Discovery of thiofulminic acid with the QUIJOTE line survey: A study of the isomers of HNCS and HNCO in TMC-1

We present the ﬁrst detection of HCNS (thiofulminic acid) in space with the QUIJOTE line survey in the direction of TMC-1. We performed a complete study of the isomers of CHNS and CHNO, including NCO and NCS. The derived column densities for HCNS, HNCS, and HSCN are (9.0 ± 0.5) × 10 9 , (3.2 ± 0.1) × 10 11 , and (8.3 ± 0.4) × 10 11 cm − 2 , respectively. The HNCS / HSCN abundance ratio is 0.38. The abundance ratios HNCO / HNCS, HCNO / HCNS, HOCN / HSCN, and NCO / NCS are 34 ± 4, 8.3 ± 0.7, 0.18 ± 0.03, and 0.78 ± 0.07, respectively. These ratios cannot be correctly reproduced by our gas-phase chemical models, which suggests that formation paths for these species are missing, and / or that the adopted dissociative recombination rates for their protonated precursors have to be revised. The isotopologues H 15 NCO, DNCO, HN 13 CO, DCNO, H 34 SCN, and DSCN have also been detected with the ultrasensitive QUIJOTE line survey.


Introduction
Among the new species discovered in recent years with the QUIJOTE 1 line survey of TMC-1 (Cernicharo et al. 2021a, 2023a,b, andreferences therein), it is worth noting that 11 of them are S-bearing species: NCS, HCCS, H 2 CCS, H 2 CCCS, C 4 S, HCSCN, HCSCCH, HC 4 S, HC 3 S + , HCCS + , and HSO (Cernicharo et al. 2021b,c,d;Cabezas et al. 2022a;Fuentetaja et al. 2022;Marcelino et al. 2023).The chemical models for Sbearing molecules in interstellar clouds poorly reproduce the observed abundances and the results are strongly dependent on the adopted depletion of sulfur (Vidal et al. 2017).Moreover, many reactions involving S + with neutrals, as well as radicals with S and CS, are missing in the chemical networks and further study is required in order to achieve better agreement between chemical models and observations of sulfur-bearing species (Petrie 1996;Bulut et al. 2021).In addition to these new species, other Sbearing molecules such as CS, SO, NS, NS + , HCS + , HSC, HCS, CCS, C 3 S, C 5 S, H 2 CS, HSCN, and HNCS are present in TMC-1 (See Table 1 in Cernicharo et al. 2021b).Despite the high depletion of sulphur in molecular clouds, most of these species are the counterpart of the same molecular structure but replacing sulphur with oxygen.Only C 4 O, HCCO + , and HC 4 O have not yet been detected in TMC-1, or indeed in any other source.
The study of isomers of the same molecule provides clues as to the formation mechanism of these species, as often they are produced from a common precursor.The recent detection of five cyano derivatives of propene, which have energies ranging from 0 to 19.6 kJ/mol (0-2357 K) but similar abundances, indicates that kinetics, and not energetics, is dominating the production of these different isomers (Cernicharo et al. 2022a).The most relevant isomers in space are HCN and HNC, which are assumed to be produced from the dissociative electronic recombination (DR) of HCNH + .In cold dark clouds, these isomers have an abundance ratio of ∼1 in spite of the much higher energy of HNC of 55 kJ/mol (Baulch et al. 2005;Mendes et al. 2012, and references therein).Their chemistry in interstellar clouds was analysed in detail by Loison et al. (2014).
Concerning the CHNS and CHNO isomers, it is assumed that the first are mainly formed from DR of H 2 NCS + and HNCSH + (Adande et al. 2010;Gronowski & Kolos 2014), and the second from DR of H 2 NCO + and HNCOH + (Marcelino et al. 2010;Quan et al. 2010).In the present work, we present the first detection of HCNS (thiofulminic acid) in space (TMC-1) and a coherent and precise study of the abundances of the isomers of CHNS and CHNO.The isotopologues H 34 SCN, DSCN, H 15 NCO, HN 13 CO, DNCO, and DCNO are also detected in TMC-1.The derived abundances for the CHNS and CHNO isomers, together with those of NCS and NCO, are compared with the results of state-of-the-art chemical models for S-bearing species.
captionObserved Ju = 3 and 4 (K a = 0) transitions of the isomers of HCNS.The bottom panel shows the hyperfine components of the J = 5/2 − 3/2 transition of NCS in its2 Π 3/2 ground state.The abscissa corresponds to the rest frequency.
The ordinate is the antenna temperature corrected for atmospheric and telescope losses, in mK.Blanked channels correspond to negative features produced when folding the frequency-switched data.The red line shows the computed synthetic spectra for these lines (see Sect. 3).The derived line parameters are given in Table A.1.

