Detection of methylisocyanate (CH3NCO) in a solar-type protostar
R. Martín-Doménech, V. Rivilla, I. Jiménez-Serra, D. Quenard, L. Testi, J. Martín-Pintado
MMNRAS , 1–5 (2017) Preprint 29 October 2018 Compiled using MNRAS L A TEX style file v3.0
Detection of methyl isocyanate (CH NCO) in a solar-typeprotostar
R. Mart´ın-Dom´enech, V. M. Rivilla, (cid:63) I. Jim´enez-Serra, D. Qu´enard, L. Testi, , , , and J. Mart´ın-Pintado, Centro de Astrobiolog´ıa (INTA-CSIC). Ctra de Ajalvir, Km. 4, Torrej´on de Ardoz, 28850 Madrid, Spain INAF/Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125, Florence, Italy School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS ESO/European Southern Observatory, Karl Schwarzschild str. 2, D-85748, Garching, Germany Excellence Cluster “Universe”, Boltzmann str. 2, D-85748 Garching bei Muenchen, Germany
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We report the detection of the prebiotic molecule CH NCO in a solar-type protostar,IRAS16293-2422 B. A significant abundance of this species on the surface of the comet67P/Churyumov-Gerasimenko has been proposed, and it has recently been detected inhot cores around high-mass protostars. We observed IRAS16293-2422 B with ALMAin the 90 GHz to 265 GHz range, and detected 8 unblended transitions of CH NCO.From our Local Thermodynamic Equilibrium analysis we derived an excitation tem-perature of 110 ±
19 K and a column density of (4.0 ± × cm − , which resultsin an abundance of ≤ (1.4 ± × − with respect to molecular hydrogen. This im-plies a CH NCO/HNCO and CH NCO/NH CHO column density ratios of ∼ NCO suggests that both ice surface and gas phaseformation reactions of this molecule are needed to explain the observations.
Key words: instrumentation:interferometers - ISM:abundances -ISM:individual(IRAS16293-2422 B) - line:identification
Understanding the origin of life is one of the main chal-lenges of modern science. It is believed that some basic pre-biotic chemistry could have developed in space, likely trans-ferring prebiotic molecules to the solar nebula and lateron to Earth. For example, comets exhibit a wide varietyof complex organic molecules (or COMs) that are com-monly detected in the ISM (see, e.g., Biver et al. 2014).Recently, the spacecraft Rosetta found evidence for the pres-ence of several COMs of prebiotic interest on the surface ofthe comet 67P/Churyumov-Gerasimenko, using the COSACmass spectrometer (as e.g. glycoladehyde, CH OHCHO, orformamide, NH CHO; Goesmann et al. 2015), and in thecoma of the comet using the ROSINA instrument (with thedetection of the amino acid glycine, and phosphorous; Al-twegg et al. 2015). Among these species, the COSAC massspectrometer suggested the presence of methyl isocyanate(CH NCO) with an abundance relatively high compared toother COMs (Goesmann et al. 2015). CH NCO is the sim-plest isocyanate, which along NH CHO contains C, N, and (cid:63)
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O atoms, and could play a key role in the synthesis of aminoacid chains known as peptides (Pascal et al. 2005). CH NCOwas subsequently detected in massive hot molecular coressuch as SgrB2 N (Halfen et al. 2015; Belloche et al. 2017)and Orion KL (Cernicharo et al. 2016). However, CH NCOremained to be reported in solar-type protostars.IRAS 16293 − ρ Ophiuchi cloud complex at a distance of 120 pc(Loinard et al. 2008), and it is considered an excellent tem-plate for astrochemical studies in low-mass protostars (e.g.,Jorgensen et al. 2011, 2016; Lykke et al. 2017). IRAS16293is composed by sources A and B, separated in the planeof the sky by ∼ (cid:48)(cid:48) ( ∼
600 AU), and whose masses are ∼ (cid:12) (Looney et al. 2000). Their emission exhibits line pro-files with linewidths of up to 8 km s − for IRAS16293 Aand < − for IRAS16293 B. The narrow emission ofIRAS16293 B, along with its rich COM chemistry, makesthis object the perfect target to search for new COMs.In this letter we report the detection of CH NCO to-wards IRAS 16293-2422 B at frequencies ≤
250 GHz usingthe Atacama Large Millimeter Array (ALMA). Our resultsare consistent with those presented by Ligterink et al. (2017)using CH NCO transitions with frequencies ≥
