A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone
Audrey Coutens, Jonathan M. C. Rawlings, Serena Viti, David A. Williams
MMNRAS , 1–10 (2016) Preprint 9 November 2018 Compiled using MNRAS L A TEX style file v3.0
A study of methanol and silicon monoxide productionthrough episodic explosions of grain mantles in the CentralMolecular Zone
A. Coutens (cid:63) , J. M. C. Rawlings, S. Viti, and D. A. Williams
Department of Physics and Astronomy, University College London, Gower St., London, WC1E 6BT, UK
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
Methanol (CH OH) is found to be abundant and widespread towards the CentralMolecular Zone, the inner few hundred parsecs of our Galaxy. Its origin is, however,not fully understood. It was proposed that the high cosmic ray ionisation rate in thisregion could lead to a more efficient non-thermal desorption of this species formed ongrain surfaces, but it would also mean that this species is destroyed in a relativelyshort timescale. In a first step, we run chemical models with a high cosmic ray ion-isation rate and find that this scenario can only reproduce the lowest abundances ofmethanol derived in this region ( ∼ − –10 − ). In a second step, we investigate anotherscenario based on episodic explosions of grain mantles. We find a good agreement be-tween the predicted abundances of methanol and the observations. We find that thedominant route for the formation of methanol is through hydrogenation of CO onthe grains followed by the desorption due to the grain mantle explosion. The cyclicaspect of this model can explain the widespread presence of methanol without requir-ing any additional mechanism. We also model silicon monoxide (SiO), another speciesdetected in several molecular clouds of the Galactic Centre. An agreement is foundwith observations for a high depletion of Si (Si/H ∼ − ) with respect to the solarabundance. Key words: astrochemistry – ISM: molecules – Galaxy: abundances – Galaxy: centre
The inner few hundred parsecs at the Centre of our Galaxy,also known as the Central Molecular Zone (CMZ), are char-acterised by specific physical conditions compared to the lo-cal interstellar medium. In particular, the dust temperatureand the gas temperature are uncoupled (G¨usten et al. 1981;Ao et al. 2013; Ginsburg et al. 2016). The dust tempera-ture is (cid:46)
30 K (Sodroski et al. 1994; Rodr´ıguez-Fern´andezet al. 2004; Longmore et al. 2012), while the gas tempera-ture is uniformly higher than 60 K (Ao et al. 2013; Ginsburget al. 2016) and possibly about 200–500 K in the most dif-fuse regions ( n <
100 cm − , Le Petit et al. 2016). This gasheating was proposed to be due to turbulence in dense gas(Ginsburg et al. 2016) or a high cosmic ray ionisation ratein diffuse regions (Le Petit et al. 2016). Indeed, the cosmicray ionisation rate in the CMZ is thought to be enhancedby several orders of magnitude with respect to the local in-terstellar medium, but its exact value is however debated. (cid:63) E-mail: [email protected]
For example, using H + observations, Le Petit et al. (2016)constrained it between 10 − and 1.1 × − s − , whilean upper limit of ζ < − s − is derived by Ginsburget al. (2016) based on H CO observations. Yusef-Zadeh et al.(2013b) used synchrotron emission and Fe K α line observa-tions to derive a cosmic ray ionisation rate of about (1–10) × − s − . There are a number of chemical consequences ofa strongly-enhanced cosmic ray ionization rate including (i)higher levels of ionization, more He + and a larger H : H ratio(Bayet et al. 2011; Meijerink et al. 2011), (ii) a stronger ra-diation field due to internally-generated cosmic-ray inducedphotons, and (iii) enhanced desorption rates for ice mantlesby cosmic ray spot heating, whole grain heating and CR-induced photodesorption.Emission from a large range of species has been de-tected towards the CMZ (e.g. Mart´ın-Pintado et al. 1997;Requena-Torres et al. 2008; Riquelme et al. 2010; Yusef-Zadeh et al. 2013c; Harada et al. 2015). Some of thesespecies are believed to be produced in the gas phase (e.g.,CO, CS, HCO + , HCN, N H + ), while others such as SiO,NH , CH OH and other complex organic molecules (e.g., c (cid:13) a r X i v : . [ a s t r o - ph . GA ] J a n A. Coutens et al. CH CHO, CH OHCHO, HCOOCH , (CH OH) ) appear torequire grain surface chemistry for their formation. In partic-ular, methanol (CH OH) was found to be widespread in theCMZ with a fractional abundance of 10 − –10 − with respectto H (Yusef-Zadeh et al. 2013c). Although the CMZ con-tains some star-forming regions such as Sgr B2, methanolis also observed towards regions devoid of star formationactivity. To explain the widespread presence of methanol,Yusef-Zadeh et al. (2013c) proposed that it would be due tothe high cosmic ray ionisation rate, which would lead to thedesorption of this grain surface species by the induced UVfield. However, the high cosmic ray ionisation rate also leadsto a destruction of methanol on a relatively short timescale(the higher the cosmic ray ionisation rate, the shorter thedestruction timescale – ∼ years for ζ = 5 × − s − vs ∼ years for ζ = 5 × − s − ) and it is uncertainif methanol can be replenished on this timescale. Based onIRAM-30m and APEX observations towards the circumnu-clear disk, Harada et al. (2015) explored the presence of 13other species (CS, CN, H CO, SO, N H + , H O + , SiO, HCN,HCO + , HNC, HC N, NO, CO) and tested whether sputter-ing due to shocks or desorption due to cosmic rays could re-produce the observed chemistry better. They concluded thatmodels with a high cosmic ray ionisation rate of ∼ − –10 − s − and a density of about 10 cm − gave the bestagreement with these observations. Alternatively, a modelwith high-velocity shocks ( >
40 km s − ) could possibly ex-plain the data but the timespan required for an agreementwith the observations is very short ( ∼ × yrs).A scenario complementary to conventional interstellarchemistry was proposed by Rawlings et al. (2013a,b) to ex-plain the presence of large organic molecules such as propy-lene in dark clouds. According to this mechanism, largemolecules could form in the ultra high density gas phase im-mediately after episodic explosions of grain mantles. Theseexplosions would be driven by the spontaneous recombina-tion of trapped hydrogen atoms, which would lead to a sud-den increase of the dust temperature. Indeed, when a criticalnumber of H-atoms on the grain is reached, a localised re-combination of the hydrogen atoms can trigger a chemicalrunaway, which will release all the chemical energy storedin the grain (Duley & Williams 2011). Similarly to ther-mal desorption, these abrupt temperature excursions wouldthen release the molecules frozen on the grains into the gasphase. However, as stated by Cecchi-Pestellini et al. (2010),just after the explosion of the grain mantle, the density inthe expanding gas may be so high that three-body reactionscould take place in a very short timescale of the order ofthe nanosecond. The radicals initially present on the grains(generated by processing by UV photons or cosmic rays)would then desorb with the other species and could reacttogether to form more complex species through three-bodyreactions. This scenario is particularly relevant for the CMZas the higher cosmic ray ionisation rate drives an increase ofboth the abundance of hydrogen atoms and the radical pro-duction rates, which consequently leads to a faster cyclingin the explosive model.In this paper, we first explore the evolution of methanolin the case of a quiescent scenario with high cosmic ray ioni-sation rates and the inclusion of the non-thermal desorptionprocesses. Yusef-Zadeh et al. (2013c) studied the variationof the methanol abundance with the cosmic ray ionisation rate but did not include the non-thermal desorption pro-cesses. Instead, they just assumed the total desorption ofthe grain species and followed the evolution of the CH OHabundance in the gas phase for different cosmic ray ionisa-tion rates. Consequently they could not study the possibilityof the replenishment of methanol with time. Secondly, we in-vestigate if a scenario based on episodic explosions of grainmantles could explain the widespread presence of methanolin the CMZ. Because of the spatial and kinematic correla-tion found between SiO (2–1) and CH OH emission in fourGalactic Centre clouds (Yusef-Zadeh et al. 2013c), we alsomodel the silicon monoxide (SiO). A description of the phys-ical and chemical model is presented in Section 2. The resultsare summarised in Section 3 and discussed in Section 4.
