Modelling the sulphur chemistry evolution in Orion KL
AAstronomy & Astrophysics manuscript no. paper˙ucl˙pdf˙SV˙ultimo c (cid:13)
ESO 2018November 5, 2018
Modelling the sulphur chemistry evolution in Orion KL
G. B. Esplugues , S. Viti , J. R. Goicoechea , and J. Cernicharo Centro de Astrobiolog´ıa (CSIC-INTA), Ctra. de Torrej´on-Ajalvir, km. 4, E-28850 Torrej´on de Ardoz, Madrid, Spaine-mail: [email protected] Department of Physics & Astronomy, University College London, Gower St. London WC1E 6BTReceived ; accepted
ABSTRACT
Context.
We present a study of the sulphur chemistry evolution in the region Orion KL along the gas and grain phases of the cloud.
Aims.
Our aim is to investigate the processes that dominate the sulphur chemistry in Orion KL and to determine how physical andchemical parameters, such as the final mass of the star and the initial elemental abundances, influence the evolution of the hot coreand of the surrounding outflows and shocked gas (the plateau).
Methods.
We independently modelled the chemistry evolution of the hot core and the plateau using the time-dependent gas-grainmodel UCL CHEM and considering two di ff erent phase calculations. Phase I starts with the collapsing cloud and the depletion ofatoms and molecules onto grain surfaces. Phase II starts when a central protostar is formed and the evaporation from grains takesplace. We show how the stellar mass, the gas density, the gas depletion e ffi ciency, the initial sulphur abundance, the shocked gastemperature, and the di ff erent chemical paths on the grains leading to di ff erent reservoirs of sulphur on the mantles a ff ect sulphur-bearing molecules at di ff erent evolutionary stages for both components. We also compare the predicted column densities with thoseinferred from observations of the species SO, SO , CS, OCS, H S, and H CS.
Results.
The models that reproduce the observations of the largest number of sulphur-bearing species in both components are thosewith an initial sulphur abundance of 0.1 times the sulphur solar abundance (0.1S (cid:12) ) and a density of at least n H = × cm − in theshocked gas region. Conclusions.
We conclude that most of the sulphur atoms were ionised during Phase I, consistent with an inhomogeneous andclumpy region where the UV interstellar radiation penetrates and leading to sulphur ionisation. We also conclude that the mainsulphur reservoir on the ice mantles was H S. In addition, we deduce that a chemical transition currently takes place in the plateaushocked gas, where SO and SO gas-phase formation reactions change from being dominated by O to being dominated by OH. Key words.
Astrochemistry - ISM: abundances - ISM: clouds - ISM: molecules
1. Introduction
Sulphur-bearing molecules can be very useful tracers of thechemistry and physical properties of complex star-forming re-gions (SFRs) located in dense molecular clouds. In particular,they are good tracers of hot cores since they are especially sen-sitive to physical and chemical variations during the lifetime ofthese hot and dense regions (e.g. Hatchell et al. 1998; Viti etal. 2004). Hot molecular cores are common in regions of high-mass star formation. They are found in the vicinity of newlyformed high-mass stars and are characterised by high temper-atures ( ∼ ∼ cm − ). Hot cores formduring the star formation process and from a chemical pointof view are characterised by the evaporation of the grain man-tle (formed during the collapse phase). The amount of eachatomic or molecular species that evaporates back to the gasphase depends mainly on the binding energies and on the totalamount present on the grains at the time the temperature startsincreasing. Once back in the gas phase, and under conditionsof high temperature and density, the chemistry evolves rapidly,transforming many of these species into more complex ones ontimescales of (cid:46) years (Viti 2005).According to previous studies (e.g. Hatchell et al. 1998;Viti et al. 2001), the abundances of sulphur-bearing speciesare strongly time-dependent, and their evolution depends onthe heating rate and therefore on the mass of the forming star.Although the levels of sulphur depletion in di ff erent environ- ments are not fully constrained (Goicoechea et al. 2006), OCSand H S ices are probably the main reservoir of sulphur on thegrains (Palumbo et al. 1997, Hatchell et al. 1998) before the dustheats up. Druard & Wakelam (2012) also obtain the same re-sult by analysing the sulphur-depletion e ffi ciency on grains un-der dense cloud conditions. In particular, they conclude that thelower the (gas and dust) temperature ( <
20 K), the greater theH S abundance obtained on the grain surfaces. Their study alsostates that H S and CS are other S-bearing molecules e ffi -ciently produced on the grains. Once these molecules start tosublimate, they rapidly initiate reactions that drive the produc-tion of other sulphuretted molecules, such as CS, SO, and SO .Therefore sulphur-bearing species can be used to investigate theevolution of the di ff erent phases of the high-mass star formation.After the gravitational collapse of a molecular cloud, theformation of a new star is accompanied by the development ofhighly supersonic outflows (Bachiller & P´erez-Guti´errez 1997).The material ejected through these jets collides with the sur-rounding cloud, compressing and heating the gas, which leadsto a drastic alteration of the chemistry because endothermic re-actions (or reactions with energy activations barriers) becomee ffi cient and dust grains are destroyed. Many observations (Wattet al. 1986, Welch 1988, Bachiller et al. 2001, Esplugues et al.2013a) have shown that sulphur-bearing species, especially theSO and SO molecules, show increased column densities (by upto three orders of magnitude) in the shocked regions with re- a r X i v : . [ a s t r o - ph . S R ] J un . B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL Table 1.
Parameter range used to model the chemistry evolution of the hot core and the plateau of Orion KL.
Size Accretion Sulphur Maximum Star Initial gas Final gas S goes into S + goes intoComponent (arcsec) e ffi ciency abundance temperature T mass density n density n H mOCS / mH S mOCS / mH S f r (S (cid:12) ) (K) (M (cid:12) ) × (cm − ) × (cm − ) (%) (%)Hot core 10 0.30-0.85 0.01-1 300 5-15 4 10-100 0-50-100 0-50-100Plateau 30 0.85 0.01-0.1 1000-2000 ... 1 0.5-5 0-50-100 0-50-100 Notes.
