Deuterium-bearing molecules in cold and warm dense interstellar clouds

Magdy Y. Amin; Ayman S. Kordi

doi:10.1016/j.jksus.2009.07.001

View/Download PDF

Buy Reprints

PDF

Translate this page into:

Original Article

21 (

); 85-92

doi:

10.1016/j.jksus.2009.07.001

Deuterium-bearing molecules in cold and warm dense interstellar clouds

Magdy Y. Amin^*, Ayman S. Kordi

Physics and Astronomy Department, College of Science, King Saud University, Riyadh, Saudi Arabia

*Corresponding author

Received: 2009-2-24, Accepted: 2009-7-4, Published: 2009-07-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

We have studied the production of key observed deuterium species for TMC-1, Orion, L134N and W3IRS4 clouds, of densities 10⁴, 10⁴, 5 × 10⁴, and 10⁶ cm⁻³, respectively, by using the pseudo-time dependent gas-phase chemistry. These clouds have temperatures in the range 10–70 K. The main results by using the more extensive chemical network with the most updating reaction rates show that the most of calculated fractional abundances are in agreement with observations, and suggest that triply-deuterated ammonia could be detectable in dark clouds. Also our models show that large abundance of NH₂D and NHD₂ can be produced in the interiors of cold dense clouds at steady state time.

Keywords

Astrochemistry

1 Introduction

Because deuterium-bearing molecules used to: probes of the physics of interstellar clouds, study the relation of connection between interstellar and cometary ices and understand the formation mechanism of isotopic composition of interstellar molecules (Roberts and Millar, 2000a; Robets et al., 2002; Shah and Wotten, 2001; van der Tak et al., 2002 ). Therefore many theoretical and observational studies which have concerned on the formation of deuterated molecules in interstellar clouds.

In the last few years the studying of doubly-deuterated molecules in the interstellar medium has gained considerable attention. This is due to a large amount of doubly-deuterated formaldehyde, D₂CO, and ammonia, NHD₂, have been observed towards low mass protostar IRAS16293-2422 (Ceccarelli, 1998), Orion cloud (Turner, 1990), very young protostellar core 16293E (Loinard et al., 2001) and in the molecular cloud L1689N (Ceccarelli et al., 2002; Loinard et al., 2001). Also NHD₂ observed in the molecular cloud L134N (Roueff et al., 2000). While triply-deuterated ammonia, ND_3, observed in the low mass protostar NGC1333-IRAS4 (van der Tak et al., 2002), in the dark cloud B1 (Lis et al., 2002). But the methanol observed in the low mass protostar IRAS16293-2422 (Parise et al., 2002). All these observations suggest that deuteration of formaldehyde and methanol is produced during the cold and dense cloud (Ceccarelli et al., 2001).

There are two chemical networks for the formation of the deuterated molecules:

First, some deuterium-bearing molecules can be formed by the gas-phase reactions (Millar et al., 1989; Rodgers and Millar, 1996; Roberts and Millar, 2000a; Robets et al., 2002 ). At low temperatures 10–70 K, the H₂D⁺ species is the key of gas-phase reaction to form DCO⁺ and N₂D⁺ (van der Tak et al., 2002). By gas-phase network, Lis et al. (2002) explained the observed abundance ratio of ND₃ in B1 cloud. Similarly, Tine et al. (2000) investigated the fractional abundance of NH₂D, which observed in L183 and TMC-1 clouds.

Second, the surface chemistry, in which deuterated molecules can be formed on dust grains. The high abundances of HDCO, D₂CO and CH₃OD in warm clouds are derived from the occurrence of grain surface chemistry during an earlier cold era followed by evaporation into the gas as temperature rise (Ceccarelli et al., 2001; Caselli et al., 2002; van der Tak et al., 2002; Parise et al., 2002 ). The formation of singly- and doubly-deuterated isotopomers of formaldehyde and singly-, doubly- and multiply-deuterated isotopomers of methanol on dust grain has been studied by Caselli et al. (2002), with a semi-empirical modified rate approach and Monte Carlo method in temperature range 10–20 K.

In the present work, we shall confine ourselves to calculate the chemical abundances of key observed deuterium species, in several different interstellar clouds and comparison with both available observation and other theoretical models which are given for justification. This paper is organized as follows, in Section 2, chemical models are given. Section 3 contains a brief description of our gas phase chemical models. The conclusions are given in Section 4.

2 Chemical models

In order to interpret the behavior of deuterium-bearing molecules on cold and warm interstellar clouds, we have carried a number of a pseudo-time dependent chemical models, which calculate the varying abundances of 408 species (130 of them containing deuterium) linked by 5320 reactions.

Our gas-phase model considers a standard gas-phase chemistry, in which we consider only reactions between gaseous species, with the exception that H₂ and HD forming on the grain surface, and we neglect the three body reactions.

Roberts and Millar (2000a,b) developed new models for the chemistry of deuterium, to investigate the fractionation of doubly-deuterated species, in interstellar molecular clouds. These models depend on a wide range of physical parameters including, density, temperature, elemental abundances and the freeze out of molecules on the dust grains. Our model is partially based on that part of gas-phase reactions of Roberts and Millar (2000a,b) for producing doubly-deuterated species. Our model extended the models of Roberts and Millar (2000a,b) to include multiply-deuterated ammonia.

The mono-deuterated reaction set is complete in the sense that for every reaction containing a hydrogen-bearing molecule there is an analogue reaction containing the equivalent mono-deuterated species. If more than one reaction product contains hydrogen atoms then uncertainly arises as to which will be the deuterium-bearing product. Due to lack of comprehensive experimental data, the usual approach is to assume statistical branching ratio between the various possibilities (Millar et al., 1989; Rodgers and Millar, 1996).

In this study we were interested in the chemistry of doubly-deuterated ammonia, NHD₂. We also interested with multiply-deuterated ammonia, which begin after the formation of NH₃, with deuterated ions XD⁺. In this model the metals (Fe, Mg, Na and Si) play an important role in determining the ionization fraction.

