arXiv subjects

M. Agúndez

Publications and source records attributed to M. Agúndez.

At least 19 recordsLinked to original sources

arXiv2026

The formation radius of HCN in O-rich asymptotic giant branch stars

Molecules detected in circumstellar envelopes around asymptotic giant branch (AGB) stars are generally well explained in terms of formation under chemical equilibrium in the stellar atmosphere or photochemistry in the outer expanding layers. However, several molecules are detected in the inner circumstellar regions with abundances orders of magnitude above the predictions of chemical equilibrium. Anomalously abundant molecules comprise H2O, NH3, SiH4, and PH3 in C-rich envelopes and HCN, CS, and NH3 in O-rich sources. These molecules are formed by some yet unknown nonequilibrium processes. Aims: We aim to shed light on the nonequilibrium process that enhances the abundance of anomalously abundant molecules in the inner regions of circumstellar envelopes. Here, we focus on HCN in O-rich stars to constrain its formation radius. Method: We observed the O-rich AGB stars RCrt and IKTau with ALMA at high angular resolution (HPBW=~0.05") in the J=4-3 line of HCN in both the ground and the ν_2=1 vibrational states. The radial distribution of the emission was modeled using a large velocity gradient radiative transfer code in which we considered an abundance profile with a central hole. Results: The models with a hole in the abundance distribution of HCN reproduce the specific shape of the radial emission distribution around the two studied O-rich stars. The best models locate the formation radius of HCN at 4-6 Rstar in RCrt and 3-5 Rstar in IKTau. Conclusion: The formation radius of HCN inferred from observations is consistent with models that invoke shocked-induced chemistry but also with models where photochemistry acts as the solely disequilibrium process. High angular resolution observations of more O-rich stars and tailored shocked-induced and/or photochemical models are needed to unambiguously unveil the underlying mechanism behind anomalously abundant molecules in AGB envelopes.

astro-ph.SR↗

arXiv2026

Chemistry in the High expansion-velocity C-rich evolved star AFGL2233. Isotopic ratios, peculiarities and evolutionary status

High expansion velocity carbon stars (HVCs) are a rare class of evolved stars whose circumstellar envelopes (CSEs) combine C-rich chemistry with unusually high expansion velocities typical of O-rich massive evolved stars. AFGL2233 has been proposed as a high-mass evolved object that exhausted hot-bottom burning. Studying its chemistry is essential to understand the nature and evolution of these objects. We characterize the chemical composition and isotopic ratios of the CSE of AFGL2233 and investigate chemical peculiarities, including the presence of N- and O-bearing species in a C-rich environment. We carried out a complete line survey at 3 mm and 1 mm using the IRAM 30m telescope, complemented by Herschel/HIFI FIR observations and interferometric maps of SiO, C2H, and HCN. Molecular emission was analyzed using rotational diagrams and radiative transfer modeling under the LVG approximation. Column densities and fractional abundances were derived for more than 30 molecular species, including isotopologues, and compared with other evolved stars. The Gaia DR3 distance of 1.236 kpc implies a luminosity of ~2 Lsun, consistent with an initial mass of 4.5-9 Msun. The molecular inventory confirms C-rich chemistry but reveals unusually high abundances of NH3, H2O, and SiN. The isotopic ratios vary among species, with 12C/13C ranging from 7 to 55. The C2H/C4H ratio is abnormally high compared with C-rich AGB stars. The presence of SiN and high NH3 may indicate N-enrichment or the influence of a companion. AFGL2233 is likely a high-mass AGB or super-AGB star with a complex evolutionary history involving nucleosynthesis, shocks, and possible binary interaction.

astro-ph.SR↗

arXiv2026

Abundant hydrocarbons in a buried galactic nucleus with signs of carbonaceous grain and polycyclic aromatic hydrocarbon processing

Hydrocarbons play a key role in shaping the chemistry of the interstellar medium (ISM), but their enrichment and relationship with carbonaceous grains and polycyclic aromatic hydrocarbons (PAHs) still lack clear observational constraints. We report JWST NIRSpec+MIRI/MRS infrared (IR; 3-28 micron) observations of the local ultra-luminous IR galaxy (ULIRG) IRAS 07251-0248, revealing the extragalactic detection of small gas-phase hydrocarbons such as benzene (C$_6$H$_6$), triacetylene (C$_6$H$_2$), diacetylene (C$_4$H$_2$), acetylene (C$_2$H$_2$), methane (CH$_4$), and methyl radical (CH$_3$) as well as deep amorphous C-H absorptions in the solid phase. The unexpectedly high abundance of these molecules indicates an extremely rich hydrocarbon chemistry, not explained by high-temperature gas-phase chemistry, ice desorption or oxygen depletion. Instead, the most plausible explanation is the erosion and fragmentation of carbonaceous grains and PAHs. This scenario is supported by the correlation between the abundance of one of their main fragmentation products, C$_2$H$_2$, and cosmic ray (CR) ionization rate for a sample of local ULIRGs. These hydrocarbons are outflowing at $\sim$160 km/s, which may represent a potential formation pathway for hydrogenated amorphous grains. Our results suggest that IRAS 07251-0248 might not be unique but represents an extreme example of the commonly rich hydrocarbon chemistry prevalent in deeply obscured galactic nuclei.