Column 9 indicates the percentage of S goes into solid OCS (mOCS) or solid H S (mH S) at the end of Phase I, and Col. 10 the percentageof S + goes into mOCS or mH S at the end of Phase I. spect to more quiescent areas of the cloud. For this reason, thesemolecules are also considered excellent tracers of shocks.In this paper we model the sulphur chemistry in the star-forming region Orion KL for two di ff erent components, the hotcore and the plateau. We present the chemical model in Sect.2. For the hot core, we simulate the formation of a star, whilein the plateau we simulate the presence of a C-type shock. Tostudy the chemical evolution in both cases, we consider severalparameters, such as the star mass and the shock temperature.First, we analyse (Sect. 3) the e ff ects of varying each parameteron the evolution of the SO and SO abundances. We then com-pare (Sect. 4) the model results with observations of di ff erentsulphur species (OCS, CS, H CS, SO, SO , and H S). We in-clude a discussion of the results in Sect. 5 and summarize themain conclusions in Sect. 6.
2. Chemical model
To study the evolution of the sulphur chemistry in Orion KL,we used the chemical model UCL CHEM (Viti 2004). The codeis a time-dependent gas-grain model that calculates the chem-istry at each time step and / or cloud depth point, providing chem-ical abundances as a function of time.The model simulates thechemical evolution of the collapsing cloud, in two-phase calcu-lations: in Phase I (cold phase) the material collapses and atomsand molecules are depleted onto grain surfaces. In this phase, thedensity increases with time (according to the so-called modifiedcollapse (Spitzer 1978, Nejad et al. 1990), which is describedin detail in Rawlings et al. 1992). In Phase II (warm phase), thedensity is constant, a central star is formed, and the sublimationfrom grains takes place due to the warming up of the region.This sublimation can be time-dependent, where mantle speciesdesorb in various temperature bands (Collings et al. 2004), orinstantaneous where all species are desorbed o ff grain surfacesin the first time step.Here we employ a time-dependent evaporation, as instanta-neous sublimation is a more appropiate approximation only ifthe mass of the central star(s) is very high ( > (cid:12) ). The pre-dicted atomic and molecular abundances are sensitive to a widerange of input parameters. The ones we investigate in this pa-per are the depletion e ffi ciency of accreted species onto grainsurfaces, f r , (that is calculated by the fraction of material thatis removed from the gas in Phase I); the initial elemental abun-dances of the main species (H, He, C, O, N, S); the maximum gastemperature; the mass of the central star; the final gas density;and the di ff erent paths of the grains leading to di ff erent reser-voirs of sulphur on the mantles. To choose the range of theseinput parameters, we considered the descriptions of the Orion-KL physical components found in detailed analyses of the re-gion (Blake et al. 1987, Genzel & Stutzki 1989, Cernicharo etal. 1994). For Phase I, the initial elemental abundances in the gas (relative to the hydrogen nuclei number density and exclud-ing metals in refractory grains) adopted in the chemical code are1.0, 0.075, 4.45 × − , 1.79 × − , 8.52 × − , and 1.43 × − for H, He, O, C, N, and S, respectively (Anders & Grevesse1989, Asplund 2005, Sofia & Meyer 2001). We also ran modelswith depleted sulphur by factors 10 and 100, since it is expectedto stick to grains and disappear from the gas phase (see Bergin etal. 2001, Pagani et al. 2005). By the end of Phase I each specieswill have depleted by di ff erent percentages. To define a deple-tion e ffi ciency, f r , we use the relationship between gaseous andsolid CO, since carbon monoxide is the most abundant speciesafter H , and also since it is now known that its abundance onices varies among objects ( ¨Oberg et al. 2011): f r = mCO / ( mCO + CO gas ) , (1)where mCO is the mantle CO at the end of the Phase I of thechemical model. In Phase II the parameter f r is switched o ff ,because we assume that the radiation from the central star willlead to 100% e ffi ciency in ice sublimation. We have modelled,independently, the chemistry evolution of the hot core and theplateau components of Orion KL. The parameters used to modeleach region are listed in Table 1. In this version of the code, non-thermal desorption is switched o ff . For low densities and pre-stellar cores observations, non-thermal desorption mechanismscannot be ignored. However, given the high densities involvedin the hot environments modelled in this paper, and since non-thermal desorption mechanisms and their e ffi ciencies are not de-termined very well experimentally (e.g. see Roberts et al. 2007),adjusting the final percentage of freeze out by reducing the ef-ficiency of the sticking to the grains (via the f r parameter) issu ffi cient, therefore avoiding extra free parameters. On the otherhand, having Phase I as a separate step calculation allows that theinitial molecular fractional abundances used in Phase II calcula-tions are computed by a real time dependence of the chemicalevolution of gas-dust interaction processes, i.e. not assumed.All figures in this paper showing model results are fromPhase II. In the model, the times are reset in each phase, sowhen we show results for Phase II, the time t = t = To model this region (with an approximated diameter of 10arcsec), we considered a time-dependent evaporation in PhaseII, where the temperature only changes due to the star formedin the central region. In Phase II, we simulated the presence ofan infrared source in the center of the core or in its vicinity byincreasing the gas and dust temperature up to T =
300 K, reachedat di ff erent times depending on the mass of the star formed (Viti
2. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. 1.