The model is largely similar to that which was used in Rawl-ings et al. (2013a), so that it includes a cold, quiescent phase,during which a standard interstellar chemistry operates andice mantles form (phase I), followed by an explosion phasein which high densities and temperatures drive a rapid rad-ical association chemistry (phase II). After a relaxation inwhich ices are re-deposited on grain surfaces and H atomsand radicals generated within them, the process is repeated.The cycle period and the triggering threshold are calculatedself-consistently and are determined by the build-up of rad-icals in the ice mantles. The frequency of the explosions isdetermined by the cloud chemistry in phase I. They are pro-duced by accumulation of H atoms on the grain surface oncetheir fractional abundance reaches a critical value f H of 0.05(Duley & Williams 2011). The fraction of H-atoms freezingout and not converting to H ( p H ) is assumed to be equal to0.1 in the standard case. The chemistry is followed througha number of cycles (5) and the results of this (stochastic)procedure are presented as the time-averaged abundancesof the various molecular species. The 2013 model has beenupdated in a number of ways: • The gas-phase chemistry of CH OH and SiO has beenincluded. In addition to the species listed in Table 2 of Rawl-ings et al. (2013a) we added in Phase I the following species:CH OH + , CH OH + (for their role in the gas phase forma-tion of methanol) and Si, Si + , SiH, SiH + , SiH , SiH + , SiH ,SiH + , SiH , SiH + , SiH + , SiO, SiO + and SiOH + (for theirrole in the formation of SiO). The gas phase reactions arelisted in Tables A1–A4 and come from the UMIST Databasefor Astrochemistry 2012 (McElroy et al. 2013). • A comprehensive list of continuous desorption mech-anisms has been incorporated. These include photo-desorption (direct and cosmic-ray photon-induced) (Robertset al. 2007; Hollenbach et al. 2009), cosmic ray spot andwhole grain heating (Hasegawa & Herbst 1993; Bringa &Johnson 2004), and enthalpy-driven desorption (Robertset al. 2007). These processes are treated self-consistently andmake clear distinctions between surface and and bulk des-orption processes. • A basic surface chemistry (for simple oxygen-bearingspecies) is also included: formation of CO through the re- MNRAS , 1–10 (2016) action of CO with O, O + , and OH, conversion of the rest ofO, O + , and OH into H O and conversion of CO into CH OH. • Thermal desorption, which is due to the dust temper-ature, is not explicitly included in our model. However, tosimulate the possible effects of thermal desorption of somevolatile species, we consider that, for a dust temperature of ∼
25 K, only 20% of CO, N and O and 30% of CH re-main frozen on the grains, the rest desorbing immediately.These results are based on the empirical findings of TPDexperiments (Viti et al. 2004). We also allow a fraction ofadsorbed species to be desorbed immediately on hydrogena-tion; e.g. C/C + can freeze-out and be converted to CH ,which is partially released back to the gas-phase. • The complex organics created in the explosion phasewere included in the gas-phase chemistry of the cold, quies-cent phase (although we do not attempt to model the for-mation of second-generation COMs in this phase).As in our previous models (and despite the desorptionrate for ice mantle species being high) the large flux of hy-drogen atoms impinging on the grains, together with thefact that the surface hydrogenation processes are extremelyrapid means that we assume that rapid and efficient conver-sion of C to CH , O to H O, and N to NH occurs. Differentradicals, which will participate in the formation of complexorganic molecules in Phase II, are assumed to form on thegrain by H abstraction of grain surface species due to theaction of the cosmic ray induced UV field with a radical for-mation rate ( R rad ) dependent on the cosmic ray ionisationrate. For example, CH leads to the radical CH , NH toNH , H O to OH, CH OH to CH OH and CH O, and SiH to SiH . During the explosion phase, the radicals that arereleased in the gas phase can react though three-body re-actions to form complex species. A total of 34 species and18 three-body reactions are considered in phase II. They arelisted in Tables 3 and 4 of Rawlings et al. (2013a). The otherspecies are considered as chemically inactive and are just re-turned to the gas-phase for the next cycle. The only relevantreaction in Phase II for this study is the rapid radical asso-ciation between CH , OH and the third reactant H O thatforms methanol.The physical parameters used for the model definedas the standard model are listed in Table 1. Most of theparameters listed in Table 1 are varied to determine theirimpact on the fractional abundances of methanol andsilicon monoxide (see Table 3). The column densities of H are typically 10 –10 cm − , implying an A V ∼ − (e.g., Longmore et al. 2012; Yusef-Zadeh et al. 2013c). Theradiation field strength G o is assumed to range between10 and 10 Habing depending on the study (Kim et al.2011; Clark et al. 2013; Harada et al. 2015; Bertram et al.2016). We adopt a value of 10 Habing, but do not expectthe chemistry to be photon-dominated for A V > . Thenominal size of the CMZ region that we study is of theorder of a few 100 pc. This has relevance when one considersthe response to the region to possibility of rapidly changingphysical conditions. Table 1.
