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Astronomy & Astrophysics Deuterium Fractionation Along the TMC-1 Ridge

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Astronomy & Astrophysics Deuterium Fractionation Along the TMC-1 Ridge A&A 376, 1054–1063 (2001) Astronomy DOI: 10.1051/0004-6361:20011022 & c ESO 2001 Astrophysics Deuterium fractionation along the TMC-1 ridge A. J. Markwick1, S. B. Charnley2, and T. J. Millar1 1 Department of Physics, UMIST, PO Box 88, Manchester M60 1QD, UK e-mail: [email protected]; [email protected] 2 Space Science Division, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA e-mail: [email protected] Received 6 February 2001 / Accepted 12 July 2001 Abstract. In this paper, we present a model to predict and explain the observed gradients in deuterium fractiona- tion and molecular abundance along the TMC-1 ridge. In TMC-1, gradients in the level of deuterium fractionation + are observed for all the molecular species for which the measurement has been made; HCO ,HC3N, C3H2 and HNC. The model is based on the assumption that the chemical evolution of the TMC-1 ridge has been affected by Alfv´en waves. The structure of the model and its comparison with observations are discussed. We find that the model qualitatively and somewhat quantitatively reproduces the observations of the aforementioned molec- ular species. We present predictions of the fractionation gradients along the TMC-1 ridge for other molecules of observational interest. Key words. MHD: waves – ISM: abundances – ISM: molecules – ISM: individual: Taurus Molecular Cloud – molecular processes 600 1. Introduction 400 The cold, dark, interstellar cloud TMC-1 is most no- table as a rich source of emission from complex organic molecules, particularly of carbon-chain species such as the 0.32 PC polyacetylenes and cyanopolyynes, and for the strong spa- 200 tial abundance gradients exhibited by them and other, simpler, molecules like SO, NH3 and CCS (e.g. Olano et al. 1988; Pratap et al. 1997; see Fig. 1). For almost 20 years it has been the testbed for many theories of interstellar 0 chemistry and yet no concensus has been reached as to IRAS 04381+2540 CP PEAK the correct explanation of the apparently distinctive or- NH PEAK DECLINATION OFFSET (ARC SEC.) 3 ganic chemistry and its associated spatial differentiation. SO PEAK TMC-1 displays large enhancements in the deuterium en- -200 richment of many molecules (Table 1), and also spatial gradients in the associated deuterium fractionation ratios 200 0 -200 -400 along the ridge (Gu´elin et al. 1982; Bell et al. 1988; Howe RIGHT ASCENSION OFFSET (ARC SEC.) et al. 1994; Hirota & Yamamoto 1998; Butner & Charnley 2001). These ratios, R(XD), are defined as quotients of Fig. 1. Schematic diagram of the TMC-1 ridge, adapted from number densities i.e. n(XD)/n(XH). Apart from the well- Fig. 6 of Olano et al. (1988). The solid contours trace HC7N emission and the dotted contours show NH . The distance known cyanopolyyne peak (CP) and ammonia peak (AP), 3 + shown is calculated assuming that the CP and AP are at the there is the DCO peak (DP). The DP is almost spatially same distance from Earth. coincident with the AP and R(DCO+) is seen to increase as one moves along the ridge from CP to AP (Gu´elin et al. 1982). Gas phase deuteration is driven by the molecular + + + Send offprint requests to: A. J. Markwick, ions H2D ,CH2D and C2HD , which, because of the e-mail: [email protected] zero-point energy difference between H2 and HD, can A. J. Markwick et al.: Deuterium fractionation along the TMC-1 ridge 1055 Table 1. Deuteration at the CP position (04 38 38, +25 35 45). IRAS 04381+2580. Markwick et al. showed that many of the molecular gradients observed by Pratap et al., includ- Species R(XD) f(XD) f(XH) Refs. ing those of the large carbon-chain compounds, could be C3HD 0.08 1.2(-9) 1.5(-8) (1) explained if simple hydrocarbons, such as methane and ≤ ≤ − − CH3CCD 0.013 2( 10) 1.7( 8) (2) acetylene, were rapidly injected from dust grain mantles. − − CH2DCCH 0.054 9.2( 10) 1.7( 8) (2) Grain heating due to the relative drift of charged and neu- − − CCD <0.03 <1.2( 11) 4( 9) (3) tral grains (at speed v ), induced by MHD waves, was as- − − D C4D 0.004 2.3( 10) 5.6( 8) (4) sumed to lead either to thermal evaporation of the most HDCS 0.02 1.2(−10) 6.1(−9) (5) volatile species, or to complete mantle removal in an ex- DC3N 0.03 6.6(−11) 2.2(−9) (6) plosive event. DC5N 0.016 - - (7) DCO+ 0.013 1.2(−10) 8.4(−9) (8) In this paper we combine the work described in Paper I with that in Charnley (1998) to model the deuterium References: (1) Bell et al. (1988); (2) Gerin et al. (1992); chemistry of TMC-1 in which gas phase deuteration + (3) Vrtilek et al. (1985); (4) Turner (1989); (5) Minowa et al. through H2D is largely suppresed in the vicinity of the (1997); (6) Langer et al. (1979); (7) Schloerb et al. (1981); CP, and simultaneously, enhanced abundances of heav- (8) Butner & Charnley (2001). ily deuterated neutral molecules have been released from grain mantles into the gas. The deuterium fractionation persist at the temperatures (∼10 K) of dark clouds. The in these mantles could have been set either by surface re- R(XD) produced by ion-molecule reactions are sensitive actions or by a previous epoch of gas phase chemistry in to the gas temperature T , and the models of Millar et al. which the products condensed out on to the dust (Tielens (1989) demonstrate that, as T increases, the deutera- 1983; Brown & Millar 1989). Since little is known about tion due to each of these ions becomes less efficient. the deuterium content of grain ice mantles, we assume Temperatures of about 20 K are sufficient to to drive the throughout this work that the ratios in the ice are the reverse reaction of the primary deuteration process same as the ratios in the gas. We are particularly inter- ested in the injection of deuterated C2H2,C2H4 and CH4 + −→ + H3 +HD H2D +H2 (1) since these molecules are mainly responsible for the gradi- ents seen in more complex organics and ions derived from and render it ineffective. However, even at 70 K, the reac- + + them, CH2D and C2HD , can enhance specific gas phase tion deuteration pathways. In this scenario the D/H ratios ob- + + served in DC3N, DC5N, CCD, CH3CCD and CH2DCCH C2H +HD−→ C2HD +H2 (2) 2 should reflect the competition between their production + produces enough C2HD to maintain some enhanced, from the surface reservoir of D in C2HD and CH3Dand though residual, deuterium fractionation (Millar et al. deuteron-hydrogen exchange in gas-phase reactions. 1989). Observations place the current gas temperature For example, deuterated acetylene should form deuter- along the TMC-1 ridge to be in the range 10–15 K or less. ated cyanoacetylene through This is insufficient to account for the spatial D/H gradi- −→ ents as being due to corresponding temperature gradients. CN + C2HD DC3N+H (3) Reaction (1) could be driven nonthermally by ambipolar and we wish to see if the observed gradient in R(DC N) diffusion caused by MHD wave motions cause ionized ma- 3 (Howe et al. 1994) can be produced in spite of D atom terial to move relative to the neutral component (Charnley exchanges. Furthermore, as it has two detectable deuter- & Roberge 1991). Based on a crude treatment of the CO ated isotopomers, CH CCH is a potentially important cooling, and associated uncertainties in the precise kinet- 3 molecule. Methyl acetylene is understood to take up D ics of reaction (1), Charnley (1998) showed that low wave in structurally different sites through gas phase reactions amplitudes, and hence sufficiently low ion-neutral drift whose rates will be enhanced by the injection of deuter- speeds, vni, such that there is negligible heating of the gas, + ated mantles, e.g. H2D production could be suppressed to significant lev- els. Hence, the declining D/H ratios running NW−→ SE + + CH2D +C2H4 −→ CH2DCCH +H2 (4) down the TMC-1 ridge (i.e. from the DP/AP region to 2 CH + +CH D −→ CH CCDH+ +H (5) the CP) could simply be due to a corresponding increase 3 2 3 3 2 in vni. Of the D/H gradients that have been determined, + + + −→ + namely those in DCO ,C3HD, DC3N, N2D (Butner & C2H2 +CH3D CH2DCCH2 +H (6) Charnley 2001) and DNC (Hirota & Yamamoto 1998), all + + C2HD +CH4 −→ CH3CCDH +H (7) are consistent with this interpretation. Markwick et al. (2000; hereafter Paper I) have modeled where we have listed only those channels incorporating a the spatial abundance gradients of TMC-1 as being due to D atom into protonated methyl acetylene. The ability to the propagation of MHD waves along the ridge. The source reproduce the CH3CCD/CH2DCCH ratio in spite of gas of these waves could either be relative clump motions phase D atom exchange reactions offers a detailed test of or, as favoured in Paper I, the embedded IRAS source the theory proposed in Paper I. 1056 A. J. Markwick et al.: Deuterium fractionation along the TMC-1 ridge The aims of this paper are to model the deuterium chemistry under the constraints imposed by the observed fractionation ratios, R(XD), as well as their spatial gradi- ents, and to predict associated values for potentially de- + tectable species like C3D, DC7NandDCS . In Sect. 2, we briefly review the relevant observations of deuterated molecules in TMC-1. The modification of our model of the chemistry along the TMC-1 ridge to include the effects of non-zero vni on the gas phase chemistry is described in Sect.
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