A study of C4H3N isomers in TMC-1: line by line detection of HCCCH2CN
AAstronomy & Astrophysics manuscript no. c4h3n_isomers_final © ESO 2021January 22, 2021 L etter to the E ditor A study of C H N isomers in TMC-1: line by line detection ofHCCCH CN (cid:63) N. Marcelino , B. Tercero , , M. Agúndez , and J. Cernicharo Grupo de Astrofísica Molecular, Instituto de Física Fundamental, CSIC, C / Serrano 123, 28006 Madrid, Spaine-mail: [email protected] Observatorio Astronómico Nacional (IGN), C / Alfonso XII 3, 28014 Madrid, Spain Observatorio de Yebes (IGN). Cerro de la Palera s / n, 19141 Yebes, Guadalajara, SpainReceived ; accepted ABSTRACT
We present Yebes 40m telescope observations of the three most stable C H N isomers towards the cyanopolyyne peak of TMC-1.We have detected 13 transitions from CH C N (A and E species), 16 lines from CH CCHCN, and 27 lines ( a -type and b -type) fromHCCCH CN. We thus provide a robust confirmation of the detection of HCCCH CN and CH CCHCN in space. We have constructedrotational diagrams for the three species, and obtained rotational temperatures between 4 − . − × cm − . Our chemical model provides abundances of the order of the observed ones, althoughit overestimates the abundance of CH CCCN and underestimates that of HCCCH CN. The similarity of the observed abundances ofthe three isomers suggests a common origin, most probably involving reactions of the radical CN with the unsaturated hydrocarbonsmethyl acetylene and allene. Studies of reaction kinetics at low temperature and further observations of these molecules in di ff erentastronomical sources are needed to draw a clear picture of the chemistry of C H N isomers in space.
Key words.
Astrochemistry – ISM: abundances – ISM: clouds, TMC-1 – ISM: molecules – line: identification
1. Introduction
Three C H N isomers have been detected in space to date.These are, in order of increasing energy, methylcyanoacetylene(CH C N), cyanoallene (CH CCHCN), and propargyl cyanide(HCCCH CN). Our knowledge of C H N isomers in the inter-stellar medium is the result of a nice multidisciplinary story withcontributions from theoretical calculations, laboratory experi-ments, and astronomical observations. The presence of cyanoal-lene in cold interstellar clouds was predicted by Balucani et al.(2000, 2002) based on crossed molecular beam experiments and ab initio calculations which indicated that the reaction of CN andCH CCH would produce CH C N, already detected in TMC-1(Broten et al. 1984), and CH CCHCN in nearly equal amounts.Laboratory experiments indeed showed that the reaction CN + CH CCH is rapid at low temperatures (Carty et al. 2001).These results motivated an astronomical search for cyanoallenein TMC-1, which turned out to be successful using the GBT (Lo-vas et al. 2006) and E ff elsberg 100m (Chin et al. 2006) tele-scopes.In their combined crossed beam and ab initio study, Balu-cani et al. (2000, 2002) studied also the reaction between CN andCH CCH (allene), a non polar metastable isomer of CH CCHwhich is thought to be also present in cold interstellar clouds.These authors found that the reaction should be rapid at lowtemperatures, something that was confirmed by Carty et al.(2001), producing cyanoallene and the third C H N isomer: (cid:63)
Based on observations with the 40-m radio telescope of the NationalGeographic Institute of Spain (IGN) at Yebes Observatory (projects19A003 and 20A014). Yebes Observatory thanks the ERC for fundingsupport under grant ERC-2013-Syg-610256-NANOCOSMOS.
HCCCH CN. This isomer was not detected in TMC-1 by Lo-vas et al. (2006), although it was later on found toward thissame source during a cm line survey with the GBT (McGuireet al. 2020). The detection of propargyl cyanide in TMC-1 bythese authors relied on four individual lines detected at a modestsignal-to-noise ratio (SNR) and was supported by line stackingof 68 transitions.Here we present an independent and robust detection ofHCCCH CN in TMC-1, with 10 lines detected with SNR above10 plus 12 lines detected above 3 σ , together with observations ofthe two other C H N isomers, CH C N and CH CCHCN. Thepresence of the latter is confirmed by the detection of a signifi-cant number of rotational lines. The high sensitivity and numberof lines detected allow us to derive precise abundances for thethree isomers in a coherent and systematic way and to revisit thechemistry of C H N isomers in TMC-1.