Observations
The observational data used in this work are part of QUIJOTE (Cernicharo et al. 2021a), a spectral line survey of TMC-1 in the Q-band carried out with the Yebes 40m telescope at the position α J2000 = 4 h 41 m 41.9 s and δ J2000 = +25 • 41 ′ 27.0 ′′ , corresponding to the cyanopolyyne peak (CP) in TMC-1.A detailed description of the system is given by Tercero et al. (2021), and details on the QUIJOTE line survey are provided by Cernicharo et al. (2021aCernicharo et al. ( , 2023a,b),b).The data analysis procedure is described by Cernicharo et al. (2022b).The total observing time on source is 1202 hours and the measured sensitivity varies between 0.07 mK at 32 GHz and 0.2 mK at 49.5 GHz.
The main beam efficiency can be given across the Q-Band by B eff =0.797 exp[−(ν(GHz)/71.1) 2 ].The forward telescope efficiency is 0.95 and the beam size at half power intensity is 54.4 ′′ and 36.4 ′′ at 32.4 and 48.4 GHz, respectively.
In addition to the Q-band data, the isomers of HNCO and HNCS were observed in the millimetre domain with the IRAM 30m telescope and consist of a 3mm line survey that covers the 71.6-116GHz domain (Cernicharo et al. 2012;Agúndez et al. 2022;Cabezas et al. 2022a;Cernicharo et al. 2023b).The final 3mm line survey has a sensitivity of 2-10 mK.However, at some selected frequencies, the sensitivity is as low as 0.6 mK.The IRAM 30m beam varies between 34 ′′ and 21 ′′ at 72 GHz and 117 GHz, respectively, while the beam efficiency takes values of 0.83 and 0.78 at the same frequencies (B e f f = 0.871 exp[−(ν(GHz)/359) 2 ]).The forward efficiency at 3mm is 0.95.
The absolute intensity calibration uncertainty is 10%.However, the relative calibration between lines within the QUIJOTE survey is certainly better because all them are observed simultaneously and have the same calibration uncertainties.The data were analysed with the GILDAS package 2 .

Results
Line identification in this work was performed using the MADEX code (Cernicharo 2012), and the CDMS and JPL catalogues (Müller et al. 2005;Pickett et al. 1998).The references for the laboratory data used by MADEX in its spectroscopic predictions are described below.The intensity scale used in this study is the antenna temperature (T * A ). Consequently, the telescope parameters and source properties were used when modelling the emission of the different species in order to produce synthetic spectra in this temperature scale.The permanent dipolar moments of the molecular species studied in this work are given in Table 1.In this work, we assumed a velocity for the source relative to the local standard of rest of 5.83 km s −1 (Cernicharo et al. 2020).The source is assumed to be circular with a uniform brightness temperature and a radius of 40 ′′ (Fossé et al. 2001).The procedure to derive line parameters is described in Appendix A. The observed line intensities were modelled using a local thermodynamical equilibrium hypothesis (LTE), or a large velocity gradient approach (LVG).In the latter case, MADEX uses the formalism described by Goldreich & Kwan (1974).

