Deuterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions
MMon. Not. R. Astron. Soc. , 1– ?? (2014) Printed 29 August 2018 (MN L A TEX style file v2.2)
Deuterium chemistry of dense gas in the vicinity oflow-mass and massive star forming regions
Zainab Awad , (cid:63) Serena Viti , Estelle Bayet , and Paola Caselli Department of Astronomy, Space Science, and Meteorology, Faculty of Science, Cairo University, Giza 11326, Egypt Department of Physics and Astronomy, University College London, London WC1E 6BT, UK Sub-Department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
Accepted 2014 June 9. Received 2014 June 7; in original form 2013 October 8
ABSTRACT
The standard interstellar ratio of deuterium to hydrogen (D/H) atoms is ∼ . × − .However, the deuterium fractionation is in fact found to be enhanced, to different de-grees, in cold, dark cores, hot cores around massive star forming regions, lukewarmcores, and warm cores ( hereafter , hot corinos) around low-mass star forming regions.In this paper, we investigate the overall differences in the deuterium chemistry be-tween hot cores and hot corinos. We have modelled the chemistry of dense gas aroundlow-mass and massive star forming regions using a gas-grain chemical model. We in-vestigate the influence of varying the core density, the depletion efficiency of gaseousspecies on to dust grains, the collapse mode and the final mass of the protostar on thechemical evolution of star forming regions. We find that the deuterium chemistry is,in general, most sensitive to variations of the depletion efficiency on to grain surfaces,in agreement with observations. In addition, the results showed that the chemistry ismore sensitive to changes in the final density of the collapsing core in hot cores than inhot corinos. Finally, we find that ratios of deuterated sulphur bearing species in densegas around hot cores and corinos may be good evolutionary indicators in a similarway as their non deuterated counterparts. Key words:
Astrochemistry - Stars: low-mass, massive, formation - ISM: abundances,molecules
Observations of local interstellar deuterated molecules havelong been used to probe the physical conditions within inter-stellar clouds. Although the interstellar ratio of deuteriumto hydrogen (D/H) atoms within the Milky Way is only ∼ . × − (Linsky et al. 1995; Oliveira et al. 2003), theobserved degree of deuterium fraction is enhanced in manyastrophysical regions such as cold, dark cores (e.g. Tin´e et al.2000; Crapsi et al. 2005), hot cores around massive starforming regions (e.g. Ehrenfreund & Charnley 2000; Fontaniet al. 2008), lukewarm cores (Sakai et al. 2009), and hotcorinos around low mass star forming regions (e.g Ceccarelliet al. 1998, 2001, 2007).Although there are only few detections of HD, the sim-plest deuterated species (e.g. Wright et al. 1999; Bertoldi (cid:63) E-mail: [email protected] The deuterium fraction is the abundance ratio of a moleculecontaining a deuterium atom (XD) to the equivalent moleculewith a hydrogen atom (XH). Deuterium fractionation is the pro-cess which leads to the enhancement of the deuterium fraction. et al. 1999; Polehampton et al. 2002; Caux et al. 2002;Neufeld et al. 2006; Yuan et al. 2012; Bergin et al. 2013),there is a growing body of observations, towards low- andhigh- mass star forming regions, for mono- as well as mul-tiply deuterated species, such as H D + (e.g. Stark et al.1999, 2004; Pillai et al. 2012), DCN (e.g. Wilson et al. 1973;van Dishoeck et al. 1995), HDO (Henkel et al. 1987; vanDishoeck et al. 1995), and DCO + (Penzias 1979; Butner& Loren 1988). The first detection of a doubly deuteratedspecies (D CO) was towards Orion KL by Turner (1990).The same molecule was then detected towards the low-massprotostar IRAS16293-2422 (Ceccarelli et al. 1998) with aD CO/H CO ratio 15 times higher than that obtained forOrion.To-date, around 32 deuterated species have been de-tected in a variety of interstellar clouds including ND ,CHD OH, CD OH, D S, D O, HD +2 , CH DOH, H D + ,C D, C HD, DC N, and DCOOCH (e.g. see reviews Cec-carelli et al. 2007; Herbst & van Dishoeck 2009; Caselli &Ceccarelli 2012; Tielens 2013 and references therein). Ta- c (cid:13) a r X i v : . [ a s t r o - ph . S R ] J un Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli
Table 1.
List of observed deuterated species in massive ‘H’ andlow-mass ‘L’ star forming regions and the corresponding refer-ences.
