arXiv subjects

Eric Herbst

Publications and source records attributed to Eric Herbst.

At least 19 recordsLinked to original sources

arXiv2026

Carbon chain diversity in L1544 and IRAS 16293-2422: an astrochemical pathfinder study for the SKAO

Astrochemical observations have revealed a surprisingly high level of chemical complexity, including long carbon chains, in the earliest stages of Sun-like star formation. The origin of these species and whether they undergo further growth, possibly contributing to the molecular complexity of planetary systems, remain open questions. We present recent observations performed using the 100-m Green Bank Telescope of the prestellar core L1544 and the protostellar system IRAS 16293-2422. In L1544, we detected several complex carbon-bearing species, including $\mathrm{C_2S}$, $\mathrm{C_3S}$, $\mathrm{C_3N}$, $\mathrm{c\text{-}C_3H}$, $\mathrm{C_4H}$, and $\mathrm{C_6H}$, complementing previously reported emission of cyanopolyynes. In IRAS 16293-2422, we detected $\mathrm{c\text{-}C_3H}$ and, for the first time, $\mathrm{HC_7N}$. Thanks to the high spectral resolution, we refine the rest frequencies of several $\mathrm{c\text{-}C_3H}$ and $\mathrm{C_6H}$ transitions. We perform radiative transfer analysis, highlighting a chemical difference between the two sources: IRAS 16293-2422 shows column densities 10-100 times lower than L1544. We perform astrochemical modeling, employing an up-to-date chemical network with revised reaction rates. The models reproduce the general trends, with cyanopolyyne and polyynyl radical abundances decreasing as molecular size increases, but they underestimate the abundances of cyanopolyynes longer than $\mathrm{HC_5N}$ by up to two orders of magnitude. Current models, which include the dominant neutral-neutral formation routes, cannot account for this discrepancy, suggesting that the chemical network is incomplete. We propose that additional ion-molecule reactions are crucial for the formation of these species. Developing a more comprehensive chemical network for long carbon chains is essential for accurately interpreting present and future observations.

astro-ph.GA↗

arXiv2025

Role of Diffusive and Non-Diffusive Grain-Surface Processes in Cold Cores: Insights from the PEGASIS Three-Phase Astrochemical Model

Cold dense cores are unique among the structures found in the interstellar medium (ISM), as they harbor a rich chemical inventory, including complex organic molecules (COMs), which will be inherited by future evolutionary stages. These molecules exist both in the gas phase and as ices accreted onto grain surfaces. To model these environments, we present Pegasis, a new, fast, and extensible three-phase astrochemical code to explore the chemistry of cold cores, with an emphasis on the role of diffusive and non-diffusive chemistry in shaping their gas and grain chemical compositions. We incorporated the 2024 KIDA chemical network and compared our results with current astrochemical models. Using a traditional rate-equation-based approach, we implemented both diffusive and non-diffusive chemistry, coupled with either an inert or chemically active ice mantle. We identify crucial reactions that enhance the production of COMs through non-diffusive mechanisms on the grain surface as well as the mechanisms through which they can accumulate in the gas phase. Across all models with non-diffusive chemistry, we observe a definite enhancement in the concentration of COMs on both the grain surface as well as in the grain mantle. Finally, our model broadly reproduces the observed abundances of multiple gas-phase species in the cold dense core TMC-1 (CP) and provides insights into its chemical age. Our work demonstrates the capabilities of Pegasis in exploring a wide range of grain-surface chemical processes and modeling approaches for three-phase chemistry in the ISM, providing robust explanations for observed abundances in TMC-1 (CP). In particular, it highlights the role of non-diffusive chemistry in the production of gas-phase COMs on grain surfaces, which are subsequently chemically desorbed, especially when the precursors involved in their formation on the surfaces are heavier than atomic hydrogen.

astro-ph.GA↗

arXiv2025

The robust detection and spatial distribution of acetaldehyde in Orion KL: ALMA observations and chemical modeling