Increase in gas temperature as a time function for threedi ff erent values of stellar masses (5, 10, and 15M (cid:12) ). These tem-peratures are calculated at a distance such that the final visualextinction, Av, is ∼
400 mag.et al. 2004). Figure 1 shows the increase in the gas tempera-ture according to the star mass formed in the center of the core.UCL CHEM treatment of the temperature is explained in detailin Viti et al. (2004). Briefly, it is assumed that the temperatureof the gas (and dust, as they are coupled at these high densi-ties) is a function of the luminosity (and therefore the age) of theprotostar. Viti et al. (2004) then used the observational luminos-ity function of Molinari et al. (2000) to correlate e ff ective tem-perature, with the age of the accreting protostar and found thata power law fitted the data. Their Table 2 lists the contractiontimes (defined as the times after which hydrogen starts burningand the star reaches the zero-age main sequence) as a functionof the mass of the star (Viti et al. 2004) and their respective vol-cano and co-desorption temperatues (which are the temperatuesat which the amorphous-to-crystalline H O ice conversion, the‘volcano’ e ff ect, and co-desorption when the H O ice desorbs,respectively).We note that A V is calculated as a function of density andsize of the core, and it represents the extinction from the irradi-ating / heating source (not along the line of sight). At the end ofPhase I, when the final density is reached, the final A V is alwayslarger than 400 mags. The plateau is the region of Orion KL a ff ected by outflowsand shocked gas. We use UCL CHEM in the same manner asfor the hot core by considering a Phase I where the materialcollapses and a Phase II where t = ∼ ∼
20 and ∼ − , respectively (Bergin et al. 1998). After the passage ofthe shock, the gas rapidly (within hundred of years) cools down. Fig. 2.
Shocked gas temperatures as a function of time duringand after the C-type shock passage ( t = ∼
30 and ∼
20 km s − , respectively.The temperature profiles were adopted from the calculations ofBergin et al. (1998), who studied the chemistry of H O and O in post-shock gas. In Fig. 2 we plot the temperature evolution inthe plateau as a function time during Phase II, where t = ff erent densities, n H = × and 5 × cm − in our models (which are the final densities atthe end of Phase I), which remain constant along the Phase II ofthe plateau.
3. Model results
In this section we describe the influence of the parametersdescribed in Sects. 2.1 and 2.2 on the abundances of SO andSO . We ran several hot core and plateau models in order tostudy the sensitivity of the evolution of SO and SO abundancesto di ff erent physical and chemical parameters. We first analysed the e ff ect of varying the mass of the formedstar on the chemical evolution of SO and SO . Figure 3 showsthe time evolution of the SO and SO abundances (in Phase II),for three di ff erent types of stars: 5, 10, and 15M (cid:12) . For a young( t < × years) hot core, the abundances of both molecules re-main unchanged independently on the mass of the star. However,as previous studies have shown (e.g. Viti et al. 2004), the evolu-tionary stages of both molecules occur at di ff erent times stronglydepending on the heating rate, i.e., on the mass of the star. In Fig.3, we observe a discontinuity in the curves (between 3 × and10 years, depending on the model) that corresponds to di ff er-ent times at which the sublimation temperature of each speciesis reached. The di ff erence found is ∼ × years between a starwith 5M (cid:12) and a star with 15M (cid:12) .Figure 4 shows how initial sulphur abundance a ff ects theevolution of SO and SO . We follow the abundances of thesespecies as a function of time for three di ff erent values of initialsulphur abundance (0.01, 0.1, and 1S (cid:12) ). For a young hot core, the
3. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. 3. Di ff erences in the SO and SO abundance evolution dur-ing Phase II for a hot core model when only the star mass is var-ied. The considered values are 5, 10, and 15M (cid:12) drawn in black,red, and blue, respectively. Fig. 4. Di ff erences in the SO and SO abundance evolution dur-ing Phase II for a hot core model when only the initial sul-phur abundance is varied. The considered values are 1, 0.1, and0.01S (cid:12) drawn in blue, black, and red, respectively.increase of one order of magnitude in the initial sulphur abun-dance also leads to the increase of one order of magnitude inthe SO abundances; however, this e ff ect is lower as the hot coreevolves. In general the SO / SO ratio increases as the initial sul-phur abundance increases.The e ff ects of varying the hydrogen density on the evolu-tion of SO and SO abundances during Phase II are shown inFig. 5. We observe the largest di ff erences during the early stage( t < × ), where the increase of one order of magnitude in thehydrogen density of the region mainly provides di ff erences ofup to one order of magnitude in the abundances of SO . For anevolved hot core, the di ff erences produced in the abundances ofboth species are much lower than one order of magnitude.Figure 6 shows the evolution of SO and SO during the warmphase (Phase II) of the hot core as a function of the gas accretione ffi ciency, f r , occurred during Phase I. We have considered twovalues of f r ; 0.3 and 0.85, which imply CO depletions of ∼ ∼ ffi ciency during Phase I is especially relevantfor the SO and SO abundances in Phase II. In particular, for a Fig. 5. Di ff erences in the SO and SO abundance evolution dur-ing Phase II for a hot core model when only the final hydrogendensity is varied. The considered values are 10 cm − (black)and 10 cm − (red). Fig. 6. Di ff erences in the SO and SO abundance evolution dur-ing Phase II for a hot core model when only the accretion ef-ficiency is varied. The considered values are 0.30 in black and0.85 in red.young hot core ( t < × years), di ff erences of more than twoorders of magnitude in the abundances of both species are founddepending on whether the accretion e ffi ciency is low (0.3) orhigh (0.85). For longer times ( t (cid:38) years), the e ff ects of varyingthis parameter become almost negligible, especially for SO .We have also looked at how abundances of SO and SO in Phase II are a ff ected by the form sulphur takes once on thegrains. Atoms, such as sulphur in gas phase accreting on thegrains from the gas, lead to the formation of new species in thesolid phase. Given that H is the most abundant element, we as-sume that the hydrogenation of S atoms is energetically viable;S atoms will tend to form H S molecules because they impingeon icy grain mantles. We have investigated the e ff ects of vary-ing the percentage of sulphur atoms (S and S + ) that freeze outto form H S and OCS on the mantles at the end of Phase I.We find that varying the amount of neutral S that goes into H Sor OCS does not significantly a ff ect the abundances of SO andSO during Phase II. We also find only a very slight change inthe abundances of OCS. For the ionised case, when we considerthat all S + atoms freeze out to form H S, we obtain similar re-sults to those obtained for the case where di ff erent percentages
4. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. 7. E ff ect on SO and SO abundances (Phase II) of varyingthe percentage of S + frozen into H S and OCS. In black, weshow the results when all S + freezes out to form H S and in red,when S + freezes out to form OCS (50%) and H S (50%) at theend of Phase I.
