Search

We report the discovery in TMC-1 of two cyano derivatives of the PAH acenaphthylene (C$_{12}$H$_8$). We have found two series of lines with the QUIJOTE line survey that we assign to 1-C$_{12}$H$_7$CN and 5-C$_{12}$H$_7$CN. For the 1-isomer, we have detected and assigned 173 rotational transitions with $J$ up to 46 and $K_a$ up to 9, corresponding to 107 independent frequencies. For the 5-isomer, the identification is based on 56 individual lines, corresponding to 117 rotational transitions with $J$ up to 40 and $K_a$ up to 8. Identification of the carriers was achieved through a careful analysis of the derived rotational constants, which permit us to focus on molecules larger than naphthalene but smaller than anthracene and phenanthrene. Moreover, the derived rotational constants indicate that the species are planar; this allows us to discard derivatives of fluorene and acenaphthene, which are non-planar species. Quantum chemical calculations and subsequent chemical synthesis of these molecules, as well as the observation of their rotational transitions in the laboratory, unequivocally support our identifications. We also confirm, via a robust line-by-line detection, the previous claimed detection of 1- and 2- cyanonaphthalene, which were obtained through statistical stacking techniques. The column densities of 1- and 5-cyanoacenaphthylene are (9.5\,$\pm$\,0.9)$\,\times$\,10$^{11}$ cm$^{-2}$, while those of 1- and 2-cyanonapthalene are (5.5\,$\pm$\,0.5)$\,\times$\,10$^{11}$ cm$^{-2}$. Hence, it seems that acenaphthylene could be a factor of 1.7 more abundant than naphthalene. These results support a scenario in which PAHs grow in cold dark clouds based on fused five- and six-membered carbon rings., Comment: Accepted as a Letter to the Editor in Astronomy and Astrophysics on 18th September 2024

While the nitrile group is by far the most prevalent one among interstellar molecules, the existence of interstellar dinitriles (molecules containing two -CN groups) has recently been proven. Here we report the discovery of two new dinitriles in the cold dense cloud TMC-1. These newly identified species are malononitrile, CH2(CN)2, and maleonitrile, the Z isomer of NC-CH=CH-CN, which can be seen as the result of substituting two H atoms with two -CN groups in methane and ethylene, respectively. These two molecules were detected using data from the ongoing QUIJOTE line survey of TMC-1 that is being carried out with the Yebes 40m telescope. We derive column densities of 1.8e11 cm-2 and 5.1e10 cm-2 for malononitrile and maleonitrile, respectively. This means that they are eight and three times less abundant than HCC-CH2-CN and (E)-HCC-CH=CH-CN, respectively, which are analog molecules detected in TMC-1 in which one -CN group is converted into a -CCH group. This is in line with previous findings in which -CCH derivatives are more abundant than the -CN counterparts in TMC-1. We examined the potential chemical pathways to these two dinitriles, and we find that while maleonitrile can be efficiently formed through the reaction of CN with CH2CHCN, the formation of malononitrile is not clear because the neutral-neutral reactions that could potentially form it are not feasible under the physical conditions of TMC-1., Comment: Accepted for publication in A&A Letters

We present the detection of the free radicals NC$_3$S and HC$_3$S towards TMC-1 with the QUIJOTE line survey. The derived column densities are (1.4$\pm$0.2)$\times$10$^{11}$ for NC$_3$S and (1.5$\pm$0.2)$\times$10$^{11}$ for HC$_3$S. We searched for NCCS, but only three transitions are within the domain of our QUIJOTE line survey and the observed lines are marginally detected at the 3$\sigma$ level, providing an upper limit to its column density of $\leq$6$\times$10$^{10}$ cm$^{-2}$. We also unsuccessfully searched for longer species of the NC$_n$S (n$\ge$4) and HC$_n$S (n$\ge$5) families in our TMC-1 data. A chemical model based on a reduced set of reactions involving HC$_3$S and NC$_3$S predicts abundances that are 10-100 times below the observed values. These calculations indicate that the most efficient reactions of formation of HC$_3$S and NC$_3$S in the model are S + C$_3$H$_2$ and N + HC$_3$S, respectively, while both radicals are very efficiently destroyed through reactions with neutral atoms., Comment: Article proposed for acceptance in Astronomy and Astrophysics on 5th July 2024

We report the first detection of the metal-bearing molecules sodium sulfide (NaS) and magnesium sulfide (MgS) and the tentative detection of calcium monoxide (CaO) in the interstellar medium (ISM) towards the Galactic Center molecular cloud G+0.693-0.027. The derived column densities are (5.0+-1.1) x 10$^{10}$ cm$^{-2}$, (6.0+-0.6) x $^{10}$ cm$^{-2}$, and (2.0+-0.5) x $^{10}$ cm$^{-2}$, respectively. This translates into fractional abundances with respect to H$_2$ of (3.7+-1.0) x $10^{-13}$, (4.4+-0.8) x $10^{-13}$, and (1.5+-0.4) x $10^{-13}$, respectively. We have also searched for other Na-, Mg- and Ca-bearing species towards this source but none of them have been detected and thus we provide upper limits for their abundances. We discuss the possible chemical routes involved in the formation of these molecules containing metals under interstellar conditions. Finally, we compare the ratio between sulfur-bearing and oxygen-bearing molecules with and without metals, finding that metal-bearing sulfur molecules are much more abundant than metal-bearing oxygen ones, in contrast with the general trend found in the ratios between other non metal- oxygen- and sulfur-bearing molecules. This further strengthen the idea that sulfur may be little depleted in G+0.693-0.027 as a result of the low velocity shocks present in this source sputtering large amounts of material from dust grains., Comment: 18 pages, 7 tables and 7 figures, final version accepted by ApJ

We present the first detection in space of 1,4-pentadiyne. It has been found towards TMC-1 with the QUIJOTE line survey in the 31-50 GHz range. We observed a total of 17 transitions with J = 2 up to 13 and Ka = 0, 1 and 2. The observed transitions allowed us to derive a rotational temperature of 9.5 +- 0.5 K and a column density of (5.0 +- 0.5) x 10^12 cm-2. This molecule was the last non-cyclic isomer of the C5H4 family that could be detected via radio astronomy. A computational chemistry study was performed to determine the energies of the five most stable isomers. The isomer (c-C3H3CCH) has a considerably higher energy than the others, and it has not yet been detected. To better understand the chemical reactions involving these species, we compared the ethynyl and cyano derivatives. The observed abundances of these species are in good agreement with the branching ratios of the formation reactions studied with our chemical model of TMC-1.

We report the first identification in space of HC$_3$N$^+$, the simplest member of the family of cyanopolyyne cations. Three rotational transitions with half-integer quantum numbers from $J$=7/2 to 11/2 have been observed with the Yebes 40m radio telescope and assigned to HC$_3$N$^+$, which has an inverted $^2\Pi$ ground electronic state. The three rotational transitions exhibit several hyperfine components due to the magnetic and nuclear quadrupole coupling effects of the H and N nuclei. We confidently assign the characteristic rotational spectrum pattern to HC$_3$N$^+$ based on the good agreement between the astronomical and theoretical spectroscopic parameters. We derived a column density of (6.0$\pm$0.6)$\times$10$^{10}$ cm$^{-2}$ and a rotational temperature of 4.5$\pm$1\,K. The abundance ratio between HC$_3$N and HC$_3$N$^+$ is 3200$\pm$320. As found for the larger members of the family of cyanopolyyne cations (HC$_5$N$^+$ and HC$_7$N$^+$), HC$_3$N$^+$ is mainly formed through the reactions of H$_2$ and the cation C$_3$N$^+$ and by the reactions of H$^+$ with HC$_3$N. In the same manner than other cyanopolyyne cations, HC$_3$N$^+$ is mostly destroyed through a reaction with H$_2$ and a dissociative recombination with electrons., Comment: Accepted as a Letter to the Editor in Astronomy & Astrophysics on June 28th 2024

