Carbon Chain Molecules Research Articles

Modeling long carbon-chain species formation with porous multiphase models

Context. Recent studies show that multiphase models trap too many volatile species such as CH4 inside ice mantles, so they usually underestimate the abundances of long carbon-chain species observed toward warm carbon-chain chemistry (WCCC) sources. Aims. We propose a new multiphase model that allows more volatile species to diffuse out of the ice mantle upon warming. The new multiphase model is used to study the synthesis of long carbon-chain molecules in WCCC sources. Methods. We included porous structure in the ice mantles. The porous structure can enlarge the active layers of ice mantles so that fewer volatile species are trapped. The porous multiphase models were simulated using an accelerated Gillespie algorithm. Results. The abundances of long carbon-chain species predicted by the porous multiphase models can be more than one order of magnitude higher than those predicted by the multiphase model at temperatures relevant to WCCC sources. Moreover, the porous multiphase models predict more abundant long carbon-chain species as the porosity of the ice mantles increases. On the other hand, the two-phase model still estimates higher long carbon-chain species abundances than the porous multiphase models do. The abundances of long carbon-chain species predicted by our porous multiphase models agree reasonably well with observations toward three WCCC sources, L483, L1527, and B228. Conclusions. Our porous multiphase model solves the problem of too many volatile species being trapped in ice mantles in the multiphase models.

Constructing potential energy surface for carbon-chain containing systems using the radial angular network with gradual expansion method.

Investigating molecular excitation induced by collisions requires the prior determination of accurate analytical potential energy surfaces for the colliding partners. For carbon-chain molecules, such as cyanopolyynes, this has been a longstanding challenge, resulting in the absence of rate coefficients for HC5N, HC7N, HC9N, and others, induced by collisions with He. To overcome this bottleneck, we introduce a new approach: the Radial Angular Network with Gradual Expansion (RANGE). This method jointly connects the construction of abinitio interaction potentials with the determination of their analytical forms. We use the HC3N-He molecular complex as a reference to assess the reliability of our method, given that its analytical potential has been derived using various methods. Additionally, we apply the RANGE approach to construct the analytical representation of the interaction potential for HC5N-He and HC7N-He. The analysis of the analytical potentials reveals three systematic trends: (i) the anisotropy increases with the length of the carbon chain, (ii) the number of local minima correlates with the number of carbon atoms, and (iii) the shallowest local minimum is consistently located at ∼30 cm-1 below the dissociation limit of the complex. Using the time-independent quantum mechanical close-coupling formalism, we briefly estimate the propensity rules governing the excitation of HC3N, HC5N, and HC7N induced by collisions with He. Consequently, the three collisional systems exhibit the same propensity rule, favoring Δj = 2 transitions.

Accretion versus core-filament collision. Implications for streamer formation in Per-emb-2

Recent millimetre and sub-millimetre observations have unveiled elongated and asymmetric structures around protostars. These structures, referred to as streamers, often exhibit coherent velocity gradients, seemingly indicating a directed gas flow towards the protostars. However, their origin and role in star formation remain uncertain. The protostellar core Per-emb-2, located in Barnard 1, has a relatively large streamer of $10^4$ au that is more prominent in emission from carbon-chain molecules. We aim to unveil the formation mechanism of this streamer. We conducted mapping observations towards Per-emb-2 using the Nobeyama 45 m telescope. We targeted carbon-chain molecular lines such as CCS, HC$_3$N, and HC$_5$N at 45 GHz. Using astrodendro we identified one protostellar and four starless cores, including three new detections, on the Herschel column density map. The starless and protostellar cores are more or less gravitationally bound. We discovered strong CCS and HC$_3$N emissions extending from the north to the south, appearing to bridge the gap between the protostellar core and the starless core to its north. This bridge spans $3 10^4$ au with velocities of 6.5 to 7.0 km $. The velocity gradient of the bridge is opposite that of the streamer. Thus, the streamer is unlikely to be connected to the bridge, suggesting that the former does not have an accretion origin. We propose that a collision between a spherical core and the filament has shaped the density structure in this region, consequently triggering star formation within the head-tail-shaped core. In this core-filament collision scenario, the collision appears to have fragmented the filament into two structures. The streamer is a bow structure, while the bridge is a remnant of the shock-compressed filament. Thus, we conclude that the Per-emb-2 streamer does not significantly contribute to the mass accumulation towards the protostar.

