CO Depletion and Deuterium Fractionation in Prestellar Cores

The standard interstellar ratio of deuterium to hydrogen (D/H) atoms is ∼ 1.5×10 −5 . However, the deuterium fractionation is in fact found to be enhanced, to different degrees, in cold, dark cores, hot cores around massive star forming regions, lukewarm cores, and warm cores (hereafter, hot corinos) around low-mass star forming regions. In this paper, we investigate the overall differences in the deuterium chemistry between hot cores and hot corinos. We have modelled the chemistry of dense gas around low-mass and massive star forming regions using a gas-grain chemical model. We investigate the influence of varying the core density, the depletion efficiency of gaseous species on to dust grains, the collapse mode and the final mass of the protostar on the chemical 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 is more sensitive to changes in the final density of the collapsing core in hot cores than in hot corinos. Finally, we find that ratios of deuterated sulphur bearing species in dense gas around hot cores and corinos may be good evolutionary indicators in a similar way as their non deuterated counterparts.

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“…This result is supported by Bacmann et al (2003), see Section (4). In hot cores, the deuterated forms of H2CO and CH3OH (Fig.…”

Section: Impact Of Varying the Depletion Efficiencysupporting

confidence: 66%

“…Caselli et al 2002;Bacmann et al 2003;Millar 2005;Crapsi et al 2005;Chen et al 2011). Therefore, we studied the influence of varying the depletion percentage on the fractional abundances of the species in hot corinos and cores.…”

Section: Impact Of Varying the Depletion Efficiencymentioning

confidence: 99%

Deuterium chemistry of dense gas in the vicinity of low-mass and massive star-forming regions

Awad

Viti

Bayet

et al. 2014

Monthly Notices of the Royal Astronomical Society

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“…Reaction (2) (Bacmann et al 2003), we find that the predicted H 2 COH + column density N mod derived from Eq. (3) is 4.1 × 10 11 −9.1 × 10 11 cm −2 (depending on the main proton donor), which is consistent with the observed value we determine in L1689B, N obs = 6.7 × 10 11 cm −2 within the uncertainties.…”

Section: Discussionmentioning

confidence: 72%

“…In dark clouds, H 2 COH + is likely the product of the protonation of H 2 CO, which is an abundant organic species in prestellar cores, with abundances generally around 10 −10 −10 −9 (Bacmann et al 2003;Guzmán et al 2011). Proton donors, such as H + 3 , or HCO + , are the most abundant ions, with abundances close to 10 −9 −10 −8 (Aikawa et al 2005;Flower et al 2005Flower et al , 2006.…”

Section: Introductionmentioning

confidence: 99%

Detection of protonated formaldehyde in the prestellar core L1689B

Bacmann

García-García

Fauré

2016

A&A

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Complex organic molecules (COMs) are detected in many regions of the interstellar medium, including prestellar cores. However, their formation mechanisms in cold (∼10 K) cores remain to this date poorly understood. The formyl radical HCO is an important candidate precursor for several O-bearing terrestrial COMs in cores, as an abundant building block of many of these molecules. Several chemical routes have been proposed to account for its formation: on grain surfaces, as an incompletely hydrogenated product of H addition to frozen-out CO molecules; and in the gas phase, either as the product of the reaction between H 2 CO and a radical or as a product of dissociative recombination of protonated formaldehyde H 2 COH + . The detection and abundance determination of H 2 COH + , if present, could provide clues as to whether this latter scenario might apply. We searched for protonated formaldehyde H 2 COH + in the prestellar core L1689B using the IRAM 30 m telescope. The H 2 COH + ion is unambiguously detected, for the first time, in a cold (∼10 K) source. The derived abundance agrees with a scenario in which the formation of H 2 COH + results from the protonation of formaldehyde. We use this abundance value to constrain the branching ratio of the dissociative recombination of H 2 COH + towards the HCO channel to ∼10−30%. This value could however be lower if HCO were efficiently formed from neutralneutral reactions in the gas phase, and we stress the need for laboratory measurements of the rate constants of these reactions at 10 K. Given the experimental difficulties in measuring branching ratios experimentally, observations can place valuable constraints on these values and provide useful input for chemical networks.

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“…Consequently, water shows a different behavior from other molecules such as methanol and formaldehyde, that need CO ices to be formed. In particular, Bacmann et al (2003Bacmann et al ( , 2007 show that the deuteration of H 2 CO and CH 3 OH increases with the CO depletion in starless dense cores, increasing itself with the H 2 density (Bacmann et al 2002). The H 2 CO and CH 3 OH deuteration is therefore sensitive to the density of the medium in which they form.…”

Section: Discussionmentioning

confidence: 97%

A study of deuterated water in the low-mass protostar IRAS 16293-2422

Coutens¹,

Vastel²,

Caux³

et al. 2012

A&A

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133

236

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Context. Water is a primordial species in the emergence of life, and comets may have brought a large fraction to Earth to form the oceans. To understand the evolution of water from the first stages of star formation to the formation of planets and comets, the HDO/H 2 O ratio is a powerful diagnostic. Aims. Our aim is to determine precisely the abundance distribution of HDO towards the low-mass protostar IRAS 16293-2422 and learn more about the water formation mechanisms by determining the HDO/H 2 O abundance ratio. Results. It is the first time that so many HDO and H 182 O transitions have been detected towards the same source with high spectral resolution. We derive an inner HDO abundance (T ≥ 100 K) of about 1.7 × 10 −7 and an outer HDO abundance (T < 100 K) of about 8 × 10 −11 . To reproduce the HDO absorption lines observed at 894 and 465 GHz, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. At a 3σ uncertainty, the HDO/H 2 O ratio is 1.4-5.8% in the hot corino, whereas it is 0.2-2.2% in the outer envelope. It is estimated at ∼4.8% in the added absorbing layer. Conclusions. Although it is clearly higher than the cosmic D/H abundance, the HDO/H 2 O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similarity of the ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol, which formed later once the CO molecules had depleted on the grains.

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CO Depletion and Deuterium Fractionation in Prestellar Cores

Cited by 194 publications

References 18 publications

Deuterium chemistry of dense gas in the vicinity of low-mass and massive star-forming regions

Deuterium chemistry of dense gas in the vicinity of low-mass and massive star-forming regions

Detection of protonated formaldehyde in the prestellar core L1689B

A study of deuterated water in the low-mass protostar IRAS 16293-2422

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