Prestellar Cores Research Articles

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.

Tracking the Ice Mantle History in the Solar-type Protostars of NGC 1333 IRAS 4

To understand the origin of the diversity observed in exoplanetary systems, it is crucial to characterize the early stages of their formation, represented by solar-type protostars. Likely, the gaseous chemical content of these objects directly depends on the composition of the dust-grain mantles formed before the collapse. Directly retrieving the ice mantle composition is challenging, but it can be done indirectly by observing the major components, such as NH3 and CH3OH at centimeter wavelengths, once they are released into the gas phase during the warm protostellar stage. We observed several CH3OH and NH3 lines toward three Class 0 protostars in NGC 1333 (IRAS 4A1, IRAS 4A2, and IRAS 4B), at high angular resolution (1″; ∼300 au) with the VLA interferometer at 24–26 GHz. Using a non-LTE LVG analysis, we derived a similar NH3/CH3OH abundance ratio in the three protostars (≤0.5, 0.015–0.5, and 0.003–0.3 for IRAS 4A1, 4A2, and 4B, respectively). Hence, we infer they were born from precollapse material with similar physical conditions. Comparing the observed abundance ratios with astrochemical model predictions, we constrained the dust temperature at the time of the mantle formation to be ∼17 K, which coincides with the average temperature of the southern NGC 1333 diffuse cloud. We suggest that a brutal event started the collapse that eventually formed IRAS 4A1, 4A2, and 4B, which, therefore, did not experience the usual prestellar core phase. This event could be the clash of a bubble with NGC 1333 South, which has previously been evoked in the literature.

Desorption of organic molecules from interstellar ices, combining experiments and computer simulations: Acetaldehyde as a case study

Context. Explaining the presence of complex organic molecules (COMs) in interstellar environments requires a thorough understanding of the physics and chemistry occurring in the interplay between the gas phase and interstellar surfaces. Experiments and computer simulations are pivotal in building a comprehensive catalogue of processes of relevance for the build up of organic molecules in those environments. Aims. We combine experiments with tailored computer simulations to study the desorption dynamics of acetaldehyde CH3CHO – an important organic precursor in cold interstellar environments - on amorphous solid water for the first time. Our goals with this paper are twofold. Firstly, we want to contextualise the role of this molecule in the evolution of organic molecules in space. Secondly, we want to suggest a joint scheme to produce quantitative information on desorption magnitudes based on the combination of computations and experiments. This scheme can be adopted to refine measurements of other molecules. Methods. We determined desorption energies and pre-exponential factors of desorption theoretically using molecular dynamics simulations that combine semi-empirical and density functional calculations. We also performed temperature-programmed desorption experiments with acetaldehyde on top of non-porous amorphous solid water. The combination of theoretical and experimental results allows us to derive reliable quantities, which are required for understanding the desorption dynamics of interstellar COMs (iCOMs) atop interstellar ices. Results. The average theoretical and experimental desorption energies found for CH3CHO desorbing from non-porous amorphous solid water (np-ASW) surfaces are 3624 K and 3774 K, respectively. The pre-exponential factor determined theoretically is vtheo = 2.4 × 1012 s−1 while from the experiments it was possible to constrain this magnitude to 1012±1 s−1. Conclusions. The comparison of the desorption energies of CH3CHO with other COMs, such as CH3NH2 or CH3NO, shows that CH3CHO is more volatile. Therefore, we suggest that, in consideration of the average binding energy, CH3CHO should undergo preferential desorption during the ice-sublimation phase in hot cores enriching the gas-phase in this particular component. In addition, the overall low binding energy suggests a possible early return to the gas phase of pre-stellar cores due to non-thermal effects (i.e. reactive desorption or cosmic-ray-induced desorption). This could explain the prevalence of CH3CHO in the gas phase of pre-stellar cores. Dedicated laboratory and theoretical efforts are required to confirm this last point.