Fig. 8. E ff ect on OCS and H S abundances (Phase II) of varyingthe percentage of S + frozen into H S and OCS. In black, weshow the results when all S + freezes out to form H S and in red,when S + freezes out to form OCS (50%) and H S (50%) at theend of Phase I.of neutral sulphur freeze out to form OCS and H S. After in-cluding in the model the possibility that part of S + also goes intoOCS on the mantles (Figs. 7 and 8), we obtain small di ff erences(lower than one order of magnitude) among the abundances ofSO, SO , and H S. The most important di ff erence is found inthe OCS evolution, where its abundance varies up to two ordersof magnitude in Phase II, when only the 50% of S + is frozenout to form H S at the end of the Phase I. All together, this sug-gests that most of the sulphur (whose ionisation potential is rel-atively low 10.36 eV) present in the di ff use phase of the cloud(Phase I) must be ionised and that it is frozen out before beingneutralised. This agrees with Ru ffl e et al. (1999), who proposedthat S + would freeze out onto dust grains during the collapsemore e ffi ciently than neutral species, given that in dense inter-stellar regions, grains typically carry one negative charge (Gail& Sedlmayr 1975, Bel et al. 1989, Druard & Wakelam 2012).The time-scale of collision of S + with grains on one order ofmagnitude smaller than those associated with the neutral speciescontaining most of the carbon, oxygen, and nitrogen. This result is also consistent with the detection of S recombination lines indark clouds (Pankonin & Walmsley 1978). Figure 9 shows the evolution of the abundance of SO andSO during Phase II when di ff erent values of gas density areconsidered. Variations of one order of magnitude in the gas den-sity can lead to di ff erences of up to two orders of magnitude inthe abundances of these species. In particular, we observe thistrend for the early stage of the plateau, especially during the gascooling after the shock (4 × (cid:46) t (cid:46) × yrs). For an evolvedplateau, the di ff erences obtained in the abundances of SO andSO are less than one order of magnitude, becoming negligiblefor t > years. Fig. 9. Di ff erences in the SO and SO abundance evolution dur-ing Phase II for a plateau model when only the final gas density isvaried. The considered values are 5 × cm − (blue) and 5 × cm − (red). Fig. 10. Di ff erences in the SO and SO abundance evolution dur-ing Phase II for a plateau model when only the initial sulphurabundance is varied. The considered values are 0.1S (cid:12) (blue) and0.01S (cid:12) (red).We have also analysed the e ff ect of varying the initial sul-phur abundance on the SO and SO evolution in the plateau.We considered two values (0.1 and 0.01S (cid:12) ). In Fig. 10 we show
5. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. 11. Di ff erences in the SO and SO abundance evolutionduring Phase II for a plateau model when only the maximumshocked gas temperature is varied. The considered values are2000 K (blue) and 1000 K (red).that the increase of one order of magnitude in the initial sulphurabundance leads to the obvious increase of up to one order ofmagnitude in the abundances of SO and SO during the warmphase. Only during the gas cooling the di ff erences found in theabundances are slightly larger than one order of magnitude.Figure 11 shows the results of varying the gas temperaturedue to the presence of C-shock on the SO and SO evolutionduring Phase II. We have considered two possible values forthe maximum temperature reached by the gas during the shock,1000 K and 2000 K. During the fast increase in the gas tempera-ture (see Fig. 2 with the temperature profiles as a time function),we find only small variations in the SO abundances, while thedi ff erences found for SO are slightly larger. In particular, sincethe start of the shock passage (at t =
0) and up to t ∼ years, wepredict that a higher maximum gas temperature reached leads tolarger SO abundances and lower SO abundances.
4. Comparison with observations
To compare model results and observations, we ran severalhot core and plateau models considering the values for the dif-ferent input parameters used in Sect. 3. From the obtained abun-dances of each species, we estimated their column densities, N x , N x = ( n x / n H ) A V . × , (2)where ( n x / n H ) is the abundance of each species, A V the visualextinction, and 1.6 × cm − the hydrogen column density at 1mag of extinction. We compared these results with the columndensities of SO, SO , CS, OCS, H CS, and H S (see Table 2)inferred from the IRAM 30m and Herschel / HIFI line surveys(Tercero et al. 2010, Crockett et al. 2014b, respectively).These source-averaged column densities were obtained bythe same method, by fitting a large number of rotational linesusing a non-local thermodynamic equilibrium (non-LTE) exci-tation and radiative transfer code, MADEX (Cernicharo 2012), The emission observed of the sulphur-bearing molecules consid-ered in this paper was analysed using the code MADEX, except forH S, whose emission was analysed using the radiative transfer codeRADEX (van der Tak et al. 2007). See more details in Crockett et al.(2014a).
Table 2.
Source-averaged column densities, N , inferred from ob-servations towards Orion KL. Plateau Hot coreSpecies N × N × References(cm − ) (cm − )SO 18 ±
10 9 ± ± ±
20 (1)CS 2.4 ± ± ± ± CS 1.0 ± ± S ... 950 ±
200 (3)
References. (1) Crockettt et al. (2014b); (2) Tercero et al. (2010); (3)Crockett et al. (2014a).
Table 3.
Parameters adopted for the hot core and the plateau ofOrion KL in MADEX.