Since the chemical structure of interstellar clouds depends on the temperature, the radiation field and density number, then we have adopted four models of initial elemental (see Table 1), to study the deuterium chemistry in TMC-1, Orion, L134N and W3 IRS4 clouds. We have neglected the chemistry of species of polycyclic aromatic hydrogen type and their reaction with smaller molecules. As Herbst and Leung (1986) we also neglected the effects of enhanced rate coefficient in ion-polar neutral reactions. The chemical scheme used here is based on that of the most recent UMIST rate file, RATE99 (Le Teuff et al., 2000), with updating the rate coefficients of some modifications by Chastaing et al. (2001). We have adopted the cosmic D/H ratio measured by Linsky et al. (1995). We used a constant ratio for C/O. We neglect the effect of X-ray ionization. The electron abundance is set equal to the sum of the ion abundances.

Table 1 Our models, where n is number density.

Model	n (m⁻³⁾	T (K)	Average (mag.)	Clouds
1		10	10	TMC-1
2	1 (4)	70	10	Orion
3	1 (4)	10	15	L134N
4	5 (4)	55	30	W3 IRS4

3 Results and discussion

We have followed a large number of runs for the four models listed in Table 1. Using a different initial elemental abundances and cosmic ray ionization rate, the best initial elemental abundances are given in Table 2.

Table 2 Initial fractional abundances.

Species	Model 1	Model 2	Model 3	Model 4
H₂	0.5	0.5	0.5	0.5
C⁺	7.3 (−5)	7.3 (−5)	7.3 (−5)	4.0 (−6)
O	1.7 (−4)	1.7 (−4)	1.7 (−4)	8.0 (−6)
N	2.14 (−5)	2.14 (−5)	2.14 (−5)	5.1 (−6)
S	1.0 (−7)	1.0 (−7)	1.0 (−7)	6.0 (−8)
Si	2.0 (−8)	2.0 (−8)	2.0 (−8)	1.5 (−10)
Fe⁺	1.0 (−8)	1.0 (−8)	1.0 (−8)	1.5 (−10)
Mg⁺	1.0 (−8)	1.0 (−8)	1.0 (−8)	1.5 (−10)
Na⁺	1.0 (−8)	1.0 (−8)	1.0 (−8)	1.5 (−10)
H³⁺	1.0 (−11)	1.0 (−11)	1.0 (−11)	0.0
HD	1.6 (−5)	1.6 (−5)	3.2 (−5)	5.0 (−6)
He	0.14	0.14	0.14	0.14

The cosmic ray ionization rate of 1.3 × 10⁻¹⁷ s⁻¹ is used for models (1)–(3) and high cosmic ray ionization rate of 1.3 × 10⁻¹⁶ s⁻¹ is used for model (4). The reduction of the initial elemental abundances and a high cosmic ray ionization rate used in model (4) is in agreement with Amin (1999).

Tables 3–6 compare the molecular D/H ratios observed toward the above four clouds, with the results from our gas-phase models at both early (10⁵ year) time and steady state (100 million year) time, and other theoretical calculations.

Table 3 A comparison of abundance ratio measured in TMC-1 cloud with predictions from our model (1) and Roberts and Millar (2000a).

Species	Observation	Our cal. M1		RM 2000		Ref.
Species	Observation	Early	Steady	Early	Steady	Ref.
DCO⁺/HCO⁺	0.02	0.018	0.082	0.019	0.087	Tine et al. (2000)
NH₂D/NH₃	0.009–0.014	0.086	0.029	0.084	0.028	Tine et al. (2000)
HDCO/H₂CO	0.0059–0.11	0.043	0.056	0.042	0.055	Turner (2001)
DCN/HCN	0.023	0.006	0.022	0.009	0.025	Turner (2001)
DNC/HNC	0.015	0.015	0.015	0.015	0.015	Wootten (1987)
C₂D/C₂H	0.01	0.012	0.028	0.011	0.027	Millar et al. (1989)
C₄D/C₄H	0.004	0.004	0.027	0.004	0.029	Turner (1989)
N₂D⁺/N₂H⁺	0.08	0.03	0.058	0.025	0.025	Tine et al. (2000)
C₃HD/C₃H₂	0.08–0.16	0.007	0.027	0.006	0.02	Bell et al. (1988)
C₃H₃D/C₃H₄	0.054–0.065	0.082	0.098	0.083	0.099	Gerin et al. (1992)
DC₃N/HC₃N	0.03–0.1	0.008	0.026	0.007	0.026	Howe et al. (1994)
DC₅N/HC₅N	0.013	0.023	0.026	0.023	0.026	MacLeod et al. (1981)
HDCS/H₂CS	0.02	0.04	0.05	0.04	0.046	Minowa et al. (1997)

Table 4 A comparison of abundance ratio measured in Orion cloud with predictions from our model (2) and Millar et al. (1989).

Species	Observation	Our cal. M2	M1998	Ref.
DCO⁺/HCO⁺	0.002	0.002	8 (−4)–8 (−5)	Penzias (1979)
NH₂D/NH₃	0.003	0.0029	4 (−4)	Walmsley et al. (1987)
HDCO/H₂CO	0.02	0.019	0.004–0.005	Loren and Wootten (1985)
DCN/HCN	0.006	0.0058	0.001–4 (−4)	Wootten (1987)
DNC/HNC	0.01	0.043	9 (−4)–2 (−4)	Wootten (1987)
C₂D/C₂H	0.045	0.032	0.003	Combes et al. (1985), Vrtilek et al. (1985)
CH₃OD/CH₃OH	0.01–0.06	0.005	0.003–0.004	Mauersberger et al. (1988)
HDO/H₂O	>0.002	0.001	0.001–2 (−4)	Henkel et al. (1987)

Note: a (−b) stands for a × 10^−b.

Table 5 A comparison of abundance ratio measured in L134N (Tine et al., 2000; Roueff et al., 2000) with predictions from our model (3) and Millar (2002).