We present the discovery with the QUIJOTE line survey of the cations HC5N+ and HC7N+ in the direction of TMC-1. Seven lines with half-integer quantum numbers from J=25/2-23/2 to 37/2-35/2 have been assigned to HC5N+ and eight lines from J=55/2-53/2 to 71/2-69/2 to HC7N+. Both species have inverted 2Pi ground electronic states with very good estimates for their B0 and AS0 constants based on optical observations. The lines with the lowest J of HC5N+ exhibit multiple components due to the hyperfine structure introduced by the H and N nuclei. However, these different components collapse for the higher J. No hyperfine structure is found for any of the lines of HC7N+. The derived effective rotational and distortion constants for HC5N+ are Beff = 1336.662+/- 0.001 MHz and Deff = 27.4+/-2.6 Hz, while for HC7N+ they are Beff = 567.85036+/-0.00037 MHz and Deff = 4.01+/-0.19 Hz. From the observed intensities, we derived Trot = 5.5+/-0.5K and N = (9.9+/-1.0)x 1010 cm-2 for HC5N+, while we obtained Trot = 8.5+/-0.5K and N = (2.3+/-0.2)x1010 cm-2 for HC7N+. The HC5N/HC5N+, C5N/HC5N+, C5N-/HC5N+, HC7N/HC7N+, HC5N+/HC7N+, and C7N-/HC7N+ abundance ratios are 670+/-80, 4.8+/-0.8, 1.2+/-0.2, 1000+/-150, 4.2+/-0.5, and 2.2+/-0.2, respectively. We have run chemical modelling calculations to investigate the formation and destruction of these new cations. We find that these species are mainly formed through the reactions of H2 and the cations C5N+ and C7N+, and by the reactions of H+ with HC5N and HC7N, while they are mostly destroyed through a reaction with H2 and a dissociative recombination with electrons. Based on the underestimation of the abundances of HC5N+ and HC7N+ by the chemical model by a factor around 20, we suggest that the rate coefficients currently assumed for the reactions of these cations with H2 could be too high by the same factor, something that will be worth investigating., Comment: Accepted as a Letter in Astronomy & Astrophysics on May 31st 2024

We present the detection of cyanothioketene, NCCHCS, in the laboratory and toward TMC-1. This transient species was produced through a discharge of a gas mixture of CH2CHCN and CS2 using argon as carrier gas, and its rotational spectrum between 9 and 40 GHz was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer. A total of 21 rotational transitions were detected in the laboratory, all of them exhibiting hyperfine structure induced by the spin of the N nucleus. The spectrum for NCCHCS was predicted in the domain of our line surveys using the derived rotational and distortion constants. The detection in the cold starless core TMC-1 was based on the QUIJOTE line survey performed with the Yebes 40m radio telescope. Twenty-three lines were detected with K_a=0, 1, and 2 and J_u=9 up to 14. The derived column density is (1.2+/-0.1)e11 cm-2 for a rotational temperature of 8.5+/-1 K. The abundance ratio of thioketene and its cyano derivative, H2CCS/NCCHCS, is 6.5+/-1.3. Although ketene is more abundant than thioketene by about 15 times, its cyano derivative NCCHCO surprisingly is not detected with a 3sigma upper level to the column density of 3.0e10 cm-2, which results in an abundance ratio H2CCO/NCCHCO > 430. Hence, the chemistry of CN derivatives seems to be more favored for S-bearing than for O-bearing molecules. We carried out chemical modeling calculations and found that the gas-phase neutral-neutral reactions CCN + H2CS and CN + H2CCS could be a source of NCCHCS in TMC-1., Comment: Accepted as a Letter in Astronomy & Astrophysics on May 2nd 2024. arXiv admin note: text overlap with arXiv:2311.13860, arXiv:2404.07585

We present a study of CH$_3$CH$_2$CCH, CH$_3$CH$_2$CN, CH$_2$CHCCH, and CH$_2$CHCN in TMC-1 using the QUIJOTE$^1$ line survey. We confirm the presence of CH$_3$CH$_2$CCH in TMC-1, which was previously reported as tentative by our group. From a detailed study of the ethynyl and cyanide derivatives of CH$_2$CH$_2$ and CH$_3$CH$_3$ in TMC-1, we found that the CH$_2$CHCCH/CH$_2$CHCN and CH$_3$CH$_2$CCH/CH$_3$CH$_2$CN abundance ratios are 1.5$\pm$0.1 and 4.8$\pm$0.5, respectively. The derived CH$_2$CHCCH/CH$_3$CH$_2$CCH abundance ratio is 15.3$\pm$0.8, and that of CH$_2$CHCN over CH$_3$CH$_2$CN is 48$\pm$5. All the single substituted isotopologs of vinyl cyanide have been detected, and we found that the first and second carbon substitutions in CH$_2$CHCN provide a $^{12}$C/$^{13}$C ratio in line with that found for other three-carbon bearing species such as HCCNC and HNCCC. However, the third $^{13}$C isotopolog, CH$_2$CH$^{13}$CN, presents an increase in its abundance similar to that found for HCCCN. Finally, we observed eight $b$-type transitions of CH$_2$CHCN, and we find that their intensity cannot be fitted adopting the dipole moment $\mu_b$ derived previously. These transitions involve the same rotational levels as those of the $a$-type transitions. From their intensity, we obtain $\mu_b$=0.80$\pm$0.03\,D, which is found to be in between earlier values derived in the laboratory using intensity measurements or the Stark effect. Our chemical model indicates that the abundances of CH$_3$CH$_2$CCH, CH$_3$CH$_2$CN, CH$_2$CHCCH, and CH$_2$CHCN observed in TMC-1 can be explained in terms of gas-phase reactions., Comment: Accepted as a Letter in Astronomy & Astrophysics on March 8th 2024

We report the first detection in space of a complete sample of nine doubly substituted isotopologues of HCCCN towards the cyanopolyyne peak of TMC-1 using observations of the QUIJOTE line survey taken with the Yebes 40 m telescope. We detected D13CCCN, DC13CCN, DCC13CN, DCCC15N, H13C13CCN, H13CC13CN, HC13C13CN, HCC13C15N, and HC13CC15N through their J=4-3 and J=5-4 lines in the 7 mm window. In addition, we present an extensive analysis of the emission of HCCCN and its singly substituted isotopologues through a large velocity gradient model of the lines detected at 7 mm and 3 mm using the Yebes 40 m and the IRAM 30 m telescopes, respectively. The derived column densities for all the isotopologues are consistent in the two spectral bands for an H2 volume density of 1e4 cm-3 and a kinetic temperature of 10 K. Whereas we observed a 13C fractionation for HCC13CN and other double isotopologues with a 13C atom adjacent to the nitrogen atom, we derived similar C/13C abundance ratios for the three 13C substituted species of DCCCN. This suggests additional chemical discrimination for deuterated isotopologues of HCCCN. Finally, we present the spatial distribution of the J=4-3 and J=5-4 lines from the singly substituted species observed with the Yebes 40 m telescope. The emission peak of the spatial distribution of DCCCN appears to be displaced by 40'' with respect to that of HCCCN and the 13C and 15N isotopologues. In addition to a different formation route for the deuterated species, we could also expect that this differentiation owing to the deuterium fractionation is more efficient at low temperatures, and therefore, that deuterated species trace a colder region of the cloud., Comment: Accepted for publication in A&A

In this work, we present the detection of twelve doublets with quantum numbers of N=12-11 to N=17-16 of the v11 vibrationally excited state of C6H towards TMC-1. This marks the first time that an excited vibrational state of a molecule has been detected in a cold starless core. The data are part of the QUIJOTE line survey gathered with the Yebes 40m radio telescope. The line intensities have been aptly reproduced with a rotational temperature of 6.2 +/- 0.4K and a column density of (1.2+/-0.2)e11 cm-2. We also analysed the ground state transitions of C6H, detecting fourteen lines with quantum numbers of J = 23/2-21/2 to J = 35/2 for each of the two 2Pi_3/2 and 2Pi_1/2 ladders. It is not possible to model the intensities of all the transitions of the ground state simultaneously using a single column density. We considered the two ladders as two different species and found that the rotational temperature is the same for both ladders, Trot(2Pi_3/2)=Trot(2Pi_1/2)=6.2+/-0.2, achieving a result that is comparable to that of the v11 state. The derived column densities are N(2Pi_3/2)(6.2+/-0.3)e12cm-2 and N(2Pi_1/2)=(8.0+/-0.4)e10cm-2. The fraction of C6H molecules in its 2Pi_3/2, 2Pi_1/2, and v11 states is 96.8 %, 1.3 %, and 1.9 %, respectively. Finally, we report that this vibrational mode has also been detected towards the cold cores Lupus-1A and L1495B, as well as the low-mass star forming cores L1527 and L483, with fractions of C6H molecules in this mode of 3.8%, 4.1%, 14.8%, and 6%, respectively., Comment: Accepted as a Letter in Astronomy & Astrophysics on 17/11/2023