The initial magnetic criticality of pre-stellar cores

ABSTRACT Direct observational measurements of the magnetic field strength in pre-stellar cores typically find supercritical mass-to-flux ratios, suggesting that the magnetic field is insufficient to prevent gravitational collapse. These measurements suffer from significant uncertainties; an alternative approach is to utilize the sensitivity of pre-stellar chemistry to the evolutionary history, and indirectly constrain the degree of magnetic support. We combine non-ideal magnetohydrodynamic simulations of pre-stellar cores with time-dependent chemistry and radiative transfer modelling, producing synthetic observations of the model cores in several commonly observed molecular lines. We find that molecules strongly affected by freeze-out, such as CS and HCN, typically have much lower line intensities in magnetically subcritical models compared to supercritical ones, due to the longer collapse time-scales. Subcritical models also produce much narrower lines for all species investigated. Accounting for a range of core properties, ages, and viewing angles, we find that supercritical models are unable to reproduce the distribution of CS and N2H+ line strengths and widths seen in an observational sample, whereas subcritical models are in good agreement with the available data. This suggests that despite presently having supercritical mass-to-flux ratios, pre-stellar cores form as magnetically subcritical objects.

A survey of deuterated ammonia in the Cepheus star-forming region L1251

ABSTRACT Understanding the chemical processes during starless core and prestellar core evolution is an important step in understanding the initial stages of star and disc formation. This project is a study of deuterated ammonia, o-NH2D, in the L1251 star-forming region towards Cepheus. Twenty-two dense cores (20 of which are starless or prestellar, and two of which have a protostar), previously identified by p-NH3 (1,1) observations, were targeted with the 12m Arizona Radio Observatory telescope on Kitt Peak. o-NH2D J$_{\rm {K_a} \rm {K_c}}^{\pm } =$$1_{11}^{+} \rightarrow 1_{01}^{-}$ was detected in 13 (59 per cent) of the NH3-detected cores with a median sensitivity of $\sigma _{T_{mb}} = 17$ mK. All cores detected in o-NH2D at this sensitivity have p-NH3 column densities >1014 cm−2. The o-NH2D column densities were calculated using the constant excitation temperature (CTEX) approximation while correcting for the filling fraction of the NH3 source size. The median deuterium fraction was found to be 0.11 (including 3σ upper limits). However, there are no strong, discernible trends in plots of deuterium fraction with any physical or evolutionary variables. If the cores in L1251 have similar initial chemical conditions, then this result is evidence of the cores physically evolving at different rates.

Discovery of CH2CCHC4H and a rigorous detection of CH2CCHC3N in TMC-1 with the QUIJOTE line survey

Using the QUIJOTE line survey in the 32.0–50.4 GHz range, we report the discovery of the molecule CH2CCHC4H towards the prestellar cold core TMC-1 in the Taurus region. We also present a rigorous detection of CH2CCHC3N, along with its detailed analysis. We identified a total of twenty rotational transitions for each one of these molecules. The rotational quantum numbers range from Ju = 17 up to 24 and Ka≤ 3. The column density for CH2CCHC4H is N = (2.2 ± 0.2) × 1012 cm−2, while for CH2CCHC3N, we derived N = (1.2 ± 0.15) × 1011 cm−2. The rotational temperature is 9.0 ± 0.5 K for both species. The abundance ratiobetween CH2CCHC4H and CH2CCHC3N is 18 ± 4. We also compared the column densities of these species with those of their isomers CH3C6H and CH3C5N, derived from their J = 20 − 10 up to J = 31 − 30 rotational transitions observed with the QUIJOTE line survey. The observed abundances for all these species are reasonably well explained by state-of-the-art chemical models of TMC-1. The observed astronomical frequencies were merged with laboratory frequencies from the literature to derive improved spectroscopic parameters.

The Effects of Cosmic Rays on the Chemistry of Dense Cores

Cosmic rays are crucial to the chemistry of molecular clouds and their evolution. They provide essential ionizations, dissociations, heating, and energy to the cold, dense cores. As cosmic rays pierce through clouds they are attenuated and lose energy, which leads to a dependency on the column density of a system. The detailed effects these particles have on the central regions still need to be fully understood. Here, we revisit how cosmic rays are treated in the UCLCHEM chemical modeling code by including both ionization rate and H2 dissociation rate dependencies alongside the production of cosmic ray induced excited species and we study in detail the effects of these treatments on the chemistry of pre-stellar cores. We find that these treatments can have significant effects on chemical abundances, up to several orders of magnitude, depending on the physical conditions. The ionization dependency is the most significant treatment influencing chemical abundances through the increased presence of ionized species, grain desorptions, and enhanced chemical reactions. Comparisons to chemical abundances derived from observations show the new treatments reproduce these observations better than the standard handling. It is clear that more advanced treatments of cosmic rays are essential to chemical models and that including this type of dependency provides more accurate chemical representations.