Species	Observation	Our cal. M2		M2002
Species	Observation	Early	Steady	Early	Steady
DCO⁺/HCO⁺	0.18	0.026	0.17	0.016	0.047
NH₂D/NH₃	0.1	0.006	0.1	0.0075	0.02
NHD₂/NH₃	5 (−3)	2.3 (−5)	5.1 (−3)	1.3 (−4)	8.4 (−5)
N₂D⁺/N₂H⁺	0.35	0.03	0.038	0.023	0.039

Table 6 A comparison of abundance ratio measured in W3IRS4 cloud by Helmich and van Dishock (1997) with predictions from our model (4).

Species	Observation	Our cal. M2
HDS/H₂S	<9.1 (−2)	1.0 (−3)
HDCO/H₂CO	<3.8 (−2)	0.02
DCN/HCN	<4.3 (−3)	0.001
DNC/HNC	7.1 (−3)	9 (−4)
C₂D/C₂H	0.045	0.032
CH₃OD/CH₃OH	<7.1 (−2)	0.006

Note: a (−b) stands for a × 10^−b.

The most important primary reactions to extract deuterium from HD involve ion-neutral isotope exchange reactions:

(1)

H_{3}^{+} + HD \to {HD}^{+} + H_{2},

(2)

{CH}_{3}^{+} + HD \to {CH}_{2} D^{+} + H_{2},

(3)

C_{2} H_{2}^{+} + HD \to C_{2} {HD}^{+} + H_{2},

where the reaction exoergicities although small – ΔE₁/k = 220 K, ΔE₂/k = 375 K, ΔE₃/k = 550 K – are much larger than the temperatures of cold interstellar clouds. At low temperatures, the reverse reactions do not occur efficiently despite the large abundance of H₂. Once formed these deuterated ions can pass on their enhanced deuterium content to other species in chemical reactions (Millar, 2002). In addition to the forward and reverse reaction in (1), H₂D⁺ can be destroyed by metals, by dissociative recombination with electron,

(4a)

H_{2 D}^{+} + e \to H + H + D,

(4b)

H_{2} D^{+} + e \to H_{2} + D,

(4c)

H_{2} D^{+} + e \to H + HD

with total rate coefficient equal to 6 × 10⁻⁸(T/300)^−0.5 cm⁻³ s⁻¹ (Larsson et al., 1996) and by reaction with neutral molecules (CO, N₂, H₂D).

The reactions for CH₂D⁺ and C₂HD⁺ are similar to that of H₂D⁺, except that each of these ions undergo a radiative association reaction with H₂ (Millar et al., 1989; Herbst et al., 1987)

(5)

{CH}_{2} D^{+} + H_{2} \to {CH}_{4} D^{+} + photon,

(6)

C_{2} {HD}^{+} + H_{2} \to C_{2} H_{3} D^{+} + photon.

Our calculated ratios for $H_{2} D^{+} / H_{3}, {CH}_{2} D^{+} / {CH}_{3}^{+}$ and $C_{2} {HD}^{+} / C_{2} H_{2}^{+}$ are all enhanced at low temperature, but H₂D⁺ is responsible for D/H ratio other than the two molecules. At high temperature H₂D⁺ is rapidly destroyed by H₂, so CH₂D⁺ and C₂HD⁺ are responsible for D/H ratio.

Our calculated ratio of C₂D/C₂H is in agreement with observations of TMC-1 at an early (10⁵ year) time and higher at the steady state (10⁸ year) time. This result is the same as that obtained by Roberts and Millar (2000a). The C₂D species is formed by:

(7)

C_{2} {HD}^{+} + e \to C_{2} D + H,

(8)

C_{2} H + D \to C_{2} D + H + 580 K

and is destroyed by

(9)

C_{2} D + H \to C_{2} H + D.

In model (2) with temperature of 70 K, the neutral–neutral rate coefficient of reaction (8) and dissociative recombination of C₂HD⁺, become more competitive, and fractional abundance of C₂D increases. This result agrees with that of Herbst et al. (1987). Also C₂D is formed from cosmic ray induced photodissociation of C₂HD. At steady state time our calculated C₂D/C₂H ratio is in agreement with observations of Orion cloud and greater than that obtained by Millar et al. (1989).

As Millar et al. (1989) C₂H₃D⁺ is precursor to C₂H₂D, so the reactions of C⁺ with C₂H₂D transfer fractionation to C₃-bearing hydrocarbons via

(10)

C_{2} H_{2} D + C^{+} \to C_{3} {HD}^{+} + H

and subsequently to C₃D via dissociative recombination. The C₃H₂ molecule is not transferred easily to the C₃HD⁺ and C₃HD is formed as (see Bell et al., 1988);

(11)

C_{2} HD + C^{+} \to C_{3} D^{+} + H,

(12)

C_{3} D^{+} + H_{2} \to C_{3} H_{2} D^{+} + photon,

(13)

C_{3} H_{2} D^{+} + e \to C_{3} HD + H.

Our calculated C₃HD/C₃H₂ ratios in both an early and the steady state times are not in agreement with observation of TMC-1 cloud.

In our model C + 3H₃D molecule is formed through the CH₂D⁺ ion, by dissociative recombination of ${CH}_{2} {DC}_{2} H_{2}^{+}$ and ${CH}_{2} {DC}_{3} H_{2}^{+}$ ions, which are themselves formed from CH₂D⁺ by ion-neutral reactions with small hydrocarbon species like methane or acetylene. C₃H₃D is destroyed by atomic and molecular ions, primarily $H_{3}^{+}$ and He⁺. Our calculated C₃H₃D/C₃H₄ ratios at an early and the steady state time are greater than the lower and upper limit of observations of TMC-1 cloud. This results is the same as that of Roberts and Millar (2000a).

In our model C₄D species is formed by the following reactions:

(14)

C_{4}^{+} + HD \to C_{4} D^{+} + H,

(15)

C_{4} D^{+} + H_{2} \to C_{4} {HD}^{+} + H,

(16)

C_{3} {HD}^{+} + C \to C_{4} {HD}^{+} + H,

(17)

C_{4} {HD}^{+} + e \to C_{4} D + H

and is destroyed by

(18a)

C_{4} D + O \to C_{3} D + CO,

(18b)

C_{4} D + H_{3}^{+} \to C_{4} {HD}^{+} + H_{2},

(18c)

C_{4} D + {HCO}^{+} \to C_{4} {HD}^{+} + CO,

(18d)

7 C_{4} D + H_{3} O^{+} \to C_{4} HD + H_{2} .

At an early time our calculated C₄D/C₄H ratio is in agreement with observations of TMC-1 clouds.

