H2CO Abundances Research Articles

Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

Gas-phase molecules in cometary atmospheres (comae) originate primarily from (1) outgassing by the nucleus, (2) sublimation of icy grains in the near-nucleus coma, and (3) coma (photo)chemical processes. However, the majority of cometary gases observed at radio wavelengths have yet to be mapped, so their production/release mechanisms remain uncertain. Here we present observations of six molecular species toward comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array during the comet’s unusually close (∼0.1 au) approach to Earth in 2018 December. Interferometric maps of HCN, CH3OH, CH3CN, H2CO, CS, and HNC were obtained at an unprecedented sky-projected spatial resolution of up to 25 km, enabling the nucleus and coma sources of these molecules to be accurately quantified. The HCN, CH3OH, and CH3CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH3OH originating from sublimation of icy grains in the near-nucleus coma (at a scale length L p = 36 ± 7 km). On the other hand, H2CO, CS, and HNC originate primarily from distributed coma sources (with L p values in the range 550–16,000 km), the identities of which remain to be established. The HCN, CH3OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H2CO, CH3CN, and CS abundances are relatively low.

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.

Physical and chemical differentiation of the luminous star-forming region W49A

The massive and luminous star-forming region W49A is a well known Galactic candidate to probe the physical conditions and chemistry similar to those expected in external starburst galaxies. We aim to probe the physical and chemical structure of W49A on a spatial scale of ~0.8 pc based on the JCMT Spectral Legacy Survey, which covers the frequency range between 330 and 373 GHz. The wide 2x2 arcminutes field and the high spectral resolution of the HARP instrument on JCMT provides information on the spatial structure and kinematics of the cloud. For species where multiple transitions are available, we estimate excitation temperatures and column densities. We detected 255 transitions corresponding to 60 species in the 330-373 GHz range at the center position of W49A. Excitation conditions can be probed for 16 molecules. The chemical composition suggests the importance of shock-, PDR-, and hot core chemistry. Many molecular lines show a significant spatial extent across the maps including high density tracers (e.g. HCN, HNC, CS, HCO+) and tracers of UV-irradiation (e.g. CN and C2H). Large variations are seen between the sub-regions with mostly blue-shifted emission toward the Eastern tail, mostly red-shifted emission toward the Northern clump, and emission peaking around the expected source velocity toward the South-west clump. A comparison of column density ratios of characteristic species observed toward W49A to Galactic PDRs suggests that while the chemistry toward the W49A center is driven by a combination of UV-irradiation and shocks, UV-irradiation dominates for the Northern Clump, Eastern tail, and South-west clump regions. A comparison to a starburst galaxy and an AGN suggests similar C2H, CN, and H2CO abundances (with respect to the dense gas tracer 34CS) between the ~0.8 pc scale probed for W49A and the >1 kpc regions in external galaxies with global star-formation.

Combined quantum chemical and modeling study of CO hydrogenation on water ice

Context. Successive hydrogenation reactions of CO on interstellar icy grain surfaces are considered one of the most e cient mechanisms in interstellar environments for the formation of H2CO and CH3OH, two of the simplest organic molecules detected in space. In the past years, several experimental and theoretical works have been focused on these reactions, providing relevant information both at the macroscopic and atomic scale. However, several questions still remain open, such as the exact role played by water in these processes, a crucial aspect because water is the dominant constituent of the ice mantles around dust grain cores. Aims. We here present a quantum chemical description of the successive H additions to CO both in the gas phase and on the surfaces of several water clusters. Methods. The hydrogenation steps were calculated by means of accurate quantum chemical methods and structural cluster models consisting of 3, 18, and 32 water molecules. Results. Our main result is that the interaction of CO and H2CO with the water cluster surfaces through H-bonds with the O atoms increases the C O polarization, thus weakening the C O bond. Consequently, the C atoms are more prone to receiving H atoms, which in turn lowers the energy barriers for the H additions compared to the gas-phase processes. The calculated energy barriers and transition frequencies associated with the reaction coordinate were adopted as input parameters in our numerical model of the surface chemistry (GRAINOBLE) to simulate the distribution of the H2CO and CH3OH ice abundances (with respect to water). Our GRAINOBLE results based on the energy barriers and transition frequencies for the reactions on the 32 water molecule cluster compare well with the observed abundances in low-mass protostars and dark cores.