Parameters in the standard modelParameter ValueHe/H 0.1C/H 2.6 × − N/H 6.1 × − O/H 4.6 × − S/H 1.0 × − Na/H 1.0 × − Si/H 1.0 × − Density ( n I ) 2 × cm − Kinetic temperature ( T k ) 200 KDust temperature ( T d ) 25 KCosmic ray ionisation rate ( ζ ) 10 − s − Visual extinction ( A V ) 10 magRadiation field ( G o ) 10 HabingStandard interstellar radiation field photon flux 10 cm − s − Photodesorption yield per photon 10 − Scaling factor for the cosmic-ray induced UV field 4.88 × − H-atom non recombination probability ( p H ) 0.1Explosion threshold abundance of H ( f H ) 0.05No. of (refractory) atoms per grain ( N g ) 10 Mantle radical formation rate ( R rad ) ( a ) − Average grain radius ( a ) 0.0083 µ mDust surface area per H-nucleon ( σ H ) 1.6 × − cm Grain albedo 0.5CO → CH OH conversion efficiency ( f CO → CH OH ) 10%Phase II: initial density ( n II ) 10 cm − Phase II: initial temperature ( T II ) 1000 KPhase II: three-body rate coefficients ( k ) 10 − cm s − Number of cycles ( n cyc . ) 5 ( a ) for a standard cosmic ray ionisation rate ζ = 1.3 × − s − OH and SiO
The formation of CH OH can be divided into three differentprocess types: • First, it can be produced in the gas-phase (see completelist of reactions in Table A1), e.g. CH + + H O → CH OH + + h ν followed by CH OH + + e − → CH OH + H . The gas-phase reactions are known to inefficiently formmethanol for local interstellar conditions. • Secondly, it can be produced in, or on, icy mantlesby surface hydrogenation (e.g., Watanabe & Kouchi 2002;Fuchs et al. 2009): CO s H −→ CH OH s followed by its non-thermal desorption into the gas-phaseduring phase I or in the explosion of the grain mantle duringphase II. The surface hydrogenation of CO is the channelthat is most usually invoked for the efficient production ofCH OH in the interstellar medium. We assume a conversionefficiency ( f CO → CH OH ) of 10% (e.g., Watanabe & Kouchi2002; ¨Oberg et al. 2011). This parameter could, however,have a different value in the CMZ (see Section 4 for moredetails). MNRAS000
The formation of CH OH can be divided into three differentprocess types: • First, it can be produced in the gas-phase (see completelist of reactions in Table A1), e.g. CH + + H O → CH OH + + h ν followed by CH OH + + e − → CH OH + H . The gas-phase reactions are known to inefficiently formmethanol for local interstellar conditions. • Secondly, it can be produced in, or on, icy mantlesby surface hydrogenation (e.g., Watanabe & Kouchi 2002;Fuchs et al. 2009): CO s H −→ CH OH s followed by its non-thermal desorption into the gas-phaseduring phase I or in the explosion of the grain mantle duringphase II. The surface hydrogenation of CO is the channelthat is most usually invoked for the efficient production ofCH OH in the interstellar medium. We assume a conversionefficiency ( f CO → CH OH ) of 10% (e.g., Watanabe & Kouchi2002; ¨Oberg et al. 2011). This parameter could, however,have a different value in the CMZ (see Section 4 for moredetails). MNRAS000 , 1–10 (2016)
A. Coutens et al. • Finally, methanol can be formed in the high density gas-phase following mantle explosion via three-body reactions: CH + OH + M → CH OH + M where M is the third body reactant (H O).The three processes may play a role, but in the case ofa region (such as the CMZ) that is subject to high cosmicray ionization rates the composition of the ice mantles maybe most unlike that pertained in normal interstellar clouds.In particular, CO could be vulnerable to efficient desorptionby cosmic-ray spot heating, in which case the ices may beCO-poor. In these circumstances, the second of the mecha-nisms above may not be as efficient as in the local interstellarmedium and the third channel may be an alternative routeto efficient CH OH formation. The objective of this study isto evaluate the efficiency of the three mechanisms for CMZconditions.The silicon chemistry is dominated by gas phase reac-tions and gas-grain interactions. In our model, Si (or Si + )sticks to grains and, assuming a similar pattern as for otherelements, is hydrogenated on the surface to silane (SiH ).Silane has a very high binding energy and is certainly re-sistant to continuous desorption processes (Turner 1991).However, any disruptive processes (such as significant heat-ing, shocks, or mantle explosions) result in its release tothe gas-phase. Thereafter it is rapidly and efficiently con-verted to SiO via a network of gas phase reactions (see Ta-bles A2–A4), none of which have activation barriers andso are efficient even at low temperatures. It therefore fol-lows that, if mantle explosions are operating, high fractionalabundances of gas-phase SiO are expected, unless silicon isdepleted. This would be consistent with observations whichshow widespread SiO emission in molecular clouds (Yusef-Zadeh et al. 2013a,c). No three-body reaction involving Si-bearing species is considered in phase II. The only relevantmechanism for these species in phase II is their desorptiondue to the grain mantle explosion, that makes them avail-able for gas phase reactions during the quiescent phase ofthe next cycle. In a first step, we test the scenario proposed by Yusef-Zadehet al. (2013c) to explain the presence of methanol by a sim-ple increase of the cosmic ray ionisation rate. The physicaland chemical attributes of this model are exactly as for thecyclic explosion model described below, but for these calcu-lations, the H-atom non recombination probability ( p H ) isequal to 0 and we just consider the chemical evolution andabundances in the quiescent phase (phase I). We find thatthe composition of the ices is largely determined by the rel-ative efficiencies of the cosmic-ray heating process. For theparameters of our standard model, the ices are mainly com-posed of H O and CH , with small amounts of NH and onlytraces ( < . ) of CO and CO .Table 2 lists the fractional abundances of gas-phasemethanol predicted at t = 10 years for different densities,temperatures and cosmic ray ionisation rates, while Figure 1shows the variation of the methanol gas phase abundance as Table 2.