Despite the organic molecule inventory detected in Orion KL, acetaldehyde (CH3CHO) -- one of the most ubiquitous interstellar aldehydes -- has not been firmly identified with mm-wave interferometry. We analyze extensive ALMA archival datasets (142-355 GHz) to search for acetaldehyde, revealing two distinct acetaldehyde emission peaks and one component with more complex kinematic structures. One peak aligns with MF10/IRc2, where emissions of other O-bearing complex organic molecules are rarely reported, while the other is coincident with the ethanol peak in the hot core-SW. The MF10/IRc2 detection suggests unique chemistry, possibly influenced by repeated heating events. In contrast, co-detection with ethanol indicates an ice origin and suggests a potential chemical relationship between the two species. We determined acetaldehyde column densities and its kinetic temperatures toward these two peaks under LTE assumptions and compare its distribution with ethanol and other molecules that have an aldehyde (HCO) group, such as methyl formate, glycolaldehyde, and formic acid. Toward the ethanol peak, observed abundance ratios between HCO-containing species are analyzed using a chemical model. The model suggests two key points: 1) the destruction of ethanol to form acetaldehyde in the ice may contribute to the observed correlation between the two species, 2) a long cold-collapse timescale and a methyl formate binding energy similar to or lower than water are needed to explain the observations. The relative abundance ratios obtained from the model are highly sensitive to the assumed kinetic temperature, which accounts for the high spatial variability of the aldehyde ratios observed toward Orion KL.

astro-ph.GA↗

arXiv2024

Detections of interstellar 2-cyanopyrene and 4-cyanopyrene in TMC-1

Polycyclic aromatic hydrocarbons (PAHs) are among the most ubiquitous compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. We report the detection of 2- and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in TMC-1.

astro-ph.GA↗

arXiv2024

Multiple chemical tracers finally unveil the intricate NGC\,1333 IRAS\,4A outflow system. FAUST XVI

The exploration of outflows in protobinary systems presents a challenging yet crucial endeavour, offering valuable insights into the dynamic interplay between protostars and their evolution. In this study, we examine the morphology and dynamics of jets and outflows within the IRAS\,4A protobinary system. This analysis is based on ALMA observations of SiO(5--4), H$_2$CO(3$_{0,3}$--2$_{0,3}$), and HDCO(4$_{1,4}$--3$_{1,3}$) with a spatial resolution of $\sim$150\,au. Leveraging an astrochemical approach involving the use of diverse tracers beyond traditional ones has enabled the identification of novel features and a comprehensive understanding of the broader outflow dynamics. Our analysis reveals the presence of two jets in the redshifted emission, emanating from IRAS\,4A1 and IRAS\,4A2, respectively. Furthermore, we identify four distinct outflows in the region for the first time, with each protostar, 4A1 and 4A2, contributing to two of them. We characterise the morphology and orientation of each outflow, challenging previous suggestions of bends in their trajectories. The outflow cavities of IRAS\,4A1 exhibit extensions of 10$''$ and 13$''$ with position angles (PA) of 0$^{\circ}$ and -12$^{\circ}$, respectively, while those of IRAS\,4A2 are more extended, spanning 18$''$ and 25$''$ with PAs of 29$^{\circ}$ and 26$^{\circ}$. We propose that the misalignment of the cavities is due to a jet precession in each protostar, a notion supported by the observation that the more extended cavities of the same source exhibit lower velocities, indicating they may stem from older ejection events.

astro-ph.GA↗

arXiv2024

Maser Activity of Organic Molecules toward Sgr B2(N)

At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH$_2$NH, HNCNH, and HCOOCH$_3$. However, the lack of spatial distribution information of these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 North (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH$_3$OH) maser at 84 GHz and the first interferometric map of the methanimine (CH$_2$NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov-Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH$_3$OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH$_2$NH maser has a close spatial relationship with the 84 GHz CH$_3$OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions.

astro-ph.GA↗

arXiv2024

PDRs4All VIII: Mid-IR emission line inventory of the Orion Bar

Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.

