A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone

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by  Audrey Coutens

MNRAS000,1–10(2016) Preprint17January2017 CompiledusingMNRASLATEXstylefilev3.0
A study of methanol and silicon monoxide production
through episodic explosions of grain mantles in the Central
Molecular 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
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ABSTRACT
a
J Methanol (CH3OH) is found to be abundant and widespread towards the Central
Molecular Zone, the inner few hundred parsecs of our Galaxy. Its origin is, however,
5
not fully understood. It was proposed that the high cosmic ray ionisation rate in this
1
region could lead to a more efficient non-thermal desorption of this species formed on

] grain surfaces, but it would also mean that this species is destroyed in a relatively
A short timescale. In a first step, we run chemical models with a high cosmic ray ion-
G isation rate and find that this scenario can only reproduce the lowest abundances of
methanolderivedinthisregion(∼10−9–10−8).Inasecondstep,weinvestigateanother
.
h scenario based on episodic explosions of grain mantles. We find a good agreement be-
p tween the predicted abundances of methanol and the observations. We find that the
-
dominant route for the formation of methanol is through hydrogenation of CO on
o
r the grains followed by the desorption due to the grain mantle explosion. The cyclic
t aspect of this model can explain the widespread presence of methanol without requir-
s
a inganyadditionalmechanism.Wealsomodelsiliconmonoxide(SiO),anotherspecies
[ detected in several molecular clouds of the Galactic Centre. An agreement is found

with observations for a high depletion of Si (Si/H∼10−8) with respect to the solar
1
abundance.
v
3 Key words: astrochemistry–ISM:molecules–Galaxy:abundances–Galaxy:centre
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. 1 INTRODUCTION For example, using H + observations, Le Petit et al. (2016)
1 3
0 constrained it between 10−14 and 1.1 × 10−13 s−1, while
TheinnerfewhundredparsecsattheCentreofourGalaxy,
7 an upper limit of ζ < 10−14 s−1 is derived by Ginsburg
alsoknownastheCentralMolecularZone(CMZ),arechar-
1 acterisedbyspecificphysicalconditionscomparedtothelo- etal.(2016)basedonH2COobservations.Yusef-Zadehetal.
: (2013b) used synchrotron emission and Fe K line observa-
v calinterstellarmedium.Inparticular,thedusttemperature α
tionstoderiveacosmicrayionisationrateofabout(1–10)×
Xi andthegastemperatureareuncoupled(Gu¨stenetal.1981; 10−15 s−1. There are a number of chemical consequences of
Ao et al. 2013; Ginsburg et al. 2016). The dust tempera-
r ture is (cid:46) 30 K (Sodroski et al. 1994; Rodr´ıguez-Fern´andez astrongly-enhancedcosmicrayionizationrateincluding(i)
a higherlevelsofionization,moreHe+andalargerH:H ratio
et al. 2004; Longmore et al. 2012), while the gas tempera- 2
(Bayetetal. 2011;Meijerinketal.2011),(ii) astronger ra-
tureisuniformlyhigherthan60K(Aoetal.2013;Ginsburg
diationfieldduetointernally-generatedcosmic-rayinduced
et al. 2016) and possibly about 200–500 K in the most dif-
photons,and(iii)enhanceddesorptionratesforicemantles
fuse regions (n < 100 cm−3, Le Petit et al. 2016). This gas
by cosmic ray spot heating, whole grain heating and CR-
heating was proposed to be due to turbulence in dense gas
induced photodesorption.
(Ginsburg et al. 2016) or a high cosmic ray ionisation rate
in diffuse regions (Le Petit et al. 2016). Indeed, the cosmic Emission from a large range of species has been de-
ray ionisation rate in the CMZ is thought to be enhanced tected towards the CMZ (e.g. Mart´ın-Pintado et al. 1997;
by several orders of magnitude with respect to the local in- Requena-Torres et al. 2008; Riquelme et al. 2010; Yusef-
terstellar medium, but its exact value is however debated. Zadeh et al. 2013c; Harada et al. 2015). Some of these
species are believed to be produced in the gas phase (e.g.,
CO, CS, HCO+, HCN, N H+), while others such as SiO,
2
(cid:63) E-mail:[email protected] NH3, CH3OH and other complex organic molecules (e.g.,
(cid:13)c 2016TheAuthors
2 A. Coutens et al.
CH CHO,CH OHCHO,HCOOCH ,(CH OH) )appearto rate but did not include the non-thermal desorption pro-
3 2 3 2 2
requiregrainsurfacechemistryfortheirformation.Inpartic- cesses. Instead, they just assumed the total desorption of
ular,methanol(CH OH)wasfoundtobewidespreadinthe the grain species and followed the evolution of the CH OH
3 3
CMZwithafractionalabundanceof10−9–10−7withrespect abundance in the gas phase for different cosmic ray ionisa-
to H (Yusef-Zadeh et al. 2013c). Although the CMZ con- tionrates.Consequentlytheycouldnotstudythepossibility
2
tains some star-forming regions such as Sgr B2, methanol ofthereplenishmentofmethanolwithtime.Secondly,wein-
is also observed towards regions devoid of star formation vestigate if a scenario based on episodic explosions of grain
activity. To explain the widespread presence of methanol, mantles could explain the widespread presence of methanol
Yusef-Zadehetal.(2013c)proposedthatitwouldbedueto in the CMZ. Because of the spatial and kinematic correla-
thehighcosmicrayionisationrate,whichwouldleadtothe tion found between SiO (2–1) and CH OH emission in four
3
desorption of this grain surface species by the induced UV Galactic Centre clouds (Yusef-Zadeh et al. 2013c), we also
field.However,thehighcosmicrayionisationratealsoleads modelthesiliconmonoxide(SiO).Adescriptionofthephys-
to a destruction of methanol on a relatively short timescale icalandchemicalmodelispresentedinSection2.Theresults
(the higher the cosmic ray ionisation rate, the shorter the are summarised in Section 3 and discussed in Section 4.
destruction timescale – ∼105 years for ζ = 5×10−16 s−1
vs ∼103 years for ζ = 5×10−14 s−1) and it is uncertain
if methanol can be replenished on this timescale. Based on
IRAM-30m and APEX observations towards the circumnu-
2 PHYSICAL AND CHEMICAL MODEL
clear disk, Harada et al. (2015) explored the presence of 13
otherspecies(CS,CN,H CO,SO,N H+,H O+,SiO,HCN, 2.1 Model
2 2 3
HCO+,HNC,HC N,NO,CO)andtestedwhethersputter-
3 ThemodelislargelysimilartothatwhichwasusedinRawl-
ingduetoshocksordesorptionduetocosmicrayscouldre-
ingsetal.(2013a),sothatitincludesacold,quiescentphase,
producetheobservedchemistrybetter.Theyconcludedthat
duringwhichastandardinterstellarchemistryoperatesand
models with a high cosmic ray ionisation rate of ∼ 10−15–
ice mantles form (phase I), followed by an explosion phase
10−13 s−1 and a density of about 104 cm−3 gave the best
inwhichhighdensitiesandtemperaturesdrivearapidrad-
agreement with these observations. Alternatively, a model
ical association chemistry (phase II). After a relaxation in
with high-velocity shocks (> 40 kms−1) could possibly ex-
which ices are re-deposited on grain surfaces and H atoms
plain the data but the timespan required for an agreement
andradicalsgeneratedwithinthem,theprocessisrepeated.
with the observations is very short (∼ 3 × 102 yrs).
