Abundances Of HCO Research Articles

The Ice Chemistry in Comets and Planet-forming Disks: Statistical Comparison of CH3OH, H2CO, and NH3 Abundance Ratios

Comets are frozen remnants of our solar system’s formation, and comparing their chemical composition to that of planet-forming systems can reveal crucial insights about our origins, potentially answering one of the most challenging questions in planetary science, i.e., whether cometary material was mainly inherited from the protosolar nebula or reprocessed during the solar system formation. Here we provide the first statistical analysis of methanol, formaldehyde, and ammonia abundances in 35 comets and 11 protostellar solar analogs and planet-forming disks. We show that comets from different dynamical families have comparable compositions on average, implying that their chemistry is preserved even after formation. While abundances retrieved from infrared and (sub)millimeter ground-based observations are in agreement, there are significant differences with those obtained via mass spectroscopy for 67P/Churyumov–Gerasimenko, target of the ESA-Rosetta mission; we discuss the implication of relying solely on the latter data for comparisons with disk abundance ratios. Finally, we find a significant difference in the [CH3OH]/[H2CO] ratio in comets observed within or farther than 1 au from the Sun, suggesting that temperature-activated mechanisms can enhance the H2CO production in the coma; this bias can strongly influence our understanding of comet chemistry in the context of planet formation. When compared to planet-forming systems, the [CH3OH]/[H2CO] and [NH3]/[CH3OH] molecular abundance ratios in comets are consistent with those measured in Class 0 hot corinos and in the inner regions of Class II disks, hence suggesting an inheritance scenario.

Ion irradiation triggers the formation of the precursors of complex organics in space

Context. Cosmic rays and solar energetic particles induce changes in the composition of compounds frozen onto dust grains in the interstellar medium (ISM), in comets, and on the surfaces of atmosphere-less small bodies in the outer Solar System. This induces the destruction of pristine compounds and triggers the formation of various species, including the precursors of complex organics. Aims. We investigate the role of energetic ions in the formation of formaldehyde (H2CO) and acetaldehyde (CH3CHO), which are observed in the ISM and in comets, and which are thought to be the precursors of more complex compounds such as hexamethylenete-tramine (HMT), which is found in carbonaceous chondrites and in laboratory samples produced after the irradiation and warm-up of astrophysical ices. Methods. We performed ion irradiation of water, methanol, and ammonia mixtures at 14–18 K. We bombarded frozen films with 40–200 keV H+ that simulate solar energetic particles and low-energy cosmic rays. Samples were analysed by infrared transmission spectroscopy. Results. Among other molecules, we observe the formation of H2CO and CH3CHO, and we find that their abundance depends on the dose and on the stoichiometry of the mixtures. We find that the H2CO abundance reaches the highest value after a dose of 10 eV/16u and then it decreases as the dose increases. Conclusions. The data suggest that surfaces exposed to high doses are depleted in H2CO. This explains why the amount of HMT in organic residues and that formed after irradiation of ices depends on the dose deposited in the ice. Because the H2CO abundance decreases at doses higher than 10 eV/16u, a lower quantity of H2CO is available to form HMT during the subsequent warm-up. The H2CO abundances caused by ion bombardment are insufficient to explain the ISM abundances, but ion bombardment can account for the abundance of CH3CHO towards the ISM and comets.

Observations of comet C/2020 F3 (NEOWISE) with IRAM telescopes

We present the results of millimetre-wave spectroscopic and continuum observations of the comet C/2020 F3 (NEOWISE) undertaken with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m and the NOrthern Extended Millimeter Array (NOEMA) telescopes on 22, 25–27 July, and 7 August 2020. Production rates of HCN, HNC, CH3OH CS, H2CO, CH3CN, H2S, and CO were determined with upper limits on six other species. The comet shows abundances within the range observed for other comets. The CO abundance is low (3.2% relative to water), while H2S is relatively abundant (1.1% relative to water). The H2CO abundance shows a steep variation with heliocentric distance, possibly related to a distributed production from the dust or macro-molecular source. The CH3OH and H2S production rates show a slower decrease post-perihelion than water. There was no detection of the nucleus point source contribution based on the interferometric map of the continuum (implying a size of r < 4.7 km), but this yielded an estimate of the dust production rate, leading to a relatively low dust-to-gas ratio of 0.7 ± 0.3 on 22.4 July 2020.