The HCNS, HSCN, and HNCS isomers
The HCNS, HNCS, and HSCN isomers were observed with high spectral resolution in the laboratory (Brünken et al. 2009b;McGuire et al. 2016).The frequencies measured in these studies were fitted using the adapted Hamiltonian for each species and the resulting rotational constants were implemented into the MADEX code to predict transition frequencies and to model the observed emission line profiles.
HNCS and HSCN are well-known molecular components of interstellar clouds (Frerking et al. 1979;Halfen et al. 2009;Adande et al. 2010;Vastel et al. 2018).They harbour strong lines in our line survey.However, the isomer HCNS has not been detected yet in space.It is the thio-form of HCNO, an isomer of HNCO and a molecule detected in several interstellar clouds (Marcelino et al. 2009).Thanks to the sensitivity of QUIJOTE, we report in this work the first detection of its two lines in the Qband (J = 3 − 2 and J = 4 − 3) in space, namely towards TMC-1.The lines are detected with a signal-to-noise ratio (S/N) of ∼12 and match the predicted frequencies within 3 kHz (see Fig. 1).The derived line parameters are given in Table A.1.The sensitivity of our line survey at 3mm with the IRAM 30m telescope is not as high as that of QUIJOTE.Nevertheless, the J = 6 − 5  2013). (j) Assumed identical to that of the main isotopologue.
line of HCNS at 73815.849MHz is detected at a 3σ level in the data (see Fig. A.1).We also observed this line with the Yebes 40m telescope using the W-band receiver described by Tercero et al. (2021).Near the frequency of the HCNS J = 6 − 5 transition, there is a line of CCC 13 CH that is detected with good S/N with both telescopes.It has an intensity of 10 mK and has been used to scale the data of the Yebes 40m and the IRAM 30m telescopes.The two sets of data were merged, weighting them as 1/σ 2 .The resulting data allow us to detect the J = 6 − 5 line of HCNS at 4σ.The results are shown in Fig. A.1.We conclude that HCNS is convincingly detected in TMC-1.A fit to the laboratory and astronomical frequencies for this species provides the following new molecular constants: B=6151.44367±0.00027MHz, D=1.712±0.016kHz, and eQq=-0.7227±0.0018MHz.
From the observed intensities of these three lines of HCNS, and using a line profile fit method (Cernicharo et al. 2021d), we obtain T rot =7.0±0.5 K and N(HCNS)=(9.0±0.5)×10 9 cm −2 .The synthetic spectra are shown in Fig. 1 and A.1, and are in excellent agreement with the observations.No collisional rates are available for HCNS, but with the aim to estimate the excitation conditions that could be expected for this molecule in the LVG approximation, we adopted the collisional rates of OCS with H 2 calculated by Green & Chapman (1978).OCS has a rotational constant similar to that of HCNS, and therefore we consider that these rates provide a better approximation to those of HCNS than the rates of HCNO (Naindouba et al. 2015), which has a rotational constant twice that of HCNS.For the typical volume densities of TMC-1 of 1×10 4 cm −3 (Agundez et al. 2023, Fuentetaja et al., in prep.), we obtain that the excitation temperatures of the observed lines could vary between 8.2 K for the J = 3 − 2 line down to 6.2 K for the J = 6 − 5 transition, in good agreement with the derived averaged rotational temperature of 7 K.