Species Region Ref (e.g.)H D + H Pillai et al. (2012)L Stark et al. (1999) N D + H Fontani et al. (2006)L Emprechtinger et al. (2009)
DCO + H Penzias (1979)L Butner & Loren (1988) CH D + H Roueff et al. (2013)
DCN
H Wilson et al. (1973)L van Dishoeck et al. (1995) † DNC
H Rodgers & Millar (1996)L van Dishoeck et al. (1995)
HDS
L van Dishoeck et al. (1995) D S L Vastel et al. (2003)
HDCO
H Loren & Wootten (1985)L van Dishoeck et al. (1995) D CO H Turner (1990)L Ceccarelli et al. (1998) CH OD H Saito et al. (1994)L Parise et al. (2002) CH DOH
H Jacq et al. (1993)L Parise et al. (2002)
CHD OH L Parise et al. (2002) CD OH L Parise et al. (2004) C D H Vrtilek et al. (1985)L van Dishoeck et al. (1995) C D L Sakai et al. (2009) C D L Sakai et al. (2009) C HD L Sakai et al. (2009) DC N L Sakai et al. (2009) DC N L Sakai et al. (2009) NH D H Rodriguez Kuiper et al. (1978)L Shah & Wootten (2001) ND L van der Tak et al. (2002)
HDO
H Henkel et al. (1987)L van Dishoeck et al. (1995) D O H Neill et al. (2013a)L Butner et al. (2007) CH DCN
H Gerin et al. (1992)
DCOOCH L Demyk et al. (2010) † Data for massive star forming regions ‘H’ is taken from Table1 in Rodgers & Millar (1996) ble (1) lists the observed deuterated species in cores aroundmassive (H) and low-mass (L) star forming regions only.In parallel to observational studies, theoretical mod-elling of deuterium chemistry has also being taken placeover the years. Early attempts at studying deuterium chem-istry used simple gas-phase models (Watson 1980) or surfacechemistry (Tielens 1983). Both models were relatively suc-cessful in reproducing the observed deuterated species, atthat time, and showed the role of grain surface reactionsin enhancing deuteration of some species such as H CO.The first gas-grain chemical models were later developed byBrown & Millar (1989a,b). They found that surface chem-istry can produce small but significant amounts of multi-deuterated molecules. Following this study, Millar et al.(1989) used a detailed numerical pseudo-time-dependentgas-phase chemical model to study deuteration in denseclouds. Their results showed that in cold clouds ( ∼
10 K) the main sources of fractionation are H D + and its daugh-ter ions DCO + and H DO + , while for warmer regions, upto 70 K, CH D + , C HD + , and associated species led to thehigh fractionation. The methodology of extending chemicalnetworks to include deuterium was presented by Rodgers& Millar (1996) in their model of the deuterium chemistryin hot cores. They based their approach on the assumptionthat both H- and D-bearing species react with the samespecies with the same rate coefficient. Where there is an un-certainty, statistical branching ratios were assumed. Theirmodel showed that the deuterium fractionation would bepreserved in hot cores for at least 10 − yrs after evapora-tion. Following Rodgers & Millar (1996), Roberts & Millar(2000a,b) developed new chemical models including, for thefirst time, the deuterated sulphur-bearing species and gas-phase chemistry of some doubly-deuterated species to in-vestigate the influence of varying a wide range of physicalparameters on the fractionation in interstellar clouds. Theyfound that the fractionation ratios are temperature and frac-tionation process ( i.e. due to H D + , CH D + or C HD + )dependent. In addition, they showed that H S and HDS aregood probes for regions where grain surface chemistry is im-portant. They also commented that HDCO and D CO aregood probes for fractionation on grain surfaces. The detec-tion of multiply deuterated isotopologues of H +3 (e.g. Caselliet al. 2003; Vastel et al. 2004) motivated Roberts et al. (2003,2004) to release a new version of their model in which multi-ply deuterated isotopologues of H +3 are taken into account.Their results revealed that the inclusion of D H + and D +3 enhances the fractionation of ionic and neutral species be-cause it allows more deuterium to transfer to other speciesin dark clouds. In addition, experimental work and chemicalmodels showed that, in cold environments, the spin state ofspecies (ortho, para, meta) affects the deuterium fractiona-tion (e.g. Gerlich et al. 2002; Walmsley et al. 2004; Floweret al. 2006; Hugo et al. 2009; Sipil¨a et al. 2010), by allowingortho-H to react with deuterated isotopologues of H +3 andreducing deuterium fractionation.It is clear that deuterium fractionation is a functionof the chemical as well as the physical conditions of starforming regions. This paper is dedicated to investigate, the-oretically, whether the evolution of deuterium chemistry dif-fers significantly between low- and high- mass star formingregions by modelling deuterium chemistry under physicalconditions likely similar to those of hot cores and corinos.Our model treats the desorption of the species, deuteratedand non-deuterated, to be temperature dependent duringthe protostellar phase (the warming up phase). This ap-proach allows for better identification of chemical tracersfor various evolutionary stages in regions where the proto-star has started to affect the surrounding gas and dust.The present model, UCL DCHEM, is an adaptation ofthe UCL CHEM model (Viti & Williams 1999; Viti et al.2004), which in its original form did not include a deuteriumnetwork. This paper is organized as follows: in § § This paper is an extension with several updates to the original,unpublished, study performed by Zainab Awad as part of herPh.D; see Awad (2010). c (cid:13) , 1–, 1–