Fractional abundances of CH OH with respect to H-nucleons obtained in the case of a model without explosion at t = 10 yrs.Model Parameters CH OH f CO → CH OH = 10%1 Standard 7.9 × − n I = 2 × cm − × − n I = 2 × cm − × − n I = 2 × cm − × − T k = 100 K 1.2 × − T k = 300 K 3.5 × − T k = 400 K 6.3 × − ζ = 10 − s − × − ζ = 10 − s − × − ζ = 10 − s − × − a function of time for four different values of the cosmic rayionisation rate (10 − –10 − s − ). For a conversion efficiencyof CO into CH OH of 10%, the fractional abundances ofmethanol with respect to H range between 10 − and 10 − ,which only corresponds to the lowest range of abundancesof methanol determined in the CMZ ( ∼ − –10 − , Yusef-Zadeh et al. 2013c). It should be noted that the predictedabundance of methanol could even be overestimated. First,the (somewhat arbitrary) CO to CH OH conversion ratemay overestimate the efficiency of the process in the CO-poor ices that are predicted to be present in the CMZ (seeSection 4 for more details). Secondly, recent studies (Bertinet al. 2016; Cruz-Diaz et al. 2016) have shown that CH OHis vulnerable to fragmentation during photodesorption - inwhich case its presence in the cold gas-phase is probablyattributable to either non-continuous (explosive) desorptionof ice mantles, or some, as yet unidentified, efficient gas-phase formation mechanism. In the absence of conversionof CO into CH OH and for a cosmic ray ionisation rate of10 − s − , the fractional abundance of methanol is only ∼ × − , confirming that the gas phase reactions are notefficient to produce methanol even for CMZ conditions.As this scenario cannot reproduce the highest fractionalabundances of methanol derived in the CMZ (Requena-Torres et al. 2008; Yusef-Zadeh et al. 2013c), we explorethereafter how the abundance of methanol is affected in thecase of episodic explosions of grain mantles. As shown by Rawlings et al. (2013a), after a few cycles ( < years. MNRAS , 1–10 (2016) time (years)10 -13 -12 -11 -10 -9 -8 -7 CH O H / H Figure 1.
Fractional abundances of gas-phase methanol with re-spect to H-nucleons as a function of time predicted by the modelwithout explosion for different cosmic ray ionisation rates (dottedline: ζ = 10 − s − , dashed line: ζ = 10 − s − , solid line: ζ =10 − s − , dotted-dashed line: ζ = 10 − s − ). The density is 2 × cm − and the gas temperature is 200 K. The parameters listed in Table 1 are varied to study theirimpact on the fractional abundances of methanol. The time-averaged abundances of methanol obtained for differentmodels during cycle 5 are listed in Table 3. In particular,to test the efficiency of the different routes of methanol inthe CMZ, a conversion efficiency f CO → CH OH of 0 (instead of10%) is used to switch off the grain surface chemistry mech-anism and a three-body reaction rate k B equal to 0 is usedto switch off the three-body reaction in phase II.We can see in Table 3 that the dominant mechanismfor methanol in the CMZ is the formation on the grain byhydrogenation of CO with an efficiency f CO → CH OH of 10%followed by the desorption due to the explosion. For the stan-dard model, when the three mechanisms are switched on, thefractional abundance of methanol with respect to H is about9 × − , while it is only 1.5 × − when the grain surfaceformation mechanism is switched off. Similar results are ob-tained for the other models. The formation of methanol ongrain surface leads to abundances that are 1–2 orders of mag-nitude higher than in the absence of this mechanism. Theonly model that shows a comparable abundance of methanolwith or without the grain surface chemistry mechanism cor-responds to a model with a very low density ( ∼ cm − ).The gas phase mechanism alone (model 23, f CO → CH OH =0) leads to a fractional abundance of methanol of 5 × − .As shown for the case without explosion, this mechanism isnegligible in the CMZ. It should be noted that the domi-nance of the hydrogenation mechanism on the grain for theformation of methanol is dependent on the conversion ef-ficiency of CO into CH OH ( f CO → CH OH ). We assumed anefficiency of 10%. If it were, however, lower ( f CO → CH OH < Table 3.
Time-averaged fractional abundances of CH OH andSiO (with respect to H) in the final cycle for different modelswith episodic explosions of grain mantles.Model Parameters CH OH ( a ) CH OH ( b ) SiO f CO → CH OH
10% 01 Standard 9.2 × − × − × − n I = 2 × cm − × − × − × − n I = 2 × cm − × − × − × − n I = 2 × cm − × − × − × − T k = 100 K 5.9 × − × − × − T k = 300 K 1.3 × − × − × − T k = 400 K 2.0 × − × − × − ζ = 10 − s − × − × − × − ζ = 10 − s − × − × − × − ζ = 10 − s − × − × − × − p H = 0.01 1.2 × − × − × − p H = 0.05 9.9 × − × − × − p H = 0.2 8.2 × − × − × − f H = 0.01 5.6 × − × − × − f H = 0.1 9.9 × − × − × − n II = 10 cm − × − × − × − n II = 10 cm − × − × − × − n II = 10 cm − × − × − × − n II = 10 cm − × − × − × − k B = 10 − cm s − × − × − × − k B = 10 − cm s − × − × − × − k B = 10 − cm s − × − × − × − k B = 0 9.1 × − × − × − σ H = 2 × − cm − × − × − × − σ H = 4 × − cm − × − × − × − σ H = 8 × − cm − × − × − × − σ H = 3.2 × − cm − × − × − × − T d = 20 K ( c ) × − × − × − T d = 30 K ( c ) × − × − × −
30 Si/H = 10 − × − × − × −
31 Si/H = 10 − × − × − × − Notes: ( a ) This column shows the abundances of methanol ob-tained when the three mechanisms for the formation of methanolare included. ( b ) This column shows the abundances of methanolobtained when we only consider the gas phase mechanismin phase I and the rapid radical association in phase II. Thehydrogenation of CO on the grains is switched off. ( c ) In thestandard case ( T d = 25 K), we consider that 20% of CO, O , andN and 30% of CH remain frozen, while for a dust temperatureof 20 K, we consider that 65% of of CO, O , and N and 100% ofCH remain frozen. In the case of T d = 30 K, we consider thatCO, O , and N do not freeze but that CH can still freeze with30% remaining on the grains. abundance of CH produced by hydrogen abstraction of CH is not sufficient to produce enough methanol. It is limited bythe mantle radical formation rate. Even if a higher cosmicray ionisation rate should increase its value, the timescaleof each cycle is too short ( ∼ years) to lead to very highabundances of CH , ii) even when the amount of CO ice isrelatively low, 10% of conversion into methanol is sufficientto produce enough methanol on grains (10 − ). MNRAS000
31 Si/H = 10 − × − × − × − Notes: ( a ) This column shows the abundances of methanol ob-tained when the three mechanisms for the formation of methanolare included. ( b ) This column shows the abundances of methanolobtained when we only consider the gas phase mechanismin phase I and the rapid radical association in phase II. Thehydrogenation of CO on the grains is switched off. ( c ) In thestandard case ( T d = 25 K), we consider that 20% of CO, O , andN and 30% of CH remain frozen, while for a dust temperatureof 20 K, we consider that 65% of of CO, O , and N and 100% ofCH remain frozen. In the case of T d = 30 K, we consider thatCO, O , and N do not freeze but that CH can still freeze with30% remaining on the grains. abundance of CH produced by hydrogen abstraction of CH is not sufficient to produce enough methanol. It is limited bythe mantle radical formation rate. Even if a higher cosmicray ionisation rate should increase its value, the timescaleof each cycle is too short ( ∼ years) to lead to very highabundances of CH , ii) even when the amount of CO ice isrelatively low, 10% of conversion into methanol is sufficientto produce enough methanol on grains (10 − ). MNRAS000 , 1–10 (2016)
A. Coutens et al.
10 100 1000time (years)10 -8 -7 -6 CH O H / H Figure 2.