astro-ph.GA↗

arXiv2026

Discovery of 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 with the QUIJOTE line survey: A new three-ringed polycyclic aromatic hydrocarbon

We report the detection of the polycyclic aromatic hydrocarbon (PAH) 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 with the QUI- JOTE line survey. We detected 22 independent lines corresponding to 88 rotational transitions with quantum numbers ranging from J=19 up to J=24 and Ka <= 5 in the Q-band range. The identification of this new PAH was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. The column density derived for 1H-cyclopent[cd]indene is (6.0 +- 0.5) x 10^12 cm-2, with a rotational temperature of 9 K. Its abundance is high, as is that of the rest of the PAHs, but it is the lowest of all those detected to date in TMC-1, being 2.66 times less abundant than indene and 4.66 times less than phenalene. This result will help us to better understand the growth of five- and six-membered rings in dark clouds. Chemical models explaining their formation through the bottom-up model are still very incomplete and require further experimental and theoretical effort. Even so, the most likely formation reactions would occur between the smallest rings with small hydrocarbons; the most probable reaction for the formation of cyclopentindene is that between indene and C2H, C2H3, and/or their cation.

astro-ph.GA↗

arXiv2025

Discovery of linear propadienone: Study of the chemistry of linear and cyclic H$_2$C$_3$O and H$_2$C$_3$S in TMC-1

We report the first detection in space of propadienone, the linear isomer (l-H$_2$C$_3$O) of cyclopropenone (c-H$_2$C$_3$O). We also report the first detection of the isotopologue c-H$_2$$^{13}$CCCO, and c-HDCCCO of c-H$_2$C$_3$O. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the frequency range 31.0-50.3 GHz, complemented with data between 71.6-116.0 GHz, and carried out with Yebes-40m and IRAM-30m telescopes, respectively. We obtain a total column density of 3.7$\times$10$^{10}$ cm$^{-2}$ for l-H$_2$C$_3$O at an excitation temperature of 4.8 K. We find that the isomer is about eight times less abundant than the cyclic one. We also report a detailed line-by-line study of cyclopropenethione (c-H$_2$C$_3$S) to compare the abundance of the O and S isomers. We find that cyclic O-isomers are more abundant than cyclic S-isomers; however, the opposite trend is found for the most stable linear isomers, with l-H$_2$C$_3$S being more than one order of magnitude more abundant than l-H$_2$C$_3$O. A comprehensive theoretical chemical analysis shows that the abundances of the H$_2$C$_3$O and H$_2$C$_3$S isomers are controlled by different formation pathways. In particular, while l-H$_2$C$_3$O is potentially produced by dissociative electron recombination reactions, ion-neutral chemistry is more effective at producing l-H$_2$C$_3$S and c-H$_2$C$_3$S.

astro-ph.GA↗

arXiv2025

Piecing together formic acid isomerism in dark clouds. Detection of cis-formic acid in TMC-1 and astrochemical modeling

The presence of molecular isomers in interstellar environments has become a topic of growing interest within the astrochemical community. Contrary to predictions based on thermodynamic equilibrium, recent observations reveal a diverse array of high-energy isomers and conformers. One of the most iconic molecular isomers detected in space, formic acid (HCOOH, FA), has been the focus of extensive theoretical research aimed at understanding its speciation into cis and trans conformers in dark clouds and photodissociation regions. In this work, we report the detection of c-FA, the higher-energy conformer, using ultrasensitive observations of TMC-1. This detection adds to previous findings in the Barnard-5 and L483 dark clouds. The derived trans-to-cis isomer ratio in TMC-1, 17.5, closely matches those observed in other sources, suggesting that the same chemical processes are at play across these environments. To investigate this, we conducted detailed astrochemical gas-grain models tailored to formic acid isomerism to explain the observed ratios. Our models successfully reproduce the observed trans/cis ratios and indicate that the presence of cis-formic acid can be attributed to the release of c-FA from grains, followed by isomerization driven by the excess energy released during the desorption process, a process that we name as isomerization upon desorption. The models also show that the isomerization of t-FA to c-FA in the gas phase is negligible at 10 K, meaning the observed ratios are a direct consequence of the formation pathways of both isomers on the surface of dust grains. However, at higher temperatures, quantum tunneling mediated direct isomerization in the gas becomes significant, and the ratios converge toward the thermodynamic equilibrium value.