10 K) the main sources of fractionation are H D + and its daugh-ter ions DCO + and H DO + , while for warmer regions, upto 70 K, CH D + , C HD + , and associated species led to thehigh fractionation. The methodology of extending chemicalnetworks to include deuterium was presented by Rodgers& Millar (1996) in their model of the deuterium chemistryin hot cores. They based their approach on the assumptionthat both H- and D-bearing species react with the samespecies with the same rate coefficient. Where there is an un-certainty, statistical branching ratios were assumed. Theirmodel showed that the deuterium fractionation would bepreserved in hot cores for at least 10 − yrs after evapora-tion. Following Rodgers & Millar (1996), Roberts & Millar(2000a,b) developed new chemical models including, for thefirst time, the deuterated sulphur-bearing species and gas-phase chemistry of some doubly-deuterated species to in-vestigate the influence of varying a wide range of physicalparameters on the fractionation in interstellar clouds. Theyfound that the fractionation ratios are temperature and frac-tionation process ( i.e. due to H D + , CH D + or C HD + )dependent. In addition, they showed that H S and HDS aregood probes for regions where grain surface chemistry is im-portant. They also commented that HDCO and D CO aregood probes for fractionation on grain surfaces. The detec-tion of multiply deuterated isotopologues of H +3 (e.g. Caselliet al. 2003; Vastel et al. 2004) motivated Roberts et al. (2003,2004) to release a new version of their model in which multi-ply deuterated isotopologues of H +3 are taken into account.Their results revealed that the inclusion of D H + and D +3 enhances the fractionation of ionic and neutral species be-cause it allows more deuterium to transfer to other speciesin dark clouds. In addition, experimental work and chemicalmodels showed that, in cold environments, the spin state ofspecies (ortho, para, meta) affects the deuterium fractiona-tion (e.g. Gerlich et al. 2002; Walmsley et al. 2004; Floweret al. 2006; Hugo et al. 2009; Sipil¨a et al. 2010), by allowingortho-H to react with deuterated isotopologues of H +3 andreducing deuterium fractionation.It is clear that deuterium fractionation is a functionof the chemical as well as the physical conditions of starforming regions. This paper is dedicated to investigate, the-oretically, whether the evolution of deuterium chemistry dif-fers significantly between low- and high- mass star formingregions by modelling deuterium chemistry under physicalconditions likely similar to those of hot cores and corinos.Our model treats the desorption of the species, deuteratedand non-deuterated, to be temperature dependent duringthe protostellar phase (the warming up phase). This ap-proach allows for better identification of chemical tracersfor various evolutionary stages in regions where the proto-star has started to affect the surrounding gas and dust.The present model, UCL DCHEM, is an adaptation ofthe UCL CHEM model (Viti & Williams 1999; Viti et al.2004), which in its original form did not include a deuteriumnetwork. This paper is organized as follows: in § § This paper is an extension with several updates to the original,unpublished, study performed by Zainab Awad as part of herPh.D; see Awad (2010). c (cid:13) , 1–, 1– ?? euterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions ran. We compare our model calculations with observationsand other models in §
4, and give our conclusions in § We have used a time-dependent gas-grain chemical modeldescribed in details in Viti et al. (2004) and Awad et al.(2010). The model is a two-phase code. The first phase(Phase I) simulates the free-fall collapse of a core as de-scribed in Rawlings et al. (1992). It starts with diffuse,mainly atomic material which has an initial number den-sity of ∼
400 cm − and temperature of 10 K. The mate-rial undergoes a free-fall collapse up to a given final den-sity considered here as a free parameter. During phase I,gas-phase chemistry and freeze-out on to grain surfaces oc-cur. Accreted atoms and molecules hydrogenate or deuteratewhen possible. The depletion (or freeze-out) efficiency is de-termined by the amount of the gas-phase material frozenonto the grains. Since CO is the most abundant species, af-ter H , and because its depletion percentage is an importantfactor in measuring the deuterium fractionation (e.g. Caselliet al. 2002; Bacmann et al. 2003), we based our calculationof the depletion efficiency on the abundance of CO species.The depletion efficiency is regulated by varying the stickingprobability of gas-phase species onto grain surfaces (Rawl-ings et al. 1992). In Phase II, we follow the chemistry ofthe remnant core, after the star is born. In this phase, thecentral star heats up the surrounding gas and dust, causingselective sublimation of the icy mantles. We adopt an iden-tical treatment for the time dependent ice sublimation toViti et al. (2004).In this work, we have extended the species set usedin Awad et al. (2010) by including all the possible mono-deuterated counterparts for H-bearing species to model deu-terium chemistry. The only doubly deuterated species in-cluded in this model is D CO and its parent ion HD CO + .The exclusion of other doubly-deuterated species, as wellas triply-deuterated species is of course a limitation of thismodel. However we note