[Paper Review] A likely planet-induced gap in the disk around T Cha

Nathanial P. Hendler, Paola Pinilla+8 more|arXiv (Cornell University)|Nov 27, 2017

Astrophysics and Star Formation Studies3 citations

TL;DR

High-resolution ALMA 3 mm observations reveal a 1.2 M_Jup planet likely carving a 18–28 au dust gap in the T Cha protoplanetary disk, with millimeter grains located farther from the star than micron-sized grains. The wavelength-dependent ring positions and gap width suggest planet-disk interactions, with multiple lower-mass planets more likely than a single massive one.

ABSTRACT

We present high resolution ($0.11" imes0.06"$) 3mm ALMA observations of the highly inclined transition disk around the star T Cha. Our continuum image reveals multiple dust structures: an inner disk, a spatially resolved dust gap, and an outer ring. When fitting sky-brightness models to the real component of the 3mm visibilities, we infer that the inner emission is compact ($\le1$au in radius), the gap width is between 18-28 au, and the emission from the outer ring peaks at $\sim36$ au. We compare our ALMA image with previously published 1.6$μ$m VLT/SPHERE imagery. This comparison reveals that the location of the outer ring is wavelength dependent. More specifically, the peak emission of the 3mm ring is at a larger radial distance than that of the 1.6$μ$m ring, suggesting that millimeter-sized grains in the outer disk are located further away from the central star than micron-sized grains. We discuss different scenarios to explain our findings, including dead zones, star-driven photoevaporation, and planet-disk interactions. We find that the most likely origin of the dust gap is from an embedded planet, and estimate --- for a single planet scenario --- that T Cha's gap is carved by a $1.2M_{jup}$ planet.

Motivation & Objective

• To determine the origin of a dust gap in the transition disk around T Cha using high-resolution ALMA 3 mm continuum observations.
• To compare millimeter and near-infrared (1.6 µm) emission structures to investigate grain size segregation and radial distribution differences.
• To assess whether the gap is caused by planet-disk interactions, photoevaporation, or other mechanisms.
• To constrain the mass and number of embedded planets consistent with the observed gap width and radial structure.
• To evaluate the consistency of the observed gap with theoretical models of planet-induced pressure bumps and dust trapping.

Proposed method

• Acquired high-angular-resolution (0.11″ × 0.06″) 3 mm ALMA continuum observations of T Cha’s disk.
• Fitted sky-brightness models to the real component of 3 mm visibilities to derive the spatial structure of inner disk, gap, and outer ring.
• Compared ALMA 3 mm data with high-resolution VLT/SPHERE 1.6 µm scattered light images to analyze wavelength-dependent radial positions of emission peaks.
• Used 2D hydrodynamical and dust evolution models (de Juan Ovelar et al. 2013) to estimate planet mass from the ratio of NIR ring radius to mm peak radius.
• Evaluated competing mechanisms—planet-disk interactions, star-driven photoevaporation, and dead zones—using gap size, accretion rate, and radial structure.
• Assessed the plausibility of single versus multiple planets by comparing observed gap width to theoretical predictions for single-planet gaps.

Experimental results

Research questions

• RQ1 What is the origin of the dust gap in the T Cha disk: planet-disk interaction, photoevaporation, or other processes?
• RQ2 Why is the peak emission of the outer ring at 3 mm located at a larger radial distance than at 1.6 µm, and what does this imply about grain size distribution?
• RQ3 What is the minimum planet mass required to carve the observed gap, assuming a single planet scenario?
• RQ4 Is the observed gap width consistent with a single planet, or does it require multiple planets with overlapping gaps?
• RQ5 How do the observed disk structures compare with theoretical models of dust trapping in planet-induced pressure bumps?

Key findings

• The ALMA 3 mm image reveals a compact inner disk (≤1 au), a dust gap 18–28 au wide, and an outer ring peaking at ∼36 au.
• The millimeter-sized grains in the outer ring are located at a larger radial distance than micron-sized grains seen in 1.6 µm SPHERE images, indicating radial segregation.
• The ratio of NIR ring radius to mm peak radius (R_wall,NIR / R_peak,mm = 0.8) implies a 1.2 M_Jup planet in a single-planet scenario.
• The observed gap width (≥18 au) exceeds the theoretical maximum for a single-planet gap (30–40% of radial location), making multiple planets more likely.
• Star-driven photoevaporation is ruled out as the gap origin due to the low accretion rate (4×10⁻⁹ M_☉/yr) and gap size, which are inconsistent with required growth timescales.
• The observed structure is most consistent with planet-disk interactions, where dust traps form at pressure maxima, with mm grains accumulating at larger radii than micron grains.