The J u = 3, 4 with K a = 0 transitions of HNCS and HSCN appear in our QUIJOTE data as features exhibiting all their hyperfine components with a very high S/N.These transitions are shown in Fig. 1.The corresponding K a = 1 lines of HNCS have upper energy levels of 66 (J u = 3) and 68 K (J u = 4) and are not detected in our data.However, the corresponding energies for HSCN are only at 16.9 and 19.2 K, respectively.We detected these latter; they are shown in Fig. A.2.Moreover, the J u = 7, 8 transitions with K a =0 of both isomers were detected with the IRAM 30m telescope (see Fig . A.2).The derived line parameters for all these lines are given in Table A.1.No collisional rates are available for either of the two isomers.We used all observed lines using the line profile fitting procedure (Cernicharo et al. 2021d) to derive their rotational temperature and column densities.For HNCS, we obtain T rot =6.0±0.5 K and N(HNCS)=(3.2±0.1)×10 11cm −2 .For HSCN, we derived T rot =5.2±0.3K and N(HSCN, K a =0) = (6.9±0.2)×10 11cm −2 , and N(HSCN, K a =1) = (1.4±0.3)×10 11cm −2 .The interladder temperature between K a =0 and 1 lines could be thermalised by collisions at the kinetic temperature of the cloud.For an energy separation between these ladders of 14.5 K, we derive a kinetic temperature for TMC-1 of 9.1±1.5 K, which is in excellent agreement with the result obtained by Agundez et al. (2023).
The isomer HSNC has been characterised in the laboratory (McGuire et al. 2016) and we searched for its lines in our survey.However, we only obtained an upper limit of 4×10 9 cm −2 for its column density.Of particular interest for the chemistry of the CHNS isomers, it is worth deriving the column density of NCS as it could participate in their formation.NCS was discovered in TMC-1 with previous QUIJOTE line data (Cernicharo et al. 2021b).The new data are shown in Fig. 1.Adopting a rotational temperature of 5.5 K, the derived column density is (9.5±0.2)×10 11cm −2 .The NCS to HNCS abundance ratio is 3.0±0.16.
Among the isomers of the CHNS family, the most abundant is HSCN.Its lines are strong enough to allow a search for its isotopologues.We observed two rotational transitions exhibiting hyperfine structure for H 34 SCN and DSCN (see Fig. A.3). Adopting the same rotational temperature as for the main isotopologue, we derive the column densities given in Table 1.The 32 S/ 34 S abundance ratio is 33±5, which agrees with previous determinations using other molecular species (see e.g.Cernicharo et al. 2021b and Fuentetaja et al. in prep., and references therein).The H/D abundance ratio is also 33±5 and represents a considerable single deuterium enhancement in HSCN, in line with that observed in other molecules in TMC-1 (Cabezas et al. 2021(Cabezas et al. , 2022b, and references therein).