300 GHz. c (cid:13) a r X i v : . [ a s t r o - ph . S R ] M a r R. Mart´ın-Dom´enech et al.
The analysis was carried out using the ALMA data from ourown project ( >
300 GHz, affecting the line intensitiesof the molecular emission; see Zapata et al. 2013; Jorgensenet al. 2016); and ii) to limit the level of line confusion, whichallows the correct subtraction of the continuum emission byselecting a suitable number of line-free channels in the ob-served spectra (see e.g. Pineda et al. 2012). All data match-ing our criteria were downloaded and re-calibrated usingstandard ALMA calibration scripts and the Common As-tronomy Software Applications package . The angular reso-lution of all datasets was sufficient to resolve source B fromsource A (with angular resolutions below 1.5”), and there-fore the emission lines arising from source B are narrowwith linewidths < − . Continuum subtraction was per-formed in the uv-plane before imaging using line-free chan-nels.Our dataset covers a total bandwidth of ∼ (cid:48)(cid:48) × (cid:48)(cid:48) to1.42 (cid:48)(cid:48) × (cid:48)(cid:48) . The velocity resolution falls between 0.14 and0.30 km s − . For the analysis, a spectrum was extractedfrom each datacube using a circular support with size ∼ (cid:48)(cid:48) centered at the position of IRAS16293 B ( RA J2000 = 16 h m s , DEC
J2000 = -24 ◦ (cid:48) (cid:48)(cid:48) ). We note that themolecular emission from IRAS16293 B for the species consid-ered in this study (e.g. CH NCO, NH CHO and HN CO),is compact and lies below 1.5” (see Figure 2 below andCoutens et al. 2016). Thus, although the ALMA datasetswere obtained with different array configurations and UVcoverage, we are confident that our extracted spectra con-tain all the emission from the hot corino and the analysedlines do not suffer from missing flux. NCO
The rotational spectrum of CH NCO, with the A and E tor-sional states, has been studied by Koput, J. (1986) (from 8to 40 GHz), and more recently by Halfen et al. (2015) (from68 to 105 GHz) and Cernicharo et al. (2016, from 40 to363 GHz). The identification of the lines was performed us-ing the software MADCUBAIJ , using the information fromthe Jet Propulsion Laboratory (JPL; Pickett et al. 1998)and the Cologne Database for Molecular Spectroscopy spec-tral catalogs (CDMS; M¨uller et al. 2005). We identified a https://casa.nrao.edu Madrid Data Cube Analysis on ImageJ is a software developedin the Center of Astrobiology (Madrid, INTA-CSIC) to visualiseand analyse single spectra and datacubes (Rivilla et al. 2016a,2017)
Table 1.