astro-ph.GA↗

arXiv2025

Analysis of the isotopologues of CS, CCS, CCCS, HCS+, HCCS+, and H2CS in TMC-1 with the QUIJOTE line survey

We performed a detailed analysis of the isotopologues with 13C, 34S, 33S, and 36S of the sulphur-bearing molecules CS, CCS, CCCS,HCS+, HCCS+, and H2CS towards TMC-1 using the QUIJOTE1. The observations were obtained with the Yebes radio telescope. Observations with the IRAM 30m of the most abundant isotopologues of these species are also presented and used to estimate volume densities and to constrain the excitation conditions. We report the first detection in space of C13C34S, CC33S, CCC33S, HC33S+, and HCC34S+. C36S is also detected for the first time in a cold object. We also complemented with maps that provide the spatial distribution of most of these species. Using the available collisional rate coefficients for each species, we modeled the observed line intensities using the large velocity gradient method for the radiative transfer. We report the most complete analysis of the column densities of the CnS family and to compare the abundance ratios of all detected isotopologues. Adopting a T_k for TMC-1 of 9K, we found that n(H2)=0.9-1.5X10^4cm-3 can explain the observed decline in intensity with increasing J. We derived the rot. constants for the C13C34S, CC33S, CCC33S, HC33S+, and HCC34S+ isotopologues from new laboratory data and complemented them with the frequencies of the observed lines. We find that all S isotopologues are consistent with solar isotopic abundance ratios. Accurate 12C/13C abundances were derived and, as previously suggested, the 13C isotopologues of CCS and CCCS show strong abundance anomalies depending on the position of the substituted carbon. Nevertheless, the 12C/13C abundance ratio is practically identical to the solar value for CS, HCS+, and H2CS. We also searched for the isotopologues of other S-bearing molecules. The expected intensities for their 34S and 13C isotopologues are too low to be detected with the present sensitivity of the QUIJOTE, however.

astro-ph.GA↗

arXiv2025

JWST reveals cosmic ray dominated chemistry in the local ULIRG IRAS 07251$-$0248

We analyse the ro-vibrational absorption bands of various molecular cations (HCO$^+$, HCNH$^+$, and N$_2$H$^+$) and neutral species (HCN, HNC, and HC$_3$N) detected in the \textit{James Webb Space Telescope}/Mid-Infrared Instrument Medium Resolution Spectrometer spectrum (4.9--27.9\,$\upmu$m) of the local ultra luminous infrared galaxy IRAS~07251$-$0248. We find that the molecular absorptions are blueshifted by 160\,km\,s$^{-1}$ relative to the systemic velocity of the target. Using local thermal equilibrium (LTE) excitation models, we derive rotational temperatures ($T_{\rm rot}$) from 42 to 185\,K for these absorption bands. This range of measured $T_{\rm rot}$ can be explained by infrared (IR) radiative pumping as a by--product of the strength, effective critical density, and opacity of each molecular band. Thus, these results suggest that these absorptions originate in a warm expanding gas shell ($\dot{M}$$\sim$90--330\,$M_\odot$\,yr$^{-1}$), which might be the base of the larger scale cold molecular outflow detected in this source. Finally, the elevated abundance of molecular cations can be explained by a high cosmic ray ionization rate, with log($ζ_{\text{H}_2}$/n$_{\rm H}\, [\text{cm}^3\, \text{s}^{-1}])$ in the range of $-$18.2 (from H$_3^+$) to $-$19.1 (inferred from HCO$^+$ and N$_2$H$^+$, which are likely tracing denser gas), consistent with a cosmic ray dominated chemistry as predicted by chemical models.

astro-ph.GA↗

arXiv2025

First detection of HS2 in a cold dark cloud

We report the first detection of HS2 towards the cold dark cloud TMC-1. This is the first observation of a chemical species containing more than one sulphur atom in this type of sources. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the Q band (31-50 GHz). The detection is confirmed by the observation of the fine and hyperfine components of two rotational transitions (2(0,2)-1(0,1) and 3(0,3)-2(0,2)). Assuming a rotational temperature of 7 K, we derived an HS2 column density of 5.7x10^11 cm-2, using a local thermodynamic equilibrium model that reproduces the observed spectra. The abundance of HS2 relative to H2 is 5.7x10^-11, which means that it is about seven times more abundant than its oxygenated counterpart HSO. We also explored the main formation and destruction mechanisms of HS2 using a chemical model, which reproduces the observed abundance of HS2 and indicates that dissociative recombination reactions from the ions H2S2+ and H3S2+ play a major role in forming HS2.

