The chemical structure of the very young starless core L1521E^★,★★

¹ Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: znagy@mpe.mpg.de
² Ural Federal University, Ekaterinburg 620002, Russia
³ Visiting Leading Researcher, Engineering Research Institute, Ventspils International Radio Astronomy Centre of Ventspils University of Applied Sciences, Inženieru 101, Ventspils 3601, Latvia
⁴ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII, 3 Madrid, Spain

Received: 28 March 2019
Accepted: 30 July 2019

Abstract

Context. L1521E is a dense starless core in Taurus that was found to have relatively low molecular depletion by earlier studies, thus suggesting a recent formation.

Aims. We aim to characterize the chemical structure of L1521E and compare it to the more evolved L1544 pre-stellar core.

Methods. We have obtained ~2.5 × 2.5 arcminute maps toward L1521E using the IRAM-30 m telescope in transitions of various species, including C¹⁷O, CH₃OH, c-C₃H₂, CN, SO, H₂CS, and CH₃CCH. We derived abundances for the observed species and compared them to those obtained toward L1544. We estimated CO depletion factors using the C¹⁷O IRAM-30 m map, an N(H₂) map derived from Herschel/SPIRE data and a 1.2 mm dust continuum emission map obtained with the IRAM-30 m telescope.

Results. Similarly to L1544, c-C₃H₂ and CH₃OH peak at different positions. Most species peak toward the c-C₃H₂ peak including C₂S, C₃S, HCS⁺, HC₃N, H₂CS, CH₃CCH, and C³⁴S. C¹⁷O and SO peak close to both the c-C₃H₂ and the CH₃OH peaks. CN and N₂H⁺ peak close to the Herschel dust peak. We found evidence of CO depletion toward L1521E. The lower limit of the CO depletion factor derived toward the Herschel dust peak is 4.3±1.6, which is about a factor of three lower than toward L1544. We derived abundances for several species toward the dust peaks of L1521E and L1544. The abundances of most sulfur-bearing molecules such as C₂S, HCS⁺, C³⁴S, C³³S, and HCS⁺ are higher toward L1521E than toward L1544 by factors of ~2–20, compared to the abundance of A-CH₃OH. The abundance of methanol is very similar toward the two cores.

Conclusions. The fact that the abundances of sulfur-bearing species toward L1521E are higher than toward L1544 suggests that significant sulfur depletion takes place during the dynamical evolution of dense cores, from the starless to pre-stellar stage. The CO depletion factor measured toward L1521E suggests that CO is more depleted than previously found. Similar CH₃OH abundances between L1521E and L1544 hint that methanol is forming at specific physical conditions in the Taurus Molecular Cloud Complex, characterized by densities of a few ×10⁴ cm⁻³ and N(H₂) ≳ 10²² cm⁻², when CO starts to catastrophically freeze-out, while water can still be significantly photodissociated, so that the surfaces of dust grains become rich in solid CO and CH₃OH, as already found toward L1544. Methanol can thus provide selective crucial information about the transition region between dense cores and the surrounding parent cloud.