An ultra wide-band, high-sensitivity Q-band receiver for single-dish telescopes, eQ: Rest-frequency determination of CCS (JN = 43–32) and SO (JN = 10–01) and high-redshift CO (J = 1–0) detection

Abstract We report on the development and commissioning of a new Q-band receiver for the Nobeyama 45 m telescope, covering 30–50 GHz with a receiver noise temperature of about 15 K. We name it eQ (extended Q-band) receiver. The system noise temperatures for observations are measured to be ∼30 K at 33 GHz and ∼75 K at 45 GHz. The half-power beam-width (HPBW) is around 38${^{\prime \prime }}$ at 43 GHz. To enhance the observation capability, we tested the smoothed bandpass calibration technique and demonstrated that the observation time can be significantly reduced compared to the standard position switch technique. The wide-bandwidth capability of this receiver provides precise determination of rest frequencies for molecular transitions with an accuracy of a few kHz through simultaneous observations of multiple transitions. Particularly, we determined the rest frequency of SO (JN = 10–01) to be 30.001542 GHz, along with the rest frequency of CCS (JN = 43–32) being 45.379033 GHz, adopting CCS (JN = 32–21) at 33.751370 GHz as a reference line. The SO profile shows a double peak shape at the Cyanopolyyne Peak (CP) position of the Taurus Molecular Cloud-1 (TMC-1). The SO peaks coincide well with the CCS sub-components located near the outer parts of the TMC-1 filament. We interpret that the gravitational infall of TMC-1 generates shocks which enhance the SO abundance. The TMC-1 map shows that carbon-chain molecules are more abundant in the southern part of the filament, whereas SO is more abundant in the northern part. The eQ’s excellent sensitivity allowed us to detect faint CO (J = 1–0) spectra from the high-redshift object at a redshift of 2.442. Our receiver is expected to open new avenues for high-sensitivity molecular line observations in the Q-band.

Decarboxylative Ketonization of Aliphatic Carboxylic Acids in a Continuous Flow Reactor Catalysed by Manganese Oxide on Silica.

The sustainable synthesis of long carbon chain molecules from carbon dioxide, water and electricity relies on the development of waste-free, highly selective C-C bond forming reactions. An example for such a power-to-chemicals process is the industrial-scale fermentation for the production of hexanoic acid. Herein, we describe how this product is transformed into 6-undecanone via decarboxylative ketonization using a heterogeneous manganese oxide/silica catalyst. The reaction reaches full conversion with near-complete selectivity when carried out in a continuous flow reactor, requires no solvent or carrier gas, and releases carbon dioxide and water as the only by-products. The reactor was operated for several weeks with no loss of reactivity, producing 7 kg of 6-undecanone from 10 g of catalyst and achieving a productivity of 1.135 kg per litre of reactor volume per hour. 6-Undecanone and other long-chain ketones accessible this way can be hydrogenated to industrially meaningful alkanes, or converted into valuable fatty acids via a hydrogenation/elimination/isomerizing hydrocarboxylation sequence.

Intermediate Phase Suppression with Long Chain Diammonium Alkane for High Performance Wide-Bandgap and Tandem Perovskite Solar Cells.

Wide bandgap (WBG) perovskite can construct tandem cells with narrow bandgap solar cells by adjusting the band gap to overcome the Shockley-Queisser limitation of single junction perovskite solar cells (PSCs). However, WBG perovskites still suffer from severe nonradiative carrier recombination and large open-circuit voltage loss. Here, this work uses an in situ photoluminescence (PL) measurement to monitor the intermediate phase evolution and crystallization process via blade coating. This work reports a strategy to fabricate efficient and stable WBG perovskite solar cells through doping a long carbon chain molecule octane-1,8-diamine dihydroiodide (ODADI). It is found that ODADI doping not only suppresses intermediate phases but also promote the crystallization of perovskite and passivate defects in blade coated 1.67eV WBG FA0.7Cs0.25MA0.05Pb(I0.8Br0.2)3 perovskite films. As a result, the champion single junction inverted PSCs deliver the efficiencies of 22.06% and 19.63% for the active area of 0.07 and 1.02 cm2, respectively, which are the highest power conversion efficiencies (PCEs) in WBG PSCs by blade coating. The unencapsulated device demonstrates excellent stability in air, which maintains its initial efficiency at the maximum power points under constant AM 1.5G illumination in open air for nearly 500h. The resulting semitransparent WBG device delivers a high PCE of 20.06%, and the 4-terminal all-perovskite tandem device delivers a PCE of 28.35%.