astro-ph.GA↗

arXiv2025

Exploring circumstellar chemistry in X-ray emitting AGB stars

Aims. Our goal is to characterize the chemistry and physical conditions of the circumstellar envelopes (CSEs) of Asymptotic Giant Branch (AGB) binary candidate stars with UV-excess and X-ray emission, in particular, to identify the effects of the internal X-ray emission in the abundance of certain key molecules. Methods. We observed the 86.0-94.0 and 260.0-272.5 GHz spectral ranges searching for rotational transitions of the X-ray sensitive molecule $HCO^{+}$ in four AGB stars, two of them detected in both UV and X-ray emission and the other two detected only in UV. We derived the CSEs's physical parameters from previous CO observations and determined the molecular abundances of the detected species using radiative transfer models. We developed chemical kinetics models that account for the effects of internal X-ray emission (as well as UV radiation) and compared our predictions with observations. Results. We report the detection of $HCO^{+}$ in the X-ray emitting C-rich AGB T\,Dra, while it remains undetected in the spectra of the other three sources. In T\,Dra we also detect SiO, HCN, HNC, $HC_{3}N$, $SiC_{2}$, $C_{2}H$ and SiS. For the other targets only HCN and SiO are detected. The high fractional abundance of $HCO^{+}$ derived for T\,Dra ($[1.5-3.0]\times 10^{-8}$) is in good agreement with the predictions from our chemical kinetics models including the effects of internal X-ray emission, and one order of magnitude higher than the values expected for C-rich AGB stars. Additionally, we identify abundance enhancements for HNC, and $HC_{3}N$ alongside a depletion of CO in the innermost regions of T\,Dra's envelope. Conclusions. An internal X-ray source can significantly alter molecular abundances in AGB CSEs, enhancing $HCO^{+}$, $N_{2}H^{+}$, HNC, and $HC_{3}N$ while depleting parent species like CO. UV radiation has a weaker effect unless the envelope is optically thin or porous.

astro-ph.SR↗

arXiv2025

Identification of the interstellar 1-cyano propargyl radical (HCCCHCN) in TMC-1

We report the first detection in interstellar medium of the 1-cyano propargyl radical, HC$_3$HCN. This species is an isomer of the 3-cyano propargyl radical (CH$_2$C$_3$N), which was recently discovered in TMC-1. The 1-cyano propargyl radical was observed in the cold dark cloud TMC-1 using data from the ongoing QUIJOTE line survey, which is being carried out with the Yebes 40m telescope. A total of seven rotational transitions with multiple hyperfine components were detected in the 31.0-50.4 GHz range. We derived a column density of (2.2$\pm$0.2)$\times$10$^{11}$ cm$^{-2}$ and a rotational temperature of 7$\pm$1\,K. The abundance ratio between HC$_3$HCN and CH$_2$C$_3$N is 1.4. The almost equal abundance of these isomers indicates that the two species may be produced in the same reaction with a similar efficiency, probably in the reaction C + CH$_2$CHCN and perhaps also in the reaction C$_2$ + CH$_3$CN and the dissociative recombination with electrons of CH$_2$C$_3$NH$^+$

astro-ph.GA↗

arXiv2024

Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

During their thermally pulsing phase, Asymptotic Giant Branch (AGB) stars eject material that forms extended dusty envelopes. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen rich stars, including W Hya and Mira. At the stellar surface level, infrared images have revealed intricate structures around the carbon semi-regular variable R Scl and in the S-type star $π^{\mathrm{1}}$ Gru. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10216, and studies of the molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures. Because of the lack of sufficient spatial resolution, however, the distribution of molecular gas in the stellar atmosphere and the dust formation zone of AGB carbon stars is not known, nor is how it is subsequently expelled. Here we report observations with a resolution of one stellar radius of the recently formed dust and molecular gas in the atmosphere of IRC+10216. Lines of HCN, SiS, and SiC$_2$ appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse. The convective cells coalesce with pulsation causing anisotropies that, together with companions, shape its circumstellar envelope.

astro-ph.SR↗

arXiv2024

Discovery of two cyano derivatives of acenaphthylene (C$_{12}$H$_8$) in TMC-1 with the QUIJOTE line survey

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.

astro-ph.GA↗

arXiv2024

More sulphur in TMC-1: Discovery of the NC$_3$S and HC$_3$S radicals with the QUIJOTE line survey

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$σ$ 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.

astro-ph.GA↗

arXiv2024

Discovery of HCCCH2CCH in TMC-1 with the QUIJOTE line survey

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.

astro-ph.GA↗

arXiv2024

Discovery of the interstellar cyanoacetylene radical cation HC$_3$N$^+$

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 $^2Π$ 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.

astro-ph.GA↗

arXiv2024

QUIJOTE discovery of the cation radicals HC5N+ and HC7N+

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.

astro-ph.GA↗

arXiv2024

Laboratory and astronomical discovery of cyanothioketene, NCCHCS, in the cold starless core TMC-1

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.

astro-ph.GA↗