2. Observations
The data presented here are part of a deep spectral line survey inthe Q band toward TMC-1, performed at the Yebes 40 m radiote-lescope (de Vicente et al. 2016), located at 990 m of altitudenear Guadalajara (Spain). The observed position corresponds tothe cyanopolyyne peak in TMC-1, at α J = h m . s and δ J = + ◦ (cid:48) . (cid:48)(cid:48) . We have covered the full Q band at the40 m telescope, between 31.1 GHz and 50.4 GHz, using the re-cently installed NANOCOSMOS HEMT Q band receiver (Ter-cero et al. 2020b) and the fast Fourier transform spectrometers(FFTS) with 8 × http://rt40m.oan.es/rt40m_en.php Article number, page 1 of 11 a r X i v : . [ a s t r o - ph . GA ] J a n & A proofs: manuscript no. c4h3n_isomers_final low a simultaneous scan of a band width of 18 GHz at a spectralresolution of 38 kHz ( ∼ − ). We observed two setups atdi ff erent central frequencies in order to fully cover the lower andupper frequencies allowed by the Q band receiver, and to checkfor spurious signals and other technical artifacts.The observations were performed in several sessions, be-tween November 2019 and February 2020, using the frequencyswitching technique with a frequency throw of 10 MHz. Theintensity scale in the spectra obtained is T ∗ A , antenna tempera-ture corrected for atmospheric absorption and spillover losses,which was calibrated using two absorbers at di ff erent tempera-tures and the atmospheric transmission model ATM (Cernicharo1985; Pardo et al. 2001). Pointing and focus were checked ev-ery hour through pseudo-continuum observations (see e.g. de Vi-cente et al. 2016; Tercero et al. 2020a) of the SiO J = − v = (cid:48)(cid:48) . System temperatures were in the range 50-250 Kdepending on the frequency, the particular weather conditions ofeach observing session (from 5 mm to 10 mm of precipitable wa-ter vapor), and the elevation of the source (from 15 ◦ to 80 ◦ ). Thefinal rms obtained is in the range 0.5-1 mK, rising up to 3 mK atthe highest frequencies. The main beam e ffi ciency of the Yebes40 m telescope ranges from 0.6 at 32 GHz to 0.43 at 49 GHz, andthe half power beam width (HPBW) ranges from 55 (cid:48)(cid:48) at 32 GHzto 37 (cid:48)(cid:48) at 49 GHz. All the data were reduced and analyzed usingthe GILDAS software.
3. Results
The high sensitivity of this line survey allowed the detectionof HCCCH CN towards TMC-1 through 17 a -type lines up toquantum numbers J = − K a = , , E u ≤
13 K),with 10 of them showing a SNR >
10. In addition, we detected10 b -type lines harbouring hyperfine structure. These lines areshown in Fig. 1 and Fig. 2 and are listed in Table A.1. Line iden-tification was performed using the MADEX catalogue (Cernicharo2012, see Table A.1), which also includes predictions for the hy-perfine structure. This detection confirms the presence of thisspecies in space, recently claimed for the first time in TMC-1 byMcGuire et al. (2020) using the Green Bank Telescope (GBT).These authors presented a 5 σ signal (18 σ in the response im-pulse function) obtained by an intensity and noise-weighted av-erage (“stack”) of the data at the expected frequencies of theHCCCH CN lines that could be present within the noise level.It is worth noting that our 40 m survey of TMC-1 in the Qband is complementary to that performed with the GBT between8 GHz and 30 GHz. Although most of the individual lines ofHCCCH CN are below the detection limit of the GBT data, fourof them are detected at 1-3 σ levels. Thanks to the high spectralresolution of these data (1.4 kHz) they distinguished three cloudcomponents in the line profiles (see Fossé et al. 2001 for a de-tailed analysis of the velocity structure of this source).In this work, a single Gaussian function was fitted to theHCCCH CN line profiles to obtain the observed line parame-ters (see Table A.1). We derived a V LSR = (5 . ± .
09) km s − and a line width ( ∆ v , full width at half maximum) of (0 . ± .
18) km s − . The former is slightly di ff erent from the value(5 . ± .