Thecycleperiodandthetriggeringthresholdarecalculated
A scenario complementary to conventional interstellar
self-consistentlyandaredeterminedbythebuild-upofrad-
chemistry was proposed by Rawlings et al. (2013a,b) to ex-
icals in the ice mantles. The frequency of the explosions is
plainthepresenceoflargeorganicmoleculessuchaspropy-
determinedbythecloudchemistryinphaseI.Theyarepro-
lene in dark clouds. According to this mechanism, large
ducedbyaccumulationofHatomsonthegrainsurfaceonce
moleculescouldformintheultrahighdensitygasphaseim-
theirfractionalabundancereachesacriticalvalue f of0.05
mediately after episodic explosions of grain mantles. These H
(Duley & Williams 2011). The fraction of H-atoms freezing
explosions would be driven by the spontaneous recombina-
outandnotconvertingtoH (p )isassumedtobeequalto
tionoftrappedhydrogenatoms,whichwouldleadtoasud- 2 H
0.1 in the standard case. The chemistry is followed through
denincreaseofthedusttemperature.Indeed,whenacritical
a number of cycles (5) and the results of this (stochastic)
number of H-atoms on the grain is reached, a localised re-
procedure are presented as the time-averaged abundances
combination of the hydrogen atoms can trigger a chemical
of the various molecular species. The 2013 model has been
runaway, which will release all the chemical energy stored
updated in a number of ways:
in the grain (Duley & Williams 2011). Similarly to ther-
maldesorption,theseabrupttemperatureexcursionswould • The gas-phase chemistry of CH OH and SiO has been
3
then release the molecules frozen on the grains into the gas included.InadditiontothespecieslistedinTable2ofRawl-
phase.However,asstatedbyCecchi-Pestellinietal.(2010), ingsetal.(2013a)weaddedinPhaseIthefollowingspecies:
just after the explosion of the grain mantle, the density in CH OH+,CH OH + (fortheirroleinthegasphaseforma-
3 3 2
theexpandinggasmaybesohighthatthree-bodyreactions tionofmethanol)andSi,Si+,SiH,SiH+,SiH ,SiH +,SiH ,
2 2 3
could take place in a very short timescale of the order of SiH +,SiH ,SiH +,SiH +,SiO,SiO+ andSiOH+ (fortheir
3 4 4 5
the nanosecond. The radicals initially present on the grains role in the formation of SiO). The gas phase reactions are
(generated by processing by UV photons or cosmic rays) listedinTablesA1–A4andcomefromtheUMISTDatabase
would then desorb with the other species and could react for Astrochemistry 2012 (McElroy et al. 2013).
together to form more complex species through three-body • A comprehensive list of continuous desorption mech-
reactions.ThisscenarioisparticularlyrelevantfortheCMZ anisms has been incorporated. These include photo-
asthehighercosmicrayionisationratedrivesanincreaseof desorption(directandcosmic-rayphoton-induced)(Roberts
boththeabundanceofhydrogenatomsandtheradicalpro- et al. 2007; Hollenbach et al. 2009), cosmic ray spot and
duction rates, which consequently leads to a faster cycling whole grain heating (Hasegawa & Herbst 1993; Bringa &
in the explosive model. Johnson 2004), and enthalpy-driven desorption (Roberts
Inthispaper,wefirstexploretheevolutionofmethanol etal.2007).Theseprocessesaretreatedself-consistentlyand
inthecaseofaquiescentscenariowithhighcosmicrayioni- make clear distinctions between surface and and bulk des-
sationratesandtheinclusionofthenon-thermaldesorption orption processes.
processes. Yusef-Zadeh et al. (2013c) studied the variation • A basic surface chemistry (for simple oxygen-bearing
of the methanol abundance with the cosmic ray ionisation species) is also included: formation of CO through the re-
2
MNRAS000,1–10(2016)
3
actionofCOwithO,O+,andOH,conversionoftherestof
Table 1.Parametersinthestandardmodel
O,O+,andOHintoH OandconversionofCOintoCH OH.
2 3
• Thermal desorption, which is due to the dust temper- Parameter Value
ature, is not explicitly included in our model. However, to
He/H 0.1
simulate the possible effects of thermal desorption of some C/H 2.6×10−4
volatile species, we consider that, for a dust temperature of N/H 6.1×10−5
∼ 25 K, only 20% of CO, N2 and O2 and 30% of CH4 re- O/H 4.6×10−4
main frozen on the grains, the rest desorbing immediately. S/H 1.0×10−7
These results are based on the empirical findings of TPD Na/H 1.0×10−7
experiments (Viti et al. 2004). We also allow a fraction of Si/H 1.0×10−7
adsorbedspeciestobedesorbedimmediatelyonhydrogena- Density(nI) 2×104 cm−3
tion; e.g. C/C+ can freeze-out and be converted to CH4, Kinetictemperature(Tk) 200K
which is partially released back to the gas-phase. Dusttemperature(T) 25K
d
Cosmicrayionisationrate(ζ) 10−14 s−1
• The complex organics created in the explosion phase
wereincludedinthegas-phasechemistryofthecold,quies- Visualextinction(AV) 10mag
cent phase (although we do not attempt to model the for- Radiationfield(Go) 103 Habing
mation of second-generation COMs in this phase). Standardinterstellarradiationfieldphotonflux 108 cm−2s−1
Photodesorptionyieldperphoton 10−3
Scalingfactorforthecosmic-rayinducedUVfield 4.88×10−5
H-atomnonrecombinationprobability(pH) 0.1
ExplosionthresholdabundanceofH(fH) 0.05
As in our previous models (and despite the desorption No.of(refractory)atomspergrain(Ng) 108
rate for ice mantle species being high) the large flux of hy- Mantleradicalformationrate(Rrad)(a) 0.01Myr−1
drogen atoms impinging on the grains, together with the
Averagegrainradius(a) 0.0083 µm
factthatthesurfacehydrogenationprocessesareextremely DustsurfaceareaperH-nucleon(σH) 1.6×10−20 cm2
rapidmeansthatweassumethatrapidandefficientconver- Grainalbedo 0.5
sionofCtoCH4,OtoH2O,andNtoNH3 occurs.Different CO→CH3OHconversionefficiency(fCO→CH3OH) 10%
radicals, which will participate in the formation of complex
organic molecules in Phase II, are assumed to form on the PhaseII:initialdensity(nII) 1020 cm−3
grain by H abstraction of grain surface species due to the PhaseII:initialtemperature(TII) 1000K
PhaseII:three-bodyratecoefficients(k3B) 10−28 cm6 s−1
actionofthecosmicrayinducedUVfieldwitharadicalfor-
mation rate (Rrad) dependent on the cosmic ray ionisation Numberofcycles(ncyc.) 5
rate. For example, CH leads to the radical CH , NH to
4 3 3 (a) forastandardcosmicrayionisationrateζ =1.3×10−17 s−1
NH ,H OtoOH,CH OHtoCH OHandCH O,andSiH
2 2 3 2 3 4
to SiH . During the explosion phase, the radicals that are
3 2.2 Chemistry of CH OH and SiO
released in the gas phase can react though three-body re- 3
actions to form complex species. A total of 34 species and TheformationofCH OHcanbedividedintothreedifferent
3
18three-bodyreactionsareconsideredinphaseII.Theyare process types:
listedinTables3and4ofRawlingsetal.(2013a).Theother
• First,itcanbeproducedinthegas-phase(seecomplete
speciesareconsideredaschemicallyinactiveandarejustre-
list of reactions in Table A1), e.g.