We report the detection of large hydrocarbon cycles toward several cold dense clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q band (31-50 GHz) using the Yebes 40m radiotelescope. Using the line stack technique, we find statistically significant evidence of benzonitrile (C$_6$H$_5$CN) in L1495B, Lupus-1A, and L483 at levels of 31.8$\,\sigma$, 15.0$\,\sigma$, and 17.2$\,\sigma$, respectively, while there is no hint of C$_6$H$_5$CN in the fourth source, L1527. The column densities derived are in the range (1.7-3.8)$\,\times\,10^{11}$ cm$^{-2}$, which is somewhat below the value derived toward the cold dense cloud TMC-1. When we simultaneously analyze all the benzonitrile abundances derived toward cold clouds in this study and in the literature, a clear trend emerges in that the higher the abundance of HC$_7$N, the more abundant C$_6$H$_5$CN is. This indicates that aromatic cycles are especially favored in those interstellar clouds where long carbon chains are abundant, which suggests that the chemical processes that are responsible for the formation of linear carbon chains are also behind the synthesis of aromatic rings. We also searched for cycles other than benzonitrile, and found evidence of indene (C$_9$H$_8$), cyclopentadiene (C$_5$H$_6$), and 1-cyano cyclopentadiene (1-C$_5$H$_5$CN) at levels of 9.3$\,\sigma$, 7.5$\,\sigma$, and 8.4$\,\sigma$, respectively, toward L1495B, which shows the strongest signal from C$_6$H$_5$CN. The relative abundances between the various cycles detected in L1495B are consistent -- within a factor of three -- with those previously found in TMC-1. It is therefore likely that not only C$_6$H$_5$CN but also other large aromatic cycles are abundant in clouds rich in carbon chains., Comment: Accepted for publication in A&A Letters. Changes with respect to previous version: language edited, error in abstract corrected, and title changed

We report the first detection of SiC$_2$ in the interstellar medium. The molecule was identified through six rotational transitions toward G\,+0.693$-$0.027, a molecular cloud located in the Galactic center. The detection is based on a line survey carried out with the GBT, the Yebes 40m, and the IRAM 30m telescopes covering a range of frequencies from 12 to 276 GHz. We fit the observed spectra assuming local thermodynamic equilibrium and derive a column density of ($1.02\pm0.04)\times10^{13}$ cm$^{-2}$, which gives a fractional abundance of $7.5\times10^{-11}$ with respect to H$_2$, and an excitation temperature of $5.9\pm0.2$ K. We conclude that SiC$_2$ can be formed in the shocked gas by a reaction between the sputtered atomic silicon and C$_2$H$_2$, or it can be released directly from the dust grains due to disruption. We also search for other Si-bearing molecules and detect eight rotational transitions of SiS and four transitions of Si$^{18}$O. The derived fractional abundances are $3.9\times10^{-10}$ and $2.1\times10^{-11}$, respectively. All Si-bearing species toward G\,+0.693$-$0.027 show fractional abundances well below what is typically found in late-type evolved stars., Comment: 8 pages, 5 figures

We report the first laboratory and interstellar detection of the alpha-cyano vinyl radical (H2CCCN). This species was produced in the laboratory by an electric discharge of a gas mixture of vinyl cyanide, CH2CHCN, and Ne, and its rotational spectrum was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer operating in the frequency region of 8-40 GHz. The observed spectrum shows a complex structure due to tunneling splittings between two torsional sublevels of the ground vibronic state, 0+ and 0-, derived from a large-amplitude inversion motion. In addition, the presence of two equivalent hydrogen nuclei makes necessary to discern between ortho- and para-H2CCCN. A least squares analysis reproduces the observed transition frequencies with a standard deviation of ca. 3 kHz. Using the laboratory predictions, this radical is detected in the cold dark cloud TMC-1 using the Yebes 40m telescope and the QUIJOTE line survey. The 404-303 and 505-404 rotational transitions, composed of several hyperfine components, were observed in the 31.0-50.4 GHz range. Adopting a rotational temperature of 6K we derive a column density of (1.4+/-0.2)e11 cm-2 and (1.1+/-0.2)e11 cm-2 for ortho-H2CCCN and para-H2CCCN, respectively. The reactions C + CH3CN, and perhaps also N + CH2CCH, emerge as the most likely routes to H2CCCN in TMC-1., Comment: Accepted for publication in A&A Letters on 18/07/2023

We report new observations of molecular anions with the Yebes 40m and IRAM 30m telescopes toward the cold dense clouds TMC-1 CP, Lupus-1A, L1527, L483, L1495B, and L1544. We detected for the first time C3N- and C5N- in Lupus-1A and C4H- and C6H- in L483. In addition, we report new lines of C6H- toward the six targeted sources, of C4H- toward TMC-1 CP, Lupus-1A, and L1527, and of C8H- and C3N- in TMC-1 CP. Excitation calculations indicate that the lines of anions accessible to radiotelescopes run from subthermally excited to thermalized as the size of the anion increases, with the degree of departure from thermalization depending on the H2 volume density and the line frequency. We noticed that the collision rate coefficients available for the radical C6H cannot explain various observational facts, which advises for a revisitation of the collision data for this species. The observations presented here, together with observational data from the literature, are used to model the excitation of interstellar anions and to constrain their abundances. In general, the anion-to-neutral ratios derived here agree within 50 % (a factor of two at most) with literature values, when available, except for the C4H-/C4H ratio, which shows higher differences due to a revision of the dipole moment of C4H. From the set of anion-to-neutral abundance ratios derived two conclusions can be drawn. First, the C6H-/C6H ratio shows a tentative trend in which it increases with increasing H2 density, as expected from theoretical grounds. And second, it is incontestable that the higher the molecular size the higher the anion-to-neutral ratio, which supports a formation mechanism based on radiative electron attachment. Nonetheless, calculated rate coefficients for electron attachment to the medium size species C4H and C3N are probably too high and too low, respectively, by more than one order of magnitude., Comment: Accepted for publication in A&A

Based on a novel laboratory method, 14 mm-wave lines of the molecular ion H$_2$CCCH$^+$ have been measured in high resolution, and the spectroscopic constants of this asymmetric rotor determined with high accuracy. Using the Yebes 40 m and IRAM 30 m radio telescopes, we detect four lines of H$_2$CCCH$^+$ towards the cold dense core TMC-1. With a dipole moment of about 0.55 Debye obtained from high-level ab initio calculations, we derive a column density of 5.4$\pm$1$\times$10$^{11}$ cm$^{-2}$ and 1.6$\pm$0.5$\times$10$^{11}$ cm$^{-2}$ for the ortho and para species, respectively, and an abundance ratio N(H$_2$CCC)/N(H$_2$CCCH$^+$)= 2.8$\pm$0.7. The chemistry of H$_2$CCCH$^+$ is modelled using the most recent chemical network for the reactions involving the formation of H$_2$CCCH$^+$. We find a reasonable agreement between model predictions and observations, and new insights into the chemistry of C$_3$ bearing species in TMC-1 are obtained.

