CH3CN Abundance Research Articles

Complex Organic Molecules in a Very Young Hot Corino, HOPS 373SW

We present the spectra of complex organic molecules (COMs) detected in HOPS 373SW with the Atacama Large Millimeter/submillimeter Array (ALMA). HOPS 373SW, which is a component of a protostellar binary with a separation of 1500au, has been discovered as a variable protostar by the JCMT transient monitoring survey with a modest (∼30%) brightness increase at submillimeter wavelengths. Our ALMA target-of-opportunity observation at ∼345 GHz for HOPS 373SW revealed extremely young chemical characteristics with strong deuteration of methanol. The dust continuum opacity is very high toward the source center, obscuring line emission from within 0.″03. The other binary component, HOPS 373NE, was detected only in C17O in our observation, implying a cold and quiescent environment. We compare the COM abundances relative to CH3OH in HOPS 373SW with those of V883 Ori, which is an eruptive disk object, as well as other hot corinos, to demonstrate the chemical evolution from envelope to disk. High abundances of singly, doubly, and triply deuterated methanol (CH2DOH, CHD2OH, and CD3OH) and a low CH3CN abundance in HOPS 373SW compared to other hot corinos suggest a very early evolutionary stage of HOPS 373SW in the hot corino phase. Since the COMs detected in HOPS 373SW would have been sublimated very recently from grain surfaces, HOPS 373SW is a promising place to study the surface chemistry of COMs in the cold prestellar phase before sublimation.

Evolutions of CH3CN abundance in molecular clumps

To investigate the effects of massive star evolution on surrounding molecules, we select nine massive clumps previously observed with the Atacama Pathfinder Experiment (APEX) telescope and the Submillimeter Array (SMA) telescope. Based on the observations of APEX, we obtain luminosity to mass ratios L clump/M clump that range from 10 to 154 L ⊙/M ⊙, where some of them embedded ultra compact (UC) HII region. Using the SMA, CH3CN (12K–11K) transitions were observed toward nine massive star-forming regions. We derive the CH3CN rotational temperature and column density using the XCLASS program, and calculate its fractional abundance. We find that CH3CN temperature seems to increase with the increase of L clump/M clump when the ratio is between 10 to 40 L ⊙/M ⊙, then decrease when L clump/M clump ≥ 40 L ⊙/M ⊙. Assuming that the CH3CN gas is heated by radiation from the central star, the effective distance of CH3CN relative to the central star is estimated. The distance ranges from ∼ 0.003 to ∼ 0.083 pc, which accounts for ∼ 1/100 to ∼ 1/1000 of clump size. The effective distance increases slightly as L clump/M clump increases (R eff ∼ (L clump/M clump)0.5±0.2). Overall, the CH3CN abundance is found to decrease as the clumps evolve, e.g., X CH3CN ∼ (L clump/M clump)−1.0 ± 0.7. The steady decline of CH3CN abundance as the clumps evolution can be interpreted as a result of photodissociation.

The TOPGöt high-mass star-forming sample

Aims. The TOPGöt project studies a sample of 86 high-mass star-forming regions in different evolutionary stages from starless cores to ultra compact HII regions. The aim of the survey is to analyze different molecular species in a statistically significant sample to study the chemical evolution in high-mass star-forming regions, and identify chemical tracers of the different phases. Methods. The sources have been observed with the IRAM 30 m telescope in different spectral windows at 1, 2, and 3 mm. In this first paper, we present the sample and analyze the spectral energy distributions (SEDs) of the TOPGöt sources to derive physical parameters such as the dust temperature, Tdust, the total column density, NH2, the mass, M, the luminosity, L, and the luminosity-to-mass ratio, L∕M, which is an indicator of the evolutionary stage of the sources. We use the MADCUBA software to analyze the emission of methyl cyanide (CH3CN), a well-known tracer of high-mass star formation. Results. We built the spectral energy distributions for ~80% of the sample and derived Tdust and NH2 values which range between 9−36 K and ~3 × 1021−7 × 1023 cm−2, respectively. The luminosity of the sources spans over four orders of magnitude from 30 to 3 × 105 L⊙, masses vary between ~30 and 8 × 103 M⊙, and the luminosity-to-mass ratio L∕M covers three orders of magnitude from 6 × 10−2 to 3 × 102 L⊙∕M⊙. The emission of the CH3CN(5K-4K) K-transitions has been detected toward 73 sources (85% of the sample), with 12 nondetections and one source not observed in the frequency range of CH3CN(5K-4K). The emission of CH3CN has been detected toward all evolutionary stages, with the mean abundances showing a clear increase of an order of magnitude from high-mass starless cores to later evolutionary stages. We found a conservative abundance upper limit for high-mass starless cores of XCH3CN < 4.0 × 10−11, and a range in abundance of 4.0 × 10−11 < XCH3CN < 7.0 × 10−11 for those sources that are likely high-mass starless cores or very early high-mass protostellar objects. In fact, in this range of abundance we have identified five sources previously not classified as being in a very early evolutionary stage. The abundance of CH3CN can thus be used to identify high-mass star-forming regions in early phases of star-formation.