Icy molecule desorption in interstellar grain collisions

ABSTRACT Observations of gaseous complex organic molecules (COMs) in cold starless and prestellar cloud cores require efficient desorption of the COMs and their parent species from icy mantles on interstellar grains. With a simple astrochemical model, we investigate if mechanical removal of ice fragments in oblique collisions between grains in two size bins (0.01 and 0.1 µm) can substantially affect COM abundances. Two grain collision velocities were considered – 10 and 50 m s−1, corresponding to realistic grain relative speeds arising from ambipolar diffusion and turbulence, respectively. From the smaller grains, the collisions are assumed to remove a spherical cap with height equal to 1/3 and 1 ice mantle thickness, respectively. We find that the turbulence-induced desorption can elevate the gas-phase abundances of COMs by several orders of magnitude, reproducing observed COM abundances within an order of magnitude. Importantly, the high gaseous COM abundances are attained for long time-scales of up to 1 Myr and for a rather low methanol ice abundance, common for starless cores. The simple model, considering only two grain size bins and several assumptions, demonstrates a concept that must be tested with a more sophisticated approach.

A semi-analytical model for the temporal evolution of the episodic disc-to-star accretion rate during star formation

ABSTRACT We develop a semi-analytical formalism for the determination of the evolution of the stellar mass accretion rate for specified density and velocity profiles that emerge from the runaway collapse of a prestellar cloud core. In the early phase, when the infall of matter from the surrounding envelope is substantial, the star accumulates mass primarily because of envelope-induced gravitational instability in a protostellar disc. In this phase, we model the envelope mass accretion rate from the isothermal free-fall collapse of a molecular cloud core. The disc gains mass from the envelope, and transports matter to the star via a disc accretion mechanism that includes episodic gravitational instability and mass accretion bursts according to the Toomre Q-criterion. In a later phase, mass is accreted on to the star due to gravitational torques within the spiral structures in the disc, in a manner that analytical theory suggests has a mass accretion rate ∝t−6/5. Our model provides a self-consistent evolution of the mass accretion rate by joining the spherical envelope accretion (dominant at the earlier stage) with the disc accretion (important at the later stage), and accounts for the presence of episodic accretion bursts at appropriate times. We show using a simple example that the burst mode can provide a good match to the observed distribution of bolometric luminosities. Our framework reproduces key elements of detailed numerical simulations of disc accretion and can aid in developing intuition about the basic physics as well as to compare theory with observations.

SOLIS

Context.Deuteration is a precious tool for investigating the origin and formation routes of interstellar complex organic molecules in the different stages of the star formation process. Methyl cyanide (CH3CN) is one of the most abundant interstellar complex organic molecules (iCOMs); it is of particular interest because it is among the very few iCOMs detected not only around protostars but also in protoplanetary disks. However, its formation pathways are not well known and only a few measurements of its deuterated isotopologue (CH2DCN) have been made to date.Aims.We studied the line emission from CH3CN and its deuterated isotopologue CH2DCN towards the prototypical Class I object SVS13-A, where the deuteration of a large number of species has already been reported. Our goal is to measure the CH3CN deuteration in a Class I protostar, for the first time, in order to constrain the CH3CN formation pathways and the chemical evolution from the early prestellar core and Class 0 to the evolved Class I stages.Methods.We imaged CH2DCN towards SVS13-A using the IRAM NOEMA interferometer at 3mm in the context of the Large Program SOLIS (with a spatial resolution of 1″.8 × 1″.2). The NOEMA images were complemented by the CH3CN and CH2DCN spectra collected by the IRAM-30m Large Program ASAI, which provided an unbiased spectral survey at 3 mm, 2 mm, and 1.3 mm. The observed line emission was analysed using local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) approaches.Results.The NOEMA/SOLIS images of CH2DCN show that this species emits in an unresolved area centred towards the SVS13-A continuum emission peak, suggesting that methyl cyanide and its isotopologues are associated with the hot corino of SVS13-A, previously imaged via other iCOMs. In addition, we detected 41 and 11 ASAI transitions of CH3CN and CH2DCN, respectively, which cover upper level energies (Eup) from 13 to 442 K and from 18 K to 200 K. The non-LTE LVG analysis of the CH3CN lines points to a kinetic temperature of (140 ± 20) K, a gas density nH2≥ 107 cm−3, and an emitting size of ~0″.3, in agreement with the hypothesis that CH3CN lines are emitted in the SVS13-A hot corino. The derived [CH2DCN]/[CH3CN] ratio is ~9%. This value is consistent with those measured towards prestellar cores and a factor 2–3 higher than those measured in Class 0 protostars.Conclusions.Contrarily to what expected for other molecular species, the CH3CN deuteration does not show a decrease in SVS13-A with respect to measurements in younger prestellar cores and Class 0 protostars. Finally, we discuss why our new results suggest that CH3CN was likely synthesised via gas-phase reactions and frozen onto the dust grain mantles during the cold prestellar phase.

