Water and Methanol Ice in L1544

TL;DR

This study investigates methanol ice in the cold starless core L1544 through spectroscopy, finding higher methanol ice abundance than typical but less than model predictions, highlighting gaps in current chemical models.

Contribution

First observational constraint on methanol ice abundance in L1544, challenging existing models of ice formation in cold cores.

Findings

Methanol ice abundance observed at 11% relative to water ice.

Observed abundance is 4.5 times lower than model predictions.

Discrepancy suggests incomplete understanding of ice chemistry in cold cores.

Abstract

Methanol and complex organic molecules have been found in cold starless cores, where a standard warm-up scenario would not work because of the absence of heat sources. A recent chemical model attributed the presence of methanol and large organics to the efficient chemical desorption and a class of neutral-neutral reactions that proceed fast at low temperatures in the gas phase. The model calls for a high abundance of methanol ice at the edge of the CO freeze-out zone in cold cloud cores. We performed medium resolution spectroscopy toward 3 field stars behind the starless core L1544 at 3 $\mu$m to constrain the methanol ice abundance and compare it with the model predictions. One of the field stars shows a methanol-ice abundance of 11% with respect to water ice. This is higher than the typical methanol abundance previously found in cold cloud cores (4%), but is 4.5 times smaller than predicted. The reason for the disagreement between the observations and the model calculations is not yet understood.