01) km s − , obtained by Cernicharo et al. (2020a) fromGaussian fits to the 50 lines of HC N and its C and N iso-topologues detected in our line survey. Note we have a largeruncertainty due to the lower number of transitions and the weak- ness of some of the lines as compared to HC N, in particular the b -type transitions.We also detected the other two C H N isomers, CH CCHCNand CH CCCN, using frequencies from the
MADEX catalogue(Cernicharo 2012, see Table A.1). The 16 lines of CH CCHCNdetected in our line survey are shown in Fig. A.1 and are listedin Table A.1. All of them are detected above a 10 σ level. Thisspecies was previously identified in TMC-1 through four linesbetween 20 GHz and 26 GHz (Lovas et al. 2006). Here we re-port the first detection of lines of CH CCHCN above 30 GHz inTMC-1. Kaifu et al. (2004) did not detect lines above the noiselimit at the CH CCHCN frequencies in their line survey between8.8 GHz and 50 GHz carried out with the Nobeyama 45 m tele-scope. As we mentioned in previous works (Cernicharo et al.2020a,b,c; Marcelino et al. 2020), the sensitivity of our observa-tions is a factor 5-10 better than that of Kaifu et al. (2004) at thesame frequencies. The derived V LSR for the CH CCHCN lines,by fitting a single Gaussian, is (5 . ± .
03) km s − , which is sim-ilar, within errors, to the one obtained for HCCCH CN. The iso-mer CH CCCN, a well known species in TMC-1 (Broten et al.1984; Kaifu et al. 2004), has been also identified in our line sur-vey through 10 strong lines ( J u from 8 to 12 and K = ,
1) plusfive K = E u >
29 K) tentatively detected (see Fig. A.2and Table A.1). These lines show a V LSR of (5 . ± .
02) km s − which matches that observed for HC N.We can estimate rotational temperatures ( T rot ) and molecu-lar column densities ( N ) for the detected species by constructingrotational diagrams (see e.g. Goldsmith & Langer 1999). Thisanalysis assumes the Rayleigh-Jeans approximation, opticallythin lines, and LTE conditions. The equation that derives the totalcolumn density under these conditions can be re-arranged asln π k B ν (cid:82) T MB d v hc A ul g u b = ln (cid:32) NQ rot T rot − T bg T rot (cid:33) − E u k B T rot , (1)where g u is the statistical weight in the upper level, A ul is theEinstein A -coe ffi cient for spontaneous emission, Q rot is the rota-tional partition function which depends on T rot , E u is the upperlevel energy, ν is the frequency of the transition, b is the dilutionfactor, and T bg is the cosmic microwave background radiationtemperature. We assume a source diameter of 80 (cid:48)(cid:48) (see Fosséet al. 2001). The first term of Eq. (1), which depends only onspectroscopic and observational line parameters, is plotted as afunction of E u / k B for the di ff erent lines detected. Thus, T rot and N can be derived by performing a linear least squares fit to thepoints (see Fig. A.3).Results for T rot and N using the population diagram proce-dure are shown in Table 1 and Fig. A.3. The uncertainties werecalculated using the statistical errors given by the linear leastsquares fit for the slope and the intercept. The individual er-rors of the data points are those derived by taking into accountthe uncertainty obtained in the determination of the observedline parameters (see Table A.1). For HCCCH CN ( a -type tran-sitions) and CH CCHCN, di ff erent hyperfine structure compo-nents of the same ( J K a , K c ) u − ( J K a , K c ) l transition are blended ina single line. Thus, to correctly determine T rot and N , the Ein-stein A -coe ffi cient for spontaneous emission and the statisticalweight were assumed as the weighted average values of the sumof the hyperfine components, and the rotational partition func-tion was calculated using this value for the statistical weight ofeach ( J K a , K c ) u − ( J K a , K c ) l transition. For CH CCCN we built in-dependent rotational diagrams for each symmetry state A and E . Article number, page 2 of 11. Marcelino et al.: C H N isomers in TMC-1
Fig. 1.
Observed lines of HCCCH CN ( a -type) toward TMC-1 (CP). The vertical dashed green line marks a radial velocity of 5.7 km s − . Table 1.
Derived rotational temperatures ( T rot ) and column densities ( N )for the C H N isomers towards TMC-1 (CP).
Species T rot (K) N (cm − )HCCCH CN 4 ± . ± . × CH CCHCN 5 . ± . . ± . × A-CH CCCN 6 . ± . . ± . × E-CH CCCN 8 . ± . . ± . × We obtained rotational temperatures between 4 − . − × cm − .