Interstellar amides have attracted significant attentions as they are potential precursors for a wide variety of organics essential to life. However, our current understanding of their formation in space is heavily based on observations in star-forming regions and hence the chemical networks lack the constraints on their early origin. In this work, unbiased sensitive spectral surveys with IRAM 30m and Yebes 40m telescopes are used to systematically study a number of amides towards a quiescent Galactic Centre molecular cloud, G+0.693-0.027. We report the first detection of acetamide (CH3C(O)NH2) and trans-N-methylformamide (CH3NHCHO) towards this cloud. In addition, with the wider frequency coverage of the survey, we revisited the detection of formamide (NH2CHO) and urea (carbamide; NH2C(O)NH2), which had been reported previously towards G+0.693-0.027. Our results are compared with those present in the literature including recent laboratory experiments and chemical models. We find constant abundance ratios independently of the evolutionary stages, suggesting that amides related chemistry is triggered in early evolutionary stages of molecular cloud and remain unaffected by the warm-up phase during the star formation process. Although a correlation between more complex amides and NH2CHO have been suggested, alternative formation routes involving other precursors such as acetaldehyde (CH3CHO), methyl isocyanate (CH3NCO) and methylamine (CH3NH2) may also contribute to the production of amides. Observations of amides together with these species towards a larger sample of sources can help to constrain the amide chemistry in the interstellar medium., Comment: 16 pages, 8 figures, 3 tables, accepted for publication in Monthly Notices of the Royal Astronomical Society

We present a highly sensitive 2D line survey of TMC-1 obtained with the Yebes 40m radio telescope in the Q-band (31.13-49.53 GHz). These maps cover a region of 320 arcsec x 320 arcsec centred on the position of the QUIJOTE line survey with a spatial sampling of 20 arcsec. The region covering 240 arcsec x 240 arcsec, where a longer integration time was used, shows a homogenous sensitivity of 2-4 mK across the band. We present in this work the first determination of the spatial extent of benzonitrile (C6H5CN), which follows that of cyanopolyynes rather well, but differs significantly from that of the radicals CnH and CnN. We definitively conclude that aromatic species in TMC-1 are formed from chemical reactions involving smaller species in the densest zones of the cloud., Comment: Accepted for publication in A&A Letters

We found four series of harmonically related lines in IRC\,+10216 with the Yebes\,40m and IRAM\,30m telescopes. The first series corresponds to a molecule with a rotational constant, $B$, of 1448.5994$\pm$0.0013 MHz and a distortion constant, $D$, of 63.45$\pm$1.15 Hz and covers upper quantum numbers from $J_u$=11 up to 33 (B1449). The second series is fitted with $B$=1446.9380$\pm$0.0098 MHz and $D$=91$\pm$23 Hz and covers upper quantum numbers from $J_u$=11 up to 17 (B1447). The third series is fitted with $B$=598.7495$\pm$0.0011 MHz and D=6.13$\pm$0.43 Hz and covers quantum numbers from $J_u$=26 up to 41 (B599). Finally, the frequencies of the last series of lines can be reproduced with $B$=594.3176$\pm$0.0026 MHz and $D$=4.92$\pm$1.16 Hz (B594). The large values of $D$ point toward four metal-bearing carriers. After exploring all plausible candidates containing Na, Al, Mg, and other metals, our ab initio calculations indicate that the cations MgC$_4$H$^+$, MgC$_3$N$^+$, MgC$_6$H$^+$, and MgC$_5$N$^+$ must be the carriers of B1449, B1447, B599, and B594, respectively. These cations could be formed by the radiative association of Mg$^+$ with C$_4$H, C$_3$N, C$_6$H, and C$_5$N, respectively. We calculated the radiative association rate coefficient of Mg$^+$ with C$_4$H, C$_3$N, C$_6$H, and C$_5$N and incorporated them in our chemical model. The results confirm that the Mg-bearing cations can be formed through these radiative association reactions in the outer layers of IRC\,+10216. This is the first time that cationic metal-bearing species have been found in space. These results provide a new paradigm on the reactivity of ionized metals with abundant radicals and open the door for further characterization of similar species in metal-rich astrophysical environments.

We report on the detection of a series of six lines in the ultra-deep Q-band integration toward IRC+10216 carried out with the Yebes 40m telescope, which are in harmonic relation with integer quantum numbers J from 12 to 18. After a detailed analysis of all possible carriers, guided by high-level quantum chemical calculations, we conclude that the lines belong to HMgCCCN, named hydromagnesium cyanoacetylide. The rotational temperature and column density derived for HMgCCCN are 17.1 +/- 2.8K and (3.0 +/- 0.6) e12 cm-2, respectively. The observed abundance ratio between MgCCCN and HMgCCCN is 3. In addition, we report the discovery in space, also toward IRC+10216, of sodium cyanoacetylide, NaCCCN, for which accurate laboratory data are available. For this species we derive a rotational temperature of 13.5 +/- 1.7K and a column density of (1.2 +/- 0.2) e11 cm-2., Comment: Accepted in Astronomy & Astrophysics as Letter to the Editor on April 3rd, 2023

We present the detection of ethanol (C2H5OH), acetone (CH3COCH3), and propanal (C2H5CHO) toward the cyanopolyyne peak of TMC-1. These three O-bearing complex organic molecules are known to be present in warm interstellar clouds, but had never been observed in a starless core. The addition of these three new pieces to the puzzle of complex organic molecules in cold interstellar clouds stresses the rich chemical diversity of cold dense cores in stages prior to the onset of star formation. The detections of ethanol, acetone, and propanal were made in the framework of QUIJOTE, a deep line survey of TMC-1 in the Q band that is being carried out with the Yebes 40m telescope. We derive column densities of (1.1 +/- 0.3)e12 cm-2 for C2H5OH, (1.4 +/- 0.6)e11 cm-2 for CH3COCH3, and (1.9 +/- 0.7)e11 cm-2 for C2H5CHO. The formation of these three O-bearing complex organic molecules is investigated with the aid of a detailed chemical model which includes gas and ice chemistry. The calculated abundances at a time around 2e5 yr are in reasonable agreement with the values derived from the observations. The formation mechanisms of these molecules in our chemical model are as follows. Ethanol is formed on grains by addition of atomic carbon on methanol followed by hydrogenation and non-thermal desorption. Acetone and propanal are produced by the gas-phase reaction between atomic oxygen and two different isomers of the C3H7 radical, where the latter follows from the hydrogenation of C3 on grains followed by non-thermal desorption. A gas-phase route involving the formation of (CH3)2COH+ through several ion-neutral reactions followed by its dissociative recombination with electrons do also contribute to the formation of acetone., Comment: Accepted for publication in A&A

We carried out an observational search for the recently discovered molecule H2NC, and its more stable isomer H2CN, toward eight cold dense clouds (L1544, L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH3, L1498, and L1641N) and two diffuse clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in different types of interstellar regions and shed light on its formation mechanism. We detected H2NC in most of the cold dense clouds targeted, 7 out of 8, while H2CN was only detected in 5 out of 8 clouds. The column densities derived for both H2NC and H2CN are in the range 1e11-1e12 cm-2 and the abundance ratio H2NC/H2CN varies between 0.51 and >2.7. The metastable isomer H2NC is therefore widespread in cold dense clouds where it is present with an abundance similar to that of H2CN. We did not detect either H2NC or H2CN in any of the two diffuse clouds targeted, which does not allow to shed light on how the chemistry of H2NC and H2CN varies between dense and diffuse clouds. We found that the column density of H2NC is correlated with that of NH3, which strongly suggests that these two molecules are chemically linked, most likely ammonia being a precursor of H2NC through the C + NH3 reaction. We performed electronic structure and statistical calculations which show that both H2CN and H2NC can be formed in the C + NH3 reaction through two different channels involving two different transition states which lie very close in energy. The predicted product branching ratio H2NC/H2CN is very method dependent but values between 0.5 and 0.8 are the most likely ones. Therefore, both the astronomical observations and the theoretical calculations support that the reaction C + NH3 is the main source of H2NC in interstellar clouds., Comment: Accepted for publication in A&A

We report the detection of methyl ketene towards TMC-1 with the QUIJOTE line survey. Nineteen rotational transitions with rotational quantum numbers ranging from J = 3 up to J = 5 and Ka =< 2 were identified in the frequency range 32.0-50.4 GHz, 11 of which arise above the 3{\sigma} level. We derived a column density for CH3CHCO of N=1.5x10^11 cm-2 and a rotational temperature of 9 K. Hence, the abundance ratio between ketene and methyl ketene, CH2CO/CH3CHCO, is 93. This species is the second C3H4O isomer detected. The other, trans-propenal (CH2CHCHO), corresponds to the most stable isomer and has a column density of N=(2.2+-0.3)x10^11 cm-2, which results in an abundance ratio CH2CHCHO/CH3CHCO of 1.5. The next non-detected isomer with the lowest energy is cis-propenal, which is therefore a good candidate for future discovery. We have carried out an in-depth study of the possible gas-phase chemical reactions involving methyl ketene to explain the abundance detected, achieving good agreement between chemical models and observations.