Fractional abundance of gas-phase methanol with re-spect to H-nucleons as a function of time in phase I for the stan-dard model with episodic explosions of grain mantles.
Some trends are observed with the change of some pa-rameters. For example, the higher the density, the higher thefractional abundance of methanol. This is expected due tothe freezing of the species (including CO) on the grains thatis more efficient at high density. The abundance of methanolslightly increases with the kinetic temperature. It is alsohigher when the cosmic ray ionisation rate is low. The vari-ation of the H-atom non recombination probability ( p H ) andthe explosion threshold abundance of H ( f H ) do not lead tosignificant variation of the abundance of methanol (lowerthan a factor 2). The variation of phase II parameters suchas the initial density ( n II ) and the three-body reaction rate( k ) do not change the fractional abundances of methanolwhen the hydrogenation of CO is included, as it is the dom-inant mechanism. In the absence of the grain surface mech-anism, methanol is, as expected, produced more efficientlywith a higher initial density and a higher three-body reactionrate. The size distribution of the grains can also be investi-gated by varying σ H , the dust surface area per H-nucleon.A high value of σ H increases the fractional abundance ofmethanol, both with and without the inclusion of the grainsurface hydrogenation of CO.We explored two cases with different dust temperaturesfor which we assumed different fractions of volatile speciesdesorbing at low temperatures. For a lower dust temperature( ∼
20 K), we assumed that, after depletion, 65% of CO, O ,and N and 100% of CH remain frozen on the grains. Inthe extreme case of a higher dust temperature ( ∼
30 K),we consider that CO, O , and N do not freeze at all, while30% of CH still remain frozen. As expected, in the lattercase, we obtain the same results as in the absence of theCO hydrogenation on the grains. For the lower temperaturecase, the abundance of methanol is a factor 3 higher due tothe highest fraction of CO that remains on the grain.Figure 2 shows the abundance of methanol as a functionof time during phase I for the standard model. The cyclehas a duration of 1.2 × years. Right after the explosionphase, the abundance of methanol in the gas-phase reachesa value of 6 × − then decreases with the time until thenext explosion. This shows that the explosion is required toreach high abundances of methanol. The main uncertainty for the silicon chemistry is the initialabundance of Si. Indeed, it is thought that Si is depletedwith respect to the solar value (Si/H ∼ × − , Asplundet al. 2009), but the depletion factor is not constrained. Inquiescent clouds like TMC-1, L183, and L1448, silicon isobserved to be heavily depleted (SiO/H (cid:46) − , Ziuryset al. 1989; Martin-Pintado et al. 1992; Requena-Torres et al.2007). The bulk of Si is expected to be in the grain cores.With the standard model (Si/H ∼ − ), the fractionalabundance of SiO is about 5 × − . The abundance of SiOis proportional to the initial Si abundance. In the case ofan initial Si abundance of 10 − , the SiO abundance wouldrange between 1 × − (for models with low density n I ∼ cm − , high density n I ∼ cm − , low cosmic ray ioni-sation rate ζ = 10 − s − or high ionisation rate ζ = 10 − s − ) and 5 × − (standard model). It should also be notedthat the abundances predicted by a model that considers thefreezing of Si into solid Si (instead of SiH ) do not changesignificantly.The main reaction that leads to the formation of SiO inour standard model is: SiOH + + e − → SiO + H , with SiOH + formed by the reactions between SiO + and H and between Si + and H O. Other reactions such as Si + O → SiO + O and Si + OH → SiO + H contribute to the formation of SiO as well. In this section, we compare the results predicted by our mod-els with observations. As the observed abundances are cal-culated with respect to H while the predicted abundancesby the chemical models are with respect to H-nucleons, wecorrected the latter to be expressed with respect to H (i.e.multiplied by a factor 2). Yusef-Zadeh et al. (2013c) estimated a fractional abun-dance of methanol in the CMZ ranging between 10 − and10 − . Requena-Torres et al. (2008) estimated even higherabundances for three molecular clouds located in the CMZ(3 × − for G–0.02–0.07, 1.1 × − for G–0.11–0.08, and5 × − for G+0.693–0.03). With a conversion efficiency ofCO into CH OH of 10%, our standard model with episodicexplosions is in agreement with the high abundances de-rived in these studies ( ∼ × − ). With high densities,the abundance is even higher (about 7 × − –1 × − ),which is consistent with the high abundances derived for themolecular clouds. On the contrary, in the absence of explo-sion and for high densities ( ≥ × cm − ), the predictedabundance of methanol is low ( ≤ × − ) compared to theobservations. It should be, however, noted that, for a highdensity of ∼ cm − , the gas and dust temperatures are MNRAS , 1–10 (2016) certainly coupled, which would mean very high dust temper-atures ( (cid:38)
100 K) and the complete absence of ices. Based onthese results, a scenario with cyclic explosions of grain man-tles is sufficient to explain the observations. No additionalmechanism is required to reproduce the observational abun-dance of methanol in the CMZ and the cyclic aspect of ourmodel can explain the continuous and widespread presenceof methanol.These results are, however, dependent on the fraction ofCO that hydrogenates into CH OH. When there is no hydro-genation of CO on the grains, the standard model predictsa fractional abundance of ∼ × − , while the abundancecan reach ∼ − for a high density. It is still consistentwith the lowest abundances derived in the CMZ, but somehydrogenation of CO on the grains is required to explainthe highest abundances. Experiments were carried out fordifferent types of ice and for different temperatures (e.g.,Watanabe et al. 2004; Fuchs et al. 2009; Cuppen et al. 2009)and the conversion efficiency seems to be quite dependent onthese parameters. For example, a range of conversion frac-tion from 5% to 100% are given as a function of the temper-ature and the H/CO gas phase abundance ratio in Cuppenet al. (2009). But none of these experiments was carried outat a temperature higher than 20 K. Based on our results, aconversion efficiency of ∼
10% appears to be needed to repro-duce the observations, which is similar to what is found forhot cores. It could also be higher if a fraction of methanoldoes not survive the evaporation. If the conversion of COinto CH OH on grains was not as efficient as ∼