The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). VII. Chemistry of Embedded Dense Cores

We present a study of the chemistry toward 294 dense cores in 12 molecular clumps, using data obtained from the ALMA Survey of 70 μm dark High-mass clumps in Early Stages. We identified 97 protostellar cores and 197 prestellar core candidates, based on the detection of outflows and molecular transitions of high upper-energy levels (E u /k > 45 K). The detection rate of the N2D+ emission toward the protostellar cores is 38%, which is higher than 9% for the prestellar cores, indicating that N2D+ does not exclusively trace prestellar cores. The detection rates of the DCO+ emission are 35% for the prestellar cores and 49% for the protostellar cores, which are higher than those for N2D+, implying that DCO+ appears more frequently than N2D+ in both prestellar and protostellar cores. Both the N2D+ and DCO+ abundances appear to decrease from the prestellar to the protostellar stage. The DCN, C2D, and 13CS emission lines are rarely seen in the dense cores of early evolutionary phases. The detection rate of the H2CO emission toward dense cores is 52%, three times higher than that for CH3OH (17%). In addition, the H2CO detection rate, abundance, line intensities, and line widths increase with the core evolutionary status, suggesting that the H2CO line emission is sensitive to protostellar activity.

Revisiting the formaldehyde masers

Context. The 4.8 GHz formaldehyde (H2CO) masers are one of a number of rare types of molecular masers in the Galaxy. There still is not agreement on the mechanism responsible for the inversion of the 110−111 transition and the conditions under which an inversion can occur, and therefore how to interpret the masers. Aims. The aim of the present calculations is to explore a larger region of parameter space to improve on our previous calculations, thereby to better understand the range of physical conditions under which an inversion of the 110−111 transition occurs. We also aim to understand recently published results that H2CO masers are radiatively pumped. Methods. We solve the rate equations of the first 40 rotational levels of o-H2CO using a fourth-order Runge-Kutta method. We consider gas kinetic temperatures between 10 and 300 K, H2 densities between 104 and 106 cm−3, and a number of different dust temperatures and grey-body spectral energy density distributions. Results. We show that when using a black body radiation field the inversion of any transition will disappear as the kinetic temperature approaches the black-body radiation temperature since the system, consisting of the gas and radiation field, approaches thermodynamic equilibrium. Using a grey-body dust radiation field appropriate for Arp 220 we find that none of 110−111, 211−212, and 312−313 transitions are inverted for kinetic temperatures less than 100 K. Our calculations also show that in theory the 110−111 transition can be inverted over a large region of explored parameter space in the presence of an external far-infrared radiation field. Limiting the abundance of H2CO to less than 10−5, however, reduces the region where an inversion occurs to H2 densities ≳105 cm−3 and kinetic temperatures ≳100 K. We propose a pumping scheme for the H2CO masers which can explain why collisions play a central role in inverting the 110−111 transition, and therefore why an external radiation field alone does not lead to an inversion. Conclusions. Collisions are an essential mechanism for the inversion of the 110−111 transition. Our results suggest that 4.8 GHz H2CO megamasers are associated with hot and dense gas typical of high mass star forming regions rather than with cold material. Although limiting the H2CO abundance to less than 10−5 significantly reduces the region in parameter space where the 110−111 is inverted, it still is not clear whether this is the only reason why these masers are so rare.

X-ray-induced chemistry of water and related molecules in low-mass protostellar envelopes