The D–N bond begins due to the lower proton (deuteron) affinity of H₂D⁺, which reacts with N₂ to form N₂D⁺ species,

(19)

H_{2} D^{+} + N_{2} \to N_{2} D^{+} + H_{2} .

In addition to reaction (19), D atom can also react with N₂H⁺ to form N₂D⁺

(20)

N_{2} H^{+} + D \to N_{2} D^{+} + H.

Reaction (20) has an exothermicity of about 550 K (Adams and Smith, 1985). Due to the proton affinity of N₂ is very small, so that N₂D⁺ reacts with CH₂D⁺ and C₂HD⁺.

IN our model N₂D⁺/N₂H⁺ ratios are less than the observations of TMC-1 and Orion clouds. This is because at dense clouds N₂D⁺ condense onto grains, i.e. N₂ D⁺is not easily predictable by gas-phase reactions and its value must be predicted through evaporation from the grain (Willacy and Millar, 1998; Millar et al., 1989; Millar, 2002 ).

After NH₃ is formed by the reaction sequence $N_{2} \overset{He +}{\to} N^{+} \overset{4 H_{2}}{\to} {NH}_{4}^{+} \overset{e}{\to} {NH}_{3}$ , deuteron transfer reaction forms NH₃D⁺ which can then recombine to give NH₂D as;

(21)

{NH}_{3} + {XD}^{+} + \to {NH}_{3} D^{+} + XH,

(22)

{NH}_{3}^{+} + e \to {NH}_{2} D + H,

where XD⁺ represents all species capable of transferring a proton or deuteron to NH₃, principally

H_{3}^{+}

, N₂H⁺, HCO⁺ and their deuterated isotopomers. Successive deuteron transfer reaction can lead eventually to NHD₂ and ND₃ as

(23)

{NH}_{2} D + {XD}^{+} \to {NH}_{2} D^{+} + + X,

(24)

{NH}_{2} D_{2}^{+} + e \to {NHD}_{2} + H,

(25)

{NHD}_{2} + {XD}^{+} \to {NHD}_{3}^{+} + X,

(26)

{NHD}_{3}^{+} + e \to {ND}_{3} + H,

(27)

{NHD}_{3}^{+} + e \to {NHD}_{2} + D.

From the last reactions (21)–(27) the relative fractional abundances depend on the XD⁺/XH⁺ and the branching ratio for dissociative recombination of the deuterated ions.

By assuming the rate coefficients of reactions (26) and (27) are equal, our calculated fractional abundances for ND₃ is about 2 × 10⁻¹¹. This value is greater than that obtained by Rodgers and Charnely (2001) by two times. The abundance of ND₃ in our model is in the same order of magnitude as given by Lis et al. (2002) for B1 cloud. From this result we can conclude that ND₃ can be detected in L134N cloud.

One species for which time dependence significant is DCO⁺. The species DCO⁺ is more readily observable than N₂D⁺, because the abundance of CO is greater than that of N₂ in interstellar clouds. So at low temperature the D–C band begins with the reaction:

(28)

H_{2} D^{+} + CO \to {DCO}^{+} + H_{2}

at an early time. Also DCO⁺ is formed by:

(29)

C^{+} + HDO \to {DCO}^{+} + H,

(30)

{HCO}^{+} + D \to {DCO}^{+} + H.

At the low temperature and density, the atomic deuterium is very abundant, so reaction (30) proceeds very rapidly and can further enhance the fractionation of DCO⁺.

At high temperature the abundance of CH₄D⁺ is greater than H₂D⁺, then DCO⁺ is formed by,

(31)

{CH}_{4} D^{+} + CO \to {DCO}^{+} + {CH}_{4} .

In addition to this reaction, there are significant contributions to DCO⁺ formation in the reactions

(32)

{CH}_{2} D^{+} + O \to {DCO}^{+} + H_{2},

(33)

C_{2} {HD}^{+} + O \to {DCO}^{+} + CH,

(34)

CD + O \to {DCO}^{+} + e,

(35)

{CH}_{2} D^{+} + O \to {DCO}^{+} + H_{2} .

The primary of D in dark clouds is the dissociative recombination of DCO⁺,

(36)

{DCO}^{+} + e \to CO + D.

Our calculated DCO⁺/HCO⁺ ratio is in agreement with observations of TMC-1 cloud at an early time. At the steady state time it is in agreement with observations of Orion and L134N clouds. This result differs than that obtained by Millar et al. (1989) and Millar (2002).

The band between D–C–O begins by the formation of deuterated formaldehyde, HDCO, which is formed from two species derived from CH₂D⁺, CH₄D⁺ and CH₂D, via the reactions;

(37)

{CH}_{4} D^{+} + O \to H_{2} {DCO}^{+} + H_{2},

followed by dissociative recombination

(38)

H_{2} {DCO}^{+} + e \to HDCO + H,

and

(39)

{CH}_{2} D^{+} + O \to HDCO + H^{+} .

HDCO also reacts with

H_{3}^{+}

to form H₂DCO⁺ which will then recombine to HDCO. The calculated HDCO/H₂CO ratio is in agreement with observations of TMC-1, Orion and W3IRS4 clouds.

Also the band between D–C–N begins by the formation of DCN, which is formed a derivative of CH₂D⁺;

(40)

{CH}_{2} D^{+} + e \to CHD + H,

(41)

{CHD}^{+} + N \to DCN + H^{+} .

The main derives of DCN at high temperature is the neutral–neutral reactions;

(42)

{DCO}^{+} + N \to DCN + O,

(43)

{HCN}^{+} + D \to DCN + H.

We found that the reaction

(44)

H^{+} + DCN \to {HCN}^{+} + D

can cycle deuterium between the atomic D and the DCN molecule. This result is in agreement with that of Schilke et al. (1992) and Hatchell et al. (1998). As shown in Tables 3, 4 and 6 our calculated DCN/HCN is in agreement with observations of TMC-1, Orion and W3IRS4 clouds.

