Importance of source structure on complex organics emission

I. Observations of CH₃OH from low-mass to high-mass protostars

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: vgelder@strw.leidenuniv.nl
² Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str.77, 50937 Köln, Germany
⁶ RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0106, Japan
⁷ Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4235, USA

Received: 27 November 2021
Accepted: 8 February 2022

Abstract

Context. Complex organic molecules (COMs) are often observed toward embedded Class 0 and I protostars. However, not all Class 0 and I protostars exhibit COM emission.

Aims. The aim is to study variations in methanol (CH₃OH) emission and use this as an observational tracer of hot cores to test if the absence of CH₃OH emission can be linked to source properties.

Methods. A sample of 148 low-mass and high-mass protostars is investigated using new and archival observations with the Atacama Large Millimeter/submillimeter Array (ALMA) that contain lines of CH₃OH and its isotopologues. Data for an additional 36 sources are added from the literature, giving a total of 184 different sources. The warm (T ≳ 100 K) gaseous CH₃OH mass, M_CH3OH, is determined for each source using primarily optically thin isotopologues and is compared to a simple toy model of a spherically symmetric infalling envelope that is passively heated by the central protostar.

Results. A scatter of more than four orders of magnitude is found for M_CH3OH among the low-mass protostars, with values ranging between 10⁻⁷ M_⊙ and ≲10⁻¹¹ M_⊙. On average, Class I protostellar systems seem to have less warm M_CH3OH(≲10⁻¹⁰ M_⊙) than younger Class 0 sources (~10⁻⁷ M_⊙). High-mass sources in our sample show more warm M_CH3OH, up to ~10⁻⁷−10⁻³ M_⊙. To take into account the effect of the source’s overall mass on M_CH3OH, a normalized CH₃OH mass is defined as M_CH3OH/M_dust,0, where M_dust,0 is the cold plus warm dust mass in the disk and inner envelope within a fixed radius measured from the ALMA dust continuum. A correlation between M_CH3OH/M_dust,0 and L_bol is found. Excluding upper limits, a simple power-law fit to the normalized warm CH₃OH masses results in M_CH3OH/M_dust,0 ∝ L_bol^0.70±0.05 over an L_bol range of 10⁻¹−10⁶ L_⊙. This is in good agreement with the toy model, which predicts that the normalized M_CH3OH increases with L_bol^0.70 due to the snow line moving outward. Sources for which the size of the disk is equivalent to or smaller than the estimated 100 K radius fall within the 3σ range of the best-fit power-law model, whereas sources with significantly larger disks show normalized warm CH₃OH masses that are up to two orders of magnitude lower.

Conclusions. The agreement between sources that are rich in CH₃OH with the toy model of a spherically symmetric infalling envelope implies that the thermal structure of the envelopes in these sources is likely not strongly affected by a disk. However, based on the disagreement between the toy model and sources that show less warm CH₃OH mass, we suggest that source structure such as a disk can result in colder gas and thus fewer COMs in the gas phase. Additionally, optically thick dust can hide the emission of COMs. Advanced modeling is necessary to quantify the effects of a disk and/or continuum optical depth on the presence of gaseous COMs in young protostellar systems.