10% at thistemperature, there is still the possibility that hydrogenationof CO could instead occur (with the rapid radical associationreactions) in the ultra high density gas phase immediatelyafter the explosion. More studies would be needed to deter-mine if this type of reaction is viable during the explosionphase.As the dominant mechanism leading to the formationof methanol in the CMZ is found to be through grain sur-face formation (instead of the rapid radical association in thehigh density gas phase following the grain mantle explosion),any other mechanism that can regularly release the molecu-lar content of the grain mantles in the gas phase should beconsidered. Shocks are consequently another option. Shockmodels can locally produce high abundances of methanolof ∼ − –10 − (e.g., Viti et al. 2011; Flower & Pineau desForˆets 2012) and there is some evidence of cloud-cloud col-lisions in the CMZ (e.g., Hasegawa et al. 1994; Tsuboi et al.2015; Tanaka 2016). Some of the methanol-rich positionsdetected by Yusef-Zadeh et al. (2013c) could correspond toshock positions, especially if they are located at the inter-section of clouds. In this case, shocks may certainly be thereason for the release of methanol in the gas phase. Several studies on silicon monoxide were carried out towardsGalactic Centre molecular clouds. In particular, Minh et al.(1992) found a SiO abundance with respect to H of 10 − –10 − for the Sgr A molecular cloud. Mart´ın-Pintado et al.(1997) derived a fractional abundance of ∼ − for someSiO-rich molecular clouds, while an upper limit of ∼ − was obtained for other molecular clouds of the Galactic Cen-tre. More recently, Minh et al. (2015) derived an abundance of ∼ − towards the circumnuclear disk. If we consider aninitial abundance of Si depleted by a factor 400 with respectto the solar abundance (Si/H = 10 − ), the SiO abundancespredicted by our models is on average 1 × − (with respectto H ), which is in good agreement with the Sgr A measure-ment by Minh et al. (1992). In a few cases (low density n I = 10 cm − , high density n I = 10 cm − , low cosmic rayionisation rate ζ = 10 − s − and high cosmic ray ionisa-tion rate ζ = 10 − s − ), it can reach fractional abundancesas low as 2 × − , which is still higher than the observa-tional values for the SiO-poor molecular clouds studied byMart´ın-Pintado et al. (1997), (cid:46) − .With a depletion factor of about 4000 with respect tothe solar value (Si/H = 10 − ), the range of abundances pre-dicted by our models (2 × − –1 × − with respect toH ) is more comparable to the observational value. Sucha high depletion factor is similar to the one used for low-metal case studies (e.g., Graedel et al. 1982; Lee et al. 1996;Jim´enez-Serra et al. 2008). A very low Si/H initial abun-dance is also in agreement with one of the suggestions raisedby Yusef-Zadeh et al. (2013a) to explain the low SiO/N H + abundance ratio of the CMZ.Finally, the spatial correlation found for CH OH andSiO by Yusef-Zadeh et al. (2013a) could be explained bythe fact that both species require the release of grain mantlespecies in the gas phase. Shocks can also produce high abun-dances of SiO (e.g., Schilke et al. 1997; Harada et al. 2015),either through the release of grain mantle species or throughgrain core sputtering from high-velocity shocks, and couldbe a possible scenario as well.
The grain mantle explosion model can also explain the factthat methanol is not so abundant outside of the CMZ. Forrelatively diffuse regions ( n I = 2 × cm − , A V ∼ ζ ∼ − s − ),the average abundance of methanol and SiO predicted bythe model is several orders of magnitude lower ( ∼ − formethanol and ∼ − for SiO). The period of a cycle is alsolonger than for the CMZ (about 8 × years) due to thelower cosmic ray ionisation rate that implies a lower abun-dance of atomic H. This explains why methanol and SiOare not widespread and abundant outside of the CMZ. Fordenser regions (10 –10 cm − ) such as prestellar cores, thepredicted abundance of methanol is higher and ranges be-tween ∼ − and ∼ − after 5 cycles. However, the timebetween 2 explosions ( ∼ × years) is comparable to orlonger than the infall timescale leading to the formationof a protostar ( ∼ –10 years, Pagani et al. 2009, 2013;Br¨unken et al. 2014). It is consequently possible that thetime is not sufficient to allow an explosion to occur at highdensity in objects such as prestellar cores. In the most con-ducive case, only one or two explosions could occur, but afterthe first explosion, the abundance of methanol is relativelylow. For densities of about 10 cm − , the average abundanceof methanol after the first explosion is about 10 − , while itis about a few 10 − for densities of about 10 cm − . Thislast value is very similar to the abundance of methanol foundin dark clouds and prestellar cores such as L1544, TMC-1or L134N ( ∼ − , Friberg et al. 1988; Vastel et al. 2014).In conclusion, although episodic explosions could theoreti- MNRAS , 1–10 (2016)
A. Coutens et al. cally occur everywhere, they are more likely to have an effectin the CMZ than in the local interstellar medium owing tothe higher cosmic ray ionisation rate that increases the fre-quency of the grain mantle explosions.
In this paper, we show that the increase of the cosmic rayionisation rate is not sufficient to explain the widespreadand abundant presence of methanol in the CMZ. A scenariowith episodic explosions of grain mantles gives, however, agood agreement between the predicted abundances and theobservations. The repetition of the explosions can also ex-plain the widespread presence of methanol on scales of a fewhundred parsecs. According to this scenario, the dominantmechanism for the formation of methanol in the CMZ is thegrain surface formation through hydrogenation of CO fol-lowed by the desorption due to the explosion. Our modelalso reproduces the SiO abundance in the case of a lowSi/H initial abundance of about 10 − . As both methanoland SiO require grain surface formation mechanisms, shockscould be another possible scenario to explain the presence ofmethanol and SiO in the CMZ. As shown by Rawlings et al.(2013b), the episodic explosion models present the advan-tage that they can explain the presence of large and com-plex molecules after several cycles. This type of model couldconsequently also explain the presence of complex organicmolecules such as glycolaldehyde and propylene oxide in thecold gas surrounding the star-forming region Sgr B2 (Holliset al. 2004; McGuire et al. 2016). More experimental andtheoretical work is however required to constrain the chem-istry during the explosion phase. ACKNOWLEDGEMENTS
The authors would like to thank an anonymous referee forvaluable comments and suggestions. The work of AC wasfunded by the STFC grant ST/M001334/1.