Context. Water is a key molecule in star- and planet-forming regions. Recent water line observations toward several low-mass protostars suggest low water gas fractional abundances (<10−6 with respect to total hydrogen density) in the inner warm envelopes (r < 102 au). Water destruction by X-rays is thought to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood. Aims. Our aim is to understand the impact of X-rays on the composition of low-mass protostellar envelopes, focusing specifically on water and related oxygen-bearing species. Methods. We computed the chemical composition of two proto-typical low-mass protostellar envelopes using a 1D gas-grain chemical reaction network. We varied the X-ray luminosities of the central protostars, and thus the X-ray ionization rates in the protostellar envelopes. Results. The protostellar X-ray luminosity has a strong effect on the water gas abundances, both within and outside the H2O snowline (Tgas ~ 102 K, r ~ 102 au). Outside, the water gas abundance increases with LX, from ~10−10 for low LX to ~10−8–10−7 at LX > 1030 erg s−1. Inside, water maintains a high abundance of ~10−4 for LX ≲ 1029–1030 erg s−1, with water and CO being the dominant oxygen carriers. For LX ≳ 1030–1031 erg s−1, the water gas abundances significantly decrease just inside the water snowline (down to ~10−8–10−7) and in the innermost regions with Tgas ≳ 250 K (~10−6). For these cases, the fractional abundances of O2 and O gas reach ~10−4 within the water snowline, and they become the dominant oxygen carriers. In addition, the fractional abundances of HCO+ and CH3OH, which have been used as tracers of the water snowline, significantly increase and decrease, respectively, within the water snowline as the X-ray fluxes become larger. The fractional abundances of some other dominant molecules, such as CO2, OH, CH4, HCN, and NH3, are also affected by strong X-ray fields, especially within their own snowlines. These X-ray effects are larger in lower-density envelope models. Conclusions. X-ray-induced chemistry strongly affects the abundances of water and related molecules including O, O2, HCO+, and CH3OH, and can explain the observed low water gas abundances in the inner protostellar envelopes. In the presence of strong X-ray fields, gas-phase water molecules within the water snowline are mainly destroyed with ion-molecule reactions and X-ray-induced photodissociation. Future observations of water and related molecules (using, e.g., ALMA and ngVLA) will access the regions around protostars where such X-ray-induced chemistry is effective.

Planck Cold Clumps in the λ Orionis Complex. III. A Chemical Probe of Stellar Feedback on Cores in the λ Orionis Cloud

Abstract Massive stars have a strong impact on their local environments. However, how stellar feedback regulates star formation is still under debate. In this context, we studied the chemical properties of 80 dense cores in the Orion molecular cloud complex composed of the Orion A (39 cores), B (26 cores), and λ Orionis (15 cores) clouds using multiple molecular line data taken with the Korean Very Long Baseline Interferometry Network 21 m telescopes. The λ Orionis cloud has an H ii bubble surrounding the O-type star λ Ori, and hence it is exposed to the ultraviolet (UV) radiation field of the massive star. The abundances of C2H and HCN, which are sensitive to UV radiation, appear to be higher in the cores in the λ Orionis cloud than in those in the Orion A and B clouds, while the HDCO to H2CO abundance ratios show the opposite trend, indicating warmer conditions in the λ Orionis cloud. The detection rates of dense gas tracers such as the N2H+, HCO+, and H13CO+ lines are also lower in the λ Orionis cloud. These chemical properties imply that the cores in the λ Orionis cloud are heated by UV photons from λ Ori. Furthermore, the cores in the λ Orionis cloud do not show any statistically significant excess in the infall signature of HCO+ (1–0), unlike those in the Orion A and B clouds. Our results support the idea that feedback from massive stars impacts star formation in a negative way by heating and evaporating dense materials, as in the λ Orionis cloud.

Interaction of HCO+ Cations With Interstellar Negative Grains. Quantum Chemical Investigation and Astrophysical Implications

In cold galactic molecular clouds, dust grains are coated by icy mantles and are prevalently charged negatively, because of the capture of the electrons in the gas. The interaction of the charged grains with gaseous cations is known to neutralize them. In this work, we focus on the chemical consequences of the neutralization process of HCO+, often the most abundant cation in molecular clouds. More specifically, by means of electronic structure calculations, we have characterized the energy and the structure of all possible product species once the HCO+ ion adsorbs on water clusters holding an extra electron. Two processes are possible: (i) electron transfer from the negative water cluster to the HCO+ ion or (ii) a proton transfer from HCO+ to the negative water cluster. Energetic considerations favor electron transfer. Assuming this scenario, two limiting cases have been considered in astrochemical models: (a) all the neutralized HCO+ is retained as neutral HCO adsorbed on the ice and (b) all the neutralized HCO+ gets desorbed to the gas phase as HCO. None of the two limiting cases appreciably contribute to the HCO abundance on the grain surfaces or in the gas.