Chemical differences among collapsing low-mass protostellar cores

Context. Organic features lead to two distinct types of Class 0/I low-mass protostars: hot corino sources exhibiting abundant saturated complex organic molecules (COMs) and warm carbon-chain chemistry (WCCC) sources exhibiting abundant unsaturated carbon-chain molecules. Some observations suggest that the chemical variations between WCCC sources and hot corino sources are associated with local environments and the luminosity of protostars. Aims. We aim to investigate the physical conditions that significantly affect WCCC and hot corino chemistry, as well as to reproduce the chemical characteristics of prototypical WCCC sources and hybrid sources, where both carbon-chain molecules and COMs are abundant. Methods. We conducted a gas-grain chemical simulation in collapsing protostellar cores, adopting a selection of typical physical parameters for the fiducial model. By adjusting the values of certain physical parameters, such as the visual extinction of ambient clouds (AVamb), cosmic-ray ionization rate (ζ), maximum temperature during the warm-up phase (Tmax), and contraction timescale of protostars (tcont), we studied the dependence of WCCC and hot corino chemistry on these physical parameters. Subsequently, we ran a model with different physical parameters to reproduce scarce COMs in prototypical WCCC sources. Results. The fiducial model predicts abundant carbon-chain molecules and COMs. It also reproduces WCCC and hot corino chemistry in the hybrid source L483. This suggests that WCCC and hot corino chemistry can coexist in some hybrid sources. Ultraviolet (UV) photons and cosmic rays can boost WCCC features by accelerating the dissociation of CO and CH4 molecules. On the other hand, UV photons can weaken the hot corino chemistry by photodissociation reactions, while the dependence of hot corino chemistry on cosmic rays is relatively complex. The value of Tmax does not affect any WCCC features, while it can influence hot corino chemistry by changing the effective duration of two-body surface reactions for most COMs. The long tcont can boost WCCC and hot corino chemistry by prolonging the effective duration of WCCC reactions in the gas phase and surface formation reactions for COMs, respectively. The scarcity of COMs in prototypical WCCC sources can be explained by insufficient dust temperatures in the inner envelopes that are typically required to activate hot corino chemistry. Meanwhile, the high ζ and the long tcont favors the explanation for scarce COMs in these sources. Conclusions. The chemical differences between WCCC sources and hot corino sources can be attributed to the variations in local environments, such as AVamb and ζ, as well as the protostellar property, tcont.

JWST Mid-infrared Spectroscopy Resolves Gas, Dust, and Ice in Young Stellar Objects in the Large Magellanic Cloud

In this work, we present spectra of 11 young stellar objects (YSOs) taken with the Mid-Infrared Instrument / Medium Resolution Spectroscopy (MRS) instrument on board the James Webb Space Telescope (JWST). The YSOs are located in the N79 region of the Large Magellanic Cloud (LMC), an active star-forming region with hundreds of Spitzer- and Herschel-identified YSOs and host to super star cluster (SSC) candidate H72.97-69.39. The three giant molecular clouds in N79 (East, West, and South) have varying star formation rates and stellar populations. MRS follow-up observations of four Spitzer-identified YSOs in N79 East, West, and South have revealed that what seemed to be a single, massive YSO is actually a cluster of YSOs. We discuss the emission and absorption lines of six YSOs that have complete or almost-complete spectral coverage from 4.9–27.9 μm. YSO Y3, located in N79 East, is the youngest source in this study and likely to be less than 10,000 yr old, as inferred from the prominent CH4, NH3, CH3OH, CH3OCHO, and CO2 ice absorption features. The most luminous source is the central ionizing YSO of SSC H72.97-69.39, Y4, which has dozens of fine-structure and H2 emission lines. Unlike the other YSOs in this work, Y4 has no polyaromatic hydrocarbon emission lines, due to the intense ionizing radiation destroying these large carbon-chain molecules. The mass accretion rates based on the H i (7-6) line luminosities of YSOs Y1, Y2, Y4, and Y9 range between 1.22 × 10−4–1.89 × 10−2 M ⊙ yr−1. For the first time in the mid-infrared, we are able to resolve individual high-mass protostars forming in small clusters in an extragalactic environment like the LMC.