turnedtothegas-phaseforthenextcycle.Theonlyrelevant
reaction in Phase II for this study is the rapid radical asso- CH+3 +H2O→CH3OH+2 +hν
ciation between CH , OH and the third reactant H O that
3 2 followed by
forms methanol.
CH OH++e−→CH OH+H.
The physical parameters used for the model defined 3 2 3
as the standard model are listed in Table 1. Most of the The gas-phase reactions are known to inefficiently form
parameters listed in Table 1 are varied to determine their methanol for local interstellar conditions.
impact on the fractional abundances of methanol and • Secondly, it can be produced in, or on, icy mantles
silicon monoxide (see Table 3). The column densities of H by surface hydrogenation (e.g., Watanabe & Kouchi 2002;
2
are typically 1022–1024cm−2, implying an A ∼10−1000 Fuchs et al. 2009):
V
(e.g., Longmore et al. 2012; Yusef-Zadeh et al. 2013c). The
H
radiation field strength G is assumed to range between COs−→CH3OHs
o
102 and 105 Habing depending on the study (Kim et al. followed by its non-thermal desorption into the gas-phase
2011; Clark et al. 2013; Harada et al. 2015; Bertram et al. duringphaseIorintheexplosionofthegrainmantleduring
2016). We adopt a value of 103 Habing, but do not expect phase II. The surface hydrogenation of CO is the channel
the chemistry to be photon-dominated for A >10. The that is most usually invoked for the efficient production of
V
nominal size of the CMZ region that we study is of the CH OHintheinterstellarmedium.Weassumeaconversion
3
orderofafew100pc.Thishasrelevancewhenoneconsiders efficiency (f ) of 10% (e.g., Watanabe & Kouchi
CO→CH3OH
theresponsetotheregiontopossibilityofrapidlychanging 2002; O¨berg et al. 2011). This parameter could, however,
physical conditions. have a different value in the CMZ (see Section 4 for more
details).
MNRAS000,1–10(2016)
4 A. Coutens et al.
• Finally,methanolcanbeformedinthehighdensitygas-
Table 2. Fractional abundances of CH3OH with respect to H-
phase following mantle explosion via three-body reactions: nucleons obtained in the case of a model without explosion at t
=107 yrs.
CH +OH+M→CH OH+M
3 3
where M is the third body reactant (H2O). Model Parameters CH3OH
The three processes may play a role, but in the case of fCO→CH3OH =10%
a region (such as the CMZ) that is subject to high cosmic 1 Standard 7.9×10−9
ray ionization rates the composition of the ice mantles may 2 nI =2×103 cm−3 7.9×10−10
be most unlike that pertained in normal interstellar clouds. 3 nI =2×105 cm−3 1.4×10−8
Inparticular,COcouldbevulnerabletoefficientdesorption 4 nI =2×106 cm−3 2.6×10−9
by cosmic-ray spot heating, in which case the ices may be
5 T =100K 1.2×10−11
CO-poor. In these circumstances, the second of the mecha- k
6 T =300K 3.5×10−9
nismsabovemaynotbeasefficientasinthelocalinterstellar k
7 T =400K 6.3×10−9
medium and the third channel may be an alternative route k
toefficientCH OHformation.Theobjectiveofthisstudyis 8 ζ =10−16 s−1 1.3×10−9
3
to evaluate the efficiency of the three mechanisms for CMZ 9 ζ =10−15 s−1 1.4×10−8
conditions. 10 ζ =10−13 s−1 7.9×10−10
The silicon chemistry is dominated by gas phase reac-
tions and gas-grain interactions. In our model, Si (or Si+)
stickstograinsand,assumingasimilarpatternasforother
elements, is hydrogenated on the surface to silane (SiH ).
4
Silane has a very high binding energy and is certainly re- afunctionoftimeforfourdifferentvaluesofthecosmicray
sistant to continuous desorption processes (Turner 1991). ionisationrate(10−16–10−13s−1).Foraconversionefficiency
However,anydisruptiveprocesses(suchassignificantheat- of CO into CH3OH of 10%, the fractional abundances of
ing, shocks, or mantle explosions) result in its release to methanol with respect to H range between 10−11 and 10−8,
the gas-phase. Thereafter it is rapidly and efficiently con- which only corresponds to the lowest range of abundances
verted to SiO via a network of gas phase reactions (see Ta- of methanol determined in the CMZ (∼10−9–10−7, Yusef-
bles A2–A4), none of which have activation barriers and Zadeh et al. 2013c). It should be noted that the predicted
so are efficient even at low temperatures. It therefore fol- abundance of methanol could even be overestimated. First,
lowsthat,ifmantleexplosionsareoperating,highfractional the (somewhat arbitrary) CO to CH3OH conversion rate
abundances of gas-phase SiO are expected, unless silicon is may overestimate the efficiency of the process in the CO-
depleted. This would be consistent with observations which poor ices that are predicted to be present in the CMZ (see
show widespread SiO emission in molecular clouds (Yusef- Section4formoredetails).Secondly,recentstudies(Bertin
Zadeh et al. 2013a,c). No three-body reaction involving Si- etal.2016;Cruz-Diazetal.2016)haveshownthatCH3OH
bearing species is considered in phase II. The only relevant is vulnerable to fragmentation during photodesorption - in
mechanism for these species in phase II is their desorption which case its presence in the cold gas-phase is probably
due to the grain mantle explosion, that makes them avail- attributabletoeithernon-continuous(explosive)desorption
able for gas phase reactions during the quiescent phase of of ice mantles, or some, as yet unidentified, efficient gas-
the next cycle. phase formation mechanism. In the absence of conversion
of CO into CH OH and for a cosmic ray ionisation rate of
3
10−14 s−1, the fractional abundance of methanol is only ∼6
× 10−15, confirming that the gas phase reactions are not
3 RESULTS
efficient to produce methanol even for CMZ conditions.
