Efficient Methanol Production on the Dark Side of a Prestellar Core

et al. 2021

ApJ

Self Cite

Observations carried out toward starless and prestellar cores have revealed that complex organic molecules are prevalent in these objects, but it is unclear what chemical processes are involved in their formation. Recently, it has been shown that complex organics are preferentially produced at an intermediate-density shell within the L1544 prestellar core at radial distances of ∼4000 au with respect to the core center. However, the spatial distribution of complex organics has only been inferred toward this core, and it remains unknown whether these species present a similar behavior in other cores. We report high-sensitivity observations carried out toward two positions in the L1498 starless core, the dust peak and a position located at a distance of ∼11,000 au from the center of the core where the emission of CH 3 OH peaks. Similarly to L1544, our observations reveal that small O-bearing molecules and N-bearing species are enhanced by factors of ∼4-14 toward the outer shell of L1498. However, unlike L1544, large O-bearing organics such as CH 3 CHO, CH 3 OCH 3 , or CH 3 OCHO are not detected within our sensitivity limits. For N-bearing organics, these species are more abundant toward the outer shell of the L1498 starless core than toward the one in L1544. We propose that the differences observed between O-bearing and N-bearing species in L1498 and L1544 are due to the different physical structure of these cores, which in turn is a consequence of their evolutionary stage, with L1498 being younger than L1544.

Section: Excitation Analysis Of Coms and Com Precursorssupporting

confidence: 81%

The Complex Organic Molecular Content in the L1498 Starless Core

Jiménez-Serra

Vasyunin

et al. 2021

ApJ

Self Cite

“…Thus, in order to model properly a complicated relation between CO and CH 3 OH in prestellar clouds, we need to utilise the reasonably detailed approach to grain chemistry and gas-grain interaction. To model the emission of methanol, we utilised the MONACO model previously applied to studies of chemistry in a number of prestellar cores (Vasyunin & Herbst 2013;Vasyunin et al 2017;Nagy et al 2019;Lattanzi et al 2020;Harju et al 2020;Scibelli et al 2021;Jiménez-Serra et al 2021). MONACO is a rate equations-based numerical code capable of simulation chemistry in interstellar medium under a three-phase approach.…”

Section: Chemical Modelling Approachesmentioning

confidence: 99%

Gas phase Elemental abundances in Molecular cloudS (GEMS) V. Methanol in Taurus

Fuente

Caselli

et al. 2021

A&A

Context. Methanol, one of the simplest complex organic molecules in the interstellar medium, has been shown to be present and extended in cold environments such as starless cores. Studying the physical conditions at which CH3OH starts its efficient formation is important to understand the development of molecular complexity in star-forming regions. Aims. We aim to study methanol emission across several starless cores and investigate the physical conditions at which methanol starts to be efficiently formed, as well as how the physical structure of the cores and their surrounding environment affect its distribution. Methods. Methanol and C18O emission lines at 3 mm have been observed with the IRAM 30 m telescope within the large programme Gas phase Elemental abundances in Molecular CloudS towards 66 positions across 12 starless cores in the Taurus Molecular Cloud. A non-LTE (local thermodynamic equilibrium) radiative transfer code was used to compute the column densities in all positions. We then used state-of-the-art chemical models to reproduce our observations. Results. We have computed N(CH3OH)/N(C18O) column density ratios for all the observed offsets, and the following two different behaviours can be recognised: the cores where the ratio peaks at the dust peak and the cores where the ratio peaks with a slight offset with respect to the dust peak (~10 000 AU). We suggest that the cause of this behaviour is the irradiation on the cores due to protostars nearby which accelerate energetic particles along their outflows. The chemical models, which do not take irradiation variations into account, can reproduce the overall observed column density of methanol fairly well, but they cannot reproduce the two different radial profiles observed. Conclusions. We confirm the substantial effect of the environment on the distribution of methanol in starless cores. We suggest that the clumpy medium generated by protostellar outflows might cause a more efficient penetration of the interstellar radiation field in the molecular cloud and have an impact on the distribution of methanol in starless cores. Additional experimental and theoretical work is needed to reproduce the distribution of methanol across starless cores.

“…When considering that HMM-1 is embedded in a non-homogeneous ISRF because of a strong illumination from the west due to the stars, for HMM-1 the differentiation between CH 3 OH and c-C 3 H 2 also follows the trend seen in L1544 and in the other cores described in this paper. The asymmetry of the methanol distribution in HMM-1 is discussed in Harju et al (2020).…”

Section: Hmm-1mentioning

confidence: 99%

Distribution of methanol and cyclopropenylidene around starless cores

Caselli

Pineda

et al. 2020

A&A

Context. The spatial distribution of molecules around starless cores is a powerful tool for studying the physics and chemistry governing the earliest stages of star formation. Aims. Our aim is to study the chemical differentiation in starless cores to determine the influence of large-scale effects on the spatial distribution of molecules within the cores. Furthermore, we want to put observational constraints on the mechanisms responsible in starless cores for the desorption of methanol from the surface of dust grains where it is efficiently produced. Methods. We mapped methanol, CH3OH, and cyclopropenylidene, c-C3H2, with the IRAM 30 m telescope in the 3 mm band towards six starless cores embedded in different environments, and in different evolutionary stages. Furthermore, we searched for correlations among physical properties of the cores and the methanol distribution. Results. From our maps we can infer that the chemical segregation between CH3OH and c-C3H2 is driven by uneven illumination from the interstellar radiation field (ISRF). The side of the core that is more illuminated has more C atoms in the gas-phase and the formation of carbon-chain molecules like c-C3H2 is enhanced. Instead, on the side that is less exposed to the ISRF the C atoms are mostly locked in carbon monoxide, CO, the precursor of methanol. Conclusions. We conclude that large-scale effects have a direct impact on the chemical segregation that we can observe at core scale. However, the non-thermal mechanisms responsible for the desorption of methanol in starless cores do not show any dependency on the H2 column density at the methanol peak.