(Invited) Electrosynthesis of Long-Chain Hydrocarbons and Oxygenates

The electroreduction of CO(2), driven by renewable electricity, can be used to sustainably generate synthetic fuels and chemical feedstocks. Long carbon chain molecules are of particular interest due to their high energy densities and utilities. However, to date, most of the CO(2) reduction products formed are limited to C2 and C3. Herein, we discuss the development of electrocatalysts for the reduction of CO(2) to long-chain oxygenates and hydrocarbons, and their underlying reaction mechanisms.CO(2) electroreduction has been previously reported to yield a range of carbonaceous products including alcohols, hydrocarbons and carboxylic acids. Interestingly, esters, an important family of organic compounds, have not been formed. We show how C3–C6 acetate esters could be produced from CO reduction in a membrane electrode assembly (MEA) cell. The near water-free reaction environment and the high local pH in the MEA played key roles in forming the esters with a Faradaic efficiency of 22 % and a current density up to −55 mA cm−2. We also disclose that inorganic nickel oxygenate (INO)-derived electrocatalysts can reduce CO2 to linear and branched C3-C6 hydrocarbons with Faradaic efficiencies up to 6.5%. We identified Niδ+ active sites which bind CO moderately. Our results show that Ni atom polarization to be important in hindering CO poisoning of nickel and thus facilitating CO2 reduction to a wider pool of valuable products.Finally, we show how MgAl layered double hydroxide (LDH) nanosheets ‘House-of-cards’ structures help to disperse CuO-derived Cu (OD-Cu) catalysts. The porous structures formed enhanced CO gas percolation through the catalyst layer. OD-Cu dispersed by the ‘House-of-cards’ structures catalysed CO electroreduction to C2+ products with a partial current density that is an order of magnitude higher than that shown by the unsupported OD-Cu.

Spectroscopic characterization and photochemistry of HC3N− and CH3C3N−: implications for ion chemistry in Titan's atmosphere

ABSTRACT Molecular ions are key intermediates in the build-up of chemical complexity in interstellar clouds. Among the more than 300 interstellar molecules, only eight negative ions, i.e. C2n+1N− (n = 0–3) and HC2n− (n = 2–5), have been astronomically observed. Understanding the formation mechanism of these ions under the interstellar conditions is essential for astrochemical modelling and establishing the astrochemical networks. Cyanopolyynes including the parent molecule HC3N are carbon-chain molecules that have been observed in a variety of astronomical objects such as the Titan's atmosphere. Herein, two cyanoacetylene anions HC3N‒ and CH3C3N‒ were generated in solid Ne matrix at 3 K and characterized with matrix-isolation infrared spectroscopy, as aided by isotopic substitutions and the ab initio calculations at the UCCSD(T)-F12a/cc-pVTZ-F12 level using second-order vibrational perturbation theory. Upon red-light irradiation at 625 nm, both ions undergo electron detachment by reformation of the neutral species. Importantly, the concomitant dehydrogenation of HC3N‒ has also been observed in the matrix, providing new insight into the intriguing mechanism for the formation of C3N‒ in the upper atmosphere of Titan.

Elusive Iron: Detection of the FeC Radical (X 3Δ i ) in the Envelope of IRC+10216

A new interstellar molecule, FeC (X 3Δ i ), has been identified in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC+10216. FeC is the second iron-bearing species conclusively observed in the interstellar medium, in addition to FeCN, also found in IRC+10216. The J = 4 → 3, 5 → 4, and 6 → 5 rotational transitions of this free radical near 160, 201, and 241 GHz, respectively, were detected in the lowest spin–orbit ladder, Ω = 3, using the Submillimeter Telescope of the Arizona Radio Observatory (ARO) for the 1 mm lines and the ARO 12 m at 2 mm. Because the ground state of FeC is inverted, these transitions are the lowest energy lines. The detected features exhibit slight U shapes with LSR velocities near V LSR ≈ −26 km s−1 and linewidths of ΔV 1/2 ≈ 30 km s−1, line parameters characteristic of IRC+10216. Radiative transfer modeling of FeC suggests that the molecule has a shell distribution with peak radius near 300 R * (∼6″) extending out to ∼500 R * (∼10″) and a fractional abundance, relative to H2, of f ∼ 6 × 10−11. The previous FeCN spectra were also modeled, yielding an abundance of f ∼ 8 × 10−11 in a larger shell situated near 800 R *. These distributions suggest that FeC may be the precursor species for FeCN. Unlike cyanides and carbon-chain molecules, diatomic carbides with a metallic element are rare in IRC+10216, with FeC being the first such detection.