Component Density Temperature (cid:52) ν FWHM v LSR (cm − ) (K) (km s − ) (km s − )Plateau 1 ×
125 25 6Hot core 5 ×
225 10 5.5 where it is assumed that the width of the lines is due to the exis-tence of large velocity gradients (LVGs) across the cloud. Theradiative coupling between two relatively close points is thusnegligible, and the excitation problem is local. Only for the hotcore, we considered LTE excitation, which means that most tran-sitions are thermalized to the same temperature. Corrections forbeam dilution were also applied for each line depending on thedi ff erent beam sizes at di ff erent frequencies. For the hot core andthe plateau of Orion KL, uniform physical conditions of density,kinetic temperature, line width, and radial velocity ( v LSR ) wereassumed taking the results obtained from Gaussians fits of theline profiles, rotational diagrams, and di ff erent maps of each re-gion into account (see Tercero et al. 2010 and Crockett et al.2014b). In addition, in the case of the density, typical valuesquoted in the literature were also considered. Table 3 lists thevalues assumed for each parameter, where only the column den-sity of each component was left as a free parameter in MADEX.To determine the uncertainty of the values of hydrogen den-sity and of the kinetic temperature, several MADEX modelswere run, varying only the values for these parameters and fixingthe rest. Comparing the intensity di ff erences between the spec-tra and the obtained line profiles, uncertainties of 20% and 15%,were deduced for the temperature and the hydrogen density, re-spectively. The column density of each species was taken fromthe model that best reproduced the majority of the observed lineprofiles from transitions covering a wide energy range within a20% of the uncertainty in the line intensity. To determine the col-umn density errors shown in Table 2 we took di ff erent sourcesof uncertainty into account, such as spatial overlap of the cloudspectral components, pointing errors during the observations,and the lack of collisional rates for some species (SO , H CS).With respect to the model results, the main uncertainties arein the treatment of the surface reactions, in the assumed initialelemental abundances in the gas (listed in Sect. 2), with uncer-tainties up to 10% (from a comparison with other works), and inthe rates for the chemical reactions available in gas-phase net-works (e.g. Wakelam et al. 2012).
6. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Table 4.
Hot core chemical models.
Star Sulphur Accretion DensityModel mass abundance e ffi ciency n H (M (cid:12) ) (S (cid:12) ) f r × (cm − )1 | | | | | | | | | | | | | |
12 15 | | | |
15 15 | | | |
18 15 | | Notes.
Models with a density of 10 cm − are noted by the numbers ofCol. 1 and adding the letter b . We list in Table 4 the hot core models run considering theinput parameters described in Sect. 2.1. In figures for Sects.4.1 and 4.2, we show (with points ) when model results repro-duce the observational results of the species SO, SO , CS, OCS,H CS, and H S.For all models with a stellar mass of 5M (cid:12) (see for exam-ple Fig. A.1, Appendix A), the observations are reproduced after t > years since the central star switches on which, if com-pared to the dynamical timescale, it is possibly too long for thehot core of Orion KL whose age is estimated to be closer to ∼ years (Walmsley et al. 1987, Plambeck & Wright 1987, Brownet al. 1988). In addition, the time range at which all sulphur-bearing species are matched well to the observations (taking theerror bars of the observed column densities into account) is upto ∼ × years, which is probably too long a time.With models where an initial solar sulphur abundance hasbeen considered (see for example Model 17 in Fig. A.2), it is alsopossible to reproduce the observations of the sulphur-bearingspecies; however this value leads to column densities of H S toohigh (see also Fig. A.1). Therefore we focus on models with ini-tial sulphur abundances of 0.1S (cid:12) and 0.01S (cid:12) .Figures A.3 and A.4 (Models 11 and 10 with 10M (cid:12) and15M (cid:12) , respectively) show the run models considering an ini-tial sulphur abundance of 0.01S (cid:12) , f r = n H = cm − .In both cases we notice that the observations of OCS, H S, andSO are not reproduced, even taking the error bars into account,since the maximum column densities obtained with these mod-els are ∼ is not reproduced either, but the di ff erence between observationsand models is less than one order of magnitude. We have alsorun Models 11 and 10 with a gas density of 10 cm − instead of10 cm − (Figs. A.5 and A.6 with Models 11 b and 10 b , respec-tively): in this case, it is possible to reproduce the observationsof the sulphur-bearing species.Model 5 is shown in Fig.12, where a stellar mass of 10M (cid:12) ,a hydrogen density of 10 cm − , and an intermediate initial sul-phur abundance of 0.1S (cid:12) have been assumed. In this case, theobservations of the six sulphur-bearing species (SO, SO , CS,OCS, H CS, and H S) are reproduced at t ∼ × years after thecentral star switches on, independently of the considered accre-tion e ffi ciency (see Model 2, Fig. A.7, with f r = Since the observations do not have measured times, their represen-tation in Figs. of Sects. 4.1 and 4.2 should be through horizontal lines.We have only plotted points (on which horizontal lines would be lo-cated) to indicate the time at which model results are matched to theobservations well.
Fig. 12.
