Magnetically regulated disk-formation in the inner 100 au region of the Class 0 young stellar object OMC-3/MMS~6 resolved by JVLA and ALMA

Abstract

We have carried out polarization calibration for archival JVLA ($\sim$9 mm) full polarization observations towards the Class 0 young stellar object (YSO) OMC-3/MMS 6 (also known as HOPS-87), and then compared with the archival ALMA 1.2 mm observations. We found that the innermost $\sim$100 au region of OMC-3/MMS 6 is likely very optically thick (e.g., $\tau \gg$1) at $\sim$1 mm wavelength such that the dominant polarization mechanism is dichroic extinction. It is marginally optically thin (e.g., $\tau \lesssim$1) at $\sim$9 mm wavelength such that the JVLA observations can directly probe the linearly polarized emission from non-spherical dust. Assuming that the projected long axis of dust grains is aligned perpendicular to magnetic field (B-field) lines, we propose that the overall B-field topology resembles an hourglass shape, while this "hourglass" appears $\sim$40${}^{\circ }$ inclined with respect to the previously reported outflow axis. The geometry of this system is consistent with a magnetically regulated dense (pseudo-)disk. Based on the observed 29.45 GHz flux density and assuming a dust absorption opacity ${\kappa }_{29.45GHz}^{abs}=$0.0096 cm${}^2$g${}^{-1}$, the derived overall dust mass within a $\sim$43 au radius is $\sim$14000 $M_{\oplus }$. From this case study, it appears to us that some previous 9 mm surveys towards Class 0/I YSOs might have systematically underestimated dust masses by one order of magnitude, owing to that they assumed the too high dust absorption opacity ($\sim$0.1 cm${}^2$ g${}^{-1}$) for $\sim$9 mm wavelengths but without self-consistently considering the dust scattering opacity.

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