3.1 Results without explosion of the grain mantles Asthisscenariocannotreproducethehighestfractional
abundances of methanol derived in the CMZ (Requena-
Inafirststep,wetestthescenarioproposedbyYusef-Zadeh
Torres et al. 2008; Yusef-Zadeh et al. 2013c), we explore
etal.(2013c)toexplainthepresenceofmethanolbyasim-
thereafterhowtheabundanceofmethanolisaffectedinthe
ple increase of the cosmic ray ionisation rate. The physical
case of episodic explosions of grain mantles.
and chemical attributes of this model are exactly as for the
cyclicexplosionmodeldescribedbelow,butforthesecalcu-
lations, the H-atom non recombination probability (p ) is
H
equal to 0 and we just consider the chemical evolution and
abundances in the quiescent phase (phase I). We find that
3.2 Results with episodic explosions of grain
thecompositionoftheicesislargelydeterminedbytherel-
mantles
ative efficiencies of the cosmic-ray heating process. For the
parametersofourstandardmodel,theicesaremainlycom- As shown by Rawlings et al. (2013a), after a few cycles (<
posedofH OandCH ,withsmallamountsofNH andonly 5), the time-averaged abundances of the different molecules
2 4 3
traces (<0.1%) of CO and CO . during a cycle reach an equilibrium and do not vary signifi-
2
Table 2 lists the fractional abundances of gas-phase cantly.Weconsequentlyshowheretheresultsobtainedafter
methanol predicted at t = 107 years for different densities, 5 cycles, which always represents a timescale shorter than
temperaturesandcosmicrayionisationrates,whileFigure1 theageofthisregion.Onaverage,thedurationofacycleis
showsthevariationofthemethanolgasphaseabundanceas about a few 103 years.
MNRAS000,1–10(2016)
5
10-7 Table 3. Time-averaged fractional abundances of CH3OH and
SiO (with respect to H) in the final cycle for different models
10-8 withepisodicexplosionsofgrainmantles.
10-9 Model Parameters CH3OH(a) CH3OH(b) SiO
H
H/ fCO→CH3OH 10% 0
O 10-10
H3 1 Standard 9.2×10−8 1.5×10−9 5.2×10−8
C
10-11 2 nI =2×103 cm−3 4.2×10−9 2.6×10−9 1.0×10−9
3 nI =2×105 cm−3 6.7×10−7 7.4×10−9 5.0×10−8
10-12 4 nI =2×106 cm−3 1.1×10−6 9.1×10−8 1.6×10−9
5 T =100K 5.9×10−8 3.6×10−9 2.5×10−8
10-13 6 Tk =300K 1.3×10−7 2.1×10−9 6.2×10−8
k
101 102 103 104 105 106 107 7 T =400K 2.0×10−7 4.0×10−9 6.7×10−8
k
time (years)
8 ζ =10−16 s−1 1.2×10−6 9.1×10−8 1.7×10−9
Figure1.Fractionalabundancesofgas-phasemethanolwithre- 9 ζ =10−15 s−1 6.7×10−7 7.4×10−9 5.0×10−8
specttoH-nucleonsasafunctionoftimepredictedbythemodel 10 ζ =10−13 s−1 3.8×10−9 2.3×10−9 1.0×10−9
withoutexplosionfordifferentcosmicrayionisationrates(dotted
line: ζ = 10−16 s−1, dashed line: ζ = 10−15 s−1, solid line: ζ = 11 pH =0.01 1.2×10−7 3.2×10−8 1.3×10−8
10−14 s−1,dotted-dashedline:ζ =10−13 s−1).Thedensityis2× 12 pH =0.05 9.9×10−8 3.5×10−9 4.1×10−8
104 cm−3 andthegastemperatureis200K. 13 pH =0.2 8.2×10−8 1.3×10−9 5.6×10−8
14 fH =0.01 5.6×10−8 1.2×10−9 5.1×10−8
15 fH =0.1 9.9×10−8 3.5×10−9 4.1×10−8
3.2.1 Methanol 16 nII =1019 cm−3 9.1×10−8 3.8×10−11 5.2×10−8
The parameters listed in Table 1 are varied to study their 17 nII =1021 cm−3 9.3×10−8 1.8×10−9 5.2×10−8
18 nII =1022 cm−3 9.3×10−8 1.8×10−9 5.2×10−8
impactonthefractionalabundancesofmethanol.Thetime- 19 nII =1023 cm−3 9.3×10−8 1.8×10−9 5.2×10−8
averaged abundances of methanol obtained for different
models during cycle 5 are listed in Table 3. In particular, 20 k3B =10−26 cm6 s−1 9.3×10−8 1.8×10−9 5.2×10−8
to test the efficiency of the different routes of methanol in 21 k3B =10−30 cm6 s−1 9.1×10−8 3.8×10−11 5.2×10−8
theCMZ,aconversionefficiency f of0(insteadof 22 k3B =10−32 cm6 s−1 9.1×10−8 9.2×10−13 5.2×10−8
10%)isusedtoswitchoffthegrainCOs→urCfHac3OeHchemistrymech- 23 k3B =0 9.1×10−8 5.4×10−13 5.2×10−8
anism and a three-body reaction rate k3B equal to 0 is used 24 σH =2×10−21 cm−2 4.7×10−9 2.1×10−10 1.9×10−8
to switch off the three-body reaction in phase II. 25 σH =4×10−21 cm−2 9.8×10−9 3.4×10−10 2.4×10−8
We can see in Table 3 that the dominant mechanism 26 σH =8×10−21 cm−2 3.0×10−8 1.0×10−9 3.4×10−8
for methanol in the CMZ is the formation on the grain by 27 σH =3.2×10−20 cm−2 2.1×10−7 4.0×10−9 5.3×10−8
hydrogenation of CO with an efficiency fCO→CH3OH of 10% 28 Td =20K(c) 3.0×10−7 5.2×10−9 5.2×10−8
followedbythedesorptionduetotheexplosion.Forthestan- 29 T =30K(c) 1.6×10−9 1.6×10−9 5.2×10−8
d
dardmodel,whenthethreemechanismsareswitchedon,the
fractionalabundanceofmethanolwithrespecttoHisabout 30 Si/H=10−8 9.2×10−8 1.5×10−9 5.2×10−9
9×10−8, while it is only 1.5×10−9 when the grain surface 31 Si/H=10−9 9.2×10−8 1.5×10−9 5.2×10−10
formationmechanismisswitchedoff.Similarresultsareob- Notes: (a) This column shows the abundances of methanol ob-
tained for the other models. The formation of methanol on tainedwhenthethreemechanismsfortheformationofmethanol
grainsurfaceleadstoabundancesthatare1–2ordersofmag- areincluded.(b) Thiscolumnshowstheabundancesofmethanol
nitude higher than in the absence of this mechanism. The obtained when we only consider the gas phase mechanism
onlymodelthatshowsacomparableabundanceofmethanol in phase I and the rapid radical association in phase II. The
withorwithoutthegrainsurfacechemistrymechanismcor- hydrogenation of CO on the grains is switched off. (c) In the
responds to a model with a very low density (∼103 cm−3). standardcase(Td =25K),weconsiderthat20%ofCO,O2,and
The gas phase mechanism alone (model 23, f = N2 and30%ofCH4 remainfrozen,whileforadusttemperature
0)leadstoafractionalabundanceofmethanolCoOf→5C×H31O0H−13. of20K,weconsiderthat65%ofofCO,O2,andN2 and100%of
As shown for the case without explosion, this mechanism is CH4 remain frozen. In the case of Td = 30 K, we consider that
CO,O2,andN2 donotfreezebutthatCH4 canstillfreezewith
negligible in the CMZ. It should be noted that the domi-
30%remainingonthegrains.