First Comet Observations with NIRSPEC-2 at Keck: Outgassing Sources of Parent Volatiles and Abundances Based on Alternative Taxonomic Compositional Baselines in 46P/Wirtanen

Abstract A major upgrade to the NIRSPEC instrument at the Keck II telescope was successfully completed in time for near-infrared spectroscopic observations of comet 46P/Wirtanen during its exceptionally close flyby of Earth in 2018 December. These studies determined the abundances of several volatiles, including C2H2, C2H6, CH3OH, NH3, HCN, H2CO, and H2O. Long-slit spatial distributions of gas rotational temperature and column density are diagnostic for the presence of icy grains in the coma and understanding if different volatiles are associated with common or distinct outgassing sources. These spatial distributions suggest that C2H2, C2H6, and HCN have a common outgassing source, whereas H2O and CH3OH have additional, more extended sources. The synergy of these findings with observations by space missions (Rosetta and EPOXI) motivates continuing studies to address whether or not C2H6, C2H2, and HCN have a common source of release (plausibly associated with CO2) in a larger sample of comets and whether systematic differences exist in the release of these species compared to H2O and CH3OH. Abundances of volatiles are reported relative to H2O, as traditionally done, as well as C2H6. While not unique, the choice of C2H6 demonstrates the value of extending the chemical taxonomy of parent volatiles in comets toward additional compositional “baselines” and, importantly, closer integration between coma abundances and the underlying volatile associations as revealed by spatial distributions. Our findings on composition and sources of outgassing include information relevant to future evaluations of 46P/Wirtanen as a prospective spacecraft target.

ATLASGAL-selected massive clumps in the inner Galaxy

Aims. We study ten molecular transitions obtained from an unbiased 3 mm molecular line survey using the IRAM 30 m telescope toward 409 compact dust clumps identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) to better understand the photodissociation regions (PDRs) associated with these clumps. The main goal of this study is to investigate whether the abundances of the selected molecules show any variations resulting from the PDR chemistry in different clump environments. Methods. We selected HCO, HOC+, C2H, c-C3H2, CN, H13CN, HC15N, and HN13C as PDR tracers, and H13CO+ and C18O as dense gas tracers. By using estimated optical depths of C2H and H13CN and assuming optically thin emission for other molecular transitions, we derived the column densities of those molecules and their abundances. To assess the influence of the presence and strength of ultra-violet radiation, we compare abundances of three groups of the clumps: HII regions, infrared bright non-HII regions, and infrared dark non-HII regions. Results. We detected C18O, H13CO+, C2H, c-C3H2, CN, and HN13C toward most of the observed dust clumps (detection rate >94%), and H13CN is also detected with a detection rate of 75%. On the other hand, HCO and HC15N show detection rates of 32 and 39%, respectively, toward the clumps, which are mostly associated with HII region sources: detection rates of HCO and HC15N toward the HII regions are 66 and 79%. We find that the abundances of HCO, CN, C2H, and c-C3H2 decrease as the H2 column density increases, indicating high visual extinction, while those of high-density tracers (i.e., H13CO+ and HC15N) are constant. In addition, N(HCO)/N(H13CO+) ratios significantly decrease as H2 column density increases, and, in particular, 82 clumps have X(HCO) ≳ 10−10 and N(HCO)/N(H13CO+) ≳ 1, which are indications of far-ultraviolet (FUV) chemistry. This suggests the observed HCO abundances are likely associated with FUV radiation illuminating the PDRs. We also find that high N(c-C3H2)/N(C2H) ratios found for HII regions that have high HCO abundances (≳10−10) are associated with more evolved clumps with high Lbol/Mclump. This trend might be associated with grain-surface processes, which determine the initial abundances of these molecules, and time-dependent effects in the clumps corresponding to the envelopes around dense PDRs and HII regions. In addition, some fraction of the measured abundances of the small hydrocarbons of the HII sources may be the result of the photodissociation of PAH molecules.