At the low temperature, the dominant route to deuterated cyanoacetylene, DC₃N, formation is thought to be

(45)

C_{3} H_{2} D^{+} + N \to HD C_{3} N^{+} + H

followed by

(46)

HD C_{3} N^{+} + e \to {DC}_{3} N + H.

At the high temperature, the deuterated acetylene should form deuterated cyanoacetylene through

(47)

C_{2} HD + CN \to {DC}_{3} N + H.

DC₃N species is only observed in TMC-1 cloud, and our calculated its ratio is in agreement with observations at steady state time.

In our models we have assumed that the species CH₃OD is formed from the radiative association reaction

(48)

{CH}_{3}^{+} + HDO \to {CH}_{3} {OHD}^{+} + photon

and

(49)

{CH}_{3} {OHD}^{+} + e \to {CH}_{3} OD + H.

The HDO species comes from the rapidly exothermic reaction

(50)

{CH}_{2} D^{+} + H_{2} O \to {CH}_{3} + HDO.

The calculated CH + 3OD/CH + 3OH ratio in our model differs from observations. This is because, at high densities most molecules condense onto grain; i.e. CH₃OH and CH₃OD is not easily predictable by gas-phase reaction and their values must be predicted through evaporation from the grains surface (Amin, 1999). Also our calculated ratio for HDO/H₂O is small, because the large value must be predicted by shock chemistry (Amin, 2001; Parise et al., 2002).

When we compare our predicted gas-phase abundances with those observed in TMC-1 and W3IRAS4 clouds, only HDCS is in agreement and other species HDS is smaller than observation. In our model after the formation of H₂S by the chain; $S^{+} \overset{H 2}{\to} {HS}^{+} \overset{H 2}{\to} H_{3} S^{+} \overset{e}{\to} H_{2} S,$ (By the last chain the calculated fractional abundance of H₂S is small) deuteron transfer reaction forms H₂DS⁺ which then recombine to give HDS as:

(51)

H_{2} D^{+} + H_{2} S \to H_{2} {DS}^{+} + H_{2},

(52)

H_{2} {DS}^{+} + e \to HDS + H.

Also DCO⁺ and H₂D⁺ react with H₂CS to form HDCS as;

(53)

{DCO}^{+} + H_{2} CS \to H_{2} {DCS}^{+} + CO,

(54)

H_{2} D^{+} + H_{2} CS \to H_{2} {DCS}^{+} + CO,

(55)

H_{2} {DCS}^{+} + e \to HDCS + H.

4 Conclusions

With a more extensive chemical network, we made a detailed study of a pseudo-time dependent chemical evolution of deuterium species in different interstellar clouds,TMC-1, Orion, L134N and W3IRS4, with different densities and temperatures. This has been done using different initial elemental abundances and without the temperature dependence of the ion–dipole molecule collisions. We have shown that large abundances of NH₂D and NHD₂ can be produced by gas phase chemistry in cold dense clouds. Ammonia is deuterated via deuteron transfer from species such as H₂D⁺, DCO⁺ and N₂D⁺, followed by dissociative recombination. We predict the abundance of ND₃ is 2 × 10⁻¹¹, by a assuming the rate coefficients of reactions ${NHD}_{3}^{+} + e \to {ND}_{3} + H$ , and ${NHD}_{3}^{+} + e \to {NHD}_{2} + D$ are equal. So we suggest that triply-deuterated ammonia could be detectable in L134N cloud. We have included the fractionation of sulphur-bearing molecules and found a good agreement with observation for HDCS. The very slow formation rates of HDS in cold gas make this molecule particularly useful in probing regions where grain surface chemistry may be important. Reduction of the initial elemental abundances and high cosmic ray ionization gave us good relative abundances for most of the observed deuterated species in W3IRS4.

