B-FROST: B-Fields and star formation across scales with TRAO

CO abundances, dynamics, and relative orientations in the translucent high latitude cloud MBM12

¹ Department of Physics, University of Helsinki, Finland
² Institut UTINAM - UMR 6213 - CNRS - Univ. Bourgogne Franche Comté, OSU THETA, 41bis avenue de l’Observatoire, 25000 Besançon, France
³ LPENS, Ecole Normale Supérieure, Université PSL, CNRS, Sorbonne Université, Université de Paris, France
⁴ IRAP, Université de Toulouse, CNRS, Toulouse, France
⁵ Physics Department, School of Sciences and Humanities, Nazarbayev University, Astana, Kazakhstan
⁶ School of Astronomy and Space Science, Nanjing University, Nanjing, PR China
⁷ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing), Ministry of Education, Nanjing, PR China
⁸ Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea
⁹ Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul, Korea
¹⁰ University of Science and Technology, Korea (UST), Daejeon, Republic of Korea
¹¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, PR China
¹² Department of Physics and Astronomy, University College London, London, UK
¹³ Institut de Ciències del Cosmos, Universitat de Barcelona, Barcelona, Spain
¹⁴ INAF-IAPS, via Fosso del Cavaliere, 100, Roma, Italy
¹⁵ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Budapest, Hungary
¹⁶ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹⁷ Institut Universitaire de France, 1 rue Descartes, Paris, France
¹⁸ Institute of Physics, University of Debrecen, Hungary

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 February 2026
Accepted: 18 May 2026

Abstract

Context. On average, in our Galaxy, the star formation efficiency (SFE) is of the order of a few percent, which is lower than theoretical predictions. Detailed observational studies of individual molecular clouds may offer insight into the contributing factors to the low galactic SFE.

Aims. We investigated the high-latitude molecular cloud MBM12 as part of the B-fields and star formation across scales (B-FROST) survey with the Taeduk Radio Astronomical Observatory (TRAO) to assess why star formation activity in MBM12 is low.

Methods. We estimated N(H₂) with Herschel dust emission and a dust opacity κ_ν derived from near-infrared extinction, 21 cm HI column densities, and far-infrared emission. With ¹²CO and ¹³CO (J = 1-0) line observations, covering an area of 2.5° × 3° at 48″ resolution, we mapped the CO column density N(CO), CO-to-H₂ factor X(CO), and abundance [CO/H₂]. We estimated the multi-scale virial parameter α_vir and constructed mass-size scaling laws of hierarchical structures with dendrograms. We computed the relative orientation between column density structures and magnetic fields using Planck observations of dust polarisation.

Results. We identified four main regions based on velocities with H₂ column densities ranging from 2 × 10²⁰−1.3 × 10²² cm⁻². The CO integrated line intensity, W(CO), increases linearly with N(H₂), providing an average X(CO) factor close to the Galactic average. At a low N(H₂), X(CO) varies below X_Gal due to the fall-off of collisional de-excitation in low-density gas, and above X_Gal due to the drop of CO abundances in poorly shielded cloud edges. The hierarchical structures follow a broken power law mass-size relation M = AR^α. The values of α_vir ranged from 3-60, with the smallest values at 0.1 pc scales. The mass-size relations for the structures with the lowest α_vir have scaling factors, A, three times larger than those of high α_vir structures, indicating external pressure one order of magnitude larger than the former. We found a transition from parallel to perpendicular relative orientations between column density structures and the magnetic field at N(H₂) = 4.5 × 10²¹ cm⁻².

Conclusions. We provide the first integrated chemical, dynamical, and magnetic field analysis of MBM12. Further investigation into the scale dependence of the mass-size relation and virial parameter can highlight the role of external pressure in regulating the star-formation efficiency.