Synthetic observations using POLARIS: an application to simulations of massive prestellar cores

Young massive stars are usually found embedded in dense and massive molecular clumps which are known for being highly obscured and distant. During their formation process, the degree of deuteration can be used as a potential indicator of the very early formation stages. This is particularly effective when employing the abundance of H2D+. However, its low abundances and large distances make detections in massive sources hard to achieve. We present an application of the radiative transfer code POLARIS, with the goal to test the observability of the ortho-H2D+ transition 1_{10}-1_{11} (∼372.42 GHz) using simulations of high-mass collapsing cores that include deuteration chemistry. We analyzed an early and a late stage of the collapse of a 60 M⊙ core, testing different source distances. For all cases, we generated synthetic single-dish and interferometric observations and studied the differences in both techniques. The column densities we derive are comparable to values reported for similar sources. These estimates depend on the extent over which they are averaged, and sources with compact emission they can be highly affected by beam dilution. Combined ALMA-ACA observations improve in signal-to-noise ratio and lead to better column density estimates as compared to ALMA alone. We confirm the feasibility to study ortho-H2D+ emission up to distances of ∼7 kpc. We provide a proof-of-concept of our framework for synthetic observations and highlight its importance when comparing numerical simulations with real observations. This work also proves how relevant it is to combine single-dish and interferometric measurements to derive appropriate source column densities.

Discovery of a 500 au Protobinary in the Massive Prestellar Core G11.92–0.61 MM2

We present high-resolution (≲160 au) Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm observations of the high-mass prestellar core candidate G11.92−0.61 MM2, which reveal that this source is in fact a protobinary system with a projected separation of 505 au. The binary components, MM2E and MM2W, are compact (radii <140 au) sources within the partially optically thick dust emission with α 0.9 cm−1.3 mm = 2.47–2.94. The 1.3 mm brightness temperatures, T b = 68.4/64.6 K for MM2E/MM2W, imply internal heating and minimum luminosities L * > 24.7 L ⊙ for MM2E and L * > 12.6 L ⊙ for MM2W. The compact sources are connected by a “bridge” of lower-surface-brightness dust emission and lie within more extended emission that may correspond to a circumbinary disk. The circumprotostellar gas mass, estimated from ∼0.″2 resolution VLA 0.9 cm observations assuming optically thin emission, is 6.8 ± 0.9 M ⊙. No line emission is detected toward MM2E and MM2W in our high-resolution 1.3 mm ALMA observations. The only line detected is 13CO J = 2–1, in absorption against the 1.3 mm continuum, which likely traces a layer of cooler molecular material surrounding the protostars. We also report the discovery of a highly asymmetric bipolar molecular outflow that appears to be driven by MM2E and/or MM2W in new deep, ∼0.″5 resolution (1685 au) ALMA 0.82 mm observations. This outflow, traced by low-excitation CH3OH emission, indicates ongoing accretion onto the protobinary system. Overall, the super-Alfvénic models of Mignon-Risse et al. agree well with the observed properties of the MM2E/MM2W protobinary, suggesting that this system may be forming in an environment with a weak magnetic field.