Parsec scales of carbon chain and complex organic molecules in AFGL 2591 and IRAS 20126

Context. There is a diverse chemical inventory in protostellar regions leading to the classification of extreme types of systems. Warm carbon chain chemistry sources, for one, are the warm and dense regions near a protostar containing unsaturated carbon chain molecules. Since the presentation of this definition in 2008, there is a growing field to detect and characterise these sources. The details are lesser known in relation to hot cores and in high-mass star-forming regions - regions of great importance in galactic evolution. Aims. To investigate the prevalence of carbon chain species and their environment in high-mass star-forming regions, we have conducted targeted spectral surveys of two sources in the direction of Cygnus X - AFGL 2591 and IRAS 20126+4104. Methods. We observed these sources in frequency ranges around 85, 96, and 290 GHz with the Green Bank Telescope and the IRAM 30m Telescope. We have constructed a local thermodynamic equilibrium (LTE) model using the observed molecular spectra to determine the physical environment in which these molecules originate. We map both the observed spatial distribution and the physical parameters found from the LTE model. We also determine the formation routes of these molecules in each source using the three-phase NAUTILUS chemical evolution code. Results. We detect several lines of propyne, CH3CCH, and cyclopropenylidene, c-C3H2 as tracers of carbon chain chemistry, as well as several lines of formaldehyde, H2CO, and methanol, CH3OH, as a precursor and a tracer of complex organic molecule chemistry, respectively. We find excitation temperatures of 20−30 K for the carbon chains and 8−85 K for the complex organics. The observed abundances, used as input for the chemical evolution code, are 10−9 to 10−10 for both CH3CCH and CH3OH. The CH3CCH abundances are reproduced by a warm-up model, consistent with warm carbon chain chemistry, while the observed CH3OH abundances require a shock mechanism sputtering the molecules into the gas phase. Conclusions. Single-dish observations are useful for studying the envelope-scale chemistry of star-forming regions, including mechanisms such as warm carbon chain chemistry. As well, LTE models lend well to the wide-band maps obtained from these telescopes. The physical and chemical environment determined for complex hydrocarbons and complex organics lends understanding to high-mass star formation.

Attosecond probing and control of charge migration in carbon-chain molecule

Attosecond probing and control of charge migration in carbon-chain molecule

A 3 mm molecular line survey toward the C-star envelope CIT 6

Abstract We present an unbiased molecular line survey toward the carbon-rich circumstellar envelope CIT 6 carried out between 90 and 116 GHz with the Arizona Radio Observatory 12 m telescope. A total of 42 lines assigned to 10 molecular species and four isotopologues are detected. Despite the absence of any newly identified circumstellar molecules, several transitions are freshly reported for this object. This work is a complement to our previous line survey toward CIT 6 in the frequency ranges of 36–49 GHz and 131–268 GHz. Based on the measurements and in combination with previously published data, we perform a rotation-diagram analysis to determine the column densities, excitation temperatures, and fractional abundances of the molecules. The excitation temperature varies along the radius. The abundance pattern of CIT 6 is broadly consistent with that of the prototype C-star IRC +10216 at the specified detection sensitivity, indicating that the molecular richness in IRC +10216 cannot be attributed to an interpretation of unusual chemistry. Nevertheless, subtle distinctions between the two C-stars are found. The higher abundance of carbon-chain molecules and lower 12C$/$13C ratio in CIT 6 compared to IRC +10216 imply that the former is more massive or in a more evolved phase than the latter.