that the aim of this work is to char-acterize the general trends of the chemical evolution of hotcores and corinos (hence gas at temperature higher than100K). In these warm regions deuteration is not as effi-cient as in dark clouds. The deuteration process is drivenby H D + , C HD + and/or CH D + ions that are formedvia radiative association reactions involving H +3 , C H +2 , andCH +3 ions, respectively. At low temperatures, H +3 is the mostabundant ion and hence fractionation of H D + is the mostimportant of the three. At temperatures higher than 25K,however, H D + is destroyed by H . CH D + and C HD + are important for deuteration in regions colder than 60 and80 K, respectively (Millar et al. 2000; Parise et al. 2009).Hence, the amount of doubly and triply deuterated formsof H +3 or CH +3 should be negligible at temperatures higheror equal to 100K. While therefore our chemistry is limitedduring Phase I (leading to the warm up phase starting witha deuteration fraction that may not be as accurate) by fo-cusing on the trends during the warm up phase, in the warmgas, a slightly lower or higher deuteration at the beginningof Phase II should not affect the trends. In fact chemicalmodels by Roberts et al. (2003) revealed that the inclusionof multiply deuterated forms of H +3 improved the results for certain ions, namely N H + . Our model supports this result(see Section 3).Our chemical network is based on the network previ-ously described by Roberts & Millar (2000a). However, weupdated several reactions following Woodall et al. (2007) ratefile and using Rodgers & Millar (1996) recipe in gen-erating the reactions involving the deuterium counterparts.Moreover, the rate coefficients for some radiative associationreactions and the binding energies for surface species werealso updated following Roberts et al. (2004) and Roberts& Millar (2007), respectively. Beside these updates, we in-cluded all the freeze-out reactions for hydrogen bearingspecies and their deuterium counterparts, assuming that theproducts will have the same branching ratios adopted fortheir hydrogen equivalents. The surface chemistry consid-ered here is simple in that it includes, besides the H and HDformation on grains, rapid hydrogenation of species, whereenergetically possible. Apart from direct hydrogenation, theonly other surface reactions we include are the formationof methanol from CO and of CH CN from the reaction ofmethane, CH , with HCN, as it has been shown that gasphase reactions are not sufficient to form these two species(e.g. Tielens & Hagen 1982; Watanabe et al. 2003; Garrodet al. 2008). As a first approximation and according to ex-perimental results of the thermal desorption non-deuteratedspecies from icy mantles (e.g. Collings et al. 2003a, 2004),we assumed that deuterated species will desorbe at the sametemperature recorded for their hydrogen counterparts ( Mc-Coustra - private communication ). Desorption temperaturesare listed in Table (2) in Viti et al. (2004) for hot cores anddetermined by Awad et al. (2010) for hot corinos.This work studies the influence of changing the finaldensity of the collapsing core, the depletion efficiency of thegaseous species onto grain surfaces, and the effect of varyingthe collapsing mode (free fall or retarded) on the chemicalevolution of deuterated species around low-mass (1 M (cid:12) ) andhigher mass (5 & 25 M (cid:12) ) protostars.In the first instance, we ran a total of 9 models, wherethe initial elemental abundances and parameters used for thegrid are listed in Tables (2) and (3). Our chemical networkconsists of 265 species (including 92 deuterated species and60 surface species) linked in 4204 reactions both in gas-phaseand on grain surfaces. We investigate the sensitivity of deuterium chemistry tochanges in the physical conditions in low- and high-massstar forming regions by comparing the reference models M1,M4 and M7 (for masses: 1, 5, and 25 M (cid:12) ; respectively -see Table (3)) to the following models: M2, M5, and M8to study the influence of changing the final density of thecollapsing cloud; and models: M3, M6, and M9 to explore This work was performed before the release of the UMIST 2012ratefile (McElroy et al. 2013). A retarded collapse means that the speed of the collapse is afactor of that of the free-fall collapsing speed which we assumeit is unity. In this model we change the speed of the collapse byvarying the collapse parameter defined in the modified collapseequation by Rawlings et al. (1992).c (cid:13) , 1– ?? Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli
Table 2.
Initial elemental abundances, with respect to the total number of hydrogen nuclei, and physical conditions assumed in ourmodel (taken from Viti et al. 2004; Awad et al. 2010). Note that for the described parameters, hereafter, the temperature is the gaskinetic temperature (in K) and the density is the gas number density (in cm − ).Initial elemental abundancesCarbon 1 . × − Oxygen 4 . × − Nitrogen 8 . × − Sulphur 1 . × − Helium 7 . × − Magnesium 5 . × − Physical parameters
The parameter Hot Corino Hot Core
Awad et al. (2010) Viti et al. (2004) † Core density (cm − ) 10 − − Core temperature (K) 100 300Core radius 150 AU 0.03 pcProtostellar Mass (M (cid:12) ) 1 5 & 25
For Both Models † Depletion percentage (%) 85 - 100 † The collapsing mode free fall - retarded † This parameter varies only during the collapsing phase (Phase I).
Table 3.