4. Discussion
The chemistry of C H N isomers in cold molecular clouds hasbeen discussed by Balucani et al. (2000) and more specificallyby Balucani et al. (2002), based on crossed molecular beamexperiments and ab initio calculations. In these studies it waspointed out that reactions of the CN radical with methyl acety-lene and allene are barrierless and exothermic when producingCH C N and CH CCHCN, in the methyl acetylene reaction, andCH CCHCN and HCCCH CN, in the reaction involving allene.Indeed, the reactions of CN with CH CCH and CH CCH weremeasured to be rapid at low temperatures (Carty et al. 2001).This chemical scheme was implemented in a chemical modelby Quan & Herbst (2007) to explain the abundance of cyanoal-lene in TMC-1. Later on, Abeysekera et al. (2015) measuredthe product branching ratios of the reaction between CN andmethyl acetylene at low temperature using a chirped-pulse uni-form flow and found that HC N is the major product, whileCH C N accounts for 22 % of the products and CH CCHCNis not formed. These results are in contrast with those obtainedfrom crossed molecular beam experiments (Huang et al. 1999;Balucani et al. 2000, 2002), where CH CCHCN is observed as
Article number, page 3 of 11 & A proofs: manuscript no. c4h3n_isomers_final
Fig. 2.
Observed lines of HCCCH CN ( b -type) toward TMC-1 (CP).Blue arrows show the position of the strongest three hyperfine com-ponents. Velocity axis refers to the frequency result of collapsing thehyperfine structure. product of the CN + CH CCH reaction. Therefore, the most sta-ble isomer CH C N can be formed in the reaction of CN andmethyl acetylene, the second most stable isomer CH CCHCNcan be formed when CN reacts with CH CCH and perhaps alsowith CH CCH, depending on whether one gives credit to thechirped-pulse uniform flow experiment or to the crossed molec-ular beam ones, and the least stable isomer HCCCH CN canonly be formed in the reaction between CN and allene. Theseneutral-neutral reactions involving CN are therefore likely routes to the three C H N isomers in cold interstellar clouds like TMC-1, where abundant CN, CH CCH, and probably CH CCH (nonpolar and thus it cannot be detected at radio wavelengths) arepresent. Moreover, the presence of HCCCH CN (and perhapsalso CH CCHCN) can be used as proxy of the non polar C H isomer allene since this isomer is only formed from CH CCH in the aforementioned reactions of CN.In the light of the recent discovery of HCCCH CN in TMC-1 and the observational study of the three C H N isomers pre-sented here, we have carried out chemical model calculations toreview the chemistry of these species in cold clouds and evalu-ate whether the mechanism proposed by Balucani et al. (2002)is in agreement with observations. We adopt typical parametersof cold dark clouds, i.e., a gas kinetic temperature of 10 K, avolume density of H nuclei of 2 × cm − , a visual extinctionof 30 mag, a cosmic-ray ionization rate of H of 1 . × − s − ,and the so-called "low-metal" elemental abundances (Agúndez& Wakelam 2013). We use the chemical network RATE12 fromthe UMIST database (McElroy et al. 2013), updated to includethe C H N isomers CH CCHCN and HCCCH CN. The reac-tionsCN + CH CCH → HCN + CH CCH , (2a) → HC N + CH , (2b) → CH C N + H , (2c) → CH CCHCN + H , (2d)CN + CH CCH → CH CCHCN + H , (3a) → HCCCH CN + H , (3b)are included with the rate constants measured by Carty et al.(2001). For the branching ratios of reaction (2) we use either thevalues measured in the chirped-pulse uniform flow experimentby Abeysekera et al. (2015), 12 %, 66 %, 22 %, and 0 % for chan-nels (a), (b), (c), and (d), respectively, or the values suggested bycrossed molecular beam experiments and quantum chemical cal-culations Balucani et al. (2000), 50 % for channels (c) and (d).For reaction (3) we adopt branching ratios of 90 % and 10 % forchannels (a) and (b), respectively, based on quantum chemicalcalculations by Balucani et al. (2002). The destruction processesof CH CCHCN and HCCCH CN are assumed to be the same asthose of CH C N, which are basically reactions with abundantcations.The calculated abundances of the three C H N isomers areshown as a function of time in Fig. A.4. It is seen that the threeisomers reach their maximum abundance at early times, in therange (1 − × yr, with CH C N being the most abun-dant and HCCCH CN being the least abundant. According tothe chemical model, the formation of CH C N occurs throughtwo routes. The first and major involves the dissociative recom-bination of the precursor ion CH C NH + with electrons and isthe responsible of the larger calculated abundance of CH C Ncompared to the two other isomers. A second and minor routeis provided by reaction (2c). Cyanoallene is formed through re-action (3), with reaction (2c) contributing to the same level ifchannel (2d) is assumed to be open. Propargyl cyanide is ex-clusively formed through reaction (3), with a lower abundancebecause it is formed with a branching ratio of just 10 %. Theimpact of using the branching ratios for reaction (2) of Balucaniet al. (2000) or those of Abeysekera et al. (2015) is modest, withthe main e ff ect being a change of less than a factor of two in theabundance of CH CCHCN (see Fig. A.4).
