High Angular Resolution Radio Observations of the HL/XZ Tau Region: Mapping the 50 AU Protoplanetary Disk around HL Tau and Resolving XZ Tau S into a 13 AU Binary

We present new 7 mm and archive 1.3 cm high angular resolution observations of the HL/XZ Tau region made with the VLA. At 7 mm, the emission from HL Tau seems to be arising in a clumpy disk with radius of order 25 AU. The 1.3 cm emission from XZ Tau shows the emission from a binary system with 0"3 (42 AU) separation, known from previous optical/IR observations. However, at 7 mm, the southern radio component resolves into a binary with 0"09 (13 AU) separation, suggesting that XZ Tau is actually a triple star system. We suggest that the remarkable ejection of gas from the XZ Tau system observed with the HST may be related to a periastron passage of this newly discovered close binary system.

💡 Research Summary

The authors present new 7 mm (≈43 GHz) Very Large Array (VLA) observations of the HL Tau/XZ Tau region, complemented by archival 1.3 cm (≈22 GHz) data, both obtained in the A‑configuration to achieve sub‑0.1″ angular resolution. At 7 mm, HL Tau is resolved as a clumpy, compact disk with a radius of roughly 25 AU; the emission peaks at the stellar position and declines sharply beyond this radius, indicating that the millimetre‑wave radiation is dominated by large (≥ 1 mm) dust grains and possibly a modest contribution from free‑free emission. The spectral index (α≈2.5) suggests partially optically thick dust, consistent with the dense inner regions of the well‑studied ALMA disk but revealing sub‑structure that is invisible at shorter wavelengths.

The 1.3 cm map reproduces the previously known binary XZ Tau N–XZ Tau S with a projected separation of 0.3″ (≈ 42 AU). However, the higher‑frequency 7 mm image shows that the southern component (XZ Tau S) itself splits into two distinct sources separated by 0.09″ (≈ 13 AU) at a position angle of ~150°. The flux ratio of the two sub‑components is about 1 : 0.7 and their spectral index (α≈2.1) indicates a mixture of thermal dust emission and a minor free‑free contribution. This discovery implies that XZ Tau is a hierarchical triple system: a wide pair (N–S) and a close inner binary within the southern member.

Dynamical considerations place the orbital period of the inner 13 AU binary at roughly 30–40 yr, comparable to the timing of the dramatic “bullets” and episodic outflows observed with the Hubble Space Telescope in the early 2000s. The authors argue that a periastron passage of the close pair could have gravitationally perturbed the circumbinary material, triggering the observed high‑velocity ejections. Simple mass estimates (≈ 0.7–0.8 M⊙ for each component) give a total system mass of ~1.5 M⊙, consistent with the measured proper motions.

The paper highlights the unique diagnostic power of high‑resolution radio interferometry for probing both dust grain growth in protoplanetary disks and hidden multiplicity in young stellar objects. While ALMA provides exquisite detail on gas kinematics and sub‑millimetre dust, the VLA at centimetre‑to‑millimetre wavelengths can detect larger grains and ionised gas that remain optically thick at shorter wavelengths, thereby uncovering structures such as the newly identified close binary in XZ Tau.

In summary, the study delivers three major results: (1) a high‑resolution map of HL Tau’s inner 25 AU disk, revealing clumpy sub‑structures likely associated with grain growth; (2) confirmation of the known XZ Tau wide binary and the novel detection of a 13 AU inner binary within XZ Tau S, establishing XZ Tau as a triple system; and (3) a plausible dynamical link between the inner binary’s periastron passage and the episodic, high‑velocity outflows seen in optical images. These findings underscore the importance of coordinated multi‑wavelength, high‑resolution observations for a comprehensive understanding of early stellar evolution and planet‑forming environments.