In-situ surface defects passivation with small carbon chain molecules for highly efficient, air-processed inorganic CsPbI2Br perovskite photovoltaics

All-inorganic CsPbI2Br perovskites have gained enormous interests owing to suitable band gap and thermal stability. However, the CsPbI2Br suffers from unsatisfactory crystal growth and poor surfaces processed under ambient conditions. Herein, we introduced small carbon chain molecules as liquid additives including diiodomethane (DIM), dibromoethane (DBM), and dichloromethane (DCM) into the precursor. The halide ions as associated with these additives played vital roles to passivate the surface defects. Interestingly, DIM additive offers multiple benefits: (i) passivate uncoordinated Pb2+ dangling bonds (ii) match band energy alignment, (iii) assist to preferential oriented crystal growth (iv) suppress halide ion vacancies and (v) improve surface morphology. As a result, 100 μl DIM-assisted perovskite solar cell (PSC) exhibited the highest power conversion efficiency (PCE) of 16.42 % which is higher than that of control (13.95 %). Notably, with additive engineering approach our champion PCE (16.42 %) is the highest to date under processing at ambient conditions. On the contrary to poor stability of control PSC (PCE drop of ∼ 50 %), the DIM assisted PSC retained ∼ 80 % of original PCE after aging for 600 hrs. Thus, our photovoltaic results demonstrated that the additive method could open an effective route for development of efficient and stable PSCs under ambient conditions.

The bending of C3: Experimentally probing the [formula omitted]-type doubling and resonance

C3, a pure carbon chain molecule that has been identified in different astronomical environments, is considered a good probe of kinetic temperatures through observation of transitions involving its low-lying bending mode (ν2) in its ground electronic state. The present laboratory work aims to investigate this bending mode with multiple quanta of excitation by combining recordings of high resolution optical and infrared spectra of C3 produced in discharge experiments. The optical spectra of rovibronic (Ã1Πu−X̃1Σg+) transitions have been recorded by laser induced fluorescence spectroscopy using a single longitude mode optical parametric oscillator as narrow bandwidth laser source at the University of Science and Technology of China. 36 bands originating from X̃(0v20), v2=0−5, are assigned. The mid-infrared spectrum of the rovibrational ν3 band has been recorded by Fourier-transform infrared spectroscopy using a globar source on the AILES beamline of the SOLEIL synchrotron facility. The spectrum reveals hot bands involving up to 5 quanta of excitation in ν2. From combining analyses of all the presently recorded spectra and literature data, accurate rotational parameters and absolute energy levels of C3, in particular for states involving the bending mode, are determined. A single PGOPHER file containing all available data involving the X̃ and à states (literature and present study) is used to fit all the data. The spectroscopic information derived from this work enables new interstellar searches for C3, not only in the infrared and optical regions investigated here but also notably in the ν2 band region (around 63 cm−1) where vibrational satellites can now be accurately predicted. This makes C3 a universal diagnostic tool to study very different astronomical environments, from dark and dense to translucent clouds.

A chemical study of nine star-forming regions with evidence of infall motion

ABSTRACT The study of the physical and chemical properties of gas infall motion in the molecular clumps helps us understand the initial stages of star formation. We used the FTS wide-sideband mode of the IRAM 30-m telescope to observe nine infall sources with significant double-peaked blue line profile. The observation frequency ranges are 83.7–91.5 GHz and 99.4–107.2 GHz. We have obtained numbers of molecular line data. Using xclass, a total of 7–27 different molecules and isotopic transition lines have been identified in these nine sources, including carbon chain molecules, such as CCH, c-C3H2 and HC3N. According to the radiation transfer model, we estimated the rotation temperatures and column densities of these sources. Chemical simulations adopting a physical model of HMSFRs are used to fit the observed molecular abundances. The comparison shows that most sources are in the early HMPO stage, with the inner temperature around several 10 K.

Astrochemical model to study the abundances of branched carbon-chain molecules in a hot molecular core with realistic binding energies