H2CO in the Horsehead PDR: photo-desorption of dust grain ice mantles

Aims: For the first time we investigate the role of the grain surface chemistry in the Horsehead Photo-dissociation region (PDR). Methods: We performed deep observations of several H2CO rotational lines toward the PDR and its associated dense-core in the Horsehead nebula, where the dust is cold ($\Td \simeq 20-30$ K). We complemented these observations with a map of the p-H2CO $~3_{03}-2_{02}$ line at 218.2 GHz (with 12" angular resolution). We determine the H2CO abundances using a detailed radiative transfer analysis and compare these results with PDR models that include either pure gas-phase chemistry or both gas-phase and grain surface chemistry. Results: The H2CO abundances ($\simeq 2-3 \times 10^{-10}$) with respect to H-nuclei are similar in the PDR and dense-core. In the dense-core the pure gas-phase chemistry model reproduces the observed H2CO abundance. Thus, surface processes do not contribute significantly to the gas-phase H2CO abundance in the core. In contrast, the formation of H2CO on the surface of dust grains and subsequent photo-desorption into the gas-phase are needed in the PDR to explain the observed gas-phase H2CO abundance, because the gas-phase chemistry alone does not produce enough H2CO. The assignments of different formation routes are strengthen by the different measured ortho-to-para ratio of H2CO: the dense-core displays the equilibrium value ($\sim3$) while the PDR displays an out-of-equilibrium value ($\sim2$). Conclusions: Photo-desorption of H2CO ices is an efficient mechanism to release a significant amount of gas-phase H2CO into the Horsehead PDR.

THE PARENT VOLATILE COMPOSITION OF 6P/d’ARREST AND A CHEMICAL COMPARISON OF JUPITER-FAMILY COMETS MEASURED AT INFRARED WAVELENGTHS

The volatile abundances in Jupiter-family comet (JFC) 6P/d'Arrest were measured on two dates (UT 2008 July 28 and August 11) using high-dispersion (λ/Δλ ~ 2.8 × 104) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory. H2O, CH3OH, and C2H6 were detected on both dates and H2CO and NH3 were detected on UT August 11. A tentative detection of HCN and an upper limit for C2H2 are also reported on UT August 11. CH3OH, H2CO, and NH3 abundances relative to H2O are in the typical range, while C2H6, C2H2, and HCN are all moderately to severely depleted compared to other comets. Gas production increased by about a factor of 2 from July 28 to August 11. Relative abundances of C2H6 and CH3OH with respect to H2O are consistent on these dates within the limitations of the measurements. Rotational temperatures were determined for H2O on UT August 11 and found to be about 40 K. The spatial distributions of H2O and C2H6 are both asymmetric with peak flux about 100 km in the anti-sunward direction compared to the more symmetric dust distribution. A comparison between the abundances of daughter fragments (e.g., C2, CN, NH, and NH2) and their suspected parents (e.g., C2H2, C2H6, HCN, and NH3) shows that while the abundance of NH3 in 6P is consistent with it being the primary source of NH and NH2, there is insufficient C2H2, C2H6, and HCN to account for C2 and CN in 6P. This is in contrast to other measured JFCs where abundances of C2 and CN are consistent with C2H2, C2H6, and HCN as their primary parents to within the accuracy of the measurements. Thus, optical taxonomic classifications of comets may not correlate well to taxonomic classifications based on parent volatile chemistry in some cases.

