Observations of gas flows inside a protoplanetary gap

Abstract: Gaseous giant planet formation is thought to occur in the first few million years following stellar birth. Models predict that giant planet formation carves a deep gap in the dust component (shallower in the gas). Infrared observations of the disk around the young star HD142527, at ~140pc, found an inner disk ~10AU in radius, surrounded by a particularly large gap, with a disrupted outer disk beyond 140AU, indicative of a perturbing planetary-mass body at ~90 AU. From radio observations, the bulk mass is molecular and lies in the outer disk, whose continuum emission has a horseshoe morphology. The vigorous stellar accretion rate would deplete the inner disk in less than a year, so in order to sustain the observed accretion, matter must flow from the outer-disk into the cavity and cross the gap. In dynamical models, the putative protoplanets channel outer-disk material into gap-crossing bridges that feed stellar accretion through the inner disk. Here we report observations with the Atacama Large Millimetre Array (ALMA) that reveal diffuse CO gas inside the gap, with denser HCO+ gas along gap-crossing filaments, and that confirm the horseshoe morphology of the outer disk. The estimated flow rate of the gas is in the range 7E-9 to 2E-7 Msun/yr, which is sufficient to maintain accretion onto the star at the present rate.

• The paper demonstrates ALMA's capability to capture gap-crossing gas flows that sustain accretion in the HD 142527 system.
• It integrates hydrodynamical simulations to confirm that filamentary structures mirror planet-induced accretion processes.
• Observations reveal a pronounced horseshoe-shaped outer disk and a steep drop in mm-sized dust, highlighting the power of high-resolution imaging.

Observations of Gas Flows Inside a Protoplanetary Gap: An Examination of HD 142527

The paper "Observations of gas flows inside a protoplanetary gap" presents a comprehensive investigation of the gas dynamics within the protoplanetary disk surrounding the young star HD 142527, utilizing data obtained from the Atacama Large Millimeter/submillimeter Array (ALMA). This study offers critical insights into the formation processes of gaseous giant planets and elucidates the mechanics of material transfer across significant gap structures in protoplanetary disks.

The authors identify a large central cavity in the HD 142527 system, characterized by the presence of diffuse CO gas and denser HCO$^+$ gas distributed along gap-crossing filaments. The estimated gas flow rates range from $7 \times 10^{-9}$ to $2 \times 10^{-7}$ M$_\odot$ yr$^{-1}$, effectively sustaining the observed stellar accretion rates. This accretion pathway supports theoretical models which posit that protoplanets can channel material from the outer disk to feed the inner disk, impacting stellar accretion through the creation of 'gap-crossing bridges'.

A salient feature of the study is its confirmation of the horseshoe morphology of the outer disk through ALMA observations. Previous infrared data had hinted at the presence of a disrupted outer disk, potentially influenced by a planetary-mass object. The analysis highlights a significant asymmetry in the dust continuum signal, a surface density reduction of mm-sized grains by at least a factor of 300 across the observed gap. Such findings underscore the potency of high-resolution observations in revealing the morphology and structure of protoplanetary disks, especially concerning the dynamics within these gaps.

The HCO$^+$ gas distribution in the disk indicates a potential interaction with UV radiation, with significant concentrations along the exposed rim of the dense outer disk and within the identified filaments. These dynamics suggest complex interplays between stellar radiation and the disk's material, influencing the disk's structure and evolution.

The integration of hydrodynamical simulations aligns with observed filaments, suggesting that these features mirror planet-induced accretion flows. The paper notes a comparison with the GG Tau system, where a similar accretion stream is observed; however, GG Tau houses a known binary pair, providing contrast to the current understanding of HD 142527, which lacks a detected stellar companion within the studied range.

High-contrast infrared imaging was employed in an attempt to detect potential protoplanets that might be driving the observed accretion flows. The authors report limits on companion mass, constrained to a few Jupiter masses depending on the region of the disk, but acknowledge that extinction from the dense material may obscure these companions even further.

Agreement between the observed gas dynamics and predictions from hydrodynamic simulations suggests that planet formation can significantly feedback on the parent disk's structure by carving gaps while allowing material inflow. This supports the theoretical framework that planets, including giant gaseous bodies, influence and are influenced by their natal environments. The work underscores the ongoing need for integrative approaches combining observation with advanced simulation to discern the processes underpinning protoplanetary disk evolution.

This research is crucial for advancing our understanding of protoplanetary disk dynamics, with implications for the broader field of planet formation and development. By leveraging advanced observational capabilities like those of ALMA, astronomers can continue to refine their models of planetary system formation and evolution. As observational technology and techniques continue to improve, further studies will likely provide deeper insights into these formative cosmic processes.