Sulphur-bearing species column densities as a timefunction (gas phase) for the hot core Model 5 with a star mass of10M (cid:12) , f r = (cid:12) initial sulphur abundance, and a densityof 10 cm − . The points indicate the observational results of SO,SO , CS, OCS, H CS, and H S.shows that Model 4 (similar to Model 5, but with a central starof 15M (cid:12) ) also provides a good fit.All together these results suggest that models with an ini-tial sulphur abundance of 0.1S (cid:12) and a hydrogen density of 10 cm − , as well as models assuming 0.01S (cid:12) and n H = cm − ,with a central star of 10-15M (cid:12) in both cases, can reproduce theobservations of the molecules SO, SO , CS, OCS, H , and H CSin the hot core of Orion KL. In Esplugues et al. (2013b), we alsomodelled the species HC N and DC N in the hot core of OrionKL. We only reproduced the observations running a model with n H = cm − , a high depletion e ffi ciency (85%), and a centralstar of 10M (cid:12) (because too fast an increase in temperature, asoccurs for 15M (cid:12) , leads to a more e ffi cient destruction chemistry,hence misses the enhancement phase for HC N and DC N). Thistherefore favours Model 5 (Fig. 12) in our present grid.In Sect. 3.1 we found that large variations in the percentageof S + ions that freeze out to form H S and OCS at the end ofPhase I mainly a ff ect the evolution of OCS, while the changes inthe SO, SO , H S abundances are lower than one order of magni-tude. From Fig. 8, we find that the more H S is formed in PhaseI, the lower the OCS column densities in Phase II and the moresimilar to the observed value in the hot core, N (OCS) = × cm − . For this reason, the models listed in Table 4 have been ob-tained assuming that S + ions freeze out to form mainly ( ∼ S at the end of Phase I. We also ran Model 5 assuming that50% of S + in Phase I forms H S and the other 50% forms OCS(see Fig. 13): we find that while the chemical evolution of CS,SO, SO , and H CS barely changes with respect to Fig. 12, asexpected, the values for OCS present a di ff erence of one orderof magnitude, although it is still possible to reproduce the obser-vations for this species. In the case of H S, we do not reproduceits observational column density with the new assumptions, al-though the di ff erence between the model results shown in Fig. 13and the H S observations is lower than one order of magnitude.
We ran eight plateau models (Table 5), varying the input pa-rameters of initial sulphur abundance, the maximum shocked gas
7. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. 13.
Sulphur-bearing species column densities as a timefunction (gas phase) for the hot core Model 5 with a star mass of10M (cid:12) , f r = (cid:12) sulphur abundance. The points indi-cate the observational results of SO, SO , CS, OCS, H CS, andH S. In this case, S + freezes out to form OCS (50%) and H S(50%) on the mantles at the end of Phase I.
Table 5.
Plateau chemical models.
Model Sulphur abundance T maxshock Density n H (S (cid:12) ) (K) (cm − )1 0.01 2000 5 × × × × × × × × Notes.
Column 1 indicates the model, Col.2 the initial sulphur abun-dance in solar units, Col.3 the maximum shocked gas temperature, andCol. 4 the gas density at Phase II. temperature ( T maxshock ), and the final gas density. Figures showingthese models correspond to the Phase II of the plateau where t = T =
10 K up to 1000and 2000 K (depending on the model). After the passage of theshock, the gas rapidly (within hundred of years) cools down to ∼
80 K following the temperature profiles shown in Fig. 2.Figures 14 and B.1 (Appendix B) show the column densitiesof sulphur-bearing molecules obtained as a function of time fromModels 4 and 8. In both cases, the hydrogen density is n H = × cm − (recall that this is the final density at the end of Phase I), theinitial sulphur abundance 0.1S (cid:12) , and only the maximum shockedgas temperature is varied. Taking the error bars for the observedcolumn densities of each species into account, we find in bothcases a time at which all species are matched well to the obser-vations.If we decrease the hydrogen density (constant along allPhase II) by one order of magnitude, i.e. to 5 × cm − , (seeFigs. B.2 and B.3 showing Models 2 and 6, respectively), themaximum SO column density is to up to one order of magni- Fig. 14.
Sulphur-species column densities as a time function atPhase II for the plateau Model 4 with a final gas density of 5 × cm − , an initial sulphur abundance of 0.1S (cid:12) , and T maxshock = Fig. 15.
Sulphur-species column densities as a time function atPhase II for a plateau model with a density of 5 × cm − , aninitial sulphur abundance of 0.1S (cid:12) , and T shock = + freezes out to form OCS (50%) and H S (50%) on themantles at the end of Phase I.tude lower than the observed one. The same is found for OCSin the case where T maxshock = T maxshock = ff erence between themodel and the observed value of OCS is less than one order ofmagnitude. On the other hand, if we consider an initial sulphurabundance of 0.01S (cid:12) (Model 3, Fig. B.4), we also obtain val-ues for SO and OCS up to one order of magnitude less than theobserved ones. This suggests that models of plateau assumingan initial sulphur abundance of 0.1S (cid:12) , as well as a final den-sity of 5 × cm − (Models 4 and 8), provide a much better fitto the observed column densities of the sulphur-bearing species.Models 4 and 8 only di ff er in the maximum temperature reachedby the shocked gas (2000 and 1000 K, respectively). Our de-rived maximum shock temperatures agree with previous works:Maret et al. (2001) found a temperature of the shocked gas inOrion KL of 1500 K (assuming an angular extent of 40 (cid:48)(cid:48) ) andSempere et al. (2000) found T maxshock =
8. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL eters representative of the H peak 1 region with 10 (cid:48)(cid:48) of size.This may suggest that the maximum shocked gas temperaturereached in the plateau of Orion KL is closer to 2000 K than to1000 K, therefore favouring Model 4 (Fig. 14) in our presentgrid. In this case, the observations are reproduced at t ∼ × years. The temperature profile (Fig. 2) for a plateau model witha maximum shocked gas temperature of 2000 K indicates that atthat time, the gas temperature is ∼
90 K, which is consistent withthe value considered in MADEX ( T k =
125 K) when the 20% ofuncertainty assumed to model the observed line profiles of thesulphur-bearing species is taken into account.As in the hot core case, the models listed in Table 5 havealso been run assuming that S + ions freeze out to form mainly( ∼ S at the end of Phase I and only ∼
5% of OCS. Onlyfor Model 4, where all species are well matched to the obser-vations, we have also assumed that 50% of S + in Phase I formsH S and the other 50% forms OCS (see Fig. 15). In this case,we see that while the column densities of most sulphur-bearingspecies do not show any significant changes, the column den-sities of OCS are almost two orders of magnitude higher thanthose obtained in Fig. 14. These results suggest that at the end ofPhase I, most of S + is frozen out as H S.