The HNCO, HCNO, and HOCN isomers
HNCO was one of the first molecules detected in the early years of millimetre radio astronomy by Snyder & Buhl (1972).Its isomer HCNO (fulminic acid) was detected by Marcelino et al. (2009) towards B1, L1544, and L183, but not in TMC-1.Another isomer, HOCN, was detected towards SgrB2, B1b, L1544, L1527, L183, and IRAS16293-2422 (Brünken et al. 2009a(Brünken et al. , 2010;;Marcelino et al. 2010).Therefore, these isomers have been detected in space in a broad sample of cold and warm molecular clouds.The level energies, transition frequencies, and intensities for HNCO and its isotopologues, as well as for HOCN, are from the JPL and CDMS catalogues, respectively.HCNO is a quasi-linear molecule and its rotational fre-quencies were obtained from a fit to available laboratory data (Winnewisser & Winnewisser 1971).The three isomers and the isotopologues 15 N, 13 C, and D of HNCO have been detected with QUIJOTE in TMC-1.The data are shown in Fig. A.4 and the derived line parameters are given in Table A.2.We note that for HN 13 CO and DNCO, our measured frequencies have a significant lower uncertainty than those in the entries of the JPL catalogue.
The HNCO lines are the strongest among those of all its isomers.We also detected the 4 04 − 3 03 and 5 05 − 4 04 lines at 3mm with the IRAM 30m radio telescope.These lines are shown in Fig . A.5. Collisional rates for HNCO with He (T K ≥30 K; Green 1986) and H 2 (T K ≥7 K; Sahnoun et al. 2018) are available.For a volume density of 1-2×10 4 , our LVG calculations using the p-H 2 collisional rates provide excitation temperatures for the 2 0,2 −1 0,1 line of HNCO of 8-9 K, decreasing to ∼5 K for the 5 05 − 4 04 transition.We therefore adopted a rotational temperature of 8 K for HNCO and its isotopologues in the Q-band, and of 4.8 K in the 3mm domain.The derived column densities are given in Table 1 and the computed synthetic spectra are shown in red in Figures A.4 and A.5.The HNCO/HN 13 CO, HNCO/DNCO, and HNCO/H 15 NCO abundance ratios are 52±8, 44±8, and 333±60.
The isomers HCNO and HOCN have dipolar moments of 3.1 and 3.7 D, respectively (see Table 1).For HCNO we used the LVG approximation using the collisional rates of HCNO with He calculated by Naindouba et al. (2015).For a density of 1-2×10 4 cm −3 , we obtain rotational temperatures in the range of 4.9-5.5 K. HCNO is detected for the first time in TMC-1.We also observed its J = 4 − 3 transition at 3mm (see Fig. A.2).The estimated column density for this isomer of HNCO is (7.8±0.2)×10 10 cm −2 .We also observed the J = 2 − 1 of DCNO in the Q-band (see Fig. A.5).The derived HCNO/DCNO abundance ratio is 25±4.For HOCN, we observed the 2 0,2 − 1 0,1 with QUIJOTE and its 4 04 − 3 03 and 5 05 − 4 04 lines at 3mm (see Fig. A.5).All the observed lines are well reproduced by the models with a rotational temperature similar to that of HCNO (T rot =5.5 K) as shown by the computed synthetic spectra shown in Figures A.4 and A.5.The derived column densities for HCNO and HOCN are given in Table 1.The fundamental transition of HOCN at 20.9 GHz was tentatively observed in TMC-1 by Brünken et al. (2009b).
We detected the four strongest hyperfine components of the J = 7/2 − 5/2 transition of NCO in its 2 Π 3/2 ground state at 3mm.This species was recently detected towards L483 by Marcelino et al. (2018) with an NCO/HNCO abundance ratio of ∼0.2.Due to the modest dipole moment of this species (µ=0.64D; Saito & Amano 1970), we modelled the TMC-1 data adopting T rot (NCO)=8 K.The derived column density is (7.4±0.5)×10 11cm −2 and the NCO/HNCO abundance ratio is 0.067, which is three times lower than towards L483.However, in TMC-1 the NCS/HNCS ratio is 45 times larger than the NCO/HNCO one.We also searched for the isomer HONC (Mladenovic et al. 2009), but only obtained upper limits (see Table 1).Finally, three lines of H 2 NCO + were observed in the Qband.These are shown in Fig. A.6 and their line parameters are given in Table A.2. Adopting a rotational temperature of 5.5 K and a o/p abundance ratio of 3:1, the derived total column density for this species is (1.8±0.4)×10 10 cm −2 .