Summary of the grid of our models in this study as described in § Models Mass Temperature † Density † DepletionM (cid:12)
K cm − % M1 × M2 × M3 × M4 × M5 × M6 × M7
25 300 1.0 × M8
25 300 1.0 × M9
25 300 1.0 × (cid:63) Models with different collapse speeds
Models speed of collapse Notesff ret 0.5 ret 0.1 † These parameters vary only during the collapsing phase (Phase I) of the chemical model. (cid:63)
These models have similar physical conditions to that of model M1. ff: free-fall collapse model, ret: retarded collapse model the influence of varying the depletion percentage of gaseousspecies onto grain surfaces. Note that the mass in the sec-ond column is that of the newly formed star. The influenceof changing the final density of the collapsing core and thedepletion efficiency were studied by adopting some standardvalues - see Table (3) - and then decreasing their values by10 and 15%, respectively. The standard values are listed inTable (3): model M1 for a solar mass hot corino and modelsM4 and M7 for 5 and 25 solar masses hot cores. The choiceof these values is arbitrary providing that the model phys-ical conditions are within the observed ranges for low- and high- mass cores. Figs. (1 - 4) show the predicted fractionalabundances as a function of time for hot corinos (panel a)and hot cores (panels b and c).The effect that the collapse timescale may have on thechemistry has been previously discussed for low-mass stel-lar cores by Sakai et al. (2008). Hence here we focus on theinfluence of varying this collapse timescale for hot corinosonly: We compare models ‘ret 0.5’ and ‘ret 0.1’ to the stan-dard ‘ff’ model of a hot corino to understand the impact ofvarying the speed of the collapse on the chemical timescalesof the region under study. Fig. (5) illustrates the fractional c (cid:13) , 1– ?? euterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions abundances of species in a hot corino as a function of timefor free-fall (solid line) and retarded collapse (dashed anddotted line) models. Generally, a change in densities affects hot corinos more thanhot cores. This result is shown in panels (a), (b) and (c) inFigs. (1) and (2). These figures represent Models M1(M2),M4(M5), and M7(M8), when the final density is 10 (10 )cm − for hot corinos, and 10 (10 ) cm − for hot cores.As expected, species in hot corinos are more abundantin denser cores (solid line) than in less dense cores (dashedline); exceptions are CH OD, CH DOH, and HDO thatshow higher abundances for less dense regions, in particularduring early times (t (cid:54) yrs). Generally, the two deuter-ated methanol counterparts show the same evolutionary pro-file, but CH OD seems to survive longer than CH DOH.The chemical analysis of those species, in dense regions (10 cm − ) at early times, reveals that they are destroyed via re-actions with H + that are more efficient at higher densities.Therefore, their abundances remain high for longer timesin less dense regions. For both models, M1 and M2, HDOis formed via reactions involving H CO (see next section)which are more efficient at lower densities (Model M2). Theabundance of HDO is enhanced for times (cid:62) yrs, whichis consistent with the desorption times of H CO from grainsurfaces in the H O co-desorption event (Awad et al. 2010).Moreover, the number of destruction pathways of heavy wa-ter in denser cores is larger than that for less dense cores,allowing more HDO to remain in the gas-phase.All the species experience a rapid and steeper declinein their abundances in less dense (10 cm − ) hot corinos(Panel a) than in denser ones (10 cm − ). The chemicaltrends in the two models remain the same except for HDCO,which shows fluctuations in its abundance for low densityhot corinos, in particular after 10 yrs. The chemical analysisof HDCO at this particular time for a lower density hotcorino reveals that the species is involved in a larger numberof gas-phase reactions than in the case of a denser core.Molecules in massive cores (with higher temperature)are less affected by the decrease in core density. However,deuterated water shows a slightly higher abundance at times (cid:54) × yrs, and both forms of deuterated methanol areless abundant in lower density cores. Observations reveal that the enhancement of the deuter-ated species to their fully hydrogenated forms arises in re-gions where the CO molecules are heavily depleted onto dustgrains (e.g. Caselli et al. 2002; Bacmann et al. 2003; Millar2005; Crapsi et al. 2005; Chen et al. 2011). Therefore, westudied the influence of varying the depletion percentage onthe fractional abundances of the species in hot corinos andcores.In this section, we discuss the results of modelling hotcorinos and cores with fully (Models M1, M4, & M7) andpartially depleted gas (Models M3, M6, & M9), respectively.The chemical evolution of deuterated molecules in a fullydepleted gas (solid line) is plotted in comparison with the case of partially depleted gas (dashed line), both as a func-tion of time, in Figs (3) and (4). The term ‘fully depleted’refers to interstellar gas which is close to 100% freeze-out bythe end of the collapsing phase (Phase I), while ‘partiallydepleted’, in this paper, means an arbitrary freeze out per-centage of ∼ × yrs and ∼ × yrs, the fractional abundances of HDO, D CO, andthe two deuterated forms of CH OH show a slight enhance-ment with lower depletion percentage. The abundance ofHDS is the least affected by a lower depletion percentage.This is probably due to the fact that HS and its deuteratedcounterpart are not formed or enhanced on the icy man-tles. Chemical analysis of HDCS reveals that prior to 9 × yrs, the formation rate of HDCS is similar in both modelswhile after that time, and when mantle species sublimate,the formation rate of HDCS increases in models with par-tial depletion. This result may indicate that this molecule ismainly formed via gas-phase reactions and is not a mantlespecies. HDO, in hot corinos (Fig. 3, panel a), with higherdepletion percentage (solid line) shows higher abundancesat late times. At times of ∼ × yrs, the abundanceof HDO shows a sudden increase which cannot be explainedin terms of mantle sublimation. The chemical analysis atthe time around this ‘jump’ reveals that it is caused by thepresence of methane (CH ) in the gas phase. The time of the‘jump’, ∼ × yrs, is associated