We report on the discovery of the C7N- anion towards the starless core TMC-1 and towards the carbon-rich evolved star IRC+10216. We used the data of the QUIJOTE line survey towards TMC-1 and found six lines in perfect harmonic frequency relation from J=27-26 up to J=32-31. The frequency of the lines can be reproduced with a rotational constant and a distortion constant of B=582.68490+/-0.00024 MHz and D=4.01+/-0.13 Hz, respectively. The standard deviation of the fit is 4 kHz. Towards IRC+10216, we identify 17 lines from J=27-26 up to J=43-42; their frequencies are also in harmonic relation, providing B=582.6827+/-0.00085 MHz and D=3.31+/-0.31 Hz. The nearly exact coincidence of the rotational and distortion constants in both sources points unambiguously to a common molecular carrier. Taking into account the chemical peculiarities of both sources, the carrier could be a radical or an anion. The radical can be discarded, as the observed lines belong to a singlet species. Hence, the most plausible carrier is an anion. High-level ab initio calculations indicate that C7N-, for which we compute a rotational constant of B=582.0 MHz and a dipole moment of 7.5 D, is the carrier of the lines in both sources. We predict the neutral C7N to have a ground electronic state 2Pi and a dipole moment around 1 D. Because of this low dipole moment value and to its much larger rotational partition function, its lines are expected to be well below the sensitivity of our data for both sources., Comment: Accepted as a Letter to the Editor in Astronomy & Astrophysics. Submitted on 29/12/2022 and accepted on 30/1/2023

The previous detection of two species related to the non polar molecule cyanogen (NCCN), its protonated form (NCCNH+) and one metastable isomer (CNCN), in cold dense clouds supported the hypothesis that dicyanopolyynes are abundant in space. Here we report the first identification in space of NC4NH+, which is the protonated form of NC4N, the second member of the series of dicyanopolyynes after NCCN. The detection was based on the observation of six harmonically related lines within the Yebes 40m line survey of TMC-1 QUIJOTE. The six lines can be fitted to a rotational constant B = 1293.90840 +/- 0.00060 MHz and a centrifugal distortion constant D = 28.59 +/- 1.21 Hz. We confidently assign this series of lines to NC4NH+ based on high-level ab initio calculations, which supports the previous identification of HC5NH+ by Marcelino et al. (2020) from the observation of a series of lines with a rotational constant 2 MHz lower than that derived here. The column density of NC4NH+ in TMC-1 is (1.1 +1.4 -0.6)e10 cm-2, which implies that NC4NH+ is eight times less abundant than NCCNH+. The species CNCN, previously reported toward L483 and tentatively in TMC-1, is confirmed in this latter source. We estimate that NCCN and NC4N are present in TMC-1 with abundances a few times to one order of magnitude lower than HC3N and HC5N, respectively. This means that dicyanopolyynes NC-(CC)n-CN are present at a lower level than the corresponding monocyanopolyynes HCC-(CC)n-CN. The reactions of the radicals CN and C3N with HNC arise as the most likely formation pathways to NCCN and NC4N in cold dense clouds., Comment: Accepted for publication in A&A Letters

We report the detection of magnesium dicarbide, MgC$_2$, in the laboratory at centimeter wavelengths and assign $^{24}$MgC$_2$, $^{25}$MgC$_2$, and $^{26}$MgC$_2$ to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC$_2$ is found to be T-shaped with a highly ionic bond between the metal atom and the C$_2$ unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC$_2$ emission lines observed in IRC+10216 yields a very low rotational temperature of $6\pm1$ K, a kinetic temperature of $22\pm13$ K, and a column density of $(1.0 \pm 0.3) \times 10^{12}$ cm$^{-2}$. The abundance of MgC$_2$ relative to the magnesium-carbon chains MgCCH, MgC$_4$H, and MgC$_6$H is $1{:}2{:}22{:}20$ and provides a new constraint on the sequential radiative association-dissociative recombination mechanisms implicated in the production of metal-bearing molecules in circumstellar environments., Comment: 15 pages, 5 figures

We report on the discovery of two new molecules, HCCCHCCC and HCCCCS, towards the starless core TMC-1 in the Taurus region from the QUIJOTE line survey in the 31.1-50.2 GHz frequency range. We identify a total of twenty-nine lines of HCCCHCCC and six rotational transitions of HCCCCS. The rotational quantum numbers range from Ju=10 up to 15 and Ka <=2 for HCCCHCCC and Ju=21/2 up to 31/2 for HCCCCS. We derived a column density for HCCCHCCC of N=(1.3+/-0.2)x10^11 cm-2 with a rotational temperature of 6+/-1 K, while for HCCCCS we derived N=(9.5+/-0.8)x10^10 cm-2 and Trot =10+/-1 K. The abundance of HCCCHCCC is higher than that of its recently discovered isomer, l-H2C6. If we compare HCCCCS with its related molecules, HCS and HCCS, we obtain abundance ratios HCS/HCCCCS=58 and HCCS/HCCCCS=7.2. We investigated the formation of these two molecules using chemical modelling calculations. The observed abundances can be accounted for by assuming standard gas-phase formation routes involving neutral-neutral reactions and ion-neutral reactions., Comment: arXiv admin note: text overlap with arXiv:2206.08992

Using the QUIJOTE line survey in the 32.0-50.4 GHz range, we report the discovery of the molecule CH2CCHC4H towards the prestellar cold core TMC-1 in the Taurus region. We also present a rigorous detection of CH2CCHC3N, along with its detailed analysis. We identified a total of twenty rotational transitions for each one of these molecules. The rotational quantum numbers range from Ju=17 up to 24 and Ka<=3. The column density for CH2CCHC4H is N=(2.2+/-0.2)x 1E12 cm-2, while for CH2CCHC3N, we derived N=(1.2+/-0.15) x 1E11 cm-2. The rotational temperature is 9.0+/-0.5 K for both species. The abundance ratio between CH2CCHC4H and CH2CCHC3N is 18+/-4. We also compared the column densities of these species with those of their isomers CH3C6H and CH3C5N, derived from their J=20-19 up to J=30-29 rotational transitions observed with the QUIJOTE line survey. The observed abundances for all these species are reasonably well explained by state-of-the-art chemical models of TMC-1. The observed astronomical frequencies were merged with laboratory frequencies from the literature to derive improved spectroscopic parameters., Comment: Astronomy and Astrophysics, in press. arXiv admin note: text overlap with arXiv:2112.01130

In recent years we have seen an important increase in the number of protonated molecules detected in cold dense clouds. Here we report the detection in TMC-1 of HCCNCH+, the protonated form of HCCNC, which is a metastable isomer of HC3N. This is the first protonated form of a metastable isomer detected in a cold dense cloud. The detection was based on observations carried out with the Yebes 40m and IRAM 30m telescopes, which revealed four harmonically related lines. We derive a rotational constant B = 4664.431891 +/- 0.000692 MHz and a centrifugal distortion constant D = 519.14 +/- 4.14 Hz. From a high-level ab initio screening of potential carriers we confidently assign the series of lines to the ion HCCNCH+. We derive a column density of (3.0 +/- 0.5)e10 cm-2 for HCCNCH+, which results in a HCCNCH+/HCCNC abundance ratio of 0.010 +/- 0.002. This value is well reproduced by a state-of-the-art chemical model, which however is subject to important uncertainties regarding the chemistry of HCCNCH+. The observational and theoretical status of protonated molecules in cold dense clouds indicate that there exists a global trend in which protonated-to-neutral abundance ratios MH+/M increase with increasing proton affinity of the neutral M, although if one restricts to species M with high proton affinities (>700 kJ/mol), MH+/M ratios fall in the range 0.001-0.1, with no apparent correlation with proton affinity. We suggest various protonated molecules that are good candidates for detection in cold dense clouds in the near future., Comment: Accepted for publication in A&A Letters