astro-ph.GA↗

arXiv2024

A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photo-dissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, impacting planet formation within the disks. We report JWST and Atacama Large Millimetere Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modelling their kinematics and excitation allows us to constrain the physical conditions within the gas. We quantify the mass-loss rate induced by the FUV irradiation, finding it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk.

astro-ph.GA↗

arXiv2023

PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar

(Abridged) We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the HII region to the atomic PDR (crossing the ionisation front (IF)), and the subsequent transition to the molecular PDR (crossing the dissociation front (DF)). We use high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science Program. The NIRSpec data reveal a forest of lines including, but not limited to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. We observe numerous smaller scale structures whose typical size decreases with distance from Ori C and IR lines from CI, if solely arising from radiative recombination and cascade, reveal very high gas temperatures consistent with the hot irradiated surface of small-scale dense clumps deep inside the PDR. The H2 lines reveal multiple, prominent filaments which exhibit different characteristics. This leaves the impression of a "terraced" transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star- and planet formation as well as galaxy evolution.

astro-ph.GA↗

arXiv2023

Explaining the Chemical Inventory of Orion KL through Machine Learning

The interplay of the chemistry and physics that exists within astrochemically relevant sources can only be fully appreciated if we can gain a holistic understanding of their chemical inventories. Previous work by Lee et al. (2021) demonstrated the capabilities of simple regression models to reproduce the abundances of the chemical inventory of the Taurus Molecular Cloud 1 (TMC-1), as well as provide abundance predictions for new candidate molecules. It remains to be seen, however, to what degree TMC-1 is a ``unicorn'' in astrochemistry, where the simplicity of its chemistry and physics readily facilitates characterization with simple machine learning models. Here we present an extension in chemical complexity to a heavily studied high-mass star forming region: the Orion Kleinmann-Low (Orion KL) nebula. Unlike TMC-1, Orion KL is composed of several structurally distinct environments that differ chemically and kinematically, wherein the column densities of molecules between these components can have non-linear correlations that cause the unexpected appearance or even lack of likely species in various environments. This proof-of-concept study used similar regression models sampled by Lee et al. (2021) to accurately reproduce the column densities from the XCLASS fitting program presented in Crockett et al. (2014).

astro-ph.GA↗

arXiv2023

Rotational spectroscopy of the thioformaldehyde isotopologues H$_2$CS and H$_2$C$^{34}$S in four interacting excited vibrational states and an account on the rotational spectrum of thioketene, H$_2$CCS

An investigation of the rotational spectrum of the interstellar molecule thioformaldehyde between 110 and 377 GHz through a pyrolysis reaction revealed a multitude of absorption lines assignable to H$_2$CS and H$_2$C$^{34}$S in their lowest four excited vibrational states besides lines of numerous thioformaldehyde isotopologues in their ground vibrational states reported earlier as well as lines pertaining to several by-products. Additional transitions of H$_2$CS in its lowest four excited vibrational states were recorded in selected regions between 571 and 1386 GHz. Slight to strong Coriolis interactions occur between all four vibrational states with the exception of the two highest lying states because both are totally symmetric vibrations. We present combined analyses of the ground and the four interacting states for our rotational data of H$_2$CS and H$_2$C$^{34}$S. The H$_2$CS data were supplemented with two sets of high-resultion IR data in two separate analyses. The $v_2 = 1$ state has been included in analyses of Coriolis interactions of low-lying fundamental states of H$_2$CS for the first time and this improved the quality of the fits substantially. We extended furthermore assignments in $J$ of transition frequencies of thioketene in its ground vibrational state.

physics.chem-ph↗

arXiv2023

PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar

(Abridged) Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 $μ$m. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR, the atomic PDR, and the HII region. We use JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extract five template spectra to represent the morphology and environment of the Orion Bar PDR. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. While the spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 $μ$m, a wealth of weaker features and sub-components are present. We report trends in the widths and relative strengths of AIBs across the five template spectra. These trends yield valuable insight into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 $μ$m AIB emission from class B$_{11.2}$ in the molecular PDR to class A$_{11.2}$ in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a "weeding out" of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called 'grandPAHs'.