with that observed pre-viously for formaldehyde (Awad et al. 2010). In this model,HDO is produced via reactions involving H CO or HDCOwith H DO + . As a consequence, we argue that the increasein the HDO abundance can be explained as a result of theenhancement in the formation of either H CO or HDCO,as follows. CH evaporates from grain surfaces at ∼ × yrs, then it undergoes many gas-phase reactions some ofwhich form H DO + and CH . These two molecules are, inturn, used to form H CO and HDCO leading to an enhance-ment in their abundances. Therefore, the jump of HDO canbe attributed, indirectly, to the evaporation of methane fromgrains surfaces.On the other hand, D CO is enhanced in hot corinomodels with full depletion. This result is supported by Bac-mann et al. (2003), see Section (4). In hot cores, the deuter-ated forms of H CO and CH OH (Fig. 4) for both modelsare abundant for longer times than in hot corinos and hencethese species may be good tracers of hot cores. For ear-lier times (i.e. t < yrs), deuterated H CO and CH OH(mainly formed on grains) can be formed, in the gas phase,via reactions involving HCN (e.g. H DCO + + HCN → HCNH + + HDCO) and NH (e.g. CH OHD + + NH → NH +4 + CH OD).Unlike the case of hot corinos, D CO is observable inhot cores even when the depletion percentage is low. Thismolecule is efficiently formed via the reaction ‘HD CO + +H O’, and is destroyed by H O + and HCO + . The latterdominates the destruction routes at times later than 1.4 × yrs, but it is less efficient in hot core models withpartial freeze out, leading to a higher D CO abundance.HDCO is abundant during most of the hot core lifetime andtherefore could be used as a good tracer for hot cores. It isinteresting to note that HDCO shows a sudden increase in its c (cid:13) , 1– ?? Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli abundance, even at low depletion efficiencies, around 4 × yrs, which cannot be explained by its sublimation from grainmantles (as in the hot corinos case). This ‘jump’ in abun-dance is related to the formation of CH , as explained abovefor hot corinos, but in hot cores H CO is mainly formed viaoxidization reactions of CH , which is formed via the effi-cient destruction of CH .DCN is another species that seems to be affected by thedegree of depletion in Phase I; Parise et al. (2009) observedand modelled the excitation of HCN and DCN in Clump 1and 3 of the Orion Bar and estimated the DCN/HCN ratios,on average, to be 0.7 and 1.1 (using the rotational diagrammethod) or 0.3 and 0.8 (using the LVG analysis), respec-tively. Our model calculations are in the range determinedby Parise et al. (2009) and indicates that, in general, fully de-pleted cores have a lower deuterium fraction ( ∼ . − . ∼ . − . +DCNH + ’ which is inefficient in fully depleted cores. More-over, DCN is extensively destroyed by the ion ‘N H + ’ incores with fully depleted gas. This route is of equal weightwith other destruction pathways in cores with partially de-pleted gas. Our results for fully depleted models are consis-tent with those obtained for Clump 1 while partial depletionresults are in better agreement with the observed values inClump 3. Our modelling suggest that the percentage of theDCN/HCN molecular ratio can be used to trace the initiallevel of depletion (i.e. before the protostar switches on) in agiven region. In their study of the chemistry of the low-mass core IRAS04368+2557 in L1527, Sakai et al. (2008) concluded that thetimescale of the collapse of the pre-stellar core affects theabundances of the species in the region under study. Theshorter the timescale (i.e. the faster the collapse) the moreabundant the species are, in particular C-bearing species(C n H m ). This result motivated us to run two extra modelsfor a solar mass core in which we vary the speed of thecollapse in Phase I. The results are then compared to thoseof a standard solar mass hot corino (Model M1 in this workwhich undergoes a free-fall collapse), see the parameters ofModel M1 and other models in Table (3). Models with longercollapse timescale (e.g. Model ret 0.1) are expected to formmore species than models with shorter collapse timescale(e.g. Model M1), and therefore show an enhancement in thefractional abundances of their molecular content in PhaseII. Figure (5) shows the chemical evolution of the deuter-ated species during Phase II, for the free fall (solid line) andthe retarded collapse models in which the speed of the col-lapse was decreased to half (dashed line) and a tenth of itsfree fall value (dotted line). From this figure, we note thatwhen species are evaporated from the grain surfaces (after ∼ × yrs): (1) the least affected species by changingthe mode of collapse are HDS and HDCO, and (2) the frac-tional abundance of HDCS and D CO decrease while thatof the rest of the species increases as the speed of the col- lapse decreases. The most affected species, and hence possi-ble good indicators of the collapsing mode, are HDCS, NH ,CH DOH, and HDO. We conclude that HDO/H O is af-fected by the collapse history of the pre-stellar core.Our results are in agreement with the findings of (Sakaiet al. 2008) in which the affected species in models withlonger collapse timescale (Model M1 - solid line) possessesthe lowest abundance of all models. We hence conclude thatthe speed of the collapse influences the fractional abun-dances of hot corinos.
Viti et al. (2004) studied the chemical evolution of hotcores around stars with various masses from 5 to 60 solarmasses. They found that ratios of sulphur bearing species(e.g. H S/SO , H S/CS, SO/CS) are good indicators of theearly stages of massive star formation while large organicmolecules such as CH OH, HCOOCH , and C H OH indi-cate late evolutionary stages. In this work, we aim to inves-tigate whether the deuterated counterparts of these evolu-tionary indicators can also be used for the same purpose.We, therefore, ran a grid of four additional models to coverthe range of stellar masses between 10 and 60 solar masses(as in Viti et al. 2004). Fig. (6) illustrates the chemical evo-lution of sulphur bearing species and their deuterium coun-terparts. Inspection of this figure shows that, as their non-deuterated counterparts, the ratios of deuterated sulphurbearing species may be good chemical evolutionary tracersof hot cores in all cases explored.