Collisional excitation of the formyl radical (HCO) by molecular hydrogen

ABSTRACT This paper addresses the need for accurate rate coefficients for transitions between fine- and hyperfine-structure resolved rotational transitions in the formyl (HCO) radical induced by collisions with the two nuclear spin modifications of H2, the dominant molecule in the interstellar medium (ISM). These rate coefficients, as well as radiative transition rate coefficients, are required for accurate determination of the abundance of HCO in the ISM. Time-independent close-coupling quantum scattering calculations have been used to compute rate coefficients for (de-)excitation of HCO in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of HCO with H2 recently computed by the explicitly correlated RCCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 5 to 400 K were calculated for all transitions among the fine and hyperfine levels associated with the first 22 rotational levels of HCO, whose energies are less than or equal to 144 K.

Simulating the circumstellar H2CO and CH3OH chemistry of young stellar objects using a spherical physical-chemical model

Context. Young stellar objects (YSOs) and their environments are generally geometrically and dynamically challenging to model, and the corresponding chemistry is often dominated by regions in non-thermal equilibrium. In addition, modern astrochemical models have to consider not only gas-phase reactions, but also solid-state reactions on icy dust grains. Solving the geometrical, physical, and chemical boundary conditions simultaneously requires a high computational effort and still runs the risk of false predictions due to the intrinsically non-linear effects that can occur. As a first step, solving problems of reduced complexity is helpful to guide more sophisticated approaches. Aims. The objective of this work is to test a model that uses shell-like structures (i.e., assuming a power-law number density and temperature gradient of the environment surrounding the YSO) to approximate the geometry and physical structure of YSOs, that in turn utilizes an advanced chemical model that includes gas-phase and solid-state reactions to model the chemical abundances of key species. A special focus is set on formaldehyde (H2CO) and methanol (CH3OH) as these molecules can be traced in the gas phase but are produced on icy dust grains. Furthermore, this kind of molecule is believed to be key to understanding the abundance of more complex species. We compare the influence of the geometry of the object on the molecular abundances with the effect induced by its chemistry. Methods. We set up a model that combines a grain-gas phase chemical model with a physical model of YSOs. The model ignores jets, shocks, and external radiation fields and concentrates on the physical conditions of spherically symmetric YSOs with a density and temperature gradient derived from available spectral energy distribution observations in the infrared. In addition, new observational data are presented using the APEX 12 m and the IRAM 30 m telescopes. Formaldehyde and methanol transitions have been searched for in three YSOs (R CrA-IRS 5A, C1333-IRAS 2A, and L1551-IRS 5) that can be categorized as Class 0 and Class 1 objects, and in the pre-stellar core L1544. The observed abundances of H2CO and CH3OH are compared with those calculated by the spherical physical-chemical model. Results. Compared to a standard “ρ and T constant” model, i.e., a homogeneous (flat) density and temperature distribution, using number density and temperature gradients results in reduced abundances for the CO hydrogenation products formaldehyde and methanol. However, this geometric effect is generally not large, and depends on the source and on the molecular species under investigation. Although the current model uses simplified geometric assumptions the observed abundances of H2CO and CH3OH are well reproduced for the quiescent Class 1 object R CrA-IRS 5A. Our model tends to overestimate formaldehyde and methanol abundances for sources in early evolutionary stages, like the pre-stellar core L1544 or NGC 1333-IRS 2A (Class 0). Observational results on hydrogen peroxide and water that have also been predicted by our model are discussed elsewhere.

On the Effects of UV Photons/X-Rays on the Chemistry of the Sgr B2 Cloud

Abstract The lines of HOC+, HCO, and CO+ are considered good tracers of photon-dominated regions (PDRs) and X-ray-dominated regions. We study these tracers toward regions of the Sgr B2 cloud selected to be affected by different heating mechanisms. We find the lowest values of the column density ratios of HCO+ versus HOC+, HCO, and CO+ in dense H ii gas, where UV photons dominate the heating and chemistry of the gas. The HOC+, HCO, and CO+ abundances and the above ratios are compared with those of chemical modeling, finding that high-temperature chemistry, a cosmic-ray ionization rate of 10−16 s−1, and timescales >105.0 yr explain well the HOC+ abundances in quiescent Sgr B2 regions, while shocks are also needed to explain the highest HCO abundances derived for these regions. The CO+ is mainly formed in PDRs, since the highest CO+ abundances of ∼(6–10) × 10−10 are found in H ii regions with electron densities >540 cm−3 and CO+ emission is undetected in quiescent gas. Among the ratios, the HCO+/HCO ratio is sensitive to the electron density, as it shows different values in dense and diffuse H ii regions. We compare SiO J = 2–1 emission maps of Sgr B2 with X-ray maps from 2004 and 2012. One known spot shown on the 2012 X-ray map is likely associated with molecular gas at velocities of 15–25 km s−1. We also derive the X-ray ionization rate of ∼10−19 s−1 for Sgr B2 regions pervaded by X-rays in 2004, which is quite low to affect the chemistry of the molecular gas.