Article number, page 4 of 11. Marcelino et al.: C H N isomers in TMC-1
The fact that the observed abundances of the three isomersare remarkably similar provides clues on the underlying chemi-cal processes at work. For example, the route to CH C N fromthe precursor ion CH C NH + is probably overestimated in thechemical model, as indicated by the too large abundance cal-culated for this species. It has become clear in recent yearsthat dissociative recombination of polyatomic ions usually re-sults in a much larger fragmentation than previously believed(Larsson et al. 2012), meaning that it would not be strange thanCH C N is a minor product in the dissociative recombination ofCH C NH + . The low branching ratio adopted for HCCCH CNformation in reaction (3) based on calculations by Balucani et al.(2002) seems also to be in conflict with the observational find-ing of similar abundances for CH CCHCN and HCCCH CN. Itwould be very interesting to measure the product branching ra-tios for the reaction of CN with allene, as was done for CN + CH CCH (Abeysekera et al. 2015), to shed light on the forma-tion routes of these two metastable C H N isomers. This willalso allow to put tight constraints on the abundance of allene incold dense clouds.In summary, the similar abundances observed for the threeC H N isomers favors a common origin through reactions (2)and (3) with similar branching ratios in the latter reaction. Ifthis scenario is correct, we can conclude that allene is as abun-dant as methyl acetylene in TMC-1. This is in fact predicted bythe chemical model, where CH CCH and CH CCH are mostlyformed during the dissociative recombination of the C H + ion(Larsson et al. 2005), with similar branching ratios assumed forthe two C H isomers.In addition to the three C H N isomers and the wellknown species HC N and CH CHCN, Balucani et al. (2000)predicted the presence of c -C H CN and the C H N isomerCH CC(CN)CH in cold interstellar clouds. It is worth notingthat all these species but CH CC(CN)CH have been identi-fied in TMC-1 (see McGuire et al. 2018 for the detection ofcyanobenzene) and are also present in our survey. Another − CNspecies, cyanocyclopentadiene ( c -C H CN), has been recentlydetected in this source (McCarthy et al. 2020). A complete studyof the molecular rings c -C H CN and c -C H CN in our datawill be published elsewhere. We searched in our data for thetwo C H N isomers CH CH CCCN and CH CHCCHCN byperforming a line stacking analysis (see, e.g., Cuadrado et al.2016; Loomis et al. 2020). We added spectra at the expected fre-quency of several lines from these species that could be presentwithin the noise level. More concreteley, we considered a -typetransitions sharing similar upper level energies, up to 15 K, andEinstein coe ffi cients. All spectra, in local standard of rest (LSR)velocity scale, are resampled to the same velocity channel res-olution before stacking. Figure A.5 shows the spectra obtainedfollowing this method. Whereas there is no evidence for thepresence of CH CH CCCN in our data, the stacked spectrumof CH CHCCHCN shows a 2 σ signal at the systemic velocityof the source. An observational e ff ort at lowest frequencies hasto be undertaken to confirm the presence of CH CHCCHCN inspace.
5. Conclusions
Using a very sensitive line survey of TMC-1 in the Q bandwe have detected multiple transitions of the three C H N iso-mers CH C N, CH CCHCN, and HCCCH CN. The presenceof the latter in TMC-1 is supported by 27 observed individ-ual lines. We have constructed rotational diagrams for the threespecies and obtained similar rotational temperatures and col- umn densities for the three isomers, in the range of 4 − . − × cm − , respectively. The observed abundances ofthe three isomers in TMC-1 suggest a similar chemical originbased on reactions of the radical CN with the isomers CH CCHand CH CCH . There are still uncertainties in the network ofreactions related to these species since our chemical model over-estimates the abundance of CH C N and underestimates the pro-duction of HCCCH CN. Further studies of these isomers in othersources could help in clarifying their chemical formation path-ways.