H2CO and CH3OH abundances in the envelopes around low-mass protostars

This paper presents the third in a series of single-dish studies of molecular abundances in the envelopes around a large sample of 18 low-mass pre- and protostellar objects. It focuses on typical grain mantle products and organic molecules, including H2CO, CH3OH and CH3CN. With a few exceptions, all H2CO lines can be fit by constant abundances of throughout the envelopes if ortho- and para lines are considered independently. The current observational dataset does not require a large H2CO abundance enhancement in the inner warm regions, but this can also not be ruled out. Through comparison of the H2CO abundances of the entire sample, the H2CO ortho-para ratio is constrained to be consistent with thermalization on grains at temperatures of 10-15 K. The H2CO abundances can be related to the empirical chemical network established on the basis of our previously reported survey of other species and is found to be closely correlated with that of the nitrogen-bearing molecules. These correlations reflect the freeze-out of molecules at low temperatures and high densities, with the constant H2CO abundance being a measure of the size of the freeze-out zone. An improved fit to the data is obtained with a “drop” abundance structure in which the abundance is typically a few 10-10 when the temperature is lower than the evaporation temperature and the density high enough so that the timescale for depletion is less than the lifetime of the core. The location of the freeze-out zone is constrained from CO observations. Outside the freeze-out zone, the H2CO abundance is typically a few . The observations show that the CH3OH lines are significantly broader than the H2CO lines, indicating that they probe kinematically distinct regions. CH3OH is moreover only detected toward a handful of sources and CH3CN toward only one, NGC 1333-IRAS2. For NGC 1333-IRAS2, CH3OH and CH3CN abundance enhancements of two-three orders of magnitude at temperatures higher than 90 K are derived. In contrast, the NGC 1333-IRAS4A and IRAS4B CH3OH data are fitted with a constant abundance and an abundance enhancement at a lower temperature of 30 K, respectively. This is consistent with a scenario where CH3OH probes the action of compact outflows on the envelopes, which is further supported by comparison to high frequency, high excitation CS and HDO line profiles which uniquely probe warm, dense gas. The extent to which the outflow dominates the abundance enhancements compared with the passively heated inner envelope depends on the filling factors of the two components in the observing beam.

Outgassing behavior and composition of comet C/1999 S4 (LINEAR) during its disruption.

The gas activity of comet C/1999 S4 (LINEAR) was monitored at radio wavelengths during its disruption. A runaway fragmentation of the nucleus may have begun around 18 July 2000 and proceeded until 23 July. The mass in small icy debris (</=30-centimeter radius) was comparable to the mass in the large fragments seen in optical images. The mass budget after breakup suggests a small nucleus ( approximately 100- to 300-meter radius) that had been losing debris for weeks. The HNC, H2CO, H2S, and CS abundances relative to H2O measured during breakup are consistent with those obtained in other comets. However, a deficiency in CH3OH and CO is observed.

The physics and chemistry of small molecular clouds in the Galactic plane. 2: H2CO

view Abstract Citations (13) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Physics and Chemistry of Small Molecular Clouds in the Galactic Plane. II. H 2CO Turner, B. E. Abstract We have made extensive observations of 2 and 6 cm H2CO in all 27 of the Clemens-Barvainis small molecular clouds for which several structural models including hydrostatic equilibrium polytropes were developed in an earlier paper based on C(18)O and (13)CO observations. We have observed the 211-110 line at 2.0 mm in 11 of the CB objects and in 10 of 11 cirrus cores earlier studied in C(18)O, (13)CO, and H2CO. As with the cirrus cores, the three H2CO lines in CB objects are all well fitted by both polytropic models and ad hoc n approximately r-1 models, using the external UV fields derived in the earlier papers. The reanalysis of the cirrus cores includes the 2 mm H2CO lines as well as treating the C-12/C-13 ratio as a variable, and yields approximately 40% higher fractional abundances than the earlier analysis, as well as giving equal preference to both centrally peaked and radially flat distributions of the H2CO fractional abundance. The same central H2CO abundances are found for the CB objects, but these objects favor radially flat abundance distributions, possibly because of beam dilution of the 6 cm lines speculated as unaccounted for in the detailed estimates made using maps of every source. As before, no clear preference is shown for polytropic or r-1 structures although r-1 is favored for a subset of 11 objects with 211-110 data. The large central abundances derived for both types of object (mean value 1.4 x 10-8 for ortho H2CO) are too large by a factor 104 to be compatible with gas-phase formation of H2CO. Grain formation is indicated, as concluded earlier for cirrus cores. It is argued that photocatalysis on grains is consistent with either peaked or flat H2CO abundance distributions, but this cannot be tested conclusively within the uncertainties of determining the structures or the abundance distributions. By including consistently the effects of UV radiation fields and electron excitation, our models fit accurately all four lines of C(18)O and (13)CO, and all three lines of H2CO so far observed. The derived extinctions and H2CO abundances are definitely determined, and significant deviations from hydrostatic equilibrium are ruled out. The CB objects and cirrus cores are indistinguishable physically or chemically in terms of these characteristics, which seem to represent the conditions to be found in all small, low mass molecular clouds regardless of Galactic latitiude. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/175029 Bibcode: 1994ApJ...437..658T Keywords: Abundance; Astronomical Models; Chemical Analysis; Interstellar Matter; Molecular Clouds; Radio Spectra; Ultraviolet Spectra; Chemical Elements; Flux Density; Hydrostatics; Photodissociation; Polytropic Processes; Radio Telescopes; Astrophysics; ISM: ABUNDANCES; ISM: CLOUDS; ISM: MOLECULES; RADIO LINES: ISM full text sources ADS | data products SIMBAD (35) Related Materials (12) Part 1: 1994ApJ...420..661T Part 3: 1995ApJ...444..708T Part 4: 1995ApJ...449..635T Part 5: 1995ApJ...455..556T Part 6: 1996ApJ...461..246T Part 7: 1996ApJ...468..694T Part 8: 1997ApJ...483..235T Part 9: 1998ApJS..115...91T Part 10: 1998ApJ...495..804T Part 11: 1998ApJ...501..731T Part 12: 1999ApJ...518..699T Part 13: 2000ApJS..126..427T