An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion

Spectral lines of ammonia, NH3, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH3 observations with the Very Large Array and Green Bank Telescope toward a prestellar core. These observations show an outer region with a fractional NH3 abundance of X(NH3) = (1.975 ± 0.005) × 10−8 (±10% systematic), but it also reveals that, after all, the X(NH3) starts to decrease above a H2 column density of ≈2.6 × 1022 cm−2. We derive a density model for the core and find that the break point in the fractional abundance occurs at the density n(H2) ∼ 2 × 105 cm−3, and beyond this point the fractional abundance decreases with increasing density, following the power law n −1.1. This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to n −1 owing to the dominance of accretion in the central parts of starless cores.

H2CS deuteration maps towards the pre-stellar core L1544

Context. Deuteration is a crucial tool for understanding the complexity of interstellar chemical processes, especially when they involve the interplay of gas-phase and grain-surface chemistry. In the case of multiple deuteration, comparing observation with the results of chemical modelling is particularly effective to study how molecules are inherited in the different stages within the process of star and planet formation. Aims. We aim to study the D/H ratio in H2CS across the prototypical pre-stellar core L1544. This study allows us to test current gas–dust chemical models involving sulfur in dense cores. Methods. Here, we present single-dish observations of H2CS, HDCS and D2CS with the IRAM 30 m telescope. We analysed their column densities and distributions and compared these observations with gas–grain chemical models. The deuteration maps of H2CS in L1544 were compared with the deuteration maps of methanol, H2CO, N2H+, and HCO+ towards the same source. Furthermore, we compared the single and double deuteration of H2CS towards the dust peak of L1544 with H2CO and c-C3H2. The difference between the deuteration of these molecules in L1544 is discussed and compared with the prediction of chemical models. Results. The maximum deuterium fractionation for the first deuteration of H2CS is N(HDCS)/N(H2CS) ~ 30% and is located towards the north-east at a distance of about 10000 AU from the dust peak. While for c-C3H2 the first and second deuteration have a similar efficiency, for H2CS and H2CO the second deuteration is more efficient, leading to D2CX/HDCX ~ 100% (with X = O or S). Conclusions. Our results imply that the large deuteration of H2CO and H2CS observed in protostellar cores as well as in comets is likely inherited from the pre-stellar phase. However, comparison with state-of-the-art chemical models suggests that the reaction network for the formation of the doubly deuterated H2CS and H2CO it is not complete yet.

Herschel Investigation of Cores and Filamentary Structures in the Perseus Molecular Cloud

Abstract Cores and filamentary structures are the prime birthplaces of stars, and play key roles in the process of star formation. Latest advances in the methods of multi-scale source and filament extraction, and in making high-resolution column density map from Herschel multi-wavelength observations enable us to detect the filamentary network structures in highly complex molecular cloud environments. The statistics for physical parameters shows that core mass strongly correlates with core dust temperature, and M/L strongly correlates with M/T, which is in line with the prediction of the blackbody radiation, and can be used to trace evolutionary sequence from unbound starless cores to robust prestellar cores. Crest column densities of the filamentary structures are clearly related with mass per unit length (M line), but are uncorrelated by three orders ranging from ∼1020 to ∼1022 cm−2 with widths. Full width at half maximum has a median value of 0.15 pc, which is consistent with the 0.1 pc typical inner width of the filamentary structures reported by previous research. We find 70% of robust prestellar cores (135/199) embedded in supercritical filaments with M line &gt; 16 M ⊙ pc−1, which implies that the gravitationally bound cores come from fragmentation of supercritical filaments. On the basis of observational evidence that the probability distribution function with power-law distribution in the Perseus south is flatter than in the north, the number of young stellar objects in the south is significantly less than that in the north, and dust temperature is different. We infer that the south region is more gravitationally bound than the north region.