We briefly discuss our results by qualitatively comparingthem to one representative hot corino and one hot core in § § Our physical conditions for models of hot corinos (modelM1) are comparable to those observed for the IRAS 16293-2422 source, while those of hot cores (model M7) are closeto observations of Orion KL hot core. Hence we briefly com-pare our results with the observed abundances of these twosources.
IRAS 16293-2422 is a nearby Class 0 protostar at dis-tance ∼
120 pc and predicted age of 10 years (Andr´e et al.1993). It has an inner small (150 AU) condensed ( ∼ × cm − ) region known as ‘hot corino’ (Ceccarelli et al. 1998).A comparison between Model M1 fractional abundances andthe calculated molecular D/H ratios of various species withthose derived from observations of IRAS 16293-2422 sourceand the times of best fit of our calculations with observa-tions are summarized in Table (4). All of our fractional abun-dances are given relative to the total number of H nucleonsin the region. Note that for most observations quoted in Ta-ble (4) we cannot distinguish the emission from the inner hotcorino region and that from the external outer region of the c (cid:13) , 1– ?? euterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions Table 4.
Comparison between observations of deuterated species and molecular D/H ratios in the IRAS 16293-2422 hot corino sourceand our model M1 calculations of Phase II.
Species Observations This Work Time of best fit RefM1 yrsDCO + ∼ (cid:54) DCN
DNC (cid:54)
HDS
HDO (cid:54) NH D HDCO (cid:54) (cid:62) (cid:62) † D CO ∼ † CH OD (cid:54) † CH DOH (cid:54) C D † DCOOCH (cid:54) RatioDCO + /HCO + ∼ DCN/HCN
DNC/HNC (cid:54) C D/C H HDCO/H CO ∼ D CO/H CO (cid:54) (cid:54) (cid:62) D CO/HDCO (cid:54) (cid:54) NH D/NH ∼ (cid:54) HDO/H O CH OD/CH OH (cid:54) (cid:62) CH DOH/CH OH (cid:62) HDS/H S ± ‡ N D + /N H + (cid:28) † This value is converted into fractional abundance assuming N(H ) = 2 × cm − as given by van Dishoeck et al. (1995).a(b) means a × b ‡ Ratio observed in NGC 1333 IRAS4 by Roberts & Millar (2007) source; an exception is the deuterated water observations byCoutens et al. (2013).Most of the species in our sample match observations attimes between 10 and 10 years, which is the assumed agefor a typical Class 0 source (Andr´e et al. 1993). Exceptionsare the ions that fit observations at times earlier than 10 years and show a rapid decrease after that time, and HDOthat does not match observations at any time and is alwaysunderestimated by our model.Deuterated ammonia matches observations at timeslater than 9 × years. Currently, our model is underesti-mating the abundances of some deuterated species by one ortwo orders of magnitude, such as CH OD, CH DOH, DNC,and DCOOCH . The main reason for this is perhaps the ex-clusion of multiply deuterated H +3 isotopologues that play arole in enhancing the deuterium fractionation via gas-phasechemistry (Roberts et al. 2003).The N D + /N H + ratio matches observations only dur-ing early times ∼ × years. In fact, Roberts & Millar(2007) surveyed the N D + /N H + ratio around a sample ofClass 0 sources and concluded that the high ratio observedrepresents the cooler gas in the extended envelope aroundthe source and not the hot gas in the core. Similar resultswere reported by Emprechtinger et al. (2009) in their surveyfor N D + /N H + in a sample of Class 0 sources. Yet our cal- culated HDO/H O ratio does not match observations. Thismust be because our model underestimates the fractiona-tion by at least a factor of two which could be a result ofour model limitations, described earlier in § The Orion Kleinmann-Low nebula (Orion KL) is the nearest massive star-forming region at ∼
410 pc. Thecore has physical conditions (n(H ) = 10 cm − , T ∼ ∼
15 M (cid:12) ; see Neill et al. 2013a; Kaufman et al.1998 and references therein) close to those of our modelM7. Most recently, Neill et al. (2013a,b) surveyed deuteratedmolecules in the Orion KL region using the Herschel/HIFIfacilities. Table (5) summarizes a comparison between frac-tional abundances and molecular ratios observed in theOrion hot core region and our model, M7, calculations. Thetable also lists times of best fit. Apart from CH DOH ourmodel seems to be consistent with the observations reportedby Neill et al. (2013a,b).
In this section, we also briefly compare our model resultswith other astrochemical modelling efforts. Albertsson et al.(2013) performed a detailed chemical study of deuteratedmolecules using gas-grain chemical models. They estimate c (cid:13) , 1– ?? Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli
Table 5.
The calculated deuterium fractional abundances and molecular D/H ratios in hot core Model M7 (Phase II) in comparisonwith observations of the Orion KL hot core.