We report the detection of the protonated form of HC7N in TMC-1. The discovery of the cation HC7NH+ was carried out via the observation of nine harmonically related lines in the Q-band using the Yebes 40m radiotelescope. The observed frequencies allowed us to obtain the rotational constants B_0=553.938802(160)MHz and D_0=3.6292(705) Hz. The identification of HC7NH+ is further supported by accurate ab initio calculations. We derived a column density of (5.5+/-0.7)e10 cm-2, which constitutes another piece of evidence for the identification of the carrier. In addition, we revised the HC7N column density and we derived a new value of (2.1+/-0.2)e13cm-2. Hence, the abundance ratio HC7N/HC7NH+ is 380, while those for HC3N/HC3NH+ and HC5N/HC5NH+ are 230 and 240, respectively. Here, we discuss these results within the framework of a chemical model for protonated molecules in cold dense clouds., Comment: Accepted as Letter to the Editor in Astronomy & Astrophysics on 06/03/2022

We report the discovery of the C5H+ cation toward TMC-1 with the QUIJOTE line survey. Four lines from J=7-6 up to J=10-9 have been identified in perfect harmonic frequency relation that can be fit with B=2411.94397 +/-0.00055 MHz and D=138+/-3 Hz. The standard deviation of the fit is 4.4 kHz. After discarding potential candidates, C5H- among them, we conclude that the carrier is C5H+, for which accurate ab initio calculations provide B=2410.3 MHz. We also report for the first time in a cold starless core the detection of the C3H+ cation. The column densities we derive for C5H+ and C3H+ are (8.8+/-0.5)e10 cm-2 and (2.4+/-0.2)e10 cm-2, respectively. Hence, the C5H+/C3H+ abundance ratio is 3.7+/-0.5. The fact that C5H+ is more abundant than C3H+ is well explained by dedicated chemical models and is due to the slow reactivity of C5H+ with H2, while C3H+ reacts with H2., Comment: Accepted for publication in A&A Letters on January 5th

We report the detection in TMC-1 of the cation HCCS+ (3Sigma-), which is the protonated form of the widespread radical CCS. This is the first time that a protonated radical has been detected in a cold dark cloud. Twenty-six hyperfine components from twelve rotational transitions have been observed with the Yebes 40m and IRAM 30m radio telescopes. We confidently assign the characteristic rotational spectrum pattern to HCCS+ based on the good agreement between the astronomical and theoretical spectroscopic parameters. The column density of HCCS+ is (1.1+/-0.1)e12 cm-2, and the CCS/HCCS+ abundance ratio is 50+/-10, which is very similar to that of CS/HCS+ (35+/-8) and CCCS/HCCCS+ (65+/-20). From a state-of-the-art gas-phase chemical model, we conclude that HCCS+ is mostly formed by reactions of proton transfer from abundant cations such as HCO+, H3O+, and H3+ to the radical CCS., Comment: Accepted for publication in A&A Letters on December 21st

We report the first detection in space of the single deuterated isotopologue of methyldiacetylene, CH2DC4H. A total of 12 rotational transitions, with J = 8-12 and Ka = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies allowed us to obtain, for the first time, the spectroscopic parameters of this deuterated isotopologue. We derived a column density of (5.5+/-0.2)e11 cm-2. The abundance ratio between CH3C4H and CH2DC4H is 24+/-2. This ratio is similar to that found for the CH3C3N/CH2DC3N analogue system, which is 22+/-2.We did not detect the deuterated species CH3C4D, which has already been observed in laboratory experiments. The detection of deuterated CH3C4H allows us to extend the discussion on the chemical mechanisms of deuterium fractionation at work in TMC-1 using a new gas-phase chemical model with multiply deuterated molecules. Introducing a possible deuterium exchange reaction between CH3CCH and atomic deuterium allows us to account for the CH3C4H/CH2DC4H abundance ratio., Comment: Accepted for publication in A&A Letters on December 20th. arXiv admin note: text overlap with arXiv:2106.03404

We report the detection of the propargyl radical (CH2CCH) in the cold dark cloud TMC-1 in the lambda 3 mm wavelength band. We recently discovered this species in space toward the same source at a wavelength of lambda 8 mm. In those observations, various hyperfine components of the 2,0,2-1,0,1 rotational transition, at 37.5 GHz, were detected using the Yebes 40m telescope. Here, we used the IRAM 30m telescope to detect ten hyperfine components of the 5,0,5-4,0,4 rotational transition, lying at 93.6 GHz. The observed frequencies differ by 0.2 MHz with respect to the predictions from available laboratory data. This difference is significant for a radioastronomical search for CH2CCH in interstellar sources with narrow lines. We thus included the measured frequencies in a new spectroscopic analysis to provide accurate frequency predictions for the interstellar search for propargyl at mm wavelengths. Moreover, we recommend that future searches for CH2CCH in cold interstellar clouds are carried out at lambda 3 mm, rather than at lambda 8 mm. The 5,0,5-4,0,4 transition is about five times more intense than the 2,0,2-1,0,1 one in TMC-1, which implies that detecting the former requires about seven times less telescope time than detecting the latter. We constrain the rotational temperature of CH2CCH in TMC-1 to 9.9 +/- 1.5 K, which indicates that the rotational levels of this species are thermalized at the gas kinetic temperature. The revised value of the column density of CH2CCH (including ortho and para species) is (1.0 +/- 0.2)e14 cm-2, and thus the CH2CCH/CH3CCH abundance ratio is revised from slightly below one to nearly one. This study opens the door for future detections of CH2CCH in other cold interstellar clouds, making possible to further investigate the role of this very abundant hydrocarbon radical in the synthesis of large organic molecules such as aromatic rings., Comment: Accepted for publication in A&A

We report on the detection, for the first time in space, of the radical HCCCO and of pentacarbon monoxide, C5O. The derived column densities are (1.6+/-0.2)e11 cm-2 and (1.5+/-0.2)e10 cm-2, respectively. We have also analysed the data for all the molecular species of the families HCnO and CnO within our QUIJOTE line survey. Upper limits are obtained for HC4O, HC6O, C4O, and C6O. We report a robust detection of HC5O and HC7O based on 14 and 12 rotational lines detected with a signal-to-noise ratio >30 and >5,respectively. The derived N(HC3O)/N(HC5O) abundance ratio is 0.09+/-0.03, while N(C3O)/N(C5O) is 80+/-2, and N(HC5O)/N(HC7O) is 2.2+/-0.3. As opposed to the cyanopolyyne family, HC2n+1N, which shows a continuous decrease in the abundances with increasing n, the CnO and HCnO species show a clear abundance maximum for n=3 and 5, respectively. They also show an odd and even abundance alternation, with odd values of n being the most abundant, which is reminiscent of the behaviour of CnH radicals, where in that case species with even values of n are more abundant. We explored the formation of these species through two mechanisms previously proposed, which are based on radiative associations between CnHm+ ions with CO and reactions of Cn- and CnH- anions with O atoms, and we found that several species, such as C5O, HC4O, and HC6O, are significantly overestimated. Our understanding of how these species are formed is incomplete as of yet. Other routes based on neutral-neutral reactions such as those of Cn and CnH carbon chains with O, OH, or HCO, could be behind the formation of these species., Comment: Accepted for publication in A&A Letters on November 26th

Context. IRC+10216, the carbon-rich envelope of the asymptotic giant branch (AGB) star CW Leo, is one of the richest molecular sources in the sky. Available spectral surveys below 51 GHz are more than 25 years old and new work is needed. Aims. Characterizing the rich molecular content of this source, specially for heavy species, requires to carry out very sensitive spectral surveys at low frequencies. In particular in this work we have achieved an rms in the range 0.2-0.6 mK per MHz. Methods. long Q-band (31.0-50.3 GHz) single dish integrations were carried out with the Yebes 40m telescope using specifically built receivers. State of the art line catalogs are used for line identification. Results. A total of 652 spectral features corresponding to 713 transitions from 81 species (we count as different the isomers, isotopologues and ortho/para species) are present in the data. Only 57 unidentified lines remain with signal to noise ratios >3. Some new species and/or vibrational modes have been discovered for the first time with this survey. Conclusions. This IRC+10216 spectral survey is, by far, the most sensitive one carried out to this date in the Q-band. It therefore provides the most complete view of IRC+10216 from 31.0 to 50.3 GHz, giving unique information on its molecular content, specially for heavy species. Rotational diagrams built from the data provide valuable information on the physical conditions and chemical content of this circumstellar envelope., Comment: Accepted for publication in Astronomy and Astrophysics on Oct. 29th 2021. Table A.1 will be removed from the published version and sent to the CDS