Acknowledgements.
We acknowledge funding support from the European Re-search Council (ERC Grant 610256: NANOCOSMOS). We also thank the Span-ish MICIU for funding support under grants AYA2016-75066-C2-1-P, PID2019-106110GB-I00, and PID2019-107115GB-C21, and PID2019-106235GB-I00.M.A. thanks MICIU for grant RyC-2014-16277.
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Appendix A: Additional figures and tables
Article number, page 6 of 11. Marcelino et al.: C H N isomers in TMC-1
Table A.1.
Observed lines of C H N isomers towards TMC-1 (CP).
Transition Rest Freq.
Eup A ij S ij (cid:82) T ∗ A d v V LSR ∆ v T ∗ A ( J K a , K c ) u − ( J K a , K c ) l (MHz) (K) (10 − s − ) (K km s − ) (km s − ) (km s − ) (K)HCCCH CN, a -type, µ a = .
87 D6 , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , CN, b -type, µ b = .
19 D3 , − , , F u − F l = − (cid:27) ∗ , − , , F u − F l = − , − , , F u − F l = − , − , , F u − F l = − (cid:27) ∗ , − , , F u − F l = − , − , , F u − F l = − , − , , F u − F l = − , − , , F u − F l = − (cid:27) ∗ , − , , F u − F l = − , − , , F u − F l = − , − , , F u − F l = − (cid:27) ∗ , − , , F u − F l = − , − , , F u − F l = − , − , , F u − F l = − (cid:27) ∗ , − , , F u − F l = − CCHCN, µ a = .
07 D6 , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , , − , CCCN, µ a = .
75 DE 8 − − − − − − − − − − ≤ − − − ≤ − − Notes. ∗ LSR velocity corresponds to the strongest hyperfine transition. Numbers in parentheses indicate the uncertainty in unitsof the last significant digits. For the observational parameters we adopted the uncertainty of the Gaussian fit provided by
GILDAS . HCCCH CN : Spectroscopic line parameters were obtained using MADEX by fitting the rotational lines reported by Demaison et al.(1985) and McNaughton et al. (1988). Dipole moments are from McNaughton et al. (1988). CH CCHCN : Spectroscopic lineparameters were obtained using
MADEX by fitting the rotational lines reported by Bouchy et al. (1973) and Schwahn et al. (1986).Dipole moment is from Bouchy et al. (1973). CH CCCN : Spectroscopic line parameters were obtained using
MADEX by fitting therotational lines reported by Moïses et al. (1982) and Bester et al. (1983). Rotation constants A and D k have been assumed to bethe same as those of CH CN. Some additional data have been taken from the CDMS ( https://cdms.astro.uni-koeln.de/ ).Dipole moment is from Bester et al. (1984). Note that the E species is 7.8 K above the A species, and energies for the E species arereferred to the lowest energy level (1,1).
Article number, page 7 of 11 & A proofs: manuscript no. c4h3n_isomers_final
Fig. A.1.
Observed lines of CH CCHCN toward TMC-1 (CP). The vertical dashed green line marks a radial velocity of 5.7 km s − .Article number, page 8 of 11. Marcelino et al.: C H N isomers in TMC-1
Fig. A.2.
Observed lines from CH CCCN towards TMC-1 (CP). Dashed green line marks a radial velocity of 5.8 km s − .Article number, page 9 of 11 & A proofs: manuscript no. c4h3n_isomers_final
Fig. A.3.
Rotational diagrams of the C H N isomers towards TMC-1 (CP). Derived values of the rotational temperature, T rot , column density, N ,and their respective uncertainties are indicated for each molecule.Article number, page 10 of 11. Marcelino et al.: C H N isomers in TMC-1 time (yr)10 a b un d a n c e r e l a t i v e t o H CH C NCH CCHCNHCCCH CN Fig. A.4.
Calculated fractional abundances of the three C H N isomersas a function of time. Solid and dashed lines correspond to two modelsin which we use branching ratios for the CN + CH CCH reaction fromAbeysekera et al. (2015) and from Balucani et al. (2000), respectively(see text). The abundances observed in TMC-1 for the three C H Nisomers (from Table 1 adopting a H column density of 10 cm − ; Cer-nicharo & Guelin 1987) are shown as horizontal dotted lines. Fig. A.5.
Stacked spectra of CH CH CCCN and CH3