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Appendix A: Spectra covering detected and non-detected lines in the 3 mm band

In the case of silyl cyanide, the three rotational transitions covered within the 3 mm band are observed, but not all rotational transitions lying in the 3 mm band (80−116 GHz) are detected for C₅S, MgCCH, and NCCP. The lines that are not detected either fall in spectral regions where the sensitivity achieved is not good enough or are blended with stronger lines of other molecules.

	Fig. A.1 Spectra covering the C5S lines in the 80−93 GHz frequency range. Detected lines are indicated by red arrows and non-detected lines by red dashed lines. LTE calculated line profiles are shown in red.
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In Fig. A.1, we show the spectra covering the J = 44−43 to J = 50−49 rotational transitions of C₅S, lying in the low frequency side (80−93 GHz) of the 3 mm band. Only the three rotational transitions lying on the lowest frequencies (J = 44−43, J = 45 −44, and J = 46−45) are detected (lines also shown in Fig. 1 and already discussed in Sect. 3.1). The spectral regions where the next higher J lines of C₅S are expected are shown in the upper panels of Fig. A.1. None of these transitions can be identified with clear emission features in the observed spectra. The spectral region around the J = 47−46 and J = 49−48 transitions are relatively noisy. The antenna temperature rms noise level per 1 MHz channel is 1.5 mK around the J = 47−46 transition, while the expected C₅S line intensity is similar, making very difficult to ascertain whether or not the weak C₅S line is present. The situation is even worse for the J = 49−48 transition, with an expected antenna temperature of just ~1 mK and a rms noise level of 1.8 mK per 1 MHz channel. The J = 48−47 and J = 50−49 transitions of C₅S are severely blended with other much stronger lines. At higher frequencies, the chances of observing a C₅S line become lower because the expected C₅S line intensities decrease as J increases (C₅S is a quite heavy rotor and levels with J> 50 become poorly populated at rotational temperatures around 20 K) and also because in our 3 mm line survey of IRC +10216 spectra usually become less sensitive with increasing frequency.

As concerns the molecule MgCCH, there are three doublets of rotational transitions lying within the 3 mm band (see Fig. A.2). The two doublets lying at lower frequencies are identified with weak emission features (see also Fig. 2 and Sect. 3.2), but the doublet lying at 109.2 GHz, corresponding to the N = 11−10 rotational transition, cannot be clearly distinguised in the 3 mm line survey data. The low frequency component of this doublet is completely blended with a stronger line arising from a hyperfine component of the N_J = 1₂−0₁ rotational transition of ¹³CN, while the high frequency component cannot be appreciated due to an insufficient sensitivity (the rms noise level is 1.8 mK per 1 MHz in this spectral region and the MgCCH component is expected with an intensity of just ~1.5 mK).

	Fig. A.2 Spectra covering the MgCCH lines in the 3 mm band. Detected lines are indicated by red arrows and non-detected lines by red dashed lines. LTE calculated line profiles are shown in red.
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	Fig. A.3 Spectra covering the NCCP lines in the 3 mm band. Detected lines are indicated by red arrows and non-detected lines by red dashed lines. LTE calculated line profiles are shown in red.
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The linear rotor NCCP has also several rotational transitions lying in the 80−116 GHz range (see Fig. A.3), from which only three (J = 15−14, J = 16−15, and J = 18−17) are identified with weak emission features (see also Fig. 3 and Sect. 3.3). The J = 17−16 transition falls in a spectral region crowded by stronger lines arising from CH¹³CN, an unidentified line at 91 953 MHz, and CH₃CN, making very difficult to infer its presence. The J = 19−18 transition is partially blended with a stronger line corresponding to C₅N⁻, and in any case, the limited sensitivity of the spectrum (rms of 1.5 mK per 1 MHz) makes it very difficult to distinguish the NCCP line from the noise. An even worse situation occurs in the case of the J = 20−19 transition, which falls in a spectral region where it partially overlaps with a line of C₂P and the data is quite noisy (rms of 2.1 mK per 1 MHz). The last transition of NCCP within the 3 mm band is the J = 21−20, which falls in a spectral region where the noise level is moderately low (rms of 1.1 mK per 1 MHz if we do not consider the much noiser region at frequencies higher than 113 600 MHz), although it is not low enough to allow for a clear detection of lines with antenna temperatures of ~1 mK, as expected for this NCCP line.

© ESO, 2014