Fevering Interstellar Ices Have More CH3OD

Monodeuterated methanol is thought to form during the prestellar core stage of star formation. Observed variations in the CH2DOH/CH3OD ratio suggest that its formation is strongly dependent on the surrounding cloud conditions. Thus, it is a potential tracer of the physical conditions before the onset of star formation. A single-point physical model representative of a typical prestellar core is coupled to chemical models to investigate potential formation pathways toward deuterated methanol at the prestellar stage. Simple addition reactions of H and D are not able to reproduce observed abundances. The implementation of an experimentally verified abstraction scheme leads to the efficient formation of methyl-deuterated methanol, but lacks sufficient formation of hydroxy-deuterated methanol. CH3OD is most likely formed at a later evolutionary stage, potentially from H–D exchange reactions in warm ices between HDO (and D2O) and CH3OH. The CH2DOH/CH3OD ratio is not an appropriate tracer of the physical conditions during the prestellar stage, but might be better suited as a tracer of ice heating.

The Central 1000 au of a Prestellar Core Revealed with ALMA. II. Almost Complete Freeze-out

Abstract Prestellar cores represent the initial conditions in the process of star and planet formation. Their low temperatures (&lt;10 K) allow the formation of thick icy dust mantles, which will be partially preserved in future protoplanetary disks, ultimately affecting the chemical composition of planetary systems. Previous observations have shown that carbon- and oxygen-bearing species, in particular CO, are heavily depleted in prestellar cores due to the efficient molecular freeze-out onto the surface of cold dust grains. However, N-bearing species such as NH3 and, in particular, its deuterated isotopologues appear to maintain high abundances where CO molecules are mainly in the solid phase. Thanks to ALMA, we present here the first clear observational evidence of NH2D freeze-out toward the L1544 prestellar core, suggestive of the presence of a “complete depletion zone” within a ≃1800 au radius, in agreement with astrochemical prestellar core model predictions. Our state-of-the-art chemical model coupled with a non-LTE radiative transfer code demonstrates that NH2D becomes mainly incorporated in icy mantles in the central 2000 au and starts freezing out already at ≃7000 au. Radiative transfer effects within the prestellar core cause the NH2D(111 − 101) emission to appear centrally concentrated, with a flattened distribution within the central ≃3000 au, unlike the 1.3 mm dust continuum emission, which shows a clear peak within the central ≃1800 au. This prevented NH2D freeze-out from being detected in previous observations, where the central 1000 au cannot be spatially resolved.

Chemical survey of Class I protostars with the IRAM-30 m

Context. Class I protostars are a bridge between Class 0 protostars (≤105 yr old), and Class II (≥106 yr) protoplanetary disks. Recent studies show gaps and rings in the dust distribution of disks younger than 1 Myr, suggesting that planet formation may start already at the Class I stage. To understand what chemistry planets will inherit, it is crucial to characterize the chemistry of Class I sources and to investigate how chemical complexity evolves from Class 0 protostars to protoplanetary disks. Aims. There are two goals: (i) to perform a census of the molecular complexity in a sample of four Class I protostars, and (ii) to compare the data with the chemical compositions of earlier and later phases of the Sun-like star formation process. Methods. We performed IRAM-30 m observations at 1.3 mm towards four Class I objects (L1489-IRS, B5-IRS1, L1455-IRS1, and L1551-IRS5). The column densities of the detected species were derived assuming local thermodynamic equilibrium (LTE) or large velocity gradients (LVGs). Results. We detected 27 species: C-chains, N-bearing species, S-bearing species, Si-bearing species, deuterated molecules, and interstellar complex organic molecules (iCOMs; CH3OH, CH3CN, CH3CHO, and HCOOCH3). Among the members of the observed sample, L1551-IRS5 is the most chemically rich source. Different spectral profiles are observed: (i) narrow lines (~1 km s−1) towards all the sources, (ii) broader lines (~4 km s−1) towards L1551-IRS5, and (iii) line wings due to outflows (in B5-IRS1, L1455-IRS1, and L1551-IRS5). Narrow c-C3H2 emission originates from the envelope with temperatures of 5–25 K and sizes of ~2′′−10′′. The iCOMs in L1551-IRS5 reveal the occurrence of hot corino chemistry, with CH3OH and CH3CN lines originating from a compact (~0.′′15) and warm (T &gt; 50 K) region. Finally, OCS and H2S seem to probe the circumbinary disks in the L1455-IRS1 and L1551-IRS5 binary systems. The deuteration in terms of elemental D/H in the molecular envelopes is: ~10−70% (D2CO/H2CO), ~5−15% (HDCS/H2CS), and ~1−23% (CH2DOH/CH3OH). For the L1551-IRS5 hot corino we derive D/H ~2% (CH2DOH/CH3OH). Conclusions. Carbon chain chemistry in extended envelopes is revealed towards all the sources. In addition, B5-IRS1, L1455-IRS1, and L1551-IRS5 show a low-excitation methanol line that is narrow and centered at systemic velocity, suggesting an origin from an extended structure, plausibly UV-illuminated. The abundance ratios of CH3CN, CH3CHO, and HCOOCH3 with respect to CH3OH measured towards the L1551-IRS5 hot corino are comparable to that estimated at earlier stages (prestellar cores, Class 0 protostars), and to that found in comets. The deuteration in our sample is also consistent with the values estimated for sources at earlier stages. These findings support the inheritance scenario from prestellar cores to the Class I phase when planets start forming.