nance of the hydrogenation mechanism on the grain for the
formation of methanol is dependent on the conversion ef-
ficiency of CO into CH OH (f ). We assumed an abundanceofCH producedbyhydrogenabstractionofCH
3 CO→CH3OH 3 4
efficiency of 10%. If it were, however, lower (f < isnotsufficienttoproduceenoughmethanol.Itislimitedby
CO→CH3OH
0.16%forthestandardmodel),therapidradicalassociation the mantle radical formation rate. Even if a higher cosmic
in the explosion phase could be more efficient than the hy- ray ionisation rate should increase its value, the timescale
drogenation of CO on the grain. The lower efficiency of the of each cycle is too short (∼103 years) to lead to very high
rapid radical association mechanism compared to the grain abundances of CH , ii) even when the amount of CO ice is
3
surfacehydrogenationofCO,evenforveryhighthree-body relatively low, 10% of conversion into methanol is sufficient
ratecoefficients,isexplainedbythefollowingreasons:i)The to produce enough methanol on grains (10−6).
MNRAS000,1–10(2016)
6 A. Coutens et al.
10-6 3.2.2 Silicon monoxide
The main uncertainty for the silicon chemistry is the initial
abundance of Si. Indeed, it is thought that Si is depleted
withrespecttothesolarvalue(Si/H∼3.9×10−5,Asplund
et al. 2009), but the depletion factor is not constrained. In
H
H/ quiescent clouds like TMC-1, L183, and L1448, silicon is
O 10-7 observed to be heavily depleted (SiO/H (cid:46) 10−12, Ziurys
H3 2
C etal.1989;Martin-Pintadoetal.1992;Requena-Torresetal.
2007). The bulk of Si is expected to be in the grain cores.
Withthestandardmodel(Si/H∼10−7),thefractional
abundance of SiO is about 5×10−8. The abundance of SiO
is proportional to the initial Si abundance. In the case of
10-8 an initial Si abundance of 10−7, the SiO abundance would
10 100 1000 range between 1×10−9 (for models with low density n ∼
time (years) I
103 cm−3, high density n ∼ 106 cm−3, low cosmic ray ioni-
I
Figure 2.Fractionalabundanceofgas-phasemethanolwithre- sationrate ζ = 10−16 s−1 or high ionisation rate ζ = 10−13
specttoH-nucleonsasafunctionoftimeinphaseIforthestan- s−1)and5×10−8 (standardmodel).Itshouldalsobenoted
dardmodelwithepisodicexplosionsofgrainmantles. thattheabundancespredictedbyamodelthatconsidersthe
freezing of Si into solid Si (instead of SiH ) do not change
4
significantly.
ThemainreactionthatleadstotheformationofSiOin
Some trends are observed with the change of some pa-
our standard model is:
rameters.Forexample,thehigherthedensity,thehigherthe
fractional abundance of methanol. This is expected due to SiOH++e−→SiO+H,
thefreezingofthespecies(includingCO)onthegrainsthat
with SiOH+ formed by the reactions between SiO+ and H
ismoreefficientathighdensity.Theabundanceofmethanol 2
and between Si+ and H O. Other reactions such as
slightly increases with the kinetic temperature. It is also 2
higherwhenthecosmicrayionisationrateislow.Thevari- Si+O →SiO+O
2
ationoftheH-atomnonrecombinationprobability(p )and
H
and
the explosion threshold abundance of H (f ) do not lead to
H
significant variation of the abundance of methanol (lower Si+OH→SiO+H
than a factor 2). The variation of phase II parameters such
as the initial density (n ) and the three-body reaction rate contribute to the formation of SiO as well.
II
(k ) do not change the fractional abundances of methanol
3B
whenthehydrogenationofCOisincluded,asitisthedom-
inant mechanism. In the absence of the grain surface mech- 4 DISCUSSION
anism, methanol is, as expected, produced more efficiently
Inthissection,wecomparetheresultspredictedbyourmod-
withahigherinitialdensityandahigherthree-bodyreaction
els with observations. As the observed abundances are cal-
rate. The size distribution of the grains can also be investi-
culated with respect to H while the predicted abundances
gated by varying σH, the dust surface area per H-nucleon. 2
by the chemical models are with respect to H-nucleons, we
A high value of σH increases the fractional abundance of
corrected the latter to be expressed with respect to H (i.e.
methanol, both with and without the inclusion of the grain 2
multiplied by a factor 2).
surface hydrogenation of CO.