REFERENCES
Ao Y., et al., 2013, A&A, 550, A135Asplund M., Grevesse N., Sauval A. J., Scott P., 2009, ARA&A,47, 481Bayet E., Williams D. A., Hartquist T. W., Viti S., 2011, MNRAS,414, 1583Bertin M., et al., 2016, ApJ, 817, L12Bertram E., Glover S. C. O., Clark P. C., Ragan S. E., KlessenR. S., 2016, MNRAS, 455, 3763Bringa E. M., Johnson R. E., 2004, ApJ, 603, 159Br¨unken S., et al., 2014, Nature, 516, 219Cecchi-Pestellini C., Rawlings J. M. C., Viti S., Williams D. A.,2010, ApJ, 725, 1581Clark P. C., Glover S. C. O., Ragan S. E., Shetty R., KlessenR. S., 2013, ApJ, 768, L34Cruz-Diaz G. A., Mart´ın-Dom´enech R., Mu˜noz Caro G. M., ChenY.-J., 2016, A&A, 592, A68Cuppen H. M., van Dishoeck E. F., Herbst E., TielensA. G. G. M., 2009, A&A, 508, 275Duley W. W., Williams D. A., 2011, ApJ, 737, L44Flower D. R., Pineau des Forˆets G., 2012, MNRAS, 421, 2786 Friberg P., Hjalmarson A., Madden S. C., Irvine W. M., 1988,A&A, 195, 281Fuchs G. W., Cuppen H. M., Ioppolo S., Romanzin C., BisschopS. E., Andersson S., van Dishoeck E. F., Linnartz H., 2009,A&A, 505, 629Ginsburg A., et al., 2016, A&A, 586, A50Graedel T. E., Langer W. D., Frerking M. A., 1982, ApJS, 48,321G¨usten R., Walmsley C. M., Pauls T., 1981, A&A, 103, 197Harada N., et al., 2015, A&A, 584, A102Hasegawa T. I., Herbst E., 1993, MNRAS, 261, 83Hasegawa T., Sato F., Whiteoak J. B., Miyawaki R., 1994, ApJL,429, L77Hollenbach D., Kaufman M. J., Bergin E. A., Melnick G. J., 2009,ApJ, 690, 1497Hollis J. M., Jewell P. R., Lovas F. J., Remijan A., 2004, ApJ,613, L45Jim´enez-Serra I., Caselli P., Mart´ın-Pintado J., Hartquist T. W.,2008, A&A, 482, 549Kim S. S., Saitoh T. R., Jeon M., Figer D. F., Merritt D., WadaK., 2011, ApJ, 735, L11Le Petit F., Ruaud M., Bron E., Godard B., Roueff E., Lan-guignon D., Le Bourlot J., 2016, A&A, 585, A105Lee H.-H., Bettens R. P. A., Herbst E., 1996, A&AS, 119, 111Longmore S. N., et al., 2012, ApJ, 746, 117Martin-Pintado J., Bachiller R., Fuente A., 1992, A&A, 254, 315Mart´ın-Pintado J., de Vicente P., Fuente A., Planesas P., 1997,ApJ, 482, L45McElroy D., Walsh C., Markwick A. J., Cordiner M. A., SmithK., Millar T. J., 2013, A&A, 550, A36McGuire B. A., Carroll P. B., Loomis R. A., Finneran I. A., JewellP. R., Remijan A. J., Blake G. A., 2016, in American Astro-nomical Society Meeting Abstracts. p. 203.04Meijerink R., Spaans M., Loenen A. F., van der Werf P. P., 2011,A&A, 525, A119Minh Y. C., Irvine W. M., Friberg P., 1992, A&A, 258, 489Minh Y. C., Liu H. B., Su Y.-N., Hsieh P.-Y., Liu S.-Y., KimS. S., Wright M., Ho P. T. P., 2015, ApJ, 808, 86¨Oberg K. I., Boogert A. C. A., Pontoppidan K. M., van den BroekS., van Dishoeck E. F., Bottinelli S., Blake G. A., Evans IIN. J., 2011, ApJ, 740, 109Pagani L., et al., 2009, A&A, 494, 623Pagani L., Lesaffre P., Jorfi M., Honvault P., Gonz´alez-Lezana T.,Faure A., 2013, A&A, 551, A38Rawlings J. M. C., Williams D. A., Viti S., Cecchi-Pestellini C.,Duley W. W., 2013a, MNRAS, 430, 264Rawlings J. M. C., Williams D. A., Viti S., Cecchi-Pestellini C.,2013b, MNRAS, 436, L59Requena-Torres M. A., Marcelino N., Jim´enez-Serra I., Mart´ın-Pintado J., Mart´ın S., Mauersberger R., 2007, ApJ, 655, L37Requena-Torres M. A., Mart´ın-Pintado J., Mart´ın S., MorrisM. R., 2008, ApJ, 672, 352Riquelme D., Bronfman L., Mauersberger R., May J., WilsonT. L., 2010, A&A, 523, A45Roberts J. F., Rawlings J. M. C., Viti S., Williams D. A., 2007,MNRAS, 382, 733Rodr´ıguez-Fern´andez N. J., Mart´ın-Pintado J., Fuente A., WilsonT. L., 2004, A&A, 427, 217Schilke P., Walmsley C. M., Pineau des Forets G., Flower D. R.,1997, A&A, 321, 293Sodroski T. J., et al., 1994, ApJ, 428, 638Tanaka K., 2016, in EAS Publications Series. pp 181–184,doi:10.1051/eas/1575033Tsuboi M., Miyazaki A., Uehara K., 2015, PASJ, 67, 90Turner B. E., 1991, ApJ, 376, 573Vastel C., Ceccarelli C., Lefloch B., Bachiller R., 2014, ApJ, 795,L2Viti S., Collings M. P., Dever J. W., McCoustra M. R. S., WilliamsMNRAS , 1–10 (2016)
D. A., 2004, MNRAS, 354, 1141Viti S., Jimenez-Serra I., Yates J. A., Codella C., Vasta M., CaselliP., Lefloch B., Ceccarelli C., 2011, ApJ, 740, L3Watanabe N., Kouchi A., 2002, ApJ, 571, L173Watanabe N., Nagaoka A., Shiraki T., Kouchi A., 2004, ApJ, 616,638Yusef-Zadeh F., Wardle M., Lis D., Viti S., Brogan C., Cham-bers E., Pound M., Rickert M., 2013a, Journal of PhysicalChemistry A, 117, 9404Yusef-Zadeh F., et al., 2013b, ApJ, 762, 33Yusef-Zadeh F., Cotton W., Viti S., Wardle M., Royster M.,2013c, ApJ, 764, L19Ziurys L. M., Friberg P., Irvine W. M., 1989, ApJ, 343, 201
APPENDIX A: LIST OF REACTIONS INPHASE I INVOLVING METHANOL ANDSILICON MONOXIDE
MNRAS000
MNRAS000 , 1–10 (2016) A. Coutens et al.
Table A1.