Structure and chemistry in the northwestern condensation of the Serpens molecular cloud core

We present single-dish and interferometric observations of gas and dust in the core of the Serpens molecular cloud, focusing on the northwestern condensation. Single-dish molecular line observations are used to probe the structure and chemistry of the condensation while high-resolution images of CS and CH_(3)0H are combined with continuum observations from λ = 1.3 mm to λ = 3.5 cm to study the subcondensations and overall distribution of dust. For the northwestern condensation, we derive a characteristic density of 3 x 10^5 cm^(-3) and an estimated total mass of approximately 70 M_⊙. We find compact molecular emission associated with the far-infrared source S68 FIRS 1, and with a newly detected subcondensation named S68 N. Comparison of the large-and small-scale emission reveals that most of the material in the northwest condensation is not directly associated with these compact sources, suggesting a youthful age for this region. CO J = 1 approaches 0 observations indicate widespread outflow activity. However, no unique association of embedded objects with outflows is possible with our observations. The SiO emission is found to be extended with the overall emission centered about S68 FIRS 1; the offset of the peak emission from all of the known continuum sources and the coincidence between the blueshifted SiO emission and blueshifted high-velocity gas traced by CO and CS is consistent with formation of SiO in shocks. Derived abundances of CO and HCO^(+) are consistent with quiescent and other star-forming regions while CS, HCN, and H2CO abundances indicate mild depletions within the condensation. Spectral energy distribution fits to S68 FIRS 1 indicate a modest luminosity (50-60 L_⊙), implying that it is a low-mass (0.5-3 M_⊙) young stellar object. Radio continuum observations of the triple source toward S68 FIRS 1 indicate that the lobe emission is varying on timescales ≤ 1 yr while the central component is relatively constant over ~14 yr. The nature of a newly detected compact emission region, S68 N, is less certain due to the absence of firm continuum detections; based on its low luminosity (<5 L_⊙) and strong molecular emission, S68 N may be prestellar subcondensation of gas and dust.

Observations of the formaldehyde emission in Orion-KL - Abundances, distribution, and kinematics of the dense gas in the Orion molecular ridge

view Abstract Citations (49) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the Formaldehyde Emission in Orion-KL: Abundances, Distribution, and Kinematics of the Dense Gas in the Orion Molecular Ridge Mangum, Jeffrey G. ; Wootten, Alwyn ; Loren, Robert B. ; Wadiak, E. James Abstract The "hot core," "plateau," and "ridge" features in the Orion-KL molecular cloud have been mapped in 2 cm H2CO emission with a 6" synthesized beam. We have also observed all ΔJ = 1 millimeter wave transitions of H2CO and several of the H213CO and H2C18O transitions in the 211 to 363 GHz window toward Orion-KL with a single antenna. The H2CO hot core emission has been identified for the first time both in high- excitation line profiles and in the 2 cm maps. We identify a velocity shift in the 2 cm H2CO emission which we attribute to the plateau or bipolar flow. This velocity shift lies at a position angle of 120° ± 30°, which is parallel to the bipolar flows seen in SiO, HCO+, H2, SO2, CO, and SO emission. The large primary beam available for these observations has provided a particularly sensitive map of the rather extended "ridge" structure in Orion-KL. The "ridge" component in our 2 cm H2CO emission maps can be decomposed into two clumps, called the "compact ridge" and "10 km s-1 feature." The compact ridge dominates the observed emission. Comparison with aperture synthesis maps in other molecules demonstrates that the 2 cm H2CO compact ridge defines an extensive high-density (≳106 cm-3) region at the southern edge of the bipolar flow near IRc2. With this multitransition, multi-isotopomer^{5 }data set, we have modeled the spatial density in the compact ridge component. We find that n(H2) = 5.0 × 106-1.0 × 107 cm-3 in the H2CO compact ridge. These models also reveal that there is no evidence for substantial variation in the H2CO abundances among the hot core, compact ridge, and 10 km s-1 feature. However, H2CO is 3-25 times more abundant in these Orion-KL components than it is in other molecular clouds (Mundy et al.; Wootten et al.). This abundance enhancement may be due to the epidemic of massive star formation in the region. Observers have noted in the past that 2 cm H2CO emission occurs only rarely in interstellar clouds, though density sufficient to cause the emission is found in many cloud cores. We suggest that the reason the emission is notably intense in Orion-KL lies in the enhancement of the H2CO abundance, as well as in the high density. Publication: The Astrophysical Journal Pub Date: January 1990 DOI: 10.1086/168262 Bibcode: 1990ApJ...348..542M Keywords: Abundance; Formaldehyde; Kinematics; Molecular Clouds; Orion Nebula; Antennas; Astronomical Models; Emission Spectra; Gas Density; Interstellar Matter; Star Formation; interstellar: molecules; nebulae: abundances; nebulae: individual (Orion Nebula); Astrophysics; INTERSTELLAR: MOLECULES; NEBULAE: ABUNDANCES; NEBULAE: ORION NEBULA full text sources ADS | data products SIMBAD (2)