5. Discussion
Although we do not have a firm detection of H S in interstel-lar ices, it does not necessarily imply that this molecule does notform, since, chemically, it should easily hydrogenate on grains.The non detection of solid H S in the interstellar medium can bedue to either a fast dissociation or may even be an observationalbias because this ice is particularly di ffi cult to observe. Geballeet al. (1985, 1991) inferred H S ice in the feature from 3.90 to3.97 µ m in W33A, a high-mass protostar; however, this detectionis not fully supported in the literature. Van der Tak et al. (2003)argue that infrared observations do not support the assumptionthat H S is the main S reservoir in grain mantles, and they pro-vide the ISO-SWS observations of W33A as an example. One ofthe problems for identifying the 3.925 µ m (2548 cm − ) band ofH S in H O-rich ice mantles was the lack of laboratory spectraof H S embedded in an H O matrix, which is expected to a ff ectthis band significantly.Mumma and Charnley (2011) present a list with the percent-age of the observed abundances (and upper limits for the non-observed ices) of several species relative to water in ices of mas-sive protostars. For H S the value is < × − -10 − and for OCSis 4 × − -2 × − (obtained by Palumbo et al. 2007 with datafrom a sample of eight dense molecular clouds). The H S andOCS abundances relative to H O that we obtain at the end ofPhase I in the hot core Model 5 (Fig. 12 where all the species arewell matched to the observations considering that S + freezes outto form mainly H S) are 3 × − and 2 × − for H S and OCS,respectively (see Table 6). Although the OCS abundance relativeto water obtained with the model is slightly lower, both results(for H S and OCS) are consistent with those listed in Mummaand Charnley (2011). See Grim & Greenberg 1987, Moore et al. 2007, and Garozzo etal. 2010 who explained the failure of its detection in the solid phasefrom irradiation experiments with 200 keV protons. They found thatH S column density has a very drop o ff at the beginning of irradiation,until almost all the H S molecules are decomposed.
Table 6.
Abundances (relative to H) of the species H S, OCS,and H O on the mantles of the dust grains at the end of Phase I.
Mantle species m(x) Abundancem(H S) 1.357 × − m(OCS) 7.300 × − m(H O) 4.278 × − m(H S) / m(H O) 3 × − m(OCS) / m(H O) 2 × − We now examine the chemistry of sulphur-bearing speciesin Model 5 in more detail. In Fig. 12, we observe how H S isevaporated at di ff erent evolutionary stages in Phase II, in agree-ment with Viti et al. (2004). Each evaporation is followed by asharp decrease in its column density owing to dissociation pro-cesses. The evolution of the column densities of SO and SO areclosely related; whereas SO is predicted to be mainly formed bythe reactionO + SH → SO + H , (3)it is destroyed by reacting with OH, leading to the e ffi cient for-mation of SO :OH + SO → SO + H . (4)This sequence triggers a continuous increase in SO , whereasSO does not stop decreasing. As a result, the SO / SO ratio in-creases with time, in agreement with the results obtained byWakelam et al. (2011), who modelled the sulphur chemistryof several massive dense cores. This is also consistent withEsplugues et al. (2013a), who concluded that SO is a bettertracer of warm gas than SO. For longer times, t (cid:38) × yearssince the star switches on, the increase in SO starts to be lim-ited as the reactionSO + C → SO + CO , (5)becomes more e ffi cient. This again produces SO and then col-umn densities of both sulphur-bearing molecules stabilise. Onthe other hand, we find that during the hot core evolution, theformation of CS and H CS is mainly the result of the reactions:CH + S → CS + H (6)andCH + S → H CS + H , (7)respectively. Although their abundances are low for a young hotcore, these two molecules become, together with SO , the mostabundant sulphur-bearing species for a more evolved hot core. Analysing the evolution chemistry of the di ff erent species inModel 4, Fig. 14, we see that for t < years, when the temper-ature of the region remains high ( T (cid:38) ff ect, the column densities of OCS, SO, and SO remain almostunchanged. In particular, during this period we find that the mainformation routes of SO and SO are through molecular oxygen:O + S → SO + O (8)
9. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL andO + SO → SO + O . (9)For t (cid:38) years, we find that the chemistry drastically changes,especially for SO and SO , which increase their column densi-ties up to five orders of magnitude. In this period where the gashas been cooled, we find that SO is mainly formed through OH:OH + S → SO + H . (10)OH is also the main one responsible for the destruction of SOand the formation of SO , through the reactionOH + SO → SO + H . (11)With this reaction all sulphur monoxide is rapidly convertedinto SO , leading to a rapid decrease in the SO / SO ratio withtime. Goicoechea et al. (2006b) found a fractional abundance of X (OH) = (0.5-1) × − in the plateau and Goldsmith et al. (2011)obtained X (O ) = (0.3-7) × − , deducing that this value is the re-sult of an enhancement of X (O ) by shocks. In fact, we have rep-resented the evolution of the OH / O ratio, Fig. 16, and our modele ff ectively predicts that during the shock action, the abundanceof O is much larger than that of OH, whereas we find the oppo-site for large times. The abundances, as well as the dominant for-mation / destruction reactions of SO and SO at di ff erent stages,indicate that a chemical transition (with similar abundances ofO and OH) is currently taking place in the plateau, where SOand SO reactions change from being dominated by O to be-ing dominated by OH. In the case of CS and H CS (which areformed through the reaction of S with CH and CH , respec-tively, for t (cid:46) years), we observe that their column densitiesincrease with time, and become, together with SO , the mostabundant sulphur-molecules in an evolved plateau. Fig. 16. OH / O and SO / SO ratios along the time for the plateauwarm phase (Phase II).
