ALMA 690 GHz observations of IRAS 16293-2422B: Infall in a highly optically-thick disk

TL;DR

This study uses ALMA observations at 690 GHz to investigate the dense, optically thick disk around IRAS 16293-2422B, revealing infall motions, a compact structure, and a possible disk orientation near the plane of the sky.

Contribution

First high-resolution ALMA 690 GHz observations of IRAS 16293-2422B showing detailed disk structure, infall signatures, and optical thickness effects.

Findings

Detected a very compact, optically thick disk of about 50 AU.

Observed inverse P-Cygni profiles indicating infall and turbulence.

Identified a small east-west velocity gradient suggesting a near-plane-of-sky disk orientation.

Abstract

We present sensitive, high angular resolution ($\sim$ 0.2 arcsec) submillimeter continuum and line observations of IRAS 16293-2422B made with the Atacama Large Millimeter/Submillimeter Array (ALMA). The 0.45 mm continuum observations reveal a single and very compact source associated with IRAS 16293-2422B. This submillimeter source has a deconvolved angular size of about 400 {\it milli-arcseconds} (50 AU), and does not show any inner structure inside of this diameter. The H$^{13}$CN, HC$^{15}$N, and CH$_{3}$OH line emission regions are about twice as large as the continuum emission and reveal a pronounced inner depression or "hole" with a size comparable to that estimated for the submillimeter continuum. We suggest that the presence of this inner depression and the fact that we do not see inner structure (or a flat structure) in the continuum is produced by very optically thick dust located in the innermost parts of IRAS 16293-2422B. All three lines also show pronounced inverse P-Cygni profiles with infall and dispersion velocities larger than those recently reported from observations at lower frequencies, suggesting that we are detecting faster, and more turbulent gas located closer to the central object. Finally, we report a small east-west velocity gradient in IRAS 16293-2422B that suggests that its disk plane is likely located very close to the plane of the sky.