The densities of the molecular clouds associated with Herbig Be/Ae and other young stars

view Abstract Citations (27) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The densities of the molecular clouds associated with Herbig Be/Ae and other young stars. Loren, R. B. Abstract The H2CO molecule has been studied toward 25 regions containing signposts of recent star formation, either Herbig Be/Ae stars, LkH alpha emission-line stars, or stars with unusual circumstellar nebulosity. The 2-cm and 2-mm lines of H2CO have been detected toward many of these regions, allowing a determination of cloud density. Core densities from 20,000 per cu cm to 150,000 per cu cm are found to be associated with more than half of the regions studied. The cloud L 43 is unusual in being cold (kinetic temperature approximately 15 K) yet having a high density (150,000 per cm) more characteristic of the hotter regions of star formation. The H2CO abundances for these clouds with embedded young stars are the same as the abundances of other clouds of similar density that do not have embedded stars. Publication: The Astronomical Journal Pub Date: January 1981 DOI: 10.1086/112857 Bibcode: 1981AJ.....86...69L Keywords: Astronomical Spectroscopy; Early Stars; Formaldehyde; Gas Density; Molecular Clouds; Abundance; Astronomical Catalogs; Astronomical Maps; Line Spectra; Astrophysics full text sources ADS | data products SIMBAD (32)

Detection of deuterated formaldehyde in interstellar clouds

view Abstract Citations (21) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of deuterated formaldehyde in interstellar clouds. Langer, W. D. ; Frerking, M. A. ; Linke, R. A. ; Wilson, R. W. Abstract The detection of deuterated formaldehyde, HDCO, in interstellar clouds by observations of the J(K-1,K+1) = 2(02) to 1(01) transition of the molecule at 128.81291 GHz is reported. Formaldehyde emission spectra were obtained of the molecular clouds L134 N and Heiles' Cloud 2 (CLD 2) of the Taurus Molecular Cloud complex using a 7-m antenna. Comparisons of HDCO abundances estimated from the observed antenna temperatures and a radiative transfer model for a uniform isothermal cloud with observed para and ortho H2CO abundances indicate an extent of deuterium fractionation comparable to that found in other deuterated species, suggesting that formaldehyde is produced primarily by gas-phase ion-molecule reactions and not grain chemistry. Examination of the HDCO and H2CO spectra from the two clouds also implies that the centimeter and millimeter lines arise from different regions within the clouds, and thus a comparison of these transitions for the derivation of the physical conditions within the clouds will lead to incorrect results. Publication: The Astrophysical Journal Pub Date: September 1979 DOI: 10.1086/183058 Bibcode: 1979ApJ...232L.169L Keywords: Deuterium Compounds; Formaldehyde; Interstellar Chemistry; Nebulae; Astronomical Spectroscopy; Centimeter Waves; Ionic Reactions; Millimeter Waves; Molecular Energy Levels; Molecular Excitation; Molecular Interactions; Molecular Spectroscopy; Astrophysics; Formaldehyde:Interstellar Clouds full text sources ADS |