References

Adams N.G., Smith D., . Laboratory studies of the reactions of HCO(+) (and DCO/+/) and N2H(+) (and N2D/+/) with D (and H) atoms – Interstellar implications. Astrophysical Journal. 1985;294(2):L63-L65.
[Google Scholar]
Amin M.Y., . Gas-phase chemistry of the star forming region W3 IRS4. Astrophysics and Space Science. 1999;260(3):405-420.
[Google Scholar]
Amin M.Y., . Gas-phase shock chemistry of the young bipolar outflow L1157. New Astronomy. 2001;6(6):393-401.
[Google Scholar]
Bell M.B., Avery L.W., Mathews H.E., . A study of C3HD in cold interstellar clouds. Astrophysical Journal. 1988;326(Part 1):924-930.
[Google Scholar]
Caselli P., Stantcheva T., Shalabiea O., . Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach. Planetary and Space Science. 2002;50(12–13):1257-1266.
[Google Scholar]
Ceccarelli C., Castets A., Loinard L., Caux E., Tielens A.G.G.M., . Detection of doubly deuterated formaldehyde towards the low-luminosity protostar IRAS 16293-2422. Astronomy and Astrophysics. 1998;338:L43-L46.
[Google Scholar]
Ceccarelli C., Loinard L., Castets A., . Extended D₂CO emission: the smoking gun of grain surface-chemistry. Astronomy and Astrophysics. 2001;372:998-1004.
[Google Scholar]
Ceccarelli C., Vastel C., Tielens A.G.G.M., Castets A., Boogert A.C.A., Loinard L., Caux E., . The puzzling detection of D₂CO in the molecular cloud L1689N. Astronomy and Astrophysics. 2002;381:L17-L20.
[Google Scholar]
Chastaing D., Le Picard S.D., Sims I.R., Smith I.W.M., . Rate coefficients for the reactions of C(³P_J) atoms with C₂H₂, C₂H₄, CH₃C⚌CH and H₂C⚌C⚌CH₂ at temperatures down to 15 K. Astronomy and Astrophysics. 2001;365:241-247.
[Google Scholar]
Combes F., Boulanger F., Encrenaz P.G., . Detection of interstellar CCD. Astronomy and Astrophysics. 1985;147(2) L25, L26
[Google Scholar]
Gerin M., Combes F., Wlodarczak G., Encrenaz P., Laurent C., . Interstellar detection of deuterated methyl acetylene. Astronomy and Astrophysics. 1992;253(2):L29-L32.
[Google Scholar]
Hatchell J., Millar T.J., Rodgers S.D., . The DCN/HCN abundance ratio in hot molecular cores. Astronomy and Astrophysics. 1998;332:695-702.
[Google Scholar]
Helmich F.D., van Dishock E.F., . Physical and chemical variations within the W3 star-forming region. II. The 345 GHz spectral line survey. A&A Supplement series. 1997;124:205-253.
[Google Scholar]
Henkel C., Mauersberger R., Wilson T.L., . Deuterated water in Orion-KL and NGC7538. Astronomy and Astrophysics. 1987;182(2):299-304.
[Google Scholar]
Herbst E., Leung C.M., . Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry – model update and sensitivity analysis. Monthly Notices of the Royal Astronomical Society. 1986;222:689-711.
[Google Scholar]
Herbst E., Adams N.G., Smith D., De Frees D.J., . Ion-molecule calculation of the abundance ratio of CCD to CCH in dense interstellar clouds. Astrophysical Journal. 1987;312(Part 1):351-357.
[Google Scholar]
Howe D.A., Millar T.J., Schilke P., Walmsley C.M., . Observations of deuterated cyanoacetylene in dark clouds. Monthly Notices of the Royal Astronomical Society. 1994;267(1/MAR1):59-68.
[Google Scholar]
Larsson M., Lepp S., Dalgarno A., . Dissociative recombination of H₂D⁺ and the cosmic abundance of deuterium. Astronomy and Astrophysics. 1996;309:L1-L3.
[Google Scholar]
Le Teuff Y., Millar T.J., Markwick A.J., . The UMIST database for astrochemistry 1999. Astronomy and Astrophysics Supplement. 2000;146:157-168.
[Google Scholar]
Linsky J.L., Diplas A., Wood B.E., . Deuterium and the local interstellar medium properties for the procyon and capella lines of sight. Astrophysical Journal. 1995;451:335-351.
[Google Scholar]
Lis D.C., Roueff E., Gerin M., . Detection of triply deuterated ammonia in the barnard 1 cloud. The Astrophysical Journal. 2002;571(1):L55-L58.
[Google Scholar]
Loinard L., Castets A., Ceccarelli C., . Doubly deuterated molecular species in protostellar environments. The Astrophysical Journal. 2001;552(2):L163-L166.
[Google Scholar]
Loren R.B., Wootten A., . High-excitation lines of deuterated formaldehyde (HDCO) in the Orion Molecular Cloud. Astrophysical Journal. 1985;299(Part 1):947-955.
[Google Scholar]
MacLeod J.M., Avery L.W., Broten N.W., . Detection of deuterated cyanodiacetylene/DC5N/ in Taurus Molecular Cloud 1. Astrophysical Journal. 1981;251(December):L33-L36.
[Google Scholar]
Mauersberger R., Henkel C., Jacq T., Walmsley C.M., . Deuterated methanol in Orion. Astronomy and Astrophysics. 1988;194(1–2):L1-L4.
[Google Scholar]
Millar T.J., . Modeling deuterium fractionation in interstellar clouds. Planetary and Space Science. 2002;50(12–13):1189-1195.
[Google Scholar]
Millar T.J., Bennett A., Herbst E., . Deuterium fractionation in dense interstellar clouds. Astrophysical Journal. 1989;340(1):906-920.
[Google Scholar]
Minowa H., Satake M., Hirota T., . Laboratory microwave spectroscopy of HDCS and its astronomical detection toward TMC-1. Astrophysical Journal Letters. 1997;491:L63-L66.
[Google Scholar]
Parise B., Ceccarelli C., Tielens A.G.G.M., . Detection of doubly-deuterated methanol in the solar-type protostar IRAS 16293-2422. Astronomy and Astrophysics. 2002;393:L49-L53.
[Google Scholar]
Penzias A.A., . Interstellar HCN, HCO/+/, and the galactic deuterium gradient. Astrophysical Journal. 1979;228(Part 1):430-434.
[Google Scholar]
Roberts H., Millar T.J., . Modelling of deuterium chemistry and its application to molecular clouds. Astronomy and Astrophysics. 2000;361:388-398.
[Google Scholar]
Roberts H., Millar T.J., . Gas-phase formation of doubly-deuterated species. Astronomy and Astrophysics. 2000;364:780-784.
[Google Scholar]
Robets H., Fuller G.A., Millar T.J., Hatchell J., Buckle J.V., . A survey of[HDCO]/[H₂CO] and[DCN]/[HCN] ratios towards low-mass protostellar cores. Astronomy and Astrophysics. 2002;381:1026-1038.
[Google Scholar]
Rodgers S.D., Charnely S.B., . Gas-phase production of NHD₂ in L134N. The Astrophysical Journal. 2001;553(2):613-617.
[Google Scholar]
Rodgers S.D., Millar T.J., . The chemistry of deuterium in hot molecular cores. Monthly Notices of the Royal Astronomical Society. 1996;280(4):1046-1054.
[Google Scholar]
Roueff E., Tine S., Coudert L.H., Des Forets P.G., Falgarone E., Gerin M., . Detection of doubly deuterated ammonia in L134N. Astronomy and Astrophysics. 2000;354:L63-L66.
[Google Scholar]
Schilke P., Walmsley C.M., De Forets G.P., Roueff E., Flower D.R., Guilloteau S., . A study of HCN, HNC and their isotopomers in OMC-1. I – Abundances and chemistry. Astronomy and Astrophysics. 1992;256(2):595-612.
[Google Scholar]
Shah R.Y., Wotten A., . Deuterated ammonia in galactic protostellar cores. The Astrophysical Journal. 2001;554(2):933-947.
[Google Scholar]
Tine S., Roueff E., Falgarone E., Gerin M., Des Forets P.G., . Deuterium fractionation in dense ammonia cores. Astronomy and Astrophysics. 2000;356:1039-1049.
[Google Scholar]
Turner B.E., . Detection of interstellar C4D – implications for ion–molecule chemistry. Astrophysical Journal. 1989;347(Part 2):L39-L42.
[Google Scholar]
Turner B.E., . Detection of doubly deuterated interstellar formaldehyde (D2CO) – an indicator of active grain surface chemistry. The Astrophysical Journal. 1990;362:L29-L33.
[Google Scholar]
Turner B.E., . Deuterated molecules in translucent and dark clouds. The Astrophysical Journal Supplement Series. 2001;136(2):579-629.
[Google Scholar]
van der Tak, F.F.S., Schilke, P., Muller, H.S.P., 2002. A. A. 388, 53.
Vrtilek J.M., Gottlieb C.A., Langer W.D., Thaddeus P., Wilson R.W., . Laboratory and astronomical detection of the deuterated ethynyl radical CCD. Astrophysical Journal. 1985;296:L35-L38.
[Google Scholar]
Walmsley C.M., Hermsen W., Henkel C., Mauersberger R., Wilson T.L., . Deuterated ammonia in the Orion hot core. Astronomy and Astrophysics. 1987;172(1–2):311-315.
[Google Scholar]
Willacy K., Millar T.J., . Desorption processes and the deuterium fractionation in molecular clouds. Monthly Notices of the Royal Astronomical Society. 1998;298(2):562-568.
[Google Scholar]
Wootten, A., 1987. In: Vardya, M.S., Tarafdar, S.P. (Eds.). ASTROCHEMISTRY Proceedings of the IAU Symposium No. 120. Goa, India 1985. Dordrecht, Reidel, p. 311.

