G11.92–0.61 MM 1: A Fragmented Keplerian Disk Surrounding a Proto-O Star

Abstract

We present high resolution (∼300 au) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the massive young stellar object G11.92−0.61 MM 1. We resolve the immediate circumstellar environment of MM 1 in 1.3 mm continuum emission and CH 3 CN emission for the first time. The object divides into two main sources -MM 1a, which is the source of a bipolar molecular outflow, and MM 1b, located 0. 57 (1920 au) to the South-East. The main component of MM 1a is an elongated continuum structure, perpendicular to the bipolar outflow, with a size of 0. 141 × 0. 050 (480 × 170 au). The gas kinematics toward MM 1a probed via CH 3 CN trace a variety of scales. The lower energy J = 12-11 K = 3 line traces extended, rotating gas within the outflow cavity, while the v8=1 line shows a clearly-resolved Keplerian rotation signature. Analysis of the gas kinematics and dust emission shows that the total enclosed mass in MM 1a is 40 ± 5 M (where between 2.2-5.8 M is attributed to the disk), while MM 1b is < 0.6 M . The extreme mass ratio and orbital properties of MM 1a and MM 1b suggest that MM 1b is one of the first observed examples of the formation of a binary star via disk fragmentation around a massive young (proto)star.

Massive discs around low-mass stars

“…For the other candidate with spirals identified in the DSHARP survey without a disc mass estimate, WaOph 6, we expect a required disc mass of 0.155 − 0.42 M for self gravity induced spirals in the limits of being optically thick and optically thin. Optimistically extrapolating our criteria to proto O stars predicts disc spirals in the fragmenting system observed by Ilee et al (2018) 3. Very low-mass stars (∼ 0.1 M ) can sustain very high disc-to-star mass ratios before becoming so gravitationally unstable that they undergo fragmentation.…”

“…Observations of such discs are challenging owing to their scarcity, distance, short lifetimes and embedded nature. However, where resolved observations are finally becoming available we do indeed seem to be detecting evidence of gravitational instability and even fragmentation (Ilee et al 2018;Jankovic et al 2019). As a simple illustration if we use the inferred stellar and disc masses from Ilee et al (2018) of 34.2 and 5.8 M respectively, as well as the inferred disc size of 850 AU in equation 9 we get a critical ratio for axisymmetry of 0.15 and observed ratio of 0.17 and so would expect the instability that they have observed.…”

Sub-arcsecond (Sub)millimeter Imaging of the Massive Protocluster G358.93−0.03: Discovery of 14 New Methanol Maser Lines Associated with a Hot Core

“…Interpreting the velocity gradient in the thermal CH 3 CN emission as arising from disk rotation, as seen in other massive protostars (e.g. Ilee et al 2018), also presents concerns. While the radius and linewidth would imply a dynamical mass of (5/sin i) M , the higher velocity channels do not peak closer to the center of the distribution but instead near the edges, which suggests a ring morphology with edge-on inclination (i ∼ 90 • ) for the thermal gas; or simply two sources at different v lsr .…”

Formation and Evolution of Disks Around Young Stellar Objects

“…Recent examples of rotating disks around high-mass YSOs are G35.20−0.74N at 2.19 kpc (Fig. 5;Sánchez-Monge et al 2013, G17.64+0.16 at 2.2 kpc (Maud et al 2019), IRAS 16547−4247 at 2.9 kpc (Zapata et al 2015(Zapata et al , 2019, G35.03+0.35 at 3.2 kpc ), G11.92−0.61 at 3.37 kpc (Ilee et al 2018), G16.59−0.05 at 3.6 kpc , and AFGL4176 at 4.2 kpc (Johnston et al 2015), ordered in distance from the nearest to the farthest disks. They exhibit probably Keplerian rotation curve observed in CH 3 CN and CH 3 OH lines including high K-ladder lines, torsionally excited transitions, and isotopologues, which preferentially trace hotter and denser regions.…”