We report the detection of two isomers of ethynyl cyclopentadiene (c-C5H5CCH), namely 1- and 2-ethynyl-1,3-cyclopentadiene, in the direction of TMC-1. We derive column densities of (1.4 +/- 0.2)e12 cm-2, and (2.0 +/- 0.4)e12 cm-2, respectively, for these two cyclopentadiene derivatives, which imply that they are about ten times less abundant than cyclopentadiene. We also report the tentative detection of ethynyl benzene (C6H5CCH), for which we estimate a column density of (2.5 +/-0.4)e12 cm-2. We derived abundances for the corresponding cyano derivatives of cyclopentadiene and benzene and found values significantly lower than previously reported. The rotational temperature of the ethynyl and cyano derivatives of these cycles is about 9 K, that is, very close to the gas kinetic temperature of the cloud. The abundance ratio of the 1- and 2- isomers of ethynyl cyclopentadiene is 1.4 +/-0.5, while for the two isomers of cyano cyclopentadiene it is 2.4 +/- 0.6. The relative abundances of CCH over CN derivatives is 7.7 +/- 2.2 for cyclopentadiene, which probably reflects the abundance ratio of the radicals CCH and CN; this ratio is only 2.1 +/- 0.5 for benzene, which suggests that additional reactions besides cyano radicals with benzene are involved in the formation of benzonitrile. The formation of these cycles is reasonably well accounted for through a chemical scheme based on neutral-neutral reactions. It is predicted that benzene should be as abundant as cyclopentadiene in TMC-1., Comment: Accepted for publication in A&A Letters

Little is known about the chemistry of isocyanates (compounds with the functional group R-N=C=O) in the interstellar medium, as only four of them have been detected so far: isocyanate radical (NCO), isocyanic acid (HNCO), N-protonated isocyanic acid (H$_2$NCO$^+$) and methyl isocyanate (CH$_3$NCO). The molecular cloud G+0.693-0.027, located in the Galactic Centre, represents an excellent candidate to search for new isocyanates since it exhibits high abundances of the simplest ones, HNCO and CH$_3$NCO. After CH$_3$NCO, the next complex isocyanates are ethyl isocyanate (C$_2$H$_5$NCO) and vinyl isocyanate (C$_2$H$_3$NCO). Their detection in the ISM would enhance our understanding of the formation of these compounds in space. We have detected C$_2$H$_5$NCO and H$_2$NCO$^+$ towards G+0.693-0.027 (the former for the first time in the interstellar medium) with molecular abundances of (4.7$-$7.3)$\times$10$^{-11}$ and (1.0$-$1.5)$\times$10$^{-11}$, respectively. A ratio CH$_3$NCO / C$_2$H$_5$NCO = 8$\pm$1 is obtained; therefore the relative abundance determined for HNCO:CH$_3$NCO:C$_2$H$_5$NCO is 1:1/55:1/447, which implies a decrease by more than one order of magnitude going progressively from HNCO to CH$_3$NCO and to C$_2$H$_5$NCO. This is similar to what has been found for e.g. alcohols and thiols and suggests that C$_2$H$_5$NCO is likely formed on the surface of dust grains. In addition, we have obtained column density ratios of HNCO / NCO > 269, HNCO / H$_2$NCO$^+$ $\sim$ 2100 and C$_2$H$_3$NCO / C$_2$H$_5$NCO~<~4. A comparison of the Methyl~/~Ethyl ratios for isocyanates (-NCO), alcohols (-OH), formiates (HCOO-), nitriles (-CN) and thiols (-SH) is performed and shows that ethyl-derivatives may be formed more efficiently for the N-bearing molecules than for the O- and S-bearing molecules., Comment: 5 pages, 5 figures; accepted in A&A

We report the detection, for the first time in space, of a new non-functionalised hydrocarbon cycle in the direction of TMC-1: o-C6H4 (ortho-benzyne). We derive a column density for this hydrocarbon cycle of (5 +/- 1)e11 cm-2. The abundance of this species is around 30 times lower than that of cyclopentadiene and indene. We compare the abundance of benzyne with that of other pure hydrocarbons, cycles or chains, and find that it could be formed from neutral-radical reactions such as C2H + CH2CHCCH and C + C5H5, and possibly through C4H + C2H4, C3H + CH2CCH2, and C3H2 + C3H3. Hence, the rich content of hydrocarbon cycles observed in TMC-1 could arise through a bottom-up scenario involving reactions of a few radicals with the abundant hydrocarbons recently revealed by the QUIJOTE line survey., Comment: Accepted for publication in A&A Letters

After the previous discovery of MgC$_3$N and MgC$_4$H in IRC+10216, a deeper Q-band (31.0-50.3 GHz) integration on this source had revealed two additional series of harmonically related doublets that we assigned on the basis of quantum mechanical calculations to the larger radicals MgC$_5$N and MgC$_6$H. The results presented here extend and confirm previous results on magnesium-bearing molecules in IRC\,+10216. We derived column densities of (4.7$\pm$1.3)$\times$10$^{12}$ for MgC$_5$N and (2.0$\pm$0.9)$\times$10$^{13}$ for MgC$_6$H, which imply that MgC$_5$N/MgC$_3$N=0.5 and MgC$_6$H/MgC$_4$H = 0.9. Therefore, MgC$_5$N and MgC$_6$H are present with column densities not so different from those of the immediately shorter analogs. The synthesis of these large magnesium cyanides and acetylides in IRC+10216 can be explained for their shorter counterparts by a two-step process initiated by the radiative association of Mg$^+$ with large cyanopolyynes and polyynes, which are still quite abundant in this source, followed by the dissociative recombination of the ionic complexes., Comment: 5 pages, 3 figures

We report the first identification in space of H2NC, a high-energy isomer of H2CN that has been largely ignored in chemical and astrochemical studies. The observation of various unidentified lines around 72.2 GHz in the cold dark cloud L483 motivated the search for, and successful detection of, additional groups of lines in harmonic relation. After an exhaustive high-level ab initio screening of possible carriers, we confidently assign the unidentified lines to H2NC based on the good agreement between astronomical and theoretical spectroscopic parameters and sound spectroscopic and astrochemical arguments. The observed frequencies are used to precisely characterize the rotational spectrum of H2NC. This species is also detected in the cold dark cloud B1-b and the z=0.89 galaxy in front of the quasar PKS1830-211. We derive H2NC/H2CN abundance ratios of 1 in L483 and B1-b and 0.27 toward PKS1830-211. Neither H2NC nor H2CN are detected in the dark cloud TMC-1, which seriously questions a previous identification of H2CN in this source. We suggest that the H2NC/H2CN ratio behaves as the HNC/HCN ratio, with values close to one in cold dense clouds and below one in diffuse clouds. The reactions N + CH3 and C + NH3 emerge as strong candidates to produce H2NC in interstellar clouds. Further studies on these two reactions are needed to evaluate the yield of H2NC. Due to the small number of atoms involved, it should be feasible to constrain the chemistry behind H2NC and H2CN, just as it has been done for HNC and HCN, as this could allow to use the H2NC/H2CN ratio as a probe of chemical or physical conditions of the host clouds., Comment: Accepted for publication in A&A

We report here the first detection in the interstellar medium of the cyanomidyl radical (HNCN). Using the Yebes 40m and the IRAM 30m telescopes, we have targeted the doublets of the $N$=2$-$1, 4$-$3, 5$-$4, 6$-$5, and 7$-$6 transitions of HNCN toward the molecular cloud G+0.693-0.027. We have detected three unblended lines of HNCN, these are the $N$=6$-$5 doublet and one line of the $N$=4$-$3 transition. Additionally we present one line of the $N$=5$-$4 transition partially blended with emission from other species. The Local Thermodynamic Equilibrium best fit to the data gives a molecular abundance of (0.91$\pm$0.05)$\times$10$^{-10}$ with respect to H$_2$. The relatively low abundance of this species in G+0.693-0.027, and its high reactivity, suggest that HNCN is possibly produced by gas-phase chemistry. Our work shows that this highly reactive molecule is present in interstellar space, and thus it represents a plausible precursor of larger prebiotic molecules with the NCN backbone such as cyanamide (NH$_2$CN), carbodiimide (HNCNH) and formamidine (NH$_2$CHNH)., Comment: Accepted in Monthly Notices of the Royal Astronomical Society Letters