Chemical and kinematic structure of extremely high-velocity molecular jets in the Serpens Main star-forming region

Context. Outflows are one of the first signposts of ongoing star formation. The fastest molecular component of protostellar outflows, extremely high-velocity (EHV) molecular jets, are still puzzling since they are seen only rarely. As they originate deep inside the embedded protostar-disk system, they provide vital information about the outflow-launching process in the earliest stages. Aims. The first aim is to analyze the interaction between the EHV jet and the slow outflow by comparing their outflow force content. The second aim is to analyze the chemical composition of the different outflow velocity components and to reveal the spatial location of molecules. Methods. The Atacama Large Millimeter/submillimeter Array 3 mm (Band 3) and 1.3 mm (Band 6) observations of five outflow sources at 0.′′3 – 0.′′6 (130–260 au) resolution in the Serpens Main cloud are presented. Observations of CO, SiO, H2CO, and HCN reveal the kinematic and chemical structure of those flows. The following three velocity components are distinguished: the slow and the fast wing, and the EHV jet. Results. Out of five sources, three have the EHV component. The comparison of outflow forces reveals that only the EHV jet in the youngest source, Ser-emb 8 (N), has enough momentum to power the slow outflow. The SiO abundance is generally enhanced with velocity, while HCN is present in the slow and the fast wing, but disappears in the EHV jet. For Ser-emb 8 (N), HCN and SiO show a bow-shock shaped structure surrounding one of the EHV peaks, thus suggesting sideways ejection creating secondary shocks upon interaction with the surroundings. Also, the SiO abundance in the EHV gas decreases with distance from this protostar, whereas it increases in the fast wing. H2CO is mostly associated with low-velocity gas, but, surprisingly, it also appears in one of the bullets in the Ser-emb 8 (N) EHV jet. No complex organic molecules are found to be associated with the outflows. Conclusions. The high detection rate suggests that the presence of the EHV jet may be more common than previously expected. The EHV jet alone does not contain enough outflow force to explain the entirety of the outflowing gas. The origin and temporal evolution of the abundances of SiO, HCN, and H2CO through high-temperature chemistry are discussed. The data are consistent with a low C/O ratio in the EHV gas versus a high C/O ratio in the fast and slow wings.

Organic molecules in the protoplanetary disk of DG Tauri revealed by ALMA

Context. Planets form in protoplanetary disks and inherit their chemical compositions. Aims. It is thus crucial to map the distribution and investigate the formation of simple organics, such as formaldehyde and methanol, in protoplanetary disks. Methods. We analyze ALMA observations of the nearby disk-jet system around the T Tauri star DG Tau in the o − H2CO 31, 2 − 21, 1 and CH3OH 3−2, 2 − 4−1, 4 E, 50, 5 − 40, 4 A transitions at an unprecedented resolution of $ {\sim}0{{\overset{\prime\prime}{.}}}{15} $, i.e., ∼18 au at a distance of 121 pc. Results. The H2CO emission originates from a rotating ring extending from ∼40 au with a peak at ∼62 au, i.e., at the edge of the 1.3 mm dust continuum. CH3OH emission is not detected down to an rms of 3 mJy beam−1 in the 0.162 km s−1 channel. Assuming an ortho-to-para ratio of 1.8−2.8 the ring- and disk-height-averaged H2CO column density is ∼0.3−4 × 1014 cm−2, while that of CH3OH is < 0.04−0.7 × 1014 cm−2. In the inner 40 au no o − H2CO emission is detected with an upper limit on its beam-averaged column density of ∼0.5−6 × 1013 cm−2. Conclusions. The H2CO ring in the disk of DG Tau is located beyond the CO iceline (RCO ∼ 30 au). This suggests that the H2CO abundance is enhanced in the outer disk due to formation on grain surfaces by the hydrogenation of CO ice. The emission peak at the edge of the mm dust continuum may be due to enhanced desorption of H2CO in the gas phase caused by increased UV penetration and/or temperature inversion. The CH3OH/H2CO abundance ratio is < 1, in agreement with disk chemistry models. The inner edge of the H2CO ring coincides with the radius where the polarization of the dust continuum changes orientation, hinting at a tight link between the H2CO chemistry and the dust properties in the outer disk and at the possible presence of substructures in the dust distribution.

