Binary Formation in a 100 $\mu$m-dark Massive Core

Abstract

We report high-resolution ALMA observations toward a massive protostellar core C1-Sa ($\sim$30 M${}_{\odot }$) in the Dragon Infrared Dark Cloud. At the resolution of 140 AU, the core fragments into two kernels (C1-Sa1 and C1-Sa2) with a projected separation of $\sim$1400 AU along the elongation of C1-Sa, consistent with a Jeans length scale of $\sim$1100 AU. Radiative transfer modeling using RADEX indicates that the protostellar kernel C1-Sa1 has a temperature of $\sim$75 K and a mass of 0.55 M${}_{\odot }$. C1-Sa1 also likely drives two bipolar outflows, one being parallel to the plane-of-the-sky. C1-Sa2 is not detected in line emission and does not show any outflow activity but exhibits ortho-H${}_2$D${}^{+}$ and N${}_2$D${}^{+}$ emission in its vicinity, thus it is likely still starless. Assuming a 20 K temperature, C1-Sa2 has a mass of 1.6 M${}_{\odot }$. At a higher resolution of 96 AU, C1-Sa1 begins to show an irregular shape at the periphery, but no clear sign of multiple objects or disks. We suspect that C1-Sa1 hosts a tight binary with inclined disks and outflows. Currently, one member of the binary is actively accreting while the accretion in the other is significantly reduced. C1-Sa2 shows hints of fragmentation into two sub-kernels with similar masses, which requires further confirmation with higher sensitivity.

7 figures, 2 tables, accepted by ApJ