Detection of protonated formaldehyde in the prestellar core L1689B

Abstract

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.

Discovery of HC3O+ in space: The chemistry of O-bearing species in TMC-1

“…In general, the abundance of polyatomic cations in cold interstellar clouds is relatively low because they react quickly with electrons. In addition to the widespread HCO + and N 2 H + , the other polyatomic cations detected in cold interstellar clouds are HCS + (Thaddeus et al 1981), HCNH + (Schilke et al 1991), HC 3 NH + (Kawaguchi et al 1994), HCO + 2 (Turner et al 1999;Sakai et al 2008), NH 3 D + (Cernicharo et al 2013), NCCNH + (Agúndez et al 2015b), H 2 COH + (Bacmann et al 2016), H 2 NCO + (Marcelino et al 2018), and HC 5 NH + (Marcelino et al 2020). These species are the protonated forms of abundant neutral species.…”

Tentative detection of HC5NH+ in TMC-1

“…The rather reduced number of them detected in space is due to the fact that a dissociative recombination with electrons is rapid and depletes cations, producing most of the neutrals observed in these objects. In addition to the widespread ions HCO + and N 2 H + , other interesting protonated species are HCS + (Thaddeus et al 1981), HCNH + (Schilke et al 1991), HC 3 NH + (Kawaguchi et al 1994), HCO + 2 (Turner et al 1999;Sakai et al 2008), NH 3 D + (Cernicharo et al 2013), NCCNH + (Agúndez et al 2015), H 2 COH + (Bacmann et al 2016), and H 2 NCO + (Marcelino et al 2018).…”

JCMT POL-2 and BISTRO Survey observations of magnetic fields in the L1689 molecular cloud

“…Lee et al 2001), has been extensively studied in terms of its chemistry (e.g. Redman et al 2002;Crapsi et al 2005;Bacmann et al 2016;Kim et al 2020), internal structure and dynamics (e.g. Lee et al 1999Lee et al , 2001Redman et al 2004;Seo et al 2013;Roy et al 2014) due to its relative isolation and simple morphology.…”