Methyl cyanide (CH3CN) and propyne (CH3CCH) in the low-mass protostar IRAS 16293–2422

Methyl cyanide (CH3CN) and propyne (CH3CCH) are two molecules commonly used as gas thermometers for interstellar gas. They are detected in several astrophysical environments and in particular towards protostars. Using data of the low-mass protostar IRAS 16293-2422 obtained with the IRAM 30m single-dish telescope, we constrained the origin of these two molecules in the envelope of the source. The line shape comparison and the results of a radiative transfer analysis both indicate that the emission of CH3CN arises from a warmer and inner region of the envelope than the CH3CCH emission. We compare the observational results with the predictions of a gas-grain chemical model. Our model predicts a peak abundance of CH3CCH in the gas-phase in the outer part of the envelope, at around 2000 au from the central star, which is relatively close to the emission size derived from the observations. The predicted CH3CN abundance only rises at the radius where the grain mantle ices evaporate, with an abundance similar to the one derived from the observations.

The physical and chemical structure of Sagittarius B2

Context. We model the emission of methyl cyanide (CH3CN) lines towards the massive hot molecular core Sgr B2(M). Aims. We aim to reconstruct the CH3CN abundance field, and investigate the gas temperature distribution as well as the velocity field. Methods. Sgr B2(M) was observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in a spectral line survey from 211 to 275 GHz. This frequency range includes several transitions of CH3CN (including isotopologues and vibrationally excited states). We employed the three-dimensional radiative transfer toolbox Pandora in order to retrieve the velocity and abundance field by modeling different CH3CN lines. For this purpose, we based our model on the results of a previous study that determined the physical structure of Sgr B2(M), i.e., the distribution of dust dense cores, ionized regions, and heating sources. Results. The morphology of the CH3CN emission can be reproduced by a molecular density field that consists of a superposition of cores with modified Plummer-like density profiles. The averaged relative abundance of CH3CN with respect to H2 ranges from 4 × 10−11 to 2 × 10−8 in the northern part of Sgr B2(M) and from 2 × 10−10 to 5 × 10−7 in the southern part. In general, we find that the relative abundance of CH3CN is lower at the center of the very dense, hot cores, causing the general morphology of the CH3CN emission to be shifted with respect to the dust continuum emission. The dust temperature calculated by the radiative transfer simulation based on the available luminosity reaches values up to 900 K. However, in some regions vibrationally excited transitions of CH3CN are underestimated by the model, indicating that the predicted gas temperature, which is assumed to be equal to the dust temperature, is partly underestimated. The determination of the velocity component along the line of sight reveals that a velocity gradient from the north to the south exists in Sgr B2(M).

Non-thermal ion desorption from an acetonitrile (CH3CN) astrophysical ice analogue studied by electron stimulated ion desorption.

The incidence of high-energy radiation onto icy surfaces constitutes an important route for leading new neutral or ionized molecular species back to the gas phase in interstellar and circumstellar environments, especially where thermal desorption is negligible. In order to simulate such processes, an acetonitrile ice (CH3CN) frozen at 120 K is bombarded by high energy electrons, and the desorbing positive ions are analyzed by time-of-flight mass spectrometry (TOF-MS). Several fragment and cluster ions were identified, including the Hn=1-3(+), CHn=0-3(+)/NHn=0-1(+); C2Hn=0-3(+)/CHn=0-3N(+), C2Hn=0-6N(+) ion series and the ion clusters (CH3CN)n=1-2(+) and (CH3CN)n=1-2H(+). The energy dependence on the positive ion desorption yield indicates that ion desorption is initiated by Coulomb explosion following Auger electronic decay. The results presented here suggest that non-thermal desorption processes, such as desorption induced by electronic transitions (DIET) may be responsible for delivering neutral and ionic fragments from simple nitrile-bearing ices to the gas-phase, contributing to the production of more complex molecules. The derived desorption yields per electron impact may contribute to chemical evolution models in different cold astrophysical objects, especially where the abundance of CH3CN is expected to be high.

Dissecting an intermediate-mass protostar

We have carried out high-angular resolution (1.4") observations in the continuum at 3.1mm and in the N2H+ 1-0, CH3CN 5_k-4_k and 13CS 2-1 lines using the Plateau de Bure Interferometer (PdBI) towards the intermediate mass (IM) protostar IRAS21391+5802 (IC1396N). In addition, we have merged the PdBI images with previous BIMA (continuum data at 1.2mm and 3.1mm) and single-dish (N2H+ 1-0) data to have a comprehensive description of the region. The combination of our data with BIMA and 30m data show that the bipolar outflow associated has completely eroded the initial molecular globule. The 1.2mm and 3.1mm continuum emissions are extended along the outflow axis tracing the warm walls of the biconical cavity. Most of the molecular gas, however, is located in an elongated feature in the direction perpendicular to the outflow. A strong chemical differentiation is detected across the molecular toroid, with the N2H+ 1-0 emission absent in the inner region.This chemical differentiation can be understood in terms of the different gas kinetic temperature. The [CH3CN]/[N2H+] ratio increases by 5 orders of magnitude with gas temperature, for temperatures between 20K and 100K. The CH3CN abundance towards IRAM 2A, the most massive protostellar core, is similar to that found in hot corinos and lower than that expected towards IM and high mass hot cores. This could indicate that IRAM 2A is a low mass or at most Herbig Ae star (IRAM 2A) instead of the precursor of a massive Be star. Alternatively, the low CH3CN abundance could also be the consequence of IRAM 2A being a Class 0/I transition object which has already formed a small photodissociation region (PDR).

