A tale of three tails: A misaligned streamer and mysterious structures around [BHB2007]1

TL;DR

This study uses ALMA Band 6 observations to map CO isotopologues around the YSO [BHB2007]-1, revealing three streamer-like structures (A, B, and C) connected to the disk. While A and B appear unbound despite kinematic ties to the disk, structure C is well described by a gravitationally bound infalling trajectory, suggesting a cloudlet-capture event that can inject mass and angular momentum comparable to the disk itself. Using the TIPSY streamer modeling framework, the authors reconstruct a 3D trajectory for C, estimate its infall time (~1.7 × 10^4 yr), and derive a mass infall rate of ~75 M_Jup Myr^-1, implying potential to double the planet-forming mass budget and tilt the disk by ~69°. The results support a dynamic, environment-driven picture of star and planet formation, where interactions with surrounding clouds can induce disk misalignments and influence planetary system architectures.

Abstract

Recent discoveries of streamer-like structures around protostellar sources challenge the traditional picture of isolated, axisymmetric star formation. Here, we present new ALMA observations of [BHB2007]1, a flat-spectrum source connected to at least three such elongated structures. Two of these features are symmetrically located to the north and south of the disk, with velocities aligned with the disk on their respective sides. However, their unbound kinematics and curved morphology make it difficult to determine their origin. Possible explanations include outflows, interactions with the nearby BHB2 system, and hyperbolic infall, but none fully account for all observed properties. In contrast, a newly identified collimated structure to the west shows clear evidence of gravitationally bound infall. Estimates of its mass, mass infall rate, and angular momentum suggest that this infalling streamer would roughly double the mass budget available to form planets and tilt the disk by a few tens of degrees. Furthermore, its misalignment with the midplane of the disk and the lack of diffuse envelope emission indicate that the streamer may have formed due to gravitational capture of cloud material unrelated to the source's natal core. Together, these findings support a more dynamic picture of star formation, one where environmental interactions continue to shape conditions for building planetary systems.

Figures (8)

• Figure 1: Integrated intensity (moment 0) maps of all three CO isotopologue emission lines imaged for this analysis: CO (2--1) (left), $^{13}$CO (2--1) (center), and C$^{18}$O (2--1) (left). The horizontal grey lines in the bottom-left corners represent the length scales of 1000 au, and the pink ellipses in the bottom-right corners represent the beam size. Green contour denote the continuum emission (5 $\sigma$) from the protoplanetary disks. Location of structures A, B, and C, and the nearby source BHB2 are marked on the maps.
• Figure 2: Intensity weighted velocity (moment 1) maps of all three CO isotopologue emission lines imaged for this analysis: CO (2--1) (left), $^{13}$CO (2--1) (center), and C$^{18}$O (2--1) (left). The horizontal grey lines in the bottom-left corners represent the length scales of 1000 au, and the pink ellipses in the bottom-right corners represent the beam size. Black contour denote the continuum emission (5 $\sigma$) from the protoplanetary disks. Location of structures A, B, and C, and the nearby source BHB2 are marked on the maps.
• Figure 3: CO (2--1) (blue), $^{13}$CO (2--1) (green), and C$^{18}$O (2--1) (red) channel maps. The colorbars indicate signal to noise ratio of the observed emission. Dotted white circles in channel maps denote the maximum bound radius ($r_{\text{POS,bound}}$), as defined in Section \ref{['sec:analysis']}. Overlap of emission from different lines is seen as cyan (CO & $^{13}$CO), yellow ($^{13}$CO & C$^{18}$O), and white (all three lines) colors. The horizontal lines in the bottom-left corners, the ellipses in the bottom-right corners, and the grey solid circles represent length scales of 500 au, beam sizes, and the field of view of the data, respectively. Location of structures A, B, and C are marked on the maps.
• Figure 4: The best fitted trajectory from TIPSY, as discussed in the Section \ref{['sec:streamer']}, compared to C$^{18}$O (2--1) data. Left panel shows the full 3D PPV diagram of the molecular-line data, with the black line denoting the best fit. Center and right panels show integrated intensity (moment 0) and position-velocity diagrams for the streamer, respectively, with green curves representing the best fit. Thinner curves in all panels represent trajectories generated from 30 parameter combinations, randomly sampled from a Gaussian distribution based on the uncertainties of the fitted trajectory. An interactive version of the left panel is available online, allowing rotation and zooming of the 3D structures.
• Figure 5: Isometric projection of the best-fit infalling trajectory (thick black line) and associated uncertainties (thin grey lines) for the streamer, as discussed in Section \ref{['sec:modeling']}, in 3D position--position--position space (RA, Decl., and LOS distance). The blue disk and red cone represent the protoplanetary disk and its angular momentum orientation, respectively, as discussed in Section \ref{['sec:disk']}. An interactive version of the left panel is available online, allowing rotation and zooming of the 3D structures.