Sampling molecular gas in the Helix planetary nebula: Variation in HNC/HCN with UV flux

Context. Observations of molecular clouds, prestellar cores, and protoplanetary disks have established that the HNC/HCN ratio may be a potent diagnostic of molecular gas physical conditions. The processes that govern the relative abundances of these molecules nevertheless remain poorly understood. Aims. We seek to exploit the wide range of UV irradiation strengths within the ∼pc diameter Helix planetary nebula to explore the potential role of UV radiation in driving HNC/HCN. Methods. We performed IRAM 30 m and APEX 12 m radio line observations across six positions within the Helix Nebula, making use of radiative transfer and photodissociation modeling codes to interpret the results for line intensities and line ratios in terms of the molecular gas properties. Results. We have obtained the first detections of the plasma-embedded Helix molecular knots (globules) in HCN, HNC, HCO+, and other trace molecules. Analysis of the HNC/HCN integrated line intensity ratio reveals an increase with radial distance from the Helix central star. In the context of molecular line ratios of other planetary nebulae from the literature, the HNC/HCN ratio appears to be anticorrelated with UV emission over four orders of magnitude in incident flux. Models of the photodissociation regions within the Helix using the RADEX and Meudon codes reveal strong constraints on the column density (1.5–2.5 × 1012 cm−2) of the molecular gas, as well as pressure and temperature. Analysis of the molecular ion HCO+ across the Helix indicates that X-ray irradiation is likely driving HCO+ production in the outer regions of planetary nebulae, where photodissociation is limited but cold gas and ionized molecules are abundant. Conclusions. Although the observational results clearly indicate that UV irradiation is important in determining the HNC/HCN ratio, our photodissociation region modeling indicates that the UV flux gradient alone cannot reproduce the observed variation in HNC/HCN across the Helix Nebula. Instead, HNC/HCN appears to be dependent on both UV irradiation and gas pressure and density.

UCLCHEMCMC: An MCMC Inference Tool for Physical Parameters of Molecular Clouds

Abstract We present the publicly available open-source code UCLCHEMCMC, designed to estimate physical parameters of an observed cloud of gas by combining Markov chain Monte Carlo (MCMC) sampling with chemical and radiative transfer modeling. When given the observed values of different emission lines, UCLCHEMCMC runs a Bayesian parameter inference, using an MCMC algorithm to sample the likelihood and produce an estimate of the posterior probability distribution of the parameters. UCLCHEMCMC takes a full forward-modeling approach, generating model observables from the physical parameters via chemical and radiative transfer modeling. While running UCLCHEMCMC, the created chemical models and radiative transfer code results are stored in an SQL database, preventing redundant model calculations in future inferences. This means that the more UCLCHEMCMC is used, the more efficient it becomes. Using UCLCHEM and RADEX, the increase oin efficiency is nearly two orders of magnitude, going from 5185.33 ± 1041.96 s for 10 walkers to take 1000 steps when the database is empty, to 68.89 ± 45.39 s when nearly all models requested are in the database. In order to demonstrate its usefulness, we provide an example inference of UCLCHEMCMC to estimate the physical parameters of mock data, and perform two inferences on the well-studied prestellar core, L1544, one of which shows that it is important to consider the substructures of an object when determining which emission lines to use.