astro-ph.GA↗

arXiv2023

PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula

The JWST has captured the most detailed and sharpest infrared images ever taken of the inner region of the Orion Nebula, the nearest massive star formation region, and a prototypical highly irradiated dense photo-dissociation region (PDR). We investigate the fundamental interaction of far-ultraviolet photons with molecular clouds. The transitions across the ionization front (IF), dissociation front (DF), and the molecular cloud are studied at high-angular resolution. These transitions are relevant to understanding the effects of radiative feedback from massive stars and the dominant physical and chemical processes that lead to the IR emission that JWST will detect in many Galactic and extragalactic environments. Due to the proximity of the Orion Nebula and the unprecedented angular resolution of JWST, these data reveal that the molecular cloud borders are hyper structured at small angular scales of 0.1-1" (0.0002-0.002 pc or 40-400 au at 414 pc). A diverse set of features are observed such as ridges, waves, globules and photoevaporated protoplanetary disks. At the PDR atomic to molecular transition, several bright features are detected that are associated with the highly irradiated surroundings of the dense molecular condensations and embedded young star. Toward the Orion Bar PDR, a highly sculpted interface is detected with sharp edges and density increases near the IF and DF. This was predicted by previous modeling studies, but the fronts were unresolved in most tracers. A complex, structured, and folded DF surface was traced by the H2 lines. This dataset was used to revisit the commonly adopted 2D PDR structure of the Orion Bar. JWST provides us with a complete view of the PDR, all the way from the PDR edge to the substructured dense region, and this allowed us to determine, in detail, where the emission of the atomic and molecular lines, aromatic bands, and dust originate.

astro-ph.GA↗

arXiv2023

Molecular Abundance of the Circumnuclear Region Surrounding an Active Galactic Nucleus in NGC 1068 based on Imaging Line Survey in the 3-mm Band with ALMA

We present an imaging molecular line survey in the 3-mm band (85-114 GHz) focused on one of the nearest galaxies with an active galactic nucleus (AGN), NGC 1068, based on observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA). Distributions of 23 molecular transitions are obtained in the central ~3 kpc region, including both the circumnuclear disk (CND) and starburst ring (SBR) with 60 and 350 pc resolution. The column densities and relative abundances of all the detected molecules are estimated under the assumption of local thermodynamic equilibrium in the CND and SBR. Then, we discuss the physical and chemical effects of the AGN on molecular abundance corresponding to the observation scale. We found that H13CN, SiO, HCN, and H13CO+ are abundant in the CND relative to the SBR. In contrast, 13CO is more abundant in the SBR. Based on the calculated column density ratios of N(HCN)/N(HCO+), N(HCN)/N(CN), and other molecular distributions, we conclude that the enhancement of HCN in the CND may be due to high-temperature environments resulting from strong shocks, which are traced by the SiO emission. Moreover, the abundance of CN in the CND is significantly lower than the expected value of the model calculations in the region affected by strong radiation. The expected strong X-ray irradiation from the AGN has a relatively lower impact on the molecular abundance in the CND than mechanical feedback.

astro-ph.GA↗

arXiv2023

Chemical Differentiation around Five Massive Protostars Revealed by ALMA -Carbon-Chain Species, Oxygen-/Nitrogen-Bearing Complex Organic Molecules-