We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH$_2$DC$_3$N. A total of fifteen rotational transitions, with $J$ = 8-12 and $K_a$ = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this deuterated isotopologue. We derive a column density of $(8.0\pm 0.4) \times 10^{10}$ cm$^{-2}$. The abundance ratio between CH$_3$C$_3$N and CH$_2$DC$_3$N is $\sim$22. We also theoretically computed the principal spectroscopic constants of $^{13}$C isotopologues of CH$_3$C$_3$N and CH$_3$C$_4$H and those of the deuterated isotopologues of CH$_3$C$_4$H for which we could expect a similar degree of deuteration enhancement. However, we have not detected either CH$_2$DC$_4$H nor CH$_3$C$_4$D nor any $^{13}$C isotopologue. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas phase chemical model where the low temperature conditions favor deuteron transfer through reactions with H$_2$D$^+$., Comment: Accepted for publication in A&A Letters

We report the first detection in space of the cumulene carbon chain $l$-H$_2$C$_5$. A total of eleven rotational transitions, with $J_{up}$ = 7-10 and $K_a$ = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8$\pm$0.5)$\times$10$^{10}$ cm$^{-2}$. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that $l$-H$_2$C$_5$ is $\sim$4.0 times less abundant than the larger cumulene carbene $l$-H$_2$C$_6$, while it is $\sim$300 and $\sim$500 times less abundant than the shorter chains $l$-H$_2$C$_3$ and $l$-H$_2$C$_4$. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C$_n$H$_2$ isomers with $n$\,$>$\,3., Comment: Accepted for publication in A&A Letters

We report the detection, for the first time in space, of cyano thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type lines, which are the strongest previously unassigned features in our Q-band line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN of 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for which the CCH-bearing species is more abundant than the CN-bearing one. We discuss the formation of these molecules in terms of neutral-neutral reactions of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals with H2CS. The calculated abundances for the sulphur-bearing species are, however, significantly below the observed values, which points to an underestimation of the abundance of atomic sulphur in the model or to missing formation reactions, such as ion-neutral reactions., Comment: Accepted for publication in A&A Letters

We report the detection for the first time in space of three new pure hydrocarbon cycles in TMC-1: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6 (cyclopentadiene) and c-C9H8 (indene). We derive a column density of 3.1e11 cm-2 for the former cycle and similar values, in the range (1-2)e13 cm-2, for the two latter molecules. This means that cyclopentadiene and indene, in spite of their large size, are exceptionally abundant, only a factor of five less abundant than the ubiquitous cyclic hydrocarbon c-C3H2. The high abundance found for these two hydrocarbon cycles, together with the high abundance previously found for the propargyl radical (CH2CCH) and other hydrocarbons like vinyl and allenyl acetylene (Agundez et al. 2021; Cernicharo et al. 2021a,b), start to allow us to quantify the abundant content of hydrocarbon rings in cold dark clouds and to identify the intermediate species that are probably behind the in situ bottom-up synthesis of aromatic cycles in these environments. While c-C3HCCH is most likely formed through the reaction between the radical CCH and c-C3H2, the high observed abundances of cyclopentadiene and indene are difficult to explain through currently proposed chemical mechanisms. Further studies are needed to identify how are five- and six-membered rings formed under the cold conditions of clouds like TMC-1., Comment: Accepted for publication in A&A Letters

We report the detection of the oxygen-bearing complex organic molecules propenal (C2H3CHO), vinyl alcohol (C2H3OH), methyl formate (HCOOCH3), and dimethyl ether (CH3OCH3) toward the cyanopolyyne peak of the starless core TMC-1. These molecules are detected through several emission lines in a deep Q-band line survey of TMC-1 carried out with the Yebes 40m telescope. These observations reveal that the cyanopolyyne peak of TMC-1, which is the prototype of cold dark cloud rich in carbon chains, contains also O-bearing complex organic molecules like HCOOCH3 and CH3OCH3, which have been previously seen in a handful of cold interstellar clouds. In addition, this is the first secure detection of C2H3OH in space and the first time that C2H3CHO and C2H3OH are detected in a cold environment, adding new pieces in the puzzle of complex organic molecules in cold sources. We derive column densities of (2.2 +/- 0.3)e11 cm-2, (2.5 +/- 0.5)e12 cm-2, (1.1 +/- 0.2)e12 cm-2, and (2.5 +/- 0.7)e12 cm-2 for C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3, respectively. Interestingly, C2H3OH has an abundance similar to that of its well known isomer acetaldehyde (CH3CHO), with C2H3OH/CH3CHO ~ 1 at the cyanopolyyne peak. We discuss potential formation routes to these molecules and recognize that further experimental, theoretical, and astronomical studies are needed to elucidate the true mechanism of formation of these O-bearing complex organic molecules in cold interstellar sources., Comment: Accepted for publication in A&A Letters

We report the detection of the sulfur-bearing species NCS, HCCS, H2CCS, H2CCCS, and C4S for the first time in space. These molecules were found towards TMC-1 through the observation of several lines for each species. We also report the detection of C5S for the first time in a cold cloud through the observation of five lines in the 31-50 GHz range. The derived column densities are N(NCS) = (7.8 +/- 0.6)e11 cm-2, N(HCCS) = (6.8 +/- 0.6)e11 cm-2, N(H2CCS) = (7.8 +/- 0.8)e11 cm-2, N(H2CCCS) = (3.7 +/- 0.4)e11 cm-2, N(C4S) = (3.8 +/- 0.4)e10 cm-2, and N(C5S) = (5.0 +/- 1.0)e10 cm-2. The observed abundance ratio between C3S and C4S is 340, that is to say a factor of approximately one hundred larger than the corresponding value for CCS and C3S. The observational results are compared with a state-of-the-art chemical model, which is only partially successful in reproducing the observed abundances. These detections underline the need to improve chemical networks dealing with S-bearing species., Comment: Accepted for publication in A&A Letters

We present the first identification in interstellar space of the propargyl radical (CH2CCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. The six strongest hyperfine components of the 2,0,2-1,0,1 rotational transition, lying at 37.46~GHz, were detected with signal-to-noise ratios in the range 4.6-12.3 sigma. We derive a column density of 8.7e13 cm-2 for CH2CCH, which translates to a fractional abundance relative to H2 of 8.7e-9. This radical has a similar abundance to methyl acetylene, with an abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thus one of the most abundant radicals detected in TMC-1, and it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud. We constructed a gas-phase chemical model and find calculated abundances that agree with, or fall two orders of magnitude below, the observed value depending on the poorly constrained low-temperature reactivity of CH2CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C2H4 reaction and by the dissociative recombination of C3Hn+ ions with n = 4-6. The propargyl radical is believed to control the synthesis of the first aromatic ring in combustion processes, and it probably plays a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds., Comment: Accepted for publication in A&A Letters

We present the discovery in TMC-1 of allenyl acetylene, H2CCCHCCH, through the observation of nineteen lines with a signal-to-noise ratio ~4-15. For this species, we derived a rotational temperature of 7 +/- 1 K and a column density of (1.2 +/- 0.2)e13 cm-2. The other well known isomer of this molecule, methyl diacetylene (CH3C4H), has also been observed and we derived a similar rotational temperature, Trot = 7.0 +/- 0.3 K, and a column density for its two states (A and E) of (6.5 +/- 0.3)e12 cm-2. Hence, allenyl acetylene and methyl diacetylene have a similar abundance. Remarkably, their abundances are close to that of vinyl acetylene (CH2CHCCH). We also searched for the other isomer of C5H4, HCCCH2CCH (1.4-pentadiyne), but only a 3sigma upper limit of 2.5e12 cm-2 to the column density can be established. These results have been compared to state-of-the-art chemical models for TMC-1, indicating the important role of these hydrocarbons in its chemistry. The rotational parameters of allenyl acetylene have been improved by fitting the existing laboratory data together with the frequencies of the transitions observed in TMC-1., Comment: Accepted for publication in A&A Letters. arXiv admin note: text overlap with arXiv:2102.10920