Formation mechanism of 1RXS J034231.8+121622 b

Determine whether the directly imaged companion 1RXS J034231.8+121622 b formed via gravitational instabilities in a protoplanetary disk or via disk fragmentation in a protostellar disk, thereby establishing the specific binary-star-related formation pathway for this system given its measured mass near the hydrogen-burning limit, moderate orbital eccentricity, and broadly solar C/O ratios.

Background

The paper characterizes the 1RXS J034231.8+121622 system using high-resolution spectroscopy (Keck/KPIC) and new astrometry (Keck/NIRC2), deriving updated atmospheric parameters, radial velocities, and orbital elements. The companion’s mass is revised upward to about 0.08 M☉, near the hydrogen-burning limit, with an orbital eccentricity around 0.41 and C/O ratios for both components broadly consistent with solar values.

Given the companion’s high mass, the authors argue that a core accretion origin is implausible, focusing instead on two formation scenarios consistent with binary-star-like outcomes: gravitational instability in a protoplanetary disk and protostellar disk fragmentation. While some system properties provide constraints, they do not uniquely distinguish between these two mechanisms in this specific case.

Resolving this open question would clarify the formation pathway for high-mass, directly imaged companions and help place this system within the broader context of companion formation across mass, separation, and age regimes.