B-Fields and Star Formation across Scales with TRAO (B-FROST): CO Abundances, Dynamics and Relative Orientations in the Translucent High Latitude Cloud MBM12

Abstract

In our Galaxy, the average star formation efficiency is of the order of a few percent. 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. We combine {\it Herschel}-based, locally $\hat{I}{º}_{\hat{I}}½$-calibrated $N$(H${}_2$) estimates with ${}^{12}$CO and ${}^{13}$CO ($J=1-0$) observations (2.5${}^{\circ }\times$3${}^{\circ }$ at 48${}^{\prime \prime }$) to map $N$(CO), $X$(CO), and [CO/H${}_2$], compute multi-scale $\hat{I}{\pm }_{\mathrm{vir}}$ and mass-size scaling laws from dendrograms, and derive the histogram of relative orientations from {\it Planck} dust polarisation. We identify four main regions based on velocities that have H${}_2$ column densities ranging from $2\times 10^{20}$ cm${}^{-2}-1.3\times 10^{22}$ cm${}^{-2}$. The average $X$(CO) is close to the galactic average, with variations below $X_{\mathrm{Gal}}$ from collisional de-excitation in low-density gas, and above $X_{\mathrm{Gal}}$ from CO photodissociation at cloud edges. The hierarchical structures follow a broken power law mass-size relation $M=A{R}^{\hat{I}}\pm$. The values of $\hat{I}{\pm }_{\mathrm{vir}}$ ranged from $3-60$, with the smallest values at 0.1 pc scales. The mass-size relations for the structures with the lowest $\hat{I}{\pm }_{\mathrm{vir}}$ have scaling factors $A$ three times larger than those of high $\hat{I}{\pm }_{\mathrm{vir}}$ structures, indicating external pressure one order of magnitude larger. We found a transition of parallel to perpendicular between column density structures and magnetic field orientations at $N$(H${}_2$) $=4.5\times 10^{21}$ cm${}^{-2}$. We provide the first integrated chemical, dynamical, and magnetic field analysis of MBM12. Scale-dependent mass-size and virial analysis can further constrain the role of external pressure in regulating the star formation efficiency.

21 pages, 16 figures, accepted in Astronomy and Astrophysics