List of reactions in Phase I involving methanolSi + + CH OH → SiOH + + CH H + + CH OH → CH OH + + HH + + CH OH → CH + + H OH + + CH OH → HCO + + H + H H + + CH OH → CH + + H O + H H + + CH OH → CH OH + + H He + + CH OH → OH + + CH + HEHe + + CH OH → OH + CH + + HEC + + CH OH → HCO + CH + CH + + CH OH → CH OH + + CCH + + CH OH → H CO + CH + CH + + H O → CH OH + + PHOTONCH + + CH OH → CH OH + + CH CH + + CH OH → CH OH + + CH N + + CH OH → CH OH + + NN + + CH OH → H CO + + NH + HN + + CH OH → NO + + CH + HN + + CH OH → NO + CH + + HO + + CH OH → CH OH + + OO + + CH OH → H CO + + H OH O + + CH OH → CH OH + + H OO2 + + CH OH → CH OH + + O2HCO + + CH OH → CH OH + + COH CO + + CH OH → CH OH + + HCOCH OH + + NH → CH OH + NH + Si + + CH OH → SiOH + + CH CH OH + + e − → CH + H O + HCH OH + + e − → H CO + H + HCH OH + + e − → OH + CH CH OH + + e − → CH + H O + HCH OH + + e − → CH + H OCH OH + + e − → CH + OH + HCH OH + + e − → CH OH + HCH OH + + e − → H CO + H + HCH OH + PHOTON → CH OH + + e − CH OH + PHOTON → H CO + H CH OH + PHOTON → OH + CH CH OH + + G → H CO + H + HCH OH + + G → CH + OH + HCH OH + G → GCH OHGCH OH + PHOTON → CH OH + GNotes : GCH OH correspond to solid methanol.This paper has been typeset from a TEX/L A TEX file prepared bythe author.
Table A2.
List of reactions in Phase I involving Si-bearingspecies O + SiH → SiH + OHSi + CO → SiO + CSi + O → SiO + OSi + CO → SiO + COSi + NO → SiO + NO + Si + → SiO + + PHOTONH + Si + → SiH + + PHOTONH + SiH + → Si + + H H + + Si → Si + + HH + + SiH → SiH + + HH + + SiH → Si + + H H + + SiH → SiH + + HH + + SiH → SiH + + H H + + SiH → SiH + + HH + + SiH → SiH + + H H + + SiH → SiH + + HH + + SiH → SiH + + H H + + SiO → SiO + + HH- + Si + → H + SiH- + SiO + → H + SiOH + + Si → SiH + + H H + + SiH → SiH + + H H + + SiH → SiH + + H H + + SiH → SiH + + H H + + SiH → SiH + + H He + + Si → Si + + HeHe + + SiH → Si + + He + HHe + + SiH → Si + + He + H He + + SiH → SiH + + He + HHe + + SiH → SiH + + He + H He + + SiH → SiH + + He + HHe + + SiH → Si + + He + H H He + + SiH → SiH + + He + H HHe + + SiO → Si + + O + HeHe + + SiO → Si + O + + HeC + SiO + → Si + + COC + + Si → Si + + CC + + SiH → SiH + + CC + + SiH → SiH + + CC + + SiO → Si + + COC − + Si + → C + SiC − + SiO + → C + SiOCO + SiH + → SiH + HCO + CO + SiO + → CO + Si + CH + SiH + → Si + CH + CH + SiO + → HCO + + SiCH + + Si → Si + + CHCH + SiO + → H CO + Si + CH + + SiH → SiH + + CH CH + + SiH → SiH + + CH + H N + SiO + → NO + + SiN + SiO + → NO + Si + NH + SiH + → Si + NH + NH + + Si → Si + + NH O + Si → SiO + PHOTONO + SiH → SiO + HO + SiH + → SiO + + HO + SiH → SiO + H O + SiH → SiO + H + HO + SiO + → O + Si + OH + Si → SiO + HOH + Si + → SiO + + HOH + + Si → SiH + + OOH + + SiH → SiH + + ONotes : G correspond to the solid form of the species.MNRAS , 1–10 (2016) Table A3.
Continuation of Table A2H O + SiH + → Si + H O + H O + SiH + → SiH + H O + H O + SiH + → SiH + H O + H O + + Si → Si + + H OH O + + Si → SiH + + H OH O + + SiH → SiH + + H OH O + + SiH → SiH + + H ONa + + Si + → Si + Na ++ HCO + SiO + → SiO + HCO + HCO + + SiH → SiH + + COHCO + + SiH → SiH + + COHCO + + SiH → SiH + + COCN + SiH → HCN + SiH NO + SiO + → SiO + NO + Si + O + → O + Si + Si + HCO + → SiH + + COSi + H CO + → H CO + Si + Si + NO + → NO + Si + Si + S + → S + Si + Si + HS + → HS + Si + Si + H S + → H S + Si + Si + CS + → CS + Si + SiH + S + → S + SiH + H + Si + → SiH + + PHOTONH + SiH + → SiH + + PHOTONH + SiH + → SiH + + PHOTONH + SiH + → SiH + + HH + SiO + → SiOH + + HH + + SiO → SiOH + + H NH + SiOH + → NH + + SiOO + SiH + → SiOH + + HO + SiH + → SiOH + + H OH + + SiO → SiOH + + OH O + Si + → SiOH + + HH O + + SiO → SiOH + + H OHCO + + SiO → SiOH + + COSi + + CH OH → SiOH + + CH SiH + + O → SiOH + + OHSi + + e − → Si + PHOTONSiH + + e − → Si + HSiH + + e − → Si + H SiH + + e − → Si + H + HSiH + + e − → SiH + HSiH + + e − → SiH + HSiH + + e − → SiH + H SiH + + e − → SiH + H SiH + + e − → SiH + HSiH + + e − → SiH + H SiH + + e − → SiH + HSiO + + e − → Si + OSiOH + + e − → Si + OHSiOH + + e − → SiO + HSi + PHOTON → Si + + e − SiH + PHOTON → Si + HSiH + + PHOTON → Si + + HSiH + PHOTON → SiH + + e − SiH + PHOTON → SiH + HSiH + PHOTON → SiH + HSiH + PHOTON → SiH + + e − SiH + PHOTON → SiH + H SiH + PHOTON → SiH + H SiH + PHOTON → SiH + HSiH + PHOTON → SiH + H + H SiO + PHOTON → Si + O
Table A4.
Continuation of Table A3SiO + PHOTON → SiO + + e − SiO + + PHOTON → Si + + OSi + + CH OH → SiOH + + CH Si + G → GSiSi + G → GSiH Si + + G → GSiSi + + G → GSiH SiH + G → GSiH SiH + + G → Si + HSiH + G → GSiH SiH + + G → Si + H + HSiH + G → GSiH SiH + + G → SiH + H SiH + G → GSiH SiH + + G → SiH + H SiH + + G → SiH + H SiO + G → GSiOSiO + + G → Si + OSiOH + + G → Si + OHGSi + PHOTON → SiGSiH + PHOTON → SiH GSiO + PHOTON → SiOMNRAS000