[1] Adams N.G., Smith D., . Laboratory studies of the reactions of HCO(+) (and DCO/+/) and N2H(+) (and N2D/+/) with D (and H) atoms – Interstellar implications. Astrophysical Journal. 1985;294(2):L63-L65.
[Google Scholar]

[2] Amin M.Y., . Gas-phase chemistry of the star forming region W3 IRS4. Astrophysics and Space Science. 1999;260(3):405-420.
[Google Scholar]

[3] Amin M.Y., . Gas-phase shock chemistry of the young bipolar outflow L1157. New Astronomy. 2001;6(6):393-401.
[Google Scholar]

[4] Bell M.B., Avery L.W., Mathews H.E., . A study of C3HD in cold interstellar clouds. Astrophysical Journal. 1988;326(Part 1):924-930.
[Google Scholar]

[5] Caselli P., Stantcheva T., Shalabiea O., . Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach. Planetary and Space Science. 2002;50(12–13):1257-1266.
[Google Scholar]

[6] Ceccarelli C., Castets A., Loinard L., Caux E., Tielens A.G.G.M., . Detection of doubly deuterated formaldehyde towards the low-luminosity protostar IRAS 16293-2422. Astronomy and Astrophysics. 1998;338:L43-L46.
[Google Scholar]

[7] Ceccarelli C., Loinard L., Castets A., . Extended D₂CO emission: the smoking gun of grain surface-chemistry. Astronomy and Astrophysics. 2001;372:998-1004.
[Google Scholar]

[8] Ceccarelli C., Vastel C., Tielens A.G.G.M., Castets A., Boogert A.C.A., Loinard L., Caux E., . The puzzling detection of D₂CO in the molecular cloud L1689N. Astronomy and Astrophysics. 2002;381:L17-L20.
[Google Scholar]

[9] Chastaing D., Le Picard S.D., Sims I.R., Smith I.W.M., . Rate coefficients for the reactions of C(³P_J) atoms with C₂H₂, C₂H₄, CH₃C⚌CH and H₂C⚌C⚌CH₂ at temperatures down to 15 K. Astronomy and Astrophysics. 2001;365:241-247.
[Google Scholar]

[10] Combes F., Boulanger F., Encrenaz P.G., . Detection of interstellar CCD. Astronomy and Astrophysics. 1985;147(2) L25, L26
[Google Scholar]

[11] Gerin M., Combes F., Wlodarczak G., Encrenaz P., Laurent C., . Interstellar detection of deuterated methyl acetylene. Astronomy and Astrophysics. 1992;253(2):L29-L32.
[Google Scholar]

[12] Hatchell J., Millar T.J., Rodgers S.D., . The DCN/HCN abundance ratio in hot molecular cores. Astronomy and Astrophysics. 1998;332:695-702.
[Google Scholar]

[13] Helmich F.D., van Dishock E.F., . Physical and chemical variations within the W3 star-forming region. II. The 345 GHz spectral line survey. A&A Supplement series. 1997;124:205-253.
[Google Scholar]

[14] Henkel C., Mauersberger R., Wilson T.L., . Deuterated water in Orion-KL and NGC7538. Astronomy and Astrophysics. 1987;182(2):299-304.
[Google Scholar]

[15] Herbst E., Leung C.M., . Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry – model update and sensitivity analysis. Monthly Notices of the Royal Astronomical Society. 1986;222:689-711.
[Google Scholar]

[16] Herbst E., Adams N.G., Smith D., De Frees D.J., . Ion-molecule calculation of the abundance ratio of CCD to CCH in dense interstellar clouds. Astrophysical Journal. 1987;312(Part 1):351-357.
[Google Scholar]

[17] Howe D.A., Millar T.J., Schilke P., Walmsley C.M., . Observations of deuterated cyanoacetylene in dark clouds. Monthly Notices of the Royal Astronomical Society. 1994;267(1/MAR1):59-68.
[Google Scholar]

[18] Larsson M., Lepp S., Dalgarno A., . Dissociative recombination of H₂D⁺ and the cosmic abundance of deuterium. Astronomy and Astrophysics. 1996;309:L1-L3.
[Google Scholar]

[19] Le Teuff Y., Millar T.J., Markwick A.J., . The UMIST database for astrochemistry 1999. Astronomy and Astrophysics Supplement. 2000;146:157-168.
[Google Scholar]

[20] Linsky J.L., Diplas A., Wood B.E., . Deuterium and the local interstellar medium properties for the procyon and capella lines of sight. Astrophysical Journal. 1995;451:335-351.
[Google Scholar]