Detected CH NCO unblended lines in IRAS16293 B.Frequency Transition logA ul E up Area(GHz) (J,K a ,K c ,m) (s − ) (K) Jy km s − − (17,2,0,3) -3.75 210 0.028 ± − (17,2,0,-3) -3.75 210 0.028 ± − (26,2,0,2) -3.22 234 0.12 ± − (26,1,0,1) -3.22 175 0.16 ± − (27,0,0,1) -3.17 181 0.18 ± − (28,3,26,0) -3.13 235 0.16 ± − (28,3,25,0) -3.13 235 0.16 ± − (28,0,28,0) -3.13 181 0.21 ± total of 22 transitions of CH NCO, 8 out of which were un-blended with upper level energies ranging from 175 to 233 K(see Table 1) using MADCUBAIJ. The remaining 14 linesappear contaminated by emission from other species. TheCH NCO lines peak at a radial velocity of v LSR = 2.7 kms − and have linewidths of ∼ − (Fig. 1), similar tothose from other molecules in IRAS16293 B (Jorgensen et al.2011). MADCUBAIJ produces synthetic spectra assumingLocal Thermodinamical Equilibrium (LTE) conditions. Thecomparison between the observed and the synthetic spec-trum for the unblended ransitions can be used to derivethe excitation temperature and total column density thatbest match the observations. We assumed a linewidth of 1.1km s − , and the source size was constrained by the contin-uum emission to 0.5 (cid:48)(cid:48) (see Figure 2), which agrees with thesource size assumed in previous works (Jorgensen et al. 2016;Coutens et al. 2016; Lykke et al. 2017). The observed spec-tra and the corresponding LTE fitted synthetic spectrumfor the 8 unblended lines detected are shown in Fig 1. AllCH NCO transitions were found to be optically thin ( τ < NCO transitions are well reproduced by an excitationtemperature of T ex =110 ±
19 K. This T ex is similar to thatfound for other COMs such as acetaldehyde or propanal inIRAS16293 B (Lykke et al. 2017). The derived column den-sity is N (CH NCO)=(4.0 ± × cm − , which agreeswith the column density reported in Ligterink et al. (2017)assuming the same source size and excitation temperaturefor the transitions with E up >
300 K detected at frequencies ≥
320 GHz. The spatial distribution of CH NCO is shownin Fig. 2 and it is coincident with the continuum emission.The measured deconvolved size is ∼ NCO abundance, we havederived the H column density by using the continuum fluxmeasured at 232 GHz (1.4 ± (cid:48)(cid:48) × (cid:48)(cid:48) ), and by assuming optically thin dust,a dust opacity of 0.009 cm g − (thin ices in a H densityof 10 cm − ; see Ossenkopf & Henning 1994) and a gas-to-dust mass ratio of 100. The estimated H column density forT dust =T ex =110 K (at these high densities, dust and gas arethermally coupled) is N (H )=2.8 × cm − , consistentwith that estimated by Jorgensen et al. (2016) at higher fre-quencies. We however caution that this value should be con- MNRAS , 1–5 (2017) etection of CH NCO in a solar-type protostar Figure 1. CH NCO unblended lines measured toward IRAS16293 B with ALMA (solid black). Transitions are shown in every panel,while their rest frequencies are reported in Table 1. The synthetic LTE spectrum generated by MADCUBAIJ is overplotted in red. sidered as a lower limit since dust may be optically thickeven at these low frequencies. The derived abundance ofCH NCO is χ (CH NCO)=(1.4 ± × − and it shouldbe considered as an upper limit.Transitions corresponding to two isomers of methylisocyanate, CH CNO and CH OCN, were not detected inour dataset, and 3 σ upper limits of 2.7 × cm − , and5.1 × cm − (respectively) were extracted assuming thesame linewidth and excitation temperature. This upper lim-its lead to column density ratios of CH NCO/CH CNO ≥ NCO/CH OCN ≥ CHO
In Orion KL, CH NCO shows the same spatial distribu-tion as HNCO and NH CHO (Cernicharo et al. 2016) andtherefore they are thought to be chemically related. Sev-eral transitions of HNCO and NH CHO, and of some oftheir isotopologues, are also covered and detected in ourdataset. The HNCO and NH CHO lines are optically thick(Coutens et al. 2016) and their column densities have beeninferred using the HNC O and NH
CHO isotopologues.Five unresolved transitions of HNC O are found at 250GHz with E up =122 K. For a fixed excitation temperature of T ex =110 K, (the T ex derived for CH NCO; see Section 3.1)we obtain a column density of N (HNC O)=(9.7 ± × cm − . By assuming an isotopic ratio O/ O = 500 (Wilson& Rood 1994), the derived total column density of HNCOis N (HNCO)=(4.9 ± × cm − , which yields an abun-dance of (1.8 ± × − .For NH CHO, three lines are detected at 156.957 GHz,157.097 GHz, and 239.628 GHz, with E up = 58 −
98 K. Theiremission is fitted with an excitation temperature of T ex =75K and a column density of N (NH CHO)=(7.6 ± × cm − . The derived T ex is slightly lower than that obtainedfor CH NCO, possibly due to the lower values of E up cov-ered by the NH CHO lines compared to those of CH NCO.We note however, that both species show the same spatial extent (see Figure 2 and Coutens et al. 2016) and therefore,they likely trace the same gas. By assuming an isotopic ratio C/ C=68 (Milam et al. 2005), the derived total columndensity is N (NH CHO)=(5.2 ± × cm − , which givesan abundance of (1.9 ± × − . As for CH NCO, theseabundances should be considered as upper limits.