ABSTRACT Straight-chain (normal-propyl cyanide, $\rm {n-C_3H_7CN}$) and branched-chain (iso-propyl cyanide, $\rm {i-C_3H_7CN}$) alkyl cyanides are recently identified in the massive star-forming regions (Sgr B2(N) and Orion). These branched-chain molecules indicate that the key amino acids (side-chain structures) may also be present in a similar region. The process by which this branching could propagate towards the higher order (butyl cyanide, $\rm {C_4H_9CN}$) is an active field of research. Since the grain catalysis process could have formed a major portion of these species, considering a realistic set of binding energies are indeed essential. We employ quantum chemical calculations to estimate the binding energy of these species considering water as a substrate because water is the principal constituent of this interstellar ice. We find significantly lower binding energy values for these species than were previously used. It is noticed that the use of realistic binding energy values can significantly change the abundance of these species. The branching is more favourable for the higher order alkyl cyanides with the new binding energies. With the inclusion of our new binding energy values and one essential destruction reaction ($\rm {i-C_3H_7CN+H \rightarrow CH_3C(CH_3)CN + H_2}$, having an activation barrier of 947 K), abundances of $\rm {t-C_4H_9CN}$ dramatically increased.

Deuteration of c-C3H2 towards the pre-stellar core L1544

Context. In the centre of pre-stellar cores, the deuterium fractionation is enhanced due to the cold temperatures and high densities. Therefore, the chemistry of deuterated molecules can be used to probe the evolution and the kinematics in the earliest stages of star formation. Aims. We analyse emission maps of cyclopropenylidene, c-C3H2, to study the distribution of the deuteration throughout the prototypical pre-stellar core L1544. Methods. We used single-dish observations of c-C3H2, c-H13CC2H, c-C3HD, and c-C3D2 towards the pre-stellar core L1544, performed at the IRAM 30 m telescope. We derived the column density and deuterium fraction maps, and compared these observations with non-local thermodynamic equilibrium radiative transfer simulations. Results. The highest deuterium fractions are found close to the dust peak at the centre of L1544, where the increased abundance of H2D+ ions drives the deuteration process. The peak values are N(c-C3HD)/N(c-C3H2) = 0.17 ± 0.01, N(c-C3D2)/N(c-C3H2) = 0.025 ± 0.003, and N(c-C3D2)/N(c-C3HD) = 0.16 ± 0.03, which is consistent with previous single-pointing observations. The distributions of c-C3HD and c-C3D2 indicate that the deuterated forms of c-C3H2 in fact trace the dust peak and not the c-C3H2 peak. Conclusions. The N(c-C3D2)/N(c-C3HD) map confirms that the process of deuteration is more efficient towards the centre of the core and demonstrates that carbon-chain molecules are still present at high densities. This is likely caused by an increased abundance of He+ ions destroying CO, which increases the number of carbon atoms in the gas phase.

Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation

Context. Isotopologue abundance ratios are important to understand the evolution of astrophysical objects and ultimately the origins of a planetary system such as our own. With nitrogen being a fundamental ingredient of pre-biotic material, understanding its chemistry and inheritance is of fundamental importance to understand the formation of the building blocks of life. Aims. We aim to study the 14N/15N ratio in HCN, HNC, and CN across the prototypical pre-stellar core L1544. This study allows us to test the proposed fractionation mechanisms for nitrogen. Methods. We present here single-dish observations of the ground state rotational transitions of the 13C and 15N isotopologues of HCN, HNC, and CN with the IRAM 30 m telescope. We analyse their column densities and compute the 14N/15N ratio map across the core for HCN. The 15N fractionation of CN and HNC is computed towards different offsets across L1544. Results. The 15N-fractionation map of HCN towards a pre-stellar core is presented here for the first time. Our map shows a very clear decrease in the 14N/15N ratio towards the southern edge of L1544, where carbon chain molecules present a peak, strongly suggesting that isotope-selective photodissociation has a strong effect on the fractionation of nitrogen across pre-stellar cores. The 14N/15N ratio in CN measured towards four positions across the core also shows a decrease towards the south-east of the core, while HNC shows the opposite behaviour. We also measured the 12CN/13CN ratio towards four positions across the core. Conclusions. The uneven illumination of the pre-stellar core L1544 provides clear evidence that 15N fractionation of HCN and CN is enhanced towards the region more exposed to the interstellar radiation field. Isotope-selective photodissociation of N2 is then a crucial process to understand 15N fractionation, as already found in protoplanetary disks. Therefore, the 15N fractionation in pre-stellar material is expected to change depending on the environment within which pre-stellar cores are embedded. The 12CN/13CN ratio also varies across the core, but its variation does not affect our conclusions as to the effect of the environment on the fractionation of nitrogen. Nevertheless, the interplay between the carbon and nitrogen fractionation across the core warrants follow-up studies.