McGuire et al. 2016
).The energy ordering in the oxygen family is similar, with HNCO being the most stable isomer, and HOCN, HCNO, and HONC lying at 12330, 34480, and 42235 K above, respectively (Schuurman et al. 2003).In spite of the similarities in the energetics, the CHNO and CHNS families show a very different behaviour in TMC-1.While HNCO is far more abundant than HOCN, the abundance of HNCS is only half of that of HSCN (see Table 2).
The chemistry of HNCS and its isomers has been discussed by Adande et al. (2010), Gronowski &Kolos (2014), andVidal et al. (2017).Here, we revisit the subject by running a gas-phase chemical model of a cold dense cloud (see details in Cernicharo et al. 2021b andFuentetaja et al. 2022).The chemical scheme of synthesis of HNCS and HSCN can be summarized as where the scheme begins with the protonation of NCS by abundant proton donors XH + , such as HCO + , H 3 O + , and H + 3 .This scheme, replacing sulfur by oxygen, is the same one that describes the formation of the oxygen analogues HNCO and HOCN (Marcelino et al. 2010;Quan et al. 2010;Lattanzi et al. 2012).The chemical schema start with the radicals NCO and NCS, which are mostly formed through neutral-neutral reactions.According to the chemical model, the main reactions forming NCO are N + HCO and CN + O 2 , while NCS is mainly formed through the analogous reactions N + HCS and CN + SO.
The abundances calculated with the chemical model are shown in Fig. 1.We focus on times around a few 10 5 yr, when most molecules reach their peak abundances.The abundances calculated for NCO and HNCO are in line with the observed values.In the case of HOCN, the calculated abundance is close Notes. (a) Column density of (1.7±0.2)×10 12 cm −2 (Cernicharo et al. 2018). (b) Column density of (2.7±0.5)×10 14cm −2 (Gerin et al. 1993).
to that of HNCO, while observations indicate that HOCN is much less abundant than HNCO.It is likely that the dissociative recombination of HNCOH + favours HNCO over HOCN (the model assumes equal branching ratios) or that the reaction between HNCO + and H 2 favours H 2 NCO + over HNCOH + (equal yields are assumed at present).Investigation into these two reactions would allow us to understand the high observed HNCO/HOCN ratio.Grain surface chemistry may also be responsible for this.Indeed, the gas-grain model of Quan et al. (2010) predicted HNCO to be somewhat more abundant than HCNO.The detection of the precursors H 2 NCO + and HNCOH + would also be very useful to shed light on this.While the H 2 NCO + cation has been detected in interstellar clouds (Gupta et al. 2013;Marcelino et al. 2018), its isomer HNCOH + is still awaiting detection in space (laboratory frequencies are available from Mladenovic et al. 2009).Only the H 2 NCO + cation is detected in our TMC-1 data.
In the case of the sulfur analogues NCS, HNCS, and HSCN, the model severely underestimates their abundances.It is likely that the model lacks efficient routes to NCS, hampering the subsequent formation of HNCS and HSCN.A similar situation was encountered in previous chemical models, with abundances relative to H 2 ≲ 10 −12 for HNCS and HSCN (Adande et al. 2010;Gronowski & Kolos 2014;Vidal et al. 2017).Apart from the general problem of abundance underestimation, the fact that HSCN is observed to be more abundant than HNCS, in contrast to the oxygen case, indicates that the branching ratios of reactions ( 2) and ( 4) must be markedly different to those of the analogous oxygen reactions.Identification of the precursor cations H 2 NCS + and HNCSH + would also help to constrain the chemistry of HNCS and HSCN, although their rotational spectrum has not yet been characterised in the laboratory.
The third observed species of the family of HNCO isomers, namely HCNO, has a different skeleton to the other two isomers and is therefore formed in a different way.According to the chemical model, the neutral-neutral reaction CH 2 + NO (Marcelino et al. 2009) is the main source of HCNO.The analogous molecule with sulfur, HCNS (the one reported here), is severely underestimated by the chemical model.The analogous reaction CH 2 + NS cannot account for HCNS because in TMC-1, NS is 160 times less abundant than NO, while HCNS is just 9 times less abundant than HCNO (see Table 2).Among the potential missing routes to HCNS, we could consider the neutralneutral reactions HCN + SH, CN + H 2 S, N + H 2 CS, NH + HCS, and NH 2 + CS.To our knowledge, these reactions have not been studied either theoretically or experimentally, and therefore we do not know whether or not they are fast at low temperatures and produce HCNS as a product.Among these reactions, the most efficient by far is N + H 2 CS, which would provide an HCNS abundance of the order of the observed one assuming that it has a rate coefficient of the order of 1×10 −10 cm 3 s −1 at low temperature.
We conclude that further experimental and theoretical work is needed to understand the chemistry of the HNCS and HNCO isomers.In particular, the enhancement of the abundance of HSCN relative to HNCS and HOCN, as well as the behaviour of the NCS/HNCS and NCO/HNCO abundance ratios, are not well reproduced by the present chemical models.The derived line parameters for all observed lines of the CHNS and CHNO families are given in Tables A.1 and A.2, respectively.The differences between the measured frequencies and the predicted ones are always of a few kHz.However, for some isotopologues, several hyperfine components are blended in the laboratory data (measurements with uncertainties ≥ 30 kHz), while in our observations the lines are well resolved and have an uncertainty of 10 kHz.
The lines of HCNS, HNCS, HSCN, and NCS are shown in Figures 1 and A