THE HOT AND CLUMPY MOLECULAR COCOON SURROUNDING THE ULTRACOMPACT H II REGION G5.89–0.39

We present observations of CH3CN (12-11) emission at a resolution of 2" toward the shell-like ultracompact HII region G5.89-0.39 with the Submillimeter Array. The integrated CH3CN emission reveals dense and hot molecular cocoon in the periphery of the HII region G5.89-0.39, with a CH3CN deficient region roughly centered at G5.89-0.39. By analyzing the CH3CN emission using population diagram analysis, we find, for the first time, a decreasing temperature structure from 150 to 40 K with the projected distance from Feldt's star, which is thought to be responsible for powering the HII region. Our results further indicate that the majority of the heating energy in the observed dense gas is supplied by the Feldt's star. From the derived CH3CN column density profile, we conclude that the dense gas is not uniformly-distributed but centrally-concentrated, with a power-law exponent of 5.5 for r < 8000 AU, and 2.0 for 8000 AU < r < 20000 AU, where r is the distance to Feldt's star. The estimated large power index of 5.5 can be attributed to an enhancement of CH3CN abundance in the close vicinity of Feldt's star.

Multiple Jets from the High‐Mass (Proto)stellar Cluster AFGL 5142

We present studies of the massive protocluster AFGL 5142 in the J ¼ 2Y1 transition of the CO isotopologues, SO, CH3OH, and CH3CN lines, as well as in the continuum at 225 GHz and 8.4 GHz. The 225 GHz continuum emission reveals at least five dust continuum peaks. The strongest peaks, MM-1 and MM-2, are associated with hot cores with temperatures of 90 � 20 and 250 � 40 K, respectively. With similar core mass, the higher temperature and CH3CN abundance in the MM-2 core suggest that it might be at a more evolved stage than the MM-1 core. A total of 22 lines fromninemoleculesaredetected.Thelinestrengthvariesremarkablyintheregion.StrongSOemissionisfoundboth in molecular outflows and cloud cores. CH3OH emission, onthe contrary, is much weaker in molecular outflows, and isdetectedtowardhotcoresMM-1andMM-2,butisabsentinthelessmassiveandperhapslessevolvedcoresMM-3, MM-4, and MM-5. The CO and SO emission reveals at least three molecular outflows originating from the center of thedustcore.Theoutflowsarewellcollimated,withterminalvelocitiesupto50kms � 1 fromthecloudvelocity.Since jetlike outflows and disk-mediated accretion process are physically connected, the well-collimated outflows indicate that even in this cluster environment, accretion is responsible for the formation of individual stars in the cluster. Subject headingg s: H ii regions — ISM: clouds — ISM: individual (AFGL 5142) — ISM: kinematics and dynamics — masers — stars: formation

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.

Determination of molecular gas properties using methyl cyanide lines

A survey has been made of 27 Galactic star-forming regions in the (CH3CN) 6K–5K, 5K–4K, and 8K–7K lines of methyl cyanide (CH3CN) at 110, 92, and 147 GHz. Twenty-five sources were detected at 110 GHz, nineteen at 92 GHz, and three at 147 GHz. The strongest CH3CN emission arises in hot cores in regions of massive star formation. The abundance of CH3CN in these objects exceeds 10−9 as a consequence of grain mantle evaporation. Weaker CH3CN lines were found in a number of sources. These can arise in either warm (30–50 K), dense (>104 cm−3) clouds or in hot regions with cooler gas.

Identification of the interstellar cyanomethyl radical (CH2CN) in the molecular clouds TMC-1 and Sagittarius B2.

We report the astronomical identification of the cyanomethyl radical, CH2CN, the heaviest nonlinear molecular radical to be identified in interstellar clouds. The complex fine and hyperfine structures of the lowest rotational transitions at about 20.12 and 40.24 GHz are resolved in TMC-1, where the abundance appears to be about 5 x 10(-9) relative to that of H2. This is significantly greater than the observed abundance of CH3CN (methyl cyanide) in TMC-1. In Sgr B2 the hyperfine structure is blended in the higher frequency transitions at 40, 80, and 100 GHz, although the spin-rotation doubling is clearly evident. Preliminary searches in other sources indicate that the distribution of CH2CN is similar to that for such carbon chain species as HC3N or C4H.