Weexploredtwocaseswithdifferentdusttemperatures
for which we assumed different fractions of volatile species
4.1 Methanol
desorbingatlowtemperatures.Foralowerdusttemperature
(∼20K),weassumedthat,afterdepletion,65%ofCO,O , Yusef-Zadeh et al. (2013c) estimated a fractional abun-
2
and N and 100% of CH remain frozen on the grains. In dance of methanol in the CMZ ranging between 10−9 and
2 4
the extreme case of a higher dust temperature (∼ 30 K), 10−7. Requena-Torres et al. (2008) estimated even higher
weconsiderthatCO,O ,andN donotfreezeatall,while abundances for three molecular clouds located in the CMZ
2 2
30% of CH still remain frozen. As expected, in the latter (3×10−7 for G–0.02–0.07, 1.1×10−6 for G–0.11–0.08, and
4
case, we obtain the same results as in the absence of the 5×10−7 for G+0.693–0.03). With a conversion efficiency of
COhydrogenationonthegrains.Forthelowertemperature CO into CH OH of 10%, our standard model with episodic
3
case, the abundance of methanol is a factor 3 higher due to explosions is in agreement with the high abundances de-
the highest fraction of CO that remains on the grain. rived in these studies (∼1.8×10−7). With high densities,
Figure2showstheabundanceofmethanolasafunction the abundance is even higher (about 7×10−7–1×10−6),
of time during phase I for the standard model. The cycle whichisconsistentwiththehighabundancesderivedforthe
has a duration of 1.2 ×103 years. Right after the explosion molecular clouds. On the contrary, in the absence of explo-
phase, the abundance of methanol in the gas-phase reaches sion and for high densities (≥ 2×105 cm−3), the predicted
a value of 6×10−7 then decreases with the time until the abundanceofmethanolislow(≤3×10−8)comparedtothe
nextexplosion.Thisshowsthattheexplosionisrequiredto observations. It should be, however, noted that, for a high
reach high abundances of methanol. density of ∼ 106 cm−3, the gas and dust temperatures are
MNRAS000,1–10(2016)
7
certainlycoupled,whichwouldmeanveryhighdusttemper- of ∼10−9 towards the circumnuclear disk. If we consider an
atures((cid:38)100K)andthecompleteabsenceofices.Basedon initialabundanceofSidepletedbyafactor400withrespect
theseresults,ascenariowithcyclicexplosionsofgrainman- to the solar abundance (Si/H = 10−7), the SiO abundances
tles is sufficient to explain the observations. No additional predictedbyourmodelsisonaverage1×10−7(withrespect
mechanismisrequiredtoreproducetheobservationalabun- toH ),whichisingoodagreementwiththeSgrAmeasure-
2
dance of methanol in the CMZ and the cyclic aspect of our ment by Minh et al. (1992). In a few cases (low density n
I
model can explain the continuous and widespread presence = 103 cm−3, high density n = 106 cm−3, low cosmic ray
I
of methanol. ionisation rate ζ = 10−16 s−1 and high cosmic ray ionisa-
Theseresultsare,however,dependentonthefractionof tionrateζ =10−13 s−1),itcanreachfractionalabundances
COthathydrogenatesintoCH OH.Whenthereisnohydro- as low as 2×10−9, which is still higher than the observa-
3
genation of CO on the grains, the standard model predicts tional values for the SiO-poor molecular clouds studied by
a fractional abundance of ∼ 3×10−9, while the abundance Mart´ın-Pintado et al. (1997), (cid:46)10−10.
can reach ∼ 10−8 for a high density. It is still consistent With a depletion factor of about 4000 with respect to
with the lowest abundances derived in the CMZ, but some thesolarvalue(Si/H=10−8),therangeofabundancespre-
hydrogenation of CO on the grains is required to explain dicted by our models (2×10−10–1×10−8 with respect to
the highest abundances. Experiments were carried out for H ) is more comparable to the observational value. Such
2
different types of ice and for different temperatures (e.g., a high depletion factor is similar to the one used for low-
Watanabeetal.2004;Fuchsetal.2009;Cuppenetal.2009) metalcasestudies(e.g.,Graedeletal.1982;Leeetal.1996;
andtheconversionefficiencyseemstobequitedependenton Jim´enez-Serra et al. 2008). A very low Si/H initial abun-
these parameters. For example, a range of conversion frac- danceisalsoinagreementwithoneofthesuggestionsraised
tionfrom5%to100%aregivenasafunctionofthetemper- byYusef-Zadehetal.(2013a)toexplainthelowSiO/N H+
2
ature and the H/CO gas phase abundance ratio in Cuppen abundance ratio of the CMZ.
etal.(2009).Butnoneoftheseexperimentswascarriedout Finally, the spatial correlation found for CH OH and
3
at a temperature higher than 20K. Based on our results, a SiO by Yusef-Zadeh et al. (2013a) could be explained by
conversionefficiencyof∼10%appearstobeneededtorepro- thefactthatbothspeciesrequirethereleaseofgrainmantle
duce the observations, which is similar to what is found for speciesinthegasphase.Shockscanalsoproducehighabun-
hot cores. It could also be higher if a fraction of methanol dancesofSiO(e.g.,Schilkeetal.1997;Haradaetal.2015),
does not survive the evaporation. If the conversion of CO eitherthroughthereleaseofgrainmantlespeciesorthrough
into CH OH on grains was not as efficient as ∼10% at this grain core sputtering from high-velocity shocks, and could
3
temperature,thereisstillthepossibilitythathydrogenation be a possible scenario as well.