First ALMA maps of HCO, an important precursor of complex organic molecules, towards IRAS 16293–2422

The formyl radical HCO has been proposed as the basic precursor of many complex organic molecules such as methanol (CH$_3$OH) or glycolaldehyde (CH$_2$OHCHO). Using ALMA, we have mapped, for the first time at high angular resolution ($\sim$1$^{\prime\prime}$, $\sim$140 au), HCO towards the Solar-type protostellar binary IRAS 16293$-$2422, where numerous complex organic molecules have been previously detected. We also detected several lines of the chemically related species H$_2$CO, CH$_3$OH and CH$_2$OHCHO. The observations revealed compact HCO emission arising from the two protostars. The line profiles also show redshifted absorption produced by foreground material of the circumbinary envelope that is infalling towards the protostars. Additionally, IRAM 30m single-dish data revealed a more extended HCO component arising from the common circumbinary envelope. The comparison between the observed molecular abundances and our chemical model suggests that whereas the extended HCO from the envelope can be formed via gas-phase reactions during the cold collapse of the natal core, the HCO in the hot corinos surrounding the protostars is predominantly formed by the hydrogenation of CO on the surface of dust grains and subsequent thermal desorption during the protostellar phase. The derived abundance of HCO in the dust grains is high enough to produce efficiently more complex species such as H$_2$CO, CH$_3$OH, and CH$_2$OHCHO by surface chemistry. We found that the main formation route of CH$_2$OHCHO is the reaction between HCO and CH$_2$OH.

3D modelling of HCO+ and its isotopologues in the low-mass proto-star IRAS16293−2422

Ions and electrons play an important role in various stages of the star formation process. By following the magnetic field of their environment and interacting with neutral species, they slow down the gravitational collapse of the proto-star envelope. This process (known as ambipolar diffusion) depends on the ionisation degree, which can be derived from the \hco abundance. We present a study of \hco and its isotopologues (H$^{13}$CO$^+$, HC$^{18}$O$^+$, DCO$^+$, and H$^{13}$CO$^+$) in the low-mass proto-star IRAS16293$-$2422. The structure of this object is complex, and the HCO$^+$ emission arises from the contribution of a young NW-SE outflow, the proto-stellar envelope and the foreground cloud. We aim at constraining the physical parameters of these structures using all the observed transitions. For the young NW-SE outflow, we derive $T_{\rm kin}=180-220$ K and $n({\rm H_2})=(4-7)\times10^6$ cm$^{-3}$ with an HCO$^+$ abundance of $(3-5)\times10^{-9}$. Following previous studies, we demonstrate that the presence of a cold ($T_{\rm kin}$$\leqslant$30 K) and low density ($n({\rm H_2})\leqslant1\times10^4$ cm$^{-3}$) foreground cloud is also necessary to reproduce the observed line profiles. We have used the gas-grain chemical code \textsc{nautilus} to derive the HCO$^+$ abundance profile across the envelope and the external regions where X(HCO$^+$)$\gtrsim1\times10^{-9}$ dominate the envelope emission. From this, we derive an ionisation degree of $10^{-8.9}\,\lesssim\,x(e)\,\lesssim\,10^{-7.9}$. The ambipolar diffusion timescale is $\sim$5 times the free-fall timescale, indicating that the magnetic field starts to support the source against gravitational collapse and the magnetic field strength is estimated to be $6-46 \mu$G.

Is the gas-phase OH+H2CO reaction a source of HCO in interstellar cold dark clouds? A kinetic, dynamic and modelling study.

