A COLD COMPLEX CHEMISTRY TOWARD THE LOW-MASS PROTOSTAR B1-b: EVIDENCE FOR COMPLEX MOLECULE PRODUCTION IN ICES

By K. Oberg, S. Bottinelli, J. Jørgensen, E. V. van Dishoeck. 2010

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Gas-phase complex organic molecules have been detected toward a range of high- and low-mass star-forming regions at abundances which cannot be explained by any known gas-phase chemistry. Recent laboratory experiments show that UV irradiation of CH3OH-rich ices may be an important mechanism for producing complex molecules and releasing them into the gas phase. To test this ice formation scenario, we mapped the B1-b dust core and nearby protostar in CH3OH gas using the IRAM 30 m telescope to identify locations of efficient non-thermal ice desorption. We find three CH3OH abundance peaks tracing two outflows and a quiescent region on the side of the core facing the protostar. The CH3OH gas has a rotational temperature of ∼10 K at all locations. The quiescent CH3OH abundance peak and one outflow position were searched for complex molecules. Narrow, 0.6–0.8 km s−1 wide, HCOOCH3 and CH3CHO lines originating in cold gas are clearly detected, CH3OCH3 is tentatively detected, and C2H5OH and HOCH2CHO are undetected toward the quiescent core, while no complex molecular lines were found toward the outflow. The core abundances with respect to CH3OH are ∼2.3% and 1.1% for HCOOCH3 and CH3CHO, respectively, and the upper limits are 0.7%–1.1%, which is similar to most other low-mass sources. The observed complex molecule characteristics toward B1-b and the pre-dominance of HCO-bearing species suggests a cold ice (below 25 K, the sublimation temperature of CO) formation pathway followed by non-thermal desorption through, e.g., UV photons traveling through outflow cavities. The observed complex gas composition together with the lack of any evidence of warm gas-phase chemistry provides clear evidence of efficient complex molecule formation in cold interstellar ices.

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Citations in the corpus

Grain Surface Models and Data for Astrochemistry (2017, background): “Recent high-sensitivity observations have suggested that the origin of gas-phase molecules at offset positions from the location of the central forming star may be due to photodesorption by stellar FUV photons along the outflow cavity walls (e.g., Öberg et al. 2010).”
Detection of complex organic molecules in a prestellar core: a new challenge for astrochemical models (2012, background): “Similar mechanisms were invoked by Öberg et al. (2010) who detected methyl formate in the protostar B1-b, possibly desorbed by UV originating from the protostar through an outflow cavity.”
H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices – an extended view on complex organic molecule formation (2016, background): “The same species have also been seen in the cold pre-stellar core B1-b (Cernicharo et al. 2012), in the cold outer envelopes of low-mass protostars (Öberg et al. 2010), and in outflow spots in dark clouds where the ice mantles are liberated by shocks (Arce et al. 2008; Öberg et al. 2011b).”
Astrochemistry and compositions of planetary systems (2020, background): “of this initial organic ice reservoir is converted into more complex organics in interstellar clouds (Öberg et al. 2010; Bacmann et al. 2012; Cernicharo et al. 2012; Vastel et al. 2014; Jiménez-Serra et al. 2016; Scibelli and Shirley 2020).”
Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “In particular, models of formation of saturated complex organic molecules that have been found in cold clouds representing the earliest stages of star formation (Öberg et al. 2010; Bacmann et al. 2012; Cernicharo et al. 2012; Vastel et al. 2014), are under active development.”
REACTIVE DESORPTION AND RADIATIVE ASSOCIATION AS POSSIBLE DRIVERS OF COMPLEX MOLECULE FORMATION IN THE COLD INTERSTELLAR MEDIUM (2013, background): “In addition to these molecules, several other simple species were observed toward B1-b and L1689b such as CO, HCO+, N2H+, NH3, CS, and HNCO/HCNO (Hirano et al. 1999; Bacmann et al. 2002; Marcelino et al. 2009; Öberg et al. 2010).”
Formation of complex organic molecules in cold objects: the role of gas-phase reactions (2015, background): “Finally, Vastel et al. (2014) argued that the COMs emission detected in L1544 also comes from the same outer shell and speculated that this is likely the case also for the other cold sources where COMs have been detected (Bacmann et al. 2012; Oberg et al. 2010; Cernicharo et al. 2012).”
THE ORIGIN OF COMPLEX ORGANIC MOLECULES IN PRESTELLAR CORES (2014, background): “Important non-detections include CH3OCH3, HCOOCH3 and CH3O, which were, on the contrary, detected in other prestellar cores (Öberg et al. 2010; Cernicharo et al. 2012; Bacmann et al. 2012).”