A Widely-Separated, Highly-Occluded Companion to the Nearby Low-Mass T Tauri Star TWA 30

TL;DR

This paper reports the discovery and detailed characterization of TWA 30B, a highly obscured, actively accreting low-mass star with an edge-on disk, providing insights into star and planet formation at the low-mass end.

Contribution

It presents the first optical detection of [C I] in a pre-main sequence star and characterizes the disk structure and outflows of TWA 30B, a rare wide-separation companion to TWA 30A.

Findings

TWA 30B is a heavily obscured, actively accreting M dwarf with an edge-on disk.

Detected [C I] emission, first in a pre-main sequence star.

Variable near-infrared excess consistent with a disk rim or warp.

Abstract

We report the discovery of TWA 30B, a wide (~3400 AU), co-moving M dwarf companion to the nearby (~42 pc) young star TWA 30. Companionship is confirmed from their statistically consistent proper motions and radial velocities, as well as a chance alignment probability of only 0.08%. Like TWA 30A, the spectrum of TWA 30B shows signatures of an actively accreting disk (H I and alkali line emission) and forbidden emission lines tracing outflowing material ([O I], [O II], [O III], [S II], and [N II]). We have also detected [C I] emission in the optical data, marking the first such detection of this line in a pre-main sequence star. Negligible radial velocity shifts in the emission lines relative to the stellar frame of rest (Delta V < 30 km/s) indicate that the outflows are viewed in the plane of the sky and that the corresponding circumstellar disk is viewed edge-on. Indeed, TWA 30B appears to be heavily obscured by its disk, given that it is 5 magnitudes fainter than TWA 30A at K-band despite having a slightly earlier spectral type (M4 versus M5). The near-infrared spectrum of TWA 30B also evinces an excess that varies on day timescales, with colors that follow classical T Tauri tracks as opposed to variable reddening (as is the case for TWA 30A). Multi-epoch data show this excess to be well-modeled by a blackbody component with temperatures ranging from 630 to 880 K and emitting areas that scale inversely with the temperature. The variable excess may arise from disk structure such as a rim or a warp at the inner disk edge located at a radial distance of ~3-5 R_sun. As the second and third closest actively accreting and outflowing stars to the Sun (after TWA 3), TWA 30AB presents an ideal system for detailed study of star and planetary formation processes at the low-mass end of the hydrogen-burning spectrum.