Letter to the Editor

The mixing of dust and gas in the high latitude translucent cloud MBM 40

¹ Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, Pisa, Italy
e-mail: monaci93@gmail.com; steven.neil.shore@unipi.it
² Department of Physics and Astronomy, University of Georgia, Athens, GA 30602-2451, Greece
e-mail: loris@uga.edu

Received: 20 September 2022
Accepted: 25 October 2022

Abstract

Context. High latitude molecular clouds (hereafter HLMCs) permit the study of interstellar gas dynamics and astrochemistry with good accuracy due to their proximity, generally clear lines of sight, and lack of internal star-forming activity which can heavily modify the physical context. MBM 40, one of the nearest HLMCs, has been extensively studied, making it a superb target to infer and study the dust-to-gas mixing ratio (DGMR).

Aims. The mixing of dust and gas in the interstellar medium remains a fundamental issue to keep track of astrochemistry evolution and molecular abundances. Accounting for both molecular and atomic gas is difficult because H₂ is not directly observable and H I spectra always show different dynamical profiles blended together which are not directly correlated with the cloud. We used two independent strategies to infer the molecular and atomic gas column densities and compute the dust-to-gas mixing ratio.

Methods. We combined H I 21 cm and ¹²CO line observations with the IRAS 100 μm image to infer the dust-to-gas mixing ratio within the cloud. The cloud 21 cm profile was extracted using a hybrid Gaussian decomposition where ¹²CO was used to deduce the total molecular hydrogen column density. Infrared images were used to calculate the dust emission.

Results. The dust-to-gas mixing ratio is nearly uniform within the cloud as outlined by the hairpin structure. The total hydrogen column density and 100 μm emissivity are linearly correlated over a range in N(H_tot) of one order of magnitude.