ofCOcouldinsteadoccur(withtherapidradicalassociation
reactions) in the ultra high density gas phase immediately
4.3 Predictions outside of the CMZ
aftertheexplosion.Morestudieswouldbeneededtodeter-
mine if this type of reaction is viable during the explosion The grain mantle explosion model can also explain the fact
phase. that methanol is not so abundant outside of the CMZ. For
As the dominant mechanism leading to the formation relatively diffuse regions (n = 2 × 103 cm−3, A ∼ 3)
I V
of methanol in the CMZ is found to be through grain sur- withastandardcosmicrayionisationrate(ζ ∼10−17 s−1),
faceformation(insteadoftherapidradicalassociationinthe the average abundance of methanol and SiO predicted by
highdensitygasphasefollowingthegrainmantleexplosion), the model is several orders of magnitude lower (∼10−10 for
anyothermechanismthatcanregularlyreleasethemolecu- methanoland∼10−12 forSiO).Theperiodofacycleisalso
lar content of the grain mantles in the gas phase should be longer than for the CMZ (about 8 × 105 years) due to the
considered. Shocks are consequently another option. Shock lower cosmic ray ionisation rate that implies a lower abun-
models can locally produce high abundances of methanol dance of atomic H. This explains why methanol and SiO
of ∼10−7–10−6 (e.g., Viti et al. 2011; Flower & Pineau des are not widespread and abundant outside of the CMZ. For
Forˆets 2012) and there is some evidence of cloud-cloud col- denser regions (104–106 cm−3) such as prestellar cores, the
lisionsintheCMZ(e.g.,Hasegawaetal.1994;Tsuboietal. predicted abundance of methanol is higher and ranges be-
2015; Tanaka 2016). Some of the methanol-rich positions tween ∼10−8 and ∼10−6 after 5 cycles. However, the time
detected by Yusef-Zadeh et al. (2013c) could correspond to between 2 explosions (∼8×105 years) is comparable to or
shock positions, especially if they are located at the inter- longer than the infall timescale leading to the formation
section of clouds. In this case, shocks may certainly be the of a protostar (∼105–106 years, Pagani et al. 2009, 2013;
reason for the release of methanol in the gas phase. Bru¨nken et al. 2014). It is consequently possible that the
time is not sufficient to allow an explosion to occur at high
density in objects such as prestellar cores. In the most con-
4.2 Silicon monoxide
ducivecase,onlyoneortwoexplosionscouldoccur,butafter
Severalstudiesonsiliconmonoxidewerecarriedouttowards the first explosion, the abundance of methanol is relatively
GalacticCentremolecularclouds.Inparticular,Minhetal. low.Fordensitiesofabout106cm−3,theaverageabundance
(1992) found a SiO abundance with respect to H of 10−7– ofmethanolafterthefirstexplosionisabout10−12,whileit
2
10−8 for the Sgr A molecular cloud. Mart´ın-Pintado et al. is about a few 10−10 for densities of about 104 cm−3. This
(1997) derived a fractional abundance of ∼10−9 for some lastvalueisverysimilartotheabundanceofmethanolfound
SiO-rich molecular clouds, while an upper limit of ∼10−10 in dark clouds and prestellar cores such as L1544, TMC-1
wasobtainedforothermolecularcloudsoftheGalacticCen- or L134N (∼10−9, Friberg et al. 1988; Vastel et al. 2014).
tre.Morerecently,Minhetal.(2015)derivedanabundance In conclusion, although episodic explosions could theoreti-
MNRAS000,1–10(2016)
8 A. Coutens et al.
callyoccureverywhere,theyaremorelikelytohaveaneffect Friberg P., Hjalmarson A., Madden S. C., Irvine W. M., 1988,
in the CMZ than in the local interstellar medium owing to A&A,195,281
the higher cosmic ray ionisation rate that increases the fre- FuchsG.W.,CuppenH.M.,IoppoloS.,RomanzinC.,Bisschop
quency of the grain mantle explosions. S. E., Andersson S., van Dishoeck E. F., Linnartz H., 2009,
A&A,505,629
GinsburgA.,etal.,2016,A&A,586,A50
Graedel T. E., Langer W. D., Frerking M. A., 1982, ApJS, 48,
5 CONCLUSION 321
Gu¨stenR.,WalmsleyC.M.,PaulsT.,1981,A&A,103,197
In this paper, we show that the increase of the cosmic ray
HaradaN.,etal.,2015,A&A,584,A102
ionisation rate is not sufficient to explain the widespread HasegawaT.I.,HerbstE.,1993,MNRAS,261,83
andabundantpresenceofmethanolintheCMZ.Ascenario HasegawaT.,SatoF.,WhiteoakJ.B.,MiyawakiR.,1994,ApJL,
with episodic explosions of grain mantles gives, however, a 429,L77
good agreement between the predicted abundances and the HollenbachD.,KaufmanM.J.,BerginE.A.,MelnickG.J.,2009,
observations. The repetition of the explosions can also ex- ApJ,690,1497
plainthewidespreadpresenceofmethanolonscalesofafew Hollis J. M., Jewell P. R., Lovas F. J., Remijan A., 2004, ApJ,
613,L45
hundred parsecs. According to this scenario, the dominant
Jim´enez-SerraI.,CaselliP.,Mart´ın-PintadoJ.,HartquistT.W.,
mechanismfortheformationofmethanolintheCMZisthe
2008,A&A,482,549
grain surface formation through hydrogenation of CO fol-
KimS.S.,SaitohT.R.,JeonM.,FigerD.F.,MerrittD.,Wada
lowed by the desorption due to the explosion. Our model
K.,2011,ApJ,735,L11
also reproduces the SiO abundance in the case of a low Le Petit F., Ruaud M., Bron E., Godard B., Roueff E., Lan-
Si/H initial abundance of about 10−8. As both methanol guignonD.,LeBourlotJ.,2016,A&A,585,A105
andSiOrequiregrainsurfaceformationmechanisms,shocks LeeH.-H.,BettensR.P.A.,HerbstE.,1996,A&AS,119,111
couldbeanotherpossiblescenariotoexplainthepresenceof LongmoreS.N.,etal.,2012,ApJ,746,117
methanolandSiOintheCMZ.AsshownbyRawlingsetal. Martin-PintadoJ.,BachillerR.,FuenteA.,1992,A&A,254,315
(2013b), the episodic explosion models present the advan- Mart´ın-Pintado J., de Vicente P., Fuente A., Planesas P., 1997,
tage that they can explain the presence of large and com- ApJ,482,L45
McElroy D., Walsh C., Markwick A. J., Cordiner M. A., Smith
plexmoleculesafterseveralcycles.Thistypeofmodelcould
K.,MillarT.J.,2013,A&A,550,A36
consequently also explain the presence of complex organic
McGuireB.A.,CarrollP.B.,LoomisR.A.,FinneranI.A.,Jewell
moleculessuchasglycolaldehydeandpropyleneoxideinthe
P.R.,RemijanA.J.,BlakeG.A.,2016,inAmericanAstro-
cold gas surrounding the star-forming region Sgr B2 (Hollis
nomicalSocietyMeetingAbstracts.p.203.04
et al. 2004; McGuire et al. 2016). More experimental and MeijerinkR.,SpaansM.,LoenenA.F.,vanderWerfP.P.,2011,
theoretical workis however requiredtoconstrainthe chem- A&A,525,A119
istry during the explosion phase. MinhY.C.,IrvineW.M.,FribergP.,1992,A&A,258,489
Minh Y. C., Liu H. B., Su Y.-N., Hsieh P.-Y., Liu S.-Y., Kim
S.S.,WrightM.,HoP.T.P.,2015,ApJ,808,86
O¨bergK.I.,BoogertA.C.A.,PontoppidanK.M.,vandenBroek
ACKNOWLEDGEMENTS S., van Dishoeck E. F., Bottinelli S., Blake G. A., Evans II
N.J.,2011,ApJ,740,109
The authors would like to thank an anonymous referee for
PaganiL.,etal.,2009,A&A,494,623
valuable comments and suggestions. The work of AC was
PaganiL.,LesaffreP.,JorfiM.,HonvaultP.,Gonza´lez-LezanaT.,
funded by the STFC grant ST/M001334/1.