We present Atacama Large Millimeter/submillimeter Array Band 3 data toward five massive young stellar objects (MYSOs), and investigate relationships between unsaturated carbon-chain species and saturated complex organic molecules (COMs). An HC$_{5}$N ($J=35-34$) line has been detected from three MYSOs, where nitrogen(N)-bearing COMs (CH$_{2}$CHCN and CH$_{3}$CH$_{2}$CN) have been detected. The HC$_{5}$N spatial distributions show compact features and match with a methanol (CH$_{3}$OH) line with an upper-state energy around 300 K, which should trace hot cores. The hot regions are more extended around the MYSOs where N-bearing COMs and HC$_{5}$N have been detected compared to two MYSOs without these molecular lines, while there are no clear differences in the bolometric luminosity and temperature. We run chemical simulations of hot-core models with a warm-up stage, and compare with the observational results. The observed abundances of HC$_{5}$N and COMs show good agreements with the model at the hot-core stage with temperatures above 160 K. These results indicate that carbon-chain chemistry around the MYSOs cannot be reproduced by warm carbon-chain chemistry, and a new type of carbon-chain chemistry occurs in hot regions around MYSOs.

astro-ph.GA↗

arXiv2023

Detection of Interstellar $E$-1-cyano-1,3-butadiene in GOTHAM Observations of TMC-1

We report the detection of the lowest energy conformer of $E$-1-cyano-1,3-butadiene ($E$-1-C$_4$H$_5$CN), a linear isomer of pyridine, using the fourth data reduction of the GOTHAM deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We performed velocity stacking and matched filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1$σ$ level. We derive a total column density of $3.8^{+1.0}_{-0.9}\times 10^{10}$ cm$^{-2}$, which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code NAUTILUS and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of $2.34 \times 10^{10}\ \mathrm{cm}^{-2}$ for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by 1--4 orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitudes. We discuss implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules.

astro-ph.GA↗

arXiv2023

FAUST VII. Detection of A Hot Corino in the Prototypical Warm Carbon-Chain Chemistry Source IRAS 15398-3359

We have observed the low-mass protostellar source, IRAS 15398$-$3359, at a resolution of 0.$''$2-0.$''$3, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST, to examine the presence of a hot corino in the vicinity of the protostar. We detect nine CH$_3$OH lines including the high excitation lines with upper state energies up to 500 K. The CH$_3$OH rotational temperature and the column density are derived to be 119$^{+20}_{-26}$ K and 3.2$^{+2.5}_{-1.0}\times$10$^{18}$ cm$^{-2}$, respectively. The beam filling factor is derived to be 0.018$^{+0.005}_{-0.003}$, indicating that the emitting region of CH$_3$OH is much smaller than the synthesized beam size and is not resolved. The emitting region of three high excitation lines, 18$_{3,15}-18_{2,16}$, A ($E_u=$447 K), 19$_{3,16}-19_{2,17}$, A ($E_u=$491 K), and 20$_{3,17}-20_{2,18}$, A ($E_u=$537 K), is located within the 50 au area around the protostar, and seems to have a slight extension toward the northwest. Toward the continuum peak, we also detect one emission line from CH$_2$DOH and two features of multiple CH$_3$OCHO lines. These results, in combination with previous reports, indicate that IRAS 15398$-$3359 is a source with hybrid properties showing both hot corino chemistry rich in complex organic molecules on small scales $\sim$10 au) and warm carbon-chain chemistry (WCCC) rich in carbon-chain species on large scales ($\sim$100-1000 au). A possible implication of the small emitting region is further discussed in relation to the origin of the hot corino activity.

astro-ph.SR↗

arXiv2023

Cyanopolyyne chemistry in the L1544 prestellar core: new insights from GBT observations

We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100m Robert C. Byrd Green Bank Telescope (GBT) we observe 3 emission lines of HC$_3$N, 9 lines of HC$_5$N, 5 lines of HC$_7$N, and 9 lines of HC$_9$N. HC$_9$N is detected for the first time towards the source. The high spectral resolution ($\sim$ 0.05 km s$^{-1}$) reveals double-peak spectral line profiles with the redshifted peak a factor 3-5 brighter. Resolved maps of the core in other molecular tracers indicates that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modelling of the HC$_5$N, HC$_7$N, and HC$_9$N lines, to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5--12 K, a source size of 80$\arcsec$, and a gas density larger than 100 cm$^{-3}$. The HC$_5$N:HC$_7$N:HC$_9$N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those towards the the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and it is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in the light of the new observations.

astro-ph.GA↗