Fig. 1 .
Fig. 1.Abundances calculated with the chemical model for molecules of the families of HNCO and HNCS.The peak calculated abundance of HCNS is around 10 −15 relative to H 2 and therefore lies outside the abundance range shown.Horizontal dashed lines correspond to the abundances observed in TMC-1.

Fig. A. 1 .
Fig. A.1.Observed J = 6 − 5 line of HCNS with the IRAM 30m (top panel) and Yebes 40m (middle panel) radio telescopes.The data of the Yebes telescope have been scaled to those of IRAM using the line N = 8 − 7 J = 15/2 − 13/2 of CCC 13 CH at 73819.18MHz that appears in both sets of data with a S/N of ∼10 in the IRAM data and of ∼5 in the Yebes data.The merged spectrum was obtained by averaging both sets of data weighted as 1/σ 2 and has a sensitivity of 1 mK.The derived line parameters for this transition of HCNS are given in TableA.1.The red line corresponds to the computed synthetic spectra (see Sect. 3).The vertical blue line corresponds to the measured frequency for a v LS R of 5.83 km s −1 .
Fig. A.2. Observed lines of HSCN and HNCS.The two upper rows show the observed lines of HSCN with K a =1.The two lower rows show the 7 07 − 6 06 and 8 08 − 7 07 lines of HSCN (third row) and HNCS (fourth row) observed at 3mm with the IRAM 30m radio telescope.The red line shows the computed synthetic spectra for these lines (see Sect. 3).The derived line parameters are given in Table A.1.

Fig. A. 4 .
Fig. A.4. Same as Fig. 1 but for the J = 2 02 − 1 01 transition of HNCO (isocyanic acid) and its isotopologues HN 13 CO, H 15 NCO, and DCNO (upper and second raw panels).The upper panel also shows a zoom to the data to exhibit the hyperfine lines of HN 13 CO.The scale of the zoom is indicated in blue on the right-Y axis.The third row shows the J = 2 − 1 line of HCNO (the J = 4 − 3 is shown in Fig. A.2) and the J = 2 02 − 1 01 line of HOCN (cyanic acid).Finally, the bottom panels show two hyperfine components of the J=7/2-5/2 line of NCO in its 2 Π 3/2 ground state.The lambda-type doubling components, e and f , are indicated.The derived line parameters are given in TableA.2.The hyperfine structure was considered in the modelling of the lines (shown by the red lines).

Fig
Fig. A.5. Observed lines of HNCO and HOCN at 3mm with the IRAM 30m radio telescope.The red line shows the synthetic spectra computed with the models described in Sect.3. The column densities are the same as those derived from the lines observed in the Q-band.The bottom panels show the DNCO 4 04 − 3 03 and DCNO J = 2 − 1 lines.The derived line parameters are given in Table A.2.

Fig
Fig. A.6.Observed J u = 2 lines of H 2 NCO + towards TMC-1.The red line shows the computed synthetic spectra for these lines (see Sect. 3).The derived line parameters are given in Table A.2.

Table 2 .
Derived abundance ratios for the isomers of HNCO and HNCS in TMC-1