FaureA.,2013,A&A,551,A38
Rawlings J. M. C., Williams D. A., Viti S., Cecchi-Pestellini C.,
DuleyW.W.,2013a,MNRAS,430,264
Rawlings J. M. C., Williams D. A., Viti S., Cecchi-Pestellini C.,
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MNRAS000,1–10(2016)
9
D.A.,2004,MNRAS,354,1141 APPENDIX A: LIST OF REACTIONS IN
VitiS.,Jimenez-SerraI.,YatesJ.A.,CodellaC.,VastaM.,Caselli PHASE I INVOLVING METHANOL AND
P.,LeflochB.,CeccarelliC.,2011,ApJ,740,L3 SILICON MONOXIDE
WatanabeN.,KouchiA.,2002,ApJ,571,L173
WatanabeN.,NagaokaA.,ShirakiT.,KouchiA.,2004,ApJ,616,
638
Yusef-Zadeh F., Wardle M., Lis D., Viti S., Brogan C., Cham-
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ChemistryA,117,9404
Yusef-ZadehF.,etal.,2013b,ApJ,762,33
Yusef-Zadeh F., Cotton W., Viti S., Wardle M., Royster M.,
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MNRAS000,1–10(2016)
10 A. Coutens et al.
Table A1.ListofreactionsinPhaseIinvolvingmethanol Table A2. List of reactions in Phase I involving Si-bearing
species
Si+ +CH3OH→SiOH+ +CH3
H+ +CH3OH→CH3OH+ +H O+SiH4 →SiH3 +OH
H+ +CH3OH→CH3+ +H2O Si+CO→SiO+C
H+ +CH3OH→HCO+ +H2 +H2 Si+O2 →SiO+O
H3+ +CH3OH→CH3+ +H2O+H2 Si+CO2 →SiO+CO
H3+ +CH3OH→CH3OH2+ +H2 Si+NO→SiO+N
He+ +CH3OH→OH+ +CH3 +HE O+Si+ →SiO+ +PHOTON
He+ +CH3OH→OH+CH3+ +HE H+Si+ →SiH+ +PHOTON
C+ +CH3OH→HCO+CH3+ H+SiH+ →Si+ +H2
CH+ +CH3OH→CH3OH2+ +C H+ +Si→Si+ +H
CH+ +CH3OH→H2CO+CH3+ H+ +SiH→SiH+ +H
CH3+ +H2O→CH3OH2+ +PHOTON H+ +SiH→Si+ +H2
CH4+ +CH3OH→CH3OH+ +CH4 H+ +SiH2 →SiH2+ +H
CH4+ +CH3OH→CH3OH2+ +CH3 H+ +SiH2 →SiH+ +H2
N+ +CH3OH→CH3OH+ +N H+ +SiH3 →SiH3+ +H
N+ +CH3OH→H2CO+ +NH+H H+ +SiH3 →SiH2+ +H2
N+ +CH3OH→NO+ +CH3 +H H+ +SiH4 →SiH4+ +H
N+ +CH3OH→NO+CH3+ +H H+ +SiH4 →SiH3+ +H2
O+ +CH3OH→CH3OH+ +O H+ +SiO→SiO+ +H
O+ +CH3OH→H2CO+ +H2O H-+Si+ →H+Si
H3O+ +CH3OH→CH3OH2+ +H2O H-+SiO+ →H+SiO
O2+ +CH3OH→CH3OH+ +O2 H3+ +Si→SiH+ +H2
HCO+ +CH3OH→CH3OH2+ +CO H3+ +SiH→SiH2+ +H2
H2CO+ +CH3OH→CH3OH2+ +HCO H3+ +SiH2 →SiH3+ +H2
CH3OH2+ +NH3 →CH3OH+NH4+ H3+ +SiH3 →SiH4+ +H2
Si+ +CH3OH→SiOH+ +CH3 H3+ +SiH4 →SiH5+ +H2
CH3OH+ +e− →CH+H2O+H He+ +Si→Si+ +He
CH3OH+ +e− →H2CO+H+H He+ +SiH→Si+ +He+H
CH3OH+ +e− →OH+CH3 He+ +SiH2 →Si+ +He+H2
CH3OH2+ +e− →CH2 +H2O+H He+ +SiH2 →SiH+ +He+H
CH3OH2+ +e− →CH3 +H2O He+ +SiH3 →SiH+ +He+H2
CH3OH2+ +e− →CH3 +OH+H He+ +SiH3 →SiH2+ +He+H
CH3OH2+ +e− →CH3OH+H He+ +SiH4 →Si+ +He+H2H2
CH3OH2+ +e− →H2CO+H2 +H He+ +SiH4 →SiH+ +He+H2H
CH3OH+PHOTON→CH3OH+ +e− He+ +SiO→Si+ +O+He
CH3OH+PHOTON→H2CO+H2 He+ +SiO→Si+O+ +He
CH3OH+PHOTON→OH+CH3 C+SiO+ →Si+ +CO
CH3OH+ +G→H2CO+H+H C+ +Si→Si+ +C
CH3OH2+ +G→CH3 +OH+H C+ +SiH2 →SiH2+ +C
CH3OH+G→GCH3OH C+ +SiH3 →SiH3+ +C
GCH3OH+PHOTON→CH3OH+G C+ +SiO→Si+ +CO
C− +Si+ →C+Si
Notes:GCH3OHcorrespondtosolidmethanol. C− +SiO+ →C+SiO
CO+SiH4+ →SiH3 +HCO+
Tthheisauptahpoerr.hasbeentypesetfromaTEX/LATEXfilepreparedby CCOH++SSiiHO++→→SCiO+2 C+HS2i++
CH+SiO+ →HCO+ +Si
CH+ +Si→Si+ +CH
CH2 +SiO+ →H2CO+Si+
CH3+ +SiH4 →SiH3+ +CH4
CH5+ +SiH4 →SiH3+ +CH4 +H2
N+SiO+ →NO+ +Si
N+SiO+ →NO+Si+
NH3 +SiH+ →Si+NH4+
NH3+ +Si→Si+ +NH3
O+Si→SiO+PHOTON
O+SiH→SiO+H
O+SiH+ →SiO+ +H
O+SiH2 →SiO+H2
O+SiH2 →SiO+H+H
O+SiO+ →O2 +Si+
OH+Si→SiO+H
OH+Si+ →SiO+ +H
OH+ +Si→SiH+ +O
OH+ +SiH→SiH2+ +O
Notes:Gcorrespondtothesolidformofthespecies.
MNRAS000,1–10(2016)