₇N, although they agree reasonably well for the ν ₁₉=1 state of HC₉N. In any case, the model satisfactorily reproduces the observed trend of increasing relative population of the vibrationally excited state with increasing molecular size.

Fig. 4.

Calculated line profiles for selected lines of HC₅N, HC₇N, and HC₉N in their ground (G.S.) and lowest energy bending vibrational states. Calculated intensities have been multiplied by 10, 8, and 2 for HC₅N, HC₇N, and HC₉N, respectively, to match the observed intensities of the ground vibrational state lines.

The observed behavior in the abundance ratio of the cyanopolyynes could introduce an important limitation in detecting longer chains. HC₁₁N could be present in the envelope but its presence in space has never been confirmed (Cordiner et al. 2017). In our sensitive Q-band data none of the expected transitions of the ground state are detected. Theoretical calculations of the vibrational modes of this species by Vichietti & Haiduke (2012) suggest that the lowest bending mode, ν ₂₃, will be at an energy of 42 K. The effect of IR pumping could be very similar for HC₉N), and within a vibrational state associated with the CH bending mode (ν ₇=1 for HC₅N, ν ₉=1 for HC₇N, and ν ₁₁=1 for HC₉N), which has the most intense fundamental band at midinfrared wavelengths, where the radiation field in IRC +10216 is high. We adopt the infrared intensities calculated in this work at the MP2 level in the anharmonic limit (see Table B.4). Since infrared intensities between excited vibrational states are not known for these molecules, we assume that molecules in the mid-IR-lying excited vibrational state decay to the lowest-lying vibrational state with the same Einstein coefficient than to the ground vibrational state. We note that this assumption is an important source of uncertainty in the models. As rate coefficients for pure rotational excitation through inelastic collisions with H₂, we adopt the approximate expression of Deguchi & Uyemura (1984), while ro-vibrational excitation through collisions is considered to be negligible, an assumption that could introduce important uncertainties in the models due to the low energy separation between the rotational levels of the ground and the lowest-lying vibrational states. In summary, IR pumping occurs through absorption in the fundamental band of the CH bending mode, which lies around 600 cm⁻¹ for the three cyanopolyynes, and further radiative decay to the ground and lowest-lying vibrational states.