The abundance of (1.4 ± × − measured for CH NCOtoward IRAS16293 B is similar to that found in SgrB2(N)(1.7 × − and 1.0 × − for the two V LSR components;see Cernicharo et al. 2016). In Table 4, we present thecomparison between the abundance ratios CH NCO/HNCOand CH NCO/NH CHO measured in IRAS16293 B withthose from the three sources where CH NCO has also beendetected (e.g. SgrB2(N), Orion KL, and 67P/Churyumov-Gerasimenko; Goesmann et al. 2015; Halfen et al. 2015; Bel-loche et al. 2017; Cernicharo et al. 2016). Since we haveestimated the column densities considering the same emit-ting region, the derived ratios are likely independent on theassumed source size and the derived H column density.From Table 4, we find that the CH NCO/HNCO col-umn density ratio in IRAS16293 B is of the same order asthose measured in SgrB2(N) and Orion KL. However, itis a factor of ∼
50 lower than in comet 67P/Churyumov-Gerasimenko. We note however that the COSAC detec-tions are tentative and therefore the abundance ratios incolumn 7 of Table 4 should be taken with caution. TheCH NCO/NH CHO column density ratio in IRAS16293 Bis similar to that observed in SgrB2(N), while it is factors 20-70 lower than those measured in Orion KL, and a factor of10 lower than that in comet 67P/Churyumov-Gerasimenko.In Section 4, we explore the formation routes for CH NCOand compare the measured ratios with those predicted bychemical modelling.
MNRAS , 1–5 (2017)
R. Mart´ın-Dom´enech et al.
Figure 2.
Integrated intensity maps of two representativeCH NCO unblended lines observed toward IRAS16293 B. Blackcontours indicate 50% and 90% of the peak line emission, whilewhite contours indicate 20%, and 80% of the continuum peakemission at 232 GHz. The rest frequency and E up of the transi-tions are shown in every panel (see also Table 1). Beam sizes areshown in the bottom right corner. For the chemical modelling of CH NCO, HNCO, andNH CHO in IRAS16293 B, we have used the gas-grain chem-ical code UCL CHEM recently re-written by Holdship et al.,(submitted) . UCL CHEM’s chemical network contains 310species (100 of them are also on the grain surface) and 3097reactions. Gas phase reactions are taken from the UMISTdatabase (McElroy et al. 2013), while dust grain surface pro-cesses include thermal desorption (as in Viti et al. 2004) andnon-thermal desorption processes such as direct UV desorp-tion, cosmic ray-induced UV photons desorption, direct cos-mic ray desorption, and H formation mechanism desorp-tion (see Roberts et al. 2007). Recently, diffusion followingthe formalism from Hasegawa et al. (1992), and chemical re-active desorption (following the experimentally-derived for-mula of Minissale et al. 2016) have also been included inUCL CHEM (Qu´enard et al. in prep.).To model the chemistry of CH NCO, HNCO andNH CHO, we expanded UCL CHEM’s chemical network byincluding recently proposed gas phase and grain surface for-mation routes. For the gas phase formation of CH NCO,Halfen et al. (2015) proposed the following reactions:HNCO / HOCN + CH −→ CH NCO + H (1)HNCO / HOCN + CH +5 −→ CH NCOH + + H (2)CH NCOH + + e − −→ CH NCO + H (3)Note that no gas-phase destruction route was proposed intheir study. We have also included the reactionCH NCOH + + e − −→ CH + HOCN , (4)to account for the fact that CH NCOH + may fragment intosmaller products. For the grain surface formation, Bellocheet al. (2017) and Cernicharo et al. (2016) proposed thatCH NCO could be formed through the grain surface reac-tions: CH + OCN −→ CH NCO (5)CH + HNCO −→ CH NCO + H . (6) UCL CHEM can be downloaded at https://uclchem.github.io/.