[21] Lis D.C., Roueff E., Gerin M., . Detection of triply deuterated ammonia in the barnard 1 cloud. The Astrophysical Journal. 2002;571(1):L55-L58.
[Google Scholar]

[22] Loinard L., Castets A., Ceccarelli C., . Doubly deuterated molecular species in protostellar environments. The Astrophysical Journal. 2001;552(2):L163-L166.
[Google Scholar]

[23] Loren R.B., Wootten A., . High-excitation lines of deuterated formaldehyde (HDCO) in the Orion Molecular Cloud. Astrophysical Journal. 1985;299(Part 1):947-955.
[Google Scholar]

[24] MacLeod J.M., Avery L.W., Broten N.W., . Detection of deuterated cyanodiacetylene/DC5N/ in Taurus Molecular Cloud 1. Astrophysical Journal. 1981;251(December):L33-L36.
[Google Scholar]

[25] Mauersberger R., Henkel C., Jacq T., Walmsley C.M., . Deuterated methanol in Orion. Astronomy and Astrophysics. 1988;194(1–2):L1-L4.
[Google Scholar]

[26] Millar T.J., . Modeling deuterium fractionation in interstellar clouds. Planetary and Space Science. 2002;50(12–13):1189-1195.
[Google Scholar]

[27] Millar T.J., Bennett A., Herbst E., . Deuterium fractionation in dense interstellar clouds. Astrophysical Journal. 1989;340(1):906-920.
[Google Scholar]

[28] Minowa H., Satake M., Hirota T., . Laboratory microwave spectroscopy of HDCS and its astronomical detection toward TMC-1. Astrophysical Journal Letters. 1997;491:L63-L66.
[Google Scholar]

[29] Parise B., Ceccarelli C., Tielens A.G.G.M., . Detection of doubly-deuterated methanol in the solar-type protostar IRAS 16293-2422. Astronomy and Astrophysics. 2002;393:L49-L53.
[Google Scholar]

[30] Penzias A.A., . Interstellar HCN, HCO/+/, and the galactic deuterium gradient. Astrophysical Journal. 1979;228(Part 1):430-434.
[Google Scholar]

[31] Roberts H., Millar T.J., . Modelling of deuterium chemistry and its application to molecular clouds. Astronomy and Astrophysics. 2000;361:388-398.
[Google Scholar]

[32] Roberts H., Millar T.J., . Gas-phase formation of doubly-deuterated species. Astronomy and Astrophysics. 2000;364:780-784.
[Google Scholar]

[33] Robets H., Fuller G.A., Millar T.J., Hatchell J., Buckle J.V., . A survey of[HDCO]/[H₂CO] and[DCN]/[HCN] ratios towards low-mass protostellar cores. Astronomy and Astrophysics. 2002;381:1026-1038.
[Google Scholar]

[34] Rodgers S.D., Charnely S.B., . Gas-phase production of NHD₂ in L134N. The Astrophysical Journal. 2001;553(2):613-617.
[Google Scholar]

[35] Rodgers S.D., Millar T.J., . The chemistry of deuterium in hot molecular cores. Monthly Notices of the Royal Astronomical Society. 1996;280(4):1046-1054.
[Google Scholar]

[36] Roueff E., Tine S., Coudert L.H., Des Forets P.G., Falgarone E., Gerin M., . Detection of doubly deuterated ammonia in L134N. Astronomy and Astrophysics. 2000;354:L63-L66.
[Google Scholar]

[37] Schilke P., Walmsley C.M., De Forets G.P., Roueff E., Flower D.R., Guilloteau S., . A study of HCN, HNC and their isotopomers in OMC-1. I – Abundances and chemistry. Astronomy and Astrophysics. 1992;256(2):595-612.
[Google Scholar]

[38] Shah R.Y., Wotten A., . Deuterated ammonia in galactic protostellar cores. The Astrophysical Journal. 2001;554(2):933-947.
[Google Scholar]

[39] Tine S., Roueff E., Falgarone E., Gerin M., Des Forets P.G., . Deuterium fractionation in dense ammonia cores. Astronomy and Astrophysics. 2000;356:1039-1049.
[Google Scholar]

[40] Turner B.E., . Detection of interstellar C4D – implications for ion–molecule chemistry. Astrophysical Journal. 1989;347(Part 2):L39-L42.
[Google Scholar]

[41] Turner B.E., . Detection of doubly deuterated interstellar formaldehyde (D2CO) – an indicator of active grain surface chemistry. The Astrophysical Journal. 1990;362:L29-L33.
[Google Scholar]

[42] Turner B.E., . Deuterated molecules in translucent and dark clouds. The Astrophysical Journal Supplement Series. 2001;136(2):579-629.
[Google Scholar]

[43] van der Tak, F.F.S., Schilke, P., Muller, H.S.P., 2002. A. A. 388, 53.

[44] Vrtilek J.M., Gottlieb C.A., Langer W.D., Thaddeus P., Wilson R.W., . Laboratory and astronomical detection of the deuterated ethynyl radical CCD. Astrophysical Journal. 1985;296:L35-L38.
[Google Scholar]

[45] Walmsley C.M., Hermsen W., Henkel C., Mauersberger R., Wilson T.L., . Deuterated ammonia in the Orion hot core. Astronomy and Astrophysics. 1987;172(1–2):311-315.
[Google Scholar]

[46] Willacy K., Millar T.J., . Desorption processes and the deuterium fractionation in molecular clouds. Monthly Notices of the Royal Astronomical Society. 1998;298(2):562-568.
[Google Scholar]

[47] Wootten, A., 1987. In: Vardya, M.S., Tarafdar, S.P. (Eds.). ASTROCHEMISTRY Proceedings of the IAU Symposium No. 120. Goa, India 1985. Dordrecht, Reidel, p. 311.

Deuterium-bearing molecules in cold and warm dense interstellar clouds

Abstract

Keywords

Astrochemistry

1 Introduction

2 Chemical models

3 Results and discussion

4 Conclusions

References

Further reading