Chemical kinetics of neutral-neutral gas-phase reactions at ultralow temperatures is a fascinating research subject with important implications on the chemistry of complex organic molecules in the interstellar medium (T∼10-100K). Scarce kinetic information is currently available for this kind of reactions at T<200 K. In this work we use the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, which means Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients (k) of the gas-phase OH+H2CO reaction between 22 and 107 K. k values greatly increase from 2.1×10-11 cm3 s-1 at 107 K to 1.2×10-10 cm3 s-1 at 22 K. This is also confirmed by quasi-classical trajectories (QCT) at collision energies down to 0.1 meV performed using a new full dimension and ab initio potential energy surface, recently developed which generates highly accurate potential and includes long range dipole-dipole interactions. QCT calculations indicate that at low temperatures HCO is the exclusive product for the OH+H2CO reaction. In order to revisit the chemistry of HCO in cold dense clouds, k is reasonably extrapolated from the experimental results at 10K (2.6×10-10 cm3 s-1). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 105-106 yrs. The different sources of production of HCO are presented and the uncertainties in the chemical networks discussed. This reaction can be expected to be a competitive process in the chemistry of prestellar cores. The present reaction is shown to account for a few percent of the total HCO production rate. Extensions to photodissociation regions and diffuse clouds environments are also commented.

H2CO Distribution and Formation in the TW HYA Disk

Abstract H2CO is one of the most readily detected organic molecules in protoplanetary disks. Yet its distribution and dominant formation pathway(s) remain largely unconstrained. To address these issues, we present ALMA observations of two H2CO lines ( and ) at 0.″5 (∼30 au) spatial resolution toward the disk around the nearby T Tauri star TW Hya. Emission from both lines is spatially resolved, showing a central depression, a peak at 0.″4 radius, and a radial decline at larger radii with a bump at ∼1″, near the millimeter continuum edge. We adopt a physical model for the disk and use toy models to explore the radial and vertical H2CO abundance structure. We find that the observed emission implies the presence of at least two distinct H2CO gas reservoirs: (1) a warm and unresolved inner component (&lt;10 au), and (2) an outer component that extends from ∼15 au to beyond the millimeter continuum edge. The outer component is further constrained by the line ratio to arise in a more elevated disk layer at larger radii. The inferred H2CO abundance structure agrees well with disk chemistry models, which predict efficient H2CO gas-phase formation close to the star, and cold H2CO grain surface formation, through H additions to condensed CO, followed by non-thermal desorption in the outer disk. The implied presence of active grain surface chemistry in the TW Hya disk is consistent with the recent detection of CH3OH emission, and suggests that more complex organic molecules are formed in disks, as well.

THE DETECTION OF A HOT MOLECULAR CORE IN THE LARGE MAGELLANIC CLOUD WITH ALMA

ABSTRACT We report the first detection of a hot molecular core outside our Galaxy based on radio observations with ALMA toward a high-mass young stellar object (YSO) in a nearby low metallicity galaxy, the Large Magellanic Cloud (LMC). Molecular emission lines of CO, C17O, HCO+, H13CO+, H2CO, NO, SiO, H2CS, 33SO, 32SO2, 34SO2, and 33SO2 are detected from a compact region (∼0.1 pc) associated with a high-mass YSO, ST11. The temperature of molecular gas is estimated to be higher than 100 K based on rotation diagram analysis of SO2 and 34SO2 lines. The compact source size, warm gas temperature, high density, and rich molecular lines around a high-mass protostar suggest that ST11 is associated with a hot molecular core. We find that the molecular abundances of the LMC hot core are significantly different from those of Galactic hot cores. The abundances of CH3OH, H2CO, and HNCO are remarkably lower compared to Galactic hot cores by at least 1–3 orders of magnitude. We suggest that these abundances are characterized by the deficiency of molecules whose formation requires the hydrogenation of CO on grain surfaces. In contrast, NO shows a high abundance in ST11 despite the notably low abundance of nitrogen in the LMC. A multitude of SO2 and its isotopologue line detections in ST11 imply that SO2 can be a key molecular tracer of hot core chemistry in metal-poor environments. Furthermore, we find molecular outflows around the hot core, which is the second detection of an extragalactic protostellar outflow. In this paper, we discuss the physical and chemical characteristics of a hot molecular core in the low metallicity environment.