These reactions have been found to be efficient experimen-tally (Ligterink et al. 2017). Furthermore, one of the possibleformation routes of N-methylformamide (N-CH NHCHO)may involve successive addition of hydrogen atoms toCH NCO: CH NCO + H −→ CH NHCO (7)CH NHCO + H −→ CH NHCHO . (8)where reaction (7) has an activation energy of ∼ ∼ A V =2 mag) by assuming a constant density of n H = 10 cm − and a temperature of T kin = 10 K for ∼ yrs. We assumean interstellar radiation field of G = 1 Habing and thestandard cosmic ray ionisation rate of 1 . × − s − . Theelemental abundances considered in this model are takenfrom Wakelam & Herbst (2008, model EA1).In Phase 1 of our model, we follow the chemistry duringthe pre-stellar core phase assuming a constant temperatureof T kin = 10 K while we increase the core’s gas density from10 cm − to 5 × cm − (this value is consistent with thatmeasured in IRAS16293 B). In Phase 2, the chemical evo-lution of the hot corino is modelled by assuming a constantH gas density (5 × cm − ) while gradually increasingthe gas temperature from 10 K to 110 K during the first 10 yrs. After then, the temperature is kept constant and thechemistry is followed during 10 yrs.The best fit to our observations is found for a dy-namical age of × yrs, with a predicted abundance forCH NCO of [CH NCO]=6 . × − (with respect to H ),i.e. a factor of 4 higher than that measured in IRAS16293B (1.4 × − ; Section 3.1). We note that these time-scalesare consistent with those estimated for this source (see,e.g., Bottinelli et al. 2014; Majumdar et al. 2016). In thismodel, HNCO (the parent molecule of CH NCO) is formedon the surface of dust grains; and once the temperaturereaches ∼
100 K, HNCO is thermally desorbed and incorpo-rated into the gas phase, allowing the gas-phase formationof CH NCO to proceed (see reactions above and Halfen etal. 2015). We note that CH NCO is also formed on grainsurfaces in our model. However, this mechanism by its ownis not sufficient to explain the observed abundances of thismolecule in IRAS16293 B. Therefore, our modelling showsthat formation both in the ices and in the gas phase is re-quired to explain the observed abundance of CH NCO inIRAS16293 B. We note that, while the HNCO abundancepredicted by our model (3.8 × − ) also agrees well withthat observed in IRAS16293 B (1.8 × − ), the abundanceof NH CHO is underproduced by a factor of 10. As a result,while the CH NCO/HNCO abundance ratio is well repro-duced by our model, the CH NCO/NH CHO ratio differsfrom that observed by a factor of ∼
40 (see Table 4). Wealso note that our model perfectly reproduces the upperlimits of CH NCO measured in cold cores such as L1544( ≤ × − ; Jim´enez-Serra et al. 2016, Qu´enard et al., inprep). MNRAS , 1–5 (2017) etection of CH NCO in a solar-type protostar Table 2.
Comparison of the CH NCO/HNCO andCH NCO/NH CHO ratios measured in IRAS16293 B, SgrB2(N),Orion KL and comet 67P. Our modelling results of IRAS16293B for T gas =110 K and time=4 × yrs, are also shown. Protostars CometMolecular Low-mass High-massratio IRAS16293 B SgrB2(N) Orion KL 67Pobs. model A BCH NCO/HNCO 0.08 0.16 0.11 0.02 0.06 4.33CH NCO/NH CHO 0.08 3.53 0.06 1.75 5.71 0.72
We carried out an additional test including the iso-mers of CH NCO, for which their upper limits have beenmeasured (see Section 3). We have assumed that CH OCNand CH CNO experience the same reactions as CH NCOat the same rates, although this assumption is highly un-certain given the lack of experimental data. The abundanceof CH NCO changes only by a factor of 1.1, but CH OCNand CH CNO are overproduced by factors ≥ NCO andits isomers will be discussed in detail in Qu´enard et al. (inprep.).
ACKNOWLEDGMENTS
This Letter makes use of the following ALMA data:ADS/JAO.ALMA
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