Species Observations This Work Time of best fit RefM7 yrsHDO ∼ (cid:62) (cid:62) NH D (cid:62) HDCO ∼ (cid:62) CH OD (cid:54) CH DOH (cid:54)
RatioHDO/H O ∼ +3 . − . (-3) 1.3-6(-3) 1.6-8.9(5) Neill et al. (2013a) NH D/NH ∼ ± HDCO/H CO (cid:54) ∼ CH OD/CH OH (cid:54) (cid:54) (cid:62) CH DOH/CH OH (cid:54) (cid:54) (cid:62) × b the abundances of HDO in hot cores and corinos to be ∼ − . This value best fits our models for times around 1-5 × yrs for hot corinos (e.g. Fig. 1-a) and time ranges of10 . -10 yrs, and 10 . -10 yrs for hot cores of 5 and 25M (cid:12) ,respectively.Aikawa et al. (2012) used a gas-grain chemical model ofan in-falling parcel of fluid to study the evolution of deuter-ated species and the deuterium fragmentation from pre- toproto-stellar cores. They found that as the gas depletion in-creases, the deuterium fraction enhances. Our calculationsfor both hot corinos, Model M3, and hot cores, Models M6and M9, showed that for most of the studied species in bothenvironments, the fractional abundances are enhanced as thegas depletion percentage increases; see Figs. (3 and 4). Theseresults are supported by the model calculations of Aikawaet al. (2012). We modelled the deuterium chemistry in hot corinos andcores using an extended, updated and improved version ofthe chemical model used by Viti et al. (2004) for hot coresand modified for hot corinos as described in Awad et al.(2010). Unlike previous studies, here we focus on the proto-stellar phase where the evaporation of mantle species occursand influences the chemistry of the core. The novelty of ourapproach is the treatment of the evaporation of deuteratedspecies adopting the experimental results of Collings et al.(2003a, 2004). We studied the influence of varying the phys-ical conditions of star forming regions, namely the densityand depletion efficiency, on the chemical evolution of thecore. In addition, we explored the effect of changing the col-lapse timescale of low-mass stellar cores, as well as the finalmass of the protostar.For both hot core and corino environments, we foundthat lowering the depletion percentage decreases the abun-dance of most of the studied deuterated species, with theexception of HDCS in both cores and D CO in hot cores.Deuterated species in hot cores are more sensitive to thechanges in the cores density than in hot corinos. Gener- ally, we find that decreasing the density of the gas reducesthe abundances of the deuterated species, in particular largespecies, in hot cores more than in hot corinos.In addition, in hot corinos, our models showed thatthe collapse time affects the abundance of HDCS, NH D,CH DOH, and HDO, so that this should also be taken intoaccount when attempting to model the deuteration of water,besides taking into account variations in physical parame-ters and dust temperature as already explored by Taquetet al. (2012) and Cazaux et al. (2011).Our model failed to reproduce the observed abundancesof large organic species such as HCOOCH , and its deuter-ated counterpart. This is most likely due to an incompletesurface chemistry as well as the lack of multiply deuteratedspecies in our chemical network. ACKNOWLEDGMENTS
Z.Awad is grateful to Professor C. Ceccarelli for her help-ful suggestions to improve the results and discussion part.E. Bayet thanks STFC astrophysics at Oxford 2010-2015(ref: ST/H002456/1) and John Fill OUP research fund”Molecules in galaxies: securing Oxford’s position in theALMA era” (ref: 0921267). The research leading to theseresults has received funding from the (European Commu-nitys) Seventh Framework Program [FP7/20072013] undergrant agreement no. 238258. P. Caselli acknowledges the fi-nancial support of the European Research Council (ERC;project PALs 320620). S. Viti and P. Caselli acknowledgesupport the UK Science and Technology Funding Council.
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The chemical evolution (from top to bottom) of HDCS, HDS, HDO, and NH D in hot corinos (panel a: 1 M (cid:12) ) and hot cores(panel b: 5 M (cid:12) , and panel c: 25 M (cid:12) ) as a function of time, for Phase II calculations. The different curves compare the evolution of thespecies at two different final densities for the collapsing cloud (see key in bottom plots, and Table 3).c (cid:13) , 1– ?? Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli
Figure 2.
Similar to Fig. 1 but for deuterated formaldehyde and methanol (see key).c (cid:13) , 1–, 1–
Similar to Fig. 1 but for deuterated formaldehyde and methanol (see key).c (cid:13) , 1–, 1– ?? euterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions Figure 3.
Chemical evolution of HDCS, HDS, HDO, and NH D as a function of time, during Phase II, at different depletion on grainsurfaces (see key text in bottom right plot, and Table 3) for different cores: (a) 1 M (cid:12) , (b) 5 M (cid:12) , and (c) 25 M (cid:12) .c (cid:13) , 1– ?? Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli
Figure 4.
Similar to Fig. 3 but for deuterated organic species (see key).c (cid:13) , 1–, 1–
Similar to Fig. 3 but for deuterated organic species (see key).c (cid:13) , 1–, 1– ?? euterium chemistry of dense gas in the vicinity of low-mass and massive star forming regions Figure 5.
Chemical evolution of a selected set of deuterated species during the the warming-up phase (Phase II) as a function of timeusing different collapsing modes: the free fall (ff: solid line), retarded; 0.5ff speed (ret 0.5: dashed line) and 0.1ff speed (ret 0.1: dottedline), see key and Table 3.c (cid:13) , 1– ?? Zainab Awad, Serena Viti, Estelle Bayet, and Paola Caselli
Figure 6.
The chemical evolution of S-bearing species and their deuterated counterparts as a function of time in warm and hot coresfor various masses as indicated on the plots (upper left corners). The fractional abundances of the studied species are represented bydifferent line styles. c (cid:13) , 1–, 1–