6. Summary and conclusions
We have modelled the sulphur chemistry of the Orion KLcloud for two components, the hot core and the plateau shockedgas. We investigated a wide range of parameters, such as the gas density, the accretion e ffi ciency, the initial sulphur abundance,the mass of the formed star, the shock temperature, and di ff erentchemical paths on the grains leading to di ff erent reservoirs ofsulphur on the ice grain mantles.We first analysed the e ff ect of varying these parametersmainly on the time evolution of the SO and SO abundances,during the warm-up phase. Our results can be summarized asfollows: • The mass of the central star : its variation barely a ff ects theSO and SO abundances at early times of the warm up phase.For t > × years, the evolution times for both moleculespresent a di ff erence of ∼ × years between stars with 5M (cid:12) and 15M (cid:12) . • E ffi ciency of gas accretion on the grains : for an early hotcore ( t < × years), the accretion e ffi ciency of species ontograin surfaces in the cold phase plays an important role in theabundances of SO and SO present in the warm gas phase,leading to di ff erences of more than two orders of magnitude.As the hot core evolves, these di ff erences become negligible. • Maximum temperature of the shocked gas in the plateau :from the time the shock starts and up to ∼ years, as themaximum temperature reached increases, lower SO abun-dances and larger SO abundances are obtained. • Depletion of S atoms versus S + ions : varying the percent-age of neutral sulphur that freezes on grains during the coldphase to form H S or OCS barely a ff ects the sulphur reser-voirs on the mantle. In contrast, varying the percentage ofS + that freezes out significantly a ff ects the OCS abundances.This suggests that most of the sulphur present in cold phasemust have been in S + ions, consistent with an inhomoge-neous and clumpy cloud where UV photons penetrate thegas, ionising S atoms. Our models indicate that S + ions de-pleted onto grains mainly form H S at the end of this phase.Finally, we compared the results obtained from the modelswith observations of sulphur-bearing molecules (SO, SO , CS,OCS, H S, and H CS) in Orion KL. It is possible to reproducethe observations of these species in the hot core by assumingmodels with an initial sulphur abundance of 0.1S (cid:12) and a hy-drogen density of 10 cm − , as well as with models assuming0.01S (cid:12) and n H = cm − , with a stellar mass higher than 5M (cid:12) inboth cases. By also considering HC N and DC N observationsfrom a previous study, we find that high (85%) accretion e ffi -ciency during the cold phase of the cloud is necessary, favouringModel 5. For t < years, we find that the main SO formationroute is through the reaction of SH with oxygen. SO is formedby destroying SO when it reacts with OH, while CS and H CS,are mainly formed through CH and CH , respectively.For the plateau, it is possible to reproduce the observationsof the sulphur-bearing molecules through models with an ini-tial sulphur abundance of 0.1S (cid:12) and a hydrogen density duringthe warm-phase of at least 5 × cm − . We deduce that sincethe start of the shock and for ∼ years, the column densitiesof SO , SO, and OCS do not significantly change. For longertimes, however, the chemistry drastically changes; the SO / SO ratio rapidly decreases with time until the SO column densitiesbecome more than two orders of magnitude lower than those ofSO , CS, and H CS. In this component, we also find that a chem-ical transition occurs, where SO and SO reactions change frombeing dominated by O to being dominated by OH.
10. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Acknowledgements.
We thank the anonymous referee for valuable commentsthat greatly improved the manuscript. We thank the Spanish MINECO forfunding support from grants CSD2009-00038, AYA2009-07304, and AYA2012-32032. G.B.E. is supported by a CSIC grant JAE PreDoc2009. J.R.G. is sup-ported by a Ram´on y Cajal research contract.
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Appendix A: Figures of hot core
The figures of this Appendix show hot core models in PhaseII. The points indicate the observational results of SO, SO , CS,OCS, H CS, and H S.
11. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. A.1.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of5M (cid:12) , a solar sulphur abundance, 1S (cid:12) , and gas density of 10 cm − (Model 18). Fig. A.2.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of10M (cid:12) , 1S (cid:12) , and density of 10 cm − (Model 17). Fig. A.3.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of10M (cid:12) , 0.01S (cid:12) , and density of 10 cm − (Model 11). Fig. A.4.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of15M (cid:12) , 0.01S (cid:12) , and density of 10 cm − (Model 10).
12. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Fig. A.5.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of10M (cid:12) , 0.01S (cid:12) , and a density of 10 cm − (Model 11 b ). Fig. A.6.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of15M (cid:12) , 0.01S (cid:12) , and a gas density of 10 cm − (Model 10 b ). Fig. A.7.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of10M (cid:12) , 0.1S (cid:12) , f r = cm − of density (Model 2). Fig. A.8.
Sulphur-bearing species column densities as a timefunction in a hot core model (gas phase) with a star mass of15M (cid:12) , 0.1S (cid:12) , f r = cm − (Model 4).
13. B. Esplugues et al.: Modelling the sulphur chemistry evolution in Orion KL
Appendix B: Figures of plateau
The figures of this Appendix show plateau models in PhaseII. The points indicate the observational results of SO, SO , CS,OCS, and H CS.
Fig. B.1.
Sulphur-species column densities as a time functionin the gas phase for a plateau model with a final gas den-sity of 5 × cm − , an initial sulphur abundance of 0.1S (cid:12) , and T maxshock = Fig. B.2.
Sulphur-species column densities as a time functionin the gas phase for a plateau model with a final gas den-sity of 5 × cm − , an initial sulphur abundance of 0.1S (cid:12) , and T maxshock = Fig. B.3.
Sulphur-species column densities as a time function inthe gas phase for a plateau model with a final hydrogen den-sity of 5 × cm − , an initial sulphur abundance of 0.1S (cid:12) , and T maxshock = Fig. B.4.
Sulphur-species column densities as a time functionin the gas phase for a plateau model with a density of 5 × cm − , an initial sulphur abundance of 0.01S (cid:12) , and T maxshock =2000K (Model 3).